U.S. patent application number 10/446779 was filed with the patent office on 2004-12-16 for distance measuring apparatus and method using pulsed electromagnetic wave.
This patent application is currently assigned to MATSUSHITA ELECTRIC WORKS, LTD.. Invention is credited to Honda, Tatsuya, Nakamura, Kuninori, Yoshimura, Kazunari.
Application Number | 20040252289 10/446779 |
Document ID | / |
Family ID | 30435662 |
Filed Date | 2004-12-16 |
United States Patent
Application |
20040252289 |
Kind Code |
A1 |
Honda, Tatsuya ; et
al. |
December 16, 2004 |
DISTANCE MEASURING APPARATUS AND METHOD USING PULSED
ELECTROMAGNETIC WAVE
Abstract
Distance measuring apparatus and method using a pulsed
electromagnetic wave for making a high speed precise distance
measurement between the apparatus and an object over a small
distance. The electromagnetic wave is projected to an object from a
single projector to provide a reflected electromagnetic wave from
the object that is received by a single receiver also receiving a
reference electromagnetic wave branched from the projected
electromagnetic wave. The reference-wave reception time at the
receiver is delayed by a delay time, so that a first time period
between the electromagnetic wave projection time and the
reference-wave reception time is longer than a second time period
between the projection time and a reflection-wave reception time at
the receiver. The distance is determined according to the delay
time and the time difference between the reference-wave reception
time and the reflection-wave reception time.
Inventors: |
Honda, Tatsuya; (Osaka-shi,
JP) ; Yoshimura, Kazunari; (Hirakata-shi, JP)
; Nakamura, Kuninori; (Owariasahi-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MATSUSHITA ELECTRIC WORKS,
LTD.
Kadoma-shi
JP
|
Family ID: |
30435662 |
Appl. No.: |
10/446779 |
Filed: |
May 29, 2003 |
Current U.S.
Class: |
356/5.01 |
Current CPC
Class: |
G01S 7/4818 20130101;
G01S 17/14 20200101 |
Class at
Publication: |
356/005.01 |
International
Class: |
G01C 003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 2002 |
JP |
2002-168871 |
Claims
1. A distance measuring apparatus using a pulsed electromagnetic
wave including: a single projector for projecting said
electromagnetic wave to an object; branch means placed between said
projector and said object to obtain a reference wave branched from
said electromagnetic wave; a single receiver for receiving said
reference wave and a reflection wave obtained when said
electromagnetic wave is reflected off said object; delay means
introduced into an optical path for said reference wave extending
from said branch means to said receiver without interacting with
said object to provide a delay time for delaying a reference-wave
reception time, at which said reference wave is received by said
receiver, such that a first time period between said reference-wave
reception time and an electromagnetic-wave projection time, at
which said electromagnetic wave is projected from said projector,
is longer than a second time period between said projection time
and a reflection-wave reception time, at which said reflection wave
is received by said receiver; and a processor for calculating a
time difference between said reference-wave reception time and said
reflection-wave reception time from outputs of said receiver, and
determining a distance between the distance measuring apparatus and
said object according to said time difference and said delay
time.
2. The distance measuring apparatus as set forth in claim 1,
wherein said electromagnetic wave is light, and said delay means is
provided by an optical fiber having a predetermined length.
3. The distance measuring apparatus as set forth in claim 1,
wherein said electromagnetic wave is light, and said delay means is
provided by a plurality of mirrors arranged so as to prolong the
optical path for said reference wave.
4. The distance measuring apparatus as set forth in claim 1,
wherein said delay means includes: a reference-wave receiving
device used only to receive said reference wave from said branch
means; a delay circuit for delaying an output signal provided from
said reference-wave receiving device; and a reference-wave emitting
device for providing an output of said delay circuit to said
receiver.
5. The distance measuring apparatus as set forth in claim 1,
wherein said delay means includes a delay-time adjuster for
changing said delay time.
6. The distance measuring apparatus as set forth in claim 1,
further including temperature compensating means for compensating
for fluctuations of said delay time, which is caused by changes in
ambient temperature.
7. The distance measuring apparatus as set forth in claim 1,
wherein said electromagnetic wave is light, and the distance
measuring apparatus further includes a light-amount adjuster for
adjusting a light amount of at least one of said reflection wave
and said reference wave received by said receiver.
8. The distance measuring apparatus as set forth in claim 1,
wherein said processor allows said projector to make a plurality of
projections of said electromagnetic wave to said object, and
calculates an average time difference between said reference-wave
reception time and said reflection-wave reception time from the
outputs of said receiver provided for each of the plurality of
projections of said electromagnetic wave to determine the distance
according to said average time difference and said delay time.
9. The distance measuring apparatus as set forth in claim 1,
wherein said delay means provides said delay time longer than a
time period required to allow said electromagnetic wave to pass
through double a maximum measurable distance of the distance
measuring apparatus.
10. A distance measuring method using a pulsed electromagnetic
wave, said method comprising the steps of: projecting said
electromagnetic wave to an object from a single projector;
receiving, by a receiver, a reflection wave obtained when said
electromagnetic wave is reflected off said object; receiving, by
the same receiver, a reference wave branched from said
electromagnetic wave before said electromagnetic wave reaches said
object; and determining a distance from said object according to
outputs of said receiver; wherein a reference-wave reception time,
at which said reference wave is received by said receiver, is
delayed by a delay time, so that a first time period between said
reference-wave reception time and an electromagnetic-wave
projection time, at which said electromagnetic wave is projected
from said projector, is longer than a second time period between
said projection time and a reflection-wave reception time, at which
said reflection wave is received by said receiver, and wherein the
distance is determined according to said delay time and a time
difference between said reference-wave reception time and said
reflection-wave reception time.
11. The distance measuring method as set forth in claim 10, wherein
said delay time is changed according to a large or small distance
from said object.
12. The distance measuring method as set forth in claim 11, wherein
a preliminary measurement of roughly determining the distance from
said object is performed by using a provisionally-determined delay
time, and then said delay time used in an actual measurement of
precisely measuring the distance is adjusted according to results
of said preliminary measurement.
13. The distance measuring method as set forth in claim 10, wherein
said electromagnetic wave is projected to said object plural times,
and an average time difference between said reference-wave
reception time and said reflection-wave reception time is
calculated from the outputs of said receiver provided every
projection of said electromagnetic wave, so that the distance is
determined according to said average time difference and said delay
time.
14. The distance measuring method as set forth in claim 10, wherein
a preliminary measurement of roughly determining the distance from
said object is performed by using a provisionally-determined delay
time, and then a light amount of at least one of said reflection
wave and said reference wave received by said receiver in an actual
measurement of precisely measuring the distance is adjusted
according to results of said preliminary measurement
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an improvement of
measurement accuracy in the case of determining a distance from an
object according to Time of Flight Method, and particularly
distance measuring apparatus and method using a pulsed
electromagnetic wave, by which a precise distance measurement can
be performed with a high-speed response even when the distance is
small.
[0003] 2. Disclosure of the Prior Art
[0004] A method of determining a distance from an object according
to a time period between a projection time, at which an pulsed
electromagnetic wave is projected to the object, and a
reflection-wave reception time, at which the electromagnetic wave
reflected from the object is received is known as Time of Flight
Method.
[0005] For example, U.S. Pat. No. 5,054,911 discloses a light wave
distance measuring instrument of the pulse type. According to this
instrument, a part of the light wave projected to an object is used
as a reference light wave, and allowed to pass an optical path
having a predetermined length without through the object. A
reflection wave obtained when the light wave is reflected off the
object and the reference wave are received by a single receiver. A
distance between the object and the instrument is determined
according to a time difference between a reference-wave reception
time, at which the reference wave is received by the receiver, and
a reflection-wave reception time, at which the reflection wave is
received by the same receiver.
[0006] By the way, as shown in FIG. 24, when the distance between
the instrument and the object is small, the reference wave "S" may
be positioned closely adjacent to the reflection wave "R" on the
time axis. In this case, it becomes difficult to clearly separate
the reference wave "S" and the reflection wave "R" from each other,
so that there is a problem that the measurement accuracy lowers. To
avoid such a problem, it was proposed to introduce an optical fiber
having a predetermined length into an optical path for the
reflection wave, and delay the reception of the lo reflection-wave
by a delay time "Td") such that the reflection wave "R" is spaced
away from the reference wave "S" on the time axis, as shown by the
dotted line in FIG. 24. Thereby, the distance can be determined
according to a time difference "Ts-R" between the receptions of the
reflection wave and the reference wave and the delay time "Td"
[0007] However, the reflectivity of the object changes according to
the color of the object surface, i.e., whether the surface color is
black or white. In addition, the reflectivity changes according to
the surface condition of the object, i.e., whether the object
surface is a mirror reflection surface or a diffuse reflection
surface. Therefore, when the reflectivity is small, there is a fear
that a considerable light loss is caused by allowing the reflection
wave to pass through the optical fiber. Needless to say, as the
length of the optical fiber increases, the problem of light loss
becomes more serious. In other hands, it brings about a
deterioration in S/N ratio. For example, to prevent the
deterioration in S/N ratio, it can be proposed to increase the gain
of an amplifier or an amount of the light wave projected to the
object. However, these lead to an increase in cost of the distance
measuring instrument. In addition, there is another problem that
the distance measuring instrument becomes a huge and complex
structure.
SUMMARY OF THE INVENTION
[0008] Therefore, in consideration of the above, a primary object
of the present invention is to provide a distance measuring
apparatus using a pulsed electromagnetic wave, by which a precise
distance measurement can be performed with a high-speed response
even when a distance between the apparatus and an object is
small.
[0009] That is, the distance measuring apparatus includes:
[0010] a single projector for projecting the electromagnetic wave
to an object;
[0011] a branch means placed between the projector and the object
to obtain a reference wave branched from the electromagnetic
wave;
[0012] a single receiver for receiving the reference wave and a
reflection wave obtained when the electromagnetic wave is reflected
off the object;
[0013] a delay means introduced into an optical path for the
reference wave extending from the branch means to the receiver
without through the object to provide a delay time for delaying a
reference-wave reception time, at which the reference wave is
received by the receiver, such that a first time period between the
reference-wave reception time and an electromagnetic-wave
projection time, at which the electromagnetic wave is projected
from the projector, is longer than a second time period between the
projection time and a reflection-wave reception time, at which the
reflection wave is received by the receiver; and
[0014] a processor for calculating a time difference between the
reference-wave reception time and the reflection-wave reception
time from outputs of the receiver, and determining a distance
between the distance measuring apparatus and the object according
to the time difference and the delay time.
[0015] According to the distance measuring apparatus of the present
invention, since the reference-wave reception time is delayed than
the reflection-wave reception time by the delay time, it is
possible to prevent the occurrence of an inconvenience that the
reference wave is overlapped with the reflection wave, as shown in
FIG. 24, so that the distance can not be measured with accuracy. In
particular, since maintaining a high strength of the reference wave
is relatively easy, a light loss of the reference wave caused by
allowing the reference wave to pass through the delay means such as
an optical fiber to delay the reference wave reception time does
not wield a large influence over the accuracy of measuring the
distance.
[0016] When the electromagnetic wave is light, it is preferred that
the delay means is provided by an optical fiber having a
predetermined length or a plurality of mirrors arranged so as to
prolong the optical path for the reference wave. In the case of
using the optical fiber, it is possible to obtain a desired delay
time according to the length of the optical fiber. In the case of
using the mirrors, it is possible to stably provide the delay time
even in the presence of disturbance factors such as changes in
ambient temperature
[0017] In addition, it is preferred that the delay means includes a
reference-wave receiving device used only to receive the reference
wave from the branch means; a delay circuit for delaying an output
signal provided from the reference-wave receiving device; and a
reference-wave emitting device for providing an output of the delay
circuit to the receiver. This embodiment is effective to downsize
the distance measuring apparatus of the present invention.
[0018] It is preferred that the delay means includes a delay-time
adjuster for changing the delay time. In this case, since an
adequate delay time can be selected in accordance with the distance
to be measured, it is possible to accurately determine the distance
with an improved response speed.
[0019] It is also preferred that the distance measuring apparatus
further includes a temperature compensating unit of compensating
for fluctuations of the delay time, which is caused by changes in
ambient temperature. In this case, it is possible to stably
maintain the measurement accuracy without the influence of ambient
temperature.
[0020] When the electromagnetic wave is light, it is preferred that
the distance measuring apparatus further includes a light-amount
adjuster for adjusting a light amount of at least one of the
reflection wave and the reference wave received by the receiver.
Since adjusting the light amount of the reflection wave and/or the
reference wave is effective to reduce noise components, it is
possible to improve the S/N ratio and to more accurately determine
the distance.
[0021] It is preferred that the processor allows the projector to
project the electromagnetic wave to the object plural times, and
calculates an average time difference between the reference-wave
reception time and the reflection-wave reception time from the
outputs of the receiver provided every projection of the
electromagnetic wave to determine the distance according to the
average time difference and the delay time. This embodiment is
particularly useful when it is needed to determine the distance
with high accuracy. In addition, this embodiment is preferably used
when a preliminary measurement of roughly determining the distance
is performed by using a provisionally-determined delay time, and
then the delay time used in an actual measurement of precisely
measuring the distance is adjusted according to results of the
preliminary measurement.
[0022] It is preferred that the delay means provides the delay time
longer than a time period required to allow the electromagnetic
wave to pass through double a maximum measurable distance of the
distance measuring apparatus. When the distance to be measured is
smaller than the maximum measurable distance, it is possible to
certainly separate the reflection wave from the reference wave.
[0023] A further object of the present invention is to provide a
distance measuring method using a pulsed electromagnetic wave, by
which the same advantages described above can be achieved. That is
method comprises the steps of:
[0024] projecting the electromagnetic wave to an object from a
single projector;
[0025] receiving, by a receiver, a reflection wave obtained when
the electromagnetic wave is reflected off the object;
[0026] receiving, by the same receiver, a reference wave branched
from the electromagnetic wave before the electromagnetic wave
reaches the object; and
[0027] determining a distance from the object according to outputs
of the receiver;
[0028] wherein the method is characterized in that a reference-wave
reception time, at which the reference wave is received by the
receiver, is delayed by a delay time, so that a first time period
between an electromagnetic-wave projection time, at which the
electromagnetic wave is emitted from the projector, and the
reference-wave reception time is longer than a second time period
between the projection time and a reflection-wave reception time,
at which the reflection wave is received by the receiver, and the
distance from the object is determined according to the delay time
and a time difference between the reference-wave reception time and
the reflection-wave reception time.
[0029] In the above method, it is preferred that the delay time is
changed according to a large or small distance from the object. By
selecting an adequate delay time, it is possible to efficiently
perform the distance measurement.
[0030] In addition, it is preferred that a preliminary measurement
of roughly determining the distance from the object is performed by
using a provisionally-determined delay time, and then the delay
time used in an actual measurement of precisely measuring the
distance is adjusted according to results of the preliminary
measurement. Since the results of the preliminary measurement are
useful to select the adequate delay time, it is possible to
smoothly proceed the actual measurement with high accuracy. In
particular, when the electromagnetic wave is projected to the
object plural times in the actual measurement, and an average time
difference between the reference-wave reception time and the
reflection-wave reception time is calculated from the outputs of
the receiver provided every projection of the electromagnetic wave,
so that the distance is determined according to the average time
difference and the delay time, it is recommend performing the
preliminary measurement.
[0031] Moreover, it is preferred that a preliminary measurement of
roughly determining the distance from the object is performed by
using a provisionally-determined delay time, and then a light
amount of at least one of the reflection wave and the reference
wave received by the receiver in an actual measurement of precisely
measuring the distance is adjusted according to results of the
preliminary measurement. By selecting the adequate light amount of
the reflection wave and/or the reference wave, it is possible to
determine the distance with an improved S/N ratio.
[0032] These and still other objects and advantages of the present
invention will become more apparent from preferred embodiments of
the invention explained below, referring to the attached
drawings.
BRIEF EXPLANATION OF THE ATTACHED DRAWINGS
[0033] FIG. 1 is a block diagram of a distance measuring apparatus
according to a preferred embodiment of the present invention;
[0034] FIGS. 2A and 2B show schematic diagrams illustrating the
principal of a distance measuring method of the present
invention;
[0035] FIG. 3 is a flow chart of the distance measuring method;
[0036] FIGS. 4A and 4B are explanatory diagrams illustrating
examples of delay means;
[0037] FIG. 5 is a block diagram of a distance measuring apparatus
with electrical delay means according to another embodiment of the
present invention;
[0038] FIG. 6 is a block diagram of a distance measuring apparatus
with a temperature compensating unit according to another
embodiment of the present invention;
[0039] FIG. 7 is a block diagram of a distance measuring apparatus
according to another embodiment of the present invention;
[0040] FIGS. 8A to 8C are explanatory diagrams illustrating
examples of the temperature compensating unit;
[0041] FIG. 9 is a block diagram of a distance measuring apparatus
with another temperature compensating unit;
[0042] FIG. 10 is a flow chart of a distance measuring method
according to a further preferred embodiment of the present
invention;
[0043] FIG. 11 is an explanatory diagram illustrating an example of
a delay time adjuster;
[0044] FIG. 12 is an explanatory diagram illustrating a further
example of the delay time adjuster;
[0045] FIGS. 13A to 13C are explanatory diagrams illustrating
another example of the delay time adjuster;
[0046] FIG. 14 is a flow chart of a distance measuring method
according to another preferred embodiment of the present
invention;
[0047] FIGS. 15A and 15B are explanatory diagrams illustrating an
example of a light amount adjuster;
[0048] FIGS. 16 is an explanatory diagram illustrating a further
example of the light amount adjuster;
[0049] FIGS. 17 is an explanatory diagram illustrating another
example of the light amount adjuster;
[0050] FIG. 18 is a block diagram of a distance measuring apparatus
according to another embodiment of the present invention;
[0051] FIGS. 19A and 19B are a block diagram of a distance
measuring apparatus using an optical switch and a waveguide
according to the present invention, and a time chart showing an
operation of the same apparatus, respectively;
[0052] FIG. 20 is a time chart showing an operation of a distance
measuring apparatus using a gate "G" according to a preferred
embodiment of the present invention
[0053] FIG. 21 is a flow chart of the operation of the distance
measuring apparatus;
[0054] FIG. 22 is a time chart showing an operation of a distance
measuring apparatus using a pair of gates "GO" and "GI" according
to another preferred embodiment of the present invention
[0055] FIG. 23 is a flow chart of the operation of the distance
measuring apparatus; and
[0056] FIG. 24 is a schematic diagram illustrating the principal of
a conventional distance measuring method.
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0057] According to the following preferred embodiments, distance
measuring apparatus and method of the present invention are
explained in detail. However, needless to say, the present
invention is not limited to these embodiments.
[0058] FIG. 1 shows a distance measuring apparatus of this
embodiment. The numeral 1 designates a projector such as a
semiconductor laser for emitting a pulsed laser light according to
an output of a drive circuit 7. A part of the emitted laser light
is branched off as a reference wave by branch means such as a beam
splitter 10, and the balance passes through the beam splitter. The
laser light passed through the beam splitter 10 is reflected by an
object 9 to provide a reflection wave. The reflection wave is
received by a receiver 2 such as a photodiode through a lens (not
shown). On the other hand, the reference wave is received by the
same receiver 2 through an optical fiber 8 used as delay means
explained later. Thus, the reference and reflection waves are
obtained from the pulsed laser light provided by the single
projector 1, and received at different timings by the single
receiver 2.
[0059] An amplifier 3 amplifies outputs of the receiver 2. The
amplified outputs are then sent to a mask unit 4 for selectively
generating a reference-wave separation mask and a reflection-wave
separation mask. When the mask unit 4 generates the reference-wave
separation mask, only signal components corresponding to the
reference wave can pass through the reference-wave separation mask.
On the contrary, when the masking unit 4 generates the
reflection-wave separation mask, only signal components
corresponding to the reflection wave can pass through the
reflection-wave separation mask. The generations of the
reference-wave separation mask and the reflection-wave separation
mask can be switched by use of an analog switch or a gate
circuit.
[0060] Outputs of the mask unit 4 are input in a counting circuit
5. The counting circuit 5 has a clock function. That is, the
counting circuit 5 measures, within a period of generating the
reference-wave separation mask, a first time period (T1) between an
laser projection time, at which the laser light is projected from
the projector 1, and a reference-wave reception time, at which the
reference wave is received by the receiver 2, and within a period
of generating the reflection-wave separation mask, a second time
period (T2) between the laser projection time and a reflection-wave
reception time, at which the reflection wave is received by the
receiver 2. The selection between the reference-wave separation
mask and the reflection-wave separation mask can be controlled by
the counting circuit 5.
[0061] By the way, when the distance between the apparatus and the
object 9 is small, a reference pulse wave S is positioned closely
adjacent to a reflection pulse wave R on the time axis, as shown in
FIG. 2A. Therefore, it is difficult to separate the reflection
pulse wave S and the reference pulse wave R from each other. In
this embodiment, an optical fiber 8 is introduced into an optical
path for the reference wave extending from the beam splitter 10 to
the receiver 2 without through the object 9. Since the optical
fiber 8 has a length sufficiently larger than the distance between
the apparatus and the object 9, the reference-wave reception time
is delayed than the reflection-wave reception time. Thus, the
optical fiber 8 functions as the delay means for providing a delay
time (Td) for delaying the reference-wave reception time, as shown
in FIG. 2B, such that the first time period (T1) between the
reference-wave reception time and the laser-light projection time
is longer than the second time period (T2) between the laser-light
projection time and the reflection-wave reception time. By
introducing the optical fiber 8 as the delay means, it is possible
to prevent a situation that the reflection pulse wave R is
overlapped with the reference pulse wave S, and therefore
accurately determine the distance even when the distance between
the apparatus and the object is relatively small.
[0062] The first time period (T1) is equal to a sum of a time
component (Ts) such as a response time of the receiver 2 and the
above-described delay time (Td), i.e., Ts+Td. The second time
period (T2) is equal to a sum of the time component (Ts) and a time
component (TL) that changes in accordance with the distance between
the apparatus and the object 9, i.e., Ts+TL. As understood from
these, a difference between the reflection-wave reception time and
the reference-wave reception time (T1-T2) is equal to "Td-TL".
Therefore, the time component TL can be determined by
"Td-(T1-T2)".
[0063] To more improve the measurement accuracy, it is preferred
that the above procedures for measuring the first and second time
periods are repeated at a specified number of times (N). For
example, when the specified number of times (N) are 10000, the
measurement of each of the first and second time periods is
performed 10000 times, so that a first accumulated time (S1)
corresponding to a total of the first time periods measured 10000
times and a second accumulated time (S2) corresponding to a total
of the second time periods measured 10000 times are obtained. The
first and second accumulated times measured by the counting circuit
5 are sent to a processor 6. In the processor 6, the distance
between the apparatus and the object 9 is determined according to
the first and second accumulated times (S1, S2), the specified
number of times (N), and the delay time (Td). That is, the distance
can be accurately determined according to the delay time (Td) and
an average time difference between the reference-wave reception
time and the reflection-wave reception time, which is obtained by
calculating (S1-S2)/N.
[0064] FIG. 3 is a schematic flowchart of the above-explained
distance measuring method. Mask unit 4, counting circuit 5,
processor 6, and a drive circuit 7 for controlling the emission
timing of a pulsed laser light from a projector 2, which were
disclosed in Japan Patent Application No. 2001-124855 A, can be
used for- the distance measuring apparatus of the present
invention.
[0065] By the way, when the optical fiber 8 having a fixed length
is used as the delay means, and the distance between the apparatus
and the object 9 is nearly equal to the fixed length of the optical
fiber, there is a fear that the reference pulse wave is overlapped
with the reflection pulse wave on the time axis. In such a case, it
is preferred that the entire length of the optical fiber 8 is
larger than double a maximum measurable distance of the distance
measuring apparatus. In other words, it is preferred that the
entire length of the optical fiber 8 is determined to provide the
delay time longer than a time period required to allow the laser
light to pass through double the maximum measurable distance of the
distance measuring apparatus
[0066] On the other hand, when the maximum measurable distance is
relatively long, an extended length of the optical fiber 8 may lead
to an increase in size of the distance measuring apparatus. In such
a case, it is preferred to use a switching mechanism for the delay
means of removing the optical fiber 8 from the light path when the
reference pulse wave is overlapped with the reflection pulse wave,
or another delay means having a delay time adjuster described
later.
[0067] In the case of using the optical fiber 8 as the delay means,
it is preferred to use an optical fiber of an optical material
having an extremely small thermal expansion coefficient such as
quartz. It is possible to minimize the influence of ambient
temperature. In addition, as the optical fiber, "SI" type and "GI"
type are mainly known. In the present invention, it is preferred to
use the "GI" type optical fiber to accurately set the delay
time.
[0068] Alternatively, as the delay means 8, it is possible to use a
plurality of mirrors 80 arranged so as to prolong the optical path
for the reference wave, or a waveguide, as shown in FIGS. 4A and
4B. In addition, as shown in FIG. 5, it is preferred that the delay
means is an electrical delay unit comprising a reference-wave
receiving device 81 used only to receive the reference wave from
the beam splitter 10, a delay circuit 82 for delaying an output
signal provided from the reference-wave receiving device by a
required time period; and a reference-wave emitting device 83 for
providing an output of the delay circuit to the receiver 2.
[0069] When the optical fiber 8 is used as the delay means, the
length of the optical fiber fluctuates in accordance with thermal
expansion and constriction of the optical fiber caused by changes
in ambient temperature, so that there is a fear that a change in
the delay time (Td) has an influence on the measurement accuracy.
In such a case, as shown in FIG. 6, it is preferred to use a
compensation circuit 12 for compensating for the delay time (Td)
according to the temperature value detected by a temperature
detector 11. The influence of ambient temperature can be
compensated in a real-time manner.
[0070] For example, as shown in FIGS. 7 and 8A, a change in length
of the optical fiber 8 caused by the thermal expansion and
constriction can be also compensated by use of a distance adjuster
13 of controlling a distance between an end of the optical fiber 8
and the receiver 2 in accordance with the temperature value
detected by the temperature detector 11. Alternatively, as shown in
FIG. 8B, the change in length of the optical fiber 8 caused by the
thermal expansion and constriction may be compensated by inserting
a required one of translucent members (e.g., 14a, 14b, 14c) having
different lengths in a space between the end of the optical fiber 8
and the receiver 2 in accordance with the temperature value
detected by the temperature detector 11. In addition, as shown in
FIG. 8C, the change in length of the optical fiber 8 caused by the
thermal expansion and constriction may be compensated by inserting
a required one of delay members (e.g., 15a, 15b, 15c) having
different refraction indexes in the space between the end of the
optical fiber 8 and the receiver 2 in accordance with the
temperature value detected by the temperature detector 11. When the
reference-wave reception time is electrically delayed, as explained
above with reference to FIG. 5, it is preferred to fine adjust the
delay time in accordance with the detected temperature value.
[0071] It is also effective to provide a temperature compensation
unit for the photodiode used as the receiver 2. For example, in an
embodiment shown in FIG. 9, a temperature detector (i.e.,
temperature sensor 11) detects the ambient temperature to output an
analog data. This analog data is sent to a microcomputer (i.e.,
controller) through an A/D converter. A compensation voltage value
output from the microcomputer is converted again into an analog
data by a D/A converter, and then applied to a compensation power
supply. The compensation power supply changes a voltage value for
the photodiode in accordance with the applied voltage value.
[0072] By the way, when a series of procedures of the projection of
the pulsed electromagnetic wave, the reception of the reflection
wave R from the object 9, and the reception of the reference wave S
are repeated the specific number of times (N) under a condition
that a relatively long delay time is set, there is a case that the
total time required to determine the distance between the apparatus
and the object becomes long. In such a case, it is preferred that
the delay means 8 has a delay-time adjuster for changing the delay
time (Td), and as shown in FIG. 10, a preliminary measurement of
roughly determining the distance between the apparatus and the
object is performed by using a provisionally-determined delay time,
and then the delay time used in an actual measurement of precisely
measuring the distance is adjusted by use of the delay-time
adjuster according to results of the preliminary measurement.
[0073] It is preferred that the delay time used in the preliminary
measurement is longer than a time period required to allow the
laser-light to pass through double the maximum measurable distance
of the distance measuring apparatus. Thereby, in most cases, the
delay time for the actual measurement becomes shorter than the
delay time for the preliminary measurement. As a result, it is
possible to more efficiently determine the distance while
maintaining the measurement accuracy, as compared with the case of
determining the distance by use of the relatively long delay time
without the preliminary measurement.
[0074] In FIG. 10, the series of procedures of the projection of
the pulsed electromagnetic wave, the reception of the reflection
wave R, and the reception of the reference wave S are repeated the
specific number of times (N) in both of the preliminary measurement
and actual measurement. In this case, it is preferred that the
specific number of times (Np) in the preliminary measurement is
smaller than the specific number of times (Na) in the actual
measurement to reduce the total time required to determine the
distance. Alternatively, the series of procedures described above
may be repeated only in the actual measurement.
[0075] When a repetition timing of projecting the pulsed
electromagnetic wave from the projector 1 is constant, the total
time required for determining the distance can not be reduced by
changing the length of the delay time. In such a case, it is
preferred that the signal components corresponding to the reference
wave, which output from the reference-wave separation mask of the
mask unit 4, or the signal components corresponding to the
reflection wave, which output from the reflection-wave separation
mask of the mask unit 4, are used for a trigger for providing the
repetition timing of projecting of the pulsed electromagnetic wave.
In addition, the trigger is not limited to the above. For example,
the reception of the reference wave by the receiver 2 may be used
as the trigger. In this case, when the series of procedures of the
projection of the pulsed electromagnetic wave, the reception of the
reflection wave R, and the reception of the reference wave S are
repeated the specific number of times (N), each of the projections
of the pulsed electromagnetic wave to the object 9 is performed
immediately when the receiver 2 receives the reference wave derived
from a previously projected electromagnetic wave. Alternatively, as
the repetition timing, the pulsed electromagnetic wave may be
projected after a lapse of a predetermined time period from the
reception of the reference wave by the receiver 2.
[0076] As an example, electrical delay unit 60 for providing the
trigger is shown in FIG. 9. The delay unit 60 receives a
reference-wave detection signal from a signal separator 61 for
separating the reference wave and reflection wave received by the
receiver 2 from each other, and then allows the projector 1 to
project the pulse electromagnetic wave according to the passage of
a predetermined time period from the reception of the
reference-wave detection signal. In the figure, the numeral 63
designates a counter for counting the repetition number of
times.
[0077] For example, as shown in FIG. 11, it is preferred that the
delay means 8 has the capability of changing the delay time (Td),
which is composed of a plurality of optical fibers having different
lengths and a waveguide 85 as a splitter. In addition, as shown in
FIG. 12, the delay means 8 having the capability of changing the
delay time (Td) may be provided by an optical-path switching unit
86 such as an optical switch and a plurality of waveguides 85.
Alternatively, as shown in FIGS. 13A to 13C, a waveguide 85 having
built-in optical switches 87 may be used.
[0078] As shown in FIG, 14, it is preferred that a light-amount
adjuster 90 for adjusting the light amount of the reference wave is
formed, and the light amount of the reference wave is automatically
adjusted by the light-amount adjuster 90 in accordance with a
signal level of the received reflection wave in the preliminary
measurement (or a distance between the apparatus and the object).
In this case, since the signal level of the received reference wave
is matched with the signal level of the received reflection wave in
the actual measurement, it is possible to reduce the noise
components generated around the reference pulse wave, and prevent
recognizing the noise components as the reference wave by mistake.
In addition, it is possible to more stably perform the distance
measurement by matching the signal levels between the reference
wave and the reflection wave.
[0079] For example, as shown in FIGS. 15A and 15B, when a pair of
waveguides 85a, 85b are placed as the delay means 8 in series in
the optical path for the reference wave extending from the beam
splitter 10 to the receiver 2, and a stage having the waveguide 85a
is slidably moved against the stage having the waveguide 85b by a
motor, 91, it is possible to adjust the light amount by changing a
contact area between ends of the waveguides 85a and 85b. In the
figure, the numeral 92 designates a high resolution encoder. A
travel amount of the stage having the waveguide 85a moved by the
motor 91 is controlled by the processor 6 and the drive circuit
7.
[0080] In addition, according to the light amount adjuster 90 shown
in FIG. 16, since a distance between the projector 1 and an end of
the optical fiber 8 used as the delay means is adjusted by use of a
motor 94, and a shield screen 95, which is movable by use of a
motor 91 to control the light amount received by the receiver 2, is
placed in front of the receiver 2, it is possible to adjust a light
amount of the reflection wave as well as the light amount of the
reference wave. Alternatively, as shown in FIG. 17, a liquid
crystal filter 96 may be used as the light amount adjuster 90.
[0081] To reduce noise components included in the reference wave
and the reflection wave, as shown in FIG. 18, it is preferred that
a band pass filter 97, through which only the pulsed
electromagnetic wave of a specific wavelength used to determine the
distance is allowed to pass, is placed in front of the receiver
2.
[0082] As the countermeasure against the noise components, it is
useful to mask the light that comes in the receiver 2. For example,
as shown in FIG. 19, to mask the reference wave, when a pair of
gates "GI" and "GO" such as optical switches are respectively
arranged at light incoming and outgoing portions of the delay means
8, these gates can be opened at the timing of passing the reference
wave. Only one of the gates "GI" and "GO" may be formed.
[0083] In the case of performing the preliminary and actual
measurements, it is preferred that a gate "G" is formed for the
reflection wave. In this case, the gate "G" can be opened at the
timing of passing the reflection wave. Since the timing of passing
the reflection wave changes according to the distance between the
apparatus and the object, it is preferred that the gate "G" is
always opened during the preliminary measurement, and in the actual
measurement, the gate "G" is opened only for a predetermined time
period around the reflection-wave reception time obtained by the
preliminary measurement, as shown in FIG. 20. FIG. 21 is a flow
chart of this case.
[0084] The reference wave and the reflection wave may be allowed to
pass through the same gate "GO". FIG. 22 shows timings of opening
and closing the gates "GI" and "GO" in the case of allowing the
reference wave and the reflection wave to pass through the same
gate "GO". In addition, a flow chart of this case is shown in FIG.
23. As the gate "GO", it is preferred to use an optical switch for
selectively introducing the reference wave and the reflection wave
into the receiver 2.
[0085] In the above preferred embodiments, the pulsed laser light
was used as the pulsed electromagnetic wave. Alternatively, a
pulsed electromagnetic wave having a different wavelength from the
laser light may be used.
[0086] In addition, to avoid the influence of noise components, it
is preferred that a housing of the distance measuring apparatus is
made of a resin material. In this case, it is possible to reduce
the influence of induction noise generated between electric parts,
particularly parts for a high-voltage power supply, and a board for
mounting those electric parts.
* * * * *