U.S. patent application number 11/157962 was filed with the patent office on 2005-10-20 for ranging apparatus, ranging method, and opto-electric conversion circuit.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Inaba, Naoto, Nagasawa, Masaya.
Application Number | 20050231709 11/157962 |
Document ID | / |
Family ID | 27531878 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050231709 |
Kind Code |
A1 |
Inaba, Naoto ; et
al. |
October 20, 2005 |
Ranging apparatus, ranging method, and opto-electric conversion
circuit
Abstract
A ranging apparatus 1 comprises a laser light emitter 3 for
emitting pulsed laser light, a reflected light receiver 4 for
receiving reflected light, a distance computer 10 for finding the
distance from the elapsed time until the reflected light is
received, and a distance display 8 for displaying this distance.
The distance computer 10 has a counter 11 for counting the
frequency when the reflected light satisfies a specific condition,
a table production component 12 for producing a frequency
distribution table corresponding to distance by adding up the
counts, a distance determiner 13 for determining as the distance to
the object of measurement the point when the frequencies in the
frequency distribution table exceed a threshold, and a distance
selector 15 for selecting a specific distance when a plurality of
distances are determined, and displaying this distance on the
distance display 8. As a result, when a plurality of distances are
calculated, these distances can be appropriately displayed,
improving the flexibility and functionality.
Inventors: |
Inaba, Naoto;
(Hiratsuka-shi, JP) ; Nagasawa, Masaya; (Tokyo,
JP) |
Correspondence
Address: |
MORGAN LEWIS & BOCKIUS LLP
1111 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20004
US
|
Assignee: |
NIKON CORPORATION
|
Family ID: |
27531878 |
Appl. No.: |
11/157962 |
Filed: |
June 22, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11157962 |
Jun 22, 2005 |
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10680174 |
Oct 8, 2003 |
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6934012 |
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10680174 |
Oct 8, 2003 |
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PCT/JP02/04122 |
Apr 25, 2002 |
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Current U.S.
Class: |
356/5.01 ;
356/5.02; 356/5.08 |
Current CPC
Class: |
G01S 17/18 20200101;
G01S 7/4873 20130101; G01S 17/10 20130101 |
Class at
Publication: |
356/005.01 ;
356/005.08; 356/005.02 |
International
Class: |
G01C 003/08; G03B
013/00; G03B 015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2001 |
JP |
2001-127551 |
May 1, 2001 |
JP |
2001-133759 |
May 1, 2001 |
JP |
2001-133760 |
May 1, 2001 |
JP |
2001-133761 |
May 1, 2001 |
JP |
2001-133762 |
Claims
1-42. (canceled)
43. An opto-electric conversion circuit in which an avalanche
photodiode is used, wherein this opto-electric conversion circuit
comprises: a reverse bias voltage regulating component for
regulating the reverse bias voltage applied to the above-mentioned
avalanche photodiode; a current measurement component for measuring
the current flowing to the above-mentioned avalanche photodiode; a
reference reverse bias voltage detecting component for regulating
the above-mentioned reverse bias voltage and detecting the reverse
bias voltage at which a specific current flows to the
above-mentioned avalanche photodiode (reference reverse bias
voltage); and a reverse bias voltage setting component for
adjusting the reverse bias voltage applied to the above-mentioned
avalanche photodiode during opto-electric conversion to a voltage
obtained by multiplying the above-mentioned detected reference
reverse bias voltage by a specific ratio.
44. A laser ranging apparatus which measures the distance to an
object of measurement by radiating laser light toward the object of
measurement and measuring the time differential between the point
when the laser light is radiated and the point when the laser light
reflected back from the object of measurement is received, wherein
the circuit for detecting the receipt of laser light reflected back
from the object of measurement has the opto-electric conversion
circuit according to claim 43.
45. The ranging apparatus according to claim 44, wherein the
above-mentioned reverse bias voltage setting component is actuated
before the start of every measurement or every time the power to
the apparatus is switched on.
Description
[0001] The present application is a continuation of PCT
International Application No. PCT/JP02/04122 filed Apr. 25, 2002,
which is hereby incorporated by reference.
TECHNICAL FIELD
[0002] This invention relates to a ranging apparatus and ranging
method with which laser light or the like is used to measure the
distance to an object of measurement by a non-contact method, and
to an opto-electric conversion circuit that can be used to
advantage in such a ranging apparatus.
BACKGROUND ART
[0003] In conventionally known ranging apparatus and methods of
this kind, pulsed measurement light (such as laser light) is
emitted toward the object of measurement, the time it takes for the
light to be reflected back from the object of measurement and
received is measured, and the distance to the object of measurement
is calculated from this elapsed time and the propagation speed of
the laser light. However, when an object of measurement is thus
irradiated with pulsed laser light and the light reflected back
from the object of measurement is then received, what is received
is not only the reflected laser light, but also natural light and
so forth, which becomes noise light. This noise light is difficult
to distinguish from the light reflected back from the object of
measurement, so the problem is that it is difficult to measure
distances accurately.
[0004] When ranging is performed in this way, as long as there is
no change in the position of the object of measurement, the
reflected light from the object of measurement is always received
in a fixed length of time after the emission of the measurement
light, but the timing at which noise light is received is random.
In view of this, a method has been proposed whereby a frequency
count is performed corresponding to distance (or to elapsed time)
when the pulsed measurement light is emitted toward the object of
measurement and the reflected light satisfies a specific condition
for each emission, the frequencies counted for all of the
measurement light emissions carried out repeatedly are added up to
produce a frequency distribution table (histogram) corresponding to
distance, and the distance at which the total count in this
frequency distribution table is at its maximum is considered to be
the distance to the object of measurement.
[0005] In the frequency distribution table produced as above, the
timing at which the reflected light from the object of measurement
is received is always constant, and the count is relatively high at
the distance (or elapsed time) indicating this position. However,
since the timing at which noise light is received is random, a
frequency count corresponding to variously changing distances (or
elapsed times) is performed for every frequency count carried out
repeatedly, and the summed count is relatively low at the various
distances (or elapsed times) in the frequency distribution table.
Accordingly, if the distance corresponding to the point when a
frequency increases in the frequency distribution table produced as
described above (such as when it exceeds a specific threshold) is
used as the distance to the object of measurement, the distance can
be measured accurately by eliminating the effect of randomly
occurring noise light.
[0006] Unfortunately, however, the following problems are
encountered with this ranging method.
[0007] The first problem is as follows: Specifically, when the
object of measurement is large and all of the laser light emitted
from the ranging apparatus irradiates the object of measurement, it
seems sufficient to calculate the distance corresponding to the
point showing a high count in the frequency distribution table, but
when the object of measurement is relatively small and the laser
light also irradiates the area surrounding the object of
measurement, so that reflected light from surrounding objects also
comes back, or when there are a plurality of objects of measurement
at varying distances within the laser light irradiation field of
the ranging apparatus, so that reflected light comes back from each
of the objects of measurement, there are a plurality of points
showing high counts in the frequency distribution table. In cases
such as these, a plurality of distances are calculated, but the
issues of how to handle the plurality of distances and how to
display them greatly affect the flexibility, functionality, and so
forth of the ranging apparatus.
[0008] The second problem is as follows: Specifically, when the
distance to the object of measurement is measured by irradiating
the object of measurement with laser light through window glass,
the laser light reflected by the window glass is also always
received corresponding to the distance to this window glass. In
general, the intensity of the light reflected from the window glass
is low, but since the intensity of the reflected light received by
a light receiver is greater for near objects than for objects
farther away, the reflected light intensity detected by the light
receiver is such that the reflected light from the object of
measurement, which is farther away, is not readily discernable from
the reflected light from the nearer window glass, so both of these
may end up being counted, or just the reflected light from the
window glass may be counted. In a case such as this, there is the
danger that the count corresponding to the distance of the window
glass will increase in the frequency distribution table, so that
the distance corresponding to the position of the window glass will
be mistakenly determined as the distance to the object of
measurement. Similarly, if tree branches or the like are in front
of the object, the reflected light from the tree branches is
received, and these branches may end up being mistakenly judged to
be the object of measurement.
[0009] The third problem is as follows: Namely, when a distance is
measured by looking at the object of measurement through window
glass, or when a distance is measured by looking at the object of
measurement through tree branches, the reflected light from the
window glass, tree branches, or the like, located in front of the
object of measurement is also constantly received. Consequently,
there is the danger that the frequency corresponding to the
distance of these obstacles will be high in the frequency
distribution table, so that the distance thereof will be determined
as the distance to the object of measurement, the result being that
the measured distance to the object of measurement is
inaccurate.
[0010] Furthermore, the position at which the frequency in the
frequency distribution table increases can be affected by the
shaking of the user's hands when the ranging apparatus is held in
the hands during measurement, atmospheric fluctuations in the
measurement environment, and other such effects, which is a problem
in that the measured distance is inconsistent, or frequencies with
an extremely large peak appearing to be noise may occur in the
frequency distribution table, and the direct use of these
frequencies results in incorrect distance measurement. Also, when
the distance to an object of measurement that spreads out
longitudinally is measured, such as when the distance to a building
is measured by looking obliquely at the walls of the building, it
is difficult to determine the distance if the frequency increases
over a wide range of distances.
[0011] The fourth problem is as follows: Specifically, the
reflected light intensity from the object of measurement varies
with the distance to the object of measurement, varies with the
type of object of measurement (this is due to differences in the
reflectivity of the object of measurement itself, for example), and
also varies with the measurement conditions (such as whether the
measurement is conducted in a bright or dark location, and whether
the measurement is conducted under weather conditions that are
clear, cloudy, rainy, foggy, etc.), so the counts in the frequency
distribution table can fluctuate greatly depending on these
factors. Consequently, it is extremely difficult to determine the
level of frequency for the distance in the frequency distribution
table to be taken as the position of the object of measurement.
[0012] In particular, to perform this determination by internal
arithmetic processing in a ranging apparatus, the general practice
is to preset a determination threshold, and automatically determine
as the distance to the object of measurement the distance having a
frequency that exceeds this determination threshold in the
frequency distribution table. In this case, if the preset
determination threshold is too high, there is a concern that no
frequency over this threshold will be found, and the distance to
the object of measurement cannot be specified, but on the other
hand, if the determination threshold is too low, many frequencies
over this threshold may be found, making it impossible to specify
which of these is the distance to the object of measurement.
[0013] Furthermore, in a laser ranging apparatus such as this, the
intensity of the reflected light weakens as the distance to the
object of measurement increases, so high sensitivity is required to
detect faint light, and an extremely short time must also be
detected accurately. Avalanche photodiodes have been used as
opto-electric conversion elements that meet these requirements.
[0014] Thus, avalanche photodiodes are often used when faint light
needs to be detected at high sensitivity (high amplification) and
at a high response rate.
[0015] However, because of their high sensitivity, avalanche
photodiodes also have the drawback of low stability. Specifically,
the proportion of current flowing with respect to light of a given
intensity (referred to as the current multiplication factor) is a
function of the reverse bias voltage being applied. Also, the
current multiplication factor tends to increase sharply as the
applied reverse bias voltage approaches the breakdown voltage.
Thus, in order to raise the light detection sensitivity, it is
preferable to use a voltage close to the breakdown voltage as the
reverse bias voltage to be applied.
[0016] However, temperature affects the breakdown voltage, so if
the reverse bias voltage is close to the breakdown voltage, a
change in the breakdown voltage will greatly alter the current
multiplication factor.
[0017] This situation is illustrated in FIG. 14, which is a graph
of the relationship between the reverse bias voltage and the
current multiplication factor (a value indicating the amount of
current flowing when a given amount of light comes in). When this
relationship is as indicated by the solid line, if the reverse bias
voltage is set at V.sub.0, then the current multiplication factor
is .alpha..sub.0. However, when a change in the breakdown voltage
causes this relationship to shift as indicated by the broken line,
the current multiplication factor changes to .alpha..sub.0'.
[0018] When this happens, if the avalanche photodiodes are used
with a laser ranging apparatus to detect reflected light from an
object of measurement, the output from the detector changes
independently of the amount of reflected light, producing an error
in the measurement timing for received reflected light, and in
extreme cases measurement may not even be possible.
[0019] Accordingly, a device that would keep the temperature
constant within the detector was added to conventional units, or
the avalanche photodiodes were used at a lower current
multiplication factor, so that the avalanche photodiodes would
operate stably. In the former case, the cost of the apparatus
increased by the cost of the device used to keep the temperature
constant, and in the latter case, because there was a limit to the
amount of light that could be detected, the measurable distance
became shorter when the avalanche photodiodes were used with a
laser ranging apparatus.
DISCLOSURE OF THE INVENTION
[0020] The present invention was conceived in an effort to solve
these problems.
[0021] Specifically, the first object of the present invention is
to provide a ranging apparatus and method with excellent
flexibility or functionality for calculating a plurality of
distances as discussed above and displaying these in an appropriate
manner.
[0022] The second object of the present invention is to allow the
distance to an object of measurement to be measured accurately,
without being affected by reflected light from an obstacle even
when window glass, tree branches, or other such obstacles are
present in front of the object of measurement.
[0023] The third object of the present invention is to allow
accurate ranging to be performed even when there are frequencies
with large peaks in the frequency distribution table, or when the
frequencies increase over a broad range, for example.
[0024] The fourth object of the present invention is to allow the
frequency corresponding to an object of measurement to be
accurately extracted using a threshold, and to accurately measure
the distance to the object of measurement, when the distance to the
object of measurement is determined to be the distance at the point
when a frequency in the frequency distribution table exceeds a
specific threshold as discussed above.
[0025] The fifth object of the present invention is to provide an
opto-electric conversion circuit that makes use of avalanche
photodiodes which operate stably at high current multiplication
factors, and a laser ranging apparatus that makes use of this
circuit.
[0026] To achieve the first object stated above, the first ranging
apparatus pertaining to the present invention comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, a
distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received,
and a distance display for displaying the distance to the object of
measurement. Furthermore, the distance computer comprises a counter
for counting the frequency corresponding to distance when the
reflected light satisfies a specific condition, a table production
component for producing a frequency distribution table
corresponding to distance by adding up the frequencies with respect
to the measurement light repeatedly emitted a specific number of
times, a distance determiner for determining as the distance to the
object of measurement the point when the total count in the
frequency distribution table exceeds a specific threshold, and a
distance selector for selecting (a) specific distance(s) from among
a plurality of distances when the distance determiner determines a
plurality of distances to the object of measurement, and displaying
the selected distance on the distance display.
[0027] To achieve the first object stated above, the second ranging
apparatus pertaining to the present invention, just as with the
above-mentioned first ranging apparatus, comprises a measurement
light emitter for emitting pulsed measurement light toward an
object of measurement, a reflected light receiver for receiving
light reflected back from the object of measurement, a distance
computer for finding the distance to the object of measurement on
the basis of the elapsed time from when the measurement light is
emitted until the reflected light is received, and a distance
display for displaying the distance to the object of measurement.
With this ranging apparatus, however, the distance computer
comprises a counter for counting the frequency corresponding to
elapsed time when the reflected light satisfies a specific
condition, a table production component for producing a frequency
distribution table corresponding to elapsed time by adding up the
frequencies with respect to the measurement light repeatedly
emitted a specific number of times, a distance determiner for
determining as the distance to the object of measurement the
elapsed time, converted to distance, at which the total count in
the frequency distribution table produced by the table production
component exceeds a specific threshold, and a distance selector for
selecting (a) specific distance(s) from among a plurality of
distances when the distance determiner determines a plurality of
distances to the object of measurement, and displaying the selected
distance on the distance display.
[0028] As discussed above, when there are a plurality of objects to
be irradiated with measurement light, reflected light comes back
from each of these objects, so a plurality of distances are
calculated. In this case, with the first and second ranging
apparatus, a specific distance is selected by the distance
selector, and a ranging apparatus with excellent flexibility and
functionality can be obtained by suitably selecting and displaying
a plurality of distances.
[0029] To select and display the proper distance as above, the
distance selector can select the longest distance and display it on
the distance display. Conversely, the distance selector may instead
select the shortest distance and display it on the distance
display. Furthermore, the distance selector may select the n-th
(where n is a positive integer) longest distance from among a
plurality of distances and display it on the distance display.
[0030] The distance selector may be constructed so that the
selection conditions are set by external operation by the user, in
which case, when the distance determiner determines a plurality of
distances to the object of measurement, a specific distance is
selected on the basis of the selection conditions set in the
distance selector, and displayed on the distance display.
[0031] When the distance determiner determines a plurality of
distances to the object of measurement, the distance selector may
select the distance according to a usage condition, etc., and
display it on the distance display. The focal point of a finder for
sighting the object of measurement can be used as the usage
condition, for example, so that the distance selector selects a
long distance when the focal point is far, and selects a short
distance when the focal point is near. The weather at the time of
ranging can also be used as the usage condition, so that the
distance selector selects a long distance when measuring the
distance to a target in the rain or snow. These usage conditions,
etc., may be switched and set by the user.
[0032] When the distance determiner determines a plurality of
distances to the object of measurement, it is also possible to
design the distance selector so that the distance selector
determines that there are a plurality of objects of measurement,
and displays a plurality of distances on the distance display. In
this case, all of the plurality of distances may be displayed at
once on the distance display, or the plurality of distances may be
displayed one after another on the distance display.
[0033] The intensity of the reflected light can also be used as the
specific condition that is employed in the counter of the
above-mentioned first and second ranging apparatus. In this case,
the counter performs a frequency count when the intensity of the
reflected light exceeds a specific intensity threshold.
[0034] To achieve the first object stated above, the first ranging
method pertaining to the present invention is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; in this method, the pulsed
measurement light is first repeatedly emitted toward the object of
measurement, a frequency count corresponding to distance is
performed when the reflected light for each emission satisfies a
specific condition, a frequency distribution table corresponding to
distance is produced by adding up the frequencies counted in all of
the measurement light emissions carried out a specific number of
times, the point when the total count in the frequency distribution
table exceeds a threshold is determined as the distance to the
object of measurement, and this distance is displayed. In this
case, when a plurality of distances to the object of measurement
are determined, a specific distance is selected and displayed from
among these distances.
[0035] To achieve the first object stated above, the second ranging
method pertaining to the present invention, just as with the
above-mentioned seventh ranging method, is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; here, the pulsed measurement
light is repeatedly emitted toward the object of measurement, a
frequency count corresponding to elapsed time is performed when the
reflected light for each emission satisfies a specific condition, a
frequency distribution table corresponding to elapsed time is
produced by adding up the frequencies counted in all of the
measurement light emissions carried out a specific number of times,
the distance is found from the elapsed time at which the total
count in the frequency distribution table exceeds a threshold, and
this distance is determined as the distance to the object of
measurement and displayed. Again with this method, when a plurality
of distances to the object of measurement are determined, a
specific distance is selected and displayed from among these
distances.
[0036] With the first and second ranging methods constituted as
above, when there are a plurality of objects being irradiated with
measurement light and a plurality of distances are calculated, a
ranging method with excellent flexibility and functionality can be
obtained, for example, by suitably selecting and displaying a
plurality of distances.
[0037] Furthermore, the intensity of the reflected light can be
used as the above-mentioned specific condition for performing a
frequency count in the above-mentioned first and second ranging
methods, and the frequency may be counted when the intensity of the
reflected light exceeds a specific intensity threshold.
[0038] With the first and second ranging apparatus and the first
and second ranging methods pertaining to the present invention as
discussed above, pulsed measurement light is repeatedly emitted
toward an object of measurement, a frequency distribution table
corresponding to distance or elapsed time is produced by adding up
the frequencies counted in all of the above-mentioned measurement
light emissions, and the point when the total count in the
frequency distribution table exceeds a threshold is determined as
the distance to the object of measurement; here, when a plurality
of distances to the object of measurement are determined, a
specific distance is selected and displayed from among the
plurality of distances. Accordingly, when there are a plurality of
objects to be irradiated with measurement light, and reflected
light comes back from each of these objects, so that a plurality of
distances are calculated, or when the distances of objects
surrounding the object of measurement are calculated, a ranging
apparatus with excellent flexibility and functionality can be
obtained by suitably selecting and displaying a specific distance
with the distance selector.
[0039] To achieve the second object stated above, the third ranging
apparatus pertaining to the present invention comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, and
a distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received.
Furthermore, the distance computer comprises a counter for counting
the frequency corresponding to distance when the reflected light
satisfies a specific condition, a table production component for
producing a frequency distribution table corresponding to distance
by adding up the frequencies with respect to the measurement light
repeatedly emitted a specific number of times, and a distance
determiner for determining as the distance to the object of
measurement the point when the total count in the frequency
distribution table produced by the table production component
exceeds a specific threshold. Here, the threshold used in the
determination made by the distance determiner is varied and set
according to distance in the frequency distribution table.
Furthermore, this threshold is preferably set so as to decrease as
the distance increases.
[0040] To achieve the second object stated above, the fourth
ranging apparatus pertaining to the present invention, just as with
the above-mentioned third ranging apparatus, comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, and
a distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received.
With this ranging apparatus, however, the distance computer
comprises a counter for counting the frequency corresponding to
elapsed time when the reflected light satisfies a specific
condition, a table production component for producing a frequency
distribution table corresponding to elapsed time by adding up the
frequencies with respect to the measurement light repeatedly
emitted a specific number of times, and a distance determiner for
determining as the distance to the object of measurement the
elapsed time, converted to distance, at which the total count in
the frequency distribution table produced by the table production
component exceeds a specific threshold. The threshold used by this
distance determiner is varied and set according to elapsed time in
the above-mentioned frequency distribution table. In this case,
furthermore, it is preferable if the threshold is set so as to
decrease as the elapsed time increases in the frequency
distribution table.
[0041] As discussed above, when an object of measurement is
irradiated with laser light and the distance to the object of
measurement is measured, the reflected light intensity from a
nearby object is generally higher. With the above-mentioned third
and fourth ranging apparatus, the distance determiner is
constituted so as to find the distance to the object of measurement
by using a threshold varied and set according to distance or
elapsed time in the frequency distribution table (preferably set so
as to decrease as the distance or elapsed time increases), so
accurate distance measurement (ranging) can be performed whether
the object of measurement is near or far.
[0042] Furthermore, when the distance to an object of measurement
is measured by looking at the object of measurement through window
glass or tree branches, the intensity of the reflected light from
the window glass, etc., is lower than the reflected light intensity
when the object of measurement is in the same position as these
obstacles, but since the window glass, etc., is located closer than
the object of measurement, the reflected light from the window
glass, etc., is sometimes counted along with that of the object of
measurement. Even in a case such as this, because the distance
determiner of the present invention determines the object of
measurement by using a threshold varied and set according to
distance or elapsed time, any window glass, etc., located nearby
will not be mistakenly identified as being the object of
measurement, so that the distance to the object of measurement can
be measured accurately.
[0043] Furthermore, the intensity of reflected light can be used as
the specific condition used by the counter of the above-mentioned
third and fourth ranging apparatus, in which case the counter
performs a frequency count when the intensity of the reflected
light exceeds a specific intensity threshold.
[0044] To achieve the second object stated above, the third ranging
method pertaining to the present invention is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; in this method, the pulsed
measurement light is first repeatedly emitted toward the object of
measurement, a frequency count corresponding to distance is
performed when the reflected light for each emission satisfies a
specific condition, a frequency distribution table corresponding to
distance is produced by adding up the frequencies counted in all of
the measurement light emissions carried out a specific number of
times, and the point when the total count in the frequency
distribution table exceeds a threshold set so as to vary according
to distance is determined as the distance to the object of
measurement. In this case, it is preferable if this threshold is
set so as to decrease as the distance increases.
[0045] To achieve the second object stated above, the fourth
ranging method pertaining to the present invention, just as with
the above-mentioned third ranging method, is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; here, the pulsed measurement
light is repeatedly emitted toward the object of measurement, a
frequency count corresponding to elapsed time is performed when the
reflected light for each emission satisfies a specific condition, a
frequency distribution table corresponding to elapsed time is
produced by adding up the frequencies counted in all of the
measurement light emissions carried out a specific number of times,
the distance is found from the elapsed time at which the total
count in the frequency distribution table exceeds a threshold set
so as to vary according to elapsed time, and this distance is
determined as the distance to the object of measurement. With this
method, it is preferable if the threshold is set so as to decrease
as the elapsed time increases.
[0046] With the third and fourth ranging methods constituted as
above, the distance to the object of measurement is determined by
using a threshold varied and set according to distance or elapsed
time in the frequency distribution table (preferably set so as to
decrease as the distance or elapsed time increases). Accordingly,
accurate distance measurement (ranging) can be performed whether
the object of measurement is near or far. Furthermore, when the
distance to an object of measurement is measured by looking at the
object of measurement through window glass or tree branches, even
if the reflected light from the window glass, etc., is counted
along with that of the object of measurement, since in these
methods the object of measurement is determined by using a
threshold varied and set according to distance or elapsed time, any
window glass, etc., located nearby will not be mistakenly
identified as being the object of measurement, allowing the
distance to the object of measurement to be measured
accurately.
[0047] In the above-mentioned third and fourth ranging methods, the
intensity of the reflected light can be used as the above-mentioned
specific condition for performing the frequency count, and the
frequency may be counted when the reflected light intensity exceeds
a specific intensity threshold.
[0048] As described above, With the above-mentioned third and
fourth ranging apparatus and third and fourth ranging methods,
pulsed measurement light is repeatedly emitted toward an object of
measurement, a frequency distribution table corresponding to
distance or elapsed time is produced by adding up the frequencies
counted in all of the above-mentioned measurement light emissions,
and the point when the total count in the frequency distribution
table exceeds a threshold set so as to vary according to distance
or elapsed time (preferably set so as to decrease as the distance
or elapsed time increases) is determined as the distance to the
object of measurement. Accordingly, accurate distance measurement
(ranging) can be performed whether the object of measurement is
near or far.
[0049] In particular, when the distance to an object of measurement
is measured by looking at the object of measurement through window
glass or tree branches, the reflected light from the window glass,
etc., is sometimes counted along with that of the object of
measurement. Even in this case, however, because the distance
determiner of these ranging apparatus determines the object of
measurement by using a threshold varied and set according to
distance or elapsed time, any window glass, etc., located nearby
will not be mistakenly identified as being the object of
measurement, allowing the distance to the object of measurement to
be measured accurately.
[0050] To achieve the third object stated above, the fifth ranging
apparatus pertaining to the present invention comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, and
a distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received.
Furthermore, the distance computer comprises a counter for counting
the frequency corresponding to distance when the reflected light
satisfies a specific condition, a table production component for
producing a frequency distribution table corresponding to distance
by adding up the frequencies with respect to the measurement light
repeatedly emitted a specific number of times, and performing
moving averaging in which the frequency at each distance added up
in this manner is replaced with an average frequency at a plurality
of distances including the distance itself and those before and
after that distance, and a distance determiner for determining as
the distance to the object of measurement the point when the total
count in the frequency distribution table produced by the table
production component exceeds a specific threshold.
[0051] To achieve the third object stated above, the sixth ranging
apparatus pertaining to the present invention, just as with the
above-mentioned fifth ranging apparatus, comprises a measurement
light emitter for emitting pulsed measurement light toward an
object of measurement, a reflected light receiver for receiving
light reflected back from the object of measurement, and a distance
computer for finding the distance to the object of measurement on
the basis of the elapsed time from when the measurement light is
emitted until the reflected light is received. With this ranging
apparatus, however, the distance computer comprises a counter for
counting the frequency corresponding to elapsed time when the
reflected light satisfies a specific condition, a table production
component for producing a frequency distribution table
corresponding to elapsed time by adding up the frequencies with
respect to the measurement light repeatedly emitted a specific
number of times, and performing moving averaging in which the
frequency at each elapsed time added up in this manner is replaced
with an average frequency at a plurality of elapsed times including
the elapsed time itself and those before and after that elapsed
time, and a distance determiner for determining as the distance to
the object of measurement the elapsed time, converted as distance,
at which the total count in the frequency distribution table
exceeds a specific threshold.
[0052] In the above-mentioned fifth and sixth ranging apparatus, it
is desirable if the number of distances or elapsed times for which
an average is calculated by moving averaging can be variably
set.
[0053] Thus, with the fifth and sixth ranging apparatus, the
frequency distribution table is produced using a count that has
undergone moving averaging, rather than using the count directly as
it comes from the counter. Accordingly, in cases where the distance
is measured by looking at the object of measurement through window
glass or tree branches, even when there is a frequency with a large
peak in the frequency distribution table due to reflected light
from the window glass, etc., or even when there is a frequency with
a large peak due to noise light, the peak can be lowered by
performing moving averaging in which an average frequency is taken
of a plurality of frequencies including of this peak and of those
before and after this peak, allowing the distance to the object of
measurement to be measured accurately.
[0054] Moreover, even if there is variance in the position at which
the frequency increases in the frequency distribution table due to
the shaking of the user's hands when the ranging apparatus is held
in the hands during measurement, atmospheric fluctuations in the
measurement environment, and other such effects, the distance can
still be measured accurately by using moving averaging to reduce
the effect of this variance. Furthermore, when measuring the
distance to an object of measurement having longitudinal spread
(that is, with depth), as is the case when measuring the distance
to a building whose walls are viewed obliquely, the count increases
over a wide range of distances, but if this frequency is subjected
to moving averaging, the middle position in this broad distance
range can be ascertained, so that accurate distance measurement
will still be possible.
[0055] Specifically, by performing moving averaging, the
frequencies with large peaks that occur in the frequency
distribution table are smoothed out, the middle part of a frequency
that increases over a broad range can be emphasized, the effect of
high noise frequencies can be eliminated, the middle part of a
broad range can be ascertained, and accurate distance measurement
will be possible even in the above-described situations.
[0056] The intensity of the reflected light can be used as the
specific condition in the counter of the above-mentioned fifth and
sixth ranging apparatus, in which case the counter performs a
frequency count when the intensity of the reflected light exceeds a
specific intensity threshold.
[0057] To achieve the third object stated above, the fifth ranging
method pertaining to the present invention is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; in this method, the pulsed
measurement light is first repeatedly emitted toward the object of
measurement, a frequency count corresponding to distance is
performed when the reflected light for each emission satisfies a
specific condition, a frequency distribution table corresponding to
distance is produced by adding up the frequencies counted in all of
the measurement light emissions carried out a specific number of
times, and by performing moving averaging in which the frequency at
each distance added up in this manner is replaced with an average
distance at a plurality of distances including the distance itself
and those before and after that distance, and the point when the
total count in the frequency distribution table exceeds a threshold
is determined as the distance to the object of measurement.
[0058] To achieve the third object stated above, the sixth ranging
method pertaining to the present invention, just as with the
above-mentioned first ranging method, is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; here, the pulsed measurement
light is repeatedly emitted toward the object of measurement, a
frequency count corresponding to elapsed time is performed when the
reflected light for each emission satisfies a specific condition, a
frequency distribution table corresponding to elapsed time is
produced by adding up the frequencies counted in all of the
measurement light emissions carried out a specific number of times,
and by replacing the frequency at each elapsed time added up in
this manner with an average frequency at a plurality of elapsed
times including the elapsed time itself and those before and after
that elapsed time, the distance is found from the elapsed time at
which the total count in the frequency distribution table exceeds a
threshold, and this distance is determined as the distance to the
object of measurement.
[0059] Again with the fifth and sixth ranging methods structured as
above, since the frequency distribution table is produced by
performing moving averaging of the count, the frequencies with
large peaks in the frequency distribution table are smoothed out,
and the middle part of a frequency that increases over a broad
range can be emphasized; consequently, the effect of high noise
frequencies can be eliminated, the middle part of a broad range can
be ascertained, and accurate distance measurement is possible.
[0060] Furthermore, in the above-mentioned fifth and sixth ranging
methods, the intensity of the reflected light can be used as the
above-mentioned specific condition for performing a frequency
count, and the frequency may be counted when the intensity of the
reflected light exceeds a specific intensity threshold.
[0061] With the fifth and sixth ranging apparatus and the fifth and
sixth ranging methods pertaining to the present invention as
discussed above, pulsed measurement light is repeatedly emitted
toward an object of measurement, a frequency distribution table is
produced by adding up the frequencies counted in all of the
above-mentioned measurement light emissions and subjecting the
frequency thus added up to moving averaging corresponding to
distance or elapsed time, and the point when the total count in
this frequency distribution table exceeds a threshold is determined
as the distance to the object of measurement. By thus performing
moving averaging, the frequencies with large peaks occurring in the
frequency distribution table are smoothed out, and the middle part
of a frequency that increases over a broad range can be emphasized.
Consequently, the effect of high noise frequencies can be
eliminated, the middle part of a broad range can be ascertained,
and accurate distance measurement is possible.
[0062] To achieve the fourth object stated above, the seventh
ranging apparatus pertaining to the present invention comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, and
a distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received.
Furthermore, the distance computer comprises a counter for counting
the frequency corresponding to distance when the reflected light
satisfies a specific condition, a table production component for
producing a frequency distribution table corresponding to distance
by adding up the frequencies with respect to the measurement light
repeatedly emitted a specific number of times, and a distance
determiner for determining as the distance to the object of
measurement the point when the total count in the frequency
distribution table produced by the table production component
exceeds a specific threshold. A plurality of types of the threshold
used in the determination by the distance determiner are set in
this case.
[0063] To achieve the fourth object stated above, the eighth
ranging apparatus pertaining to the present invention, just as with
the above-mentioned seventh ranging apparatus, comprises a
measurement light emitter for emitting pulsed measurement light
toward an object of measurement, a reflected light receiver for
receiving light reflected back from the object of measurement, and
a distance computer for finding the distance to the object of
measurement on the basis of the elapsed time from when the
measurement light is emitted until the reflected light is received.
With this ranging apparatus, though, the distance computer
comprises a counter for counting the frequency corresponding to
elapsed time when the reflected light satisfies a specific
condition, a table production component for producing a frequency
distribution table corresponding to elapsed time by adding up the
frequencies with respect to the measurement light repeatedly
emitted a specific number of times, and a distance determiner for
determining as the distance to the object of measurement the
elapsed time, converted to distance, at which the total count in
the frequency distribution table produced by the table production
component exceeds a specific threshold. A plurality of types of the
threshold used in this distance determiner are set.
[0064] When the frequency distribution table is produced by adding
up the frequencies counted by the counter, the reflected light
intensity varies with the distance to the object of measurement,
with the type of object of measurement (the reflectivity of the
measurement light), and with the measurement conditions (such as
brightness and weather); accordingly, ranging of the same object of
measurement can yield markedly different counts in the frequency
distribution table depending on the above variations. Consequently,
with the seventh and eighth ranging apparatus described above, a
plurality of types of threshold are set, and the threshold is
switched according to the above-mentioned conditions, which allows
the distance having the frequency corresponding to the object of
measurement to be accurately determined.
[0065] Because of this, it is preferable with the above-mentioned
seventh and eighth ranging apparatus if the distance computer is
provided with a threshold selector used to select from among the
plurality of types of threshold according to the determination of
the distance determiner. In this case, it is preferable if when
none of the total count in the frequency distribution table exceeds
the threshold selected by the threshold selector, the threshold
selector switches to a threshold with a lower value than the
selected threshold, and when there are a plurality of (or many)
counts out of the total counts in the frequency distribution table
that exceed the threshold selected by the threshold selector, the
threshold selector switches to a threshold with a higher value than
the selected threshold.
[0066] To achieve the fourth object stated above, the seventh
ranging method pertaining to the present invention is one in which
pulsed measurement light is emitted toward an object of
measurement, and the distance to the object of measurement is
determined on the basis of the elapsed time until the light
reflected back from the object of measurement is received; in this
method, the pulsed measurement light is first repeatedly emitted
toward the object of measurement, a frequency count corresponding
to distance is performed when the reflected light for each emission
satisfies a specific condition, a frequency distribution table
corresponding to distance is produced by adding up the frequencies
counted in all of the measurement light emissions carried out a
specific number of times, and the point when the total count in the
frequency distribution table exceeds a threshold is determining as
the distance to the object of measurement. In this case, a
plurality of types of threshold are set, and the plurality of types
of threshold are selected and used.
[0067] Furthermore, the intensity of the reflected light can be
used as the specific condition used by the counter in the
above-mentioned seventh and eighth ranging apparatus, in which case
the counter performs a frequency count when the intensity of the
reflected light exceeds a specific intensity threshold.
[0068] To achieve the fourth object stated above, the eighth
ranging method pertaining to the present invention, just as with
the above-mentioned ranging method, is one in which pulsed
measurement light is emitted toward an object of measurement, and
the distance to the object of measurement is determined on the
basis of the elapsed time until the light reflected back from the
object of measurement is received; here, the pulsed measurement
light is repeatedly emitted toward the object of measurement, a
frequency count corresponding to elapsed time is performed when the
reflected light for each emission satisfies a specific condition, a
frequency distribution table corresponding to elapsed time is
produced by adding up the frequencies counted in all of the
measurement light emissions carried out a specific number of times,
the distance is found from the elapsed time at which the total
count in the frequency distribution table exceeds a threshold, and
this distance is determined as the distance to the object of
measurement. Again with this method, a plurality of types of
threshold are set, and the plurality of types of threshold are
selected and used.
[0069] When the frequency distribution table is produced by adding
up the frequencies counted by the counter, since the intensity of
the reflected light varies with the distance to the object of
measurement, with the type of object of measurement (the
reflectivity of the measurement light), and with the measurement
conditions (such as brightness and weather), the counts in the
frequency distribution table vary greatly with the above
variations. However, the distance having the frequency
corresponding to the object of measurement can be accurately
determined with the seventh and eighth ranging methods by switching
among the plurality of types of threshold according to the distance
determination conditions.
[0070] Because of this, it is preferable that in the
above-mentioned seventh and eighth ranging methods, when none of
the total counts in the frequency distribution table exceed the
selected specific threshold, this threshold be switched to a
threshold with a lower value, and that when a plurality of (or
many) total counts in the frequency distribution table exceed the
selected threshold, this threshold be switched to a threshold with
a higher value.
[0071] Furthermore, the intensity of the reflected light can be
used as the above-mentioned specific condition for counting the
frequency in the above-mentioned seventh and eighth ranging
methods, and the frequency may be counted when the intensity of the
reflected light exceeds a specific intensity threshold.
[0072] With the seventh and eighth ranging apparatus and the
seventh and eighth ranging methods pertaining to the present
invention as discussed above, pulsed measurement light is
repeatedly emitted toward an object of measurement, a frequency
distribution table corresponding to distance or elapsed time is
produced by adding up the frequencies counted in all of the
above-mentioned measurement light emissions, and a plurality of
types of threshold are set for determining as the distance to the
object of measurement the point when the total count in the
frequency distribution table exceeds a threshold. Accordingly, even
if the frequencies in the frequency distribution table vary greatly
with the distance to the object of measurement, with the type of
object of measurement (the reflectivity of the measurement light),
and with the measurement conditions (such as brightness and
weather), the distance having the frequency corresponding to the
object of measurement can be accurately determined by switching the
threshold according to the conditions during the determination of
the distance to the object of measurement.
[0073] To achieve the fifth object stated above, the opto-electric
conversion circuit pertaining to the present invention is an
opto-electric conversion circuit in which an avalanche photodiode
is used, comprising a reverse bias voltage regulating component for
regulating the reverse bias voltage applied to the avalanche
photodiode, a measurement component for measuring the current
flowing to the avalanche photodiode, a reference reverse bias
voltage detecting component for regulating the above-mentioned
reverse bias voltage and detecting the reverse bias voltage at
which a specific current flows to the avalanche photodiode
(reference reverse bias voltage), and a reverse bias voltage
setting component for adjusting the reverse bias voltage applied to
the above-mentioned avalanche photodiode during opto-electric
conversion to a voltage obtained by multiplying the above-mentioned
detected reference reverse bias voltage by a specific ratio.
[0074] With such an opto-electric conversion circuit, in a
non-measurement state, the reverse bias voltage applied to the
avalanche photodiode is varied by the reverse bias voltage
regulating component, and the reverse bias voltage at which a
predetermined specific current flows to the avalanche photodiode is
detected by the reference reverse bias voltage detecting component.
This is equivalent to measuring the breakdown voltage. During
measurement, furthermore, the reverse bias voltage regulating
component is driven by the reverse bias voltage setting component,
and the reverse bias voltage applied to the avalanche photodiode is
adjusted to a voltage obtained by multiplying the above-mentioned
detected voltage by a specific ratio. As a result, even if the
breakdown voltage varies with temperature or other factors, the
current multiplication factor can be kept constant, affording
stable light detection. Thus, the circuit can be used at a reverse
bias voltage having a high current multiplication factor, and faint
light can be stably detected.
[0075] The above-mentioned specific ratio can be set greater than
1, but setting this ratio to less than or equal to 1, detecting the
voltage at which a large current flows, and using the circuit at a
voltage lower than this is usually a more stable usage method.
Furthermore, if the specific ratio is set at 1, a voltage
regulating component is not driven, and the reference reverse bias
voltage is used directly as the reverse bias voltage during
opto-electric conversion.
[0076] To achieve the fifth object stated above, the ninth ranging
apparatus pertaining to the present invention is a laser ranging
apparatus which measures the distance to an object of measurement
by radiating laser light toward the object of measurement and
measuring the time differential between the point when the laser
light is radiated and the point when the laser light reflected back
from the object of measurement is received, wherein the circuit for
detecting the receipt of laser light reflected back from the object
of measurement has the above-mentioned opto-electric conversion
circuit.
[0077] With this ranging apparatus, because the above-mentioned
opto-electric conversion circuit is used for the circuit that
detects the laser light reflected back from the object of
measurement, reflected light can be detected at a high current
multiplication factor that is always stable. Therefore, any
measurement error or states in which detection is impossible due to
instability of a photodetection circuit can be prevented, and since
stable detection of even faint light is possible, the measurable
distance can be increased.
[0078] With this ranging apparatus, the reverse bias voltage
setting component can be actuated at various points in time, such
as every time the power is turned on to the laser ranging
apparatus, every time measurement is commenced, at specific time
intervals, or every time the temperature changes by at least a
specific amount, but it is preferable to actuate this component
every time measurement is commenced or every time the power to the
apparatus is turned on, because this will yield the most accurate
results.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] FIG. 1 is an oblique view showing the external appearance of
the ranging apparatus in one embodiment of the present
invention;
[0080] FIG. 2 is a block diagram illustrating the structure of the
above-mentioned ranging apparatus;
[0081] FIG. 3 is a diagram illustrating how distance measurement is
performed with the above-mentioned ranging apparatus by looking at
an object of measurement through window glass;
[0082] FIG. 4 is a flow chart illustrating a ranging method carried
out using the above-mentioned ranging apparatus;
[0083] FIG. 5 is a flow chart illustrating a ranging method carried
out using the above-mentioned ranging apparatus;
[0084] FIG. 6 consists of graphs of the reflected light intensity
versus elapsed time when reflected light is received by the
above-mentioned ranging apparatus, and a diagram illustrating the
state when flags are set in time zones in which this reflected
light intensity exceeds an intensity threshold;
[0085] FIG. 7 is a diagram of a count table formed by the counter
constituting part of the distance computer of the above-mentioned
ranging apparatus;
[0086] FIG. 8 illustrates a frequency distribution table formed by
the table production component constituting part of the
above-mentioned distance computer;
[0087] FIG. 9 illustrates a frequency distribution table before
moving averaging is performed, and a frequency distribution table
when moving averaging is performed;
[0088] FIG. 10 is a simplified diagram illustrating the
opto-electric conversion circuit in one embodiment of the present
invention;
[0089] FIG. 11 is a flow chart illustrating the simplified
operation of an MPU;
[0090] FIG. 12 is a graph of the relationship between the detected
current value and the reverse bias voltage applied to an APD 3;
[0091] FIG. 13 is a graph of the relationship between the current
multiplication factor and the reverse bias voltage applied to the
APD 3; and
[0092] FIG. 14 is a graph of the relationship between the reverse
bias voltage and the current multiplication factor.
BEST MODE FOR CARRYING OUT THE INVENTION
[0093] Embodiments considered to be the best mode for carrying out
the present invention are described below through reference to
drawings, but these descriptions should not be construed to limit
the scope of the present invention.
[0094] FIG. 1 shows a ranging apparatus 1 that is a first
embodiment of the present invention. This ranging apparatus 1
consists of a laser light emitter 3 and a reflected light receiver
4 housed in a case 2. A laser light emission window 3a through
which pulsed laser light (measurement light) from the laser light
emitter 3 is emitted, and a reflected light reception window 4a
through which reflected light is received, are provided to the case
2. A first control button 5 for switching the power on and off and
commencing ranging, and a second control button 6 for display
selection, are provided on the upper surface of the case 2. A
finder window 2a (see FIG. 3) is provided on the back surface of
the case 2, and the user (who uses this ranging apparatus 1 to
perform ranging) measures the distance to an object of measurement
by looking at the object of measurement through the finder window
2a.
[0095] FIG. 2 shows the simplified internal structure of this
ranging apparatus 1. In addition to the structure described above,
also provided are a controller 7 having a distance computer 10, and
a distance display 8 that displays distances by receiving display
signals from the controller 7. The distance computer 10 comprises a
counter 11, a table production component 12, a distance determiner
13, a threshold selector 14, and a distance selector 15, the
details of which will be described below. The distance display 8
performs distance display in the interior of the finder window 2a,
and is designed so that when the user looks into the finder window
2a, the distance is displayed within the field of vision thereof.
Furthermore, a distance display that performs liquid crystal
display, for example, may be provided on the outside of the case 2.
The controller 7 is designed to receive the input of operation
signals from the first and second control buttons 5 and 6. The
laser light emitter 3 comprises a pulse generation circuit 31, a
light emitting element (semiconductor laser) 32, and a collimating
lens 33, while the reflected light receiver 4 comprises a signal
receiving circuit 41, a light receiving element (photodiode) 42,
and a focusing lens 43.
[0096] The operation when the distance to an object of measurement
is measured using the ranging apparatus 1 structured as above will
be described below through reference to the flow charts shown in
FIGS. 4 and 5. The flows in FIGS. 4 and 5 are connected where
indicated by the circled A, and together constitute a single
flow.
[0097] The example described here will be for a case in which, as
shown in FIG. 3, the distance to a far-away object of measurement
OB is measured through window glass WG using the ranging apparatus
1. When the ranging apparatus 1 is used to measure the distance to
the object of measurement OB, first, as shown in FIG. 3, the user
operates the first control button 5 while looking through the
finder window 2a and seeing the object of measurement OB through
the window glass WG. As a result, the power is switched on, an
operation signal is inputted from the first control button 5 to the
controller 7, and distance measurement operation commences (step
S2). The corresponding pre-processing shown in step S4 is
performed, and initialization processing such as clearing the
various memories is carried out.
[0098] Next, a single measurement timer is started (step S6), and
an intensity threshold TL is set (step S8). Then, a timer counter
is started (step S10), and the pulse generation circuit 31 is
actuated by the controller 7 so that pulsed laser light is emitted
from the light emitting element 32 (step S12). This laser light is
emitted through the collimating lens 33 and from the laser light
emission window 3a toward the object of measurement (the laser
light indicated by arrow A in FIGS. 2 and 3).
[0099] The laser light A emitted from the ranging apparatus 1 in
this manner first hits the window glass WG located nearby, and some
of the light is reflected (arrow B2). The rest of the laser light
reaches the object of measurement OB. The laser light that reaches
the object of measurement OB here is reflected as indicated by
arrow B1. Part of the light reflected by the window glass WG
(indicated by arrow B2) and the light reflected by the object of
measurement OB (indicated by arrow B1) (this part is the light
reflected toward the ranging apparatus 1) is then incident inside
the reflected light reception window 4a (see arrow B in FIG. 2),
where it is focused by the focusing lens 43 before reaching the
light receiving element 42. When the light receiving element 42 is
thus irradiated with the reflected light, a signal corresponding to
the intensity of the reflected light is sent to the signal
receiving circuit 41, and this signal receiving circuit 41
amplifies or otherwise processes this signal before sending it on
to the controller 10.
[0100] Thus, in the controller 10, a reflected light signal as
shown in FIG. 6 (A1) is received (step S14), and the distance to
the object of measurement OB is measured from this received signal
by the distance computer 10 as follows. In FIG. 6 (A1), the
horizontal axis indicates the elapsed time, the origin of which is
the point when pulsed laser light is emitted from the laser light
emitter 3, and the vertical axis indicates the intensity of the
reflected light that is received. Specifically, FIG. 6 (A1) shows
the change over time in the intensity of the reflected light
received by the reflected light receiver 4 from the time when the
pulsed laser light is emitted from the laser light emitter 3 in
step S12.
[0101] When this reflected light is detected, the system searches
for a point at which the reflected light intensity is over the
intensity threshold TL set in step S8, and the time zone in which
this point is located is recorded (step S16). These time zones are
formed by division into fixed time intervals (such as 12.5 ns), as
shown in FIG. 6 (B), on the basis of the count of the timer counter
started in step S10. Accordingly, when the reflected light
intensity is as shown in FIG. 6 (A1), for instance, flags are set
up as shown in the first row in FIG. 6 (B) in the time zones
including the locations of peaks P.sub.11 to P.sub.17 that exceed
the intensity threshold TL (indicated by one-dot chain lines in the
drawings), and the time zones Z.sub.5, Z.sub.6, Z.sub.8, Z.sub.11,
Z.sub.16, Z.sub.17, and Z.sub.18 in which these flags have been set
up are recorded in step S16.
[0102] The elapsed time from the point when the pulsed laser light
is emitted from the laser light emitter 3 until the reflected light
is received by the reflected light receiver 4 can be converted into
distance, and the above-mentioned time zones converted into
corresponding distance zones, by using the spatial propagation
speed of laser light. In the description herein, the time zones and
distance zones are both indicated by the same symbols as Z.sub.1,
Z.sub.2, . . . , with corresponding zones being numbered the same.
Then, as shown in FIG. 7, one count is added and recorded in each
distance zone where one of the above-mentioned flags was set up in
the count table formed corresponding to the various distance zones
Z.sub.1, Z.sub.2, . . . by the counter 11 constituting part of the
distance computer 10 of the controller 7. In the above case, one
count is recorded in each of the distance zones Z.sub.5, Z.sub.6,
Z.sub.8, Z.sub.11, Z.sub.16, Z.sub.17, and Z.sub.18.
[0103] In this example, furthermore, the window glass WG in FIG. 3
is in distance zone Z.sub.5, and the object of measurement OB is
near distance zone Z.sub.16. Accordingly, it is believed that the
peaks P.sub.11 and P.sub.12 in FIG. 6 (A1) are reflected light from
the window glass WG, and that the peaks P.sub.15, P.sub.16, and
P.sub.17 are reflected light from the target OB, and it is believed
that the other peaks P.sub.13 and P.sub.14 are the result of
natural light or the like being detected as noise light.
[0104] In this example, the flow from the above-mentioned step S6
to step S18 consists of a total of 520 iterations, and a judgment
is made in step S20 as to whether 520 measurements have been
completed. At the stage where irradiation with the first laser
pulse is performed as described above, the flow moves to step S22,
waits for the single measurement timer to elapse (after 1 ms, for
instance), and then moves on to step S24, at which point the single
measurement timer is stopped.
[0105] The flow then moves to step S6, the single measurement timer
is restarted, and measurement by irradiation with the second laser
pulse is commenced. Thereafter, the setting of the intensity
threshold TL (step S8), the start of the timer counter (step S10),
and the emission of pulsed laser light (step S12) are carried out
the same as the first time, and the reflected light is received
(step S14). Thus, FIG. 6 (A2) shows the change in intensity over
the elapsed time of the received reflected light for the second
irradiation with pulsed laser light. Here again, flags are set up
as shown in the second row in FIG. 6 (B) in the time zones
including the locations of peaks P.sub.21 to P.sub.25 that exceed
the intensity threshold TL set in step S8, and the time zones
Z.sub.5, Z.sub.6, Z.sub.10, Z.sub.14, and Z.sub.15 in which these
flags have been set up are recorded in step S16.
[0106] Then, just as with the first irradiation with pulsed laser
light, one count is added and recorded in each distance zone where
one of the above-mentioned flags was set up in the count table
shown in FIG. 7. In this case, one count is added and recorded in
each of the distance zones Z.sub.5, Z.sub.6, Z.sub.10, Z.sub.14,
and Z.sub.15; since one count has been recorded in distance zones
Z.sub.5 and Z.sub.6 the first time around, the recorded count in
these distance zones is 2.
[0107] FIG. 7 shows the count in the count table when 520
irradiations of pulsed laser light have been performed at the set
time interval (such as 1 ms) of the single measurement timer. Once
520 irradiations of pulsed laser light have been completed in this
manner, the flow moves to step S26, where the count in the various
distance zones is subjected to moving averaging. This moving
averaging is processing in which the average count at the distance
zones Z.sub.n-1, Z.sub.n, and Z.sub.n+1 including the n-th distance
zone Z.sub.n and the zones before and after this Z.sub.n, for
example, is reset as the count for the distance zone Z.sub.n in the
count table in FIG. 7. The goal, effect, and so forth of this
moving averaging will be discussed below.
[0108] The table production component 12 of the distance computer
10 produces the frequency distribution table (histogram) shown in
FIG. 8 from the count table that has undergone this moving
averaging. In the frequency distribution table thus produced, the
count is greater in distance zone Z.sub.5 corresponding to the
location of the window glass WG and distance zone Z.sub.16
corresponding to the location of the object of measurement OB,
where it is highly probable that reflected light will be always
generated.
[0109] The distance determiner 13 then determines whether there is
a frequency that exceeds a determination threshold P that varies
with distance (distance zone) in this frequency distribution table,
and sets up a flag in a distance zone where the determination
threshold P is exceeded (steps S28 and S30). Here, since the count
in the frequency distribution table is greater in distance zone
Z.sub.5 corresponding to the location of the window glass WG and
distance zone Z.sub.16 corresponding to the location of the object
of measurement OB, if the determination threshold Q with a constant
value, indicated by the broken line in FIG. 8, is used to determine
frequencies exceeding this value, flags will be set up in both
distance zone Z.sub.5 corresponding to the location of the window
glass WG and distance zone Z.sub.16 corresponding to the location
of the object of measurement OB.
[0110] Accordingly, the determination is made using the
determination threshold P set to vary with distance as shown by the
one-dot chain line P in FIG. 8 (that is, set to decrease as the
distance increases). As a result, no flag is set up in distance
zone Z.sub.5 corresponding to the location of the window glass WG,
and a flag is only set up in distance zone Z.sub.16 corresponding
to the location of the object of measurement OB, thereby affording
more accurate measurement of the distance to the object of
measurement OB. However, either determination threshold P or Q may
be used in the ranging apparatus and method of the present
invention.
[0111] The flow then moves on to step S32, in which the flag
position, that is, the distance zone where a flag has been set up,
is detected. At this point, there may be no flags whatsoever set up
if the count is low with respect to the size of the determination
threshold P, and conversely, if the count is high with respect to
the size of the determination threshold P, the counts in a
plurality of distance zones may exceed the determination threshold
P, and a plurality of flags may be set up. This is why the
threshold selector 14 is provided to the distance computer 10, and
a plurality of types of threshold are preset as the determination
threshold P. For instance, the determination threshold P shown in
FIG. 8 is set to a determination threshold (a type of determination
threshold having a large value) P' that has been moved upward in
parallel, and a determination threshold (a type of determination
threshold having a small value) P" that has been moved downward in
parallel.
[0112] Then, in the threshold selector 14, if there are no flags,
the flow moves from step S34 to step S38, the determination
threshold P" of the type having a small value is selected as the
determination threshold P, and steps S26 to S32 are repeated. On
the other hand, if there are too many flags, the flow moves from
step S36 to step S38, the determination threshold P' of the type
having a large value is selected, and steps S26 to S32 are
repeated. This adjusts the set-up flags to a suitable number.
[0113] The above description was for a case in which a plurality of
types of threshold had been preset as the determination threshold
P, but another possibility is to predetermine the initial
determination threshold and the incremental and decremental widths
of the determination threshold, so that when there are too many
flags, the determination threshold value is incremented by 1 in
step S38, and when there are too few flags, the determination
threshold value is decremented by 1 in step S38, with steps S26 to
S32 being repeated until the desired number of flags are
obtained.
[0114] Then, the center of gravity position corresponding to
distance zones in which flags have been set up is found by
performing weighted averaging on the basis of the counts of the
distance zones before and after the distance zones at positions
where flags have been set up (step S40), this center of gravity
position is calculated as the distance to the object of measurement
OB (step S42), and this calculated distance is displayed by the
distance display 8 (step S44).
[0115] Furthermore, when a plurality of flags have been set up in
the above flow, the distance selector 14 is actuated according to
operation of the second control button 6, a specific flag is
selected from among the plurality of flags, and the distance at the
center of gravity position of that flag is displayed by the
distance display 8.
[0116] When the distance to the object of measurement OB was
measured with the ranging apparatus 1 as described above, the count
table shown in FIG. 7 was formed by converting time zones to
distance zones. However, the count table may instead be produced by
using the time zones "as is." In this case, time zones can be used
for the horizontal axis in the frequency distribution table in FIG.
8 as well, and the distance to the object of measurement OB can be
calculated from the elapsed time at the position where a flag is
set up. Moreover, the intensity threshold TL is a constant value in
FIGS. 6 (A1) and (A2), but may instead be an intensity threshold
that varies with elapsed time. More specifically, the intensity
threshold that decreases as the elapsed time increases may also be
used.
[0117] In addition, in the above-mentioned embodiment, the
determination threshold was changed and selected according to the
number of flags when the determination threshold P was used, but
the determination threshold P may instead be changed manually.
Furthermore, the determination threshold P used initially may be
pre-varied according to external conditions. For example, when it
is bright and there is much natural light that would become noise,
such as during the day, the determination threshold P may be set
high, and at night the determination threshold P may be set
low.
[0118] The above description is of an example of the simple
modeling of a single object of measurement OB, but when the ranging
apparatus 1 is actually used to view an object of measurement
through the finder 2a, there will be objects in the vicinity of the
object of measurement to be measured. Accordingly, the laser light
emitted from the laser light emitter 3 irradiates not only the
object of measurement, but also its surrounding objects, and the
reflected light receiver 4 also receives reflected light from these
objects. Therefore, the count is greater in a plurality of distance
zones in the frequency distribution table shown in FIG. 8, and
there are a plurality of distance zone that exceed the
determination threshold. Furthermore, there may be cases in which a
plurality of close objects of measurement are viewed at the same
time through the finder 2a and the goal is to measure the distances
to the plurality of objects of measurement, and here again, there
will be a plurality of distance zones that exceed the determination
threshold.
[0119] In situations such as these, in this embodiment, the flow
moves to step S40 while flags are still set up in a plurality of
distance zones, the center of gravity position for each flag is
calculated, and a plurality of distances are determined. Then, a
specific distance is selected from among the above-mentioned
plurality of distances by the distance selector 14, and this is
displayed by the distance display 8.
[0120] Thus, the selection made by the distance selector 14 can,
for instance, consist of selecting the greatest distance and
displaying it on the distance display 8, selecting the least
distance and displaying it on the distance display 8, or selecting
the n-th greatest distance (where n is a positive integer) and
displaying it on the distance display 8. Which of these selection
methods is to be employed may be programmed in ahead of time, but
the system can also be designed so that the method is selected and
set by operating the second control button 6.
[0121] Conceivable conditions for selecting the distance as above
include the type of object being ranged, the weather conditions
during ranging, and other such usage conditions, and the system can
also be designed to allow these to be switched and set by operation
of the second control button 6. In this case, the distance selector
14 selects a specific distance on the basis of the selection
conditions set by operation of the second control button 6 by the
user, and this distance is displayed on the distance display 8.
[0122] When the distance selector 14 selects a distance and
displays it on the distance display 8 according to a usage
condition, etc., this usage condition can be, for example, the
focal position (such as the position of the focal ring) of the
finder 2a through which the object of measurement is viewed. In
this case, the distance selector 15 will select a greater distance
when the finder 2a is focused farther away, the distance selector
15 will select a shorter distance when the focus is nearer, and
this distance will be displayed on the distance display 8.
Furthermore, the weather during ranging can also be used as a usage
condition. For example, when the distance to a target is measured
in the rain or snow, reflected light from raindrops or snowflakes
will be admixed, but since the reflected light from the closer
raindrops or snowflakes has a greater effect, the distance selector
14 selects a greater distance. Moreover, these usage conditions,
etc., can be switched and set as desired by the user by operating
the second control button 6.
[0123] The system may also be designed so that when the distance
determiner 14 determines a plurality of distances to an object of
measurement, the distance selector 14 determines that there are a
plurality of objects of measurement, and the plurality of distances
are displayed on the distance display 8. In this case, all of the
plurality of distances may be displayed at once on the distance
display 8, or the plurality of distances may be switched in order
and displayed one after the other. In this case, the switching of
the display mode, and the switching of the plurality of distances
in the display in order may be accomplished by operating the second
control button 6.
[0124] Furthermore, as described above, in the distance measurement
that makes use of the ranging apparatus 1, moving averaging is
performed on the count in each distance zone in step S26 in the
count table shown in FIG. 7 and formed by adding up the count
values for 520 times. This will be described below.
[0125] FIG. 9 (A) shows an example of producing a frequency
distribution table by directly using the frequencies in a count
table that has not undergone moving averaging. With the frequency
distribution table in this figure, the count has a large peak in
distance zone Z.sub.5, and the count increases over the range of
distance zones Z.sub.14 to Z.sub.18. This is because, for example,
a peak was generated in distance zone Z.sub.5 by reflected light
from tree branches or the like in front of the object of
measurement OB. Because tree branches and so forth have no
longitudinal depth, the count is high only in distance zone
Z.sub.5, while the count is low in the preceding and subsequent
distance zones Z.sub.4 and Z.sub.6. The object of measurement OB
here is one with some depth longitudinally, so the count is high
over a wide range of distance zones Z.sub.14 to Z.sub.18.
[0126] If the determination threshold P is used to detect a
distance zone having a frequency over this threshold in the
frequency distribution table shown in FIG. 9 (A), that is, a
frequency distribution table that has not undergone moving
averaging, then flags are set up in all of the distance zones
Z.sub.5 and Z.sub.14 to Z.sub.18, which show high frequencies as
indicated in the figure, and all of these are determined to be
distances to an object of measurement. The problem is therefore
that the determination of distance to the object of measurement is
ambiguous, resulting in inaccurate measurement.
[0127] Because of this, moving averaging is performed in step S26
in the present embodiment. This involves, for example, performing
moving averaging in which an average value is found for the n-th
distance zone Z.sub.n in the frequency distribution table in FIG. 9
(A), including one distance zone before (Z.sub.n-1) and one
distance zone after (Z.sub.n+1) this distance zone. For instance,
if the count of the n-th distance zone Z.sub.n is C.sub.n, the
count of the (n-1)-th distance zone Z.sub.n-1 is C.sub.n-1, and the
count of the (n+1)-th distance zone Z.sub.n+1 is C.sub.n+1, then
the count C.sub.n of the n-th distance zone Z.sub.n is replaced
with the value (C.sub.n-1+C.sub.n+C.sub.n+1)/3.
[0128] FIG. 9 (B) shows the results of this moving averaging,
wherein the peak in the distance zone Z.sub.5 becomes lower, and
the frequencies in the distance zones Z.sub.14 to Z.sub.18 become
lower on both sides (i.e., at the distance zones Z.sub.14 and
Z.sub.18), which emphasizes the middle part. If a determination
threshold P is used to detect a distance zone having a frequency
over this threshold in the frequency distribution table in FIG. 9
(B), which has thus undergone moving averaging, then flags will
only be set up in the range of distance zones Z.sub.15 to Z.sub.17,
which results in accurate measurement of the distance to the object
of measurement.
[0129] Here, the moving averaging is performed by using distance
zones Z.sub.n-1 and Z.sub.n+1, which are one behind and one ahead,
respectively, of the specified distance zone Z.sub.n, but moving
averaging may instead be performed using two or more distance zones
ahead and behind. When this is done, even if the frequency
increases over a wider range of distance zones, the middle part can
still be emphasized; therefore, the distance to the object of
measurement can be measured accurately.
[0130] The embodiment described above combines all of the
following:
[0131] (1) the function of selecting and displaying a specific
distance to an object of measurement from among a plurality of
distances when it is determined that such a plurality exists
(distance selector),
[0132] (2) the function of varying, according to distance (elapsed
time), and setting the threshold used for determining as the
distance to the object of measurement the point when the total
count exceeds a specific threshold,
[0133] (3) the function of moving averaging, in which the frequency
at each distance added up is replaced with an average frequency at
a plurality of distances including the distance itself and those
before and after that distance, and
[0134] (4) the function of changing or selecting a plurality of
thresholds used for determining as the distance to the object of
measurement the point when the total count exceeds a specific
threshold (threshold selector).
[0135] However, it is sufficient if at least one of these functions
(1) to (4) is selected as needed, and all of the functions are not
necessarily required as in the above-mentioned embodiment.
[0136] The opto-electric conversion circuit in an embodiment of the
present invention will be described below through reference to the
drawings.
[0137] FIG. 10 is a simplified diagram illustrating the
opto-electric conversion circuit constituting one embodiment of the
present invention. This circuit consists mainly of an MPU
(microprocessor unit) 71, and constitutes part of the laser ranging
apparatus 1 in FIG. 2. The MPU 71 is part of the controller 7, and
also controls distance measurement. Furthermore, an APD (avalanche
photodiode) 42 corresponds to the light receiving element 42 in
FIG. 2.
[0138] An APD voltage setting circuit 44 receives commands from the
MPU 71, and applies reverse bias voltage of the designated value to
the APD 42. This results in current corresponding to the applied
reverse bias voltage flowing to the APD 42, and this current is
detected by a current detection circuit 45, converted to a digital
value by an A/D converter 46, and inputted to the MPU 71.
[0139] To show the relation to the Claims, the APD voltage setting
circuit 44 corresponds to the reverse bias voltage regulating
component, the current detection circuit 45 and the A/D converter
46 correspond to the means for measuring the current flowing to the
avalanche photodiode, and the program in the MPU 71 corresponds to
the reference reverse bias voltage detecting component and the
reverse bias voltage setting component.
[0140] The operation of this circuit will be described through
reference to FIGS. 11 and 12. FIG. 11 is a flow chart illustrating
the simplified operation of the MPU 71, and FIG. 12 is a graph
illustrating the relationship of the detected current value and the
current multiplication factor to the reverse bias voltage applied
to the APD 42.
[0141] The operation described in the flow chart of FIG. 11 starts
up every time a distance measurement command is inputted. The MPU
71 first measures the reverse bias voltage at which the current
flowing through the APD 42 will reach a specific value. In concrete
terms, the command voltage value given to the APD voltage setting
circuit 44 is increased in stages, which increases in stages the
reverse bias voltage applied to the APD 42, and each time this
happens, the current flowing through the APD 42 is measured via the
current detection circuit 45. Then, the reverse bias voltage when
the difference between the detected current value and the specified
value is within a permissible range is termed the reference reverse
bias voltage.
[0142] Referring to FIG. 12, when the above-mentioned specific
current value is I.sub.1 and the characteristics are as indicated
by the solid line, the reference reverse bias voltage is V.sub.1,
but when the characteristics are as indicated by the broken line,
the reference reverse bias voltage is V.sub.1'.
[0143] Next, the MPU 71 decides a voltage by multiplying the
reverse bias voltage thus found (the reference reverse bias
voltage) by a specific ratio, and this voltage is applied to the
APD 42 via the APD voltage setting circuit 44 as the reverse bias
voltage to be used in measurement. As a result, as shown in FIG.
12, when the characteristics are in the state indicated by the
solid line, the reverse bias voltage becomes V.sub.0, whereas the
reverse bias voltage becomes V.sub.0' when the characteristics are
in the state indicated by the broken line. The detected current
value remains constant whether the characteristics of the APD are
in the state indicated by the solid line or in the state indicated
by the broken line. In this case, as shown in FIG. 13, the current
multiplication factor of the APD 42 is kept at a constant value of
.alpha..sub.0 regardless of whether the characteristics of the APD
42 are in the state indicated by the solid line or in the state
indicated by the broken line.
[0144] In the above description, the operation described in the
flow chart of FIG. 11 was started up every time a distance
measurement command was inputted, but may instead be started up at
another appropriate timing, such as every time the power is
switched on to the laser ranging apparatus, at specific time
intervals, or every time the temperature changes by at least a
specific amount.
[0145] The MPU 71 executes distance measurement in this state.
Specifically, pulsed laser light from the laser diode 32 (FIG. 2)
is directed at the object of measurement, the reflected light is
detected by the APD 42, and the distance to the object of
measurement is found from the time lag between the point when the
pulsed laser light is emitted and the point when the reflected
light is detected. Stable measurement is possible here because the
current multiplication factor of the APD 42 is kept at a constant
value of .alpha..sub.0 even if the temperature changes.
INDUSTRIAL APPLICABILITY
[0146] The ranging apparatus and ranging method pertaining to the
present invention can be utilized in the fields of surveying and so
forth, as well as in the measurement of distances between vehicles,
the auto-focusing of cameras, and so on. Furthermore, the
opto-electric conversion circuit pertaining to the present
invention can be utilized in ranging apparatus and so forth.
* * * * *