U.S. patent application number 15/876669 was filed with the patent office on 2018-10-04 for controlling pulse timing to compensate for motor dynamics.
The applicant listed for this patent is LUMINAR TECHNOLOGIES, INC.. Invention is credited to Scott R. Campbell, Rodger W. Cleye, Jason M. Eichenholz, Lane A. Martin, Austin K. Russell, Melvin L. Stauffer, Matthew D. Weed.
Application Number | 20180284225 15/876669 |
Document ID | / |
Family ID | 63638687 |
Filed Date | 2018-10-04 |
United States Patent
Application |
20180284225 |
Kind Code |
A1 |
Weed; Matthew D. ; et
al. |
October 4, 2018 |
CONTROLLING PULSE TIMING TO COMPENSATE FOR MOTOR DYNAMICS
Abstract
To compensate for motor dynamics in a scanner in a lidar system,
a light source transmits light pulses at a variable pulse rate in
accordance with a scan speed of the scanner. More specifically, the
pulse rate may be directly related to the scan speed so that the
light source transmits light pulses uniformly across a field of
regard. A controller may determine the scan speed and provide a
control signal to the light source adjusting the pulse rate
accordingly.
Inventors: |
Weed; Matthew D.; (Winter
Park, FL) ; Campbell; Scott R.; (Sanford, FL)
; Martin; Lane A.; (Sunnyvale, CA) ; Eichenholz;
Jason M.; (Orlando, FL) ; Russell; Austin K.;
(Palo Alto, CA) ; Cleye; Rodger W.; (Aliso Viejo,
CA) ; Stauffer; Melvin L.; (Orlando, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LUMINAR TECHNOLOGIES, INC. |
Orlando |
FL |
US |
|
|
Family ID: |
63638687 |
Appl. No.: |
15/876669 |
Filed: |
January 22, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62478258 |
Mar 29, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 7/4817 20130101;
G01S 17/08 20130101; G01S 7/484 20130101; G01S 17/10 20130101; G01S
17/42 20130101 |
International
Class: |
G01S 7/484 20060101
G01S007/484; G01S 7/481 20060101 G01S007/481; G01S 17/08 20060101
G01S017/08 |
Claims
1. A method of controlling pulse rate in lidar systems in which a
scanner scans in a forward-scanning direction and a
reverse-scanning direction, to compensate for motor dynamics of the
scanner, the method comprising: generating light pulses by a light
source in a lidar system; scanning, by a scanner in the lidar
system, a field of view of the light source across a field of
regard of the lidar system in a forward-scanning direction and a
reverse-scanning direction, including: directing the light pulses
toward different points within the field of regard, and decreasing
a scan speed of the scanner as the scanner changes direction
between the forward-scanning direction and the reverse-scanning
direction at respective turnaround points; adjusting a pulse rate
at which the lidar system generates the light pulses in accordance
with the scan speed, including transmitting the light pulses at a
first pulse rate for a first scan speed and at a second pulse rate
for a second scan speed; and detecting, by a receiver of the lidar
system, light from some of the light pulses scattered by one or
more remote targets to generate respective pixels.
2. The method of claim 1, wherein adjusting the pulse rate includes
decreasing the pulse rate when decreasing the scan speed, so that
the scanner distributes the light pulses uniformly across the field
of regard.
3. The method of claim 1, wherein adjusting the pulse rate includes
increasing the pulse rate when decreasing the scan speed, so that
the pixel density is higher at a turnaround point.
4. The method of claim 1, wherein scanning across the field of
regard includes using two or more scanners to concurrently
illuminate two or more different portions of the field of regard,
with each of the two or more scanners having an inner turnaround
point and an outer turnaround point.
5. The method of claim 4, wherein adjusting the pulse rate includes
increasing the pulse rate as the scanner approaches the inner
turnaround point and decreasing the pulse rate as the scanner
approaches the outer turnaround point.
6. The method of claim 4, wherein adjusting the pulse rate includes
decreasing the pulse rate as the scanner approaches the inner
turnaround point and increasing the pulse rate as the scanner
approaches the outer turnaround point.
7. The method of claim 4, wherein the emitted light pulses are
split to produce two or more output beams which are scanned by the
respective two or more scanners.
8. The method of claim 4, wherein using two or more scanners
includes scanning simultaneously in opposite directions.
9. The method of claim 4, wherein using two or more scanners
includes scanning simultaneously in a same direction.
10. The method of claim 1, further comprising: adjusting an amount
of average power of the emitted light pulses or an amount of energy
per light pulse provided by the light source based on the pulse
rate.
11. The method of claim 10, wherein the amount energy per light
pulse is higher for slower pulse rates.
12. The method of claim 1, further comprising: identifying, using a
controller, the scan speed of the scanner, and providing a control
signal to the light source to adjust the pulse rate according to
the scan speed.
13. A lidar system comprising: a light source configured to emit
light pulses; a scanner configured to scan a field of view of the
light source across a field of regard of the lidar system in a
forward-scanning direction and a reverse-scanning direction,
including: direct the light pulses toward different points within
the field of regard; and decrease a scan speed as the scanner
changes direction between the forward-scanning direction and the
reverse-scanning direction at respective turnaround points; a
controller configured to adjust a pulse rate at which the light
pulses are emitted by the light source, including: when the scanner
scan at a first scan speed, cause the light source to emit the
light pulses at a first pulse rate, and when the scanner scans at a
second scan speed, cause the light source to emit the light pulses
at a second pulse rate; and a detector configured to detect light
from some of the light pulses scattered by one or more remote
targets to generate respective pixels.
14. The lidar system of claim 13, wherein to adjust the pulse rate
the controller is configured to decrease the pulse rate when the
scan speed decreases, so that the scanner distributes the light
pulses uniformly across the field of regard.
15. The lidar system of claim 13, wherein to adjust the pulse rate
the controller is configured to increase the pulse rate when the
scan speed decreases, so that the pixel density is higher in at a
turnaround point.
16. The lidar system of claim 13, wherein the controller is further
configured to receive an indication of the scan speed of the
scanner and to provide a control signal to the light source to
adjust the pulse rate in accordance with the scan speed.
17. The lidar system of claim 13, wherein the scanner includes two
or more scanners to concurrently illuminate two or more different
portions of the field of regard, with each of the two or more
scanners having an inner turnaround point and an outer turnaround
point.
18. The lidar system of claim 17, wherein for each of the two or
more scanners, to adjust the pulse rate the controller is
configured to increase the pulse rate as the scanner approaches the
inner turnaround point and decrease the pulse rate as the scanner
approaches the outer turnaround point.
19. The lidar system of claim 17, wherein for each of the two or
more scanners, to adjust the pulse rate the controller is
configured to decrease the pulse rate as the scanner approaches the
inner turnaround point and increase the pulse rate as the scanner
approaches the outer turnaround point.
20. The lidar system of claim 17, wherein the two or more scanners
scan simultaneously in opposite directions.
21. The lidar system of claim 17, wherein the two or more scanners
scan simultaneously in a same direction.
22. The lidar system of claim 13, wherein the controller is further
configured to adjust an amount of average power of the emitted
light pulses or an amount of energy per light pulse provided by the
light source based on the pulse rate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to provisional U.S.
application Ser. No. 62/478,258, filed on Mar. 29, 2017, entitled
"Controlling Pulse Timing To Compensate for Motor Dynamics" the
entire disclosure of which is hereby expressly incorporated by
reference herein.
FIELD OF TECHNOLOGY
[0002] This disclosure generally relates to lidar systems and, more
particularly, to varying the pulse rate at which light pulses are
transmitted in the lidar system.
BACKGROUND
[0003] The background description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description that
may not otherwise qualify as prior art at the time of filing, are
neither expressly nor impliedly admitted as prior art against the
present disclosure.
[0004] Light detection and ranging (lidar) is a technology that can
be used to measure distances to remote targets. Typically, a lidar
system includes a light source and an optical receiver. The light
source can be, for example, a laser which emits light having a
particular operating wavelength. The operating wavelength of a
lidar system may lie, for example, in the infrared, visible, or
ultraviolet portions of the electromagnetic spectrum. The light
source emits light toward a target which then scatters the light.
Some of the scattered light is received back at the receiver. The
system determines the distance to the target based on one or more
characteristics associated with the returned light. For example,
the system may determine the distance to the target based on the
time of flight of a returned light pulse.
SUMMARY
[0005] One example embodiment of the techniques of this disclosure
is a method of controlling pulse rate in lidar systems in which a
scanner scans in a forward-scanning direction and a
reverse-scanning direction, to compensate for motor dynamics of the
scanner. The method includes generating light pulses by a light
source in a lidar system and scanning, by a scanner in the lidar
system, a field of view of the light source across a field of
regard of the lidar system in a forward-scanning direction and a
reverse-scanning direction, including: directing the light pulses
toward different points within the field of regard and decreasing a
scan speed of the scanner as the scanner changes direction between
the forward-scanning direction and the reverse-scanning direction
at respective turnaround points. The method further includes
setting a pulse rate at which the lidar system generates the light
pulses in accordance with the scan speed, including transmitting
the light pulses at a first pulse rate for a first scan speed and
at a second pulse rate for a second scan speed and detecting, by a
receiver of the lidar system, light from the light pulses scattered
by one or more remote targets to generate respective pixels.
[0006] Another example embodiment of the techniques of this
disclosure is a lidar system including a light source configured to
emit light pulses and a scanner configured to scan a field of view
of the light source across a field of regard of the lidar system in
a forward-scanning direction and a reverse-scanning direction. More
specifically, the scanner is configured to direct the light pulses
toward different points within the field of regard and decrease a
scan speed as the scanner changes direction between the
forward-scanning direction and the reverse-scanning direction at
respective turnaround points. The lidar system further includes a
controller configured to adjust a pulse rate at which the light
pulses are emitted by the light source, including when the scanner
scan at a first scan speed, the controller causes the light source
to emit the light pulses at a first pulse rate. When the scanner
scans at a second scan speed, the controller causes the light
source to emit the light pulses at a second pulse rate.
Additionally, the lidar system includes a detector configured to
detect light from some of the light pulses scattered by one or more
remote targets to generate respective pixels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a block diagram of an example light detection and
ranging (lidar) system in which the techniques of this disclosure
can be implemented;
[0008] FIG. 2 illustrates in more detail several components that
can operate in the system of FIG. 1;
[0009] FIG. 3 illustrates an example configuration in which the
components of FIG. 1 scan a 360-degree field of regard through a
window in a rotating housing;
[0010] FIG. 4 illustrates another configuration in which the
components of FIG. 1 scan a 360-degree field of regard through a
substantially transparent stationary housing;
[0011] FIG. 5 illustrates an example scan pattern which the lidar
system of FIG. 1 can produce when identifying targets within a
field of regard;
[0012] FIG. 6 illustrates an example scan pattern which the lidar
system of FIG. 1 can produce when identifying targets within a
field of regard using multiple beams;
[0013] FIG. 7 schematically illustrates fields of view (FOVs) of a
light source and a detector that can operate in the lidar system of
FIG. 1;
[0014] FIG. 8 illustrates an example configuration of the lidar
system of FIG. 1 or another suitable lidar system, in which a laser
is disposed away from sensor components;
[0015] FIG. 9 illustrates an example vehicle in which the lidar
system of FIG. 1 can operate;
[0016] FIG. 10 illustrates an example InGaAs avalanche photodiode
which can operate in the lidar system of FIG. 1;
[0017] FIG. 11 illustrates an example photodiode coupled to a
pulse-detection circuit, which can operate in the lidar system of
FIG. 1;
[0018] FIG. 12 is a timing diagram of an example technique for
transmitting light pulses upon detection of return pulses, which
can be implemented in the lidar system of FIG. 1;
[0019] FIG. 13 illustrates an example technique for varying the
pulse rate in accordance with the orientation of transmitted light
pulses with respect to a vehicle, which can operate in the lidar
system of FIG. 1;
[0020] FIG. 14 illustrates another example technique for varying
the pulse rate in accordance with the orientation of transmitted
light pulses with respect to a vehicle, which can be implemented in
the lidar system of FIG. 1;
[0021] FIG. 15 illustrates another example technique for varying
the pulse rate in accordance with a scan speed of a scanner that
can operate in the lidar system of FIG. 1; and
[0022] FIG. 16 illustrates a flow diagram of an example method for
varying the pulse rate in accordance with the scan speed of the
scanner.
DETAILED DESCRIPTION
Overview
[0023] Generally speaking, a light source (e.g., a fiber laser or a
laser diode) in a lidar system transmits light pulses at a variable
pulse rate in view of various conditions and/or events. One
technique includes operating the light source so that after
transmitting light pulse N, the light source transmits subsequent
light pulse N+1 upon detecting a return light pulse that
corresponds to the light pulse N, or upon expiration of the time
period during which a light pulse can travel to a target at a
maximum supported distance and back. In this manner, the effective
pulse rate for the light source may be faster than a fixed pulse
rate selected based on the time it takes a light pulse to travel to
a target at the maximum range (e.g., 200 m) and return to the lidar
system. For example, the fixed pulse rate may be about 750 kHz to
allow for approximately 1.33 .mu.s time of flight to a target at
200 m and back. The effective or average pulse rate may be
significantly faster when some of the light pulses return from
targets much closer than 200 m away (e.g., 5 m away), and new light
pulses are transmitted in response.
[0024] In an example implementation, a controller is
communicatively coupled to the light source that transmits light
pulses as well as to a receiver that detects return light pulses
scattered by remote targets. For example, a pulse-detection circuit
at the receiver may generate an indication that a return pulse has
been received when a voltage of incoming light exceeds a certain
threshold.
[0025] The receiver may determine one or more of the peak power for
the return light pulse, the average power for the return light
pulse, the pulse energy of the return light pulse, the pulse
duration of the return light pulse, or any other measurable
characteristics of the return light pulse. The receiver then may
provide an appropriate indication of the detected characteristics
of the return light pulse to the controller.
[0026] Next, the controller may compare the peak power, average
power, or pulse energy of the return light pulse to a power or
energy threshold to determine whether the return light pulse
corresponds to an emitted light pulse scattered by a soft target or
a hard target. In some implementations, the controller may combine
the peak or average power, pulse energy, and the pulse duration in
any suitable manner (e.g., by determining a ratio between the peak
power and the pulse duration) and compare the combined metric to a
combined threshold.
[0027] When the return light pulse exceeds the power or energy
threshold and/or the combined threshold, the controller may provide
a control signal to the light source to transmit another light
pulse. This process may be repeated before transmitting each light
pulse.
[0028] In some implementations, the pulse rate varies according to
the orientation of the beam with respect to the vehicle. For
example, under certain conditions, targets within a field of regard
of the lidar system are more likely to be closer to the middle of
the field of regard than to the periphery. Additionally, the lidar
system in some cases may be more concerned with objects directly in
front of the vehicle or the lidar system than objects not directly
in front of the vehicle or the lidar system (e.g., to better avoid
accidents). To compensate for the uneven distribution of data
points, the lidar system in this implementation transmits light
pulses at a variable pulse rate such that the pulse rate is slower
when scanning at orientations near the front of the vehicle or the
lidar system and faster at the periphery of the field of
regard.
[0029] In this manner, the lidar system may increase the power and
range farther directly in front of the vehicle or the lidar system
and then decrease the power and corresponding range as the lidar
system scans toward the sides. For example, directly in front of
the lidar system, the lidar system may range 200 m whereas at an
orientation near the periphery (e.g., .+-.60 degrees from the front
of the lidar system), the lidar system may range 50 m.
Additionally, by increasing the pulse rate around the periphery,
the lidar system can collect more data points in areas or portions
of the field of regard where there are fewer targets than the
number of targets directly in front of the lidar system. The number
of targets in a region or portion of a field of regard may be
referred to herein as "information density." For example, the
information density may be high directly in front of the vehicle
where there are several vehicles, road signs, and other objects.
Information density may be low around the periphery of the vehicle
where other vehicles are less likely to be present. In some
scenarios, the lidar system may increase the pulse rate thereby
increasing the resolution and pixel density in areas where
information density is low to identify small objects in such areas
where objects are sparsely located. Additionally, the pulse rate
may be increased to conserve power or energy in these areas where
the lidar system is unlikely to identify objects near a maximum
range and/or unlikely to approach such objects. Still further, the
pulse rate may be increased around the periphery based on an
upcoming vehicle maneuver such as a lane charge or turn. For
example, when the vehicle is about to make a left turn, the lidar
system may increase the pulse rate near the left side of the
vehicle to more clearly identify objects in an area that the
vehicle is approaching.
[0030] In some implementations, the controller identifies the
orientations at which the light pulses are transmitted by
processing the corresponding signal from the scanner, for example.
The controller may provide control signals to the light source to
increase the pulse rate as the angle defining the orientation
increases. In some implementations, the controller may compare the
orientation angle to a threshold orientation angle. Then the
controller may provide a control signal to the light source to
adjust the pulse rate when the orientation angle rises above (or
falls below) a threshold orientation angle. For example, at zero
degrees relative to the line along which the vehicle is currently
moving, the pulse rate may be 750 kHz to allow for a 1.33 .mu.s
time of flight to reach a target at a maximum range of 200 m. Then,
when the orientation angle exceeds a first threshold orientation
angle with respect to the vehicle (e.g., 30 degrees), the pulse
rate may increase to a second pulse rate (1.5 MHz). The lidar
system accordingly may decrease the power for the light pulses, as
the faster pulse rate may not allow for the light pulses to reach
targets at the maximum range. Then, when the orientation exceeds a
second threshold orientation with respect to the vehicle (e.g., 45
degrees), the pulse rate may increase to a third pulse rate, and so
on. In some implementations, the controller determines the
threshold orientations. The threshold orientations may be static
and predetermined or may be dynamically set by the controller. For
example, a threshold orientation may be determined based on point
cloud data from a previous scan line or scan frame. In another
example, the controller may set a threshold orientation according
to an upcoming vehicle maneuver such as a lane charge or turn. For
example, when the vehicle is about to make a left turn, the
controller may set a threshold orientation near the left side of
the lidar system (e.g., when the lidar system is positioned in
front of the vehicle) to increase the pulse rate near the left side
of the vehicle to more clearly identify objects in an area that the
vehicle is approaching. Still further, the controller may set the
threshold orientation based on road conditions, visibility due to
fog, snow, or rain, for example, or based on any other suitable
condition to adjust the pulse rate above or below the threshold
orientation.
[0031] In other implementations, the lidar system increases the
pulse rate when the beam scans areas ahead of the vehicle or the
lidar system and decreases the pulse rate when the beam scans areas
near the periphery of the field of regard, i.e., not directly ahead
of the vehicle or the lidar system. The resolution or pixel density
thus is higher for the areas directly ahead of the vehicle, in
these implementations. By increasing the pulse rate when scanning
directly ahead of the vehicle, the lidar system can collect more
data points when information density is higher. In some scenarios,
the lidar system may increase the resolution when information
density is higher to pinpoint each of the objects in an area where
the objects are densely located. Additionally, the lidar system may
increase the pulse rate in this area to more clearly identify
objects in an area that the vehicle is approaching.
[0032] In some implementations, the lidar system includes two or
more light sources, each scanning in opposite directions with
respect to the direction directly ahead of the vehicle (e.g., from
0 degrees to 60 degrees and from 0 degrees to -60 degrees). In this
manner, the horizontal field of regard may double compared to a
lidar system having one light source that scans across a 60-degree
horizontal field of regard. In some scenarios, the two-light source
implementation doubles the resolution when compared to a lidar
system having one light source that scans across a 120-degree
horizontal field of regard. In other implementations, the two or
more light sources scan in the same direction and are offset by a
predetermined phase angle. For example, two light sources may be
phased apart by 60 degrees, such that one light source scans back
and forth from 0 degrees to 60 degrees and the other light source
scans back and forth from -60 degrees to 0 degrees in the same
scanning direction as the first light source. In some
implementations, the lidar system includes one light source, and
the pulses of light emitted by the light source are split to
produce two or more output beams which are scanned by two or more
respective scanners.
[0033] Further, a lidar system may adjust the pulse rate in
accordance with the scan speed of a scanner to compensate for motor
dynamics at the scanner. More specifically, a lidar system
operating in a vehicle may include a scanner that scans in the
forward-scanning and reverse-scanning directions. The forward and
reverse-scanning directions may be substantially horizontal
scanning directions, such that the forward-scanning direction is to
the right and the reverse-scanning direction is to the left (or
vice versa). Furthermore, the forward and reverse-scanning
directions may be substantially vertical scanning directions, such
that the forward-scanning direction is up and the reverse-scanning
direction is down (or vice versa). Additionally, the forward and
reverse-scanning directions may be scanning directions having any
suitable combination of horizontal and vertical components. In any
event, as the scanner changes directions, the scan speed may
decrease as the field of view of the light source approaches the
peripheries. Motor dynamics may refer to the movement (e.g., speed
or velocity) or change in movement (e.g., acceleration) of a
scanning mirror or a motor that drives a scanning mirror. In an
example implementation, the lidar system includes two or more scan
heads (also referred to above as sensor heads), each concurrently
scanning in opposite directions with respect to the front of the
lidar system (e.g., from 0 degrees to 60 degrees and from 0 degrees
to -60 degrees). In another example implementation, the scan heads
concurrently scan in the same direction with respect to the front
of the lidar system. When one of the light sources approaches 0
degrees or 60 degrees, the scanner may slow down from 50 to 60
degrees and then speed up from 60 degrees on its way back to 50
degrees, for example.
[0034] To compensate for these motor dynamics, the lidar system in
some implementations transmits light pulses at a variable pulse
rate such that the pulse rate relates to the scan speed (e.g., the
pulse rate decreases when the scan speed decreases and the pulse
rate increases when the scan speed increases). In this manner, the
light source transmits light pulses uniformly across the field of
regard. The lidar system accordingly may adjust the power or
energy, because a faster pulse rate may not allow for the light
pulses to reach targets at the maximum range.
[0035] In some implementations, a controller may determine the scan
speed and provide control signals to the light source to decrease
the pulse rate as the scan speed decreases and increase the pulse
rate as the scan speed increases. In some implementations, the
controller may compare the scan speed to a threshold speed. Then
the controller may provide a control signal to the light source to
adjust the pulse rate when the scan speed increases above or
decreases below the threshold speed. For example, at a first scan
speed the pulse rate may be 600 kHz. Then, when the scan speed
exceeds a threshold speed, the pulse rate may increase to a second
pulse rate of 750 kHz. As the scanner approaches the periphery and
is about to change directions the scan speed may drop below the
threshold speed and accordingly the pulse rate may decrease back to
the first pulse rate of 600 kHz.
[0036] In some implementations, the lidar system may include two or
more light sources each concurrently scanning in opposite
directions with respect to the direction directly in front of or
orthogonal to the lidar system (e.g., from 0 degrees to 60 degrees
and from 0 degrees to -60 degrees). In other implementations, the
two or more light sources concurrently scan in the same direction
and are offset by a predetermined phase angle to illuminate
different portions of the field of regard. For example, two light
sources may be phased apart by 60 degrees, such that one light
source scans back and forth from 0 degrees to 60 degrees and the
other light source scans back and forth from -60 degrees to 0
degrees in the same scanning direction as the first light source.
In this manner, each light source may change directions at 0
degrees and at .+-.60 degrees with respect to the vehicle. The
lidar system may increase the pulse rate or keep the pulse rate the
same as one of the light sources approaches 0 degrees to increase
the resolution or pixel density near the front of the lidar system.
Then the lidar system may reduce the pulse rate as the light source
approaches the periphery (e.g. .+-.60 degrees) to compensate for
the slower scan speed. In any event, the lidar system may adjust
the pulse rate based on a combination of the scan speed and
orientations of the light pulses. In other implementations, the
lidar system includes one light source, and the pulses of light
emitted by the light source are split to produce two or more output
beams which are scanned by two or more respective scanners.
[0037] In yet other implementations, the lidar system may include a
single light source and may decrease the pulse rate as the light
source approaches .+-.60 degrees with respect to the front of the
lidar system.
[0038] An example lidar system in which these techniques can be
implemented is considered next with reference to FIGS. 1-4,
followed by a discussion of the techniques which the lidar system
can implement to scan a field of regard and generate individual
pixels (FIGS. 5-7). An example implementation in a vehicle is then
discussed with reference to FIGS. 8 and 9. Then, an example photo
detector and an example pulse-detection circuit are discussed with
reference to FIGS. 10 and 11.
System Overview
[0039] FIG. 1 illustrates an example light detection and ranging
(lidar) system 100. The lidar system 100 may be referred to as a
laser ranging system, a laser radar system, a LIDAR system, a lidar
sensor, or a laser detection and ranging (LADAR or ladar) system.
The lidar system 100 may include a light source 110, a mirror 115,
a scanner 120, a receiver 140, and a controller 150. The light
source 110 may be, for example, a laser which emits light having a
particular operating wavelength in the infrared, visible, or
ultraviolet portions of the electromagnetic spectrum. As a more
specific example, the light source 110 may include a laser with an
operating wavelength between approximately 1.2 .mu.m and 1.7
.mu.m.
[0040] In operation, the light source 110 emits an output beam of
light 125 which may be continuous-wave, pulsed, or modulated in any
suitable manner for a given application. The output beam of light
125 is directed downrange toward a remote target 130 located a
distance D from the lidar system 100 and at least partially
contained within a field of regard of the system 100. Depending on
the scenario and/or the implementation of the lidar system 100, D
can be between 1 m and 1 km, for example.
[0041] Once the output beam 125 reaches the downrange target 130,
the target 130 may scatter or, in some cases, reflect at least a
portion of light from the output beam 125, and some of the
scattered or reflected light may return toward the lidar system
100. In the example of FIG. 1, the scattered or reflected light is
represented by input beam 135, which passes through the scanner
120, which may be referred to as a beam scanner, optical scanner,
or laser scanner. The input beam 135 passes through the scanner 120
to the mirror 115, which may be referred to as an overlap mirror,
superposition mirror, or beam-combiner mirror. The mirror 115 in
turn directs the input beam 135 to the receiver 140. The input 135
may contain only a relatively small fraction of the light from the
output beam 125. For example, the ratio of average power, peak
power, or pulse energy of the input beam 135 to average power, peak
power, or pulse energy of the output beam 125 may be approximately
10.sup.-1, 10.sup.-2, 10.sup.-3, 10.sup.-4, 10.sup.-5, 10.sup.-6,
10.sup.-7, 10.sup.-8, 10.sup.-9, 10.sup.-11, 10.sup.-11, or
10.sup.-12. As another example, if a pulse of the output beam 125
has a pulse energy of 1 microjoule (.mu.J), then the pulse energy
of a corresponding pulse of the input beam 135 may have a pulse
energy of approximately 10 nanojoules (nJ), 1 nJ, 100 picojoules
(pJ), 10 pJ, 1 pJ, 100 femtojoules (fJ), 10 fJ, 1 fJ, 100
attojoules (aJ), 10 aJ, or 1 aJ.
[0042] The output beam 125 may be referred to as a laser beam,
light beam, optical beam, emitted beam, or just beam; and the input
beam 135 may be referred to as a return beam, received beam, return
light, received light, input light, scattered light, or reflected
light. As used herein, scattered light may refer to light that is
scattered or reflected by the target 130. The input beam 135 may
include light from the output beam 125 that is scattered by the
target 130, light from the output beam 125 that is reflected by the
target 130, or a combination of scattered and reflected light from
target 130.
[0043] The operating wavelength of a lidar system 100 may lie, for
example, in the infrared, visible, or ultraviolet portions of the
electromagnetic spectrum. The Sun also produces light in these
wavelength ranges, and thus sunlight can act as background noise
which can obscure signal light detected by the lidar system 100.
This solar background noise can result in false-positive detections
or can otherwise corrupt measurements of the lidar system 100,
especially when the receiver 140 includes SPAD detectors (which can
be highly sensitive).
[0044] Generally speaking, the light from the Sun that passes
through the Earth's atmosphere and reaches a terrestrial-based
lidar system such as the system 100 can establish an optical
background noise floor for this system. Thus, in order for a signal
from the lidar system 100 to be detectable, the signal must rise
above the background noise floor. It is generally possible to
increase the signal-to-noise (SNR) ratio of the lidar system 100 by
raising the power level of the output beam 125, but in some
situations it may be desirable to keep the power level of the
output beam 125 relatively low. For example, increasing transmit
power levels of the output beam 125 can result in the lidar system
100 not being eye-safe.
[0045] In some implementations, the lidar system 100 operates at
one or more wavelengths between approximately 1400 nm and
approximately 1600 nm. For example, the light source 110 may
produce light at approximately 1550 nm.
[0046] In some implementations, the lidar system 100 operates at
frequencies at which atmospheric absorption is relatively low. For
example, the lidar system 100 can operate at wavelengths in the
approximate ranges from 980 nm to 1110 nm or from 1165 nm to 1400
nm.
[0047] In other implementations, the lidar system 100 operates at
frequencies at which atmospheric absorption is high. For example,
the lidar system 100 can operate at wavelengths in the approximate
ranges from 930 nm to 980 nm, from 1100 nm to 1165 nm, or from 1400
nm to 1460 nm.
[0048] According to some implementations, the lidar system 100 can
include an eye-safe laser, or the lidar system 100 can be
classified as an eye-safe laser system or laser product. An
eye-safe laser, laser system, or laser product may refer to a
system with an emission wavelength, average power, peak power, peak
intensity, pulse energy, beam size, beam divergence, exposure time,
or scanned output beam such that emitted light from the system
presents little or no possibility of causing damage to a person's
eyes. For example, the light source 110 or lidar system 100 may be
classified as a Class 1 laser product (as specified by the 60825-1
standard of the International Electrotechnical Commission (IEC)) or
a Class I laser product (as specified by Title 21, Section 1040.10
of the United States Code of Federal Regulations (CFR)) that is
safe under all conditions of normal use. In some implementations,
the lidar system 100 may be classified as an eye-safe laser product
(e.g., with a Class 1 or Class I classification) configured to
operate at any suitable wavelength between approximately 1400 nm
and approximately 2100 nm. In some implementations, the light
source 110 may include a laser with an operating wavelength between
approximately 1400 nm and approximately 1600 nm, and the lidar
system 100 may be operated in an eye-safe manner. In some
implementations, the light source 110 or the lidar system 100 may
be an eye-safe laser product that includes a scanned laser with an
operating wavelength between approximately 1530 nm and
approximately 1560 nm. In some implementations, the lidar system
100 may be a Class 1 or Class I laser product that includes a fiber
laser or solid-state laser with an operating wavelength between
approximately 1400 nm and approximately 1600 nm.
[0049] The receiver 140 may receive or detect photons from the
input beam 135 and generate one or more representative signals. For
example, the receiver 140 may generate an output electrical signal
145 that is representative of the input beam 135. The receiver may
send the electrical signal 145 to the controller 150. Depending on
the implementation, the controller 150 may include one or more
processors, an application-specific integrated circuit (ASIC), a
field-programmable gate array (FPGA), and/or other suitable
circuitry configured to analyze one or more characteristics of the
electrical signal 145 to determine one or more characteristics of
the target 130, such as its distance downrange from the lidar
system 100. More particularly, the controller 150 may analyze the
time of flight or phase modulation for the beam of light 125
transmitted by the light source 110. If the lidar system 100
measures a time of flight of T (e.g., T represents a round-trip
time of flight for an emitted pulse of light to travel from the
lidar system 100 to the target 130 and back to the lidar system
100), then the distance D from the target 130 to the lidar system
100 may be expressed as D=cT/2, where c is the speed of light
(approximately 3.0.times.10.sup.8 m/s).
[0050] As a more specific example, if the lidar system 100 measures
the time of flight to be T=300 ns, then the lidar system 100 can
determine the distance from the target 130 to the lidar system 100
to be approximately D=45.0 m. As another example, the lidar system
100 measures the time of flight to be T=1.33 .mu.s and accordingly
determines that the distance from the target 130 to the lidar
system 100 is approximately D=199.5 m. The distance D from lidar
system 100 to the target 130 may be referred to as a distance,
depth, or range of the target 130. As used herein, the speed of
light c refers to the speed of light in any suitable medium, such
as for example in air, water, or vacuum. The speed of light in
vacuum is approximately 2.9979.times.10.sup.8 m/s, and the speed of
light in air (which has a refractive index of approximately 1.0003)
is approximately 2.9970.times.10.sup.8 m/s.
[0051] The target 130 may be located a distance D from the lidar
system 100 that is less than or equal to a maximum range R.sub.MAX
of the lidar system 100. The maximum range R.sub.MAX (which also
may be referred to as a maximum distance) of a lidar system 100 may
correspond to the maximum distance over which the lidar system 100
is configured to sense or identify targets that appear in a field
of regard of the lidar system 100. The maximum range of lidar
system 100 may be any suitable distance, such as for example, 25 m,
50 m, 100 m, 200 m, 500 m, or 1 km. As a specific example, a lidar
system with a 200-m maximum range may be configured to sense or
identify various targets located up to 200 m away. For a lidar
system with a 200-m maximum range (R.sub.MAX=200 m), the time of
flight corresponding to the maximum range is approximately
2R.sub.MAXc.apprxeq.1.33 .mu.s.
[0052] In some implementations, the light source 110, the scanner
120, and the receiver 140 may be packaged together within a single
housing 155, which may be a box, case, or enclosure that holds or
contains all or part of a lidar system 100. The housing 155
includes a window 157 through which the beams 125 and 135 pass. In
one example implementation, the lidar-system housing 155 contains
the light source 110, the overlap mirror 115, the scanner 120, and
the receiver 140 of a lidar system 100. The controller 150 may
reside within the same housing 155 as the components 110, 120, and
140, or the controller 150 may reside remotely from the
housing.
[0053] Moreover, in some implementations, the housing 155 includes
multiple lidar sensors, each including a respective scanner and a
receiver. Depending on the particular implementation, each of the
multiple sensors can include a separate light source or a common
light source. The multiple sensors can be configured to cover
non-overlapping adjacent fields of regard or partially overlapping
fields of regard, depending on the implementation.
[0054] The housing 155 may be an airtight or watertight structure
that prevents water vapor, liquid water, dirt, dust, or other
contaminants from getting inside the housing 155. The housing 155
may be filled with a dry or inert gas, such as for example dry air,
nitrogen, or argon. The housing 155 may include one or more
electrical connections for conveying electrical power or electrical
signals to and/or from the housing.
[0055] The window 157 may be made from any suitable substrate
material, such as for example, glass or plastic (e.g.,
polycarbonate, acrylic, cyclic-olefin polymer, or cyclic-olefin
copolymer). The window 157 may include an interior surface (surface
A) and an exterior surface (surface B), and surface A or surface B
may include a dielectric coating having particular reflectivity
values at particular wavelengths. A dielectric coating (which may
be referred to as a thin-film coating, interference coating, or
coating) may include one or more thin-film layers of dielectric
materials (e.g., SiO.sub.2, TiO.sub.2, Al.sub.2O.sub.3,
Ta.sub.2O.sub.5, MgF.sub.2, LaF.sub.3, or AlF.sub.3) having
particular thicknesses (e.g., thickness less than 1 .mu.m) and
particular refractive indices. A dielectric coating may be
deposited onto surface A or surface B of the window 157 using any
suitable deposition technique, such as for example, sputtering or
electron-beam deposition.
[0056] The dielectric coating may have a high reflectivity at a
particular wavelength or a low reflectivity at a particular
wavelength. A high-reflectivity (HR) dielectric coating may have
any suitable reflectivity value (e.g., a reflectivity greater than
or equal to 80%, 90%, 95%, or 99%) at any suitable wavelength or
combination of wavelengths. A low-reflectivity dielectric coating
(which may be referred to as an anti-reflection (AR) coating) may
have any suitable reflectivity value (e.g., a reflectivity less
than or equal to 5%, 2%, 1%, 0.5%, or 0.2%) at any suitable
wavelength or combination of wavelengths. In particular
embodiments, a dielectric coating may be a dichroic coating with a
particular combination of high or low reflectivity values at
particular wavelengths. For example, a dichroic coating may have a
reflectivity of less than or equal to 0.5% at approximately
1550-1560 nm and a reflectivity of greater than or equal to 90% at
approximately 800-1500 nm.
[0057] In some implementations, surface A or surface B has a
dielectric coating that is anti-reflecting at an operating
wavelength of one or more light sources 110 contained within
enclosure 155. An AR coating on surface A and surface B may
increase the amount of light at an operating wavelength of light
source 110 that is transmitted through the window 157.
Additionally, an AR coating at an operating wavelength of the light
source 110 may reduce the amount of incident light from output beam
125 that is reflected by the window 157 back into the housing 155.
In an example implementation, each of surface A and surface B has
an AR coating with reflectivity less than 0.5% at an operating
wavelength of light source 110. As an example, if the light source
110 has an operating wavelength of approximately 1550 nm, then
surface A and surface B may each have an AR coating with a
reflectivity that is less than 0.5% from approximately 1547 nm to
approximately 1553 nm. In another implementation, each of surface A
and surface B has an AR coating with reflectivity less than 1% at
the operating wavelengths of the light source 110. For example, if
the housing 155 encloses two sensor heads with respective light
sources, the first light source emits pulses at a wavelength of
approximately 1535 nm and the second light source emits pulses at a
wavelength of approximately 1540 nm, then surface A and surface B
may each have an AR coating with reflectivity less than 1% from
approximately 1530 nm to approximately 1545 nm.
[0058] The window 157 may have an optical transmission that is
greater than any suitable value for one or more wavelengths of one
or more light sources 110 contained within the housing 155. As an
example, the window 157 may have an optical transmission of greater
than or equal to 70%, 80%, 90%, 95%, or 99% at a wavelength of
light source 110. In one example implementation, the window 157 can
transmit greater than or equal to 95% of light at an operating
wavelength of the light source 110. In another implementation, the
window 157 transmits greater than or equal to 90% of light at the
operating wavelengths of the light sources enclosed within the
housing 155.
[0059] Surface A or surface B may have a dichroic coating that is
anti-reflecting at one or more operating wavelengths of one or more
light sources 110 and high-reflecting at wavelengths away from the
one or more operating wavelengths. For example, surface A may have
an AR coating for an operating wavelength of the light source 110,
and surface B may have a dichroic coating that is AR at the
light-source operating wavelength and HR for wavelengths away from
the operating wavelength. A coating that is HR for wavelengths away
from a light-source operating wavelength may prevent most incoming
light at unwanted wavelengths from being transmitted through the
window 117. In one implementation, if light source 110 emits
optical pulses with a wavelength of approximately 1550 nm, then
surface A may have an AR coating with a reflectivity of less than
or equal to 0.5% from approximately 1546 nm to approximately 1554
nm. Additionally, surface B may have a dichroic coating that is AR
at approximately 1546-1554 nm and HR (e.g., reflectivity of greater
than or equal to 90%) at approximately 800-1500 nm and
approximately 1580-1700 nm.
[0060] Surface B of the window 157 may include a coating that is
oleophobic, hydrophobic, or hydrophilic. A coating that is
oleophobic (or, lipophobic) may repel oils (e.g., fingerprint oil
or other non-polar material) from the exterior surface (surface B)
of the window 157. A coating that is hydrophobic may repel water
from the exterior surface. For example, surface B may be coated
with a material that is both oleophobic and hydrophobic. A coating
that is hydrophilic attracts water so that water may tend to wet
and form a film on the hydrophilic surface (rather than forming
beads of water as may occur on a hydrophobic surface). If surface B
has a hydrophilic coating, then water (e.g., from rain) that lands
on surface B may form a film on the surface. The surface film of
water may result in less distortion, deflection, or occlusion of an
output beam 125 than a surface with a non-hydrophilic coating or a
hydrophobic coating.
[0061] With continued reference to FIG. 1, the light source 110 may
include a pulsed laser configured to produce or emit pulses of
light with a certain pulse duration. In an example implementation,
the pulse duration or pulse width of the pulsed laser is
approximately 10 picoseconds (ps) to 100 nanoseconds (ns). In
another implementation, the light source 110 is a pulsed laser that
produces pulses with a pulse duration of approximately 1-4 ns. In
yet another implementation, the light source 110 is a pulsed laser
that produces pulses at a pulse repetition frequency of
approximately 100 kHz to 5 MHz or a pulse period (e.g., a time
between consecutive pulses) of approximately 200 ns to 10 .mu.s.
The light source 110 may have a substantially constant or a
variable pulse repetition frequency, depending on the
implementation. As an example, the light source 110 may be a pulsed
laser that produces pulses at a substantially constant pulse
repetition frequency of approximately 640 kHz (e.g., 640,000 pulses
per second), corresponding to a pulse period of approximately 1.56
.mu.s. As another example, the light source 110 may have a pulse
repetition frequency that can be varied from approximately 500 kHz
to 3 MHz. As used herein, a pulse of light may be referred to as an
optical pulse, a light pulse, or a pulse, and a pulse repetition
frequency may be referred to as a pulse rate.
[0062] In general, the output beam 125 may have any suitable
average optical power, and the output beam 125 may include optical
pulses with any suitable pulse energy or peak optical power. Some
examples of the average power of the output beam 125 include the
approximate values of 1 mW, 10 mW, 100 mW, 1 W, and 10 W. Example
values of pulse energy of the output beam 125 include the
approximate values of 0.1 .mu.J, 1 .mu.J, 10 .mu.J, 100 .mu.J, and
1 mJ. Examples of peak power values of pulses included in the
output beam 125 are the approximate values of 10 W, 100 W, 1 kW, 5
kW, 10 kW. An example optical pulse with a duration of 1 ns and a
pulse energy of 1 .mu.J has a peak power of approximately 1 kW. If
the pulse repetition frequency is 500 kHz, then the average power
of the output beam 125 with 1-.mu.J pulses is approximately 0.5 W,
in this example.
[0063] The light source 110 may include a laser diode, such as a
Fabry-Perot laser diode, a quantum well laser, a distributed Bragg
reflector (DBR) laser, a distributed feedback (DFB) laser, or a
vertical-cavity surface-emitting laser (VCSEL). The laser diode
operating in the light source 110 may be an
aluminum-gallium-arsenide (AlGaAs) laser diode, an
indium-gallium-arsenide (InGaAs) laser diode, or an
indium-gallium-arsenide-phosphide (InGaAsP) laser diode, or any
other suitable diode. In some implementations, the light source 110
includes a pulsed laser diode with a peak emission wavelength of
approximately 1400-1600 nm. Further, the light source 110 may
include a laser diode that is current-modulated to produce optical
pulses.
[0064] In some implementations, the light source 110 includes a
pulsed laser diode followed by one or more optical-amplification
stages. For example, the light source 110 may be a fiber-laser
module that includes a current-modulated laser diode with a peak
wavelength of approximately 1550 nm, followed by a single-stage or
a multi-stage erbium-doped fiber amplifier (EDFA). As another
example, the light source 110 may include a continuous-wave (CW) or
quasi-CW laser diode followed by an external optical modulator
(e.g., an electro-optic modulator), and the output of the modulator
may be fed into an optical amplifier. In other implementations, the
light source 110 may include a laser diode which produces optical
pulses that are not amplified by an optical amplifier. As an
example, a laser diode (which may be referred to as a direct
emitter or a direct-emitter laser diode) may emit optical pulses
that form an output beam 125 that is directed downrange from a
lidar system 100. In yet other implementations, the light source
110 may include a pulsed solid-state laser or a pulsed fiber
laser.
[0065] In some implementations, the output beam of light 125
emitted by the light source 110 is a collimated optical beam with
any suitable beam divergence, such as a divergence of approximately
0.1 to 3.0 milliradian (mrad). Divergence of the output beam 125
may refer to an angular measure of an increase in beam size (e.g.,
a beam radius or beam diameter) as the output beam 125 travels away
from the light source 110 or the lidar system 100. The output beam
125 may have a substantially circular cross section with a beam
divergence characterized by a single divergence value. For example,
the output beam 125 with a circular cross section and a divergence
of 1 mrad may have a beam diameter or spot size of approximately 10
cm at a distance of 100 m from the lidar system 100. In some
implementations, the output beam 125 may be an astigmatic beam or
may have a substantially elliptical cross section and may be
characterized by two divergence values. As an example, the output
beam 125 may have a fast axis and a slow axis, where the fast-axis
divergence is greater than the slow-axis divergence. As another
example, the output beam 125 may be an astigmatic beam with a
fast-axis divergence of 2 mrad and a slow-axis divergence of 0.5
mrad.
[0066] The output beam of light 125 emitted by light source 110 may
be unpolarized or randomly polarized, may have no specific or fixed
polarization (e.g., the polarization may vary with time), or may
have a particular polarization (e.g., the output beam 125 may be
linearly polarized, elliptically polarized, or circularly
polarized). As an example, the light source 110 may produce
linearly polarized light, and the lidar system 100 may include a
quarter-wave plate that converts this linearly polarized light into
circularly polarized light. The lidar system 100 may transmit the
circularly polarized light as the output beam 125, and receive the
input beam 135, which may be substantially or at least partially
circularly polarized in the same manner as the output beam 125
(e.g., if the output beam 125 is right-hand circularly polarized,
then the input beam 135 may also be right-hand circularly
polarized). The input beam 135 may pass through the same
quarter-wave plate (or a different quarter-wave plate), resulting
in the input beam 135 being converted to linearly polarized light
which is orthogonally polarized (e.g., polarized at a right angle)
with respect to the linearly polarized light produced by light
source 110. As another example, the lidar system 100 may employ
polarization-diversity detection where two polarization components
are detected separately. The output beam 125 may be linearly
polarized, and the lidar system 100 may split the input beam 135
into two polarization components (e.g., s-polarization and
p-polarization) which are detected separately by two photodiodes
(e.g., a balanced photoreceiver that includes two photodiodes).
[0067] With continued reference to FIG. 1, the output beam 125 and
input beam 135 may be substantially coaxial. In other words, the
output beam 125 and input beam 135 may at least partially overlap
or share a common propagation axis, so that the input beam 135 and
the output beam 125 travel along substantially the same optical
path (albeit in opposite directions). As the lidar system 100 scans
the output beam 125 across a field of regard, the input beam 135
may follow along with the output beam 125, so that the coaxial
relationship between the two beams is maintained.
[0068] The lidar system 100 also may include one or more optical
components configured to condition, shape, filter, modify, steer,
or direct the output beam 125 and/or the input beam 135. For
example, lidar system 100 may include one or more lenses, mirrors,
filters (e.g., bandpass or interference filters), beam splitters,
polarizers, polarizing beam splitters, wave plates (e.g., half-wave
or quarter-wave plates), diffractive elements, or holographic
elements. In some implementations, lidar system 100 includes a
telescope, one or more lenses, or one or more mirrors to expand,
focus, or collimate the output beam 125 to a desired beam diameter
or divergence. As an example, the lidar system 100 may include one
or more lenses to focus the input beam 135 onto an active region of
the receiver 140. As another example, the lidar system 100 may
include one or more flat mirrors or curved mirrors (e.g., concave,
convex, or parabolic mirrors) to steer or focus the output beam 125
or the input beam 135. For example, the lidar system 100 may
include an off-axis parabolic mirror to focus the input beam 135
onto an active region of receiver 140. As illustrated in FIG. 1,
the lidar system 100 may include the mirror 115, which may be a
metallic or dielectric mirror. The mirror 115 may be configured so
that the light beam 125 passes through the mirror 115. As an
example, mirror 115 may include a hole, slot, or aperture through
which the output light beam 125 passes. As another example, the
mirror 115 may be configured so that at least 80% of the output
beam 125 passes through the mirror 115 and at least 80% of the
input beam 135 is reflected by the mirror 115. In some
implementations, the mirror 115 may provide for the output beam 125
and the input beam 135 to be substantially coaxial, so that the
beams 125 and 135 travel along substantially the same optical path,
in opposite directions.
[0069] Generally speaking, the scanner 120 steers the output beam
125 in one or more directions downrange. The scanner 120 may
include one or more scanning mirrors and one or more actuators
driving the mirrors to rotate, tilt, pivot, or move the mirrors in
an angular manner about one or more axes, for example. For example,
the first mirror of the scanner may scan the output beam 125 along
a first direction, and the second mirror may scan the output beam
125 along a second direction that is substantially orthogonal to
the first direction. Example implementations of the scanner 120 are
discussed in more detail below with reference to FIG. 2.
[0070] The scanner 120 may be configured to scan the output beam
125 over a 5-degree angular range, 20-degree angular range,
30-degree angular range, 60-degree angular range, or any other
suitable angular range. For example, a scanning mirror may be
configured to periodically rotate over a 15-degree range, which
results in the output beam 125 scanning across a 30-degree range
(e.g., a 0-degree rotation by a scanning mirror results in a
20-degree angular scan of the output beam 125). A field of regard
(FOR) of the lidar system 100 may refer to an area, region, or
angular range over which the lidar system 100 may be configured to
scan or capture distance information. When the lidar system 100
scans the output beam 125 within a 30-degree scanning range, the
lidar system 100 may be referred to as having a 30-degree angular
field of regard. As another example, a lidar system 100 with a
scanning mirror that rotates over a 30-degree range may produce the
output beam 125 that scans across a 60-degree range (e.g., a
60-degree FOR). In various implementations, the lidar system 100
may have a FOR of approximately 10.degree., 20.degree., 40.degree.,
60.degree., 120.degree., or any other suitable FOR. The FOR also
may be referred to as a scan region.
[0071] The scanner 120 may be configured to scan the output beam
125 horizontally and vertically, and the lidar system 100 may have
a particular FOR along the horizontal direction and another
particular FOR along the vertical direction. For example, the lidar
system 100 may have a horizontal FOR of 10.degree. to 120.degree.
and a vertical FOR of 2.degree. to 45.degree..
[0072] The one or more scanning mirrors of the scanner 120 may be
communicatively coupled to the controller 150 which may control the
scanning mirror(s) so as to guide the output beam 125 in a desired
direction downrange or along a desired scan pattern. In general, a
scan pattern may refer to a pattern or path along which the output
beam 125 is directed, and also may be referred to as an optical
scan pattern, optical scan path, or scan path. As an example, the
scanner 120 may include two scanning mirrors configured to scan the
output beam 125 across a 60.degree. horizontal FOR and a 20.degree.
vertical FOR. The two scanner mirrors may be controlled to follow a
scan path that substantially covers the 60.degree..times.20.degree.
FOR. The lidar system 100 can use the scan path to generate a point
cloud with pixels that substantially cover the
60.degree..times.20.degree. FOR. The pixels may be approximately
evenly distributed across the 60.degree..times.20.degree. FOR.
Alternately, the pixels may have a particular non-uniform
distribution (e.g., the pixels may be distributed across all or a
portion of the 60.degree..times.20.degree. FOR, and the pixels may
have a higher density in one or more particular regions of the
60.degree..times.20.degree. FOR).
[0073] In operation, the light source 110 may emit pulses of light
which the scanner 120 scans across a FOR of lidar system 100. The
target 130 may scatter one or more of the emitted pulses, and the
receiver 140 may detect at least a portion of the pulses of light
scattered by the target 130.
[0074] The receiver 140 may be referred to as (or may include) a
photoreceiver, optical receiver, optical sensor, detector,
photodetector, or optical detector. The receiver 140 in some
implementations receives or detects at least a portion of the input
beam 135 and produces an electrical signal that corresponds to the
input beam 135. For example, if the input beam 135 includes an
optical pulse, then the receiver 140 may produce an electrical
current or voltage pulse that corresponds to the optical pulse
detected by the receiver 140. In an example implementation, the
receiver 140 includes one or more avalanche photodiodes (APDs) or
one or more single-photon avalanche diodes (SPADs). In another
implementation, the receiver 140 includes one or more PN
photodiodes (e.g., a photodiode structure formed by a p-type
semiconductor and a n-type semiconductor) or one or more PIN
photodiodes (e.g., a photodiode structure formed by an undoped
intrinsic semiconductor region located between p-type and n-type
regions).
[0075] The receiver 140 may have an active region or an
avalanche-multiplication region that includes silicon, germanium,
or InGaAs. The active region of receiver 140 may have any suitable
size, such as for example, a diameter or width of approximately
50-500 .mu.m. The receiver 140 may include circuitry that performs
signal amplification, sampling, filtering, signal conditioning,
analog-to-digital conversion, time-to-digital conversion, pulse
detection, threshold detection, rising-edge detection, or
falling-edge detection. For example, the receiver 140 may include a
transimpedance amplifier that converts a received photocurrent
(e.g., a current produced by an APD in response to a received
optical signal) into a voltage signal. The receiver 140 may direct
the voltage signal to pulse-detection circuitry that produces an
analog or digital output signal 145 that corresponds to one or more
characteristics (e.g., rising edge, falling edge, amplitude, or
duration) of a received optical pulse. For example, the
pulse-detection circuitry may perform a time-to-digital conversion
to produce a digital output signal 145. The receiver 140 may send
the electrical output signal 145 to the controller 150 for
processing or analysis, e.g., to determine a time-of-flight value
corresponding to a received optical pulse.
[0076] The controller 150 may be electrically coupled or otherwise
communicatively coupled to one or more of the light source 110, the
scanner 120, and the receiver 140. The controller 150 may receive
electrical trigger pulses or edges from the light source 110, where
each pulse or edge corresponds to the emission of an optical pulse
by the light source 110. The controller 150 may provide
instructions, a control signal, or a trigger signal to the light
source 110 indicating when the light source 110 should produce
optical pulses. For example, the controller 150 may send an
electrical trigger signal that includes electrical pulses, where
the light source 110 emits an optical pulse in response to each
electrical pulse. Further, the controller 150 may cause the light
source 110 to adjust one or more of the frequency, period,
duration, pulse energy, peak power, average power, or wavelength of
the optical pulses produced by light source 110.
[0077] The controller 150 may determine a time-of-flight value for
an optical pulse based on timing information associated with when
the pulse was emitted by light source 110 and when a portion of the
pulse (e.g., the input beam 135) was detected or received by the
receiver 140. The controller 150 may include circuitry that
performs signal amplification, sampling, filtering, signal
conditioning, analog-to-digital conversion, time-to-digital
conversion, pulse detection, threshold detection, rising-edge
detection, or falling-edge detection.
[0078] As indicated above, the lidar system 100 may be used to
determine the distance to one or more downrange targets 130. By
scanning the lidar system 100 across a field of regard, the system
can be used to map the distance to a number of points within the
field of regard. Each of these depth-mapped points may be referred
to as a pixel or a voxel. A collection of pixels captured in
succession (which may be referred to as a depth map, a point cloud,
or a frame) may be rendered as an image or may be analyzed to
identify or detect objects or to determine a shape or distance of
objects within the FOR. For example, a depth map may cover a field
of regard that extends 60.degree. horizontally and 15.degree.
vertically, and the depth map may include a frame of 100-2000
pixels in the horizontal direction by 4-400 pixels in the vertical
direction.
[0079] The lidar system 100 may be configured to repeatedly capture
or generate point clouds of a field of regard at any suitable frame
rate between approximately 0.1 frames per second (FPS) and
approximately 1,000 FPS. For example, the lidar system 100 may
generate point clouds at a frame rate of approximately 0.1 FPS, 0.5
FPS, 1 FPS, 2 FPS, 5 FPS, 10 FPS, 20 FPS, 100 FPS, 500 FPS, or
1,000 FPS. In an example implementation, the lidar system 100 is
configured to produce optical pulses at a rate of 5.times.10.sup.5
pulses/second (e.g., the system may determine 500,000 pixel
distances per second) and scan a frame of 1000.times.50 pixels
(e.g., 50,000 pixels/frame), which corresponds to a point-cloud
frame rate of 10 frames per second (e.g., 10 point clouds per
second). The point-cloud frame rate may be substantially fixed or
dynamically adjustable, depending on the implementation. For
example, the lidar system 100 may capture one or more point clouds
at a particular frame rate (e.g., 1 Hz) and then switch to capture
one or more point clouds at a different frame rate (e.g., 10 Hz).
In general, the lidar system can use a slower frame rate (e.g., 1
Hz) to capture one or more high-resolution point clouds, and use a
faster frame rate (e.g., 10 Hz) to rapidly capture multiple
lower-resolution point clouds.
[0080] The field of regard of the lidar system 100 can overlap,
encompass, or enclose at least a portion of the target 130, which
may include all or part of an object that is moving or stationary
relative to lidar system 100. For example, the target 130 may
include all or a portion of a person, vehicle, motorcycle, truck,
train, bicycle, wheelchair, pedestrian, animal, road sign, traffic
light, lane marking, road-surface marking, parking space, pylon,
guard rail, traffic barrier, pothole, railroad crossing, obstacle
in or near a road, curb, stopped vehicle on or beside a road,
utility pole, house, building, trash can, mailbox, tree, any other
suitable object, or any suitable combination of all or part of two
or more objects.
[0081] Now referring to FIG. 2, a scanner 162 and a receiver 164
can operate in the lidar system of FIG. 1 as the scanner 120 and
the receiver 140, respectively. More generally, the scanner 162 and
the receiver 164 can operate in any suitable lidar system.
[0082] The scanner 162 may include any suitable number of mirrors
driven by any suitable number of mechanical actuators. For example,
the scanner 162 may include a galvanometer scanner, a resonant
scanner, a piezoelectric actuator, a polygonal scanner, a
rotating-prism scanner, a voice coil motor, a DC motor, a brushless
DC motor, a stepper motor, or a microelectromechanical systems
(MEMS) device, or any other suitable actuator or mechanism.
[0083] A galvanometer scanner (which also may be referred to as a
galvanometer actuator) may include a galvanometer-based scanning
motor with a magnet and coil. When an electrical current is
supplied to the coil, a rotational force is applied to the magnet,
which causes a mirror attached to the galvanometer scanner to
rotate. The electrical current supplied to the coil may be
controlled to dynamically change the position of the galvanometer
mirror. A resonant scanner (which may be referred to as a resonant
actuator) may include a spring-like mechanism driven by an actuator
to produce a periodic oscillation at a substantially fixed
frequency (e.g., 1 kHz). A MEMS-based scanning device may include a
mirror with a diameter between approximately 1 and 10 mm, where the
mirror is rotated using electromagnetic or electrostatic actuation.
A voice coil motor (which may be referred to as a voice coil
actuator) may include a magnet and coil. When an electrical current
is supplied to the coil, a translational force is applied to the
magnet, which causes a mirror attached to the magnet to move or
rotate.
[0084] In an example implementation, the scanner 162 includes a
single mirror configured to scan an output beam 170 along a single
direction (e.g., the scanner 162 may be a one-dimensional scanner
that scans along a horizontal or vertical direction). The mirror
may be a flat scanning mirror attached to a scanner actuator or
mechanism which scans the mirror over a particular angular range.
The mirror may be driven by one actuator (e.g., a galvanometer) or
two actuators configured to drive the mirror in a push-pull
configuration. When two actuators drive the mirror in one direction
in a push-pull configuration, the actuators may be located at
opposite ends or sides of the mirror. The actuators may operate in
a cooperative manner so that when one actuator pushes on the
mirror, the other actuator pulls on the mirror, and vice versa. In
another example implementation, two voice coil actuators arranged
in a push-pull configuration drive a mirror along a horizontal or
vertical direction.
[0085] In some implementations, the scanner 162 may include one
mirror configured to be scanned along two axes, where two actuators
arranged in a push-pull configuration provide motion along each
axis. For example, two resonant actuators arranged in a horizontal
push-pull configuration may drive the mirror along a horizontal
direction, and another pair of resonant actuators arranged in a
vertical push-pull configuration may drive mirror along a vertical
direction. In another example implementation, two actuators scan
the output beam 170 along two directions (e.g., horizontal and
vertical), where each actuator provides rotational motion along a
particular direction or about a particular axis.
[0086] The scanner 162 also may include one mirror driven by two
actuators configured to scan the mirror along two substantially
orthogonal directions. For example, a resonant actuator or a
galvanometer actuator may drive one mirror along a substantially
horizontal direction, and a galvanometer actuator may drive the
mirror along a substantially vertical direction. As another
example, two resonant actuators may drive a mirror along two
substantially orthogonal directions.
[0087] In some implementations, the scanner 162 includes two
mirrors, where one mirror scans the output beam 170 along a
substantially horizontal direction and the other mirror scans the
output beam 170 along a substantially vertical direction. In the
example of FIG. 2, the scanner 162 includes two mirrors, a mirror
180-1 and a mirror 180-2. The mirror 180-1 may scan the output beam
170 along a substantially horizontal direction, and the mirror
180-2 may scan the output beam 170 along a substantially vertical
direction (or vice versa). Mirror 180-1 or mirror 180-2 may be a
flat mirror, a curved mirror, or a polygon mirror with two or more
reflective surfaces.
[0088] The scanner 162 in other implementations includes two
galvanometer scanners driving respective mirrors. For example, the
scanner 162 may include a galvanometer actuator that scans the
mirror 180-1 along a first direction (e.g., vertical), and the
scanner 162 may include another galvanometer actuator that scans
the mirror 180-2 along a second direction (e.g., horizontal). In
yet another implementation, the scanner 162 includes two mirrors,
where a galvanometer actuator drives one mirror, and a resonant
actuator drives the other mirror. For example, a galvanometer
actuator may scan the mirror 180-1 along a first direction, and a
resonant actuator may scan the mirror 180-2 along a second
direction. The first and second scanning directions may be
substantially orthogonal to one another, e.g., the first direction
may be substantially vertical, and the second direction may be
substantially horizontal. In yet another implementation, the
scanner 162 includes two mirrors, where one mirror is a polygon
mirror that is rotated in one direction (e.g., clockwise or
counter-clockwise) by an electric motor (e.g., a brushless DC
motor). For example, mirror 180-1 may be a polygon mirror that
scans the output beam 170 along a substantially horizontal
direction, and mirror 180-2 may scan the output beam 170 along a
substantially vertical direction. A polygon mirror may have two or
more reflective surfaces, and the polygon mirror may be
continuously rotated in one direction so that the output beam 170
is reflected sequentially from each of the reflective surfaces. A
polygon mirror may have a cross-sectional shape that corresponds to
a polygon, where each side of the polygon has a reflective surface.
For example, a polygon mirror with a square cross-sectional shape
may have four reflective surfaces, and a polygon mirror with a
pentagonal cross-sectional shape may have five reflective
surfaces.
[0089] To direct the output beam 170 along a particular scan
pattern, the scanner 162 may include two or more actuators driving
a single mirror synchronously. For example, the two or more
actuators can drive the mirror synchronously along two
substantially orthogonal directions to make the output beam 170
follow a scan pattern with substantially straight lines. In some
implementations, the scanner 162 may include two mirrors and
actuators driving the two mirrors synchronously to generate a scan
pattern that includes substantially straight lines. For example, a
galvanometer actuator may drive the mirror 180-2 with a
substantially linear back-and-forth motion (e.g., the galvanometer
may be driven with a substantially sinusoidal or triangle-shaped
waveform) that causes the output beam 170 to trace a substantially
horizontal back-and-forth pattern, and another galvanometer
actuator may scan the mirror 180-1 along a substantially vertical
direction. The two galvanometers may be synchronized so that for
every 64 horizontal traces, the output beam 170 makes a single
trace along a vertical direction. Whether one or two mirrors are
used, the substantially straight lines can be directed
substantially horizontally, vertically, or along any other suitable
direction.
[0090] The scanner 162 also may apply a dynamically adjusted
deflection along a vertical direction (e.g., with a galvanometer
actuator) as the output beam 170 is scanned along a substantially
horizontal direction (e.g., with a galvanometer or resonant
actuator) to achieve the straight lines. If a vertical deflection
is not applied, the output beam 170 may trace out a curved path as
it scans from side to side. In some implementations, the scanner
162 uses a vertical actuator to apply a dynamically adjusted
vertical deflection as the output beam 170 is scanned horizontally
as well as a discrete vertical offset between each horizontal scan
(e.g., to step the output beam 170 to a subsequent row of a scan
pattern).
[0091] With continued reference to FIG. 2, an overlap mirror 190 in
this example implementation is configured to overlap the input beam
172 and output beam 170, so that the beams 170 and 172 are
substantially coaxial. In FIG. 2, the overlap mirror 190 includes a
hole, slot, or aperture 192 through which the output beam 170
passes, and a reflecting surface 194 that reflects at least a
portion of the input beam 172 toward the receiver 164. The overlap
mirror 190 may be oriented so that input beam 172 and output beam
170 are at least partially overlapped.
[0092] In some implementations, the overlap mirror 190 may not
include a hole 192. For example, the output beam 170 may be
directed to pass by a side of mirror 190 rather than passing
through an aperture 192. The output beam 170 may pass alongside
mirror 190 and may be oriented at a slight angle with respect to
the orientation of the input beam 172. As another example, the
overlap mirror 190 may include a small reflective section
configured to reflect the output beam 170, and the rest of the
overlap mirror 190 may have an AR coating configured to transmit
the input beam 172.
[0093] The input beam 172 may pass through a lens 196 which focuses
the beam onto an active region 166 of the receiver 164. The active
region 166 may refer to an area over which receiver 164 may receive
or detect input light. The active region may have any suitable size
or diameter d, such as for example, a diameter of approximately 25
.mu.m, 50 .mu.m, 80 .mu.m, 100 .mu.m, 200 .mu.m, 500 .mu.m, 1 mm, 2
mm, or 5 mm. The overlap mirror 190 may have a reflecting surface
194 that is substantially flat or the reflecting surface 194 may be
curved (e.g., the mirror 190 may be an off-axis parabolic mirror
configured to focus the input beam 172 onto an active region of the
receiver 140).
[0094] The aperture 192 may have any suitable size or diameter
.PHI..sub.1, and the input beam 172 may have any suitable size or
diameter .PHI..sub.2, where .PHI..sub.2 is greater than
.PHI..sub.1. For example, the aperture 192 may have a diameter
.PHI..sub.1 of approximately 0.2 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 5
mm, or 10 mm, and the input beam 172 may have a diameter
.PHI..sub.2 of approximately 2 mm, 5 mm, 10 mm, 15 mm, 20 mm, 30
mm, 40 mm, or 50 mm. In some implementations, the reflective
surface 194 of the overlap mirror 190 may reflect 70% or more of
input beam 172 toward the receiver 164. For example, if the
reflective surface 194 has a reflectivity R at an operating
wavelength of the light source 160, then the fraction of input beam
172 directed toward the receiver 164 may be expressed as
R.times.[1-(.PHI..sub.1/.PHI..sub.2).sup.2]. As a more specific
example, if R is 95%, .PHI..sub.1 is 2 mm, and .PHI..sub.2 is 10
mm, then approximately 91% of the input beam 172 may be directed
toward the receiver 164 by the reflective surface 194.
[0095] FIG. 3 illustrates an example configuration in which several
components of the lidar system 100 may operate to scan a 360-degree
view of regard. Generally speaking, the field of view of a light
source in this configuration follows a circular trajectory and
accordingly defines a circular scan pattern on a two-dimensional
plane. All points on the trajectory remain at the same elevation
relative to the ground level, according to one implementation. In
this case, separate beams may follow the circular trajectory with
certain vertical offsets relative to each other. In another
implementation, the points of the trajectory may define a spiral
scan pattern in three-dimensional space. A single beam can be
sufficient to trace out the spiral scan pattern but, if desired,
multiple beams can be used.
[0096] In the example of FIG. 3, a rotating scan module 200
revolves around a central axis in one or both directions as
indicated. An electric motor may drive the rotating scan module 200
around the central axis at a constant speed, for example. The
rotating scan module 200 includes a scanner, a receiver, an overlap
mirror, etc. The components of the rotating module 200 may be
similar to the scanner 120, the receiver 140, and the overlap
mirror 115. In some implementations, the subsystem 200 also
includes a light source and a controller. In other implementations,
the light source and/or the controller are disposed apart from the
rotating scan module 200 and/or exchange optical and electrical
signals with the components of the rotating scan module 200 via
corresponding links.
[0097] The rotating scan module 200 may include a housing 210 with
a window 212. Similar to the window 157 of FIG. 1, the window 212
may be made of glass, plastic, or any other suitable material. The
window 212 allows outbound beams as well as return signals to pass
through the housing 210. The arc length defined by the window 212
can correspond to any suitable percentage of the circumference of
the housing 210. For example, the arc length can correspond to 5%,
20%, 30%, 60%, or possibly even 100% of the circumference.
[0098] Now referring to FIG. 4, a rotating scan module 220 is
generally similar to the rotating scan module 200. In this
implementation, however, the components of the rotating scan module
220 are disposed on a platform 222 which rotates inside a
stationary circular housing 230. In this implementation, the
circular housing 230 is substantially transparent to light at the
lidar-system operating wavelength to pass inbound and outbound
light signals. The circular housing 230 in a sense defines a
circular window similar to the window 212, and may be made of
similar material.
Generating Pixels within a Field of Regard
[0099] FIG. 5 illustrates an example scan pattern 240 which the
lidar system 100 of FIG. 1 can produce. The lidar system 100 may be
configured to scan output optical beam 125 along one or more scan
patterns 240. In some implementations, the scan pattern 240
corresponds to a scan across any suitable field of regard (FOR)
having any suitable horizontal FOR (FOR.sub.H) and any suitable
vertical FOR (FOR.sub.V). For example, a certain scan pattern may
have a field of regard represented by angular dimensions (e.g.,
FOR.sub.H.times.FOR.sub.V) 40.degree..times.30.degree.,
90.degree..times.40.degree., or 60.degree..times.15.degree.. As
another example, a certain scan pattern may have a FOR.sub.H
greater than or equal to 10.degree., 25.degree., 30.degree.,
40.degree., 60.degree., 90.degree., or 120.degree.. As yet another
example, a certain scan pattern may have a FOR.sub.V greater than
or equal to 2.degree., 5.degree., 10.degree., 15.degree.,
20.degree., 30.degree., or 45.degree.. In the example of FIG. 5,
reference line 246 represents a center of the field of regard of
scan pattern 240. The reference line 246 may have any suitable
orientation, such as, a horizontal angle of 0.degree. (e.g.,
reference line 246 may be oriented straight ahead) and a vertical
angle of 0.degree. (e.g., reference line 246 may have an
inclination of 0.degree.), or the reference line 246 may have a
nonzero horizontal angle or a nonzero inclination (e.g., a vertical
angle of +10.degree. or -10.degree.) In FIG. 5, if the scan pattern
240 has a 60.degree..times.15.degree. field of regard, then the
scan pattern 240 covers a .+-.30.degree. horizontal range with
respect to reference line 246 and a .+-.7.5.degree. vertical range
with respect to reference line 246. Additionally, the optical beam
125 in FIG. 5 has an orientation of approximately -15.degree.
horizontal and +3.degree. vertical with respect to reference line
246. The beam 125 may be referred to as having an azimuth of
-15.degree. and an altitude of +3.degree. relative to the reference
line 246. An azimuth (which may be referred to as an azimuth angle)
may represent a horizontal angle with respect to the reference line
246, and an altitude (which may be referred to as an altitude
angle, elevation, or elevation angle) may represent a vertical
angle with respect to the reference line 246.
[0100] The scan pattern 240 may include multiple pixels 242, and
each pixel 242 may be associated with one or more laser pulses and
one or more corresponding distance measurements. A cycle of scan
pattern 240 may include a total of P.sub.x.times.P.sub.y pixels 242
(e.g., a two-dimensional distribution of P.sub.x by P.sub.y
pixels). For example, the scan pattern 240 may include a
distribution with dimensions of approximately 100-2,000 pixels 242
along a horizontal direction and approximately 4-400 pixels 242
along a vertical direction. As another example, the scan pattern
240 may include a distribution of 1,000 pixels 242 along the
horizontal direction by 64 pixels 242 along the vertical direction
(e.g., the frame size is 1000.times.64 pixels) for a total of
64,000 pixels per cycle of scan pattern 240. The number of pixels
242 along a horizontal direction may be referred to as a horizontal
resolution of the scan pattern 240, and the number of pixels 242
along a vertical direction may be referred to as a vertical
resolution of the scan pattern 240. As an example, the scan pattern
240 may have a horizontal resolution of greater than or equal to
100 pixels 242 and a vertical resolution of greater than or equal
to 4 pixels 242. As another example, the scan pattern 240 may have
a horizontal resolution of 100-2,000 pixels 242 and a vertical
resolution of 4-400 pixels 242.
[0101] Each pixel 242 may be associated with a distance (e.g., a
distance to a portion of a target 130 from which the corresponding
laser pulse was scattered) or one or more angular values. As an
example, the pixel 242 may be associated with a distance value and
two angular values (e.g., an azimuth and altitude) that represent
the angular location of the pixel 242 with respect to the lidar
system 100. A distance to a portion of the target 130 may be
determined based at least in part on a time-of-flight measurement
for a corresponding pulse. An angular value (e.g., an azimuth or
altitude) may correspond to an angle (e.g., relative to reference
line 246) of the output beam 125 (e.g., when a corresponding pulse
is emitted from lidar system 100) or an angle of the input beam 135
(e.g., when an input signal is received by lidar system 100). In
some implementations, the lidar system 100 determines an angular
value based at least in part on a position of a component of the
scanner 120. For example, an azimuth or altitude value associated
with the pixel 242 may be determined from an angular position of
one or more corresponding scanning mirrors of the scanner 120.
[0102] In some implementations, the lidar system 100 concurrently
directs multiple beams across the field of regard. In the example
implementation of FIG. 6, the lidar system generates output beams
250A, 250B, 250C, . . . 250N etc., each of which follows a linear
scan pattern 254A, 254B, 254C, . . . 254N. The number of parallel
lines can be 2, 4, 12, 20, or any other suitable number. The lidar
system 100 may angularly separate the beams 250A, 250B, 250C, . . .
250N, so that, for example, the separation between beams 250A and
250B at a certain distance may be 30 cm, and the separation between
the same beams 250A and 250B at a longer distance may be 50 cm.
[0103] Similar to the scan pattern 240, each of the linear scan
patterns 254A-N includes pixels associated with one or more laser
pulses and distance measurements. FIG. 6 illustrates example pixels
252A, 252B and 252C along the scan patterns 254A, 254B and 254C,
respectively. The lidar system 100 in this example may generate the
values for the pixels 252A-252N at the same time, thus increasing
the rate at which values for pixels are determined.
[0104] Depending on the implementation, the lidar system 100 may
output the beams 250A-N at the same wavelength or different
wavelengths. The beam 250A for example may have the wavelength of
1540 nm, the beam 250B may have the wavelength of 1550 nm, the beam
250C may have the wavelength of 1560 nm, etc. The number of
different wavelengths the lidar system 100 uses need not match the
number of beams. Thus, the lidar system 100 in the example
implementation of FIG. 6 may use M wavelengths with N beams, where
1.ltoreq.M.ltoreq.N.
[0105] Next, FIG. 7 illustrates an example light-source field of
view (FOV.sub.L) and receiver field of view (FOV.sub.R) for the
lidar system 100. The light source 110 may emit pulses of light as
the FOV.sub.L and FOV.sub.R are scanned by the scanner 120 across a
field of regard (FOR). The light-source field of view may refer to
an angular cone illuminated by the light source 110 at a particular
instant of time. Similarly, a receiver field of view may refer to
an angular cone over which the receiver 140 may receive or detect
light at a particular instant of time, and any light outside the
receiver field of view may not be received or detected. For
example, as the scanner 120 scans the light-source field of view
across a field of regard, the lidar system 100 may send the pulse
of light in the direction the FOV.sub.L is pointing at the time the
light source 110 emits the pulse. The pulse of light may scatter
off the target 130, and the receiver 140 may receive and detect a
portion of the scattered light that is directed along or contained
within the FOV.sub.R.
[0106] In some implementations, the scanner 120 is configured to
scan both a light-source field of view and a receiver field of view
across a field of regard of the lidar system 100. The lidar system
100 may emit and detect multiple pulses of light as the scanner 120
scans the FOV.sub.L and FOV.sub.R across the field of regard while
tracing out the scan pattern 240. The scanner 120 in some
implementations scans the light-source field of view and the
receiver field of view synchronously with respect to one another.
In this case, as the scanner 120 scans FOV.sub.L across a scan
pattern 240, the FOV.sub.R follows substantially the same path at
the same scanning speed. Additionally, the FOV.sub.L and FOV.sub.R
may maintain the same relative position to one another as the
scanner 120 scans FOV.sub.L and FOV.sub.R across the field of
regard. For example, the FOV.sub.L may be substantially overlapped
with or centered inside the FOV.sub.R (as illustrated in FIG. 7),
and the scanner 120 may maintain this relative positioning between
FOV.sub.L and FOV.sub.R throughout a scan. As another example, the
FOV.sub.R may lag behind the FOV.sub.L by a particular, fixed
amount throughout a scan (e.g., the FOV.sub.R may be offset from
the FOV.sub.L in a direction opposite the scan direction).
[0107] The FOV.sub.L may have an angular size or extent
.THETA..sub.L that is substantially the same as or that corresponds
to the divergence of the output beam 125, and the FOV.sub.R may
have an angular size or extent O.sub.R that corresponds to an angle
over which the receiver 140 may receive and detect light. The
receiver field of view may be any suitable size relative to the
light-source field of view. For example, the receiver field of view
may be smaller than, substantially the same size as, or larger than
the angular extent of the light-source field of view. In some
implementations, the light-source field of view has an angular
extent of less than or equal to 50 milliradians, and the receiver
field of view has an angular extent of less than or equal to 50
milliradians. The FOV.sub.L may have any suitable angular extent
.THETA..sub.L, such as for example, approximately 0.1 mrad, 0.2
mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2 mrad, 3 mrad, 5 mrad, 10 mrad,
20 mrad, 40 mrad, or 50 mrad. Similarly, the FOV.sub.R may have any
suitable angular extent .THETA..sub.R, such as for example,
approximately 0.1 mrad, 0.2 mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2
mrad, 3 mrad, 5 mrad, 10 mrad, 20 mrad, 40 mrad, or 50 mrad. The
light-source field of view and the receiver field of view may have
approximately equal angular extents. As an example, .THETA..sub.L
and .THETA..sub.R may both be approximately equal to 1 mrad, 2
mrad, or 3 mrad. In some implementations, the receiver field of
view is larger than the light-source field of view, or the
light-source field of view is larger than the receiver field of
view. For example, .THETA..sub.L may be approximately equal to 1.5
mrad, and .THETA..sub.R may be approximately equal to 3 mrad.
[0108] A pixel 242 may represent or correspond to a light-source
field of view. As the output beam 125 propagates from the light
source 110, the diameter of the output beam 125 (as well as the
size of the corresponding pixel 242) may increase according to the
beam divergence .THETA..sub.L. As an example, if the output beam
125 has a .THETA..sub.L of 2 mrad, then at a distance of 100 m from
the lidar system 100, the output beam 125 may have a size or
diameter of approximately 20 cm, and a corresponding pixel 242 may
also have a corresponding size or diameter of approximately 20 cm.
At a distance of 200 m from the lidar system 100, the output beam
125 and the corresponding pixel 242 may each have a diameter of
approximately 40 cm.
A Lidar System Operating in a Vehicle
[0109] As indicated above, one or more lidar systems 100 may be
integrated into a vehicle. In one example implementation, multiple
lidar systems 100 may be integrated into a car to provide a
complete 360-degree horizontal FOR around the car. As another
example, 4-10 lidar systems 100, each system having a 45-degree to
90-degree horizontal FOR, may be combined together to form a
sensing system that provides a point cloud covering a 360-degree
horizontal FOR. The lidar systems 100 may be oriented so that
adjacent FORs have an amount of spatial or angular overlap to allow
data from the multiple lidar systems 100 to be combined or stitched
together to form a single or continuous 360-degree point cloud. As
an example, the FOR of each lidar system 100 may have approximately
1-15 degrees of overlap with an adjacent FOR. In particular
embodiments, a vehicle may refer to a mobile machine configured to
transport people or cargo. For example, a vehicle may include, may
take the form of, or may be referred to as a car, automobile, motor
vehicle, truck, bus, van, trailer, off-road vehicle, farm vehicle,
lawn mower, construction equipment, forklift, robot, golf cart,
motorhome, taxi, motorcycle, scooter, bicycle, skateboard, train,
snowmobile, watercraft (e.g., a ship or boat), aircraft (e.g., a
fixed-wing aircraft, helicopter, or dirigible), or spacecraft. In
particular embodiments, a vehicle may include an internal
combustion engine or an electric motor that provides propulsion for
the vehicle.
[0110] In some implementations, one or more lidar systems 100 are
included in a vehicle as part of an advanced driver assistance
system (ADAS) to assist a driver of the vehicle in the driving
process. For example, a lidar system 100 may be part of an ADAS
that provides information or feedback to a driver (e.g., to alert
the driver to potential problems or hazards) or that automatically
takes control of part of a vehicle (e.g., a braking system or a
steering system) to avoid collisions or accidents. The lidar system
100 may be part of a vehicle ADAS that provides adaptive cruise
control, automated braking, automated parking, collision avoidance,
alerts the driver to hazards or other vehicles, maintains the
vehicle in the correct lane, or provides a warning if an object or
another vehicle is in a blind spot.
[0111] In some cases, one or more lidar systems 100 are integrated
into a vehicle as part of an autonomous-vehicle driving system. In
an example implementation, the lidar system 100 provides
information about the surrounding environment to a driving system
of an autonomous vehicle. An autonomous-vehicle driving system may
include one or more computing systems that receive information from
the lidar system 100 about the surrounding environment, analyze the
received information, and provide control signals to the vehicle's
driving systems (e.g., steering wheel, accelerator, brake, or turn
signal). For example, the lidar system 100 integrated into an
autonomous vehicle may provide an autonomous-vehicle driving system
with a point cloud every 0.1 seconds (e.g., the point cloud has a
10 Hz update rate, representing 10 frames per second). The
autonomous-vehicle driving system may analyze the received point
clouds to sense or identify targets 130 and their respective
locations, distances, or speeds, and the autonomous-vehicle driving
system may update control signals based on this information. As an
example, if the lidar system 100 detects a vehicle ahead that is
slowing down or stopping, the autonomous-vehicle driving system may
send instructions to release the accelerator and apply the
brakes.
[0112] An autonomous vehicle may be referred to as an autonomous
car, driverless car, self-driving car, robotic car, or unmanned
vehicle. An autonomous vehicle may be a vehicle configured to sense
its environment and navigate or drive with little or no human
input. For example, an autonomous vehicle may be configured to
drive to any suitable location and control or perform all
safety-critical functions (e.g., driving, steering, braking,
parking) for the entire trip, with the driver not expected to
control the vehicle at any time. As another example, an autonomous
vehicle may allow a driver to safely turn their attention away from
driving tasks in particular environments (e.g., on freeways), or an
autonomous vehicle may provide control of a vehicle in all but a
few environments, requiring little or no input or attention from
the driver.
[0113] An autonomous vehicle may be configured to drive with a
driver present in the vehicle, or an autonomous vehicle may be
configured to operate the vehicle with no driver present. As an
example, an autonomous vehicle may include a driver's seat with
associated controls (e.g., steering wheel, accelerator pedal, and
brake pedal), and the vehicle may be configured to drive with no
one seated in the driver's seat or with little or no input from a
person seated in the driver's seat. As another example, an
autonomous vehicle may not include any driver's seat or associated
driver's controls, and the vehicle may perform substantially all
driving functions (e.g., driving, steering, braking, parking, and
navigating) without human input. As another example, an autonomous
vehicle may be configured to operate without a driver (e.g., the
vehicle may be configured to transport human passengers or cargo
without a driver present in the vehicle). As another example, an
autonomous vehicle may be configured to operate without any human
passengers (e.g., the vehicle may be configured for transportation
of cargo without having any human passengers onboard the
vehicle).
[0114] In some implementations, a light source of a lidar system is
located remotely from some of the other components of the lidar
system such as the scanner and the receiver. Moreover, a lidar
system implemented in a vehicle may include fewer light sources
than scanners and receivers.
[0115] FIG. 8 illustrates an example configuration in which a
laser-sensor link 320 includes an optical link 330 and an
electrical link 350 coupled between a laser 300 and a sensor 310.
The laser 300 may be configured to emit pulses of light and may be
referred to as a laser system, laser head, or light source. The
laser 300 may include, may be part of, may be similar to, or may be
substantially the same as the light source 110 illustrated in FIG.
1 and discussed above. Further, the scanner 302, the receiver 304,
the controller 306, and the mirror 308 may be similar to the
scanner 120, the receiver 140, the controller 150, and the mirror
115 discussed above. In the example of FIG. 8, the laser 300 is
coupled to the remotely located sensor 310 by a laser-sensor link
320 (which may be referred to as a link). The sensor 310 may be
referred to as a sensor head and may include the mirror 308, the
scanner 302, the receiver 304, and the controller 306. In an
example implementation, the laser 300 includes a pulsed laser diode
(e.g., a pulsed DFB laser) followed by an optical amplifier, and
light from the laser 300 is conveyed by an optical fiber of the
laser-sensor link 320 of a suitable length to the scanner 120 in a
remotely located sensor 310.
[0116] The laser-sensor link 320 may include any suitable number of
optical links 330 (e.g., 0, 1, 2, 3, 5, or 10) and any suitable
number of electrical links 350 (e.g., 0, 1, 2, 3, 5, or 10). In the
example configuration depicted in FIG. 8, the laser-sensor link 320
includes one optical link 330 from the laser 300 to an output
collimator 340 and one electrical link 350 that connects the laser
300 to the controller 150. The optical link 330 may include optical
fiber (which may be referred to as fiber-optic cable or fiber) that
conveys, carries, transports, or transmits light between the laser
300 and the sensor 310. The optical fiber may be, for example,
single-mode (SM) fiber, multi-mode (MM) fiber, large-mode-area
(LMA) fiber, polarization-maintaining (PM) fiber, photonic-crystal
or photonic-bandgap fiber, gain fiber (e.g., rare-earth-doped
optical fiber for use in an optical amplifier), or any suitable
combination thereof. The output collimator 340 receives optical
pulses conveyed from the laser 300 by the optical link 330 and
produces a free-space optical beam 312 that includes the optical
pulses. The output collimator 340 directs the free-space optical
beam 312 through the mirror 308 and to the scanner 302.
[0117] The electrical link 350 may include electrical wire or cable
(e.g., a coaxial cable or twisted-pair cable) that conveys or
transmits electrical power and/or one or more electrical signals
between the laser 300 and the sensor 310. For example, the laser
300 may include a power supply or a power conditioner that provides
electrical power to the laser 300, and additionally, the power
supply or power conditioner may provide power to one or more
components of the sensor 310 (e.g., the scanner 304, the receiver
304, and/or the controller 306) via the one or more electrical
links 350. The electrical link 350 in some implementations may
convey electrical signals that include data or information in
analog or digital format. Further, the electrical link 350 may
provide an interlock signal from the sensor 310 to the laser 300.
If the controller 306 detects a fault condition indicating a
problem with the sensor 310 or the overall lidar system, the
controller 306 may change a voltage on the interlock line (e.g.,
from 5 V to 0 V) indicating that the laser 300 should shut down,
stop emitting light, or reduce the power or energy of emitted
light. A fault condition may be triggered by a failure of the
scanner 302, a failure of the receiver 304, or by a person or
object coming within a threshold distance of the sensor 310 (e.g.,
within 0.1 m, 0.5 m, 1 m, 5 m, or any other suitable distance).
[0118] As discussed above, a lidar system can include one or more
processors to determine a distance D to a target. In the
implementation illustrated in FIG. 8, the controller 306 may be
located in the laser 300 or in the sensor 310, or parts of the
controller 150 may be distributed between the laser 300 and the
sensor 310. In an example implementation, each sensor head 310 of a
lidar system includes electronics (e.g., an electronic filter,
transimpedance amplifier, threshold detector, or time-to-digital
(TDC) converter) configured to receive or process a signal from the
receiver 304 or from an APD or SPAD of the receiver 304.
Additionally, the laser 300 may include processing electronics
configured to determine a time-of-flight value or a distance to the
target based on a signal received from the sensor head 310 via the
electrical link 350.
[0119] Next, FIG. 9 illustrates an example vehicle 354 with a lidar
system 351 that includes a laser 352 with multiple sensor heads 360
coupled to the laser 352 via multiple laser-sensor links 370. The
laser 352 and the sensor heads 360 may be similar to the laser 300
and the sensor 310 discussed above, in some implementations. For
example, each of the laser-sensor links 370 may include one or more
optical links and/or one or more electrical links. The sensor heads
360 in FIG. 9 are positioned or oriented to provide a greater than
30-degree view of an environment around the vehicle. More
generally, a lidar system with multiple sensor heads may provide a
horizontal field of regard around a vehicle of approximately
30.degree., 45.degree., 60.degree., 90.degree., 120.degree.,
180.degree., 270.degree., or 360.degree.. Each of the sensor heads
may be attached to or incorporated into a bumper, fender, grill,
side panel, spoiler, roof, headlight assembly, taillight assembly,
rear-view mirror assembly, hood, trunk, window, or any other
suitable part of the vehicle.
[0120] In the example of FIG. 9, four sensor heads 360 are
positioned at or near the four corners of the vehicle (e.g., the
sensor heads may be incorporated into a light assembly, side panel,
bumper, or fender), and the laser 352 may be located within the
vehicle (e.g., in or near the trunk). The four sensor heads 360 may
each provide a 90.degree. to 120.degree. horizontal field of regard
(FOR), and the four sensor heads 360 may be oriented so that
together they provide a complete 360-degree view around the
vehicle. As another example, the lidar system 351 may include six
sensor heads 360 positioned on or around a vehicle, where each of
the sensor heads 360 provides a 60.degree. to 90.degree. horizontal
FOR. As another example, the lidar system 351 may include eight
sensor heads 360, and each of the sensor heads 360 may provide a
45.degree. to 60.degree. horizontal FOR. As yet another example,
the lidar system 351 may include six sensor heads 360, where each
of the sensor heads 360 provides a 70.degree. horizontal FOR with
an overlap between adjacent FORs of approximately 10.degree.. As
another example, the lidar system 351 may include two sensor heads
360 which together provide a horizontal FOR of greater than or
equal to 30.degree..
[0121] Data from each of the sensor heads 360 may be combined or
stitched together to generate a point cloud that covers a greater
than or equal to 30-degree horizontal view around a vehicle. For
example, the laser 352 may include a controller or processor that
receives data from each of the sensor heads 360 (e.g., via a
corresponding electrical link 370) and processes the received data
to construct a point cloud covering a 360-degree horizontal view
around a vehicle or to determine distances to one or more targets.
The point cloud or information from the point cloud may be provided
to a vehicle controller 372 via a corresponding electrical,
optical, or radio link 370. In some implementations, the point
cloud is generated by combining data from each of the multiple
sensor heads 360 at a controller included within the laser 352 and
provided to the vehicle controller 372. In other implementations,
each of the sensor heads 360 includes a controller or process that
constructs a point cloud for a portion of the 360-degree horizontal
view around the vehicle and provides the respective point cloud to
the vehicle controller 372. The vehicle controller 372 then
combines or stitches together the points clouds from the respective
sensor heads 360 to construct a combined point cloud covering a
360-degree horizontal view. Still further, the vehicle controller
372 in some implementations communicates with a remote server to
process point cloud data.
[0122] In any event, the vehicle 354 may be an autonomous vehicle
where the vehicle controller 372 provides control signals to
various components 390 within the vehicle 354 to maneuver and
otherwise control operation of the vehicle 354. The components 390
are depicted in an expanded view in FIG. 9 for ease of illustration
only. The components 390 may include an accelerator 374, brakes
376, a vehicle engine 378, a steering mechanism 380, lights 382
such as brake lights, head lights, reverse lights, emergency
lights, etc., a gear selector 384, and/or other suitable components
that effectuate and control movement of the vehicle 354. The gear
selector 384 may include the park, reverse, neutral, drive gears,
etc. Each of the components 390 may include an interface via which
the component receives commands from the vehicle controller 372
such as "increase speed," "decrease speed," "turn left 5 degrees,"
"activate left turn signal," etc. and, in some cases, provides
feedback to the vehicle controller 372.
[0123] In some implementations, the vehicle controller 372 receives
point cloud data from the laser 352 or sensor heads 360 via the
link 370 and analyzes the received point cloud data to sense or
identify targets 130 and their respective locations, distances,
speeds, shapes, sizes, type of target (e.g., vehicle, human, tree,
animal), etc. The vehicle controller 372 then provides control
signals via the link 370 to the components 390 to control operation
of the vehicle based on the analyzed information. For example, the
vehicle controller 372 may identify an intersection based on the
point cloud data and determine that the intersection is the
appropriate location at which to make a left turn. Accordingly, the
vehicle controller 372 may provide control signals to the steering
mechanism 380, the accelerator 374, and brakes 376 for making a
proper left turn. In another example, the vehicle controller 372
may identify a traffic light based on the point cloud data and
determine that the vehicle 354 needs to come to a stop. As a
result, the vehicle controller 372 may provide control signals to
release the accelerator 374 and apply the brakes 376.
Example Receiver Implementation
[0124] FIG. 10 illustrates an example InGaAs avalanche photodiode
(APD) 400. Referring back to FIG. 1, the receiver 140 may include
one or more APDs 400 configured to receive and detect light from
input light such as the beam 135. More generally, the APD 400 can
operate in any suitable receiver of input light. The APD 400 may be
configured to detect a portion of pulses of light which are
scattered by a target located downrange from the lidar system in
which the APD 400 operates. For example, the APD 400 may receive a
portion of a pulse of light scattered by the target 130 depicted in
FIG. 1, and generate an electrical-current signal corresponding to
the received pulse of light.
[0125] The APD 400 may include doped or undoped layers of any
suitable semiconductor material, such as for example, silicon,
germanium, InGaAs, InGaAsP, or indium phosphide (InP).
Additionally, the APD 400 may include an upper electrode 402 and a
lower electrode 406 for coupling the ADP 400 to an electrical
circuit. The APD 400 for example may be electrically coupled to a
voltage source that supplies a reverse-bias voltage V to the APD
400. Additionally, the APD 400 may be electrically coupled to a
transimpedance amplifier which receives electrical current
generated by the APD 400 and produces an output voltage signal that
corresponds to the received current. The upper electrode 402 or
lower electrode 406 may include any suitable electrically
conductive material, such as for example a metal (e.g., gold,
copper, silver, or aluminum), a transparent conductive oxide (e.g.,
indium tin oxide), a carbon-nanotube material, or polysilicon. In
some implementations, the upper electrode 402 is partially
transparent or has an opening to allow input light 410 to pass
through to the active region of the APD 400. In FIG. 10, the upper
electrode 402 may have a ring shape that at least partially
surrounds the active region of the APD 400, where the active region
refers to an area over which the APD 400 may receive and detect the
input light 410. The active region may have any suitable size or
diameter d, such as for example, a diameter of approximately 25
.mu.m, 50 .mu.m, 80 .mu.m, 100 .mu.m, 200 .mu.m, 500 .mu.m, 1 mm, 2
mm, or 5 mm.
[0126] The APD 400 may include any suitable combination of any
suitable semiconductor layers having any suitable doping (e.g.,
n-doped, p-doped, or intrinsic undoped material). In the example of
FIG. 10, the InGaAs APD 400 includes a p-doped InP layer 420, an
InP avalanche layer 422, an absorption layer 424 with n-doped
InGaAs or InGaAsP, and an n-doped InP substrate layer 426.
Depending on the implementation, the APD 400 may include separate
absorption and avalanche layers, or a single layer may act as both
an absorption and avalanche region. The APD 400 may operate
electrically as a PN diode or a PIN diode, and, during operation,
the APD 400 may be reverse-biased with a positive voltage V applied
to the lower electrode 406 with respect to the upper electrode 402.
The applied reverse-bias voltage V may have any suitable value,
such as for example approximately 5 V, 10 V, 20 V, 30 V, 50 V, 75
V, 100 V, or 200 V.
[0127] In FIG. 10, photons of the input light 410 may be absorbed
primarily in the absorption layer 424, resulting in the generation
of electron-hole pairs (which may be referred to as photo-generated
carriers). For example, the absorption layer 424 may be configured
to absorb photons corresponding to the operating wavelength of the
lidar system 100 (e.g., any suitable wavelength between
approximately 1400 nm and approximately 1600 nm). In the avalanche
layer 422, an avalanche-multiplication process occurs where
carriers (e.g., electrons or holes) generated in the absorption
layer 424 collide with the semiconductor lattice of the absorption
layer 424, and produce additional carriers through impact
ionization. This avalanche process can repeat numerous times so
that one photo-generated carrier may result in the generation of
multiple carriers. As an example, a single photon absorbed in the
absorption layer 424 may lead to the generation of approximately
10, 50, 100, 200, 500, 1000, 10,000, or any other suitable number
of carriers through an avalanche-multiplication process. The
carriers generated in an APD 400 may produce an electrical current
that is coupled to an electrical circuit which may perform signal
amplification, sampling, filtering, signal conditioning,
analog-to-digital conversion, time-to-digital conversion, pulse
detection, threshold detection, rising-edge detection, or
falling-edge detection.
[0128] The number of carriers generated from a single
photo-generated carrier may increase as the applied reverse bias V
is increased. If the applied reverse bias V is increased above a
particular value referred to as the APD breakdown voltage, then a
single carrier can trigger a self-sustaining avalanche process
(e.g., the output of the APD 400 is saturated regardless of the
input light level). The APD 400 that is operated at or above a
breakdown voltage may be referred to as a single-photon avalanche
diode (SPAD) and may be referred to as operating in a Geiger mode
or a photon-counting mode. The APD 400 that is operated below a
breakdown voltage may be referred to as a linear APD, and the
output current generated by the APD 400 may be sent to an amplifier
circuit (e.g., a transimpedance amplifier). The receiver 140 (see
FIG. 1) may include an APD configured to operate as a SPAD and a
quenching circuit configured to reduce a reverse-bias voltage
applied to the SPAD when an avalanche event occurs in the SPAD. The
APD 400 configured to operate as a SPAD may be coupled to an
electronic quenching circuit that reduces the applied voltage V
below the breakdown voltage when an avalanche-detection event
occurs. Reducing the applied voltage may halt the avalanche
process, and the applied reverse-bias voltage may then be re-set to
await a subsequent avalanche event. Additionally, the APD 400 may
be coupled to a circuit that generates an electrical output pulse
or edge when an avalanche event occurs.
[0129] In some implementations, the APD 400 or the APD 400 along
with transimpedance amplifier have a noise-equivalent power (NEP)
that is less than or equal to 100 photons, 50 photons, 30 photons,
20 photons, or 10 photons. For example, the APD 400 may be operated
as a SPAD and may have a NEP of less than or equal to 20 photons.
As another example, the APD 400 may be coupled to a transimpedance
amplifier that produces an output voltage signal with a NEP of less
than or equal to 50 photons. The NEP of the APD 400 is a metric
that quantifies the sensitivity of the APD 400 in terms of a
minimum signal (or a minimum number of photons) that the APD 400
can detect. The NEP may correspond to an optical power (or to a
number of photons) that results in a signal-to-noise ratio of 1, or
the NEP may represent a threshold number of photons above which an
optical signal may be detected. For example, if the APD 400 has a
NEP of 20 photons, then the input beam 410 with 20 photons may be
detected with a signal-to-noise ratio of approximately 1 (e.g., the
APD 400 may receive 20 photons from the input beam 410 and generate
an electrical signal representing the input beam 410 that has a
signal-to-noise ratio of approximately 1). Similarly, the input
beam 410 with 100 photons may be detected with a signal-to-noise
ratio of approximately 5. In some implementations, the lidar system
100 with the APD 400 (or a combination of the APD 400 and
transimpedance amplifier) having a NEP of less than or equal to 100
photons, 50 photons, 30 photons, 20 photons, or 10 photons offers
improved detection sensitivity with respect to a conventional lidar
system that uses a PN or PIN photodiode. For example, an InGaAs PIN
photodiode used in a conventional lidar system may have a NEP of
approximately 10.sup.4 to 10.sup.5 photons, and the noise level in
a lidar system with an InGaAs PIN photodiode may be 10.sup.3 to
10.sup.4 times greater than the noise level in a lidar system 100
with the InGaAs APD detector 400.
[0130] Referring back to FIG. 1, an optical filter may be located
in front of the receiver 140 and configured to transmit light at
one or more operating wavelengths of the light source 110 and
attenuate light at surrounding wavelengths. For example, an optical
filter may be a free-space spectral filter located in front of APD
400 of FIG. 10. This spectral filter may transmit light at the
operating wavelength of the light source 110 (e.g., between
approximately 1530 nm and 1560 nm) and attenuate light outside that
wavelength range. As a more specific example, light with
wavelengths of approximately 400-1530 nm or 1560-2000 nm may be
attenuated by any suitable amount, such as for example, by at least
5 dB, 10 dB, 20 dB, 30 dB, or 40 dB.
[0131] Next, FIG. 11 illustrates an APD 502 coupled to an example
pulse-detection circuit 504. The APD 502 can be similar to the APD
400 discussed above with reference to FIG. 10, or can be any other
suitable detector. The pulse-detection circuit 504 can operate in
the lidar system of FIG. 1 as part of the receiver 140. Further,
the pulse-detection circuit 504 can operate in the receiver 164 of
FIG. 2, the receiver 304 of FIG. 8, or any other suitable receiver.
The pulse-detection circuit 504 alternatively can be implemented in
the controller 150, the controller 306, or another suitable
controller. In some implementations, parts of the pulse-detection
circuit 504 can operate in a receiver and other parts of the
pulse-detection circuit 504 can operate in a controller. For
example, components 510 and 512 may be a part of the receiver 140,
and components 514 and 516 may be a part of the controller 150.
[0132] The pulse-detection circuit 504 may include circuitry that
receives a signal from a detector (e.g., an electrical current from
the APD 502) and performs current-to-voltage conversion, signal
amplification, sampling, filtering, signal conditioning,
analog-to-digital conversion, time-to-digital conversion, pulse
detection, threshold detection, rising-edge detection, or
falling-edge detection. The pulse-detection circuit 504 may
determine whether an optical pulse has been received by the APD 502
or may determine a time associated with receipt of an optical pulse
by the APD 502. Additionally, the pulse-detection circuit 504 may
determine a duration of a received optical pulse. In an example
implementation, the pulse-detection circuit 504 includes a
transimpedance amplifier (TIA) 510, a gain circuit 512, a
comparator 514, and a time-to-digital converter (TDC) 516.
[0133] The TIA 510 may be configured to receive an
electrical-current signal from the APD 502 and produce a voltage
signal that corresponds to the received electrical-current signal.
For example, in response to a received optical pulse, the APD 502
may produce a current pulse corresponding to the optical pulse. The
TIA 510 may receive the current pulse from the APD 502 and produce
a voltage pulse that corresponds to the received current pulse. The
TIA 510 may also act as an electronic filter. For example, the TIA
510 may be configured as a low-pass filter that removes or
attenuates high-frequency electrical noise by attenuating signals
above a particular frequency (e.g., above 1 MHz, 10 MHz, 20 MHz, 50
MHz, 100 MHz, 200 MHz, or any other suitable frequency).
[0134] The gain circuit 512 may be configured to amplify a voltage
signal. As an example, the gain circuit 512 may include one or more
voltage-amplification stages that amplify a voltage signal received
from the TIA 510. For example, the gain circuit 512 may receive a
voltage pulse from the TIA 510, and the gain circuit 512 may
amplify the voltage pulse by any suitable amount, such as for
example, by a gain of approximately 3 dB, 10 dB, 20 dB, 30 dB, 40
dB, or 50 dB. Additionally, the gain circuit 512 may also act as an
electronic filter configured to remove or attenuate electrical
noise.
[0135] The comparator 514 may be configured to receive a voltage
signal from the TIA 510 or the gain circuit 512 and produce an
electrical-edge signal (e.g., a rising edge or a falling edge) when
the received voltage signal rises above or falls below a particular
threshold voltage V.sub.T. As an example, when a received voltage
rises above V.sub.T, the comparator 514 may produce a rising-edge
digital-voltage signal (e.g., a signal that steps from
approximately 0 V to approximately 2.5 V, 3.3 V, 5 V, or any other
suitable digital-high level). As another example, when a received
voltage falls below V.sub.T, the comparator 514 may produce a
falling-edge digital-voltage signal (e.g., a signal that steps down
from approximately 2.5 V, 3.3 V, 5 V, or any other suitable
digital-high level to approximately 0 V). The voltage signal
received by the comparator 514 may be received from the TIA 510 or
the gain circuit 512 and may correspond to an electrical-current
signal generated by the APD 502. For example, the voltage signal
received by the comparator 514 may include a voltage pulse that
corresponds to an electrical-current pulse produced by the APD 502
in response to receiving an optical pulse. The voltage signal
received by the comparator 514 may be an analog signal, and an
electrical-edge signal produced by the comparator 514 may be a
digital signal.
[0136] The time-to-digital converter (TDC) 516 may be configured to
receive an electrical-edge signal from the comparator 514 and
determine an interval of time between emission of a pulse of light
by the light source and receipt of the electrical-edge signal. The
output of the TDC 516 may be a numerical value that corresponds to
the time interval determined by the TDC 516. In some
implementations, the TDC 516 has an internal counter or clock with
any suitable period, such as for example, 5 ps, 10 ps, 15 ps, 20
ps, 30 ps, 50 ps, 100 ps, 0.5 ns, 1 ns, 2 ns, 5 ns, or 10 ns. The
TDC 516 for example may have an internal counter or clock with a 20
ps period, and the TDC 516 may determine that an interval of time
between emission and receipt of a pulse is equal to 25,000 time
periods, which corresponds to a time interval of approximately 0.5
microseconds. Referring back to FIG. 1, the TDC 516 may send the
numerical value "25000" to a processor or controller 150 of the
lidar system 100, which may include a processor configured to
determine a distance from the lidar system 100 to the target 130
based at least in part on an interval of time determined by a TDC
516. The processor may receive a numerical value (e.g., "25000")
from the TDC 516 and, based on the received value, the processor
may determine the distance from the lidar system 100 to a target
130.
Example Techniques for Varying Pulse Rate
[0137] As mentioned above with reference to FIG. 1, the light
source 110 may be a pulsed laser that produces pulses at a pulse
repetition frequency of approximately 100 kHz to 5 MHz or a pulse
period (e.g., a time between consecutive pulses) of approximately
200 ns to 10 .mu.s. The light source 110 may have a variable pulse
repetition frequency, depending on the implementation. As an
example, the light source 110 may have a pulse repetition frequency
that can be varied from approximately 500 kHz to 3 MHz. In some
embodiments, the controller 150 may provide instructions, a control
signal, or a trigger signal to the light source 110 indicating when
the light source 110 should produce optical pulses. For example,
the controller 150 may send an electrical trigger signal that
includes electrical pulses, where the light source 110 emits an
optical pulse in response to each electrical pulse.
[0138] In some embodiments, the controller 150 may communicate with
the receiver 140 as shown in FIG. 1 that detects a received light
signal scattered by a remote target 130. For example, the
pulse-detection circuit 504 may detect the received light signal as
shown in FIG. 11. The pulse detection circuit 504 may compare a
voltage from the received light signal to a voltage threshold
V.sub.T and, when the voltage exceeds the voltage threshold
V.sub.T, the pulse detection circuit 504 (and/or another component
of the receiver 140) can register detection of a return light
pulse. In response to receiving an indication of a return light
pulse, the controller 150 may provide a control signal or other
trigger signal to the light source 110, indicating that the light
source 110 should produce a new light pulse. In response, the light
source 110 emits a subsequent light pulse. The lidar system 150 in
this manner may increase the effective pulse rate.
[0139] When the receiver 140 does not detect a return light pulse
within a threshold time period (e.g., 1.33 .mu.s) corresponding to
a maximum range (e.g., 200 m) at which a target can be located
relative to the lidar system 100, the controller 150 may provide a
control signal to the light source 110 to produce a light pulse. To
this end, the controller 150 can implement a timer in hardware,
firmware, software, or any suitable combination thereof.
[0140] In some implementations, in addition to providing an
indication of a received light signal to the controller 150 when a
voltage from the received light signal exceeds a voltage threshold
V.sub.T, the receiver 140 provides additional characteristics of
the received light signal to the controller 150. The additional
characteristics may include indications of the peak power for the
received light signal, the average power for the received light
signal, the pulse energy of the received light signal, the pulse
duration of the received light signal, or any other suitable
characteristics of the received light signal. The controller 150
may then analyze these characteristics to determine whether the
received light signal scattered from a "hard" target or a "soft"
target. A "hard target" may be an object such as a vehicle,
building, person, etc. that scatters a large portion of the light
pulse. A "soft" target may be fog or rain that scatters a small
portion of the light pulse. When the controller 150 determines that
the received light signal scattered from a hard target, the
controller 150 provides the control signal or other trigger signal
to the light source 100 to produce a light pulse. On the other
hand, when the controller 150 determines that the received light
signal scattered from a soft target, the controller 150 may not
provide a control signal or other trigger signal to the light
source 100 and may continue to wait until a hard target is detected
or the threshold time period (e.g., 1.33 .mu.s) has expired. This
process may be repeated before transmitting each light pulse.
[0141] To determine whether the received light signal scattered
from a hard target or a soft target, the controller 150 may compare
the peak power, average power, or pulse energy for the received
light signal to a power or energy threshold. In some
implementations, the controller may combine two or more of the peak
or average power, pulse energy, the distance measurement, and the
pulse duration in any suitable manner (e.g., by determining a ratio
between the peak power and the pulse duration) and compare the
combined metric to a combined threshold.
[0142] FIG. 12 illustrates example timing of outbound pulses in the
lidar system 100. The pulse timing diagram 600 schematically
illustrates when the controller 150 provides signals to the light
source 110 to trigger emission of light pulses. As shown in the
pulse timing diagram 600, the period between light pulses varies
based on when the receiver 140 detects a return pulse corresponding
to the previous light pulse.
[0143] In the illustrated example, after the lidar system 100 emits
pulse N, the receiver 140 detects a return pulse corresponding to
pulse N after a time interval T1. The controller 150 generates a
signal 610 in response to the determination that the receiver 140
has received pulse N. The signal 610 causes the lidar system 100 to
emit pulse N+1. For clarity, FIG. 12 also illustrates a short delay
between the time pulse N returns and pulse N+1 leaves the lidar
system 100. This delay corresponds to the time it takes the signal
610 to propagate through the lidar system 100.
[0144] The lidar system 100 in the scenario of FIG. 12 emits pulse
N+1 but does not receive a return pulse corresponding to pulse N+1
in the time T2 it takes a light pulse to travel to a target
disposed at the maximum range and return to the lidar system 100.
The lidar system 100 in this case generates signal 612 and emits
next pulse N+2 upon expiration of a time period of duration T2. As
FIG. 12 further illustrates, the receiver 140 receives a return
pulse corresponding to the emitted pulse N+2 after a time period
T3. Because T1<T2 and T3<T2 in this case, the lidar system
achieves a higher pulse rate than a fixed pulse rate in which each
pair of adjacent pulses is separated by a time interval of duration
T2.
[0145] Additionally or alternatively, the lidar system 100 may vary
the pulse rate according to the orientation of the light pulses
with respect to the direction of the front of the lidar system 100.
FIG. 13 illustrates an example vehicle 700, which may be similar to
the vehicle 354 of FIG. 9, and in which the lidar system 100 may
operate. The scanner 120 directs light pulses transmitted by a
light source 110 across a 120.degree. horizontal FOR. For purposes
of this illustration, 0.degree. may refer to the horizontal
orientation that is directly in front of the lidar system 100 and
35 60.degree. may refer to horizontal orientations at or outside
the peripheries of the lidar system 100. In some scenarios, the
light source 110 may have a pulse repetition frequency that is
slower near the front of the lidar system 100 and faster near the
periphery. In this manner, the lidar system can increase the power
and range farther directly in front of the lidar system 100 and
then decrease the power and corresponding range as the lidar system
scans toward the sides. In the example implementation illustrated
in FIG. 13, the lidar system 100 is positioned near the front of
the vehicle 700 such that the 0.degree. horizontal orientation
directly in front of the lidar system 100 is also directly in front
of the vehicle 700. In other implementations, one or several lidar
systems 100 or sensor heads may be positioned at the sides of the
vehicle 700, near the back of the vehicle 700, in the corners of
the vehicle 700, or placed in any other suitable manner with
respect to the vehicle 700.
[0146] FIG. 13 includes a graph 710 to illustrate the relationship
between information density and the horizontal orientation of a
light pulse, and a graph 720 to illustrate the relationship between
the pulse rate and the horizontal orientation of a light pulse.
Information density may refer to the number of targets at a
particular orientation or portion of the field of regard. For
example, targets within a field of regard of the lidar system are
more likely to be in front of the lidar system 100 (e.g., when the
lidar system is positioned near the front of the vehicle 700) than
at the periphery so the information density may be higher near the
front of the lidar system 100. In the graphical representation 710,
the information density is highest near the front of the lidar
system 100 at 0.degree. and trails off near the periphery at
60.degree.. In the graphical representation 720, the pulse rate may
be the lowest between 0.degree. and 15.degree. before rapidly
increasing and then plateauing around 45.degree..
[0147] In other implementations, the light source operating in the
vehicle 700 may have a pulse repetition frequency that is faster
near the front of the lidar system 100 and slower near the
periphery. FIG. 14 includes a graph 810 that illustrates the
relationship between information density and the horizontal
orientation of a light pulse, and a graph 820 that illustrates the
relationship between the pulse rate and the horizontal orientation
of a light pulse. The graph 810 may be similar to the graph 710
discussed above. However, the graph 820 illustrates a generally
opposite relationship between the pulse rate and the horizontal
orientation of a light pulse as compared to the graph 720 of FIG.
13. The resolution or pixel density is higher for the area in front
of the lidar system 100. By increasing the pulse rate near the
front of the lidar system 100, the lidar system can collect more
data points when information density is higher.
[0148] The controller 150 may identify the orientations at which
the light pulses are transmitted (e.g., by communicating with the
scanner 120). The controller 150 may provide control signals to the
light source 110 to increase or decrease the pulse rate as the
orientation increases. In other implementations, the controller 150
may compare the orientation to a threshold orientation and may
provide a control signal to the light source 110 adjusting the
pulse rate when the orientation increases above or decreases below
a threshold orientation.
[0149] For example, at 0 degrees the pulse rate may be 750 kHz to
allow for a 1.33 .mu.s time of flight to reach a target at a
maximum range of 200 m. When the orientation exceeds a first
threshold angle with respect to the lidar system 100 (e.g., 30
degrees), the pulse rate may increase to a second pulse rate (1.5
MHz). The light source 110 may decrease the power or energy
accordingly, as the faster pulse rate may not allow for the light
pulses to reach targets at the maximum range. In some
implementations, the light source 110 may operate in a constant
power mode, where the average power of the emitted output beam 125
remains substantially constant over a period of time. Accordingly,
the amount of energy per light pulse may decrease when the pulse
rate increases. Additionally, the amount of energy per light pulse
may increase when the pulse rate decreases. In other
implementations, the light source 110 may operate in a constant
energy mode, where the amount of energy per light pulse remains
substantially constant over a period of time. Accordingly, the
average power of the emitted output beam 125 may increase when the
pulse rate increases. Additionally, the average power of the
emitted output beam 125 may decrease when the pulse rate decreases.
When the orientation exceeds a second threshold angle with respect
to the lidar system 100 (e.g., 45 degrees), the pulse rate may
increase to a third pulse rate, and so on. In some implementations,
the controller 150 determines the threshold orientations. The
threshold orientations may be static and predetermined or may be
dynamically set by the controller 150. For example, a threshold
orientation may be determined based on point cloud data from a
previous scan line or scan frame. In another example, the
controller 150 may set a threshold orientation according to an
upcoming vehicle maneuver such as a lane charge or turn. For
example, when the vehicle is about to make a left turn, the
controller 150 may set a threshold orientation near the left side
of the lidar system 100 (e.g., when the lidar system 100 is
positioned in front of the vehicle) to increase the pulse rate near
the left side of the vehicle to more clearly identify objects in an
area that the vehicle is approaching. Still further, the controller
150 may set the threshold orientation based on road conditions,
visibility due to fog, snow, or rain, for example, or based on any
other suitable condition to adjust the pulse rate above or below
the threshold orientation.
[0150] In some implementations, the lidar system 100 may include
two or more scanners, each scanning in opposite directions with
respect to the front of the lidar system 100 (e.g., from 0 degrees
to 60 degrees and from 0 degrees to -60 degrees). In this manner,
the horizontal field of regard of the lidar system 100 may double.
In other implementations, the two or more scanners scan in the same
direction and are offset by a predetermined phase angle (e.g., 60
degrees).
[0151] The techniques discussed with reference to FIGS. 13 and 14
can be used with the implementations in which the lidar system 100
scans a FOV with the FOR.sub.H of less than 360 degrees (e.g., 60
degrees) as well as the implementations in which the lidar system
conducts a circular scan (see FIGS. 3 and 4).
[0152] In yet other implementations, the lidar system 100 may
adjust the pulse rate in accordance with the scan speed of the
scanner 120 to compensate for motor dynamics at the scanner 120. As
mentioned above, the scanner 120 may scan in the forward-scanning
and reverse-scanning directions. The scanner 120 changes the
direction of the scan from the forward-scanning direction to the
reverse-scanning direction at a first turnaround point, and from
the reverse-scanning direction to the forward-scanning direction at
a second turnaround point. When the field of regard is centered at
a front-facing direction with respect to the lidar system 100, the
turnaround points are positioned symmetrically relative to the
front-facing direction, and each of the turnaround points is a
side-facing turnaround point. As mentioned above, the forward and
reverse-scanning directions may be substantially horizontal
scanning directions, substantially vertical scanning directions, or
scanning directions having any suitable combination of horizontal
and vertical components. Accordingly, the turnaround points may be
horizontal turnaround points where the scanner 120 changes the
horizontal direction of the scan, or vertical turnaround points
where the scanner 120 changes the vertical direction of the
scan.
[0153] As the scanner approaches a turnaround point and the beam
accordingly traverses the periphery of the field of regard, the
scan speed may decrease. The lidar system also may include two
scanners, with the shared light source 110 or separate respective
light sources, each scanning in opposite directions with respect to
the front-facing direction of the lidar system 100 (e.g., from 0
degrees to 60 degrees and from 0 degrees to -60 degrees). In other
implementations, the two scanners scan in the same direction and
are offset by a predetermined phase angle. When one of the beams
approaches 0 degrees or 60 degrees, the scanner 120 may slow down
for example from 50 to 60 degrees and then speed up from 60 degrees
on its way back to 50 degrees. In this example two-scanner
implementation, one of the turnaround points of each scanner is on
the axis corresponding to the front-facing direction, and may be
referred to as the front-facing turnaround point, the center
turnaround point, or the inner turnaround point. The other
turnaround point of each scanner is at the periphery of the field
of regard, and may be referred to as the side-facing turnaround
point, the edge turnaround point, or the outer turnaround
point.
[0154] To compensate for motor dynamics, the lidar system 100
transmits light pulses at a variable pulse rate related to the scan
speed (e.g., the pulse rate decreases when the scan speed decreases
and the pulse rate increases when the scan speed increases). In
this manner, the light source 110 transmits light pulses uniformly
across the FOR. The lidar system 100 may adjust the average power
or the energy per light pulse provided by the light source 110
accordingly since a faster pulse rate may not allow for the light
pulses to reach targets at the maximum range. In some
implementations, the light source 110 may operate in a constant
power mode, where the average power of the emitted output beam 125
remains substantially constant over a period of time. Accordingly,
the amount of energy per light pulse may decrease when the pulse
rate increases. Additionally, the amount of energy per light pulse
may increase when the pulse rate decreases. In other
implementations, the light source 110 may operate in a constant
energy mode, where the amount of energy per light pulse remains
substantially constant over a period of time. Accordingly, the
average power of the emitted output beam 125 may increase when the
pulse rate increases. Additionally, the average power of the
emitted output beam 125 may decrease when the pulse rate
decreases.
[0155] FIG. 15 includes graphs 910, 920 that illustrate varying the
pulse rate in accordance with a scan speed of the scanner 120. The
graph 910 illustrates the relationship between the scan speed of
the scanner 120 and the horizontal orientation of the scanner 120.
When the scanner 120 scans back and forth between -60 degrees and
60 degrees, the scan speed is lowest at these orientations. More
specifically, the scan speed may go down to zero at the outer
turnaround points, disposed respectively on the -60 and 60-degree
axes. Then the scan speed steadily increases from -60 degrees
before reaching the maximum and, in some implementations,
plateauing at about 0 degrees. The scan speed then begins to
decrease until the scanner 120 reaches the 60-degree turnaround
point. The scan speed begins to increase again as the scanner 120
directs light pulses in the reverse-scanning direction. The graph
920 illustrates the relationship between the pulse rate and
horizontal orientation of the scanner 120. The graphs 910 and 920
may be generally similar, with the pulse rate varying directly with
the scan speed in this example implementation. This allows the
light source 110 to transmit light pulses uniformly across the
FOR.
[0156] When two scanners scan the field of view, with one scanner
for example scanning between -60 degrees and the front-facing
direction of zero degrees, and the other scanner scanning between
zero degrees and 60 degrees, the controller 150 may vary the scan
speed differently than in the single-scanner implementation. As
graph 930 illustrates, the scan speed for one of the two scanners
is at zero approximately at zero degrees and 60 degrees, but the
first turnaround point corresponds to the front-facing direction,
while the second turnaround point corresponds to the periphery of
the field of view. In an example implementation, the pulse rate is
at its highest at the first turnaround point and at its lowest at
the second turnaround point (see graph 940). As a more specific
example, the pulse rate can correspond to a sinusoid having half
the frequency of the sinusoidal scan speed.
[0157] In an alternative implementation of a two-scanner system,
the pulse rate is at its lowest at the first turnaround point and
at its highest at the second turnaround point. In this manner, the
scanners transmit light pulses uniformly across the combined field
of regard.
[0158] The controller 150 may identify the scan speed and provide
control signals to the light source 110 to decrease the pulse rate
as the scan speed decreases and increase the pulse rate as the scan
speed increases. In other implementations, the controller 150 may
compare the scan speed to a threshold speed. The controller 150
then may provide a control signal to the light source 110 to adjust
the pulse rate when the scan speed increases above or decreases
below the threshold speed. For example, at a first scan speed the
pulse rate may be 600 kHz. Then, when the scan speed exceeds a
threshold speed, the pulse rate may increase to a second pulse rate
(750 kHz). As the scanner 120 approaches the periphery and is about
to change direction of the scan, the scan speed may drop below the
threshold speed and, accordingly, the pulse rate may decrease back
to the first pulse rate of 600 kHz. More generally, the controller
150 in some implementations can implement any desired number of
discrete pulse rate bands (e.g., 700 kHz, 720 kHz, 750 kHz, 770
kHz).
[0159] In other implementations, the controller 150 may vary the
pulse rate based on a combination of the orientation of the light
pulses and the scan speed of the scanner 120. For example, when the
scanner 120 directs light pulses across a horizontal FOR from 0
degrees to 60 degrees with respect to the frontal orientation of
the lidar system 100, the controller 150 may increase the pulse
rate or keep the pulse rate the same as the scanner 120 approaches
0 degrees and as the scan speed decreases, so as to increase the
resolution or pixel density near the front of the lidar system 100.
The controller 150 may then reduce the pulse rate as the scanner
120 approaches 60 degrees and as the scan speed decreases, so as to
compensate for the slower scan speed. While the graphs 710, 720,
810, 820, 910, 920, 930, and 940 shown in FIGS. 13-15 illustrate
the relationship between pulse rate, scan speed, and horizontal
orientation, these are merely examples for ease of illustration
only. The pulse rate and scan speed may also vary according to the
vertical orientation of the scanner 120. For example, as the
scanner approaches a vertical turnaround point and the beam
accordingly traverses the periphery of the field of regard, the
scan speed may decrease. As a result, the pulse rate may also
decrease in accordance with the decrease in the scan speed, thereby
allowing the light source 110 to transmit light pulses uniformly
across the vertical FOR.
[0160] Further, the controller 150 in some implementations may
adjust the pulse rate in view of the timing of previously detected
pulses as well as in view of the orientation. Referring back to
FIG. 12, the controller 150 may transmit pulse N+1 in response to
detecting the previous pulse N, which may occur immediately or
shortly after detecting the pulse N. The value of T2 corresponds to
the time it takes a light pulse to travel to a target disposed at
the maximum range and return to the lidar system 100, as discussed
above. The controller 150 may control the duration of interval T2
in view of the current scan direction. For example, the controller
150 may adjust the duration of T2 upward or downward, depending on
the implementation, when the scanner approaches an outer turnaround
point. Similarly, the controller 150 may adjust the duration of T2
upward or downward, depending on the implementation, when the
scanner approaches the front-facing direction.
[0161] In other implementations, such as the implementations
illustrated in FIG. 3 and FIG. 4, the scan speed remains constant
as the scanner 120 directs light pulses across the FOR. While the
scan speed remains constant, the pulse rate may vary according to
the scan direction, the timing of previously detected pulses, or in
any other suitable manner. For example, the pulse rate may vary
across different sectors of the FOR.sub.H (e.g., 0 to 30 degrees,
30 to 60 degrees, 60 to 90 degrees, 90 to 120 degrees, etc.).
Example Method for Varying Pulse Rate
[0162] FIG. 16 depicts a flow diagram of an example method 1000 for
varying the pulse rate in accordance with the scan speed of the
scanner. The method 1000 may be implemented by various components
of the lidar system 100 as shown in FIG. 1 including the light
source 110, the scanner 120, the receiver 140, and the controller
150. For ease of illustration only, some of the steps of the method
1000 may be described below with reference to a particular
component of the lidar system 100. However, each of the method
steps may be implemented by any suitable component in any suitable
manner. In some embodiments, the method or a portion thereof can be
implemented in a set of instructions stored on a computer-readable
memory and executable on one or more processors or the controller
150.
[0163] At block 1002, light pulses are emitted by the light source
110. In some implementations, the controller 150 directs the light
source 110 to emit light pulses by providing instructions, a
control signal, or a trigger signal to the light source 110
indicating when the light source 110 should produce optical pulses.
The light pulses are then emitted at a particular pulse rate or
pulse repetition frequency.
[0164] At block 1004, the emitted light pulses are directed, via
the scanner 120, at various scan angles or orientations relative to
a front-facing direction with respect to the lidar system 100. In
this manner, the emitted light pulses are scanned across a
horizontal FOR (e.g., from -60 degrees to +60 degrees with respect
to the front-facing direction of the lidar system 100). In some
implementations, the controller 150 provides a drive signal to the
scanner 120 for rotating the scanning mirror across a horizontal
FOR to direct light pulses toward different points within the
horizontal FOR. Also in some implementations, the scanner 120
includes one or several scanning mirrors that rotate back and forth
in the horizontal direction (e.g., from -60 degrees to +60 degrees,
from -60 degrees to 0 degrees, from 0 degrees to +60 degrees,
etc.). To rotate back and forth the one or several scanning mirrors
slow down before coming to a stop at respective turnaround points.
Then after a scanning mirror reaches a turnaround point (e.g., at 0
degrees, at .+-.60 degrees, etc.) and turns around, the scanning
mirror speeds up and scans in the opposite direction. As shown in
the graph 910 of FIG. 15, the scan speed may be zero at the outer
turnaround points, disposed respectively on the -60 and +60-degree
axes. Then the scan speed steadily increases from -60 degrees
before reaching the maximum and, in some implementations,
plateauing at about 0 degrees. The scan speed then begins to
decrease until the scanner 120 reaches the +60-degree turnaround
point.
[0165] The emitted light pulses may also be directed, via the
scanner 120 across a vertical FOR (e.g., from -15 degrees vertical
to +15 degrees vertical). In some implementations, the controller
150 provides a drive signal to the scanner 120 for rotating the
same scanning mirror or another scanning mirror across a vertical
FOR to direct light pulses toward different points within the
vertical FOR. For example, the scanner 120 may direct light pulses
across a horizontal FOR at a first vertical orientation (e.g., +15
degrees vertical) to generate a scan line. Then the scanner 120 may
direct light pulses across the horizontal FOR at another vertical
orientation (e.g., +14 degrees vertical) to generate another scan
line. Also in some implementations, the scanner 120 includes one or
several scanning mirrors that rotate back and forth in the vertical
direction (e.g., from -15 degrees to +15 degrees). To rotate back
and forth the one or several scanning mirrors slow down before
coming to a stop at respective turnaround points. Then after a
scanning mirror reaches a turnaround point (e.g., at .+-.15
degrees) and turns around, the scanning mirror speeds up and scans
in the opposite direction. The scan speed may be zero at the
turnaround points, disposed respectively on the -15 and +15-degree
axes.
[0166] At block 1006, the method 1000 includes identifying the scan
speed of the scanner. For example, the controller 150 may identify
the scan speed by communicating with the scanner 120. In
implementations in which the controller 150 provides the drive
signal to the scanner 120 for rotating the scanning mirror, the
controller 150 may determine the scan speed based on the drive
signal.
[0167] Then at block 1008, the scan speed is compared to a
threshold scan speed or one or several scan speed ranges. When the
scanner rotates at a first scan speed, the light pulses may be
emitted at a first pulse rate (block 1010). When the scanner
rotates at a second scan speed, the light pulses may be emitted at
a second pulse rate (block 1012) which is different from the first
pulse rate. For example, as described above the pulse rate may be
lowest when the scan speed is the highest. In another example, the
pulse rate may be lowest when the scan speed is the lowest to
distribute the light pulses uniformly across the field of regard.
Additionally, when the scanner rotates at an nth scan speed, the
light pulses may be emitted at an nth pulse rate (block 1014) which
is higher than both of the first and second pulse rates when the
pulse rate increases in accordance with the scan speed or lower
than both of the first and second pulse rates when the pulse rate
decreases in accordance with the scan speed. The light source 110
or the controller 150 also may adjust the power or energy, because
a faster pulse rate may not allow for the light pulses to reach
targets at the maximum range. Accordingly, the amount of power or
energy may be higher for slower pulse rates.
[0168] In other implementations, the pulse rate may be varied in
accordance with a combination of the scan speed of the scanner and
the orientations of the light pulses. For example, the lidar system
100 may include two scanners 120 with a shared light source 110 or
separate respective light sources, where the two scanners 120 scan
from -60 degrees (side-facing, edge, or outer turnaround point) to
0 degrees (front-facing, center, or inner turnaround point) and
from 0 degrees (front-facing, center, or inner turnaround point) to
+60 degrees (side-facing, edge, or outer turnaround point),
respectively. In the implementations described above, the pulse
rate may decrease or increase at the turnaround points to account
for the reduced scan speeds at the respective turnaround points. In
an alternative implementation, the pulse rate may decrease at the
outer turnaround point while increasing or remaining constant at
the inner turnaround point to increase pixel density at the inner
turnaround point.
[0169] In some implementations, the lidar system 100 may identify a
"portion of interest" of the field of regard that is an orientation
range at which to increase the pulse rate. When light pulses are
directed at orientations within the identified portion of interest,
the light source 110 may emit the light pulses at an increased
pulse rate. Then when subsequent light pulses are directed at
orientations outside of the identified portion of interest, the
light source 110 may decrease the pulse rate for the subsequent
light pulses. For example, the lidar system 100 may identify a
portion of interest corresponding to an upcoming maneuver of the
vehicle. In some implementations, the vehicle controller 372
transmits an indication of an upcoming maneuver to the lidar
controller 150 or provides a request for increased resolution
within a particular orientation range. The lidar controller 150
then identifies the portion of interest based on the upcoming
maneuver or the request. For example, when the upcoming maneuver is
a right turn, the lidar controller 150 may identify the portion of
the field of regard to the right of the vehicle as the portion of
interest (e.g., having an orientation range of +40 degrees to +60
degrees). Then the lidar controller 150 may provide control signals
to the light source 110 to increase the pulse rate within the
identified portion of interest. In these implementations, the pulse
rate may increase or remain constant at a turnaround point having
an orientation within the portion of interest to increase pixel
density at the turnaround point.
[0170] While scan speeds are described as being compared to a
threshold scan speed or threshold scan speed range, the pulse rate
also may vary continuously in accordance with the scan speed. For
example, the pulse rate may increase linearly as the scan speed
increases, as shown in graphs 910, 920 of FIG. 15.
[0171] In some implementations, the controller 150 provides control
signals to the light source 110 to adjust the pulse rate to the
first pulse rate, second pulse rate, nth pulse rate, or any other
suitable pulse rate. For example, when the scan speed exceeds a
threshold scan speed, the controller 150 may provide a control
signal to the light source 110 directing the light source 110 to
switch the pulse rate from the first pulse rate to the second pulse
rate.
[0172] At block 1016, light from some of the light pulses is
scattered by remote targets such as the target 130, as shown in
FIG. 1 and detected by the receiver 140, for example.
Characteristics of the received light signals are then used to
generate a point cloud having respective pixels.
General Considerations
[0173] In some cases, a computing device may be used to implement
various modules, circuits, systems, methods, or algorithm steps
disclosed herein. As an example, all or part of a module, circuit,
system, method, or algorithm disclosed herein may be implemented or
performed by a general-purpose single- or multi-chip processor, a
digital signal processor (DSP), an ASIC, a FPGA, any other suitable
programmable-logic device, discrete gate or transistor logic,
discrete hardware components, or any suitable combination thereof.
A general-purpose processor may be a microprocessor, or, any
conventional processor, controller, microcontroller, or state
machine. A processor may also be implemented as a combination of
computing devices, e.g., a combination of a DSP and a
microprocessor, a plurality of microprocessors, one or more
microprocessors in conjunction with a DSP core, or any other such
configuration.
[0174] In particular embodiments, one or more implementations of
the subject matter described herein may be implemented as one or
more computer programs (e.g., one or more modules of
computer-program instructions encoded or stored on a
computer-readable non-transitory storage medium). As an example,
the steps of a method or algorithm disclosed herein may be
implemented in a processor-executable software module which may
reside on a computer-readable non-transitory storage medium. In
particular embodiments, a computer-readable non-transitory storage
medium may include any suitable storage medium that may be used to
store or transfer computer software and that may be accessed by a
computer system. Herein, a computer-readable non-transitory storage
medium or media may include one or more semiconductor-based or
other integrated circuits (ICs) (such, as for example,
field-programmable gate arrays (FPGAs) or application-specific ICs
(ASICs)), hard disk drives (HDDs), hybrid hard drives (HHDs),
optical discs (e.g., compact discs (CDs), CD-ROM, digital versatile
discs (DVDs), blue-ray discs, or laser discs), optical disc drives
(ODDs), magneto-optical discs, magneto-optical drives, floppy
diskettes, floppy disk drives (FDDs), magnetic tapes, flash
memories, solid-state drives (SSDs), RAM, RAM-drives, ROM, SECURE
DIGITAL cards or drives, any other suitable computer-readable
non-transitory storage media, or any suitable combination of two or
more of these, where appropriate. A computer-readable
non-transitory storage medium may be volatile, non-volatile, or a
combination of volatile and non-volatile, where appropriate.
[0175] In some cases, certain features described herein in the
context of separate implementations may also be combined and
implemented in a single implementation. Conversely, various
features that are described in the context of a single
implementation may also be implemented in multiple implementations
separately or in any suitable sub-combination. Moreover, although
features may be described above as acting in certain combinations
and even initially claimed as such, one or more features from a
claimed combination may in some cases be excised from the
combination, and the claimed combination may be directed to a
sub-combination or variation of a sub-combination.
[0176] While operations may be depicted in the drawings as
occurring in a particular order, this should not be understood as
requiring that such operations be performed in the particular order
shown or in sequential order, or that all operations be performed.
Further, the drawings may schematically depict one more example
processes or methods in the form of a flow diagram or a sequence
diagram. However, other operations that are not depicted may be
incorporated in the example processes or methods that are
schematically illustrated. For example, one or more additional
operations may be performed before, after, simultaneously with, or
between any of the illustrated operations. Moreover, one or more
operations depicted in a diagram may be repeated, where
appropriate. Additionally, operations depicted in a diagram may be
performed in any suitable order. Furthermore, although particular
components, devices, or systems are described herein as carrying
out particular operations, any suitable combination of any suitable
components, devices, or systems may be used to carry out any
suitable operation or combination of operations. In certain
circumstances, multitasking or parallel processing operations may
be performed. Moreover, the separation of various system components
in the implementations described herein should not be understood as
requiring such separation in all implementations, and it should be
understood that the described program components and systems may be
integrated together in a single software product or packaged into
multiple software products.
[0177] Various implementations have been described in connection
with the accompanying drawings. However, it should be understood
that the figures may not necessarily be drawn to scale. As an
example, distances or angles depicted in the figures are
illustrative and may not necessarily bear an exact relationship to
actual dimensions or layout of the devices illustrated.
[0178] The scope of this disclosure encompasses all changes,
substitutions, variations, alterations, and modifications to the
example embodiments described or illustrated herein that a person
having ordinary skill in the art would comprehend. The scope of
this disclosure is not limited to the example embodiments described
or illustrated herein. Moreover, although this disclosure describes
or illustrates respective embodiments herein as including
particular components, elements, functions, operations, or steps,
any of these embodiments may include any combination or permutation
of any of the components, elements, functions, operations, or steps
described or illustrated anywhere herein that a person having
ordinary skill in the art would comprehend.
[0179] The term "or" as used herein is to be interpreted as an
inclusive or meaning any one or any combination, unless expressly
indicated otherwise or indicated otherwise by context. Therefore,
herein, the expression "A or B" means "A, B, or both A and B." As
another example, herein, "A, B or C" means at least one of the
following: A; B; C; A and B; A and C; B and C; A, B and C. An
exception to this definition will occur if a combination of
elements, devices, steps, or operations is in some way inherently
mutually exclusive.
[0180] As used herein, words of approximation such as, without
limitation, "approximately, "substantially," or "about" refer to a
condition that when so modified is understood to not necessarily be
absolute or perfect but would be considered close enough to those
of ordinary skill in the art to warrant designating the condition
as being present. The extent to which the description may vary will
depend on how great a change can be instituted and still have one
of ordinary skill in the art recognize the modified feature as
having the required characteristics or capabilities of the
unmodified feature. In general, but subject to the preceding
discussion, a numerical value herein that is modified by a word of
approximation such as "approximately" may vary from the stated
value by .+-.0.5%, .+-.1%, .+-.2%, .+-.3%, .+-.4%, .+-.5%, .+-.10%,
.+-.12%, or .+-.15%.
[0181] As used herein, the terms "first," "second," "third," etc.
may be used as labels for nouns that they precede, and these terms
may not necessarily imply a particular ordering (e.g., a particular
spatial, temporal, or logical ordering). As an example, a system
may be described as determining a "first result" and a "second
result," and the terms "first" and "second" may not necessarily
imply that the first result is determined before the second
result.
[0182] As used herein, the terms "based on" and "based at least in
part on" may be used to describe or present one or more factors
that affect a determination, and these terms may not exclude
additional factors that may affect a determination. A determination
may be based solely on those factors which are presented or may be
based at least in part on those factors. The phrase "determine A
based on B" indicates that B is a factor that affects the
determination of A. In some instances, other factors may also
contribute to the determination of A. In other instances, A may be
determined based solely on B.
* * * * *