JPH0395402A - Laser vision sensor - Google Patents

Abstract

PURPOSE:To enable measurement of a distance at a high speed and with high precision by enabling modulation of a laser light by a plurality of modulation frequencies, by conducting primary measurement of a long distance by a low modulation frequency and by conducting highly-precise distance measurement by a high modulation frequency. CONSTITUTION:A laser light emitted from a laser oscillator 1 is modulated in intensity by a light modulator 2. In order to conduct modulation by a plurality of frequencies, two light-modulating elements 202 and 203 are provided. A distance range for measure ment is divided in a plurality and a long distance is measured primarily at a low modulation frequency, while the range of a relatively short distance is measured with high precision at a high modulation frequency. By combining these two measurements, it is made possible to measure the long distance with high precision. By raising the ratio of the modulation frequencies to (n)th power of 2, in addition, a value of the distance measured by the low modulation frequency is recorded in upper bits, while a value of the distance measured by the high modulation frequency is recorded in lower bits, and they are read out as continuous data, whereby distance measurement of a wide dynamic range is conducted. As the result, high-speed and highly-precise measurement of distances can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a laser visual sensor for obtaining three-dimensional information using laser light.

BACKGROUND OF THE INVENTION Measuring the absolute distance on the object's surface using a laser has recently attracted attention as a source of congratulatory information for remote control of robots. Such laser distance measuring devices are broadly divided into those that use trigonometry and those that measure the round trip time of light, and the latter are further divided into those that use pulsed lasers and those that modulate the intensity of continuous wave lasers. A continuous wave laser intensity modulation method is suitable for measuring relatively short distances such as robot vision, and this method has been described by, for example, David Nitza.
n) et al. Proceedings of the I.E.I., Vol. 65, p. 206, 1977 (Pr.
oc. IEEEVo 1. ．． 65 P 206,
(1977). Hereinafter, a conventional intensity modulation type laser visual sensor will be explained with reference to FIG.

As shown in Figure 5, the output light from laser oscillator 1 is transferred to oscillator 3.
modulation frequency fY by optical modulator 2 driven by
The intensity is modulated by 1'L. The intensity-modulated laser beam passes through a hole in a perforated mirror 4 and is irradiated onto an object 6 by a scanner 5. Scattered light from the object 6 passes through the scanner 5, is reflected by the perforated mirror 4, and is focused by the condensing lens 7 onto the photodetector 8. The photodetector 8 performs photoelectric conversion and obtains a detection signal i.

The resulting detection signal i has a frequency f. is an AC signal of
The phase is object 6! f. The delay is proportional to the distance. Therefore, by measuring the phase difference φ between the reference signal r from the oscillator 3 and the detection signal i using the phase detector 9, the distance to the object 61 can be measured. The distance L is determined by the following equation, where C is the speed of light.

L=c·φ/4πfyyt A distance image represented by the distance L is obtained by scanning a two-dimensional plane with a laser beam using the scanner 5 and measuring the distance.

The distance measurement accuracy and resolution of the obtained distance L are determined by the detection accuracy and resolution of the phase difference φ. Therefore, the modulation frequency fYW may be increased in order to improve ranging accuracy and resolution. In that case, the distance L at which the phase difference φ becomes 2π becomes shorter, making it unsuitable for long-distance measurement. As an example, the modulation frequency fffi is 5MH
Table 1 shows the distance accuracy at which the phase difference φ becomes 2π when the detection accuracy of the phase difference φ is 1 degree between z and 100 MHz.

Table 1 Modulation frequency and ranging accuracy Modulation frequency f. If you set the modulation frequency fm to 5 MHz, you can measure the distance unambiguously and unambiguously, but the distance accuracy is only 83 degrees, and if the modulation frequency fm is 100 MHz, the distance accuracy is 4.2 degrees, but it is 1.5 degrees. The distance can only be determined uniquely by m i.

Therefore, in laser distance measuring devices that do not scan laser light, methods are used to measure long distances with high precision by combining multiple modulation frequencies (for example, Koichi Matsumoto, ``Amplitude modulated light "Usage Measurement", O Plus E,
No. 98, P1'l9 (1988)).

6.＼-7 Problems to be solved by the invention However, conventional laser vision sensors measure distance without scanning the laser beam and capture the distance as an image, so they cannot be used at high speeds. It is necessary to perform signal processing, and it is not possible to measure using multiple modulation frequencies, as is the case with laser distance measuring devices that do not scan laser beams, and distance measurement with high viscosity has not been performed.

An object of the present invention is to perform distance measurement at high speed and with high accuracy.

Means for Solving the Problems In order to achieve the above object, the technical means applicable to the present invention are as follows. In the inventions according to claims 1 and 2, the laser beam can be modulated at a plurality of modulation frequencies, and unique distance measurement over a long distance is performed at a low modulation frequency, and highly accurate distance measurement is performed at a high modulation frequency. This is what we do. In the invention described in claims 3 and 4, the ratio of the modulation frequencies of the laser beams is multiplied by 2 to the nth power, and the value measured at the lower modulation frequency is placed in the upper range, and the higher modulation frequency 7.
・The value measured in units of 1/2 is recorded in the lower bits to perform distance measurement with a wide dynamic range. In the invention according to claim 5, the scanning method of the laser beam is one sub-scanning step for each plurality of main scans, distance measurement is performed using a plurality of modulation frequencies for each scanning line, and high-speed and high-precision distance measurement is performed. It is something to do. According to the sixth aspect of the invention, frequency beatdown is performed by mixing the detector signal and a mixed signal of a plurality of modulated frequency signals.

[Claim] According to the invention described in the lawsuit and 2, the distance range for distance measurement is divided into a plurality of parts by modulating the laser beam at a plurality of frequencies. It uniquely measures a relatively short distance range at a high modulation frequency with high precision. By combining these two, it becomes possible to measure long distances with high accuracy.

According to the inventions described in claims 3 and 4, by increasing the ratio of modulation frequencies to the nth power of 2, low The distance measurement value based on the modulation frequency is recorded in the upper bits, the distance measurement value based on the higher modulation frequency is recorded in the lower bits, and distance measurement with a wide range of accuracy is performed by reading out the data as continuous data.

For example, if distance measurement is performed using 8 bits for each of the upper and lower bits and read out as continuous data, it is equivalent to performing distance measurement using 16 bits.

According to the invention described in claim 5, the scanning of the laser beam is performed by moving one sub-scan step for each plurality of main scans, and distance measurement is performed using a plurality of modulation frequencies for each scanning line. That is, distance measurement is performed for each line from coarse distance measurement to highly accurate distance measurement using a plurality of modulation frequencies, and an image is formed by sub-scanning. This realizes high-speed and highly accurate distance measurement.

In the invention described in claim 6, the frequency beatdown is performed by mixing the detection signal from the detector and the mixed signal from the oscillator. As a result, the detection signal becomes an alternating current signal with the same frequency as the modulation frequency, and signal processing circuits such as phase comparison for distance measurement are required for the road system depending on each frequency. By beating down the frequencies to make them the same frequency, processing can be performed with one signal processing circuit system for a single frequency band.

EXAMPLE A first example of the present invention will be described below with reference to FIG. The intensity of the laser light emitted from the laser oscillator (2) is modulated by the optical modulator (2). In this embodiment, two light modulation elements 202 and 203 are provided to perform modulation at a plurality of frequencies. Here, an EOM (electro-optic modulator) was used as the optical modulator. The linearly polarized laser beam is passed through the λ/4 plate 2.
The light is circularly polarized at 01, the polarization angle is rotated by an electric field at light modulators 202 and 203, and linearly polarized light is selected by an analyzer 204 to perform intensity modulation. Since the value of the EOM tuning circuit differs depending on the modulation frequency, the light modulation element 20
2 and 203 are equipped with dedicated tuning circuits 24 and 25. There are multiple modulation signals, for example two of 5MHz and 80MHz.
A modulation signal is used, which is output from two oscillators 21 and 22, and is amplified by 9 by an RF amplifier 23.
7 Enters the key circuits 24 and 25.

The modulation frequency is selected by the mode selector 10. According to the signal from the mode selector 10, Suinochi 101, 1
02 is switched.

The intensity-modulated laser beam passes through a hole in a perforated mirror 4 and is irradiated onto an object 6 by a scanner 5. The scattered light reflected from the object 6 passes through the scanner 5 and is reflected by the perforated mirror 4.
The light is focused on a photodetector 8 by a focusing lens 7 and converted into an electrical signal.

The laser beam is scanned by the scanner 5 over a two-dimensional plane 53'' to form an image, and the scanner 5 is controlled by a scanner driver 50. As for the scanning method, in the case of high-speed mode, scanning is performed in the main scanning direction 51 as shown in FIG. When the scanning of the thousand planes 53 is completed, the sub-scanning is reversed and the formation of the next screen continues. In this case, distance measurement is performed using only a low modulation frequency.

The scanner driver 50 is controlled by the mode selector 10, and when the high precision mode is selected, the scanner driver 50 is controlled by the mode selector 10.
- The scanning method is changed as shown in FIG. 2(b).

In this embodiment, since the modulation frequencies are 5 MHz and 80 MHz, the sub-scan moves by one step every two main scans. The switch 101 is set so that the modulation frequency is 5 MHz for odd-numbered main scans and 80 MHz for even-numbered main scans.
, 102 are switched, and respective distance measurements are performed. When using two or more types of modulation frequencies and setting them more precisely, the number of repetitions of main scanning is increased by the number of modulation frequencies.

After the detection signal i from the photodetector 8 is amplified by the broadband amplifier 81, the 5MH signal is demultiplexed from the oscillator 21 by the phase detector 9.
The phase difference φ between z and the reference signal r is measured, and the distance is calculated.

In the case of high-precision distance measurement, the reference signals separated from the oscillators 21 and 22 are mixed in a DBM (double balanced mixer) 27, and the difference frequency between the two modulation frequencies is extracted by beatdown. In this embodiment, it is 75 MHz. A 5 MHz signal can be obtained by mixing this difference frequency signal with an 80 MHz detection signal in the DBM28. switch 103 controlled by mode selector 10;
When modulated at 5MHz by 104, it is through.
When the signal is modulated at 80 MHz, if the signal is passed through the DBM 28, the phase detector 9 can process the signal using only one of the 5 MHz frequency bands.

The output of the phase detector 9 is converted into a digital signal by an AD converter 91. Here, the modulation frequency is set to 5MH by the switch 105 controlled by the mode selector 10.
When the frequency is 80 MHz, the data is switched to the upper bits, and when the frequency is 80 MHz, the lower bits are switched and recorded in the memory 11. When reading data from the memory 11, it is treated as continuous bits. In this embodiment, an 8-bit AD converter 91 was used for input, and a 12-bit ODA converter 92 was used for output.

In the high-speed mode, only the upper 8 bits of the memory 11 are used and O is stored in the lower 4 bits. Therefore, distance measurement is performed over 30 m in 256 steps with a resolution of 117 im.

In high precision mode, 5MHz is applied to the upper 4 bits of memory 11.
distance measurement data, lower 8 bits are 80 MHz1
3. Distance data at \-7 is recorded. How many bits to allocate to memory 11 is determined by the ratio of the two modulation frequencies. In the case of this embodiment, 80/5=16, which is 4 bits. Therefore, at 5 MHz, distance measurement is performed in 16 steps, and at 80 MHz, distance measurement is performed in 256 steps. Since the readout is performed in 12 bits, the distance of 30m is measured with a resolution of 5.2 mm in 4096 steps.

The speed of image formation is determined by the speed of the scanner 5, but in this embodiment, a 10-sided polygon scanner is used.
Main scanning was performed by rotating at 000 rpm.

A galvanometer mirror scanner is used for sub-scanning, and in high-speed mode it scans 10 screens per second. Mode selector 1
If the high-precision mode is selected with 0, the speed is 5 screens per second because it moves one line in the sub-scan with two main scans. Here, the memory 1 performs distance measurement using multiple modulation frequencies for each line.
This is because it is easy to transfer data to 1 and because an object 6 that has some movement can be captured as an image. In this example, the forming time for one line is 17500 even in high precision mode.
14 in seconds, and the pixel shift when forming one line can be almost ignored.

A second embodiment of the present invention will be explained in FIG. This embodiment uses one EOM 205 as an optical modulator, and is equipped with a tuning circuit 26 with a switching tap. The other parts are the same as those shown in FIG. 1, and illustration and description thereof will be omitted. The signals from the oscillators 21 and 22 amplified by the RF amplifier 23 are inputted to the taps which are controlled by the mode selector 10 and are controlled by the switch 102 and 106 to provide a suitable tuning condition of l+Q. In this example, EO
Since the number of M is one, it is advantageous for downsizing the senna.

A third embodiment of the present invention will be explained in FIG. In this embodiment, a semiconductor laser is used as the laser oscillator 1 to perform internal modulation.

The modulated signals output from the oscillators 21 and 22 are amplified by an amplifier 30 and input to the laser oscillator 1 through a tuning circuit 29. Selection of the modulation frequency is performed by a switch 101 controlled by a mode selector 10. The tuning circuit 29 is a high-pass filter, and transmits frequencies 15.\-7 of 5 MHz or higher without loss. A DC voltage is applied as a bias to the laser oscillator 1, and when a modulation signal is applied, oscillation is started, and laser light whose intensity is modulated by the modulation signal is extracted. The other parts are the same as the configuration shown in FIG.

According to this embodiment, since the external optical modulator 2 is not required,
It becomes a very small light source. Since scanning and detection of laser light and signal processing are the same as those in the embodiment shown in FIG. 1, their explanations will be omitted here.

Effects of the Invention As described above, according to the present invention, firstly, the laser beam is modulated at multiple frequencies and distance measurement is performed by dividing a long distance into multiple distance ranges. becomes possible.

Second, by increasing the modulation frequency ratio to the nth power of 2, data conversion becomes possible using an AD converter with a small number of bits. When writing data in the high precision mode, data with a low modulation frequency is recorded in the upper bits, data with a high modulation frequency is recorded in the lower bits, and reading is treated as continuous data, allowing measurement over a wide dynamic range. In the high speed mode, only the upper bits may be used and the lower bits may be set to O. In the laser beam scanning in the high precision mode, the laser beam moves by one step in the sub-scan for every plural main scans, and ranges from rough distance measurement to high-precision distance measurement and distance measurement using a plurality of modulation frequencies for each line. The measurement time for one line at this time is 1/500 seconds assuming that two modulation frequencies are used.
It is now possible to perform high-precision distance measurement at high speed.

Third, frequency beatdown is performed by mixing the detection signal from the detector and the reference signal from the oscillator, and the frequency of the beatdown signal is made one for multiple modulation frequencies, so it is possible to measure The signal processing circuit system for distance measurement only needs to use a single frequency band, resulting in simpler circuits, lower costs, and smaller sensors. Moreover, since the mode selector automatically switches the modulation frequency and the signal processing circuit automatically performs frequency beatdown, high-speed and highly accurate distance measurement is possible.

[Brief explanation of drawings]

l7・\-/ FIG. 1 is a block diagram showing the overall configuration of a laser visual sensor in the first embodiment of the present invention, FIG. 2 is a timing diagram showing a laser beam scanning method in the configuration of FIG. 1, and FIG. 3
The figure is a block diagram showing the structure of the light modulation system of a laser visual sensor according to the second embodiment of the present invention, and FIG. 4 is a block diagram of the laser light modulation circuit system according to the third embodiment of the present invention. FIG. 5 is a block diagram showing the structure of a conventional laser peach sensor.

Claims (6)

[Claims] (1) A laser oscillator, a means for modulating the laser beam from the laser oscillator at a plurality of modulation frequencies, a means for irradiating the modulated laser beam onto an object to be measured, and detecting the reflected laser beam from the object to be measured. means for detecting a phase difference between the output of the photodetector and an electrical signal of one modulation frequency among the plurality of modulation frequencies, and means for selecting one of the plurality of modulation frequencies. A laser visual sensor characterized by comprising: (2) The means for selecting one of a plurality of modulation frequencies is the mode selection means for high-speed mode and high-precision mode, and in high-speed mode, the laser beam is modulated at a low modulation frequency to uniquely measure distances over long distances. 2. The laser visual sensor according to claim 1, wherein the laser beam is modulated at a high modulation frequency in the high-precision mode to perform high-precision distance measurement. (3) The laser visual sensor according to claim 1, wherein the ratio of the plurality of modulation frequencies is 2 to the nth power. (4) The laser visual sensor according to claim 2, wherein the distance measurement data in the high-speed mode is recorded in the upper bits, and the distance measurement data in the high-precision mode is recorded in the lower bits. (5) In distance measurement in high-precision mode, the laser beam scans by one sub-scan step for each main scan, and one main scan is used for distance measurement using a low modulation frequency, and the other times are used for distance measurement using a high modulation frequency. The laser visual sensor according to claim 1, wherein the distance is a distance. (6) The laser vision sensor according to claim 1, wherein the beatdown is performed by mixing the detection signal from the detector with a mixed signal of a plurality of modulated frequency signals.