Distance measuring equipment

Disclosed is a distance measuring equipment comprising: a light emitting element for generating a pulse beam; and a light receiving element for receiving a reflected pulse beam from an object under measurement with respect to the pulse beam generated from the light emitting element and converting the reflected pulse beam into an electric light receiving signal. The distance measuring equipment also comprises: an object detecting element for detecting the under-measurement object on the basis of a level of an output signal of the light receiving signal through the light receiving element; and a calculating element for calculating, when the object detecting element detects the under-measurement object, a distance to the under-measurement object on the basis of a time obtained by subtracting a predetermined time corresponding to a half-value of a pulse width of the light receiving signal from a delay time from a generation of the pulse beam through the light emitting element to a reach of a peak of the light receiving signal through the light receiving element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a distance measuring equipment for 
obtaining a distance to an object by irradiating the object with a pulse 
beam, receiving the pulse beam reflected from the object and thus 
measuring a time spent from the irradiation and the reception thereof. 
2. Related Background Art 
This type of known distance measuring equipment is an optical radar system 
as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2-228579. 
FIG. 7 is a diagram illustrating a construction of a conventional distance 
measuring equipment. A light sending device 1 generates a pulse beam P2 by 
actuating a light emitting element such as a laser diode, etc. A clock 
generator 2, which has its output side connected to an input side of the 
light sending device 1, generates a clock pulse P1 serving as a generation 
timing for the pulse beam P2. The light sending device 1 inputs the clock 
pulse P1. A light receiving device 3 receives the pulse beam reflected 
from the object 7 irradiated with the pulse beam P2. The light receiving 
device 3 converts the pulse beam into an electric signal P4. A sample 
pulse generator 4, which has its input side connected to the other output 
side of the clock generator 2, counts the clock pulses P1 inputted from 
the clock generator 2. The sample pulse generator 4, at the same time, 
generates a sample pulse P3. A sample hold circuit 5 is connected to an 
output side of the light receiving device 3 and to an output side of the 
sample pulse generator 4. The sample hold circuit 5 performs sampling of 
the output signals P4 of the light receiving device 3 by use of the sample 
pulses P3 of the sample pulse generator 4. A processor 6 is connected to 
output sides of the sample pulse generator 4 and of the sample hold 
circuit 5. The processor 6 inputs a clock pulse count value from the 
sample pulse generator 4 and also an output signal from the sample hold 
circuit 5. The processor 6 thus detects a distance to the object 7. 
Next, an operation of the thus constructed conventional equipment will be 
explained with reference to FIGS. 8 and 9. FIG. 8 is a diagram 
illustrating operating waveforms within a clock pulse period of the clock 
generator 2. FIG. 9 is a diagram illustrating operating waveforms at a 
time interval when this distance measuring equipment measures the distance 
once. The clock generator 2 generates the clock pulse P1 at a time 
interval T longer than a time corresponding to the maximum measured 
distance. This clock pulse P1 is inputted to the light sending device 1. 
The light sending device 1 generates the pulse beam P2 in synchronism with 
this clock pulse P1. The light receiving device 3 receives this pulse beam 
P2 reflected from the object 7. The light receiving device 3 
photoelectrically converts the reflected pulse beam into the electric 
signal and performs a high-frequency amplification thereof. The output 
signal P4 therefrom is inputted to the sample hold circuit 5. On the other 
hand, the sample pulse generator 4 counts the clock pulses P1 given from 
the clock generator 2. The sample pulse generator 4 repeats counting, 
wherein one period is a predetermined clock pulse count value M larger 
than a value obtained by dividing the maximum measured distance by a 
distance resolving power. At the same time, the sample pulse generator 4 
generates a sample pulse P3 in which the clock pulse P1 is delayed by a 
time given by multiplying a minute time .DELTA.T corresponding to the 
distance resolving power by a clock pulse count value n. The sample hold 
circuit 5 samples a pulse signal of the reflected beam with the above 
sample pulse P3. The sample hold circuit 5 holds a signal level thereof up 
to the next sample pulse P3. This held signal P5 is a signal in which a 
waveform of the high-frequency reflected pulse beam is frequency-converted 
into a low-frequency signal. The processor 6 compares the low-frequency 
output signal P5 of the sample hold circuit 5 with a threshold value Vth 
for detecting the reflected pulse beam and thus detects signals (A and B 
in FIG. 9) larger than the threshold value. A distance L between the 
distance measuring equipment and the object is obtained from a clock pulse 
count value N of the sample pulse generator 4 at this time in accordance 
with the following formula: 
EQU L=N.times..DELTA.T.times.C/2 (1) 
where C is the velocity of light. Namely, the distance L is 1/2 of a pulse 
beam travel obtained by multiplying a light emission-through-receipt time 
given from the clock pulse count value N by the light velocity C. The 
pulse count value N is, when the count value becomes a value corresponding 
to the maximum detected distance, reset to 0. The above actions are 
defined as a measuring one period, and the distance is continuously 
obtained with repetitions thereof. 
Problems inherent in this type of distance measuring equipment will be 
explained with reference to FIG. 10. A waveform shown in FIG. 10 indicates 
an output signal P5 of the sample hold circuit 5. The symbol LT designates 
a predetermined threshold value for detecting a reflected pulse beam. The 
processor 6 in the conventional equipment performs a detection by 
comparing, with the threshold value LT, a level of the light receiving 
signal with respect to the reflected pulse beam from the object. The 
processor 6 calculates a distance L by use of a count value N of the clock 
pulses P1 at that time. With this processing, as illustrated in FIG. 10, 
an error .DELTA.N is produced in the count value N of the clock pulses P1, 
depending on a magnitude of the level of the light receiving signal. 
Consequently, the measured distance L has an error given by 
.DELTA.N.times..DELTA.t.times.C/2. For this reason, if a reflectivity of 
the object 7 serving as an object for measurement is different, and even 
when existing at the same distance, an intensity of the reflected beam 
differs. For the reason given above, there arises a problem wherein a 
different distance L is to be measured, resulting in an error caused in 
the measured distance L. For example, this type of distance measuring 
equipment is mounted on a car. If the distance measuring equipment is 
utilized for a system for keeping a safe car-to-car distance by measuring 
a distance to a foregoing car, danger is present because the detected 
distance varies according to the type of the foregoing car. Namely, a 
measured distance error arises due to the reflectivity of the object, and 
becomes a serious problem which affects the safety and reliability of a 
system incorporating this type of equipment. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention, which has been devised to 
obviate the problems described above, to provide a distance measuring 
equipment capable of measuring a distance irrespective of a magnitude of a 
level of a reflected beam from an object with respect to a pulse beam 
generated from the distance measuring equipment. 
To accomplish this object, according to one aspect of the present 
invention, there is provided a distance measuring equipment comprising: a 
light emitting element for generating a pulse beam; a light receiving 
element for receiving a reflected pulse beam from an object under 
measurement with respect to the pulse beam generated from the light 
emitting element and converting the reflected pulse beam into an electric 
light receiving signal; an object detecting element for detecting the 
under-measurement object on the basis of a level of an output signal of 
the light receiving signal through the light receiving element; and a 
calculating element for calculating, when the object detecting element 
detects the under-measurement object, a distance to the under-measurement 
object on the basis of a time obtained by subtracting a predetermined time 
corresponding to a half-value of a pulse width of the light receiving 
signal from a delay time from a generation of the pulse beam through the 
light emitting element to a reach of a peak of the light receiving signal 
through the light receiving element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment 1 of this invention will be explained with reference to the 
drawings. Turning to FIG. 1, a light sending device 1 generates a pulse 
beam P2 by actuating a light emitting element such as a laser diode, etc. 
A clock generator 2 connected to an input side of the light sending device 
1 generates a clock pulse P1 serving as a pulse beam emitting timing of 
the light sending device 1. A light receiving device 3 is disposed in 
side-by-side relationship with the light sending device 1. A sample pulse 
generator 4 connected to the clock generator 2 counts the clock pulses P1 
and generates a sample pulse P3 in which the above clock pulse P1 is 
delayed by a time interval corresponding to the count value thereof. A 
sample hold circuit 5 is connected to output sides of the sample pulse 
generator 4 and of the light receiving device 3. The sample hold circuit 5 
performs sampling of the output signals P4 of the light receiving device 3 
by use of the sample pulses P3 generated by the sample pulse generator 4. 
A processor 6A is connected to output sides of the sample pulse generator 
4 and of the sample hold circuit 5. The processor 6A is, at the same time, 
connected to the light sending device 1. The processor 6A compares the 
output signal P5 of the sample hold circuit 5 with a predetermined level. 
The processor 6A thus detects an object 7 and calculates a distance 
thereto. This processor 6A comprises a light sending device drive 
controlling element 6a, a sample pulse generator drive controlling element 
6b, a peak determining element 6c, an object detection determining element 
6d and a distance calculating element 6e. The light sending device drive 
controlling element 6a connected to a light sending device 1 
drive-controls the light sending device 1. The sample pulse generator 
drive controlling element 6b connected to a sample pulse generator 4 
drive-controls the sample pulse generator. The peak determining element 6c 
connected to a sample hold circuit 5 determines a peak of a light 
receiving signal P4 on the basis of an output signal P5 from this sample 
hold circuit 5. The object detection determining element 6d connected to 
the sample hold circuit 5 determines a detection of an object by comparing 
a level of the light receiving signal P4 with a predetermined level LT on 
the basis of the output signal P5 from this sample hold circuit 5. The 
distance calculating element 6e is connected to this object detection 
determining element 6d and to the peak determining element 6c as well. The 
distance calculating element 6e is also connected to an output side of the 
light sending device drive controlling element 6a and obtains a distance 
to an object 7 for detection. 
Next, operations of the thus configured embodiment 1 will be discussed. A 
clock generator 2 generates a clock pulse P1. The light sending device 1 
emits a pulse beam P2 in synchronism therewith. This pulse beam P2 is 
reflected by the object 7 and received by a light receiving device 3. The 
light receiving device 3 photoelectrically converts this reflected pulse 
beam and thereafter high-frequency-amplifies the pulse beam. The light 
receiving device 3 then outputs an electric signal P4 to the sample hold 
circuit 5. On the other hand, the sample pulse generator 4 counts the 
clock pulses P1 given from the clock generator 2. The sample pulse 
generator 4 generates a sample pulse P3 delayed from the clock pulse P1 by 
a time given by multiplying a count value N thereof by .DELTA.t 
corresponding to a distance resolving power. The sample hold circuit 5 
samples the light receiving signal P4 outputted from the light receiving 
device 3 by use of this sample pulse P3 and holds it up to a generation of 
the next sample pulse. The object detection determining element 6d in the 
processor 6A compares the output signal P5 from this sample hold circuit 5 
with the threshold value LT for detecting the reflected pulse beam. The 
object detection determining element 6d determines a detection of the 
object 7 by detecting a signal larger than the threshold value LT. 
Actions of the processor 6A after the detection will be explained with 
reference to FIG. 2. Note that a waveform shown in FIG. 2 indicates an 
output signal of the sample hold circuit 5, i.e., the light receiving 
signal P5. The symbol LT represents a threshold value for detecting the 
reflected pulse beam, and N designates a then-obtained clock pulse count 
value of the sample pulse generator 4. The peak determining element 6c in 
the processor 6A, after detecting the object 7, sequentially reads a level 
of the light receiving signal P5 in synchronism with the sample pulse P3 
and thus searches a peak of the waveform. When detecting the peak, a 
detection signal is outputted to the distance calculating element 6e. This 
distance calculating element 6e obtains NS=NP-NW, which corresponds to a 
delay time up to a rise starting point of the light receiving signal PS. 
This is given by subtracting a count value NW corresponding to a 
half-value of a pulse width of the light receiving signal P5 from a count 
value NP of the clock pulses P1 at that time. This equation is substituted 
into the following formula (2) to obtain a distance L. The calculated 
distance L is outputted to the outside by means of the distance 
calculating element 6e. 
EQU L=NS.times..DELTA.t.times.C/2 (2) 
Note that the half-value of the pulse width of the light receiving signal 
P5 is obtained as a half-value of an emitted pulse length from the light 
sending device 1. The count value N of the clock pulses P1 is, when the 
count value N reaches a value corresponding to the maximum detected 
distance, reset to 0. The above operations are defined as a measuring one 
period, and the distance is continuously obtained with repetitions 
thereof. 
Embodiment 2: 
Given further is an explanation of a case where the peak is undetectable 
even by reading the level of the light receiving signal for a 
predetermined or longer time in the above-described embodiment 1 in 
conjunction with FIG. 3. As illustrated in the FIG. 3, when N is a count 
value of the clock pulses P1, the light receiving signal P5 larger than 
the threshold value LT is detected. Thereafter, however, the peak can not 
be detected even when the peak determining element 6c reads the light 
receiving signal P5 over a predetermined time corresponding to a count 
value NC. In this case, the light receiving device 3 receives an excessive 
input, and, therefore, the distance calculating element 6e determines that 
the light receiving signal P5 reaches a saturation level LC. An 
inclination of the light receiving signal P5 at that time is steep, and an 
error of measurement is small. On this assumption, the distance L is 
obtained by use of the count value N of the clock pulses P1 when detecting 
the light receiving signal. The distance can be thereby measured with a 
high accuracy even if the peak is undetectable. 
Embodiment 3: 
The following is an arrangement of this embodiment 3. In the embodiment 1 
discussed above, the distance calculating element 6e calculates a distance 
L to the object 7 on the basis of a count value NS of the clock pulses P1. 
On the other hand, the distance calculating element 6e calculates an 
inclination K of the light receiving signal P5 from a rise over the 
detection level LT to a time when the peak is reached. As illustrated in 
FIG. 4, when the count value of the clock pulses P1 comes to N, the light 
receiving signal P5 larger than the threshold value LT is detected. When a 
count value NP of the clock pulses P1 becomes NP thereafter, the peak is 
detected. Here, this peak value is set as LP. Then, the inclination K of 
the light receiving signal P5 can be given by the following formula (3). 
EQU K=(LP-LT)/(NP-N) (3) 
Next, when a plurality of objects exist, as shown in FIG. 4, this 
inclination K at this time is smaller than a predetermined value. Then, 
the distance calculating element 6e is capable of determining that the 
plurality of objects exist on the basis of output signals of the peak 
determining element 6c and of the object detection determining element 6d. 
This is done on condition that this inclination K is smaller than the 
predetermined value. Then, the measured distance L obtained from the count 
value NP of the clock pulses up to the peak is determined as an error at 
this time. The distance L is obtained by use of the count value N of the 
clock pulses P1 when detecting the light receiving signal. 
Embodiment 4: 
FIG. 5 illustrates a distance measuring equipment of embodiment 4. In 
accordance with this embodiment 4, the processor 6A in the embodiment 1 is 
replaced with a processor 6B including a scattered substance determining 
element 6f connected to the distance calculating element 6e. The scatted 
substance determining element 6f compares a measured distance L calculated 
by the distance calculating element 6e with a predetermined value. If the 
measured distance L is smaller than the predetermined value, a level of 
the light receiving signal P5 does not reach the saturation level LC 
irrespective of a short distance. The scattered substance determining 
element 6f therefore determines that the object detected is related to a 
scattered substance such as a fog, etc. The scattered substance 
determining element 6f judges that the then-measured distance L is 
erroneous and outputs this result. 
Note that the distance calculating element 6e may determine an existence 
and non-existence of the scattered substance without separately providing 
the scattered substance determining element 6f. 
Embodiment 5: 
If the scattered substance determining element 6f determines that the 
detection is erroneous due to a scattered substance such as fog, etc. in 
the embodiment 4, the distance calculating element 6e detects the next 
peak of the light receiving signal P5, and the distance may be newly 
measured based on this peak. For instance, as illustrated in FIG. 6, the 
measurement based on the count value NP at the first peak detection after 
detecting the light receiving signal P5 is determined as an erroneous 
detection. In this case, the distance is to be measured based on a count 
value NP2 at the next peak detection. Then, if the distance calculated 
based on the count value NP2 is equal to or greater than a predetermined 
value, this measured distance is outputted as a correct value. 
It is apparent that, in this invention, a wide range of different working 
modes can be formed based on the invention without deviating from the 
spirit and scope of the invention. This invention is not restricted by its 
specific working modes except being limited by the appended claims.