JP2765530B2 - Laser 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 laser distance measuring apparatus using a laser beam, and more particularly to a laser distance measuring apparatus for measuring a distance at an extremely long distance.

[0002]

2. Description of the Related Art Conventionally, a pulse laser is used for long distance measurement, and a laser beam emitted from an apparatus hits a target.
The time until the light is reflected back is measured by a counter circuit or the like, and the distance is calculated from the light propagation speed. In this case, as the distance increases, the amount of returning light decreases, the received light power decreases, and the signal is masked by noise or the like of the photodetector, which makes detection difficult, so that the laser output is increased or the noise is reduced. The device was devised such as using a detector.

However, increasing the laser output has problems such as safety in addition to the limitation due to the size of the apparatus. On the other hand, there is a limit to the noise reduction of the detector, and in so-called analog measurement in which the measurement counter is stopped when the intensity of the received light exceeds a certain level, the detection level is lowered in response to a low received signal. If it is set, the peak value of the noise of the detector may be higher than the detection level, which may cause a malfunction, so that there is a disadvantage that it cannot be used in the case of a low input.

It is also conceivable to apply a single-photon counting (single-photon counting) technique used as a means for detecting extremely weak light in analysis and other applications, but has the following problems. ing.

Although photon counting by a photomultiplier tube is common, there is little attenuation along the way, and the sensitivity of the photomultiplier tube to infrared light that is a wavelength safe for eyes (detection of incident photons). (It is often expressed by the quantum efficiency, which is the rate at which pulses are emitted), which is extremely low and not practical.

An optical semiconductor detector having a high sensitivity (quantum efficiency) to infrared light cannot provide a sufficient multiplication factor, making it difficult to perform photon counting.

[0007] Regarding the multiplication factor in the above term, the photodetector is broken down by a slight stimulus (the state in which the current flows even after the stimulus is removed, and stops unless a measure such as applying a voltage is taken). A "Geiger mode" is used, which uses a detector to which a voltage is applied so high that even if one photon is incident, it becomes a stimulus and breaks down, causing a large current to flow. The method has been devised to detect a single photon, and a product that uses it has been marketed. However, at present, only one company sells it, and it also has some problems. doing.

The biggest problem in the term is what is called a dark count, which means that when a high voltage is applied, a signal similar to that when a photon enters by breaking down due to thermions or the like is output. . That is, it cannot be distinguished whether a photon has entered or a malfunction has occurred due to thermal electrons or the like.

When used for analysis or the like, the total number of incident photons is counted, and the accuracy is obtained by subtracting the dark count number generated at a certain probability from the counting result to obtain the number of actual photon incidences. Although it is possible to improve it, when using weak reflected light from a long-distance target as a stop pulse for distance measurement, if there is a dark count before the reflected light arrives, the counter stops there and Can not be used because it will.

If the problem is merely that the counter is stopped in advance, a conventional distance measuring device may use a method such as two-target distance measurement using a plurality of counters, except for the case where the approximate distance is known. It is not possible to distinguish which of the two measured values is correct.

The dark count of a commercially available detector is 10
0 to 200 counts / sec, average 1 to 5 to 10 ms
There are two error pulses. In the measurement of about 10 km, the light propagation time is several tens of μs, and the probability of occurrence of dark count noise during that time is small. ms, that is, about the same as the average occurrence time of the dark count, and it is not practical because there is a possibility that the distance may be erroneously measured with a probability of about 1/2 in any measurement.

[0012]

In the conventional laser distance measuring apparatus, as long as analog measurement is used, the receivable power is limited, and ultra-long distance measurement is difficult.

On the other hand, in the case of using a technique called photon counting (photon measurement), the photomultiplier tube has extremely low sensitivity to infrared light which is relatively safe for the eyes, and solid-state light detection with high sensitivity There was a dark count problem with the vessel.

Accordingly, it is an object of the present invention to make it possible to measure an ultra-long distance while avoiding a problem caused by dark counting, while taking advantage of the feature that the solid-state photodetector has high sensitivity (quantum efficiency). An object of the present invention is to provide a laser distance measuring device.

[0015]

SUMMARY OF THE INVENTION A laser distance measuring apparatus according to the present invention includes a laser oscillator, a transmission / reception optical system, a counter circuit, and a calculation display circuit provided in a conventional laser distance measuring apparatus. A branch optical system as a means for dividing at least the reflected light of at least three, a plurality of photodetectors, and a time coincidence of the outputs thereof are checked, so that an accidental like a dark count is obtained. And a determination means for removing a malfunction due to a phenomenon occurring in the above and displaying only a correct count value.

[0016]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention. The light emitted from the laser oscillator 1 is
After being set to a required beam divergence angle by the transmission / reception optical system 2, the beam is emitted into space and hits a target 11 at an extremely long distance. The light reflected by the target 11 is condensed by the transmission / reception optical system 2, is divided into three by the branching optical system 3, and is received by the plurality of photodetectors 41, 42, and 43. Here, if each detector has an input of several photons (photons) to tens of photons, the 1 / (quantum efficiency)
Even if there is a problem that it does not always work (photoelectrons are generated) even if there are photons,
The photodetector can be operated reliably. In addition, even if photons are excessively incident, only extra photoelectrons increase during breakdown, and there is no problem in operation.

Next, a plurality of counter circuits 71, 72, 73 receiving the outputs of the photodetectors 41, 42, 43 are used to calculate the time until the reflected light arrives, that is, the distance to the target. In the embodiment, when the photodetector is operated earlier due to the dark count, only one circuit displays a value that is significantly different from the other circuits, so that it is possible to determine. Although there are problems such as the necessity of a plurality of counter circuits, "Depending on the waveform of the emitted laser light, depending on which photon at which position (time) of the received reflected light caused the breakdown,
The operation time of the counter circuit is different '' is an advantage that the more accurate distance measurement value can be obtained by averaging the measurement values of a plurality of counter circuits excluding the dark count. have.

FIG. 2 shows the outputs of the photodetectors 41, 42, and 43 input to the AND circuits 51, 52, 53 and the OR circuit 6, and when two or more photodetectors operate almost in time, the counter is activated. 9 is an example showing a second embodiment of the present invention in which a signal to a circuit 7 is output. If the photodetector 43
If only the light source breaks down early, the light detectors 41 and 42 operate at different times from the light detectors 41 and 42 operated by the reflected light from the target. No output. On the other hand, AN
The D circuit 51 is a combination of the outputs of the photodetectors 41 and 42 that have normally operated. When the reflected light is received, a signal is input at the same time, and an output is obtained at that time. AND circuit 5
If a signal is obtained from any of 1, 52, and 53,
The R circuit 6 operates to output a signal for stopping the counter circuit 7.

In FIG. 2, if the response of the AND circuit is made somewhat earlier (approximately the time corresponding to the accuracy required for distance measurement), the occurrence timing of the dark count that occurs by accident is such a short time. Since it is unlikely that they coincide in time, a signal for stopping the counter circuit 7 at an erroneous time is not issued. On the other hand, it cannot be said that there is no possibility of malfunction (breakdown) before two or more photodetectors receive the reflected light, in which case there is no output from any of the AND circuits, A signal for stopping the circuit 7 cannot be obtained.

In the case of measuring a very long distance, the probability of malfunction of two or more photodetectors increases. Therefore, it is not sufficient to use a minimum number of photodetectors as shown in FIG. It is conceivable that four to five or more photodetectors may be required. On the other hand, if the time required for the laser light to reciprocate to the target 11 is sufficiently shorter than the average dark count generation time, only the extremely rare case where one of the photodetectors accidentally breaks down before that time should be considered. Since it is sufficient, even with a minimum of three units, a measurement error can hardly be considered. In designing the device, the required number of photodetectors may be determined from the relationship between the performance of photodetectors (average time of dark count generation) and the maximum distance measurement distance. Compared with the method of FIG. 1, there is an advantage that only one counter circuit 7 is required and the time required for discrimination is unnecessary, but there is a problem that averaging cannot be performed. It is only necessary to determine which method is to be used depending on whether it is necessary to reduce the time required for comparing and determining each output. The input range is too weak for analog measurement, but too large for photon counting.Therefore, if you can expect the reception input to be in this range, increase the number of branches and add several detectors. Can be set as a set, and the accuracy of averaging can be improved and the determination time can be shortened at the same time by a method such as averaging the count values of each set through an AND and OR circuit.

[0021]

As described above, according to the present invention, even when photon counting is performed by a solid-state photodetector having a high quantum efficiency in dark distance measurement, dark count due to malfunction of the detector can be obtained. A laser ranging device that can be avoided can be provided.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing another embodiment of the present invention, particularly an example in which an output process of a photodetector is configured by an AND circuit and an OR circuit.

[Explanation of symbols]

 Reference Signs List 1 laser oscillator 2 transmission / reception optical system 3 branch optical system 41, 42, 43 photodetector 51, 52, 53 AND circuit 6 OR circuit 7, 71, 72, 73 counter circuit 8 discriminating circuit 9 display circuit 11 Target

Claims (5)

(57) [Claims] 1. A laser oscillator, a branch optical system that splits a light beam received from a target into a plurality of light beams, a plurality of photodetectors respectively receiving the split light beams from the branch optical system, and a plurality of light beams. A plurality of counter circuits each receiving the output of the detector and counting the time from the transmission of the laser beam to the reception of the laser light, a discriminating circuit for examining the time coincidence of the count outputs of the plurality of counter circuits, and excluding a count value having a small correlation; Laser ranging device having a. 2. A laser oscillator, a branching optical system for splitting a light receiving target reflected light into a plurality of light beams, a plurality of light detectors respectively receiving the divided light receiving outputs, and outputs of the plurality of light detectors. A logic circuit that inhibits a signal from the photodetector that has been generated due to a malfunction caused by a phenomenon that occurs accidentally, and a counter circuit that receives the output of the logic circuit and counts the time from transmission of the laser light to reception of the laser light. Laser ranging device. 3. The laser distance measuring apparatus according to claim 2, wherein said logic circuit includes an AND circuit receiving two of a plurality of outputs from a plurality of photodetector circuits. 4. The logic circuit has a plurality of AND circuits.
4. The laser distance measuring apparatus according to claim 3, further comprising an OR circuit for ORing outputs of a plurality of AND circuits. 5. The laser distance measuring apparatus according to claim 1, wherein said photodetector performs detection by photocounting.