JP2967656B2 - Distance measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distance measuring device,
In particular, the present invention relates to a high-precision distance measuring device using a pulse laser.

[0002]

2. Description of the Related Art Generally, as shown in FIG. 5, a distance measuring apparatus using a pulse laser includes a laser oscillator 1, a transmission pulse monitor circuit 12, a transmission / reception optical system 3, a photodetector 4, an amplifier 5, a discriminator. It comprises a circuit 9, a counter circuit 7, a display 8, and the like. The basic operation is that a part of the light emitted from the laser oscillator 1 is detected by the transmission pulse monitor circuit 12 and used as a start signal of the counter circuit 7, and most of the light is transmitted via the transmission / reception optical system 3. A part of the reflected light is received by the transmission / reception optical system 3 and is converted into an electric signal by the photodetector 4, amplified by the amplifier 5, and then a signal exceeding a predetermined level is extracted by the discrimination circuit 9. , The stop signal of the counter circuit 7. The light propagation distance is calculated from the time from when the counter circuit 7 starts to when it stops, and the distance to the target is displayed on the display 8 or the like.
As shown in FIG. 6, since the laser waveform takes a certain amount of time to rise, if the intensity of the received light level differs as in the waveform A or B due to the difference in the distance to the target or the difference in the reflectance of the target. When the discrimination circuit 9 is operated at a constant input V, the time T1 at which the counter circuit 7 stops
And T2 are different, and the distance to the target is calculated differently. In order to solve this problem, various measures described below have been proposed and put to practical use. Under the respective limited conditions, the corresponding effect is exhibited, but under other conditions, there may be a problem. Hereinafter, an outline of a method of removing a distance measurement error caused by a laser waveform, particularly a rising portion, and advantages and problems will be described.

[0003] 1) Measuring method in which a pulse width measuring circuit is added See JP-A-3-81687. A pulse width measuring circuit 10 is added as shown in FIG. 7 (a) and transmitted as shown in FIG. 7 (b). This is a method of measuring the time between the threshold levels of the rise and fall of each pulse and the time of reception, and performing an operation equivalent to measuring the intermediate point between them, that is, the interval corresponding to the peak of the pulse.

In this method, as shown in FIG. 7 (b), a pulse width T1 of a light detection signal of an emitted laser light signal, a light receiving system output signal T2 of an input laser light signal to the light receiving system, and detection of these signals. The time interval TD between the signal and the output signal is represented by TD-1 /
By calculating 2 * T1 + 1/2 * T2, the peak interval T between the emitted laser light and the received laser light is measured. The advantage of this method is that it is relatively simple compared to other methods, but the rising and falling of the transmission pulse is asymmetric, When the falling edge of the signal becomes gentle, the center of the time cut at the threshold level does not coincide with the peak of the pulse, which causes a problem that the measurement time changes depending on the input level. As a similar method, there is a method of preparing two sets of counters and measuring the interval between rising and falling edges as described in JP-A-51-22465, but has the same advantages and problems. doing.

[0005] 2) A method in which a counter circuit is always operated at a fixed position of a pulse waveform by using both a delay circuit and an attenuation circuit, regardless of the intensity of a received pulse.

[0006] Constant Fraction Discriminator (Constant Fraction Di)
Using a circuit generally called a CFD (hereinafter abbreviated as a CFD in the present invention), a pulse is used regardless of the asymmetry of the transmission pulse or the gradual fall of the reception pulse. There is a method that can detect the peak position. For example, "January 1977, Applied Optics, Vol. 16, No. 1, page 16-
Page 18 (Applied Optics, Vol. 1).
6, No. 1. January, 1977) ". As shown in FIG. 8A, instead of a simple discrimination circuit 9, C
The FD circuit 6 is used. As shown in FIG. 8B, the CFD circuit divides a received pulse into two, one of which is attenuated by half by an attenuation circuit 6a, and the other is a delay circuit 6
b, the delay is delayed by half the pulse width, that is, the time from the half point of the peak to the peak of the pulse, and then the discrimination circuit (discriminator) 6c
And a signal for stopping the counter circuit 7 at the moment when the two have the same intensity, that is, at the time corresponding to the peak position of the received pulse. Actually, it is conceivable that the CFD operates even with a slight noise.
As shown in (c), two discriminating circuits are used, and a discriminating circuit 6d is used.
Therefore, a method is considered in which the AND circuit 6e operates and a signal for stopping the counter circuit 7 is obtained only when a signal having a level exceeding the reference voltage is obtained. The advantage of this method is that the discrimination circuit 6c can always be operated at the peak position of the pulse even if the transmission pulse is asymmetric or the fall of the reception pulse becomes gentle due to reflection from the rear of the target 11. That is.
However, in order for this condition to be satisfied, the delay time of the delay circuit 6b must exactly match the time between the half point of the pulse and the peak of the pulse. For a laser such as a semiconductor laser capable of obtaining a laser waveform substantially equal to the waveform to be driven, the fluctuation of the pulse waveform may not be a problem, but a solid-state laser excited by a flash lamp, In a three-level laser in which the state of the level changes and the oscillation waveform is different, the time from the pulse width or the half of the peak to the peak of the pulse changes depending on the temperature. If the temperature of the laser substance only changes with the outside air temperature, it may be possible to adjust the delay time using a delay circuit or the like that changes the delay time according to the outside air temperature. As the laser material rises and the pulse waveform changes as the light is emitted, the correction is not easy. Furthermore, the laser waveform is affected not only by the problem that the state of the upper level changes depending on the temperature, but also by the distortion due to the unevenness of the heat distribution. Therefore, even if the temperature is accurately monitored, the pulse waveform must be estimated. It is difficult.

3) Other methods In the case of a known pulse waveform, the intensity of the received pulse is measured using a peak hold circuit or the like, and the counter circuit 7 is determined from the pulse waveform and the received intensity.
A method of calculating how early the device operates and correcting the time equivalent is disclosed in JP-A-3-38898. A method of storing a pulse waveform including a delay time corresponding to a propagation distance with a digital oscilloscope having a high sampling rate and calculating a distance from a time until a peak of a received waveform may be considered, but details are omitted.

When considering the problem that the pulse waveform fluctuates, not only the time fluctuation occurs on the receiving side, but also the time fluctuation occurs on the transmitting side, that is, the generation of the start pulse signal as described below. In general, in the case of a solid-state laser excited by a flash lamp, even if excited by the same energy, the pulse width increases and the peak output decreases as the temperature of the laser material increases. If the pulse width increases with the same peak output, the trigger point, that is, if the output threshold level is exceeded, moves forward, but the peak output decreases, so even if the pulse width increases, the trigger point does not always move forward. Absent. For example, as shown in FIG. 9A or 9B, the triggering time ΔT2 is earlier than the original ΔT1 due to the relative relationship between the pulse waveform and the threshold level, even if the pulse width is similarly widened. Or slow down. Therefore, in a laser whose pulse width fluctuates, the relationship between the peak position of the pulse to be transmitted and the time for triggering the transmission pulse monitor circuit 12 to generate a start pulse to the counter circuit 7 fluctuates, and the error of the distance measurement varies. Cause.

Here, the CFD circuit 6 will be described in more detail. As shown in various documents and data of commercially available CFD circuits, when a laser having a pulse width of several ns to several tens of ns is used, even if the level of the input signal differs by two digits or more,
As long as the pulse width is constant, the movement of the trigger position (called time walk) is ns or less (about 100 ps). On the other hand, FIG. 10 shows an example in which the trigger position moves when the pulse width changes. When the delay time of the signal 100 to be delayed is equal to the time from the half point of the peak to the peak as shown in (a), the point where the CFD operates coincides with the peak of the pulse 101 which is not delayed. The point where the CFD operates when the pulse width is widened with the delay time unchanged as in ()) comes before the peak of the pulse 101 which is not delayed. That is, the measurement time becomes shorter, and the distance is calculated to be shorter. On the other hand, if the pulse width is reduced while the delay time remains unchanged as in (c), the point at which the CFD operates is later, that is, the measurement time is longer, and the distance is calculated longer. As a specific calculation example, the movement amount of the CFD operation point when the pulse width is widened to 40 ns or narrowed to 20 ns is determined so that the delay time is set to 15 ns so that the pulse width becomes 30 ns in FIG. A table is shown. Pulse waveform assuming a Gaussian distribution i.e. normal distribution, y = exp (- (2.3548 × t / W) 2/2)
<T: time from peak (ns) W; full width at half maximum of pulse (ns) y: pulse intensity at point distant from peak by t> Looking at the column of 40 ns in FIG. 11, the intensity of the signal attenuated by で with a delay time of 0 is equal to the intensity of the signal delayed by 15 ns.
Where the intensity is around 0.472, ie -5 ns and -6 n
s. Specifically, the counter circuit stops about 5.7 ns earlier than when the pulse width is 30 ns.
Looking at the column of 20 ns in FIG. 11, the intensity of the signal attenuated by に with a delay time of 0 and the intensity of the signal delayed by 15 ns become equal at the intensity around 0.444, that is, between 4 ns and 5 ns. between. Specifically, pulse width 30
The counter circuit stops about 4.2 ns later than at the time of ns.

[0010]

As described above,
The CFD circuit has an effect that the trigger position does not fluctuate with respect to the fluctuation of the input level due to the distance to the target 11 and the reflectance of the target 11, but the fluctuation of the pulse width of the laser oscillated by itself. Can not adapt to Actually, the pulse width does not vary as much as ± 10 ns as in the above calculation example, but the stop time of the counter circuit 7 is 1 ns.
Even if the distance is shifted by s, a distance measurement error of 15 cm occurs, so that a pulse width fluctuation cannot be ignored in high-accuracy distance measurement. Further, the fluctuation of the pulse width also affects the generation time of the start pulse of the counter circuit 7, and the distance measurement error has a problem that two complex factors are complicatedly combined.

[0011]

According to a first aspect of the present invention, there is provided a laser oscillator, a transmission / reception optical system for irradiating a target object with laser light from the laser oscillator and receiving the reflected light, and a laser from the laser oscillator. A transmission pulse monitoring circuit incorporating a first CFD circuit for receiving a part of light and detecting a peak position of a pulse signal; and a photodetector for converting target reflected light from the transmission / reception optical system into an electric signal. A second CFD circuit that receives a signal from the photodetector and detects a peak position of a received signal, and starts counting by receiving an output from the monitor circuit, and outputs an output from the second CFD circuit. And a display circuit for displaying the distance to a target calculated from the count value of the counter circuit. .

As a second invention, in the distance measuring apparatus according to the first invention, there is provided an introducer for directly inputting a part of laser light from the laser oscillator to the photodetector, and the counter circuit is provided with the counter circuit. A distance measuring device is provided, wherein the counting is started by the first input from the CFD circuit 2 and stopped by the next input.

According to a third aspect of the present invention, there is provided a laser oscillator, a transmission / reception optical system for irradiating a target object with laser light from the laser oscillator and receiving the reflected light, and receiving a part of the laser light from the laser oscillator. A transmission pulse monitor circuit that performs
A matching circuit for matching a signal from the monitor circuit, a photodetector for converting target reflected light from the transmission / reception optical system into an electric signal, and a signal from the photodetector and an output from the matching circuit. , Which detects the peak position of the received signal
FD circuit, a counter circuit that starts counting by the first input from the CFD circuit, and stops counting by the next input, and a display that displays the distance to the target calculated from the count value of the counter circuit. A distance measuring device comprising a circuit.

According to a fourth aspect, in the distance measuring apparatus of the first to third aspects, the photodetector is a photodetector having an amplifying action, and a detection signal from the photodetector is directly transmitted to a CFD circuit. The distance measuring device is characterized in that the distance is input to the distance measuring device.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of one embodiment of the present invention. A part of the light emitted from the laser oscillator 1 is detected by the first transmission pulse monitor circuit 2 with a built-in CFD circuit, and is used as a start signal of the counter circuit 7.
And a part of the reflected light is received by the transmission / reception optical system 3, converted into an electric signal by the photodetector 4, amplified by the amplifier 5, and then by the second CFD circuit 6. A signal appearing at a position equal to the peak of the received pulse is extracted and used as a stop signal for the counter circuit 7. The light propagation distance is calculated from the time from when the counter circuit 7 starts to when it stops, and the distance to the target is displayed on the display 8 or the like.
When the delay time of the delay circuit 6b of the second CFD circuit 6 is equal to the time from 1/2 of the peak of the laser pulse to the peak, as shown in FIG. Because it comes out of the position that matches
Considering the circuit-specific delay time, etc., a signal that detects light that has propagated over the distance R is generated with a delay of 2R ÷ C <<C; speed of light >>. Therefore, the distance R can be calculated by measuring the time difference between the start signal and the stop signal. On the other hand, if the delay time of the delay circuit 6b of the second CFD circuit 6 is different from the time from half the peak of the laser pulse to the peak due to the increase in the pulse width, the operating point of the second CFD circuit 6 Does not coincide with the peak of the pulse, and the receiving side can generate a stop signal earlier by ΔT. However, since the CFD is also used for the transmission pulse monitor circuit, the delay circuit 6b also has a delay time and the first CF incorporated in the transmission pulse monitor circuit.
If the delay times of the D circuit 2c are exactly the same, the start signal will also appear earlier by the same ΔT. Therefore, ΔT + (2R ÷ C−ΔT) = 2R ÷ C, and the operation time of the counter circuit 7 is the time corresponding to the distance R, even if the operation time varies depending on the pulse width variation. Becomes As described above, the feature of the present invention is that the distance can be accurately measured irrespective of the pulse width variation by using two CFD circuits that cannot cope with the pulse width variation alone. Also, by using the CFD circuit on the transmitting side, the generation time of the start signal changes with respect to the change in the reflectivity of the beam splitter 2a (shown in FIG. 3) due to the change in the laser output and the change in the polarization component. The effect of not being added is also added. FIG. 3 shows the first CFD
FIG. 2 shows a detailed block diagram of a transmission pulse monitor circuit 2 with a built-in circuit. A part of the light emitted from the laser oscillator 1 is split by a beam splitter 2a and converted into an electric signal by a photodetector 2b. Since the split light is much stronger than the light reflected by the target 11, the photodetector 2b may have low sensitivity. Rather, an attenuator or the like may be inserted before the detector as needed. The output from the photodetector 2b enters the first CFD circuit 2c and generates a start signal. The first CFD circuit 2c is the second CF shown in FIG.
Since the operation is the same as that of the D circuit 6, the description of the operation is omitted.
In order to take advantage of the features of the present invention, it is necessary that the delay time of the first CFD circuit 2c and the delay time of the second CFD circuit 6 exactly match within a required accuracy range. It is also conceivable to use a single CFD circuit for transmission and reception as in the other embodiment shown in FIG. FIG. 4A shows a method in which a part of light from the laser oscillator 1 is directly input to the photodetector 4 by the optical fiber 13 or the like. Although the device configuration may be simplified and the price may be reduced, since the light from the laser oscillator 1 is extremely strong, the photodetector 4 may saturate or burn out. Care must be taken. FIG. 4B shows a method in which the output from the photodetector 2b in FIG. First
Since the circuit in which the CFD circuit 2c is omitted is the same as the transmission pulse monitor circuit 12 in FIG. 5, the display is unified. If a common CFD circuit is used, there is only one path to the counter circuit 7. Therefore, in practice, a reset pulse or the like is provided separately, and then consideration is given, such as starting with the first signal and then stopping with the next signal. Is also required, but has no direct relation to the description of the present invention, so that detailed description is omitted. Since the reception signal from the target 11 is extremely weak, it is not advisable to divide the signal even after amplification. FIG.
When the signal from the transmission pulse monitor circuit 12 is directly connected to the CFD circuit 16 as shown in (b), a part of the signal from the amplifier 5 is divided into the transmission pulse monitor circuit 12 or the signal itself is reflected by the reflection therefrom. In practice, the matching circuit 14 may be distorted, as shown in FIG.
Will be connected via. This matching circuit 14 is C
When viewed from the FD circuit 16, the impedance must be high, and when viewed from the transmission pulse monitor circuit 12, the impedance must be low considering the response of a high-speed pulse. Since a large output is obtained from the transmission pulse monitor circuit 12,
Attenuation by the matching circuit 14 is not a major problem.

[0016]

As described above, the present invention employs substantially two CFD circuits, so that the time during which the counter circuit for calculating the distance operates while the size of the received input signal is large or small. Even if the pulse width of the laser does not change, the operation time of the counter circuit for calculating the distance does not change, so that the distance can be measured accurately.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing that the use of two CFD circuits cancels out pulse width fluctuation.

FIG. 3 is a detailed block diagram of a transmission pulse monitor circuit with a built-in CFD circuit.

FIG. 4 is a block diagram of another embodiment of the present invention.

FIG. 5 is a block diagram of a general distance measuring device.

FIG. 6 is an explanatory diagram showing that the stop time of the counter circuit varies depending on the magnitude of a reception input.

FIG. 7 is an explanatory diagram of a method having a pulse width measurement circuit.

FIG. 8 is an explanatory diagram of a method using a CFD circuit and a CFD circuit.

FIG. 9 is an explanatory diagram showing that the start pulse generation time is shifted forward or backward due to the spread of the pulse width.

FIG. 10 is an explanatory diagram of a shift in the operation position of the CFD circuit when the delay time and the time from 1/2 of the pulse to the peak are different.

FIG. 11 is a table used for calculating an operation position shift of the CFD circuit when a delay time and a time from a half of a pulse to a peak are different.

[Explanation of symbols]

 Reference Signs List 1 laser oscillator 2 first transmission pulse monitor circuit with built-in CFD circuit 2a beam splitter 2b photodetector 2c first CFD circuit 3 transmission / reception optical system 4 photodetector 5 amplifier 6 second CFD circuit 6a attenuation circuit 6b delay circuit 6c Discrimination circuit 6d Discrimination circuit 6e AND circuit 7 Counter circuit 8 Display circuit 9 Discrimination circuit 10 Pulse width measurement circuit 11 Target 12 Transmission pulse monitor circuit 13 Optical fiber 14 Matching circuit

Claims (3)

(57) [Claims] 1. A laser oscillator, a transmission / reception optical system for irradiating a target object with laser light from the laser oscillator and receiving the reflected light, and receiving a part of the laser light from the laser oscillator to generate a pulse signal A transmission pulse monitoring circuit incorporating a first CFD circuit for detecting a peak position of the light, a photodetector for converting target reflected light from the transmission / reception optical system into an electric signal, and receiving a signal from the photodetector. A second CFD circuit for detecting a peak position of a received signal, and receiving an output from the monitor circuit, starting counting,
A distance circuit comprising: a counter circuit that receives an output from the second CFD circuit to stop the counting; and a display circuit that displays a distance to a target calculated from a count value of the counter circuit. measuring device. 2. A laser oscillator, and a target object is irradiated with laser light from the laser oscillator.
A transmission / reception optical system for receiving reflected light of the laser beam, and directly inputting a part of laser light from the laser oscillator
An introducer, the laser light introduced by the introducer and the transmission / reception optical system
Photodetector that sequentially converts the reflected light of these targets into electrical signals
And electrical signals sequentially output from the photodetector,
Circuit for sequentially detecting the peak position of these electric signals
And the first output of the outputs sequentially output from the CFD circuit.
Start counting by receiving force and count by receiving the next output.
And a target circuit calculated from the count value of this counter circuit.
And a display circuit for displaying the distance of the object. 3. A laser oscillator, a transmitting / receiving optical system for irradiating a target with laser light from the laser oscillator and receiving the reflected light, and a transmission pulse monitor for receiving a part of the laser light from the laser oscillator. A matching circuit that matches a signal from the monitor circuit; a photodetector that converts target reflected light from the transmission / reception optical system into an electric signal; and an output signal from the matching circuit and the photodetector. No electricity
C which sequentially receives the air signal and sequentially detects the peak position of the signal
An FD circuit, a counter circuit that receives detection signals sequentially detected by the CFD circuit, starts counting by a first detection signal, and stops counting by the next input, and a target calculated from the count value of the counter circuit. A distance measuring device comprising: a display circuit for displaying a distance to an object.