JP7257105B2 - Absolute distance measuring device and method - Google Patents

Description

The present invention relates to a distance measuring device and a distance measuring method in the field of production of precision equipment and precision machined parts such as the machinery industry and the electrical industry, and more particularly to absolute distance measurement for measuring the absolute distance to a desired object using a laser beam. Apparatus and method.

Conventionally, an optical means is suitable as a method for non-contact length measurement and distance measurement to an object to be measured. The principle of an absolute distance meter using light can be broadly divided into the intensity modulation method, in which the intensity of light is modulated and the phase difference detected by interference with a reference light, and the method in which a pulse of light is projected onto the object to be measured, and the reciprocating motion is performed. Time of flight (TOF), which is a method of measuring distance based on time, is known.

The intensity modulation method is not practical due to the fact that the intensity of light is vulnerable to disturbance, that it requires a relatively large-scale device, and that it takes a long time to measure the absolute distance to the object to be measured. Field applications in mechanical and electrical factories are limited.

The time-of-flight method has a simple principle and can measure distance relatively easily. Therefore, it is not sufficient for measuring the shape and position of precision equipment and precision-machined members that require high measurement accuracy, so its application in the field is limited.

In addition, continuous wave (CW) laser beams with different frequencies superimposed using a reflector and a partial reflector are projected onto an object, and the distance to the object is determined by the phase difference between the beat signals of the reflected or scattered laser beams. It is known to measure and is described in US Pat.

Furthermore, in order to increase the measurement range and improve the resolution, the beat signals from multiple continuous wave (CW) laser beams reflected or scattered by the light source and the object are electrically mixed, A beat signal is generated by a plurality of reflected or scattered continuous waves (CW). Then, the phase of the beat signal between multiple continuous wave (CW) beat signals reflected or scattered by the light source and object is compared with the phase of the multiple continuous wave (CW) beat signals before irradiating the light source and object. Patent Document 2 describes that the distance to the object is measured by

JP-A-61-138191 Japanese Unexamined Patent Application Publication No. 2011-203188

In the prior art disclosed in Patent Document 1, when detecting the beat signal, the amount of light is insufficient, the required signal-to-noise ratio cannot be obtained, and it is difficult to extract the phase difference. Moreover, when the surface of the object to be measured has a low reflectance or is a scattering surface, a large-scale and expensive system is required to measure a long distance with high accuracy.

The method described in Patent Document 2 is similar, and requires a plurality of continuous wave (CW) laser beams. Therefore, as a continuous wave (CW) laser beam, a laser beam comb in which a large number of coherent frequency components are arranged at regular intervals like teeth of a comb is required. An optical comb generator is itself expensive and a large system for absolute distance measurement.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems of the prior art, and to obtain an absolute distance measuring apparatus and method using light that can accurately measure the absolute distance to an object with a simpler and less expensive configuration.

To achieve the above object, the present invention generates a reference clock signal in an absolute distance measuring device that measures distance based on the round-trip time of light projected onto an object to be measured, reflected, and returned. a clock generation unit, a measurement pulse generation unit that generates a periodic measurement pulse based on the clock signal, a detection pulse generation unit that generates a detection pulse with a period different from that of the measurement pulse, and a an optical pulse transmitter for projecting the measuring light onto the object to be measured; a light receiving part for detecting the measuring light reflected by the surface of the object to be measured and outputting it as a received signal; and the detection pulse and the received signal. The measurement light is reflected from the surface of the object to be measured after being projected from the optical pulse transmitting section to the object to be measured by a logical product circuit that performs logic operation and synthesizing, and the signal synthesized by the logical product circuit. a signal processing unit for obtaining a propagation time until detection by the light-receiving unit, and calculating an absolute distance to the object to be measured.

A periodic measurement pulse and a detection pulse with a period different from that of the measurement pulse are generated, the measurement light according to the measurement pulse is projected onto the object to be measured, and the reflected received signal is detected. Further, the propagation time of the measurement light is obtained by logically operating the detected pulse and the received signal, and the absolute distance to the object to be measured is calculated.

Therefore, the period of the AND of the measurement pulse and the detection pulse is equal to the frequency difference between the two, which is equivalent to the sine wave beat phenomenon. As a result, the phase of the beat waveform changes greatly even with a slight delay such as the propagation time, so the absolute distance to the target can be measured with high accuracy using light with a simpler and cheaper configuration. can.

Further, in the above, it is preferable that the detection pulse generator generates the detection pulse based on the clock signal.

Furthermore, in the above apparatus, it is preferable that the optical pulse transmitting section projects measurement light onto the object to be measured by a laser diode that utilizes recombination light emitted from a semiconductor.

Furthermore, in the above, it is desirable that the difference between the period of the measurement pulse and the period of the detection pulse is 0.3 to 3% of the period of the measurement pulse.

Furthermore, in the above, it is preferable that the ratio is such that the period _T1 of the detection pulse is 99 when the period _T2 of the measurement pulse is 100.

Furthermore, the present invention comprises absolute distance measuring devices arranged on both sides of the object to be measured, and calculates the thickness of the object to be measured from the distance between the absolute distance measuring devices measured in advance.

Furthermore, the absolute distance measuring method of the present invention is a method for measuring distance based on the round-trip time of light projected onto an object to be measured, reflected, and returned. A periodic measurement pulse and a detection pulse having a period different from that of the measurement pulse are generated based on the measurement pulse, and the measurement light is projected onto the object to be measured in accordance with the measurement pulse, and reflected from the surface of the object to be measured. The measuring light is detected as a received signal, the detected pulse and the received signal are logically operated to obtain the propagation time of the measuring light, and the absolute distance to the object to be measured is calculated.

Moreover, in the above, it is preferable that the detection pulse is generated based on the clock signal.

According to the present invention, a periodic measurement pulse and a detection pulse with a period different from that of the measurement pulse are generated, the measurement light according to the measurement pulse is projected onto the object under test, and the reflected received signal is detected. . Further, the propagation time of the measurement light is obtained by logically operating the detected pulse and the received signal, and the absolute distance to the object to be measured is calculated.

Therefore, the period of the AND of the measurement pulse and the detection pulse is equal to the frequency difference between the two, which is equivalent to the sine wave beat phenomenon. As a result, the phase of the beat waveform changes greatly even with a slight delay such as the propagation time, so the absolute distance to the target can be measured with high accuracy using light with a simpler and cheaper configuration. can.

1 is a block diagram of an absolute distance measuring device according to one embodiment of the present invention; FIG. Graph showing amplitude versus time of a measured pulse in one embodiment 4 is a graph showing amplitude versus time of detected pulses in one embodiment. 4 is a graph showing the amplitude versus time of the AND signal of the measurement pulse and the detection pulse in one embodiment; Graph showing amplitude versus time of received signal and detected pulse in one embodiment General view showing an example of applying the absolute distance measuring device of FIG. 1 to a thickness measuring device

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Measuring instruments that measure the distance between two points without contact include the ultrasonic type and the optical type, which are used according to the application. The ultrasonic type measures the time it takes for an ultrasonic wave to be emitted toward an object, reflected, and returned, and is simpler and cheaper than the optical type. The ultrasonic method has high environmental resistance and does not care about the material of the object, but the measurement distance is about several meters, and the target (object size) must be large (several tens of millimeters). There are drawbacks.

On the other hand, the optical type has low environmental resistance, but has the characteristics that the measurement distance is about 10 m to several hundred meters, the target is as small as several mm, and the measurement point is visible. In addition, an expensive optical comb generator is required to measure the absolute distance with higher accuracy, resulting in a large-scale system. Therefore, an inexpensive absolute rangefinder is realized by transmitting light in the form of pulses and measuring the distance based on the round trip time.

FIG. 1 is a block diagram of an absolute distance measuring device according to an embodiment of the present invention, FIG. 2 is a graph showing time versus amplitude of a measurement pulse, and FIG. 3 is a graph showing time versus amplitude of a detection pulse. Reference numeral 1 denotes a clock generation unit that generates a clock signal that serves as a reference when generating a signal for measurement. Reference numeral 2 denotes a measurement pulse generator that generates periodic measurement pulses shown in FIG. 2 based on the clock signal from the clock generator 1 .

3 is a detection pulse generator that generates a detection pulse. Similar to the measurement pulse generator 2, the detection pulse generator 3 generates detection pulses having a period slightly different from that of the measurement pulse based on the clock signal from the clock generator 1, as shown in FIG. Since the measurement pulse and the detection pulse are generated based on the same clock signal, an accurate signal can be generated even if the period difference is slight, and the measurement accuracy can be improved. Here, the detection pulse generator may generate the detection pulse independently without being based on the clock signal. It is desirable because it can be done.

The period of the measurement pulse shown in FIG. 2 is 100 μs, and the electrical signal is turned ON every 100 μs. The period of the detection pulse shown in FIG. 3 is 99 μs, and the electrical signal is turned ON every 99 μs. Therefore, as can be seen from FIG. 3, the generation timings of the measurement pulse and the detection pulse deviate with the lapse of time. However, in this example, to simplify the explanation, the period of the measurement pulse is 100 μs, the period of the detection pulse is 99 μs, and the period of the clock signal is 1 μs.

Assuming that the period of the clock signal is 0.8 μs, the period of the measurement pulse is set to 80 μs, and the period of the detection pulse is set to 79.2 μs. In this example, the period of the measurement pulse is 100 μs and the period of the detection pulse is 99 μs, but this ratio may be set arbitrarily. Hereinafter, the period will be described without a unit.

Reference numeral 4 denotes an optical pulse transmitter, which projects measurement light as a pulse laser onto the object 5 to be measured according to the measurement pulse, which is a digital signal shown in FIG. Since the measurement light is generated based on the digital signal, the optical pulse transmitter 4 does not need to phase-modulate the continuous wave laser light or use an optical comb using a synchronous laser.

A pulsed laser is a laser that repeatedly blinks at short time intervals, and one laser irradiation time of the pulsed laser is the pulse width. Laser output is controlled by pulsatingly lighting an excitation source such as an LD (laser diode) and electrically controlling the excitation source lighting time width and the current value to the excitation source. Continuous wave (CW) laser light means that the excitation source is lit continuously.

The measurement light from the pulse laser is reflected by the surface of the object 5 to be measured and detected by the light receiving section 6 . The received signal 7 obtained by the light receiving section 6 is detected as a signal delayed by the propagation time (round trip time) Δt of the measurement pulse. The propagation time is the difference between the time when the measurement light is emitted from the optical pulse transmitter 4 and the time when the light receiver 6 detects the measurement light reflected by the object 5 to be measured.

On the other hand, the detection pulse generated from the clock generation unit 1 with a period slightly different from that of the measurement pulse and the received signal delayed by Δt obtained by reflection from the surface of the object under test 5 are received by the AND circuit 8. They are synthesized by a logical operation (AND operation). The combined signal enters the signal processing section 9 to obtain the propagation time .DELTA.t with high precision and to calculate the absolute distance from the optical pulse transmitting section 4 to the device under test 5. FIG.

The measurement and detection pulses have slightly different frequencies. If the detection pulse is replaced by a sine wave of frequency f ₁ and the measurement pulse is replaced by a sine wave of frequency f ₂ , then the period of the AND of the measurement pulse and the detection pulse is the beat waveform (envelope) of the sine wave of frequency f ₁ and frequency f ₂ becomes a cycle of Therefore, the phase coincidence point of the initial phase in the beat waveform corresponds to the coincidence of both pulses at the time of the common multiple of the period of the detection pulse and the period of the measurement pulse.

The received signal, which is a signal in which the measurement pulse is delayed by Δt, greatly changes the initial phase of the beat waveform. Even a slight propagation time delay causes a large change in the phase of the beat waveform. By solving the timing of the signal based on the AND of the received signal and the detected pulse in the signal processing unit 9, it is possible to obtain the propagation time with higher accuracy, thereby making it possible to measure the absolute distance to the object. . The change in initial phase is magnified by the ratio of the frequency of the measurement pulse to the frequency difference between the measurement and detection pulses, as follows.

として表現できる。このときｃｏｓの項はうなり波形のエンベロープを示し、ｓｉｎの項はエンベロープ内部の波形を示している。 Details of the beat waveform and changes in phase will be described. Details of the beat waveform and changes in phase will be described by replacing the detection pulse with a sine wave of F ₁ and the measurement pulse with a sine wave of F ₂ . Assuming that the detection pulse has a frequency f ₁ , the measurement pulse has a frequency f ₂ , the respective angular frequencies ω ₁ and ω ₂ , and the respective initial phases φ ₁ and φ ₂ , then ω ₁ =2πf ₁ and ω ₂ =2πf _{2 .} Therefore, F ₁ =sin(ω ₁ t+φ ₁ ) and F ₂ =sin(ω ₂ t+φ ₂ ) . The beat waveform F generated by superimposing F ₁ and F ₂ is

can be expressed as At this time, the cos term indicates the envelope of the beat waveform, and the sin term indicates the waveform inside the envelope.

Here, the propagation time of _F2 is slightly delayed, and the delay time is Δt. However, if this delay time Δt is set within the period (1/f ₂ ) of F ₂ , F ₂ can be expressed by Equation 2 using time t.

式３において、ｃｏｓの項はうなり波形のエンベロープを示し、ｓｉｎの項はエンベロープ内部の波形を示している。したがって、Ｆ_２の遅れがうなり波形Ｆのエンベロープの位相に影響を与えていることが分かる。 Then, the generated beat waveform F is represented by Equation (3).

In Equation 3, the cos term indicates the envelope of the beat waveform, and the sin term indicates the waveform inside the envelope. Therefore, it can be seen that the delay of _F2 affects the phase of the envelope of the beat waveform F.

In order to clarify the degree of influence of the frequencies f ₁ and f ₂ , Equation 3 is transformed into Equation 4.

In Equation 4, the cos term also indicates the envelope of the beat waveform, so if the propagation time of _F2 is delayed by Δt, the envelope of the beat waveform F will have a phase shift of ω ₂ /(ω ₁ −ω ₂ ) Δt. It has a distorted shape. Also, ω ₂ /(ω ₁ −ω ₂ ) Δt= f ₂ /(f ₁ −f ₂ ) Δt, and since f ₂ =1/T ₂ and f ₁ =1/T ₁ , ω ₂ / (ω ₁ −ω ₂ ) Δt=T ₁ /(T ₂ −T ₁ ) Δt.

Assuming that T ₁ =99 and T ₂ =100, the phase change of the beat waveform is T ₁ /(T ₂ −T ₁ ) Δt=99Δt, and if the propagation time of the measurement pulse F ₂ is delayed by Δt, 99 It can be seen that detection can be performed at double magnification.

FIG. 4 is a graph showing the time vs. amplitude of the AND signal of the measurement pulse and the detection pulse, and FIG. 5 is a graph showing the time vs. amplitude of the AND signal of the received signal and the detection pulse. In the above description, the detection pulse is F ₁ and the measurement pulse is a sine wave of F ₂ . However, digital signals will be described with reference to FIGS. 2 to 5 . In FIG. 2, the period _T2 of the measurement pulse is 100, and the period _T1 of the detection pulse in FIG.

FIG. 4 shows the AND signal of the measurement pulse and the detection pulse, the period of which is the period of the beat waveform (envelope) when replaced with a sine wave, and the phase coincidence point is the period of the detection pulse and the measurement pulse. This corresponds to the coincidence of both pulses at the time of the common multiple of the period. Therefore, the period of the AND signal of the measurement pulse and the detection pulse is 9900 as shown in FIG.

FIG. 5 shows the logical AND signal of the received signal, which is the measurement pulse with period T ₂ =100 delayed by Δt=1 (1% of period T ₂ ), and the detection pulse with period T ₁ =99. . According to the above equation 4, the change in the phase of the beat waveform is [T ₁ /(T ₂ −T ₁ )]Δt=99Δt. It shifts to time 9801 .

That is, the delay time Δt=1 is enlarged by 99 times. This ratio of 99 times is equal to the ratio of the difference between the measured pulses of period T ₂ =100 and the detected pulses of period T ₁ =99. Since the period of the measurement pulse and the detection pulse can be changed arbitrarily, the enlargement ratio can be arbitrarily set. However, the shift amount, which is the difference between the period _T2 of the measurement pulse and the period _T1 of the detection pulse, should be 0.3 to 3% with respect to the period _T2 in order to make the AND signal more accurate. It is desirable from the viewpoint of sensitivity, and in particular, it is practical and good to set the ratio of the period _T1 of the detection pulse to 99 with respect to the period _T2 of the measurement pulse=100.

Therefore, for example, when the measurement pulse period T ₂ =100 μs, the received signal delay Δt=1 μs, and the detection pulse period T ₁ =99 μs, and the speed of light is 300000 km/s, the signal processing unit 9 calculates It is possible to obtain the round trip distance of the light with respect to the object to be measured. Therefore, the one-way distance of light, that is, the distance to the object to be measured is halved to 150 m.

At this time, the signal processing unit 9 can obtain a signal obtained by expanding the delay Δt of the received signal by 99 times from the logical product of the signal received by the light receiving unit and the detection pulse, and based on this, the delay of the received signal can be obtained. Δt can be calculated. The signal processing unit 9 can calculate the distance to the measurement object by performing such calculations.

In addition, since interference is not used, an inexpensive configuration is possible regardless of the quality of the light source. Furthermore, since the synthesis is performed using digital signals, there is no need to use a coherent laser comb regardless of the coherence of the light itself. Therefore, it becomes possible to use a laser diode or the like that utilizes recombination light emission of an inexpensively available semiconductor.

FIG. 6 is an overall view showing an example of applying the above-described absolute distance measuring device to a thickness measuring device. Reference numeral 25 denotes an object to be measured, for example, a silicon wafer having a diameter of 300 mm, a thickness of 0.775 mm, and a thickness tolerance of about ±0.025 mm. An object 25 to be measured is placed on a stage 28 . The upper rangefinder 20 is provided above the object 25 to be measured, and the lower rangefinder 21 is provided below the object 25 to be measured.

When measuring the thickness of the object 25 to be measured, the distance between the upper rangefinder 20 and the lower rangefinder 21 is accurately measured in advance as the distance between the rangefinders. The upper rangefinder 20 and the lower rangefinder 21 are each composed of the optical pulse transmitter 4, the light receiver 6, etc., as shown in FIG. The measurement light from the optical pulse transmitter 4 of the upper rangefinder 20 is projected onto the upper surface of the object 25 to be measured, reflected by the upper surface of the object 25 to be detected by the light receiving unit 6 .

The same is true for the lower surface of the object 25 to be measured. The upper rangefinder 20 measures the absolute distance to the upper surface, and the lower rangefinder 21 measures the absolute distance to the lower surface. The thickness of the object to be measured 25 is the absolute distance from the distance between the rangefinders measured in advance to the upper surface measured by the upper rangefinder 20, and the absolute distance to the lower surface measured by the lower rangefinder 21. Subtract the sum.

As described above, if absolute distance measuring devices are placed on both sides of a disk-shaped object and the distance between the absolute distance measuring devices is measured in advance, the thickness of even an extremely thin object can be measured. can be done accurately. With such a thickness measuring instrument, it is possible to perform non-contact thickness measurement with a simpler and less expensive configuration, high environmental resistance, high accuracy, and a visible measurement point.

REFERENCE SIGNS LIST 1 clock generator 2 measurement pulse generator 3 detection pulse generator 4 optical pulse transmitter 5, 25 device under test 6 light receiver 7 received signal 8 AND circuit 9 signal processor 20 upper rangefinder 21 lower rangefinder 26 upper arm 27 lower arm 28 stage

Claims (6)

An absolute distance measuring device that measures distance based on the round-trip time of projecting measurement light onto an object to be measured, reflecting it, and returning it,
a clock generator that generates a reference clock signal;
a measurement pulse generator that generates periodic measurement pulses based on the clock signal;
a detection pulse generating unit that generates a detection pulse having a period different from the measurement pulse and having a period difference of 0.3 to 3% with respect to the period of the measurement pulse;
an optical pulse transmitter that projects the measurement light onto the object under test according to the measurement pulse;
a light receiving unit that detects the measurement light reflected by the surface of the object to be measured and outputs it as a received signal pulse;
a logical product circuit that outputs a logical product signal whose period is a common multiple of the period of the detected pulse and the period of the received signal pulse ;
The measurement light is projected from the optical pulse transmitter onto the device under test based on the time lag of the logical product signal of the received signal pulse and the detection pulse with respect to the logical product signal of the measurement pulse and the detection pulse. a signal processing unit that obtains a propagation time from the surface of the object to be measured until it is detected by the light receiving unit, and calculates the absolute distance to the object to be measured;
An absolute distance measuring device comprising: 2. The absolute distance measuring device according to claim 1, wherein the detection pulse generator generates the detection pulse based on the clock signal. 2. The absolute distance measuring device according to claim 1, wherein the ratio is such that the period _T1 of the detection pulse is 99 when the period _T2 of the measurement pulse is 100. 4. The absolute distance measuring device according to any one of claims 1 to 3 arranged on both sides of an object to be measured, wherein the thickness of the object to be measured is determined from the distance between the absolute distance measuring devices measured in advance. A thickness measuring instrument characterized by computing. An absolute distance measurement method for measuring a distance based on the round-trip time of projecting a measurement light onto an object to be measured, reflecting it, and returning it,
periodic measurement pulses based on a reference clock signal;
generating a detection pulse having a period different from the measurement pulse and having a period difference of 0.3 to 3% with respect to the period of the measurement pulse ;
projecting the measurement light onto the object to be measured according to the measurement pulse and detecting the measurement light reflected from the surface of the object to be measured as a received signal pulse;
Based on a time shift of a logical product signal having a period of a common multiple time of the received signal pulse and the detection pulse with respect to a logical product signal having a period of a common multiple time of the measurement pulse and the detection pulse An absolute distance measuring method comprising obtaining a propagation time and calculating an absolute distance to the object to be measured. 6. The absolute distance measuring method according to claim 5 , wherein said detection pulse is generated based on said clock signal.

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