JPH07139907A - Method for measuring distance by laser, and circuit for processing signal - Google Patents

Abstract

PURPOSE:To continuously output measuring values by turning a frequency of a beat signal to a high frequency and relatively reducing influences of phase noises. CONSTITUTION:A detected beat signal is input to a delay detection circuit 1 and a reset terminal of a flip flop 2. The delay detection circuit 1 outputs a delay signal obtained by delaying the beat signal to the flip flop 2. The flip flop 2 feeds an output signal of a pulse string having a duty ratio corresponding to a frequency of the input signal to a low pass filter 3. The low pass filter 3 cuts high frequency components of not smaller than a predetermined level. Accordingly, an analog output showing a distance corresponding to the frequency of the beat signal is obtained and input to a distance-operating means. A semiconductor laser 5 is controlled and driven by a driving circuit 4 to set a frequency of a triangular wave from the relationship between the frequency of the beat signal and a modulating frequency of the triangular wave. Accordingly, outputs of distance values are continuously generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for a laser distance measuring method using a laser distance measuring device used in fields such as AF (auto focus) and FA (factory automation) of cameras, and its implementation. The present invention relates to a signal processing circuit, and more particularly to a laser distance measuring method that enables highly accurate measurement even when an object to be measured vibrates (swings), and a signal processing circuit used for the method.

[0002]

2. Description of the Related Art As a laser distance measuring method, a laser beam emitted from a semiconductor laser is temporally frequency-modulated and is incident on an interference optical system, and then the incident light is split into two beams, one of which ( The light of the measurement system) is projected onto the object to be measured, and the other light (the light of the reference system) is projected onto the reflector consisting of a reflection mirror, and the reflected light from each is combined to perform coherent heterodyne detection. Therefore, the difference in distance between the two lights,
That is, the beat signal is detected from the frequency difference according to the optical path difference,
A method of measuring (calculating) the distance to the object to be measured based on this beat signal is known.

As a conventional example of such a laser distance measuring method, the method described in Japanese Patent Laid-Open No. 61-223577 and the document "FM heterodyne length measuring method using a semiconductor laser" Takao Kobayashi et al., Electronic Information The Institute of Communication Engineers Technical Research Report Meeting OQE87-153 ”is available. This distance measuring method has the advantages that the dynamic range of the measuring distance can be wide and highly accurate length measurement is possible.

FIG. 8 shows the structure of a laser distance measuring device used for implementing such a laser distance measuring method. In this distance measuring method, the injection current of the semiconductor laser 201, which serves as a light source,
After being modulated into a triangular wave, the laser light whose optical frequency is modulated by the triangular wave is collimated by the collimator lens 202 by the thermal effect of this injection current,
The light is split by the beam splitter 203. In addition,
FIG. 9 shows the frequency modulation amount of the laser light and the waveform of the beat signal.

One of the split beams of light enters an optical system (reference optical system) which is stable and has an optical path difference which serves as a reference for measurement. This reference optical system includes a beam splitter 20.
It is composed of four and two reflectors 205, 205 and a detector 207, and operates as follows.

First, the beam splitter 204 splits the incident light into a reference optical system and a measurement optical system, and the respective reflectors 2
Beam splitter 2 for the light reflected by 05 and 205
The signal is combined again at 04, and then the detector 207 performs coherent heterodyne detection. A beat signal is obtained by this detection.

On the other hand, the other light split by the beam splitter 203 is incident on the distance measuring optical system. The structure of the distance measuring optical system is substantially the same as the structure of the reference optical system.
That is, the configuration is the same as that of the reference optical system except that the measurement light is guided to the object to be measured 216 instead of being guided to the reflector, and the beam splitter 214, the reflector 215, and the detector 217 are similarly provided. ing. Similarly, the detector 217 of this distance measuring optical system also performs coherent heterodyne detection to obtain a beat signal.

In FIG. 9, R _r represents the optical path length of the reference light of the standard optical system, and L _r represents the optical path length of the measurement light of the standard optical system.

Here, the frequency f _b of the beat signal is expressed by the following equation (1).

F _b = (dν / dt) · τ = 4f _m · Δν · (R−L) / c (1) where ν: frequency of laser light τ: time delay between measurement light and reference light f _m : Modulation frequency Δν (GHz / mA): Frequency modulation amount of laser light R: Distance of reference light of the distance measurement optical system (optical path length of reference light) L: Distance of measurement light (distance to distance measurement target) c : It is the speed of light.

In this optical system, the distance L to the object 216 to be measured is expressed by the following equation (2).

L = (f _s / f _r ) · (R _r −L _r ) + R (2) where f _{s is} the frequency of the beat signal of the ranging optical system f _{r is} the frequency of the beat signal of the reference optical system Is.

The distance calculation is performed by the computer 211 including a personal computer or the like. More specifically, the beat signals obtained by the detectors 207 and 217 as described above are amplified to a predetermined level by the amplifiers 208 and 218 connected to the detectors 207 and 217, respectively, and then the waveform shaper 209. Two
After the waveform is shaped by 19, the frequency is counted by the counter 210. Then, the computer 211 calculates (measures) the distance L based on this count value.

However, in the above method, since the phase portion of the beat signal of the reference optical system is measured based on the clock pulse emitted from the clock generator, the measurement accuracy is determined by the clock accuracy. .
Explaining a little now, the signal processing circuit calculates the phase angle corresponding to the wave number within one cycle of the beat signal of the reference optical system, that is, the wave number below the decimal point, by calculating the number of clock pulses and phase angle for one cycle. It is performed by the ratio of the decimal part of the wave number by the counting phase counter.

Here, the frequency stability of a clock generator which is usually used is about ± 50 ppm. Therefore, the level of the measurement frequency error is about 10 _−4, which is relatively large due to this error, and there is a limit in performing highly accurate measurement.

In order to solve such a problem, there is a signal processing circuit previously proposed by the present applicant in Japanese Patent Application No. 5-181739. FIG. 10 shows a counter 210 of this signal processing circuit. The circuit configuration will be described below together with the operation.

The beat signal of the reference optical system is supplied to the amplifier 208.
After being amplified to a predetermined level by the waveform shaper 10
9 is input, waveform shaping is performed here, and FIG.
It is converted into the rectangular wave shown in (a). Similarly, the beat signal of the distance measuring optical system is waveform-shaped by the waveform shaper 119 and converted into a rectangular wave shown in FIG.

The signal converted into the rectangular wave by the waveform shaper 109 is input to the wave number counter 124 of the reference system. An integer set value is preset in the wave number counter 124 by the wave number setting device 126 connected thereto. The wave number counter 124 is used for the waveform shaper 10.
The number of waves of the input signal from 9 is counted, and the gate is opened until the count value reaches the set value.
More specifically, during this period, the wave number counter 124 outputs the "H" level gate open signal (reference system gate signal) to the reference system time measurement counter 130 at the timing shown in FIG. 11B. It has become so.

The clock pulse of the waveform shown in FIG. 11C is input from the clock generator 122 to the time measuring counter 130. The time measurement counter 130 counts the number of clock pulses from the clock generator 122 and measures the time while the reference system gate signal from the wave number counter 124 is in the gate open state (FIG. 11 (d. )reference).

Further, the reference system gate signal is the synchronizing circuit 1
It is input to 31. The synchronizing circuit 131 generates a distance measuring system gate signal (see FIG. 11 (f)) synchronized with the reference beat signal based on the reference system gate signal and supplies it to the wave number counter 123 and the time measuring counter 132 of the distance measuring system.
The output of the waveform shaper 119 is input to the synchronization circuit 131. This output is also input to the time measurement counter 132.

The wave number counter 123 counts the wave number (see FIG. 11 (g)) of the distance measurement beat signal while the distance measurement system gate signal is in the "H" level gate open state. Further, the time measurement counter 132 has clock pulses from the same clock generator 122 (see FIG. 11 (h)).
Is input, and the distance measurement system gate signal is
While the gate is open at H ”level, this clock
Pulses are counted and time is measured.

With the above count values, the beat frequencies of the beat signals of the distance measuring optical system and the reference optical system are measured,
In the same manner as the above-mentioned conventional method, the computer 111 calculates the distance L to the measurement object by performing the calculation of the following expression (2) ′.

L = [(N _clr / N _r ) / (N _cls / N _s )] (2) ′ where N _{s is} the frequency of the beat signal of the distance measuring optical system N _{r is} the beat signal of the reference optical system Frequency N _clr : number of pulses of clock generator of reference optical system for time measurement N _cls : number of pulses of clock generator of distance measurement optical system for time measurement

According to such a signal processing system, the time measurement of the beat signals respectively generated by the reference optical system and the distance measuring optical system is performed almost simultaneously according to the clock pulse generated from the same clock generator 122. ,
Even if the clock pulse generated from the clock generator 122 changes due to environmental changes such as temperature, that is, it fluctuates with time, the influence appears equally in both optical systems. Therefore, the error with respect to the measurement value of each optical system is canceled and canceled, which enables highly accurate measurement,
This has the effect of significantly improving the reliability of the laser range finder.

[0025]

By the way, for example, AF
The focusing method of the lens used for this purpose is to perform the wobbling operation of the lens and move the lens so that the magnitude of the harmonic component of the spatial frequency of the image is maximized to adjust the focus position. That is, when the harmonic component of the spatial frequency is large, the state is in the best focus.

When a laser range finder using the above signal processing method is applied to such a device for focusing a lens, phase noise caused by wobbling operation of the lens causes a measurement error. There is a problem that a large error occurs. That is, the above laser distance measuring device is premised on measuring the distance to the distance measuring object in a stationary state with high accuracy.

Therefore, even when the above laser distance measuring device is applied to an FA, there is a problem that a large error similarly occurs in the measurement when the object to be measured vibrates.

The cause of this problem will be described in detail below. When the object to be measured changes a minute distance by the wobbling operation, the initial phase of the beat signal changes according to the distance difference between the two lights. Therefore, the output of the beat signal actually observed is expressed by the following equation (3). As shown, the beat signal corresponding to the amount of change in the actual distance represented by f _b + Δf _b is
The frequency corresponding to the second term in the equation (3), which corresponds to the phase change due to the vibration of the object to be measured, is the output I that is FM-modulated.

I = 2ab cos [2π (f _b + Δf _b ) t + 2πν ₀ (α / c) cos 2πf · t + 2πν ₀ τ ₀ ] (3) where f _b : beat frequency (frequency of beat signal) Δf _b : Amount of change in beat frequency depending on vibration distance α: Vibration width of distance measuring object f: Frequency of vibration of distance measuring object ν ₀ : Optical frequency of semiconductor laser τ ₀ : Distance difference c: Light speed .

FIG. 12 shows the influence of the above equation (3).
When the object to be measured is not vibrating, that is, when the object to be measured is not vibrated, the beat frequency has a temporally stable waveform as shown in FIG. On the other hand, when the object to be measured vibrates, it is FM-modulated due to the influence of the vibration of the object to be measured, so that its waveform changes greatly as shown in FIG.

Therefore, when the object to be measured is vibrating, the beat frequency changes more than the amount of change in the beat frequency according to the distance of the vibration amount of the object to be actually measured. In the laser distance measuring method of the signal processing method, the large measurement error occurs. Therefore, there is a limit to the practical application.

In the above laser distance measuring apparatus proposed by the applicant of the present application, such a problem can be solved by suppressing the vibration of the distance measuring object. However, when such a laser distance measuring device is applied to FA, it is impossible to reduce the vibration to 10 μm or less because of vibration of the device and the like. Therefore, it is difficult to apply the above laser range finder to FA. For this reason, the usage of the application was limited.

Also, if a large number of beat signals are measured at a measurement rate sufficiently faster than the vibration frequency (f) of the object to be measured, and if time averaged, that is, if measured in a long window, the object to be measured will vibrate. It is also possible to reduce the influence of. However, such a measuring method is not practical because the measuring time becomes long.

Further, in the above laser distance measuring device, there is a problem in the output method of the measured distance value for the following reason, so that it is difficult to apply it to a control system such as AF of a camera. Is. In the frequency measurement method using the frequency counter, the number of clock pulses and the number of beat signals having a constant gate time are counted, and the beat frequency is calculated from these count values. In this method, the higher the number of clock pulse counts is measured, that is, the longer the gate time is, the more the measurement accuracy can be improved. However, it is impossible to continuously output the distance output value in order to count the clock pulses of arbitrary gate times. Therefore, it is difficult to apply it to a control system that requires continuous output of distance values such as AF of a camera.

In addition, in the optical system using the semiconductor laser as the light source as described above, there is a problem that the measurement accuracy is deteriorated due to the deterioration of the coherence due to the returning light. This problem will be explained a little now with reference to FIG. Now, as shown in FIG. 7A, it is assumed that the frequency modulation amount of the triangular wave is small, for example, the frequency modulation amount is 100 GHz, the modulation frequency is 50 KHz, and the distance to the object to be measured is 50 cm. In this case, the difference between the oscillation frequencies of the light emitted from the semiconductor laser and the return light is the optical frequency of the laser emission light at this time, where f _a is the optical frequency of the return light, as shown in FIG. Is the frequency after a time τ corresponding to the distance difference, that is, f _b, and therefore changes only at most 30 KHz under the above distance measuring conditions. Therefore, in this case, the difference between the oscillation frequencies of the emitted light and the returned light is
This means that the normal laser light has a spectral line width separated by no more than 10 MHz. For this reason, the emitted light and the returned light are combined, resulting in a decrease in coherence.
The strength of the beat signal weakens and the S / N ratio decreases (Fig. 1
4 (b)). Therefore, since noise is captured,
The measurement error of the frequency of the beat signal becomes large. That is, since the strong noise component becomes larger than the weak signal component,
This caused a problem that the measurement error was large. The present invention solves such a problem of the prior art, and can measure the distance with high accuracy even when the object to be measured vibrates, and can be applied to FA and AF of a camera. It is to provide a laser distance measuring method that enables

Another object of the present invention is to provide a signal processing circuit capable of continuously outputting measured values and suitable for application to AF and FA of a camera.

[0037]

According to a laser distance measuring method of the present invention, a laser light source is temporally frequency-modulated, the modulated light is incident on an interference optical system, and then the light is split by a light splitting means. One of the two lights is projected onto the object to be measured, the other light is projected onto the reflector, and the reflected light from the object to be measured and the reflected light from the reflector are heterodyne-detected to thereby obtain two optical paths. A laser distance measuring method for detecting a beat signal due to an optical frequency difference according to a difference and measuring a distance to the distance measuring object or a displacement to the distance measuring object based on the beat signal. When the object vibrates, the frequency of the beat signal satisfies the following condition: (4π / laser light source oscillation wavelength) × vibration width of the distance measurement object × vibration frequency of the distance measurement object ≦ Beat signal frequency x relative accuracy Optical modulation frequency of the laser light source or Adjusts the amount of injected current to perform the measurement, which achieves the above object.

Preferably, the triangular wave drive frequency or the FM modulation amount is increased and the frequency of the beat signal is set so as to satisfy the above condition.

Preferably, a DBR laser with three electrodes is used as the laser light source. Further, the signal processing circuit of the present invention is provided with a self-delay detection circuit that performs self-delay detection of a beat signal, and an analog distance value output corresponding to the frequency of the beat signal is obtained by the output of the self-delay detection circuit. Therefore, the above object is achieved.

Preferably, the self-delay detecting circuit sets a delay amount to a beat signal input from an optical system, a beat detecting circuit from the optical system, and a delay beat signal from the delay circuit. And a flip-flop circuit that outputs an output of a pulse train corresponding to the analog distance value output from an output terminal.

Further, the signal processing circuit of the present invention comprises a phase comparator and an analog frequency discriminating circuit, the analog frequency discriminating circuit performs frequency synchronization of the frequency of the beat signal, and performs phase synchronization in a state where the frequencies match. Then, an output signal corresponding to the distance value is output, whereby the above object is achieved.

Preferably, a PLL circuit is used as the analog frequency discriminating circuit.

Further, the signal processing circuit of the present invention is provided with a frequency divider that divides the beat signals from the reference optical system and the distance measuring optical system, respectively, thereby achieving the above object.

[0044]

As described above, according to the method of the present invention, the frequency of the beat signal is increased so as to obtain a desired relative accuracy,
As a result, the influence of the phase noise is made relatively small, and the problems of the above-mentioned conventional technique are solved. That is,
Specifically, it is measured under the condition of the following formula (4).

(4π / laser light oscillation wavelength) × vibration width of the object to be measured × vibration frequency of the object to be measured ≦ beat signal frequency × relative accuracy (4) More concretely, for example, a semiconductor The oscillation wavelength of the laser is 0.8 μm, and the vibration width of the object to be measured is 0.1 μm.
m, in order to obtain the relative accuracy of 10 ⁻⁶ when the vibration frequency of the object to be measured is 10 Hz, it can be seen from the above formula (4) that the frequency of the beat signal is 2 × 10 ⁷ Hz or higher. .

In the present invention, the triangular wave drive frequency (see FIG. 6) or the FM modulation amount is increased so that the frequency of the beat signal becomes equal to or higher than the above value, and the frequency of the beat signal is set to a desired frequency. Further, as a signal processing circuit for implementing the method of the present invention, a delay detection circuit and a PLL (P
By using a circuit equipped with an analog frequency discriminating circuit such as a has Locked Loop circuit, by detecting the frequency of the beat signal, it is possible to continuously output measured values to a distance calculating means including a computer such as a personal computer. it can.

By the way, as a method for increasing the frequency of the beat signal, as shown in FIG.
Method of raising from the state shown in (a), that is, method of increasing the triangular wave driving frequency itself As shown in FIG. 6 (b), the triangular wave driving frequency is kept constant and the FM modulation amount of the semiconductor laser is increased. Therefore, there is a method of making the triangular wave of the optical frequency steep.

However, with the method of increasing the triangular wave drive frequency itself, the response of the injection current to the semiconductor laser due to heat is at most several MHz, and it is difficult to obtain a sufficient modulation amount.

Further, in the method of increasing the FM modulation amount, it is necessary to greatly change the injection current when a normal Fabry-Perot laser is used as the semiconductor laser, but this is impossible because there is a limit due to the maximum light output. Is.

Therefore, in the present invention, a solution is sought by using a three-electrode DBR laser and changing the refractive index due to the plasma effect of carriers due to the injection current. Explaining a little now, since the response speed due to the plasma effect of the carrier is about several GHz, the triangular wave driving frequency can be set to an arbitrary frequency. Regarding the response speed due to the plasma effect of carriers, see, for example, 1993 Spring Applied Physics Society 30p-C-9 “AlGaAs / GaAs MQW-D.
FM response characteristics of FB laser ”Toshihiko Onouchi Since it is described in detail elsewhere, it is omitted here.

Next, the case where the method of the present invention is applied to the wobbling operation of an AF lens will be described. AF
In the wobbling operation of the lens for the laser, the wavelength of the semiconductor laser is 0.8 μm and the vibration width of the object to be measured is 10 μm.
m, the frequency of vibration of the object to be measured is about 50 Hz, and the required relative accuracy is 10 ⁻⁴ , so the frequency of the beat signal is set to 8 × 10 ⁷ Hz or more from the above formula (4). There is a need to. Then, by setting the triangular wave drive frequency or the injection current for the semiconductor laser so that the frequency of the beat signal becomes 8 × 10 ⁷ Hz or more, desired relative accuracy can be obtained.

A little explanation will now be given with reference to FIGS. 7A to 7D. For example, when the vibration range of the object to be measured is 10 μm, the relative accuracy is 10 as shown in FIG. ^-To obtain ^-4 or more, the vibration frequency of the object is 10
^{When 1} (= 10) Hz, the beat signal frequency is 2
× 10 ⁷ Hz or more is required. The vibration frequency is 10 ²
When Hz, the beat signal frequency is 2 × 10 ⁸ H
z or more is required. Further, as shown in FIG. 7B, in order to obtain a vibration width of 1 μm and a relative accuracy of 10 ⁻⁴ or more, when the vibration frequency is 10 ² Hz, the frequency of the beat signal must be 2 × 10 ⁷ Hz or more. There, when the vibration frequency ^103, the frequency of the beat signal is required 2 × 10 ⁸ or more.

From this result, it is understood that the relative accuracy is deteriorated equally when the vibration frequency of the object to be measured is low and the vibration width is large and when the vibration frequency is high and the vibration width is small. That is, these two effects are equal. In other words, the change in the initial phase of the beat signal frequency (brightness and darkness of interference fringes) due to the vibration width of the object to be measured and the change in the initial phase of the beat signal due to the increase in the vibration frequency are equal. There is.

In the above conditional expression (4), although the relative accuracy is deteriorated due to the influence of the vibration of the object to be measured, if the frequency of the beat signal is increased so that this accuracy becomes a desired value, the accuracy of the accuracy becomes higher. It shows that deterioration can be reduced. Also,
By increasing the frequency of the beat signal, it is possible to reduce deterioration in measurement accuracy due to fluctuations in the refractive index due to air temperature. This is because the effect of the fluctuation of the refractive index is the same as the swing of the object to be measured, and it is detected as the frequency of the beat signal FM-modulated.

In the method of the present invention, when the modulation amount is 10 THz, the modulation frequency is 5 KHz, and the distance difference to the object to be measured is 50 cm, as shown in FIG. When the modulation amount is large, the optical frequency difference is about 300 MHz, and therefore the frequency difference between the emitted light and the return light becomes equal to or larger than the line width of normal laser light. Therefore, since the emitted light and the returned light are not combined, the deterioration of the S / N ratio due to the coherence can be reduced.

Further, according to the signal processing circuit of the present invention, by performing the self-delay detection by the delay detection circuit and the frequency detection by the frequency discrimination by the PLL, it becomes possible to continuously output the measured value in analog.

[0057]

Embodiments of the present invention will be described below with reference to the drawings.

Since the structure of the optical system used in the laser distance measuring method of the present invention is the same as that of the conventional example described above, the description and drawings of the optical system are omitted. However, in the present invention, a three-electrode DBR laser having the characteristics described in the above section is used as the laser light source.

(Embodiment 1) In the present embodiment 1, as described in the section of the above operation, as shown in FIG.
The triangular wave driving frequency of the semiconductor laser is changed so that the frequency of the beat signal can obtain a desired measurement accuracy with respect to 16 vibrations (see FIG. 8). As an example, the oscillation wavelength of the semiconductor laser is 0.8 μm,
When 16 is vibrating with a vibration width of 1 μm and a vibration frequency of 10 Hz, the distance to the object 216 is measured with a relative accuracy of 10
^In order to measure at ^-4 , it is necessary to set the frequency of the beat signal to 2 MHz or higher according to the equation (4).

FIG. 1 shows a signal processing circuit used in the first embodiment of the present invention. This signal processing circuit includes a differential detection circuit 1, a flip-flop 2, a low-pass filter 3, and a laser triangular wave drive circuit 4, and a semiconductor laser 5 is used to set a frequency of a beat signal that allows measurement with desired relative accuracy. It has a system configuration for drive control.

Explaining a little more concretely, the laser triangular wave is set so as to set the triangular wave frequency from the relationship between the frequency of the beat signal of the above formula (1) and the triangular wave modulation frequency so that the frequency of the desired beat signal is obtained. The drive circuit 4 drives and controls the semiconductor laser 5. Alternatively, the triangular wave oscillation frequency is kept constant and Δν in the above equation (1) is changed. That is, the injection current to the semiconductor laser 5 is increased by the laser triangular wave drive circuit 4 to change the FM modulation amount, and thereby the frequency of the desired beat signal is controlled. According to such a drive control method, the distance measuring object 2 vibrating under the above conditions
It was confirmed that the relative accuracy when measuring 16 distances could be 10 ⁻⁴ .

Next, a method of outputting the distance value based on the frequency of the beat signal using this signal processing circuit will be described. The beat signal (see FIG. 2A) detected by the optical system is input to the delay detection circuit 1 and the reset terminal of the flip-flop 2. The delay detection circuit 1 gives an arbitrary delay amount (delay time) τ to the frequency of the input beat signal, and outputs the delayed signal (see FIG. 2B) to the set terminal of the flip-flop 2.

When such an input signal is applied, the flip-flop 2 outputs from the output terminal Q a pulse train output signal having a duty ratio corresponding to the frequency of the input signal (see FIG. 2).
(See (c)) is applied to the low-pass filter 3. That is, the combination of the delay detection circuit 1 and the flip-flop 2 performs the self-delay detection of the beat signal. The low-pass filter 3 cuts high-frequency components above a predetermined level.
As a result, an analog distance value output (see FIG. 2D) corresponding to the frequency of the beat signal is obtained. This distance value output is input to a computer which is a distance calculating means, and finally the distance to the distance measuring object 216 or the displacement to the distance measuring object is measured.

Here, there is a problem that the optical frequency of the semiconductor laser 5 lowers to 0 Hz at the apex of the triangular wave drive frequency, but the output time corresponding to this time is about 10 nsec at the most. If the output value is peak-held in synchronization with the triangular wave drive frequency during this time, the output value can be continuously output.
Therefore, such a configuration enables continuous output of distance values.

The difference in FM modulation efficiency can be canceled by performing calibration based on the analog distance value signal obtained from the reference optical system with known distance difference shown in FIG.

(Second Embodiment) FIG. 3 shows a second embodiment of the present invention. In the second embodiment, the triangular wave drive frequency or FM
The triangular wave drive frequency or the FM modulation quantity is set by the laser triangular wave drive circuit 4 so that the beat signal has a desired relative accuracy with respect to the vibration of the object 216 by changing the modulation quantity. It is configured to drive and control the laser 5. The distance value is output by PLL detection of the analog output value of the beat signal. Below P
A circuit configuration for performing LL detection will be briefly described together with the operation.

The beat signal detected by the optical system is input to the phase comparator 6, where the phase of the frequency of the beat signal is compared, and the comparison output (difference output) is obtained by the loop filter 7.
To the amplifier 8. The loop filter 7 cuts a predetermined noise component. Phase discrimination output signal V (t), which is the output signal amplified to a predetermined level by the amplifier 8.
Is applied to a voltage controlled oscillator (VCO) 9 where phase discrimination is performed. In this manner, the signal V _c (t) that minimizes the amplified phase discrimination output signal V (t) is input to the voltage controlled oscillator 9, and this signal V _c (t) is input to the phase comparator 6.
To a loop filter 7 and an amplifier 8 and then output as a distance value.

That is, with such a circuit configuration, the beat signal is frequency-synchronized, and then the phase-locking process is performed in the state where the frequencies match each other.
The output voltage of corresponds to the distance value.

(Embodiment 3) FIGS. 4 and 5 show Embodiment 3 of the present invention. FIG. 3 shows the circuit configuration of the signal processing circuit used in the third embodiment, but most of the devices are
Since the applicant of the present invention has common features with the signal processing circuit previously proposed in Japanese Patent Application No. 5-181739, common parts will be denoted by the same reference numerals and description thereof will be omitted, and only different parts will be described below. To do. Further, the description of the common parts to the waveforms shown in FIG. 5 will be omitted.

This signal processing circuit includes a waveform shaper 109 to which a reference system beat signal is input and a wave number counter 124.
1 / N frequency divider 140, 1 / N between the waveform shaper 119 to which the beat signal of the distance measurement system is input and the wave number counter
The signal processing circuit is the same as the signal processing circuit described above except that the frequency dividers 141 are respectively connected.

In the third embodiment, as shown in FIG. 7, a semiconductor is formed by, for example, a laser triangular wave drive circuit so that the beat signal frequency can obtain a desired measurement accuracy with respect to the vibration of the object 216 to be measured. By changing the triangular wave driving frequency of the laser, the frequency of this beat signal is divided by 1 / N frequency dividers 140, 14
Divide by 1 to 1 / N (Fig. 4 (d), (e)
As a result, the frequency is lowered to a frequency that can be measured by the wave number counters (frequency counters) 124 and 123. Therefore, there is an advantage that the frequency can be measured by a general-purpose type frequency counter.

In addition, at this time, the fluctuation component of the beat signal caused by the vibration of the object 216 is also set to 1 / N, and the relative accuracy can be improved accordingly.

[0073]

According to the above laser distance measuring method of the present invention,
The frequency of the beat signal satisfies the following condition: (4π / laser light oscillation wavelength) × vibration width of the object to be measured ×
Vibration frequency of distance measuring object ≦ beat signal frequency × relative accuracy Since the optical modulation frequency of the laser light or the amount of injected current is adjusted, a beat signal frequency sufficiently higher than the vibration of the distance measuring object can be obtained. . Therefore, the influence of phase noise caused by the vibration of the object to be measured can be reduced, and highly accurate measurement can be performed.

Further, in the above adjustment, when the frequency modulation amount is increased, it is possible to prevent the deterioration of the coherence due to the returning light, so that the measurement can be performed with higher accuracy.

In particular, according to the signal processing circuits of claims 4 to 8, since an analog continuous output signal corresponding to the distance value can be obtained, it can be applied to AF and FA of the camera. The effect is that the application can be expanded.

[Brief description of drawings]

FIG. 1 is a block diagram showing a signal processing circuit used in a first embodiment of the present invention.

FIG. 2 is a waveform diagram showing a signal waveform at that time.

FIG. 3 is a block diagram showing a signal processing circuit used in a second embodiment of the present invention.

FIG. 4 is a block diagram showing a signal processing circuit used in a third embodiment of the present invention.

FIG. 5 is a waveform diagram showing a signal waveform at that time.

FIG. 6 is a signal waveform diagram showing the principle of the method of the present invention.

FIG. 7 is a graph showing the relationship between the beat frequency (the frequency of the beat signal) and the vibration frequency or relative accuracy of the object to be measured, showing the principle of the method of the present invention.

FIG. 8 is a schematic view showing an optical system used in a conventional example and the present invention.

FIG. 9 is a graph showing a triangular wave drive frequency and a beat signal.

FIG. 10 is a block diagram showing a signal processing circuit used in the laser distance measuring device previously proposed by the applicant of the present application.

FIG. 11 is a signal waveform diagram thereof.

FIG. 12 is a graph for explaining the influence of the vibration of the object to be measured on the measurement accuracy.

FIG. 13 is a graph for explaining deterioration of measurement accuracy due to a decrease in coherence due to returning light.

FIG. 14 is a graph showing the quality of the S / N ratio.

[Explanation of symbols]

 1 Delay Detection Circuit 2 Flip Flop 3 Low Pass Filter 4 Laser Triangular Wave Drive Circuit 5 Semiconductor Laser 6 Phase Comparator 7 Loop Filter 8 Amplifier 9 Voltage Controlled Oscillator 109, 119 Waveform Shaper 111, 210 Computer 122 Clock Generator 123, 124 Wave Count Counter 130, 132 Time measurement counter 131 Synchronous circuit 140, 141 1 / N frequency divider 205, reflector 207 Detector 216 Distance measurement object

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location G02B 7/32 G03B 13/36 (72) Inventor Atsushi Shimonaka 22 No. 22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka No.22 Sharp Co., Ltd.

Claims (8)

[Claims] 1. A laser light source is temporally optical frequency modulated,
The modulated light is made incident on the interference optical system, and subsequently, one of the two lights split by the light splitting means is projected on the object to be measured, and the other light is projected on the reflector to measure the distance. The beat signal due to the optical frequency difference corresponding to the optical path difference between the two lights is detected by performing heterodyne detection on the reflected light from the object and the reflector, and the distance to the object to be measured or the distance to the object to be measured based on the beat signal. A laser distance measuring method for measuring displacement to a distance measuring object, wherein when the distance measuring object vibrates, the frequency of the beat signal satisfies the following condition: (4π / laser light source Oscillation frequency) × vibration width of distance measuring object × vibration frequency of distance measuring object ≦ beat signal frequency × relative accuracy A laser distance measuring method for performing measurement by adjusting the optical modulation frequency of the laser light source or the injected current amount. 2. The laser distance measuring method according to claim 1, wherein the triangular wave driving frequency or the FM modulation amount is increased and the frequency of the beat signal is set so as to satisfy the above condition. 3. The DBR having three electrodes as the laser light source
The laser distance measuring method according to claim 1 or 2, wherein a laser is used. 4. A signal processing circuit for carrying out the laser distance measuring method according to claim 1, further comprising a self-delay detecting circuit for self-delay detecting a beat signal, the self-delay detecting circuit comprising: A signal processing circuit that obtains an analog distance value output according to the frequency of the beat signal by output. 5. The delay detection circuit, wherein the self-delay detection circuit sets an arbitrary delay amount to a beat signal input from an optical system, the beat signal from the optical system, and the delayed beat signal from the delay circuit. 5. The signal processing circuit according to claim 4, further comprising a flip-flop circuit which is input and outputs an output of a pulse train corresponding to the analog distance value output from an output terminal. 6. A signal processing circuit for carrying out the laser distance measuring method according to claim 1, comprising a phase comparator and an analog frequency discriminating circuit, wherein the beat signal is detected by the analog frequency discriminating circuit. A signal processing circuit that performs frequency synchronization of frequencies and performs phase synchronization when the frequencies match and outputs an output signal corresponding to the distance value. 7. The signal processing circuit according to claim 6, wherein the analog frequency discrimination circuit is a PLL circuit. 8. A signal processing circuit for carrying out the laser distance measuring method according to claim 1, wherein the frequency divider divides the beat signals from the reference optical system and the distance measuring optical system, respectively. Signal processing circuit provided with.