U.S. patent application number 17/482947 was filed with the patent office on 2022-09-29 for hyper temporal lidar with dynamic laser control using marker shots.
The applicant listed for this patent is AEYE, Inc.. Invention is credited to Joel Benscoter, Luis Dussan, Philippe Feru, Alex Liang, Igor Polishchuk, Allan Steinhardt.
Application Number | 20220308169 17/482947 |
Document ID | / |
Family ID | 1000006268413 |
Filed Date | 2022-09-29 |
United States Patent
Application |
20220308169 |
Kind Code |
A1 |
Feru; Philippe ; et
al. |
September 29, 2022 |
HYPER TEMPORAL LIDAR WITH DYNAMIC LASER CONTROL USING MARKER
SHOTS
Abstract
A lidar system that includes a laser source can be controlled to
fire laser pulse shots from the laser source at a variable rate of
firing those laser pulse shots. The fired laser pulse shots can
include scheduled laser pulse shots that are targeted at range
points in the field of view. The fired laser pulse shots can also
include marker shots that bleed energy out of the laser source in
order to avoid reaching a threshold for available energy in the
laser source and/or regulate energy amounts for the targeted laser
pulse shots. A laser energy model that models how much energy is
available from the laser source for laser pulse shots over time can
be used to model future available energies for the laser source and
determine whether any marker shots should be fired.
Inventors: |
Feru; Philippe; (Dublin,
CA) ; Dussan; Luis; (Dublin, CA) ; Benscoter;
Joel; (Dublin, CA) ; Liang; Alex; (Dublin,
CA) ; Polishchuk; Igor; (Dublin, CA) ;
Steinhardt; Allan; (Dublin, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AEYE, Inc. |
Dublin |
CA |
US |
|
|
Family ID: |
1000006268413 |
Appl. No.: |
17/482947 |
Filed: |
September 23, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63166475 |
Mar 26, 2021 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 7/484 20130101;
G01S 17/89 20130101; G01S 7/4817 20130101; G01S 7/4814 20130101;
G01S 17/931 20200101 |
International
Class: |
G01S 7/484 20060101
G01S007/484; G01S 7/481 20060101 G01S007/481; G01S 17/931 20060101
G01S017/931; G01S 17/89 20060101 G01S017/89 |
Claims
1. A lidar apparatus comprising: a laser source; and a control
circuit that (1) controls a variable rate firing of laser pulse
shots by the laser source that are targeted toward a plurality of
range points in the field of view, (2) evaluates an upcoming
schedule of the targeted laser pulse shots based on a laser energy
model that models how much energy is available from the laser
source for the targeted laser pulse shots over time, and (3)
includes a plurality of marker shots among the fired laser pulse
shots in response to the evaluation in order to (i) avoid a future
state of the laser source where an amount of available energy in
the laser source would exceed a threshold and/or (ii) regulate
energy amounts for laser the targeted laser pulse shots.
2. The apparatus of claim 1 wherein the control circuit (1)
determines from the laser energy model and the upcoming schedule
whether there is a future state of the laser source where the laser
source's modeled available energy would exceed the threshold and
(2) schedules a marker shot in order to bleed energy out of the
laser source to avoid the future state of the laser source where
the laser source's modeled available energy would exceed the
threshold.
3. The apparatus of claim 1 wherein the control circuit regulates
energy amounts for the targeted laser pulse shots by scheduling the
marker shots to achieve a consistency in energy amounts for the
targeted laser pulse shots on the upcoming schedule.
4. The apparatus of claim 1 wherein the control circuit regulates
energy amounts for the targeted laser pulse shots by scheduling the
marker shots to achieve desired energy amounts for the targeted
laser pulse shots on the upcoming schedule.
5. The apparatus of claim 1 wherein the control circuit controls
the variable rate firing by generating firing commands for the
laser source in accordance with the upcoming schedule for the
targeted laser pulse shots and the marker shots.
6. The apparatus of claim 1 wherein the laser energy model (1)
models a depletion of energy in the laser source in response to
each laser pulse shot, (2) models a retention of energy in the
laser source after laser pulse shots, and (3) models a buildup of
energy in the laser source between laser pulse shots.
7. The apparatus of claim 1 wherein the laser source comprises an
optical amplification laser source.
8. The apparatus of claim 7 wherein the optical amplification laser
source comprises a pulsed fiber laser source.
9. The apparatus of claim 8 wherein the pulsed fiber laser source
comprises a seed laser, a pump laser, and a fiber amplifier, and
wherein laser energy model models (1) seed energy for the pulsed
fiber laser source over time and (2) energy stored in the fiber
amplifier over time.
10. The apparatus of claim 9 wherein the laser energy model models
the available energy for laser pulse shots according to a
relationship of EF(t+.delta.)=aS(t+.delta.)+bEF(t), wherein a+b=1
so that a and b reflect how much energy is drained from and remains
in the fiber amplifier when laser pulse shots are fired, wherein
EF(t) represents laser energy for a laser pulse shot fired at time
t, wherein EF(t+.delta.) represents laser energy for a laser pulse
shot fired at time t+.delta., wherein S(t+.delta.) represents an
amount of energy deposited by the pump laser into the fiber
amplifier over time duration .delta., wherein t represents a fire
time for a laser pulse shot, and wherein the time duration .delta.
represents intershot spacing in time.
11. The apparatus of claim 7 wherein the laser energy model (1)
models depletion of energy in an optical amplifier of the optical
amplification laser source in response to each laser pulse shot,
(2) models retention of energy in the optical amplifier after laser
pulse shots, and (3) models buildup of energy in the optical
amplifier between laser pulse shots.
12. The apparatus of claim 1 wherein the laser energy model models
available laser energy for laser pulse shots at time intervals in a
range between 10 nanoseconds to 100 nanoseconds.
13. The apparatus of claim 1 further comprising: a mirror that is
scannable to define where the lidar apparatus is aimed along an
axis within a field of view, wherein the mirror is optically
downstream from the laser source; and wherein the fired laser pulse
shots are targeted toward the range points in the field of view via
the mirror.
14. The apparatus of claim 13 wherein the mirror is scannable
through a plurality of scan angles to define where the lidar
apparatus is aimed along the axis in the field of view; wherein the
range points to be targeted with the laser pulse exhibit
corresponding scan angles along the axis; and wherein the control
circuit schedules the targeted laser pulse shots with respect to
the upcoming schedule based on a mirror motion model in combination
with the laser energy model, wherein the mirror motion model models
the scan angles for the mirror over time.
15. The apparatus of claim 14 wherein the mirror motion model
models the scan angles for the scannable mirror as a plurality of
corresponding time slots, and wherein the upcoming schedule
identifies time slots corresponding to the targeted laser pulse
shots, and wherein the evaluations are performed with respect to
the laser energy model for the identified time slots.
16. The apparatus of claim 15 wherein the time slots reflect time
intervals in a range between 5 nanoseconds and 50 nanoseconds.
17. The apparatus of claim 14 wherein the control circuit schedules
the targeted laser pulse shots with respect to the upcoming
schedule based on the mirror motion model in combination with the
laser energy model as compared to a plurality of energy
requirements relating to the targeted laser pulse shots.
18. The apparatus of claim 14 wherein the mirror motion model
models the scan angles according to a cosine oscillation.
19. The apparatus of claim 13 wherein the control circuit drives
the mirror to scan along the axis in a resonant mode.
20. The apparatus of claim 19 wherein the mirror comprises a first
mirror, wherein the axis comprises a first axis, the apparatus
further comprising a second mirror that is scannable along a second
axis within the field of view, and wherein the control circuit
drives the second mirror to scan in a point-to-point mode based on
the range points in the field of view to be targeted with the fired
laser pulse shots.
21. The apparatus of claim 13 wherein the control circuit drives
the mirror to scan along the axis at a frequency between 100 Hz and
20 kHz.
22. The apparatus of claim 13 wherein the control circuit drives
the mirror to scan along the axis at a frequency between 10 kHz and
15 kHz.
23. The apparatus of claim 13 wherein the control circuit comprises
(1) a system controller and (2) a beam scanner controller; wherein
the system controller performs the evaluations; and wherein the
beam scanner controller (1) provides firing commands to the laser
source in accordance with the variable rate firing of laser pulse
shots and (2) controls a scanning of the mirror.
24. A method for dynamic control of lidar transmissions, the method
comprising: maintaining a laser energy model that models available
energy for laser pulses from a laser source over time, wherein the
laser source generates laser pulse shots for transmission into a
field of view for a lidar transmitter; comparing the available
energy as modeled by the laser energy model with a threshold; and
in response to a determination that the modeled available energy
will exceed the threshold, providing a firing command to the laser
source to trigger a marker shot that reduces the available energy
for the laser source below the threshold before the available
energy exceeds the threshold.
25. A method for dynamic control of lidar transmissions, the method
comprising: maintaining a laser energy model that models available
energy for laser pulses from a laser source over time, wherein the
laser source generates laser pulse shots for transmission into a
field of view for a lidar transmitter; supplementing a schedule of
laser pulse shots that target a plurality of range points in the
field of view with a plurality of marker shots that bleed energy
out of the laser source and regulate energy amounts for the
targeted laser pulse shots ; and firing the targeted laser pulse
shots and the marker shots from the laser source in accordance with
the supplemented schedule.
26. An article of manufacture for control of a lidar transmitter,
wherein the lidar transmitter comprises a laser source, the article
comprising: machine-readable code that is resident on a
non-transitory machine-readable storage medium, wherein the code
defines processing operations to be performed by a processor to
cause the processor to: control a variable rate firing of laser
pulse shots by the laser source that are targeted toward a
plurality of range points in the field of view; evaluate an
upcoming schedule of the targeted laser pulse shots based on a
laser energy model that models how much energy is available from
the laser source for the targeted laser pulse shots over time; and
includes a plurality of marker shots among the fired laser pulse
shots in response to the evaluation in order to (i) avoid a future
state of the laser source where an amount of available energy in
the laser source would exceed a threshold and/or (ii) regulate
energy amounts for laser the targeted laser pulse shots.
Description
CROSS-REFERENCE AND PRIORITY CLAIM TO RELATED PATENT
APPLICATIONS
[0001] This patent application claims priority to U.S. provisional
patent application 63/166,475, filed Mar. 26, 2021, and entitled
"Hyper Temporal Lidar with Dynamic Laser Control", the entire
disclosure of which is incorporated herein by reference.
[0002] This patent application is related to (1) U.S. patent
application Ser. No. ______, filed this same day, and entitled
"Hyper Temporal Lidar with Dynamic Laser Control Using a Laser
Energy Model" (said patent application being identified by Thompson
Coburn Attorney Docket Number 56976-213171), (2) U.S. patent
application Ser. No. ______, filed this same day, and entitled
"Hyper Temporal Lidar with Dynamic Laser Control Using Laser Energy
and Mirror Motion Models" (said patent application being identified
by Thompson Coburn Attorney Docket Number 56976-213172), (3) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Dynamic Laser Control for Scan
Line Shot Scheduling" (said patent application being identified by
Thompson Coburn Attorney Docket Number 56976-213173), (4) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Dynamic Laser Control Using
Safety Models" (said patent application being identified by
Thompson Coburn Attorney Docket Number 56976-213174), (5) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Shot Scheduling for Variable
Amplitude Scan Mirror" (said patent application being identified by
Thompson Coburn Attorney Docket Number 56976-213175), (6) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Dynamic Control of Variable
Energy Laser Source" (said patent application being identified by
Thompson Coburn Attorney Docket Number 56976-213176), (7) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Dynamic Laser Control and Shot
Order Simulation" (said patent application being identified by
Thompson Coburn Attorney Docket Number 56976-213177), (8) U.S.
patent application Ser. No. ______, filed this same day, and
entitled "Hyper Temporal Lidar with Elevation-Prioritized Shot
Scheduling" (said patent application being identified by Thompson
Coburn Attorney Docket Number 56976-213179), (9) U.S. patent
application Ser. No. ______, filed this same day, and entitled
"Hyper Temporal Lidar with Dynamic Laser Control Using Different
Mirror Motion Models for Shot Scheduling and Shot Firing" (said
patent application being identified by Thompson Coburn Attorney
Docket Number 56976-213180), and (10) U.S. patent application Ser.
No. ______, filed this same day, and entitled "Hyper Temporal Lidar
with Detection-Based Adaptive Shot Scheduling" (said patent
application being identified by Thompson Coburn Attorney Docket
Number 56976-213181), the entire disclosures of each of which are
incorporated herein by reference
INTRODUCTION
[0003] There is a need in the art for lidar systems that operate
with low latency and rapid adaptation to environmental changes.
This is particularly the case for automotive applications of lidar
as well as other applications where the lidar system may be moving
at a high rate of speed or where there is otherwise a need for
decision-making in short time intervals. For example, when an
object of interest is detected in the field of view for a lidar
transmitter, it is desirable for the lidar transmitter to rapidly
respond to this detection by firing high densities of laser pulses
at the detected object. However, as the firing rate for the lidar
transmitter increases, this places pressure on the operational
capabilities of the laser source employed by the lidar transmitter
because the laser source will need re-charging time.
[0004] This issue becomes particularly acute in situations where
the lidar transmitter has a variable firing rate. With a variable
firing rate, the laser source's operational capabilities are not
only impacted by periods of high density firing but also periods of
low density firing. As charge builds up in the laser source during
a period where the laser source is not fired, a need arises to
ensure that the laser source does not overheat or otherwise exceed
its maximum energy limits.
[0005] The lidar transmitter may employ a laser source that uses
optical amplification to support the generation of laser pulses.
Such laser sources have energy characteristics that are heavily
impacted by time and the firing rate of the laser source. These
energy characteristics of a laser source that uses optical
amplification have important operational impacts on the lidar
transmitter when the lidar transmitter is designed to operate with
fast scan times and laser pulses that are targeted on specific
range points in the field of view.
[0006] As a technical solution to these problems in the art, the
inventors disclose that a laser energy model can be used to model
the available energy in the laser source over time. The timing
schedule for laser pulses fired by the lidar transmitter can then
be determined using energies that are predicted for the different
scheduled laser pulse shots based on the laser energy model. This
permits the lidar transmitter to reliably ensure at a highly
granular level that each laser pulse shot has sufficient energy to
meet operational needs, including when operating during periods of
high density/high resolution laser pulse firing. The laser energy
model is capable of modeling the energy available for laser pulses
in the laser source over very short time intervals as discussed in
greater detail below. With such short interval time modeling, the
laser energy modeling can be referred to as a transient laser
energy model.
[0007] Moreover, the inventors disclose that the laser energy model
can be consulted to determine whether any marker shots should be
scheduled and fired. A marker shot is a laser pulse shot from the
laser source that is fired in order to bleed energy out of the
laser source and need not target any particular range point in the
field of view. By bleeding energy out of the laser source in a
controlled manner via the marker shots, the system can (1) avoid
potential damage to the laser source by preventing the amount of
available energy in the laser source from exceeding a threshold
and/or (2) regulate the energy amounts that are included in the
scheduled laser pulse shots that are targeted toward specific range
points in the field of view.
[0008] Furthermore, the inventors also disclose that mirror motion
can be modeled so that the system can also reliably predict where a
scanning mirror is aimed within a field of view over time. This
mirror motion model is also capable of predicting mirror motion
over short time intervals as discussed in greater detail below. In
this regard, the mirror motion model can also be referred to as a
transient mirror motion model. The model of mirror motion over time
can be linked with the model of laser energy over time to provide
still more granularity in the scheduling of laser pulses that are
targeted at specific range points in the field of view. Thus, a
control circuit can translate a list of arbitrarily ordered range
points to be targeted with laser pulses into a shot list of laser
pulses to be fired at such range points using the modeled laser
energy coupled with the modeled mirror motion. In this regard, the
"shot list" can refer to a list of the range points to be targeted
with laser pulses as combined with timing data that defines a
schedule or sequence by which laser pulses will be fired toward
such range points.
[0009] Through the use of such models, the lidar system can provide
hyper temporal processing where laser pulses can be scheduled and
fired at high rates with high timing precision and high spatial
targeting/pointing precision. This results in a lidar system that
can operate at low latency, high frame rates, and intelligent range
point targeting where regions of interest in the field of view can
be targeted with rapidly-fired and spatially dense laser pulse
shots.
[0010] These and other features and advantages of the invention
will be described in greater detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 depicts an example lidar transmitter that uses a
laser energy model to schedule laser pulses.
[0012] FIG. 2A depicts an example process flow the control circuit
of FIG. 1.
[0013] FIG. 2B-2D depict additional examples of lidar transmitters
that use a laser energy model to schedule laser pulses.
[0014] FIG. 3 depicts an example lidar transmitter that uses a
laser energy model and a mirror motion model to schedule laser
pulses.
[0015] FIGS. 4A-4D illustrate how mirror motion can be modeled for
a mirror that scans in a resonant mode.
[0016] FIG. 4E depicts an example process flow for controllably
adjusting an amplitude for mirror scanning.
[0017] FIG. 5 depicts an example process flow for the control
circuit of FIG. 3.
[0018] FIG. 6A and 6B depict example process flows for shot
scheduling using the control circuit of FIG. 3.
[0019] FIG. 7A depicts an example process flow for simulating and
evaluating different shot ordering candidates based on the laser
energy model and the mirror motion model.
[0020] FIG. 7B depicts an example of how time slots in a mirror
scan can be related to the shot angles for the mirror using the
mirror motion model.
[0021] FIG. 7C depicts an example process flow for simulating
different shot ordering candidates based on the laser energy
model.
[0022] FIGS. 7D-7F depict different examples of laser energy
predictions produced by the laser energy model with respect to
different shot order candidates.
[0023] FIG. 8 depicts an example lidar transmitter that uses a
laser energy model and a mirror motion model to schedule laser
pulses, where the control circuit includes a system controller and
a beam scanner controller.
[0024] FIG. 9 depicts an example process flow for inserting marker
shots into a shot list.
[0025] FIG. 10 depicts an example process flow for using an eye
safety model to adjust a shot list.
[0026] FIG. 11 depicts an example lidar transmitter that uses a
laser energy model, a mirror motion model, and an eye safety model
to schedule laser pulses.
[0027] FIG. 12 depicts an example process flow for simulating
different shot ordering candidates based on the laser energy model
and eye safety model.
[0028] FIG. 13 depicts another example process for determining shot
schedules using the models.
[0029] FIG. 14 depicts an example lidar system where a lidar
transmitter and a lidar receiver coordinate their operations with
each other.
[0030] FIG. 15 depicts another example process for determining shot
schedules using the models.
[0031] FIG. 16 illustrates how the lidar transmitter can change its
firing rate to probe regions in a field of view with denser
groupings of laser pulses.
[0032] FIGS. 17A-17F depict example process flows for prioritized
selections of elevations with respect to shot scheduling.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0033] FIG. 1 shows an example embodiment of a lidar transmitter
100 that can be employed to support hyper temporal lidar. In an
example embodiment, the lidar transmitter 100 can be deployed in a
vehicle such as an automobile. However, it should be understood
that the lidar transmitter 100 described herein need not be
deployed in a vehicle. As used herein, "lidar", which can also be
referred to as "ladar", refers to and encompasses any of light
detection and ranging, laser radar, and laser detection and
ranging. In the example of FIG. 1, the lidar transmitter 100
includes a laser source 102, a mirror subsystem 104, and a control
circuit 106. Control circuit 106 uses a laser energy model 108 to
govern the firing of laser pulses 122 by the laser source 102.
Laser pulses 122 transmitted by the laser source 102 are sent into
the environment via mirror subsystem 104 to target various range
points in a field of view for the lidar transmitter 100. These
laser pulses 122 can be interchangeably referred to as laser pulse
shots (or more simply, as just "shots"). The field of view will
include different addressable coordinates (e.g., {azimuth,
elevation} pairs) which serve as range points that can be targeted
by the lidar transmitter 100 with the laser pulses 122.
[0034] In the example of FIG. 1, laser source 102 can use optical
amplification to generate the laser pulses 122 that are transmitted
into the lidar transmitter's field of view via the mirror subsystem
104. In this regard, a laser source 102 that includes an optical
amplifier can be referred to as an optical amplification laser
source 102. In the example of FIG. 1, the optical amplification
laser source 102 includes a seed laser 114, an optical amplifier
116, and a pump laser 118. In this laser architecture, the seed
laser 114 provides the input (signal) that is amplified to yield
the transmitted laser pulse 122, while the pump laser 118 provides
the power (in the form of the energy deposited by the pump laser
118 into the optical amplifier 116). So, the optical amplifier 116
is fed by two inputs--the pump laser 118 (which deposits energy
into the optical amplifier 116) and the seed laser 114 (which
provides the signal that stimulates the energy in the optical
amplifier 116 and induces pulse 122 to fire).
[0035] Thus, the pump laser 118, which can take the form of an
electrically-driven pump laser diode, continuously sends energy
into the optical amplifier 116. The seed laser 114, which can take
the form of an electrically-driven seed laser that includes a pulse
formation network circuit, controls when the energy deposited by
the pump laser 118 into the optical amplifier 116 is released by
the optical amplifier 116 as a laser pulse 122 for transmission.
The seed laser 114 can also control the shape of laser pulse 122
via the pulse formation network circuit (which can drive the pump
laser diode with the desired pulse shape). The seed laser 114 also
injects a small amount of (pulsed) optical energy into the optical
amplifier 116.
[0036] Given that the energy deposited in the optical amplifier 116
by the pump laser 118 and seed laser 114 serves to seed the optical
amplifier 116 with energy from which the laser pulses 122 are
generated, this deposited energy can be referred to as "seed
energy" for the laser source 102.
[0037] The optical amplifier 116 operates to generate laser pulse
122 from the energy deposited therein by the seed laser 114 and
pump laser 118 when the optical amplifier 116 is induced to fire
the laser pulse 122 in response to stimulation of the energy
therein by the seed laser 114. The optical amplifier 116 can take
the form of a fiber amplifier. In such an embodiment, the laser
source 102 can be referred to as a pulsed fiber laser source. With
a pulsed fiber laser source 102, the pump laser 118 essentially
places the dopant electrons in the fiber amplifier 116 into an
excited energy state. When it is time to fire laser pulse 122, the
seed laser 114 stimulates these electrons, causing them to emit
energy and collapse down to a lower (ground) state, which results
in the emission of pulse 122. An example of a fiber amplifier that
can be used for the optical amplifier 116 is a doped fiber
amplifier such as an Erbium-Doped Fiber Amplifier (EDFA).
[0038] It should be understood that other types of optical
amplifiers can be used for the optical amplifier 116 if desired by
a practitioner. For example, the optical amplifier 116 can take the
form of a semiconductor amplifier. In contrast to a laser source
that uses a fiber amplifier (where the fiber amplifier is optically
pumped by pump laser 118), a laser source that uses a semiconductor
amplifier can be electrically pumped. As another example, the
optical amplifier 116 can take the form of a gas amplifier (e.g., a
CO.sub.2 gas amplifier). Moreover, it should be understood that a
practitioner may choose to include a cascade of optical amplifiers
116 in laser source 102.
[0039] In an example embodiment, the pump laser 118 can exhibit a
fixed rate of energy buildup (where a constant amount of energy is
deposited in the optical amplifier 116 per unit time). However, it
should be understood that a practitioner may choose to employ a
pump laser 118 that exhibits a variable rate of energy buildup
(where the amount of energy deposited in the optical amplifier 116
varies per unit time).
[0040] The laser source 102 fires laser pulses 122 in response to
firing commands 120 received from the control circuit 106. In an
example where the laser source 102 is a pulsed fiber laser source,
the firing commands 120 can cause the seed laser 114 to induce
pulse emissions by the fiber amplifier 116. In an example
embodiment, the lidar transmitter 100 employs non-steady state
pulse transmissions, which means that there will be variable timing
between the commands 120 to fire the laser source 102. In this
fashion, the laser pulses 122 transmitted by the lidar transmitter
100 will be spaced in time at irregular intervals. There may be
periods of relatively high densities of laser pulses 122 and
periods of relatively low densities of laser pulses 122. Examples
of laser vendors that provide such variable charge time control
include Luminbird and ITF. As examples, lasers that have the
capacity to regulate pulse timing over timescales corresponding to
preferred embodiments discussed herein and which are suitable to
serve as laser source 102 in these preferred embodiments are
expected to exhibit laser wavelengths of 1.5 .mu.m and available
energies in a range of around hundreds of nano-Joules to around
tens of micro-Joules, with timing controllable from hundreds of
nanoseconds to tens of microseconds and with an average power range
from around 0.25 Watts to around 4 Watts.
[0041] The mirror subsystem 104 includes a mirror that is scannable
to control where the lidar transmitter 100 is aimed. In the example
embodiment of FIG. 1, the mirror subsystem 104 includes two
mirrors--mirror 110 and mirror 112. Mirrors 110 and 112 can take
the form of MEMS mirrors. However, it should be understood that a
practitioner may choose to employ different types of scannable
mirrors. Mirror 110 is positioned optically downstream from the
laser source 102 and optically upstream from mirror 112. In this
fashion, a laser pulse 122 generated by the laser source 102 will
impact mirror 110, whereupon mirror 110 will reflect the pulse 122
onto mirror 112, whereupon mirror 112 will reflect the pulse 122
for transmission into the environment. It should be understood that
the outgoing pulse 122 may pass through various transmission optics
during its propagation from mirror 112 into the environment.
[0042] In the example of FIG. 1, mirror 110 can scan through a
plurality of mirror scan angles to define where the lidar
transmitter 100 is targeted along a first axis. This first axis can
be an X-axis so that mirror 110 scans between azimuths. Mirror 112
can scan through a plurality of mirror scan angles to define where
the lidar transmitter 100 is targeted along a second axis. The
second axis can be orthogonal to the first axis, in which case the
second axis can be a Y-axis so that mirror 112 scans between
elevations. The combination of mirror scan angles for mirror 110
and mirror 112 will define a particular {azimuth, elevation}
coordinate to which the lidar transmitter 100 is targeted. These
azimuth, elevation pairs can be characterized as {azimuth angles,
elevation angles} and/or {rows, columns} that define range points
in the field of view which can be targeted with laser pulses 122 by
the lidar transmitter 100.
[0043] A practitioner may choose to control the scanning of mirrors
110 and 112 using any of a number of scanning techniques. In a
particularly powerful embodiment, mirror 110 can be driven in a
resonant mode according to a sinusoidal signal while mirror 112 is
driven in a point-to-point mode according to a step signal that
varies as a function of the range points to be targeted with laser
pulses 122 by the lidar transmitter 100. In this fashion, mirror
110 can be operated as a fast-axis mirror while mirror 112 is
operated as a slow-axis mirror. When operating in such a resonant
mode, mirror 110 scans through scan angles in a sinusoidal pattern.
In an example embodiment, mirror 110 can be scanned at a frequency
in a range between around 100 Hz and around 20 kHz. In a preferred
embodiment, mirror 110 can be scanned at a frequency in a range
between around 10 kHz and around 15 kHz (e.g., around 12 kHz). As
noted above, mirror 112 can be driven in a point-to-point mode
according to a step signal that varies as a function of the range
points to be targeted with laser pulses 122 by the lidar
transmitter 100. Thus, if the lidar transmitter 100 is to fire a
laser pulse 122 at a particular range point having an elevation of
X, then the step signal can drive mirror 112 to scan to the
elevation of X. When the lidar transmitter 100 is later to fire a
laser pulse 122 at a particular range point having an elevation of
Y, then the step signal can drive mirror 112 to scan to the
elevation of Y. In this fashion, the mirror subsystem 104 can
selectively target range points that are identified for targeting
with laser pulses 122. It is expected that mirror 112 will scan to
new elevations at a much slower rate than mirror 110 will scan to
new azimuths. As such, mirror 110 may scan back and forth at a
particular elevation (e.g., left-to-right, right-to-left, and so
on) several times before mirror 112 scans to a new elevation. Thus,
while the mirror 112 is targeting a particular elevation angle, the
lidar transmitter 100 may fire a number of laser pulses 122 that
target different azimuths at that elevation while mirror 110 is
scanning through different azimuth angles. U.S. Pat. Nos.
10,078,133 and 10,642,029, the entire disclosures of which are
incorporated herein by reference, describe examples of mirror scan
control using techniques and transmitter architectures such as
these (and others) which can be used in connection with the example
embodiments described herein.
[0044] Control circuit 106 is arranged to coordinate the operation
of the laser source 102 and mirror subsystem 104 so that laser
pulses 122 are transmitted in a desired fashion. In this regard,
the control circuit 106 coordinates the firing commands 120
provided to laser source 102 with the mirror control signal(s) 130
provided to the mirror subsystem 104. In the example of FIG. 1,
where the mirror subsystem 104 includes mirror 110 and mirror 112,
the mirror control signal(s) 130 can include a first control signal
that drives the scanning of mirror 110 and a second control signal
that drives the scanning of mirror 112. Any of the mirror scan
techniques discussed above can be used to control mirrors 110 and
112. For example, mirror 110 can be driven with a sinusoidal signal
to scan mirror 110 in a resonant mode, and mirror 112 can be driven
with a step signal that varies as a function of the range points to
be targeted with laser pulses 122 to scan mirror 112 in a
point-to-point mode.
[0045] As discussed in greater detail below, control circuit 106
can use a laser energy model 108 to determine a timing schedule for
the laser pulses 122 to be transmitted from the laser source 102.
This laser energy model 108 can model the available energy within
the laser source 102 for producing laser pulses 122 over time in
different shot schedule scenarios. By modeling laser energy in this
fashion, the laser energy model 108 helps the control circuit 106
make decisions on when the laser source 102 should be triggered to
fire laser pulses. Moreover, as discussed in greater detail below,
the laser energy model 108 can model the available energy within
the laser source 102 over short time intervals (such as over time
intervals in a range from 10-100 nanoseconds), and such a short
interval laser energy model 108 can be referred to as a transient
laser energy model 108.
[0046] Control circuit 106 can include a processor that provides
the decision-making functionality described herein. Such a
processor can take the form of a field programmable gate array
(FPGA) or application-specific integrated circuit (ASIC) which
provides parallelized hardware logic for implementing such
decision-making. The FPGA and/or ASIC (or other compute
resource(s)) can be included as part of a system on a chip (SoC).
However, it should be understood that other architectures for
control circuit 106 could be used, including software-based
decision-making and/or hybrid architectures which employ both
software-based and hardware-based decision-making. The processing
logic implemented by the control circuit 106 can be defined by
machine-readable code that is resident on a non-transitory
machine-readable storage medium such as memory within or available
to the control circuit 106. The code can take the form of software
or firmware that define the processing operations discussed herein
for the control circuit 106. This code can be downloaded onto the
control circuit 106 using any of a number of techniques, such as a
direct download via a wired connection as well as over-the-air
downloads via wireless networks, which may include secured wireless
networks. As such, it should be understood that the lidar
transmitter 100 can also include a network interface that is
configured to receive such over-the-air downloads and update the
control circuit 106 with new software and/or firmware. This can be
particularly advantageous for adjusting the lidar transmitter 100
to changing regulatory environments with respect to criteria such
as laser dosage and the like. When using code provisioned for
over-the-air updates, the control circuit 106 can operate with
unidirectional messaging to retain function safety.
Modeling Laser Energy Over Time:
[0047] FIG. 2A shows an example process flow for the control
circuit 106 with respect to using the laser energy model 108 to
govern the timing schedule for laser pulses 122. At step 200, the
control circuit 106 maintains the laser energy model 108. This step
can include reading the parameters and expressions that define the
laser energy model 108, discussed in greater detail below. Step 200
can also include updating the laser energy model 108 over time as
laser pulses 122 are triggered by the laser source 102 as discussed
below.
[0048] In an example embodiment where the laser source 102 is a
pulsed fiber laser source as discussed above, the laser energy
model 108 can model the energy behavior of the seed laser 114, pump
laser 118, and fiber amplifier 116 over time as laser pulses 122
are fired. As noted above, the fired laser pulses 122 can be
referred to as "shots". For example, the laser energy model 108 can
be based on the following parameters: [0049] CE(t), which
represents the combined amount of energy within the fiber amplifier
116 at the moment when the laser pulse 122 is fired at time t.
[0050] EF(t), which represents the amount of energy fired in laser
pulse 122 at time t; [0051] E.sub.P, which represents the amount of
energy deposited by the pump laser 118 into the fiber amplifier 116
per unit of time. [0052] S(t+.delta.), which represents the
cumulative amount of seed energy that has been deposited by the
pump laser 118 and seed laser 114 into the fiber amplifier 116 over
the time duration .delta., where .delta. represents the amount of
time between the most recent laser pulse 122 (for firing at time t)
and the next laser pulse 122 (to be fired at time t+.delta.).
[0053] F(t+.delta.), which represents the amount of energy left
behind in the fiber amplifier 116 when the pulse 122 is fired at
time t (and is thus available for use with the next pulse 122 to be
fired at time t+.delta.). [0054] CE(t+.delta.), which represents
the amount of combined energy within the fiber amplifier 116 at
time t+.delta. (which is the sum of S(t+.delta.) and F(t+.delta.))
[0055] EF(t+.delta.), which represents the amount of energy fired
in laser pulse 122 fired at time t+.delta. [0056] a and b, where
"a" represents a proportion of energy transferred from the fiber
amplifier 116 into the laser pulse 122 when the laser pulse 122 is
fired, where "b" represents a proportion of energy retained in the
fiber amplifier 116 after the laser pulse 122 is fired, where
a+b=1.
[0057] While the seed energy (S) includes both the energy deposited
in the fiber amplifier 116 by the pump laser 118 and the energy
deposited in the fiber amplifier 116 by the seed laser 114, it
should be understood that for most embodiments the energy from the
seed laser 114 will be very small relative to the energy from the
pump laser 118. As such, a practitioner can choose to model the
seed energy solely in terms of energy produced by the pump laser
118 over time. Thus, after the pulsed fiber laser source 102 fires
a laser pulse at time t, the pump laser 118 will begin re-supplying
the fiber amplifier 116 with energy over time (in accordance with
E.sub.P) until the seed laser 116 is triggered at time t+.delta. to
cause the fiber amplifier 116 to emit the next laser pulse 122
using the energy left over in the fiber amplifier 116 following the
previous shot plus the new energy that has been deposited in the
fiber amplifier 116 by pump laser 118 since the previous shot. As
noted above, the parameters a and b model how much of the energy in
the fiber amplifier 116 is transferred into the laser pulse 122 for
transmission and how much of the energy is retained by the fiber
amplifier 116 for use when generating the next laser pulse 122.
[0058] The energy behavior of pulsed fiber laser source 102 with
respect to the energy fired in laser pulses 122 in this regard can
be expressed as follows:
EF(t)=aCE(t)
F(t+.delta.)=bCE(t)
S(t+.delta.)=.delta.E.sub.P
CE(t+.delta.)=S(t+.delta.)+F(t+.delta.)
EF(t+.delta.)=aCE(t+.delta.)
[0059] With these relationships, the value for CE(t) can be
re-expressed in terms of EF(t) as follows:
CE .times. ( t ) = E .times. F .function. ( t ) a ##EQU00001##
[0060] Furthermore, the value for F(t+.delta.) can be re-expressed
in terms of EF(t) as follows:
F .function. ( t + .delta. ) = b .times. E .times. F .function. ( t
) a ##EQU00002##
[0061] This means that the values for CE(t+.delta.) and
EF(t+.delta.) can be re-expressed as follows:
CE .times. ( t + .delta. ) = .delta. .times. E P + b .times. E
.times. F .function. ( t ) a .times. EF .function. ( t + .delta. )
= a .function. ( .delta. .times. E P + b .times. E .times. F
.function. ( t ) a ) ##EQU00003##
[0062] And this expression for EF(t+.delta.) shortens to:
EF(t+.delta.)=a.delta.E.sub.P+bEF(t)
[0063] It can be seen, therefore, that the energy to be fired in a
laser pulse 122 at time t+.delta. in the future can be computed as
a function of how much energy was fired in the previous laser pulse
122 at time t. Given that a, b, E.sub.P, and EF(t) are known
values, and .delta. is a controllable variable, these expressions
can be used as the laser energy model 108 that predicts the amount
of energy fired in a laser pulse at select times in the future (as
well as how much energy is present in the fiber amplifier 116 at
select times in the future).
[0064] While this example models the energy behavior over time for
a pulsed fiber laser source 102, it should be understood that these
models could be adjusted to reflect the energy behavior over time
for other types of laser sources.
[0065] Thus, the control circuit 106 can use the laser energy model
108 to model how much energy is available in the laser source 102
over time and can be delivered in the laser pulses 122 for
different time schedules of laser pulse shots. With reference to
FIG. 2A, this allows the control circuit 106 to determine a timing
schedule for the laser pulses 122 (step 202). For example, at step
202, the control circuit 106 can compare the laser energy model 108
with various defined energy requirements to assess how the laser
pulse shots should be timed. As examples, the defined energy
requirements can take any of a number of forms, including but not
limited to (1) a minimum laser pulse energy, (2) a maximum laser
pulse energy, (3) a desired laser pulse energy (which can be per
targeted range point for a lidar transmitter 100 that selectively
targets range points with laser pulses 122), (4) eye safety energy
thresholds, and/or (5) camera safety energy thresholds. The control
circuit 106 can then, at step 204, generate and provide firing
commands 120 to the laser source 102 that trigger the laser source
102 to generate laser pulses 122 in accordance with the determined
timing schedule. Thus, if the control circuit 106 determines that
laser pulses should be generated at times t1, t2, t3, . . . , the
firing commands 120 can trigger the laser source to generate laser
pulses 122 at these times.
[0066] A control variable that the control circuit 106 can evaluate
when determining the timing schedule for the laser pulses is the
value of .delta., which controls the time interval between
successive laser pulse shots. The discussion below illustrates how
the choice of .delta. impacts the amount of energy in each laser
pulse 122 according to the laser energy model 108.
[0067] For example, during a period where the laser source 102 is
consistently fired every .delta. units of time, the laser energy
model 108 can be used to predict energy levels for the laser pulses
as shown in the following toy example. [0068] Toy Example 1, where
E.sub.P=1 unit of energy; .delta.=1 unit of time; the initial
amount of energy stored by the fiber laser 116 is 1 unit of energy;
a=0.5 and b=0.5:
TABLE-US-00001 [0068] Shot Number 1 2 3 4 5 Time t + 1 t + 2 t + 3
t + 4 t + 5 Seed Energy from Pump Laser (S) 1 1 1 1 1 Leftover
Fiber Energy (F) 1 1 1 1 1 Combined Energy (S + F) 2 2 2 2 2 Energy
Fired (EF) 1 1 1 1 1
[0069] If the rate of firing is increased, this will impact how
much energy is included in the laser pulses. For example, relative
to Toy Example 1, if the firing rate is doubled (.delta.=0.5 units
of time) (while the other parameters are the same), the laser
energy model 108 will predict the energy levels per laser pulse 122
as follows below with Toy Example 2. [0070] Toy Example 2, where
E.sub.P=1 unit of energy; .delta.=0.5 units of time; the initial
amount of energy stored by the fiber laser 116 is 1 unit of energy;
a=0.5 and b=0.5:
TABLE-US-00002 [0070] Shot Number 1 2 3 4 5 Time t + 0.5 t + 1 t +
1.5 t + 2 t + 3.5 Seed Energy 0.5 0.5 0.5 0.5 0.5 from Pump Laser
(S) Leftover Fiber 1 0.75 0.625 0.5625 0.53125 Energy (F) Combined
1.5 1.25 1.125 1.0625 1.03125 Energy (S + F) Energy Fired 0.75
0.625 0.5625 0.53125 0.515625 (EF)
[0071] Thus, in comparing Toy Example 1 with Toy Example 2 it can
be seen that increasing the firing rate of the laser will decrease
the amount of energy in the laser pulses 122. As example
embodiments, the laser energy model 108 can be used to model a
minimum time interval in a range between around 10 nanoseconds to
around 100 nanoseconds. This timing can be affected by both the
accuracy of the clock for control circuit 106 (e.g., clock skew and
clock jitter) and the minimum required refresh time for the laser
source 102 after firing.
[0072] If the rate of firing is decreased relative to Toy Example
1, this will increase how much energy is included in the laser
pulses. For example, relative to Toy Example 1, if the firing rate
is halved (.delta.=2 units of time) (while the other parameters are
the same), the laser energy model 108 will predict the energy
levels per laser pulse 122 as follows below with Toy Example 3.
[0073] Toy Example 3, where E.sub.P=1 unit of energy; .delta.=2
units of time; the initial amount of energy stored by the fiber
laser 116 is 1 unit of energy; a=0.5 and b=0.5:
TABLE-US-00003 [0073] Shot Number 1 2 3 4 5 Time t + 2 t + 4 t + 6
t + 8 t + 10 Seed Energy from 2 2 2 2 2 Pump Laser (S) Leftover
Fiber Energy (F) 1 1.5 1.75 1.875 1.9375 Combined Energy (S + F) 3
3.5 3.75 3.875 3.9375 Energy Fired (EF) 1.5 1.75 1.875 1.9375
1.96875
[0074] If a practitioner wants to maintain a consistent amount of
energy per laser pulse, it can be seen that the control circuit 106
can use the laser energy model 108 to define a timing schedule for
laser pulses 122 that will achieve this goal (through appropriate
selection of values for .delta.).
[0075] For practitioners that want the lidar transmitter 100 to
transmit laser pulses at varying intervals, the control circuit 106
can use the laser energy model 108 to define a timing schedule for
laser pulses 122 that will maintain a sufficient amount of energy
per laser pulse 122 in view of defined energy requirements relating
to the laser pulses 122. For example, if the practitioner wants the
lidar transmitter 100 to have the ability to rapidly fire a
sequence of laser pulses (for example, to interrogate a target in
the field of view with high resolution) while ensuring that the
laser pulses in this sequence are each at or above some defined
energy minimum, the control circuit 106 can define a timing
schedule that permits such shot clustering by introducing a
sufficiently long value for .delta. just before firing the
clustered sequence. This long .delta. value will introduce a
"quiet" period for the laser source 102 that allows the energy in
seed laser 114 to build up so that there is sufficient available
energy in the laser source 102 for the subsequent rapid fire
sequence of laser pulses. As indicated by the decay pattern of
laser pulse energy reflected by Toy Example 2, increasing the
starting value for the seed energy (S) before entering the time
period of rapidly-fired laser pulses will make more energy
available for the laser pulses fired close in time with each
other.
[0076] Toy Example 4 below shows an example shot sequence in this
regard, where there is a desire to fire a sequence of 5 rapid laser
pulses separated by 0.25 units of time, where each laser pulse has
a minimum energy requirement of 1 unit of energy. If the laser
source has just concluded a shot sequence after which time there is
1 unit of energy retained in the fiber laser 116, the control
circuit can wait 25 units of time to allow sufficient energy to
build up in the seed laser 114 to achieve the desired rapid fire
sequence of 5 laser pulses 122, as reflected in the table below.
[0077] Toy Example 4, where E.sub.P=1 unit of energy;
.delta..sub.LONG=25 units of time; .delta..sub.SHORT=0.25 units of
time; the initial amount of energy stored by the fiber laser 116 is
1 unit of energy; a=0.5 and b=0.5; and the minimum pulse energy
requirement is 1 unit of energy:
TABLE-US-00004 [0077] Shot Number 1 2 3 4 5 Time t + 25 t + 25.25 t
+ 25.5 t + 25.75 t + 26 Seed Energy 25 0.25 0.25 0.25 0.25 from
Pump Laser (S) Leftover Fiber 1 13 6.625 3.4375 1.84375 Energy (F)
Combined Energy 26 13.25 6.875 3.6875 2.09375 (S + F) Energy Fired
13 6.625 3.4375 1.84375 1.046875 (EF)
[0078] This ability to leverage "quiet" periods to facilitate
"busy" periods of laser activity means that the control circuit 106
can provide highly agile and responsive adaptation to changing
circumstances in the field of view. For example, FIG. 16 shows an
example where, during a first scan 1600 across azimuths from left
to right at a given elevation, the laser source 102 fires 5 laser
pulses 122 that are relatively evenly spaced in time (where the
laser pulses are denoted by the "X" marks on the scan 1600). If a
determination is made that an object of interest is found at range
point 1602, the control circuit 106 can operate to interrogate the
region of interest 1604 around range point 1602 with a higher
density of laser pulses on second scan 1610 across the azimuths
from right to left. To facilitate this high density period of
rapidly fired laser pulses within the region of interest 1604, the
control circuit 106 can use the laser energy model 108 to determine
that such high density probing can be achieved by inserting a lower
density period 1606 of laser pulses during the time period
immediately prior to scanning through the region of interest 1604.
In the example of FIG. 16, this lower density period 1604 can be a
quiet period where no laser pulses are fired. Such timing schedules
of laser pulses can be defined for different elevations of the scan
pattern to permit high resolution probing of regions of interest
that are detected in the field of view.
[0079] The control circuit 106 can also use the energy model 108 to
ensure that the laser source 102 does not build up with too much
energy. For practitioners that expect the lidar transmitter 100 to
exhibit periods of relatively infrequent laser pulse firings, it
may be the case that the value for .delta. in some instances will
be sufficiently long that too much energy will build up in the
fiber amplifier 116, which can cause problems for the laser source
102 (either due to equilibrium overheating of the fiber amplifier
116 or non-equilibrium overheating of the fiber amplifier 116 when
the seed laser 114 induces a large amount of pulse energy to exit
the fiber amplifier 116). To address this problem, the control
circuit 106 can insert "marker" shots that serve to bleed off
energy from the laser source 102. Thus, even though the lidar
transmitter 100 may be primarily operating by transmitting laser
pulses 122 at specific, selected range points, these marker shots
can be fired regardless of the selected list of range points to be
targeted for the purpose of preventing damage to the laser source
102. For example, if there is a maximum energy threshold for the
laser source 102 of 25 units of energy, the control circuit 106 can
consult the laser energy model 108 to identify time periods where
this maximum energy threshold would be violated. When the control
circuit 106 predicts that the maximum energy threshold would be
violated because the laser pulses have been too infrequent, the
control circuit 106 can provide a firing command 120 to the laser
source 102 before the maximum energy threshold has been passed,
which triggers the laser source 102 to fire the marker shot that
bleeds energy out of the laser source 102 before the laser source's
energy has gotten too high. This maximum energy threshold can be
tracked and assessed in any of a number of ways depending on how
the laser energy model 108 models the various aspects of laser
operation. For example, it can be evaluated as a maximum energy
threshold for the fiber amplifier 116 if the energy model 108
tracks the energy in the fiber amplifier 116 (S+F) over time. As
another example, the maximum energy threshold can be evaluated as a
maximum value of the duration .delta. (which would be set to
prevent an amount of seed energy (S) from being deposited into the
fiber amplifier 116 that may cause damage when taking the values
for E.sub.P and a presumed value for F into consideration.
[0080] While the toy examples above use simplified values for the
model parameters (e.g. the values for E.sub.P and .delta.) for the
purpose of ease of explanation, it should be understood that
practitioners can select values for the model parameters or
otherwise adjust the model components to accurately reflect the
characteristics and capabilities of the laser source 102 being
used. For example, the values for E.sub.P, a, and b can be
empirically determined from testing of a pulsed fiber laser source
(or these values can be provided by a vendor of the pulsed fiber
laser source). Moreover, a minimum value for .delta. can also be a
function of the pulsed fiber laser source 102. That is, the pulsed
fiber laser sources available from different vendors may exhibit
different minimum values for .delta., and this minimum value for
.delta. (which reflects a maximum achievable number of shots per
second) can be included among the vendor's specifications for its
pulsed fiber laser source.
[0081] Furthermore, in situations where the pulsed fiber laser
source 102 is expected or observed to exhibit nonlinear behaviors,
such nonlinear behavior can be reflected in the model. As an
example, it can be expected that the pulsed fiber laser source 102
will exhibit energy inefficiencies at high power levels. In such a
case, the modeling of the seed energy (S) can make use of a
clipped, offset (affine) model for the energy that gets delivered
to the fiber amplifier 116 by pump laser 118 for pulse generation.
For example, in this case, the seed energy can be modeled in the
laser energy model 108 as:
S(t+.delta.)=E.sub.Pmax (a.sub.1.delta.+a.sub.0, offset)
[0082] The values for a.sub.1, a.sub.0, and offset can be
empirically measured for the pulsed fiber laser source 102 and
incorporated into the modeling of S(t+.delta.) used within the
laser energy model 108. It can be seen that for a linear regime,
the value for a.sub.1 would be 1, and the values for a.sub.0 and
offset would be 0. In this case, the model for the seed energy
S(t-.delta.) reduces to .delta.E.sub.P as discussed in the examples
above.
[0083] The control circuit 106 can also update the laser energy
model 108 based on feedback that reflects the energies within the
actual laser pulses 122. In this fashion, laser energy model 108
can better improve or maintain its accuracy over time. In an
example embodiment, the laser source 102 can monitor the energy
within laser pulses 122 at the time of firing. This energy amount
can then be reported by the laser source 102 to the control circuit
106 (see 250 in FIG. 2B) for use in updating the model 108. Thus,
if the control circuit 106 detects an error between the actual
laser pulse energy and the modeled pulse energy, then the control
circuit 106 can introduce an offset or other adjustment into model
108 to account for this error.
[0084] For example, it may be necessary to update the values for a
and b to reflect actual operational characteristics of the laser
source 102. As noted above, the values of a and b define how much
energy is transferred from the fiber amplifier 116 into the laser
pulse 122 when the laser source 102 is triggered and the seed laser
114 induces the pulse 122 to exit the fiber amplifier 116. An
updated value for a can be computed from the monitored energies in
transmitted pulses 122 (PE) as follows:
a=argmin.sub.a(.SIGMA..sub.k=1 . . .
N|PE(t.sub.k+.delta..sub.k)-aPE(t.sub.k)-(1-a).delta.t.sub.k|.sup.2)
[0085] In this expression, the values for PE represent the actual
pulse energies at the referenced times (t.sub.k or
t.sub.k+.delta..sub.k). This is a regression problem and can be
solved using commercial software tools such as those available from
MATLAB, Wolfram, PTC, ANSYS, and others. In an ideal world, the
respective values for PE(t) and PE(t+.delta.) will be the same as
the modeled values of EF(t) and EF(t+.delta.), However, for a
variety of reasons, the gain factors a and b may vary due to laser
efficiency considerations (such as heat or aging whereby back
reflections reduce the resonant efficiency in the laser cavity).
Accordingly, a practitioner may find it useful to update the model
108 over time to reflect the actual operational characteristics of
the laser source 102 by periodically computing updated values to
use for a and b.
[0086] In scenarios where the laser source 102 does not report its
own actual laser pulse energies, a practitioner can choose to
include a photodetector at or near an optical exit aperture of the
lidar transmitter 100 (e.g., see photodetector 252 in FIG. 2C). The
photodetector 252 can be used to measure the energy within the
transmitted laser pulses 122 (while allowing laser pulses 122 to
propagate into the environment toward their targets), and these
measured energy levels can be used to detect potential errors with
respect to the modeled energies for the laser pulses so model 108
can be adjusted as noted above. As another example for use in a
scenario where the laser source 102 does not report its own actual
laser pulse energies, a practitioner derives laser pulse energy
from return data 254 with respect to returns from known fiducial
objects in a field of view (such as road signs which are regulated
in terms of their intensity values for light returns) (see 254 in
FIG. 2D) as obtained from a point cloud 256 for the lidar system.
Additional details about such energy derivations are discussed
below. Thus, in such an example, the model 108 can be periodically
re-calibrated using point cloud data for returns from such
fiducials, whereby the control circuit 106 derives the laser pulse
energy that would have produced the pulse return data found in the
point cloud 256. This derived amount of laser pulse energy can then
be compared with the modeled laser pulse energy for adjustment of
the laser energy model 108 as noted above.
Modeling Mirror Motion Over Time:
[0087] In a particularly powerful example embodiment, the control
circuit 106 can also model mirror motion to predict where the
mirror subsystem 104 will be aimed at a given point in time. This
can be especially helpful for lidar transmitters 100 that
selectively target specific range points in the field of view with
laser pulses 122. By coupling the modeling of laser energy with a
model of mirror motion, the control circuit 106 can set the order
of specific laser pulse shots to be fired to targeted range points
with highly granular and optimized time scales. As discussed in
greater detail below, the mirror motion model can model mirror
motion over short time intervals (such as over time intervals in a
range from 5-50 nanoseconds). Such a short interval mirror motion
model can be referred to as a transient mirror motion model.
[0088] FIG. 3 shows an example lidar transmitter 100 where the
control circuit 106 uses both a laser energy model 108 and a mirror
motion model 308 to govern the timing schedule for laser pulses
122.
[0089] In an example embodiment, the mirror subsystem 104 can
operate as discussed above in connection with FIG. 1. For example,
the control circuit 106 can (1) drive mirror 110 in a resonant mode
using a sinusoidal signal to scan mirror 110 across different
azimuth angles and (2) drive mirror 112 in a point-to-point mode
using a step signal to scan mirror 112 across different elevations,
where the step signal will vary as a function of the elevations of
the range points to be targeted with laser pulses 122. Mirror 110
can be scanned as a fast-axis mirror, while mirror 112 is scanned
as a slow-axis mirror. In such an embodiment, a practitioner can
choose to use the mirror motion model 308 to model the motion of
mirror 110 as (comparatively) mirror 112 can be characterized as
effectively static for one or more scans across azimuth angles.
[0090] FIGS. 4A-4C illustrate how the motion of mirror 110 can be
modeled over time. In these examples, (1) the angle theta (.theta.)
represents the tilt angle of mirror 110, (2) the angle phi (.PHI.)
represents the angle at which a laser pulse 122 from the laser
source 102 will be incident on mirror 110 when mirror 110 is in a
horizontal position (where .theta. is zero degrees--see FIG. 4A),
and (3) the angle mu (.mu.) represents the angle of pulse 422 as
reflected by mirror 110 relative to the horizontal position of
mirror 110. In this example, the angle .mu. can represent the scan
angle of the mirror 110, where this scan angle can also be referred
to as a shot angle for mirror 110 as angle .mu. corresponds to the
angle at which reflected laser pulse 122' will be directed into the
field of view if fired at that time.
[0091] FIG. 4A shows mirror 110, where mirror 110 is at "rest" with
a tilt angle .theta. of zero degrees, which can be characterized as
the horizon of mirror 110. Laser source 102 is oriented in a fixed
position so that laser pulses 122 will impact mirror 110 at the
angle .PHI. relative to the horizontal position of mirror 110.
Given the property of reflections, it should be understood that the
value of the shot angle .mu. will be the same as the value of angle
.PHI. when the mirror 110 is horizontal (where .theta.=0).
[0092] FIG. 4B shows mirror 110 when it has been tilted about pivot
402 to a positive non-zero value of .theta.. It can be seen that
the tilting of mirror to angle .theta. will have the effect of
steering the reflected laser pulse 122' clockwise and to the right
relative to the angle of the reflected laser pulse 122' in FIG. 4A
(when mirror 110 was horizontal).
[0093] Mirror 110 will have a maximum tilt angle that can be
referred to as the amplitude A of mirror 110. Thus, it can be
understood that mirror 110 will scan through its tilt angles
between the values of -A (which corresponds to -.theta..sub.max)
and +A (which corresponds to +.theta..sub.Max). It can be seen that
the angle of reflection for the reflected laser pulse 122' relative
to the actual position of mirror 110 is the sum of .theta.+.PHI. as
shown by FIG. 4B. In then follows that the value of the shot angle
.mu. will be equal to 2 .theta.+.PHI., as can be seen from FIG.
4B.
[0094] When driven in a resonant mode according to sinusoidal
control signal, mirror 110 will change its tilt angle .theta.
according to a cosine oscillation, where its rate of change is
slowest at the ends of its scan (when it changes its direction of
tilt) and fastest at the mid-point of its scan. In an example where
the mirror 110 scans between maximum tilt angles of -A to +A, the
value of the angle .theta. as a function of time can be expressed
as:
.theta.=Acos(2.pi.ft)
where f represents the scan frequency of mirror 110 and t
represents time. Based on this model, it can be seen that the value
for .theta. can vary from A (when t=0) to 0 (when t is a value
corresponding to 90 degrees of phase (or 270 degrees of phase) to
-A (when t is a value corresponding to 180 degrees of phase).
[0095] This means that the value of the shot angle .mu. can be
expressed as a function of time by substituting the cosine
expression for .theta. into the expression for the shot angle of
.mu.=2 .theta.+.PHI. as follows:
.mu.=2 Acos(2.pi.ft)+.phi.
[0096] From this expression, one can then solve for t to produce an
expression as follows:
t = arccos .function. ( .mu. - .phi. 2 .times. A ) 2 .times. .pi.
.times. f ##EQU00004##
[0097] This expression thus identifies the time t at which the scan
of mirror 110 will target a given shot angle .mu.. Thus, when the
control circuit 106 wants to target a shot angle of .mu., the time
at which mirror 110 will scan to this shot angle can be readily
computed given that the values for .PHI., A, and f will be known.
In this fashion, the mirror motion model 308 can model that shot
angle as a function of time and predict the time at which the
mirror 110 will target a particular shot angle.
[0098] FIG. 4C shows mirror 110 when it has been tilted about pivot
402 to a negative non-zero value of -.theta.. It can be seen that
the tilting of mirror to angle -.theta. will have the effect of
steering the reflected laser pulse 122' counterclockwise and to the
left relative to the angle of the reflected laser pulse 122' in
FIG. 4A (when mirror 110 was horizontal). FIG. 4C also demonstrates
a constraint for a practitioner on the selection of the value for
the angle .PHI.. Laser source 102 will need to be positioned so
that the angle .PHI. is greater than the value of A to avoid a
situation where the underside of the tilted mirror 110 occludes the
laser pulse 122 when mirror is tilted to a value of .theta. that is
greater than .PHI.. Furthermore, the value of the angle .PHI.
should not be 90.degree. to avoid a situation where the mirror 110
will reflect the laser pulse 122 back into the laser source 102. A
practitioner can thus position the laser source 102 at a suitable
angle .PHI. accordingly.
[0099] FIG. 4D illustrates a translation of this relationship to
how the mirror 110 scans across a field of view 450. The mirror 110
will alternately scan in a left-to-right direction 452 and
right-to-left direction 454 as mirror 110 tilts between its range
of tilt angles (e.g., .theta.=-A through +A). For the example of
FIG. 4A where the value for .theta. is zero, this means that a
laser pulse fired at the untilted mirror 110 will be directed as
shown by 460 in FIG. 4D, where the laser pulse is directed toward a
range point at the mid-point of scan. The shot angle .mu. for this
"straight ahead" gaze is .PHI. as discussed above in connection
with FIG. 4A. As the angle .theta. increases from .theta.=0, this
will cause the laser pulses directed by mirror 110 to scan to the
right in the field of view until the mirror 110 tilts to the angle
.theta.=+A. When .theta.=+A, mirror 110 will be at the furthest
extent of its rightward scan 452, and it will direct a laser pulse
as shown by 462. The shot angle .mu. for this rightmost scan
position will be the value .mu.=2 A+.PHI.. From that point, the
mirror 110 will begin scanning leftward in direction 454 by
reducing its tilt angle .theta.. The mirror 110 will once again
scan through the mid-point and eventually reach a tilt angle of
.theta.=-A. When .theta.=-A, mirror 110 will be at the furthest
extent of its leftward scan 452, and it will direct a laser pulse
as shown by 464. The shot angle .mu. for this leftmost scan
position will be the value .mu.=.PHI.-2 A. From that point, the
mirror 110 will begin tilting in the rightward direction 450 again,
and the scan repeats. As noted above, due to the mirror motion
model 308, the control circuit 106 will know the time at which the
mirror 110 is targeting a shot angle of .mu..sub.i to direct a
laser pulse as shown by 466 of FIG. 4D.
[0100] In an example embodiment, the values for +A and -A can be
values in a range between +/-10 degrees and +/-20 degrees (e.g.,
+/-16 degrees) depending on the nature of mirror chosen as mirror
110. In an example where A is 16 degrees and mirror 110 scans as
discussed above in connection with FIGS. 4A-4D, it can be
understood that the angular extent of the scan for mirror 110 would
be 64 degrees (or 2 A from the scan mid-point in both the right and
left directions for a total of 4 A).
[0101] In some example embodiments, the value for A in the mirror
motion model 308 can be a constant value. However, some
practitioners may find it desirable to deploy a mirror 110 that
exhibits an adjustable value for A (e.g., a variable amplitude
mirror such as a variable amplitude MEMS mirror can serve as mirror
110). From the relationships discussed above, it can be seen that
the time required to move between two shot angles is reduced when
the value for amplitude A is reduced. The control circuit 106 can
leverage this relationship to determine whether it is desirable to
adjust the amplitude of the mirror 110 before firing a sequence of
laser pulses 122. FIG. 4E shows an example process flow in this
regard. At step 470, the control circuit 106 determines the settle
time (ts) for changing the amplitude from A to A' (where A'<A).
It should be understood that changing the mirror amplitude in this
fashion will introduce a time period where the mirror is relatively
unstable, and time will need to be provided to allow the mirror to
settle down to a stable position. This settling time can be
empirically determined or tracked for the mirror 110, and the
control circuit 106 can maintain this settle time value as a
control parameter. At step 472, the control circuit 106 determines
the time it will take to collect a shot list data set in a
circumstance where the amplitude of the mirror is unchanged
(amplitude remains A). This time can be referenced as collection
time tc. This value for tc can be computed through the use of the
laser energy model 108 and mirror motion model 308 with reference
to the shots included in a subject shot list. At step 474, the
control circuit 106 determines the time it will take to collect the
same shot list data set in a circumstance where the amplitude of
the mirror is changed to A'. This time can be referenced as
collection time tc'. This value for tc' can be computed through the
use of the laser energy model 108 and mirror motion model 308 (as
adjusted in view of the reduced amplitude of A') with reference to
the shots included in the subject shot list. At step 476, the
control circuit compares tc with the sum of tc' and ts. If the sum
(tc'+ts) is less than tc, this means that it will be time efficient
to change the mirror amplitude to A'. In this circumstance, the
process flow proceeds to step 478, and the control circuit 106
adjusts the amplitude of mirror 110 to A'. If the sum (tc'+ts) is
not less than tc, then the control circuit 106 leaves the amplitude
value unchanged (step 480).
Model-Based Shot Scheduling:
[0102] FIG. 5 shows an example process flow for the control circuit
106 to use both the laser energy model 108 and the mirror motion
model 308 to determine the timing schedule for laser pulses 122.
Step 200 can operate as described above with reference to FIG. 2A
to maintain the laser energy model 108. At step 500, the control
circuit 106 maintains the mirror motion model 308. As discussed
above, this model 308 can model the shot angle that the mirror will
target as a function of time. Accordingly, the mirror motion model
308 can predict the shot angle of mirror 110 at a given time t. To
maintain and update the model 308, the control circuit 108 can
establish the values for A, .PHI., and f to be used for the model
308. These values can be read from memory or determined from the
operating parameters for the system.
[0103] At step 502, the control circuit 106 determines a timing
schedule for laser pulses 122 using the laser energy model 108 and
the mirror motion model 308. By linking the laser energy model 108
and the mirror motion model 308 in this regard, the control circuit
106 can determine how much energy is available for laser pulses
targeted toward any of the range points in the scan pattern of
mirror subsystem 104. For purposes of discussion, we will consider
an example embodiment where mirror 110 scans in azimuth between a
plurality of shot angles at a high rate while mirror 112 scans in
elevation at a sufficiently slower rate so that the discussion
below will assume that the elevation is held steady while mirror
110 scans back and forth in azimuth. However, the techniques
described herein can be readily extended to modeling the motion of
both mirrors 110 and 112.
[0104] If there is a desire to target a range point at a Shot Angle
A with a laser pulse of at least X units of energy, the control
circuit 106, at step 502, can consult the laser energy model 108 to
determine whether there is sufficient laser energy for the laser
pulse when the mirror 110's scan angle points at Shot Angle A. If
there is sufficient energy, the laser pulse 122 can be fired when
the mirror 110 scans to Shot Angle A. If there is insufficient
energy, the control circuit 106 can wait to take the shot until
after mirror 110 has scanned through and back to pointing at Shot
Angle A (if the laser energy model 108 indicates there is
sufficient laser energy when the mirror returns to Shot Angle A).
The control circuit 106 can compare the shot energy requirements
for a set of shot angles to be targeted with laser pulses to
determine when the laser pulses 122 should be fired. Upon
determination of the timing schedule for the laser pulses 122, the
control circuit 106 can generate and provide firing commands 120 to
the laser source 102 based on this determined timing schedule (step
504).
[0105] FIG. 6A and 6B show example process flows for implementing
steps 502 and 504 of FIG. 5 in a scenario where the mirror
subsystem 104 includes mirror 110 that scans through azimuth shot
angles in a resonant mode (fast-axis) and mirror 112 that scans
through elevation shot angles in a point-to-point mode (slow-axis).
Lidar transmitter 100 in these examples seeks to fire laser pulses
122 at intelligently selected range points in the field of view.
With the example of FIG. 6A, the control circuit 106 schedules
shots for batches of range points at a given elevation on whichever
scan direction of the mirror 110 is schedulable for those range
points according to the laser energy model 108. With the example of
FIG. 6B, the control circuit 106 seeks to schedule shots for as
many range points as it can at a given elevation for each scan
direction of the mirror 110 in view of the laser energy model 108.
For any shots at the subject elevation that cannot be scheduled for
a given scan direction due to energy model constraints, the control
circuit 106 then seeks to schedule those range points on the
reverse scan (and so on until all of the shots are scheduled).
[0106] The process flow of FIG. 6A begins with step 600. At step
600, the control circuit 106 receives a list of range points to be
targeted with laser pulses. These range points can be expressed as
(azimuth angle, elevation angle) pairs, and they may be ordered
arbitrarily.
[0107] At step 602, the control circuit 106 sorts the range points
by elevation to yield sets of azimuth shot angles sorted by
elevation. The elevation-sorted range points can also be sorted by
azimuth shot angle (e.g., where all of the shot angles at a given
elevation are sorted in order of increasing azimuth angle (smallest
azimuth shot angle to largest azimuth shot angle) or decreasing
azimuth angle (largest azimuth shot angle to smallest azimuth shot
angle). For the purposes of discussing the process flows of FIGS.
6A and 6B, these azimuth shot angles can be referred to as the shot
angles for the control circuit 106. Step 602 produces a pool 650 of
range points to be targeted with shots (sorted by elevation and
then by shot angle).
[0108] At step 604, the control circuit 106 selects a shot
elevation from among the shot elevations in the sorted list of
range points in pool 650. The control circuit 106 can make this
selection on the basis of any of a number of criteria. The order of
selection of the elevations will govern which elevations are
targeted with laser pulses 122 before others.
[0109] Accordingly, in an example embodiment, the control circuit
106 can prioritize the selection of elevations at step 604 that are
expected to encompass regions of interest in the field of view. As
an example, some practitioners may find the horizon in the field of
view (e.g., a road horizon) to be high priority for targeting with
laser pulses 122. In such a case, step 604 can operate as shown by
FIG. 17A to determine the elevation(s) which correspond to a
horizon in the field of view (e.g. identify the elevations at or
near the road horizon) (see step 1702) and then prioritize the
selection of those elevations from pool 650 (see step 1702). Step
1702 can be performed by analyzing lidar return point cloud data
and/or camera images of the field of view to identify regions in
the field of view that are believed to qualify as the horizon
(e.g., using contrast detection techniques, edge detection
techniques, and/or other pattern processing techniques applied to
lidar or image data).
[0110] As another example, the control circuit 106 can prioritize
the selection of elevations based on the range(s) to detected
object(s) in the field of view. Some practitioners may find it
desirable to prioritize the shooting of faraway objects in the
field of view. Other practitioners may find it desirable to
prioritize the shooting of nearby objects in the field of view.
Thus, in an example such as that shown by FIG. 17B, the range(s)
applicable to detected object(s) is determined (see step 1706).
This range information will be available from the lidar return
point cloud data. At step 1708, the control circuit sorts the
detected object(s) by their determined range(s). Then, at step
1708, the control circuit 106 prioritizes the selection of
elevations from pool 650 based on the determined range(s) for
object(s) included in those elevations. With step 1708,
prioritization can be given to larger range values than for smaller
range values if the practitioner wants to shoot faraway objects
before nearby objects. For practitioners that want to shoot nearby
objects before faraway objects, step 1708 can give priority to
smaller range values than for larger range values. Which objects
are deemed faraway and which are deemed nearby can be controlled
using any of a number of techniques. For example, a range threshold
can be defined, and the control circuit 106 can make the elevation
selections based on which elevations include sorted objects whose
range is above (or below as the case may be) the defined range
threshold. As another example, the relative ranges for the sorted
objects can be used to control the selection of elevations (where
the sort order of either farthest to nearest or nearest to farthest
governs the selection of elevations which include those
objects).
[0111] As yet another example, the control circuit 106 can
prioritize the selection of elevations based on the velocity(ies)
of detected object(s) in the field of view. Some practitioners may
find it desirable to prioritize the shooting of fast-moving objects
in the field of view. FIG. 17C shows an example process flow for
this. At step 1714, the velocity is determined for each detected
object in the field of view. This velocity information can be
derived from the lidar return point cloud data. At step 1716, the
control circuit 106 can sort the detected object(s) by the
determined velocity(ies). The control circuit 106 can then use
determined velocities for the sorted objects as a basis for
prioritizing the selection of elevations which contain those
detected objects (step 1718). This prioritization at step 1718 can
be carried out in any of a number of ways. For example, a velocity
threshold can be defined, and step 1718 can prioritize the
selection of elevation include an object moving at or above this
defined velocity threshold. As another example, the relative
velocities of the sorted objects can be used where an elevation
that includes an object moving faster than another object can be
selected before an elevation that includes the another (slower
moving) object.
[0112] As yet another example, the control circuit 106 can
prioritize the selection of elevations based on the directional
heading(s) of detected object(s) in the field of view. Some
practitioners may find it desirable to prioritize the shooting of
objects in the field of view that moving toward the lidar
transmitter 100. FIG. 17D shows an example process flow for this.
At step 1720, the directional heading is determined for each
detected object in the field of view. This directional heading can
be derived from the lidar return point cloud data. The control
circuit 1722 can then prioritize the selection of elevation(s) that
include object(s) that are determined to be heading toward the
lidar transmitter 100 (within some specified degree of tolerance
where the elevation that contains an object heading near the lidar
transmitter 100 would be selected before an elevation that contains
an object moving away from the lidar transmitter 100).
[0113] Further still, some practitioners may find it desirable to
combine the process flows of FIG. 17C and 17D to prioritize the
selection of fast-moving objects that are heading toward the lidar
transmitter 100. An example for this is shown by FIG. 17E. With
FIG. 17E, steps 1714 and 1720 can be performed as discussed above.
At step 1724, the detected object(s) are sorted by their
directional headings (relative to the lidar transmitter 100) and
then by the determined velocities. At step 1726, the elevations
which contain objected deemed to be heading toward the lidar
transmitter 100 (and moving faster than other such objects) are
prioritized for selection.
[0114] In another example embodiment, the control circuit 106 can
select elevations at step 604 based on eye safety or camera safety
criteria. For example, eye safety requirements may specify that the
lidar transmitter 100 should not direct more than a specified
amount of energy in a specified spatial area over of a specified
time period. To reduce the risk of firing too much energy into the
specified spatial area, the control circuit 106 can select
elevations in a manner that avoids successive selections of
adjacent elevations (e.g., jumping from Elevation 1 to Elevation 3
rather than Elevation 2) to insert more elevation separation
between laser pulses that may be fired close in time. This manner
of elevation selection may optionally be implemented dynamically
(e.g., where elevation skips are introduced if the control circuit
106 determines that the energy in a defined spatial area has
exceeded some level that is below but approaching the eye safety
thresholds). Furthermore, it should be understood that the number
of elevations to skip (a skip interval) can be a value selected by
a practitioner or user to define how many elevations will be
skipped when progressing from elevation-to-elevation. As such, a
practitioner may choose to set the elevation skip interval to be a
value larger than 1 (e.g., a skip interval of 5, which would cause
the system to progress from Elevation 3 to Elevation 9).
Furthermore, similar measures can be taken to avoid hitting cameras
that may be located in the field of view with too much energy. FIG.
17F depicts an example process flow for this approach. At step
1730, the control circuit 106 selects Elevation X.sub.t (where this
selected elevation is larger (or smaller) than the preceding
selected elevation (Elevation X.sub.t-1) by the defined skip
interval. Then, the control circuit 106 schedules the shots for the
selected elevation (step 1732), and the process flow returns to
step 1730 where the next elevation (Elevation X.sub.t+1) is
selected (according to the skip interval relative to Elevation
X.sub.t).
[0115] Thus, it should be understood that step 604 can employ a
prioritized classification system that decides the order in which
elevations are to be targeted with laser pulses 122 based on the
criteria of FIGS. 17A-17F or any combinations of any of these
criteria.
[0116] At step 606, the control circuit 106 generates a mirror
control signal for mirror 112 to drive mirror 112 so that it
targets the angle of the selected elevation. As noted, this mirror
control signal can be a step signal that steps mirror 112 up (or
down) to the desired elevation angle. In this fashion, it can be
understood that the control circuit 106 will be driving mirror 112
in a point-to-point mode where the mirror control signal for mirror
112 will vary as a function of the range points to be targeted with
laser pulses (and more precisely, as a function of the order of
range points to be targeted with laser pulses).
[0117] At step 608, the control circuit 106 selects a window of
azimuth shot angles that are in the pool 650 at the selected
elevation. The size of this window governs how many shot angles
that the control circuit 106 will order for a given batch of laser
pulses 122 to be fired. This window size can be referred to as the
search depth for the shot scheduling. A practitioner can configure
the control circuit 106 to set this window size based on any of a
number of criteria. While the toy examples discussed below use a
window size of 3 for purposes of illustration, it should be
understood that practitioners may want to use a larger (or smaller)
window size in practice. For example, in an example embodiment, the
size of the window may be a value in a range between 2 shots and 12
shots. However, should the control circuit 106 have larger
capacities for parallel processing or should there be more lenient
time constraints on latency, a practitioner may find it desirable
to choose larger window sizes. Furthermore, the control circuit 106
can consider a scan direction for the mirror 110 when selecting the
shot angles to include in this window. Thus, if the control circuit
106 is scheduling shots for a scan direction corresponding to
increasing shot angles, the control circuit 106 can start from the
smallest shot angle in the sorted pool 650 and include
progressively larger shot angles in the shot angle sort order of
the pool 650. Similarly, if the control circuit 106 is scheduling
shots for a scan direction corresponding to decreasing shot angles,
the control circuit 106 can start from the largest shot angle in
the sorted pool 650 and include progressively smaller shot angles
in the shot angle sort order of the pool 650.
[0118] At step 610, the control circuit 106 determines an order for
the shot angles in the selected window using the laser energy model
108 and the mirror motion model 308. As discussed above, this
ordering operation can compare candidate orderings with criteria
such as energy requirements relating to the shots to find a
candidate ordering that satisfies the criteria. Once a valid
candidate ordering of shot angles is found, this can be used as
ordered shot angles that will define the timing schedule for the
selected window of laser pulses 122. Additional details about
example embodiments for implementing step 610 are discussed
below.
[0119] Once the shot angles in the selected window have been
ordered at step 610, the control circuit 106 can add these ordered
shot angles to the shot list 660. As discussed in greater detail
below, the shot list 660 can include an ordered listing of shot
angles and a scan direction corresponding to each shot angle.
[0120] At step 612, the control circuit 106 determines whether
there are any more shot angles in pool 650 to consider at the
selected elevation. In other words, if the window size does not
encompass all of the shot angles in the pool 650 at the selected
elevation, then the process flow can loop back to step 608 to grab
another window of shot angles from the pool 650 for the selected
elevation. If so, the process flow can then perform steps 610 and
612 for the shot angles in this next window.
[0121] Once all of the shots have been scheduled for the shot
angles at the selected elevation, the process flow can loop back
from step 612 to step 604 to select the next elevation from pool
650 for shot angle scheduling. As noted above, this selection can
proceed in accordance with a defined prioritization of elevations.
From there, the control circuit 106 can perform steps 606-614 for
the shot angles at the newly selected elevation.
[0122] Meanwhile, at step 614, the control circuit 106 generates
firing commands 120 for the laser source 102 in accordance with the
determined order of shot angles as reflected by shot list 660.
[0123] By providing these firing commands 120 to the laser source
102, the control circuit 106 triggers the laser source 102 to
transmit the laser pulses 122 in synchronization with the mirrors
110 and 112 so that each laser pulse 122 targets its desired range
point in the field of view. Thus, if the shot list includes Shot
Angles A and C to be fired at during a left-to-right scan of the
mirror 110, the control circuit 106 can use the mirror motion model
308 to identify the times at which mirror 110 will be pointing at
Shot Angles A and C on a left-to-right scan and generate the firing
commands 120 accordingly. The control circuit 106 can also update
the pool 650 to mark the range points corresponding to the firing
commands 120 as being "fired" to effectively remove those range
points from the pool 650.
[0124] In the example of FIG. 6B, as noted above, the control
circuit 106 seeks to schedule as many shots as possible on each
scan direction of mirror 110. Steps 600, 602, 604, and 606 can
proceed as described above for FIG. 6A.
[0125] At step 620, the control circuit 106 selects a scan
direction of mirror 110 to use for scheduling. A practitioner can
choose whether this scheduling is to start with a left-to-right
scan direction or a right-to-left scan direction. Then, step 608
can operate as discussed above in connection with FIG. 6A, but
where the control circuit 106 uses the scan direction selected at
step 620 to govern which shot angles are included in the selected
window. Thus, if the selected scan direction corresponds to
increasing shot angles, the control circuit 106 can start from the
smallest shot angle in the sorted pool 650 and include
progressively larger shot angles in the shot angle sort order of
the pool 650. Similarly, if the selected scan direction corresponds
to decreasing shot angles, the control circuit 106 can start from
the largest shot angle in the sorted pool 650 and include
progressively smaller shot angles in the shot angle sort order of
the pool 650.
[0126] At step 622, the control circuit 106 determines an order for
the shot angles based on the laser energy model 108 and the mirror
motion model 308 as discussed above for step 610, but where the
control circuit 106 will only schedule shot angles if the laser
energy model 108 indicates that those shot angles are schedulable
on the scan corresponding to the selected scan direction. Scheduled
shot angles are added to the shot list 660. But, if the laser
energy model 108 indicates that the system needs to wait until the
next return scan (or later) to take a shot at a shot angle in the
selected window, then the scheduling of that shot angle can be
deferred until the next scan direction for mirror 110 (see step
624). This effectively returns the unscheduled shot angle to pool
650 for scheduling on the next scan direction if possible.
[0127] At step 626, the control circuit 106 determines if there are
any more shot angles in pool 650 at the selected elevation that are
to be considered for scheduling on the scan corresponding to the
selected scan direction. If so, the process flow returns to step
608 to grab another window of shot angles at the selected elevation
(once again taking into consideration the sort order of shot angles
at the selected elevation in view of the selected scan
direction).
[0128] Once the control circuit 106 has considered all of the shot
angles at the selected elevation for scheduling on the selected
scan direction, the process flow proceeds to step 628 where a
determination is made as to whether there are any more unscheduled
shot angles from pool 650 at the scheduled elevation. If so, the
process flow loops back to step 620 to select the next scan
direction (i.e., the reverse scan direction). From there, the
process flow proceeds through steps 608, 622, 624, 626, and 628
until all of the unscheduled shot angles for the selected elevation
have been scheduled and added to shot list 660. Once step 628
results in a determination that all of the shot angles at the
selected elevation have been scheduled, the process flow can loop
back to step 604 to select the next elevation from pool 650 for
shot angle scheduling. As noted above, this selection can proceed
in accordance with a defined prioritization of elevations, and the
control circuit 106 can perform steps 606, 620, 608, 622, 624, 626,
628, and 614 for the shot angles at the newly selected
elevation.
[0129] Thus, it can be understood that the process flow of FIG. 6B
will seek to schedule all of the shot angles for a given elevation
during a single scan of mirror 110 (from left-to-right or
right-to-left as the case may be) if possible in view of the laser
energy model 108. However, should the laser energy model 108
indicate that more time is needed to fire shots at the desired shot
angles, then some of the shot angles may be scheduled for the
return scan (or subsequent scan) of mirror 110.
[0130] It should also be understood that the control circuit 106
will always be listening for new range points to be targeted with
new laser pulses 122. As such, steps 600 and 602 can be performed
while steps 604-614 are being performed (for FIG. 6A) or while
steps 604, 606, 620, 608, 622, 624, 626, 628, and 614 are being
performed (for FIG. 6B). Similarly, step 614 can be performed by
the control circuit 106 while the other steps of the FIG. 6A and 6B
process flows are being performed. Furthermore, it should be
understood that the process flows of FIG. 6A and 6B can accommodate
high priority requests for range point targeting. For example, as
described in U.S. Pat. No. 10,495,757, the entire disclosure of
which is incorporated herein by reference, a request may be
received to target a set of range points in a high priority manner.
Thus, the control circuit 106 can also always be listening for such
high priority requests and then cause the process flow to quickly
begin scheduling the firing of laser pulses toward such range
points. In a circumstance where a high priority targeting request
causes the control circuit 106 to interrupt its previous shot
scheduling, the control circuit 106 can effectively pause the
current shot schedule, schedule the new high priority shots (using
the same scheduling techniques) and then return to the previous
shot schedule once laser pulses 122 have been fired at the high
priority targets.
[0131] Accordingly, as the process flows of FIGS. 6A and 6B work
their way through the list of range points in pool 650, the control
circuit 106 will provide improved scheduling of laser pulses 122
fired at those range points through use of the laser energy model
108 and mirror motion model 308 as compared to defined criteria
such as shot energy thresholds for those shots. Moreover, by
modeling laser energy and mirror motion over short time intervals
on the order of nanoseconds using transient models as discussed
above, these shot scheduling capabilities of the system can be
characterized as hyper temporal because highly precise shots with
highly precise energy amounts can be accurately scheduled over
short time intervals if necessary.
[0132] While FIGS. 6A and 6B show their process flows as an
iterated sequence of steps, it should be understood that if the
control circuit 106 has sufficient parallelized logic resources,
then many of the iterations can be unrolled and performed in
parallel without the need for return loops (or using a few number
of returns through the steps). For example, different windows of
shot angles at the selected elevation can be processed in parallel
with each other if the control circuit 106 has sufficient
parallelized logic capacity. Similarly, the control circuit 106 can
also work on scheduling for different elevations at the same time
if it has sufficient parallelized logic capacity.
[0133] FIG. 7A shows an example process flow for carrying out step
610 of FIG. 6A. At step 700, the control circuit 106 creates shot
angle order candidates from the shot angles that are within the
window selected at step 608. These candidates can be created based
on the mirror motion model 308.
[0134] For example, as shown by FIG. 7B, the times at which the
mirror 110 will target the different potential shot angles can be
predicted using the mirror motion model 308. Thus, each shot angle
can be assigned a time slot 710 with respect to the scan of mirror
110 across azimuth angles (and back). As shown by FIG. 7B, if
mirror 110 starts at Angle Zero at Time 1, it will then scan to
Angle A at Time 2, then scan to Angle B at Time 3, and so on
through its full range of angles (which in the example of FIG. 7B
reaches Angle J before the mirror 110 begins scanning back toward
Angle Zero). The time slots for these different angles can be
computed using the mirror motion model 308. Thus, if the window of
shot angles identifies Angle A, Angle C, and Angle I as the shot
angles, then the control circuit 106 will know which time slots of
the mirror scan for mirror 110 will target those shot angles. For
example, according to FIG. 7B, Time Slots 1, 3, and 9 will target
Angles A, C, and I. On the return scan, Time Slot 11 will also
target Angle I (as shown by FIG. 7B), while Time Slots 17 and 19
will also target Angles C and A respectively.
[0135] As example embodiments, the time slots 710 can correspond to
time intervals in a range between around 5 nanoseconds and around
50 nanoseconds, which would correspond to angular intervals of
around 0.01 to 0.1 degrees if mirror 110 is scanning at 12 kHz over
an angular extent of 64 degrees (where +/-A is +/-16 degrees).
[0136] To create the order candidates at step 700, the control
circuit 106 can generate different permutations of time slot
sequences for different orders of the shot angles in the selected
window. Continuing with an example where the shot angles are A, C,
and I, step 700 can produce the following set of example order
candidates (where each order candidate can be represented by a time
slot sequence):
TABLE-US-00005 Order Time Slot Candidate Sequence Comments
Candidate 1 1, 3, 9 This would correspond to firing laser pulses in
the shot angle order of ACI during the first scan for mirror 110
(which moves from left-to-right) Candidate 2 1, 9, 17 This would
correspond to firing laser pulses in the shot angle order of AIC,
where laser pulses are fired at Shot Angles A and I during the
first scan for mirror 110 and where the laser pulse is fired at
Shot Angle C during the second (return) scan for mirror 110 (where
this second scan moves from right-to-left). Candidate 3 3, 9, 19
This would correspond to firing laser pulses in the shot angle
order of CIA, here laser pulses are fired at Shot Angles C and I
during the first scan for mirror 110 and where the laser pulse is
fired at Shot Angle A during the second (return) scan for mirror
110. Candidate 4 3, 9, 21 This would correspond to firing laser
pulses in the shot angle order of CIA, where laser pulses are fired
at Shot Angles C and I during the first scan for mirror 110 and
where the laser pulse is fired at Shot Angle A during the third
scan for mirror 110 (which moves from left-to-right) . . . . . . .
. .
[0137] It should be understood that the control circuit 106 could
create additional candidate orderings from different permutations
of time slot sequences for Shot Angles A, C, and I. A practitioner
can choose to control how many of such candidates will be
considered by the control circuit 106.
[0138] At step 702, the control circuit 106 simulates the
performance of the different order candidates using the laser
energy model 108 and the defined shot requirements. As discussed
above, these shot requirements may include requirements such as
minimum energy thresholds for each laser pulse (which may be
different for each shot angle), maximum energy thresholds for each
laser pulse (or for the laser source), and/or desired energy levels
for each laser pulse (which may be different for each shot
angle).
[0139] To reduce computational latency, this simulation and
comparison with shot requirements can be performed in parallel for
a plurality of the different order candidates using parallelized
logic resources of the control circuit 106. An example of such
parallelized implementation of step 702 is shown by FIG. 7C. In the
example of FIG. 7C, steps 720, 722, and 724 are performed in
parallel with respect to a plurality of the different time slot
sequences that serve as the order candidates. Thus, steps 720a,
722a, and 724a are performed for Time Slot Sequence 1; steps 720b,
722b, and 724b are performed for Time Slot Sequence 2; and so on
through steps 720n, 722n, and 724n for Time Slot Sequence n.
[0140] At step 720, the control circuit 106 uses the laser energy
model 108 to predict the energy characteristics of the laser source
and resultant laser pulse if laser pulse shots are fired at the
time slots corresponding to the subject time slot sequence. These
modeled energies can then be compared to criteria such as a maximum
laser energy threshold and a minimum laser energy threshold to
determine if the time slot sequence would be a valid sequence in
view of the system requirements. At step 722, the control circuit
106 can label each tested time slot sequence as valid or invalid
based on this comparison between the modeled energy levels and the
defined energy requirements. At step 724, the control circuit 106
can compute the elapsed time that would be needed to fire all of
the laser pulses for each valid time slot sequence. For example,
Candidate 1 from the example above would have an elapsed time
duration of 9 units of time, while Candidate 2 from the example
above would have an elapsed time duration of 17 units of time.
[0141] FIGS. 7D, 7E, and 7F show examples of such simulations of
time slot sequences for our example where the shot angles to be
scheduled with laser pulses are Shot Angles A, C, and I. In this
scenario, we will assume that the laser energy model 108 will
employ (1) the value for E.sub.S as a constant value of 1 unit of
energy per unit of time and (2) the values for a and b as 0.5 each.
Furthermore, we will assume that there are 3 units of energy left
in the fiber laser 116 when the scan begins (and where the scan
begins at Angle Zero while moving from left-to-right). Moreover,
for the purposes of this example, the energy requirements for the
shots can be defined as (8,3,4) for minimum shot energies with
respect to shot angles A, C, and I respectively, and where the
maximum laser energy for the laser source can be defined as 20
units of combined seed and stored fiber energy (which would
translate to a maximum laser pulse energy of 10 units of
energy).
[0142] FIG. 7D shows an example result for simulating the time slot
sequence of laser pulses at time slots 1, 3, and 9. In this
example, it can be seen that this time slot sequence is invalid
because the shot energy for Time Slot 1 (targeting Shot Angle A) is
only 2 units of energy, which is below the minimum energy threshold
of 8 units for Shot Angle A. This time slot sequence also fails
because the shot energy for Time Slot 3 (targeting Shot Angle C) is
only 2 units of energy, which is below the minimum energy threshold
of 3 units for Shot Angle C.
[0143] FIG. 7E shows an example result for simulating the time slot
sequence of laser pulses at time slots 1, 9, and 17. In this
example, it can be seen that this time slot sequence is invalid
because the shot energy for Time Slot 1 (targeting Shot Angle A) is
too low.
[0144] FIG. 7F shows an example result for simulating the time slot
sequence of laser pulses at time slots 3, 9, and 21. In this
example, it can be seen that this time slot sequence is valid
because the shot energies for each time slot are at or above the
minimum energy thresholds for their corresponding shot angles (and
none of the time slots would violate the maximum energy threshold
for the laser source). It can be further surmised from FIG. 7F that
a simulation of a Time Slot Sequence of (3,9,19) also would have
failed because there is insufficient energy in a laser pulse that
would have been fired at Shot Angle A.
[0145] Accordingly, the simulation of these time slot sequences
would result in a determination that the time slot sequence of
(3,9,21) is a valid candidate, which means that this time slot
sequence can define the timing schedule for laser pulses fired
toward the shot angles in the selected window. The elapsed time for
this valid candidate is 21 units of time.
[0146] Returning to FIG. 7A, at step 704, the control circuit 106
selects the valid order candidate which has the lowest elapsed
time. Thus, in a scenario where the simulations at step 702 would
have produced two or more valid order candidates, the control
circuit 106 will select the order candidate that will complete its
firing of laser pulses the soonest which helps improve the latency
of the system.
[0147] For example embodiments, the latency with which the control
circuit 106 is able to determine the shot angle order and generate
appropriate firing commands is an important operational
characteristic for the lidar transmitter 100. To maintain high
frame rates, it is desirable for the control circuit 106 to carry
out the scheduling operations for all of the shot angles at a
selected elevation in the amount of time it takes to scan mirror
110 through a full left-to-right or right-to-left scan if feasible
in view of the laser energy model 108 (where this time amount is
around 40 microseconds for a 12 kHz scan frequency). Moreover, it
is also desirable for the control circuit 106 to be able to
schedule shots for a target that is detected based on returns from
shots on the current scan line during the next return scan (e.g.,
when a laser pulse 122 fired during the current scan detects
something of interest that is to be interrogated with additional
shots (see FIG. 16 discussed above)). In this circumstance, the
detection path for a pulse return through a lidar receiver and into
a lidar point cloud generator where the target of interest is
detected will also need to be taken into account. This portion of
the processing is expected to require around 0.4 to 10
microseconds, which leaves around 30 microseconds for the control
circuit 106 to schedule the new shots at the region of interest
during the next return scan if possible. For a processor of the
control circuit 106 which has 2 Gflops of processing per second
(which is a value available from numerous FPGA and ASIC vendors),
this amounts to 50 operations per update, which is sufficient for
the operations described herein. For example, the control circuit
106 can maintain lookup tables (LUTs) that contain pre-computed
values of shot energies for different time slots within the scan.
Thus, the simulations of step 702 can be driven by looking up
precomputed shot energy values for the defined shot angles/time
slots. The use of parallelized logic by the control circuit 106 to
accelerate the simulations helps contribute to the achievement of
such low latency. Furthermore, practitioners can adjust operational
parameters such as the window size (search depth) in a manner to
achieve desired latency targets.
[0148] FIG. 8 shows an example embodiment for the lidar transmitter
100 where the control circuit 106 comprises a system controller 800
and a beam scanner controller 802. System controller 800 and beam
scanner controller 802 can each include a processor and memory for
use in carrying out its tasks. The mirror subsystem 104 can be part
of beam scanner 810 (which can also be referred to as a lidar
scanner). Beam scanner controller 802 can be embedded as part of
the beam scanner 810. In this example, the system controller 800
can carry out steps 600, 602, 604, 608, 610, and 612 of FIG. 6A if
the control circuit 106 employs the FIG. 6A process flow (or steps
600, 602, 604, 620, 608, 622, 624, 626, and 628 of FIG. 6B if the
control circuit 106 employs the FIG. 6B process flow), while beam
scanner controller 802 carries out steps 606 and 614 for the FIG.
6A and 6B process flows. Accordingly, once the system controller
800 has selected the elevation and the order of shot angles, this
information can be communicated from the system controller 800 to
the beam scanner controller 802 as shot elevation 820 and ordered
shot angles 822.
[0149] The ordered shot angles 822 can also include flags that
indicate the scan direction for which the shot is to be taken at
each shot angle. This scan direction flag will also allow the
system to recognize scenarios where the energy model indicates
there is a need to pass by a time slot for a shot angle without
firing a shot and then firing the shot when the scan returns to
that shot angle in a subsequent time slot. For example, with
reference to the example above, the scan direction flag will permit
the system to distinguish between Candidate 3 (for the sequence of
shot angles CIA at time slots 3, 9, and 19) versus Candidate 4 (for
the same sequence of shot angles CIA but at time slots 3, 9, and
21). A practitioner can explicitly assign a scan direction to each
ordered shot angle by adding the scan direction flag to each
ordered shot angle if desired, or a practitioner indirectly assign
a scan direction to each ordered shot angle by adding the scan
direction flag to the ordered shot angles for which there is a
change in scan direction. Together, the shot elevations 802 and
order shot angles 822 serve as portions of the shot list 660 used
by the lidar transmitter 100 to target range points with laser
pulses 122.
[0150] The beam scanner controller 802 can generate control signal
806 for mirror 112 based on the defined shot elevation 820 to drive
mirror 112 to a scan angle that targets the elevation defined by
820. Meanwhile, the control signal 804 for mirror 110 will continue
to be the sinusoidal signal that drives mirror 110 in a resonant
mode. However, some practitioners may choose to also vary control
signal 804 as a function of the ordered shot angles 822 (e.g., by
varying amplitude A as discussed above).
[0151] In the example of FIG. 8, the mirror motion model 308 can
comprise a first mirror motion model 808a maintained and used by
the beam scanner controller 802 and a second mirror motion model
808b maintained and used by the system controller 800. With FIG. 8,
the task of generating the firing commands 120 can be performed by
the beam scanner controller 802. The beam scanner controller 810
can include a feedback system 850 that tracks the actual mirror
tilt angles .theta. for mirror 110. This feedback system 850
permits the beam scanner controller 802 to closely monitor the
actual tilt angles of mirror 110 over time which then translates to
the actual scan angles .mu. of mirror 110. This knowledge can then
be used to adjust and update mirror motion model 808a maintained by
the beam scanner controller 802. Because model 808a will closely
match the actual scan angles for mirror 110 due to the feedback
from 850, we can refer to model 808a as the "fine" mirror motion
model 808a. In this fashion, when the beam scanner controller 802
is notified of the ordered shot angles 822 to be targeted with
laser pulses 122, the beam scanner controller 802 can use this
"fine" mirror motion model 808a to determine when the mirror has
hit the time slots which target the ordered shot angles 822. When
these time slots are hit according to the "fine" mirror motion
model 808a, the beam scanner controller 802 can generate and
provide corresponding firing commands 120 to the laser source
102.
[0152] Examples of techniques that can be used for the scan
tracking feedback system 850 are described in the above-referenced
and incorporated U.S. Pat. No. 10,078,133. For example, the
feedback system 850 can employ optical feedback techniques or
capacitive feedback techniques to monitor and adjust the scanning
(and modeling) of mirror 110. Based on information from the
feedback system 850, the beam scanner controller 802 can determine
how the actual mirror scan angles may differ from the modeled
mirror scan angles in terms of frequency, phase, and/or maximum
amplitude. Accordingly, the beam scanner controller 802 can then
incorporate one or more offsets or other adjustments relating the
detected errors in frequency, phase, and/or maximum amplitude into
the mirror motion model 808a so that model 808a more closely
reflects reality. This allows the beam scanner controller 802 to
generate firing commands 120 for the laser source 102 that closely
match up with the actual shot angles to be targeted with the laser
pulses 122.
[0153] Errors in frequency and maximum amplitude within the mirror
motion model 808a can be readily derived from the tracked actual
values for the tilt angle .theta. as the maximum amplitude A should
be the maximum actual value for .theta., and the actual frequency
is measurable based on tracking the time it takes to progress from
actual values for A to -A and back.
[0154] Phased locked loops (or techniques such as PID control, both
available as software tools in MATLAB) can be used to track and
adjust the phase of the model 808a as appropriate. The expression
for the tilt angle .theta. that includes a phase component (p) can
be given as:
.theta.=Acos(2.pi.ft+p)
[0155] From this, we can recover the value for the phase p by the
relation:
.theta..apprxeq.Acos(2.pi.ft)-Asin(2.pi.ft)p
[0156] Solving for p, this yields the expression:
p = A .times. cos .times. ( 2 .times. .pi. .times. f .times. t ) -
.theta. A .times. sin .times. ( 2 .times. .pi. .times. f .times. t
) ##EQU00005##
[0157] Given that the tracked values for A, f, t, and .theta. are
each known, the value for p can be readily computed. It should be
understood that this expression for p assumes that the value of the
p is small, which will be an accurate assumption if the actual
values for A, f, t, and .theta. are updated frequently and the
phase is also updated frequently. This computed value of p can then
be used by the "fine" mirror motion model 808a to closely track the
actual shot angles for mirror 110, and identify the time slots that
correspond to those shot angles according to the expression:
t = arccos .function. ( .mu. - .phi. 2 .times. A ) - p 2 .times.
.pi. .times. f ##EQU00006##
[0158] While a practitioner will find it desirable for the beam
scanner controller 802 to rely on the highly accurate "fine" mirror
motion model 808a when deciding when the firing commands 120 are to
be generated, the practitioner may also find that the shot
scheduling operations can suffice with less accurate mirror motion
modeling. Accordingly, the system controller 800 can maintain its
own model 808b, and this model 808b can be less accurate than model
808a as small inaccuracies in the model 808b will not materially
affect the energy modeling used to decide on the ordered shot
angles 822. In this regard, model 808b can be referred to as a
"coarse" mirror motion model 808b. If desired, a practitioner can
further communicate feedback from the beam scanner controller 802
to the system controller 800 so the system controller 800 can also
adjusts its model 808b to reflect the updates made to model 808a.
In such a circumstance, the practitioner can also decide on how
frequently the system will pass these updates from model 808a to
model 808b.
Marker Shots to Bleed Off and/or Regulate Shot Energy:
[0159] FIG. 9 depicts an example process flow for execution by the
control circuit 106 to insert marker shots into the shot list in
order to bleed off energy from the laser source 102 when needed. As
discussed above, the control circuit 106 can consult the laser
energy model 108 as applied to the range points to be targeted with
laser pulses 122 to determine whether a laser energy threshold
would be violated. If so, the control circuit 106 may insert a
marker shot into the shot list to bleed energy out of the laser
source 102 (step 902). In an example embodiment, this threshold can
be set to define a maximum or peak laser energy threshold so as to
avoid damage to the laser source 102. In another example
embodiment, this threshold can be set to achieve a desired
consistency, smoothness, and/or balance in the energies of the
laser pulse shots.
[0160] For example, one or more marker shots can be fired to bleed
off energy so that a later targeted laser pulse shot (or set of
targeted shots) exhibits a desired amount of energy. As an example
embodiment, the marker shots can be used to bleed off energy so
that the targeted laser pulse shots exhibit consistent energy
levels despite a variable rate of firing for the targeted laser
pulse shots (e.g., so that the targeted laser pulse shots will
exhibit X units of energy (plus or minus some tolerance) even if
those targeted laser pulse shots are irregularly spaced in time).
The control circuit 106 can consult the laser energy model 108 to
determine when such marker shots should be fired to regulate the
targeted laser pulse shots in this manner.
Modeling Eye and Camera Safety Over Time:
[0161] FIG. 10 depicts an example process flow for execution by the
control circuit 106 where eye safety requirements are also used to
define or adjust the shot list. To support these operations, the
control circuit 106 can also, at step 1000, maintain an eye safety
model 1002. Eye safety requirements for a lidar transmitter 100 may
be established to define a maximum amount of energy that can be
delivered within a defined spatial area in the field of view over a
defined time period. Since the system is able to model per pulse
laser energy with respect to precisely targeted range points over
highly granular time periods, this allows the control circuit 106
to also monitor whether a shot list portion would violate eye
safety requirements. Thus, the eye safety model 1002 can model how
much aggregated laser energy is delivered to the defined spatial
area over the defined time period based on the modeling produced
from the laser energy model 108 and the mirror motion model 308. At
step 1010, the control circuit 106 uses the eye safety model 1002
to determine whether the modeled laser energy that would result
from a simulated sequence of shots would violate the eye safety
requirements. If so, the control circuit can adjust the shot list
to comply with the eye safety requirements (e.g., by inserting
longer delays between ordered shots delivered close in space, by
re-ordering the shots, etc.)
[0162] FIG. 11 shows an example lidar transmitter 100 that is
similar in nature to the example of FIG. 8, but where the system
controller 800 also considers the eye safety model 1002 when
deciding on how to order the shot angles. FIG. 12 shows how the
simulation step 702 from FIG. 7A can be performed in example
embodiments where the eye safety model 1002 is used. As shown by
FIG. 12, each parallel path can include steps 720, 722, and 724 as
discussed above. Each parallel path can also include a step 1200 to
be performed prior to step 722 where the control circuit 106 uses
the eye safety model 1002 to test whether the modeled laser energy
for the subject time slot sequence would violate eye safety
requirements. If the subject time slot sequence complies with the
criteria tested at steps 720 and 1200, then the subject time slot
sequence can be labeled as valid. If the subject time slot sequence
violates the criteria tested at steps 720 or 1200, then the subject
time slot sequence can be labeled as invalid.
[0163] Similar to the techniques described for eye safety in
connection with Figured 10, 11, and 12, it should be understood
that a practitioner can also use the control circuit to model and
evaluate whether time slot sequences would violate defined camera
safety requirements. To reduce the risk of laser pulses 122
impacting on and damaging cameras in the field of view, the control
circuit can also employ a camera safety model in a similar manner
and toward similar ends as the eye safety model 1002. In the camera
safety scenario, the control circuit 106 can respond to detections
of objects classified as cameras in the field of view by monitoring
how much aggregated laser energy will impact that camera object
over time. If the model indicates that the camera object would have
too much laser energy incident on it in too short of a time period,
the control circuit can adjust the shot list as appropriate.
[0164] Moreover, as noted above with respect to the laser energy
model 108 and the mirror motion model 308, the eye safety and
camera safety models can track aggregated energy delivered to
defined spatial areas over defined time periods over short time
intervals, and such short interval eye safety and camera safety
models can be referred to as transient eye safety and camera safety
models.
Additional Example Embodiments:
[0165] FIG. 13 shows another example of a process flow for the
control circuit 106 with respect to using the models to dynamically
determine the shot list for the transmitter 100.
[0166] At step 1300, the laser energy model 108 and mirror motion
model 308 are established. This can include determining from
factory or calibration the values to be used in the models for
parameters such as E.sub.P, a, b, and A. Step 1300 can also include
establishing the eye safety model 1002 by defining values for
parameters that govern such a model (e.g. parameters indicative of
limits for aggregated energy for a defined spatial area over a
defined time period). At step 1302, the control law for the system
is connected to the models established at step 1300.
[0167] At step 1304, the seed energy model used by the laser energy
model 108 is adjusted to account for nonlinearities. This can
employ the clipped, offset (affine) model for seed energy as
discussed above.
[0168] At step 1306, the laser energy model 108 can be updated
based on lidar return data and other feedback from the system. For
example, as noted above in connection with FIG. 2D, the actual
energies in laser pulses 122 can be derived from the pulse return
data included in point cloud 256. For example, the pulse return
energy can be modeled as a function of the transmitted pulse energy
according to the following expression (for returns from objects
that are equal to or exceed the laser spot size and assuming modest
atmospheric attenuation):
Pulse .times. Return .times. Energy = ( PEAperture R .times. e
.times. ceiver .pi. .times. R 2 ) .times. Reflectivity
##EQU00007##
[0169] In this expression, Pulse Return Energy represents the
energy of the pulse return (which is known from the point cloud
256), PE represents the unknown energy of the transmitted laser
pulse 122, Aperture.sub.Receiver represents the known aperture of
the lidar receiver (see 1400 in FIG. 14), R represents the measured
range for the return (which is known from the point cloud 256), and
Reflectivity represents the percentage of reflectivity for the
object from which the return was received. Therefore, one can solve
for PE so long as the reflectivity is known. This will be the case
for objects like road signs whose reflectivity is governed by
regulatory agencies. Accordingly, by using returns from known
fiducials such as road signs, the control circuit 106 can derive
the actual energy of the transmitted laser pulse 122 and use this
value to facilitate determinations as to whether any adjustments to
the laser energy model 108 are needed (e.g., see discussions above
re updating the values for a and b based on PE values which
represent the actual energies of the transmitted laser pulses
122).
[0170] Also, at step 1308, the laser health can be assessed and
monitored as a background task. The information derived from the
feedback received for steps 1306 and 1308 can be used to update
model parameters as discussed above. For example, as noted above,
the values for the seed energy model parameters as well as the
values for a and b can be updated by measuring the energy produced
by the laser source 102 and fitting the data to the parameters.
Techniques which can be used for this process include least
squares, sample matrix inversion, regression, and multiple
exponential extensions. Further still, as noted above, the amount
of error can be reduced by using known targets with a given
reflectivity and using these to calibrate the system. This is
helpful because the reflectivity of a quantity that is known, i.e.
a fiducial, allows one to explicitly extract shot energy (after
backing out range dependencies and any obliquity). Examples of
fiducials that may be employed include road signs and license
plates.
[0171] At step 1310, the lidar return data and the coupled models
can be used to ensure that the laser pulse energy does not exceed
safety levels. These safety levels can include eye safety as well
as camera safety as discussed above. Without step 1310, the system
may need to employ a much more stringent energy requirement using
trial and error to establish laser settings to ensure safety. For
example if we only had a laser model where the shot energy is
accurate to only .+-.3/per shot around the predicted shot, and
maximum shot energy is limited to 8, we could not use any shots
predicted to exceed 5. However, the hyper temporal modeling and
control that is available from the laser energy model 108 and
mirror motion model 308 as discussed herein allows us to obtain
accurate predictions within a few percent error, virtually erasing
the operational lidar impact of margin.
[0172] At step 1312, the coupled models are used with different
orderings of shots, thereby obtaining a predicted shot energy in
any chosen ordered sequence of shots drawn from the specified list
of range points. Step 1312 may employ simulations to predict shot
energies for different time slots of shots as discussed above.
[0173] At step 1314, the system inserts marker shots in the timing
schedule if the models predict that too much energy will build up
in the laser source 102 for a given shot sequence. This reduces the
risk of too much energy being transferred into the fiber laser 116
and causing damage to the fiber laser 116.
[0174] At step 1316, the system determines the shot energy that is
needed to detect targets with each shot. These values can be
specified as a minimum energy threshold for each shot. The value
for such threshold(s) can be determined from radiometric modeling
of the lidar, and the assumed range and reflectivity of a candidate
target. In general, this step can be a combination of modeling
assumptions as well as measurements. For example, we may have
already detected a target, so the system may already know the range
(within some tolerance). Since the energy required for detection is
expected to vary as the square of the range, this knowledge would
permit the system to establish the minimum pulse energy thresholds
so that there will be sufficient energy in the shots to detect the
targets.
[0175] Steps 1318 and 1320 operate to prune the candidate ordering
options based on the energy requirements (e.g., minimum energy
thresholds per shot) (for step 1318) and shot list firing
completion times (to favor valid candidate orderings with faster
completion times) (for step 1320).
[0176] At step 1322, candidate orderings are formed using elevation
movements on both scan directions. This allows the system to
consider taking shots on both a left-to-right scan and a
right-to-left scan. For example, suppose that the range point list
has been completed on a certain elevation, when the mirror is close
to the left hand side. Then it is faster to move the elevation
mirror at that point in time and begin the fresh window of range
points to be scheduled beginning on this same left hand side and
moving right. Conversely, if we deplete the range point list when
the mirror is closer to the right hand side it is faster to move
the mirror in elevation whilst it is on the right hand side.
Moreover, in choosing an order from among the order candidates, and
when moving from one elevation to another, movement on either side
of the mirror motion, the system may move to a new elevation when
mirror 110 is at one of its scan extremes (full left or full
right). However, in instances where a benefit may arise from
changing elevations when mirror 110 is not at one of its scan
extremes, the system may implement interline skipping as described
in the above-referenced and incorporated U.S. Pat. No. 10,078,133.
The mirror motion model 308 can also be adjusted to accommodate
potential elevation shift during a horizontal scan.
[0177] At step 1324, if processing time allows the control circuit
106 to implement auctioning (whereby multiple order candidates are
investigated, the lowest "cost" (e.g., fastest lidar execution
time) order candidate is selected by the control circuit 106
(acting as "auctioneer"). A practitioner may not want the control
circuit to consider all of the possible order candidates as this
may be too computationally expensive and introduce an undue amount
of latency. Thus, the control circuit 106 can enforce maximums or
other controls on how many order candidates are considered per
batch of shots to be ordered. Greedy algorithms can be used when
choosing ordering shots. Generally, the system can use a search
depth value (which defines how many shots ahead the control circuit
will evaluate) in this process in a manner that is consistent with
any real time consideration in shot list generation. At step 1326,
delays can be added in the shot sequence to suppress a set of shots
and thus increase available shot energy to enable a finer (denser)
grid as discussed above. The methodology for sorting through
different order candidates can be considered a special case of the
Viterbi algorithm which can be implemented using available software
packages such as Mathworks. This can also be inferred using
equivalence classes or group theoretic methods. Furthermore, if the
system detects that reduced latency is needed, the search depth can
be reduced (see step 1328).
[0178] FIG. 14 depicts an example embodiment for a lidar
transmitter 100 that shows how the system controller 800 can
interact with the lidar receiver 1400 to coordinate system
operations. The lidar receiver 1400 can receive and process pulse
returns 1402 to compute range information for objects in the field
of view impacted by the laser pulses 122. This range information
can then be included in the point cloud 1404 generated by the lidar
system. Examples of suitable technology for use as the lidar
receiver 1400 are described in U.S. Pat. Nos. 9,933,513 and
10,754,015, the entire disclosures of which are incorporated herein
by reference. In the example of FIG. 14, the system controller 800
can use the point cloud 1404 to intelligently select range points
for targeting with laser pulses, as discussed in the
above-referenced and incorporated patents. For example, the point
cloud data can be used to determine ranges for objects in the field
of view that are to be targeted with laser pulses 122. The control
circuit 106 can use this range information to determine desired
energy levels for the laser pulses 122 which will target range
points that are believed to correspond to those objects. In this
fashion, the control circuit 106 can controllably adjust the laser
pulse energy as a function of the estimated range of the object
being targeted so the object is illuminated with a sufficient
amount of light energy given its estimated range to facilitate
adequate detection by the lidar receiver 1400. Further still, the
beam scanner controller 802 can provide shot timing information
1410 to the receiver 1400 and the system controller 800 can provide
shot data 1412 (such as data identifying the targeting range
points) to the receiver 1400. The combination of this information
informs the receiver how to control which pixels of the receiver
1400 should be activated for detecting pulse returns 1402
(including when those pixels should be activated). As discussed in
the above-referenced and incorporated '513 and '015 patents, the
receiver can select pixels for activation to detect pulse returns
1402 based on the locations of the targeted range points in the
field of view. Accordingly, precise knowledge of which range points
were targeted and when those range points were targeted helps
improve the operations of receiver 1400. Although not shown in FIG.
14, it should also be understood that a practitioner may choose to
also include a camera that images the field of view, and this
camera can be optically co-axial (co-bore sighted) with the lidar
transmitter 100. Camera images can also be used to facilitate
intelligent range point selection among other tasks.
[0179] FIG. 15 shows another example of a process flow for the
control circuit 106 with respect to using the models to dynamically
determine the shot list for the transmitter 100. At step 1500, the
laser energy model 108 and mirror motion model 308 are established.
This can operate like step 1300 discussed above. At step 1502, the
model parameters are updated using pulse return statistics (which
may be derived from point cloud 1404 or other information provided
by the receiver 1400) and mirror scan position feedback (e.g., from
feedback system 850). At step 1504, the models are coupled so that
shot angles are assigned to time slots according to the mirror
motion model 308 for which shot energies can be predicted according
to the laser energy model 108. These coupled models can then be
embedded in the shot scheduling logic used by control circuit 106.
At step 1506, a list of range points to be targeted with laser
pulses 122 is received. At step 1508, a selection is made for the
search depth that governs how far ahead the system will schedule
shots.
[0180] Based on the listed range points and the defined search
depth, the order candidates for laser pulse shots are created (step
1510). The mirror motion model 308 can assign time slots to these
order candidates as discussed above. At step 1512, each candidate
is tested using the laser energy model 108. This testing may also
include testing based on the eye safety model 1002 and a camera
safety model. This testing can evaluate the order candidates for
compliance with criteria such as peak energy constraints, eye
safety constraints, camera safety constraints, minimum energy
thresholds, and completion times. If a valid order candidate is
found, the system can fire laser pulses in accordance with the
timing/sequencing defined by the fastest of the valid order
candidates. Otherwise, the process flow can return to step 1510 to
continue the search for a valid order candidate.
[0181] While the invention has been described above in relation to
its example embodiments, various modifications may be made thereto
that still fall within the invention's scope.
[0182] For example, while the example embodiments discussed above
involve a mirror subsystem architecture where the resonant mirror
(mirror 110) is optically upstream from the point-to-point step
mirror (mirror 112), it should be understood that a practitioner
may choose to position the resonant mirror optically downstream
from the point-to-point step mirror.
[0183] As another example, while the example mirror subsystem 104
discussed above employs mirrors 110 and 112 that scan along
orthogonal axes, other architectures for the mirror subsystem 104
may be used. As an example, mirrors 110 and 112 can scan along the
same axis, which can then produce an expanded angular range for the
mirror subsystem 104 along that axis and/or expand the angular rate
of change for the mirror subsystem 104 along that axis. As yet
another example, the mirror subsystem 104 can include only a single
mirror (mirror 110) that scans along a first axis. If there is a
need for the lidar transmitter 100 to also scan along a second
axis, the lidar transmitter 100 could be mechanically adjusted to
change its orientation (e.g., mechanically adjusting the lidar
transmitter 100 as a whole to point at a new elevation while mirror
110 within the lidar transmitter 100 is scanning across
azimuths).
[0184] As yet another example, a practitioner may find it desirable
to drive mirror 110 with a time-varying signal other than a
sinusoidal control signal. In such a circumstance, the practitioner
can adjust the mirror motion model 308 to reflect the time-varying
motion of mirror 110.
[0185] As still another example, it should be understood that the
techniques described herein can be used in non-automotive
applications. For example, a lidar system in accordance with any of
the techniques described herein can be used in vehicles such as
airborne vehicles, whether manned or unmanned (e.g., airplanes,
drones, etc.). Further still, a lidar system in accordance with any
of the techniques described herein need not be deployed in a
vehicle and can be used in any lidar application where there is a
need or desire for hyper temporal control of laser pulses and
associated lidar processing.
[0186] These and other modifications to the invention will be
recognizable upon review of the teachings herein.
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