JPH06118173A - Range finder - Google Patents

Abstract

PURPOSE:To prevent range-finding accuracy from being deteriorated even if the reception input level of a range finder using pulse laser fluctuates and at the same time the pulse width of the laser fluctuates. CONSTITUTION:When a pulse width is constant. two CFD circuits 6 for constantly generating a trigger signal at the peak position of an input waveform are used for transmission and reception, thus canceling the deviation of the trigger signal of the CFD circuits generated when the pulse width of a laser oscillator 1 fluctuates and hence preventing the operation time of a counter circuit 7 from being affected. Also, one CFD circuit 6 is shared for obtaining the same effect.

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, it relates to a highly accurate distance measuring device using a pulse laser.

[0002]

2. Description of the Related Art In general, a distance measuring device using a pulse laser has a laser oscillator 1, a transmission pulse monitor circuit 12, a transmission / reception optical system 3, a photodetector 4, an amplifier 5, a discrimination device as shown in FIG. 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 for the counter circuit 7, and most of the light is transmitted via the transmission / reception optical system 3. After hitting the target object 11, 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 a signal exceeding a predetermined level is taken out by the discrimination circuit 9. , As a stop signal of the counter circuit 7. The propagation distance of light is calculated from the time from the start of the counter circuit 7 to the stop, and the distance to the target is displayed on the display 8 or the like.
As shown in FIG. 6, since the laser waveform requires a certain amount of time to rise, if the intensity of the received light differs as in waveform A or B due to the difference in the distance to the target and the difference in the reflectance of the target. , When the discrimination circuit 9 is operated with 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, the following various measures have been proposed and put into practical use. Under each limited condition, the corresponding effect is exhibited, but under other conditions, there may be a problem. The outline, advantages, and problems of the method for removing the error in the distance measurement caused by the laser waveform, especially the rising portion, will be described below.

1) Measuring method with a pulse width measuring circuit added See Japanese Patent Application Laid-Open No. 3-81687. A pulse width measuring circuit 10 is added as shown in FIG. 7A and transmitted as shown in FIG. 7B. This is a method of performing an operation corresponding to measuring the time between the threshold levels of the pulse rise and the fall of each of the reception and the reception, and measuring the interval at the intermediate point, that is, the position corresponding to the peak of the pulse.

In this method, as shown in FIG. 7B, the pulse width T1 of the light detection signal of the emitted laser light signal, the light receiving system output signal T2 of the input laser light signal to the light receiving system, and their detection. The time interval TD between the signal and the output signal is TD-1 /
2 * T1 + 1/2 * T2 is calculated to measure the peak interval T between the emitted laser light and the received laser light. The advantage of this method is that it is relatively simple compared to other methods, but the rising and falling edges of the transmission pulse are not targeted, or the irradiated light hits other than the target 11 and the received pulse is received. If the trailing edge of the pulse becomes gradual, the center of the time cut at the threshold level does not match the peak of the pulse, which causes a problem that the measurement time changes depending on the strength of the input level. As a method similar to this, there is a method, as described in JP-A-51-22465, in which two sets of counters are prepared and the interval between rising and falling is measured, but there are similar advantages and problems. is doing.

2) A method in which a delay circuit and an attenuator circuit are used in combination and the counter circuit is always operated at a fixed position of the pulse waveform regardless of the intensity of the received pulse.

Constant Fraction Discriminator (Constant Fraction Di)
A circuit called a "scrimilator" and generally used as an abbreviated CFD (hereinafter also abbreviated as CFD in the present invention) is used to generate a pulse 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, "Applied Optics, Vol. 16, No. 1, p. 16-
Page 18 (Applied Optics, Vol. 1)
6, No. 1. January, 1977) ". As shown in FIG. 8A, C is used instead of the simple discrimination circuit 9.
The FD circuit 6 is used. As shown in FIG. 8B, the CFD circuit divides the received pulse into two parts, one of which is attenuated to 1/2 by the attenuation circuit 6a and the other of which is delayed by the delay circuit 6a.
In general, after delaying by 1/2 the pulse width, that is, the time from the half point of the peak to the peak of the pulse, the discriminating circuit (discriminator) 6c
Then, a signal for stopping the counter circuit 7 is sent at the moment when the two have the same intensity, that is, at the time corresponding to the position of the peak of the received pulse. Actually, it is possible that the CFD operates even with a slight noise, so that FIG.
As shown in (c), two discrimination circuits are used, and the discrimination circuit 6d
Therefore, the method is taken into consideration such that the AND circuit 6e operates to obtain the signal for stopping the counter circuit 7 only when the signal of the 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 asymmetrical 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 coincide with the half point of the pulse and the time until the peak of the pulse. For semiconductor lasers and the like that can obtain a laser waveform that is almost equal to the driving waveform, the fluctuation of the pulse waveform does not actually pose a problem, but a solid-state laser excited by a flash lamp, in particular, temperature In a three-level laser in which the level state changes and the oscillation waveforms are different, the pulse width or the time from 1/2 of the peak to the peak of the pulse changes depending on the temperature. Also, 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 that changes the delay time according to the outside air temperature. As the laser substance rises and the pulse waveform changes as is fired, the correction is not easy. Furthermore, the laser waveform is affected not only by the problem that the state of the upper level changes with temperature but also by the distortion caused by the uneven heat distribution. Therefore, even if the temperature is accurately monitored, the pulse waveform should 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.
The method of calculating how early the sigma moves and correcting the amount corresponding to the time is described in Japanese Patent Laid-Open No. 3-38898. A method of storing a pulse waveform including a delay time corresponding to the propagation distance with a digital oscilloscope having a high sample rate and calculating the distance from the time to the peak of the received waveform can be considered, but the details are omitted.

Considering the problem that the pulse waveform fluctuates, not only time fluctuation occurs on the receiving side, but also time fluctuation occurs on the transmitting side, that is, when the start pulse signal is generated, as described below. Generally, in the case of a solid-state laser excited by a flash lamp, even if it is excited by the same energy, the pulse width increases and the peak output decreases as the temperature of the laser material rises. If the pulse width widens at the same peak output, it moves forward when it exceeds the trigger point, that is, the output threshold level, but since the peak output decreases, the trigger point does not always move forward even if the pulse width widens. Absent. For example, as shown in FIG. 9A or FIG. 9B, due to the relative relationship between the pulse waveform and the threshold level, the triggering time ΔT2 becomes earlier than the original ΔT1 even when the pulse width similarly expands. Or slow down. Therefore, in the case of 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 the start pulse to the counter circuit 7 fluctuates and causes an error in distance measurement. Cause.

The CFD circuit 6 will now be described in more detail. As shown in various documents and commercially available CFD circuit data, when a laser having a pulse width of several ns to several tens of ns is used, even if the input signal level differs by two digits or more,
As long as the pulse width is constant, the movement of the trigger position (called a 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 in (a), the point where the CFD operates coincides with the peak of the pulse 101 which is not delayed, but (b) The point at which the CFD operates when the pulse width is widened while the delay time remains unchanged as shown in (1) comes before the peak of the pulse 101 that is not delayed. That is, the measurement time is shortened, and the distance is calculated shorter. On the other hand, when the pulse width becomes narrow while the delay time remains unchanged as shown in (c), the point at which the CFD operates becomes behind, that is, the measurement time becomes longer, and the distance is calculated longer. As a specific calculation example, in FIG. 11, when the delay time is set to 15 ns and the pulse width is expanded to 40 ns or narrowed to 20 ns, the amount of movement of the CFD operating point is calculated so as to be optimal when the pulse width is 30 ns. Show the table. Pulse waveform assuming a Gaussian distribution i.e. normal distribution, y = exp (- (2.3548 × t / W) 2/2)
<< t: time from peak (ns) W; pulse full width at half maximum (ns) y; pulse intensity at a point separated by t from peak >>. Looking at the 40 ns column in FIG. 11, it can be seen that the intensity of what is attenuated to 1/2 at a delay time of 0 and the intensity of what is delayed for 15 ns are equal to each other.
At an intensity around 0.472, that is, -5ns and -6n
between 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 one attenuated to 1/2 at the delay time 0 and the intensity of the one delayed for 15 ns are equal at the intensity around 0.444, that is, at 4 ns and 5 ns. between. Specifically, pulse width 30
The counter circuit stops about 4.2 ns later than 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 the CFD circuit. Can not adapt to. Actually, the pulse width rarely fluctuates by ± 10 ns as in the above calculation example, but the stop time of the counter circuit 7 is 1 m.
Since a distance measurement error of 15 cm occurs even if the distance is shifted by s, the fluctuation of the pulse width cannot be ignored in the highly accurate distance measurement. In addition, 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 is a complex combination of two complex factors.

[0011]

As a first invention, a laser oscillator, a transmitting / receiving optical system for irradiating a target with laser light from the laser oscillator, and receiving reflected light thereof, and a laser from the laser oscillator are provided. A transmission pulse monitor circuit that receives a part of light and detects a peak value of a pulse signal, and a photodetector that converts a target reflected light from the transmission / reception optical system into an electric signal. Second, which receives the signal from the photodetector and detects the peak value of the received signal
CFD circuit, a counter circuit that receives an output from the monitor circuit to start counting, and a counter circuit that receives an output from the second CFD circuit to stop the counting, and a target calculated from the count value of the counter circuit. Provided is a distance measuring device including a display circuit for displaying a distance to an object.

As a second invention, in the distance measuring apparatus of the first invention, an introducing device for directly inputting a part of laser light from the laser oscillator to the photodetector is provided, and the counter circuit is the first invention. There is provided a distance measuring device characterized by starting counting by the first input from the CFD circuit of No. 2 and stopping counting by the next input.

As a third invention, a laser oscillator, a transmission / reception optical system for irradiating a target with laser light from the laser oscillator and receiving reflected light thereof, and a part of the laser light from the laser oscillator are received. A transmission pulse monitor circuit for
A matching circuit for matching the signal from the monitor circuit, a photodetector for converting the reflected light of the target object from the transmitting / receiving optical system into an electric signal, and a signal from the photodetector and an output from the matching circuit. , CF for detecting peak value of received signal
D circuit, a counter circuit that starts counting by the first input from this CFD circuit, and stops counting by the next input, and a display that displays the distance to the target object calculated from the count value of this counter circuit A distance measuring device comprising: a circuit.

A fourth invention is the distance measuring device according to the first to third inventions, wherein the photodetector is a photodetector having an amplifying action, and a detection signal from the photodetector is directly fed to a CFD circuit. There is provided a distance measuring device characterized in that

[0015]

The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an 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 used as a start signal for the counter circuit 7, and most of the light is transmitted / received by the transmission / reception optical system 3.
Then, a part of the reflected light is received by the transmission / reception optical system 3, converted into an electric signal by the photodetector 4 and 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 taken out and used as a stop signal for the counter circuit 7. The propagation distance of light is calculated from the time from the start of the counter circuit 7 to the stop, 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 the half of the peak of the laser pulse to the peak, as shown in FIG. It will come out from the position that matches
If the delay time peculiar to the circuit is neglected, the signal detecting the light propagating through the distance R is delayed by 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, when the delay time of the delay circuit 6b of the second CFD circuit 6 is different from the time from the half of the laser pulse peak to the peak due to the widened 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 in 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 circuits 2c are exactly the same, the start signal also comes out earlier by the same ΔT. Therefore, ΔT + (2R ÷ C−ΔT) = 2R ÷ C, and the counter circuit 7 is operating for a time corresponding to the distance R even if there are operating time fluctuations that occur according to pulse width fluctuations. Becomes As described above, the feature of the present invention resides in that the distance can be accurately measured regardless of the fluctuation of the pulse width by using the two CFD circuits which cannot individually cope with the fluctuation of the pulse width. Further, by using the CFD circuit on the transmitting side as well, the generation time of the start signal changes with respect to the change in the reflectance 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 doing so is also added. Figure 1 shows the first CFD
The detailed block diagram of the circuit built-in transmission pulse monitor circuit 2 is shown. A part of the light emitted from the laser oscillator 1 is split by the beam splitter 2a and converted into an electric signal by the photodetector 2b. Since the split light is extremely strong as compared with the light reflected by the target object 11, the photodetector 2b may have low sensitivity. Rather, an attenuator or the like may be inserted in front of the detector if necessary. 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 it is the same as the D circuit 6, the explanation of the operation is omitted.
In order to obtain the characteristics 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 the required accuracy range. As in the other embodiment shown in FIG. 4, it is conceivable that one CFD circuit for transmission / reception also serves as one. FIG. 4A shows a method of directly entering a part of the light from the laser oscillator 1 into the photodetector 4 by using the optical fiber 13 or the like. Although the device configuration may be simplified and the cost may be reduced, since the light from the laser oscillator 1 is extremely strong, the photodetector 4 may be saturated or burned out. You need to be careful. FIG. 4B shows a method in which the output from the photodetector 2b in FIG. 3 is directly input to the CFD circuit 16. First
The display without a CFD circuit 2c is unified because it is the same as the transmission pulse monitor circuit 12 of FIG. If the CFD circuit is shared, the number of paths to the counter circuit 7 becomes one, so in practice, a reset pulse or the like should be given separately, and then the first input signal should start and the next signal should stop. However, detailed description thereof is omitted because it is not directly related to the description of the present invention. Since the received signal from the target object 11 is extremely weak, it is not a good idea to divide it even after amplifying it. Figure 4
When the signal from the transmission pulse monitor circuit 12 is directly connected to the CFD 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 distorted due to reflection from the transmission pulse monitor circuit 12. Therefore, the connection is actually made via the matching circuit 14 as shown in FIG. This matching circuit 14 is a CFD
The circuit 16 must have a high impedance and the transmission pulse monitor circuit 12 must have a low impedance in view of the response of a high speed pulse. Since a large output can be obtained from the transmission pulse monitor circuit 12, the attenuation by the matching circuit 14 is not a big problem.

[0016]

As described above, according to the present invention, substantially two CFD circuits are used. Therefore, the operation time of the counter circuit for calculating the distance is irrespective of the magnitude of the received input signal. Even if the pulse width of the laser does not change, the operating time of the counter circuit for calculating the distance does not change, so that the distance can be accurately measured.

[Brief description of drawings]

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

FIG. 2 is an explanatory view that the use of two CFD circuits cancels a pulse width variation.

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 counter circuit stop time varies depending on the size of a reception input.

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

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

FIG. 9 is an explanatory diagram for the start pulse generation time being shifted before or after the pulse width is expanded.

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

[Explanation of symbols]

 1 Laser Oscillator 2 First CFD Circuit Built-in Transmission Pulse Monitor 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 (4)

[Claims] 1. A laser oscillator, a transmission / reception optical system for irradiating a target with laser light from the laser oscillator, and receiving reflected light thereof, and a pulse signal for receiving a part of the laser light from the laser oscillator. A transmission pulse monitor circuit having a built-in first CFD circuit for detecting the peak value of, a photodetector for converting the reflected light of the target object from the transmission / reception optical system into an electric signal, and a signal from the photodetector. A second CFD circuit that detects the peak value of the received signal, and starts counting when receiving an output from the monitor circuit,
A distance including a counter circuit that stops the counting upon receiving an output from the second CFD circuit, and a display circuit that displays the distance to the target object calculated from the count value of the counter circuit. measuring device. 2. The distance measuring device according to claim 1, further comprising an introducer for directly inputting a part of the laser light from the laser oscillator to the photodetector, wherein the counter circuit is a first circuit from the second CFD circuit. 2. The distance measuring device according to claim 1, wherein the counting is started by the input of and the counting is stopped by the next input. 3. A laser oscillator, a transmission / reception optical system for irradiating a target with laser light from the laser oscillator, and receiving reflected light thereof, and a transmission pulse monitor for receiving a part of the laser light from the laser oscillator. A circuit, a matching circuit for matching the signal from the monitor circuit, a photodetector for converting the reflected light of the target object from the transmitting / receiving optical system into an electric signal, a signal from the photodetector and a signal from the matching circuit. A CFD circuit that receives the output and detects the peak value of the received signal, a counter circuit that starts counting by the first input from this CFD circuit, and stops counting by the next input, and the count value of this counter circuit And a display circuit for displaying the calculated distance to the target object. 4. The distance measuring device according to claim 1, wherein the photodetector is a photodetector having an amplifying action, and a detection signal from the photodetector is directly input to a CFD circuit. The described distance measuring device.