JPH0550711B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a distance measuring method using a laser beam.

``Prior Art'' Conventionally, various distance measurement methods using laser beams have already been proposed, and among them, a method in which a part of the laser beam is irradiated onto the object to be measured via a beam splitter has been proposed. the reflected light obtained by irradiating another part of the laser beam through the beam splitter onto a reference reflector spaced a predetermined distance from the beam splitter; At the same time, the frequency of the laser beam is modulated to generate a measurement beat wave in the superimposed reflected light, and the measurement beat wave is measured to measure the distance to the object to be measured. Are known.

To explain the principle of this distance measurement method, in Fig. 3, 1 receives a DC drive current from a DC power supply 2 and oscillates a single mode laser beam, and corresponds to the magnitude of the drive current. The laser beam oscillated from this semiconductor laser 1 is transmitted through a convex lens 3, a first beam splitter 4, and a lens group 5 consisting of a concave lens and a convex lens, and is directed to an object to be measured having a rough surface. 6 is irradiated. The reflected light from the object to be measured 6 is reflected by the first beam splitter 4, passes through the convex lens 7, is received by a light receiving element 8 such as a photodiode, and the signal is sent to the measuring device 9. be introduced.

On the other hand, the remainder of the laser beam oscillated by the semiconductor laser 1 and separated and reflected by the first beam splitter 4 passes through a reference reflector 10 such as a mirror or prism, which is provided at a predetermined distance _L1 from here. The reflected light reflected by the reference reflector 10 passes through the first beam splitter 4, where it is superimposed on the reflected light from the object to be measured 6, and is transmitted through the convex lens 7 and received. The light is received by element 8.

Therefore, the signals of the two reflected lights introduced into the measuring device 9 have a time delay due to the optical path difference between the two, so specifically, the distance L between the first beam splitter 4 and the reference reflector 10 ₁ and the distance L ₂ between the first beam splitter 4 and the object to be measured 6, which is twice the difference, resulting in a phase difference.

In this state, the magnitude of the drive current supplied from the DC power supply 2 to the semiconductor laser 1 is controlled by the modulator 11, and a modulated current having a linear waveform is applied to the semiconductor laser 1 as shown in FIG. When the oscillation frequency is linearly modulated in the same manner as the modulation current, the frequency of the laser beam also changes linearly, as shown in FIG. The phase difference changes in response to a change in the frequency of the laser beam, thereby generating a beat wave.

The measuring device 9 measures the wave number Nb, where one period of the measured beat wave is one wave number, and if the wave number Nb of the beat wave can be measured, the distance to the object to be measured 6 can be calculated and measured.

That is, the time difference of the superimposed reflected light is τ, the difference between the distances L ₂ and L ₁ is R, and the speed of light is c
Then, the time difference τ is expressed by the following equation.

τ=2R/c ......(1) Also, as shown in FIG. time T ₀ and the frequency deviation amount δ of the modulation frequency fm,
The following proportional relationship exists between the beat frequency fb and the beat frequency fb.

τ:fb=T ₀ :δ (2″) The following equation is obtained from this equation (2″).

fb=δ・τ/T ₀ ......(2') Also, the wave number Nb of the beat wave is obtained by Nb=fb・T ₀ , so the following equation can be obtained from this equation and equation (2'). It will be done.

Nb＝δ・τ……(2) Therefore, the following equation can be obtained from equations (1) and (2).

R=c·Nb(2δ) (3) By the way, the above-mentioned frequency deviation amount δ is the amplitude of the laser beam that changes periodically, and is the value obtained by subtracting the minimum frequency from the maximum frequency. Since the frequency is the reciprocal of the wavelength, and the wavelength of the maximum frequency and the wavelength of the minimum frequency can be measured using a spectrum analyzer, the above frequency deviation amount δ should be calculated in advance by that measurement. I can do it.

In this way, the frequency displacement amount δ can be measured in advance by modulating the semiconductor laser 1, so if the wave number Nb of the beat wave is measured by the measuring device 9, the difference R Therefore, the distance _L2 to the object to be measured 6 can be calculated and measured.

"Problem to be Solved by the Invention" However, although the frequency deviation amount δ can be measured in advance by modulating the semiconductor laser 1, semiconductor lasers are susceptible to unstable factors such as temperature changes. Therefore, even if the modulation applied to the semiconductor laser is constant, the amount of frequency deviation δ is always constant.
It was difficult to obtain this, and this was one of the causes of large measurement errors.

"Means for Solving the Problem" In view of such circumstances, the present invention provides a measurement optical meter that obtains the above-mentioned measurement beat waves with a reference optical system that obtains a reference beat wave for reference. The distance _L2 to the object to be measured 6 can be calculated and measured without being influenced by the value of the deviation amount δ.

That is, in the present invention, a part of the laser beam is separated by the first beam splitter, and reflected light is obtained by irradiating one of the separated laser beams onto the object to be measured, and at the same time, the first beam splitter A reflected light is obtained by irradiating the other laser beam separated by the first beam splitter onto a reference reflector separated by a predetermined distance from the first beam splitter, and both reflected lights are reflected by the first beam splitter. The laser beams are superimposed and the frequency of the laser beams is modulated to generate a measurement beat wave in the superimposed reflected light, and another part of the laser beams is separated by a second beam splitter, and one of the separated laser beams is separated. A reflected light is obtained by irradiating a laser beam onto a first reference reflector separated by a predetermined distance from the second beam splitter, and at the same time, the other laser beam separated by the second beam splitter is directed to the first reference reflector. A reflected light is obtained by irradiating the second reference reflector from the second beam splitter to a second reference reflector separated by a predetermined distance different from the first reference reflector, and both reflected lights are sent to the second beam splitter. When the frequency of the laser beam is superimposed and the frequency of the laser beam is modulated, a reference beat wave is also generated in the superimposed reflected light at the same time, and the measured beat wave is generated within a certain period of time, with one period of each beat wave being one wave number. and the wave number of the reference beat wave, and calculate and measure the distance to the object to be measured from the ratio of both wave numbers and the known distance between each of the beam splitters and each reflector. This is how it was done.

"Function" According to such a distance measuring method, it is clear that the above-mentioned equation (3) can be obtained in the measurement optical system, and the following equation can be obtained in the reference optical system.

Rr=c・Nr/(2δ) ......(4) However, Rr is the difference in distance between the first reference reflector and the second reference reflector based on the second beam splitter in the reference optical system, This difference is of course known. Further, Nr is the wave number of the reference beat wave.

From the above equations (3) and (4), R=Rr・Nb/Nr (5) is obtained, and from this equation, the above frequency that includes an unstable element in the calculation formula for calculating the distance difference R. There is no need to use the amount of deviation δ, and therefore more accurate distance measurement can be performed.

"Embodiments" The present invention will be described below with reference to illustrated embodiments. In FIG. 1, the same parts as in FIG. 3 are denoted by the same reference numerals as in FIG.
5 is further provided with a reference optical system 16 for obtaining a reference beat wave for reference.

The laser beam introduced into this reference optical system 16 is
A third beam splitter 17 provided between the convex lens 3 of the measurement optical system 15 and the first beam splitter 4 introduces a portion of the laser beam from the semiconductor laser 1, and A portion of the light beam is separated and reflected by the second beam splitter 18 and irradiated onto a first reference reflector 19 such as a mirror or prism separated by a predetermined distance, and the laser beam separated by the second beam splitter 18 The remainder is transmitted through the second beam splitter 18 and irradiated onto a second reference reflector 20, such as a mirror or prism, which is spaced a predetermined distance apart. Note that the distances between the first reference reflector 19 and the second reference reflector 20 with respect to the second beam splitter 18 are made different.

The reflected light reflected by the first reference reflector 19 is transmitted to the second beam splitter 18 and the convex lens 2.
1 and is received by the light receiving element 22, and the second
The reflected light reflected by the reference reflector 20 is reflected by the second beam splitter 18 and superimposed on the reflected light from the first reference reflector 19.
The light passes through and is received by the light receiving element 22. The signal from the light receiving element 22 is then introduced into the measuring device 23, and the signal from the light receiving element 8 of the measurement optical system 15 is also introduced into the measuring device 23.

Therefore, when a modulated current having a linear waveform is applied to the semiconductor laser 1 by the modulator 11, the measurement device 23 receives a measurement beat wave signal from the measurement optical system 15, and a measurement beat wave signal from the reference optical system 16. Reference beat waves will now be input at the same time.

However, in this embodiment, the measuring device 23 modulates the frequency of the laser beam for a predetermined time, specifically, for the time required to count a predetermined number of integer waves of the measurement beat signal,
Therefore, if the distance _L2 to the object to be measured 6 differs, the frequency of the measurement beat signal will differ, so the above predetermined time will vary depending on the distance _L2 , but in any case, the measurement beat wave for the unknown predetermined time The wave number is fixed in advance.

On the other hand, the measuring device 23 starts counting the wave number of the reference beat signal at the same time as it starts counting the wave number of the measurement beat signal, and when a predetermined number of integer waves of the measurement beat signal are counted, that is, an unknown predetermined time has elapsed. After the elapsed time, stop counting the number of reference beat waves. Since the wave number of this reference beat signal is usually not an integer, the measuring device 23 is designed to be able to measure the wave number of the reference beat signal down to the decimal point.

In this way, if the wave number of the measured beat signal and the wave number of the reference beat signal for the unknown predetermined time, and therefore the ratio of both wave numbers, can be obtained, even if the predetermined time is unknown, the above equation (5) can be used. Therefore, the distance _L2 to the object to be measured 6 can be calculated.

FIG. 2 is a block diagram showing a more specific configuration of the measuring device 23. This measuring device 23 converts the reference beat signal and measurement beat signal from each of the light receiving elements 8 and 22 into a waveform shaping circuit 30 and a waveform shaping circuit 30, respectively. 31, the waveform is shaped, and the waveform-shaped signals are input to integer wave counters 32 and 33, respectively, which count the number of integer waves in each beat wave.

Integer wave counter 3 into which the measurement beat signal is input
2 is connected to an integer wave setter 34, and the number of integer waves of the measurement beat signal counted by the integer wave counter 32, that is, the wave number of the measurement beat wave, is the set value preset by the integer wave setter 34. Then, the integer wave counter 32 outputs a count stop command signal 35 to the integer wave counter 33 of the reference system, whereby the integer wave counter 33 stops counting the number of integer waves of the reference beat signal.

At this time, since the integer wave counter 33 can only count the wave number of the reference beat wave an integer number of times, a clock generator 36 is provided to measure the wave number below the decimal point. This clock generator 36 generates a clock at a much higher frequency than the frequency of the reference beat wave, and the clock counter 3
7 can count the clock.

Each time the integer wave counter 33 counts an integer wave of the reference beat wave, the clock counter 33
The clock counter 37 is reset by outputting a reset signal 38 to the clock counter 7. Then, the integer wave counter 32 of the measurement system receives a count stop command signal 35.
When this is output, the clock counter 37 inputs it and stops counting. In this state, the clock counter 37 counts the number of waves within one cycle, that is, the number of waves below the decimal point. are doing.

The distance to each of the reference reflectors 19 and 20 is always constant, and therefore the frequency of the beat signal does not change, so the count number of the clock counter 37 for one cycle of the reference beat signal is constant in advance. As a result, the wave number of the reference beat signal can be obtained below the decimal point from the count number for one cycle and the count number counted by the clock counter 37 when the count stop command signal 35 is output.

The calculation circuit 39 thus calculates the wave number below the decimal point, inputs the signals from the integer wave counter 33 and the integer wave setter 34, calculates the distance _L2 to the object to be measured 6, and displays it. vessel 40
to be displayed. At this time, the wave number of the reference beat wave calculated by the display 40 may be displayed down to the decimal point.

In addition, in the above embodiment, the wave number of the measured beat signal and the wave number of the reference beat signal are directly measured to find the ratio of the two, but the frequency of the measured beat signal and the frequency of the reference beat signal can be adjusted as appropriate. The ratio between the wave number of the measured beat signal and the wave number of the reference beat signal may be determined indirectly from the ratio of both frequencies by measuring with a frequency measuring device. Moreover, if the distance is determined by two-dimensionally sweeping the measurement point of the object to be measured in the X-Y direction, the three-dimensional shape of the object to be measured can be measured.

"Effects of the Invention" As described above, according to the present invention, distance measurement can be performed with higher accuracy because it is not affected by the amount of frequency deviation including unstable elements when calculating distance. This effect can be obtained.

[Brief explanation of drawings]

Fig. 1 is a system diagram for explaining one embodiment of the method of the present invention, Fig. 2 is a block diagram showing the measuring device 23 of Fig. 1, and Fig. 3 is a system diagram for explaining the conventional distance measuring method. 4 is an explanatory diagram for explaining the current waveform applied to the semiconductor laser 1, and FIG. 5 shows a state in which the frequency of the laser beam changes linearly based on the current waveform applied to the semiconductor laser 1. It is an explanatory diagram. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 4... First beam splitter, 6... Measured object, 8, 22... Light receiving element, 10...
Reference reflector, 15...Measuring optical meter, 16...Reference optical meter, 18...Second beam splitter, 19...First reference reflector, 20...Second reference reflector, 23...Measuring device.

Claims (1)

[Scope of Claims] 1. Part of the laser beam is separated by a first beam splitter, and reflected light is obtained by irradiating one of the separated laser beams onto an object to be measured. The other laser beam separated by the splitter is irradiated onto a reference reflector separated by a predetermined distance from the first beam splitter to obtain reflected light, and both reflected lights are sent to the first beam splitter. The laser beam was superimposed by a vine, and the frequency of the laser beam was modulated to generate a measurement beat wave in the superimposed reflected light, and another part of the laser beam was separated by a second beam splitter. One laser beam is irradiated to a first reference reflector spaced a predetermined distance from the second beam splitter to obtain reflected light, and at the same time, the other laser beam separated by the second beam splitter A reflected light is obtained by irradiating a light beam from the second beam splitter to a second reference reflector separated by a predetermined distance different from the first reference reflector, and both reflected lights are sent to the second beam splitter. When the frequency of the laser beam is superimposed by a pritter and the frequency of the laser beam is modulated, a reference beat wave is also generated at the same time in the superimposed reflected light. Measure the ratio of the wave number of the measurement beat wave and the wave number of the reference beat wave, and calculate the distance to the object to be measured from the ratio of both wave numbers and the known distance between each beam splitter and each reflector. A distance measurement method that uses a laser beam and is characterized by calculation measurement.