JPH10246782A - Laser distance meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure the distance from a stationary object to be measured without being affected by surrounding environment of the object to be measured. SOLUTION: Provided are a laser element 2 emitting laser light to an object to be measured, a laser drive circuit 6 driving the laser element 2 based on a specific modulation drive signal, a detection means 4 receiving the laser light reflected on the object to be measured and detecting the distance to the object based on the saw-tee wave self-mixed with the modulated laser light. Also, the detection means 4 is provided with an A/D converter converting the laser light reflected by the object to be measured to digital signal and a frequency analyzer analyzing the digital signal for a certain time converted with this A/D converter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser range finder, and more particularly to a laser range finder for measuring a distance to a stationary object using a self-mixing laser.

[0002]

2. Description of the Related Art Conventionally, as a method of measuring a distance to a stationary object, such as measuring a liquid level in a test tube, there is a method using an ultrasonic wave as shown in FIG. The ultrasonic distance meter 54 measures the distance L to the liquid surface 52 in the test tube 50 based on the propagation speed of the ultrasonic wave.

In addition to the non-contact method using ultrasonic waves, there is a contact method. FIG. 7 is a diagram showing an example of a contact-type liquid level measurement using energization of electrodes and water pressure.
As shown in FIG. 7 and FIG.
And the pressure sensor 58 are moved to calculate a moving distance L until the liquid sensor comes into contact with the liquid surface.

On the other hand, in distance measurement using a laser, there is a method based on the principle of triangulation. As shown in FIG.
This conventional example includes a laser 60 and a laser beam 62 that receives the laser beam reflected by the liquid surface 52, and calculates the distance L based on the principle of triangulation based on the angles of the light receiving element and the laser with respect to the liquid surface. I do.

[0005] Distance measurement using the self-mixing effect of a laser is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-112784. As shown in FIG. 10, in this conventional example, a laser element 72 that emits a laser beam and receives self-mixed reflected light from an object to be measured is received, and the laser beam from the laser element is modulated at a predetermined cycle. A modulation drive current generating circuit 80 for amplifying the output current output from the laser element 70, and pulsating the output signal amplified by the amplifier 82 for each mode hopping and counting the number of pulses. Counter and other measurement circuits 76
And a personal computer 7 that calculates a distance L to the measurement target 51 based on the count value and the like measured by the measurement circuit 76.
8 is provided.

[0006]

However, the above-described conventional examples have the following disadvantages.

[0007] In the conventional distance measurement using ultrasonic waves, the wavelength is as large as 0.8 mm, and the measurement requires several times the wavelength. Therefore, there is an inconvenience that there is a limit in improving the accuracy. Further, since the propagation speed of the ultrasonic wave itself is easily affected by the temperature, there is a certain limit in improving the measurement accuracy depending on the measurement target also from this point.

Further, in the measurement using ultrasonic waves, if a wall or the like is present in the vicinity of the object to be measured, irregular reflection of the ultrasonic waves occurs, and the measurement accuracy is not improved.
In particular, in detecting the liquid level of a test tube, the wall surface becomes closer due to the inclination of the test tube, and the reflection state changes.

In the method using triangulation by laser,
When the distance to the object to be measured cannot be sufficiently set, there is a disadvantage that accuracy is reduced. In particular, in the case of distance measurement in a test tube, this effect is large.

Further, in the method of obtaining the distance by the mode hop period appearing in the photodiode output using the self-mixing effect of the FM-modulated semiconductor laser, the dependence on the mode hop generation period is large. Since the distance has to be calculated for each of the ascending section and the descending section of the modulation at the time of light emission, there is a disadvantage that it is disadvantageous in terms of resolution.

As described above, in the conventional example, there is a disadvantage that the distance to the stationary measurement object cannot be measured more accurately than the distance measurement using ultrasonic waves.

[0012]

SUMMARY OF THE INVENTION It is an object of the present invention to improve the disadvantages of the prior art, and in particular, to accurately measure the distance to a stationary measuring object without being affected by the surrounding environment of the measuring object. It is an object of the present invention to provide a laser range finder that can be used.

[0013]

Therefore, according to the present invention, there is provided a laser element for irradiating a laser beam to an object to be measured, a laser drive circuit for driving the laser element based on a predetermined modulation drive signal, and Detecting means for receiving the laser light reflected by the laser beam and detecting the distance to the object to be measured based on the sawtooth wave self-mixed with the modulation of the laser light according to the modulation drive signal. Moreover, the detecting means converts the laser light reflected by the object to be measured into a digital signal.
An A / D conversion section and a frequency analysis section for frequency-analyzing the digital signal for a predetermined time converted by the A / D conversion section are employed. This aims to achieve the above-mentioned object.

Since the laser drive circuit drives the laser element based on the modulation drive signal, the laser element irradiates a modulated laser beam to the object to be measured. When the laser light is modulated and irradiated, the irradiated laser light and the laser light reflected by the measurement object cause self-conflict, and a sawtooth wave appears according to the distance to the measurement object. The detection means receives the laser light in which the self-mixing has occurred and converts it into a digital signal.
Further, the frequency analysis unit analyzes the frequency of the digital signal for a predetermined time. When this frequency analysis is performed, a large number of frequency components of the sawtooth wave in which self-mixing has occurred appear. Therefore, the distance is obtained in a non-contact manner from the value of the frequency component of the sawtooth wave proportional to the distance to the object to be measured. Since the distance is obtained from the measured frequency based on the proportional relationship between the frequency value and the distance, if the relation between the reference distance and the frequency is obtained in advance, the calculation after the frequency analysis becomes easy.

In general, in the case of a self-mixing type measurement,
There is a problem that when the measuring object moves, the moving direction changes, and when the measuring object is stationary, the sawtooth waveform is disturbed when the driving current is reversed. In the present invention, a plurality of sawtooth waves indicating the same object (same distance) are measured, and the distance is measured based on the maximum frequency component. The distance is measured without performing noise processing and without considering the inversion timing of the drive current.

According to the frequency analysis, the change in the liquid level appears as the wavelength (frequency) of the sawtooth wave, so that the change in the object to be measured can be captured linearly.

A typical frequency analysis is a fast Fourier transform (FFT), and in some embodiments,
An FFT analyzer is employed as the detection means. However, the present invention is not limited to this, and the detecting unit may be any unit that analyzes the frequency component of the digital signal. Further, the frequency analysis includes a frequency component of the modulation by the laser driving circuit, but the frequency band of this modulation does not overlap with the frequency band of the sawtooth wave. However, since the frequency of the modulation at the time of light emission is predetermined, a filter for removing the frequency component at the time of light emission may be provided in some embodiments.

The present invention is particularly preferably used for measuring the liquid level in a test tube. However, the present invention is not limited to this.When the measurement target is stationary, and the measurement target is surrounded by a wall surface or has unpredictable dirt, or when measuring from a very short distance to a far distance. It is suitable for.

[0019]

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

FIG. 1 is a block diagram showing a configuration of a laser distance meter according to the present invention. The laser rangefinder includes a laser element 2 that emits a laser beam to a measurement target 51, a laser driving circuit 6 that drives the laser element 2 based on a modulation drive signal, and a laser beam reflected by the measurement target 51. Detecting means 4 for receiving the light and detecting the distance to the object to be measured based on the sawtooth wave of the laser light;

In addition, the detecting means 4 includes an A / D converter 16 for converting the sawtooth wave into a digital signal, and a frequency analyzer for frequency-analyzing the digital signal for a predetermined time converted by the A / D converter 16. 18 is provided. The frequency analysis unit 18 may be constituted by an FFT analyzer that performs a fast Fourier transform.

The laser element 2 includes a laser driving circuit 6
A laser diode 12 for irradiating the object to be measured with a laser beam based on a laser drive signal output from the device, and a photodiode 10 for receiving the laser beam reflected from the object to be measured via the laser diode 12. The photodiode 10 converts the laser light that has undergone self-mixing by the laser diode 12 into an electric signal and outputs the electric signal. In the present embodiment, an amplifier 14 that amplifies the electric signal output from the photodiode 10 is provided at a stage before the input to the detection unit 4.

FIG. 2 is an explanatory view showing an example in which the liquid level 52 in the test tube 50 is measured. Here, the laser rangefinder is
It operates as a liquid level measuring device in a test tube. Laser light is
Since the wavelength is shorter and the straightness is higher than that of ultrasonic waves, distance measurement can be performed without deteriorating accuracy even for a measurement object surrounded by a wall such as the liquid level 52 in the test tube 50. It can be carried out.

This will be described in detail. In this embodiment,
Even if the measurement target is a liquid such as water, it can be measured favorably. The laser light output from the laser diode 12 reaches the liquid surface 52 via air. Since there is a difference in the refractive index between air and liquid, the laser light is transmitted and reflected. The self-mixing effect is obtained by receiving the light reflected on the liquid surface by the photodiode 10 via the laser diode 12.

In this case, the laser driving circuit 6
As shown in (A), the drive current of the semiconductor laser for distance measurement is a triangular wave current, and the oscillation frequency of the laser is modulated. In the distance measuring method using the self-mixing effect, the frequency f of the sawtooth wave appearing in the photodiode 10 increases in proportion to the distance L between the laser diode 12 and the object to be measured. If the measuring object moves, a saw-tooth wave is generated according to the amount of the displacement.In this embodiment, however, the measuring object is stationary. Is received by the self-mixing effect. Therefore, the drive modulation current is
The modulation is not limited to the triangular wave current, and may be any modulation that has a frequency component such that the sawtooth wave self-mixes.

In the self-mixing method, the oscillation light from the semiconductor laser and the reflected return light from the object to be measured interfere with each other in the semiconductor laser element to generate a sawtooth signal. FIG.
(B) and (C) are diagrams showing examples of waveforms in which a sawtooth wave is mixed with a modulation component. Here, a frequency modulation is performed on the drive current of the laser to create a difference between the frequencies of the return light and the oscillation light, and the self-mixing effect is obtained by the difference.

If the frequency at the maximum FFT point other than 0 Hz of the sawtooth wave is f, the following equation (1) holds.
Using this proportional relationship, the distance L can be obtained from the reference distance L0 and the sawtooth wave frequency f0 at that time, as in the following equation (2).

[0028]

(Equation 1)

Where c is the speed of light, im is the modulation drive current,
fm is the modulation frequency, and df / di is the frequency modulation efficiency.

Therefore, if the frequency of the sawtooth wave is obtained,
The distance to the liquid level can be known. In this embodiment,
As shown in FIG. 3D, the PD output current shown in FIG. 3C is filtered to remove the modulation frequency component at the time of light emission, and only the sawtooth wave is extracted. Further, a frequency analysis of the sawtooth wave after the filtering is as shown in FIG. Since the frequency component that becomes the peak of the power spectrum is the main frequency of the sawtooth wave, the distance to the object to be measured is determined from the value of this frequency.

FIGS. 4 and 5 are diagrams showing an example in which the frequency analysis of the PD output current is performed without removing the frequency component due to the modulation. In the example shown in FIG.
9.250 kHz, and in this case, the distance from the laser element 2 to the liquid level 52 is calculated to be about 80 mm by the above-described equations (1) and (2). The frequency component indicated by reference numeral 31 in FIG. 4 is the frequency component of the modulated triangular wave. In the example shown in FIG. 4, the averaging is performed 30 times.

In the example shown in FIG. 5, the averaging is performed 100 times, and the frequency of the peak 32 is 17.8125 kHz. In this case, the distance from the laser element 2 to the liquid level 52 is calculated to be about 91 mm.

As described above, according to the present embodiment, since the distance is measured by the laser beam, the distance can be measured without being affected by contamination on the wall surface of the test tube or the like. Also,
Compared to ultrasonic measurement, the laser wavelength is small, so the accuracy in principle is high, and the FFT waveform changes linearly with the change in distance, so the resolution does not depend on mode hop. The distance can be measured accurately.

[0034]

According to the present invention, the laser drive circuit drives the laser element based on the modulation drive signal, and the laser light reflected from the object to be measured is measured. The sawtooth wave according to the distance to the object causes self-mixing, and the frequency analysis unit analyzes the frequency of the digital signal for a certain period of time.Therefore, many frequency components of the sawtooth wave in which self-mixing occurs appear. The distance can be obtained in a non-contact manner from the value of the frequency component of the sawtooth wave, which is proportional to the distance to the object to be measured, and the distance is obtained by frequency analysis. Continuous measurement without
Moreover, when the distance is obtained from the peak of the frequency, the average value is obtained from a large number of measurements. Therefore, it is possible to provide an unprecedented excellent laser range finder that is resistant to noise, stable, and can accurately obtain the distance. it can.

[Brief description of the drawings]

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

FIG. 2 is an explanatory diagram showing a configuration for measuring the liquid level of a test tube.

FIGS. 3A and 3B are waveform diagrams showing various signal waveforms used in FIGS. 1 and 2; FIG. 3A is a diagram showing an example of a driving current of a laser element; FIG. 3 (C) is an enlarged view of the PD output current waveform shown in FIG. 3 (B), and FIG. 3 (D) is an enlarged view of the PD output current waveform shown in FIG. 3 (C).
FIG. 3 is a diagram showing an example in which the waveform shown in FIG.
FIG. 4E is a diagram showing an example of a sawtooth wave spectrum obtained by performing frequency analysis of the sawtooth wave shown in FIG.

FIG. 4 is an explanatory diagram showing an example of a power spectrum obtained by frequency-analyzing a PD output current without performing a filtering process.

FIG. 5 is an explanatory diagram showing another example of a power spectrum obtained by frequency-analyzing a PD output current without performing a filtering process.

FIG. 6 is an explanatory view schematically showing a conventional ultrasonic range finder.

FIG. 7 is an explanatory view showing an example of a conventional distance meter using electrodes.

FIG. 8 is an explanatory view showing an example of a distance meter using a conventional pressure sensor.

FIG. 9 is an explanatory diagram showing an example of a conventional rangefinder based on the principle of triangulation using a laser light source and a light receiving element.

FIG. 10 is an explanatory view showing a configuration of a conventional self-mixing type laser distance meter.

[Explanation of symbols]

 Reference Signs List 2 laser element 4 detecting means 6 laser driving circuit 8 lens 10 photodiode (PD) 12 laser diode (LD) 14 amplifier 16 A / D converter 18 frequency analysis unit (FFT analyzer)

Claims (1)

[Claims] 1. A laser device for irradiating a laser beam to an object to be measured, a laser drive circuit for driving the laser device based on a predetermined modulation drive signal, and a laser device for receiving the laser beam reflected by the object to be measured. A laser range finder comprising: a modulation unit that modulates the laser beam according to the modulation drive signal and a detection unit that detects a distance to the measurement target based on a self-mixed sawtooth wave. An A / D converter for converting a laser beam reflected by an object into a digital signal, and a frequency analyzer for frequency-analyzing the digital signal for a predetermined time converted by the A / D converter are provided. Laser rangefinder.