JPH03130617A - Optical displacement detector - Google Patents

Abstract

PURPOSE:To make it possible to detect an optical spot stably by using a reference level which is fluctuated in response to the change in intensity of the optical spot, and evaluating the detected pulses. CONSTITUTION:An optical spot is formed with output luminous flux emitted from a standard optical element (holographic lens grating 4). The optical spot is received and detected through a wide slit and a narrow slit at the same time in the divided mode. Thus a corresponding reference pulse signal and a corresponding detected pulse signal are obtained. The reference and detected pulses are outputted in the mutually overlapped state. The reference pulse has the larger pulse width in comparison with the detected pulse. When the detected pulse is compared with the reference pulse and evaluated, the standard- pulse signal can be stably outputted regardless of the fluctuation in pulse height. The passage of an encoder plate 2 through the standard position can be detected. Since the standard pulse has the narrow pulse in comparison with the reference pulse, the standard position of the encoder plate 2 can be determined with high resolution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical displacement detection device having a configuration in which an encoder plate is arranged between a light source and a light receiving element, and particularly relates to a technique for detecting a reference position of an encoder plate.

[Conventional technology]

Conventionally, various types of optical encoders have been known for detecting displacement of moving parts in a non-contact manner.

For example, Japanese Patent Application Laid-Open No. 63-47616 discloses an optical encoder that utilizes a diffraction phenomenon caused by a spherical wave from a coherent point light source. When a diffraction image from a point light source is used, the diffraction image moves as a one-dimensional diffraction grating formed on an encoder plate moves in the same way as in the case of a silhouette. In addition, the diffraction image in this case can be magnified by changing the ratio of the distances between the light source and the diffraction grating, and between the diffraction grating and the light receiving element, making it possible to easily detect minute movements of the diffraction grating without the need for a magnifying optical system. . Taking advantage of this fact, a high-performance and high-resolution laser encoder can be obtained using a semiconductor laser and a one-dimensional diffraction grating with several pitches.

FIG. 5 is a schematic diagram for explaining the principle of a laser encoder. A coherent incident light beam having a wavelength λ is emitted from a point light source O made of a semiconductor laser. An encoder plate movable in both directions is arranged at a distance in front of the point light source O, as shown by the arrow. A one-dimensional diffraction grating having a pitch T is formed on this encoder plate. When this moving encoder plate is irradiated with a coherent incident beam, an interference notch is imaged at a distance M in front of the one-dimensional diffraction grating. The interference pattern consists of bright and dark striped patterns arranged at a predetermined spatial period P. This interference pattern is apparently an enlarged projected image of the diffraction grating and is displaced in accordance with the movement of the diffraction grating. By the way, in order to obtain a clear interference pattern, it is necessary to satisfy the following relational expression according to the so-called Fresnel diffraction theory.

MLλ H By setting the parameters L, M, λ, and T of the laser encoder so as to satisfy this relational expression, an interference pattern with high clarity can be obtained. The displacement information of the encoder plate can be obtained by receiving the emitted light beam from the one-dimensional diffraction grating through a spatial filter having a spatial period corresponding to the spatial period of the interference pattern, that is, the fringe interval P, and converting it into an AC detection signal. .

That is, the frequency of the AC detection signal represents the displacement speed of the encoder plate, and the number of waves represents the amount of displacement.

As shown in FIG. 5, a reference slit S is formed on the encoder plate corresponding to the reference position. When the reference slit S crosses the incident light flux emitted from the common point light source O as the encoder plate moves, a light spot is formed in front of the reference slit. Information on the reference position of the encoder plate can be obtained by arranging a light receiving element at this light spot position and detecting the light beam emitted from the reference slit. In this way, the laser encoder detects the absolute movement amount of the encoder plate based on both the displacement information of the moving interference pattern and the reference position information of the light spot.

[Problem that the invention seeks to solve]

By the way, the wavelength λ of a coherent light beam emitted by a semiconductor laser used as a light source is temperature dependent and is not necessarily constant. For example, when a commercially available semiconductor laser is used, the wavelength changes by about 0.2 to 0.26 nm per time. As is clear from the above relational expression, when the wavelength λ of the incident light beam irradiated onto the encoder plate changes, a clear interference pattern cannot be obtained and the peak intensity of the interference pattern decreases. Similarly, if the reference position slit is formed, for example, by a holographic lens grating, the peak intensity of the light spot will also be reduced. FIG. 6 is a graph showing the temperature dependence of peak intensity.

The vertical axis shows the peak intensity vs. value, and the horizontal axis shows the temperature difference ΔT from the reference temperature. As shown in the figure, the larger the temperature change, the lower the peak intensity. However, the peak position hardly changes.

Regarding the interference pattern, even if its peak intensity fluctuates,
This does not adversely affect detection accuracy that much. This is because the frequency components of the AC detection signal obtained by receiving and detecting the interference pattern are preserved even if the peak intensity of the interference pattern varies. However, if the peak intensity of the light spot that provides the reference position information of the encoder plate fluctuates, it may have a serious adverse effect on the reference position detection and cause false detection. Conventionally, pulses were obtained by receiving and detecting a light spot. The reference position of the encoder plate was detected by comparing the voltage level of the pulse with a predetermined reference voltage level. When the peak intensity of the light spot decreases, the voltage level of the pulse also decreases, and when the voltage level falls below a certain reference voltage level, there is a problem in that the reference position cannot be detected.

[Means for solving problems]

In view of the problems of the conventional laser encoder described above, it is an object of the present invention to provide an improved laser encoder that does not cause false detection of the reference position even if the wavelength of the incident light beam changes.

For this purpose, the optical displacement detection device according to the present invention includes a fixed light source for generating an incident light beam, an encoder plate that moves across the incident light beam, and a reference position of the encoder plate. a reference optical element for emitting an emitted light beam each time the light beam passes; a mask plate having a pair of wide slits and narrow slits arranged to block the emitted light beam and having different effective aperture widths; , one light receiving element receives the emitted light beam through a wide slit and outputs a reference pulse signal with a wide pulse width, and the other receives the emitted light beam simultaneously through a narrow slit and outputs a detection pulse signal with a narrow pulse width. and a detection circuit for outputting a reference pulse signal and detecting passage of the encoder plate base position by comparing and evaluating the detected pulse signal with the reference pulse signal.

Preferably, the detection circuit includes a comparison circuit that forms a gate pulse signal by comparing a reference pulse signal with a predetermined reference level, and a gate circuit that outputs a reference pulse signal in response to the gate pulse signal. There is.

More preferably, the fixed light source comprises a semiconductor laser that emits a coherent incident beam, and the reference optical element comprises a holographic lens grating for diffracting the coherent incident beam and imaging the output beam into a light spot. In addition, the encoder plate has a one-dimensional diffraction grating formed along its moving direction. do. A spatial filter having a spatial frequency corresponding to the pitch of the interference pattern is arranged. A light receiving element receives the emitted light flux through the spatial filter and outputs a corresponding AC detection signal, and an encoder plate processes the AC detection signal. and a circuit that outputs a displacement signal representing the displacement of.

[For production]

According to the present invention, a light spot formed by an output light beam emitted from a reference optical element is simultaneously and separately received and detected through a wide slit and a narrow slit to obtain a corresponding reference pulse signal and a detection pulse signal. ing. The reference pulse and the detection pulse are output in a state where they are overlapped with each other,
Moreover, the reference pulse has a larger pulse width than the detection pulse. Therefore, the detection pulse exists within the reference pulse width. Although the voltage levels, that is, the pulse heights of the reference pulse and the detection pulse vary in response to changes in the intensity of the light spot, the relative relationship between the two pulse heights is maintained. Therefore, by comparing and evaluating the detection pulse with respect to the reference pulse, the reference pulse signal can be stably output regardless of fluctuations in pulse height, and passage of the encoder plate reference position can be detected. Since the reference pulse has a narrower pulse width than the reference pulse, the encoder plate reference position can be determined with high resolution.

As described above, according to the present invention, the detected pulse is not evaluated based on a detailed reference level, but the detected pulse is evaluated using a reference level that fluctuates according to changes in the intensity of the light spot. is not affected by changes in the intensity of the light spot.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view showing an embodiment in which an optical displacement device according to the present invention is applied to a laser rotary encoder. The laser rotary encoder has a fixed light source 1. The fixed light source 1 is composed of, for example, a semiconductor laser and generates a coherent incident light beam. However, the wavelength varies due to temperature changes and other factors. A rotary encoder plate 2 is arranged in front of the fixed light source 1.

As shown by the arrow, the encoder plate 2 is rotatable in both directions so as to cross the incident light beam. A one-dimensional diffraction grating 3 is formed in a ring shape over the entire inner circumference of the encoder plate 2. The one-dimensional diffraction grating 3 is formed along the moving direction of the encoder plate 2, and diffracts a coherent incident light beam to image an interference pattern at a predetermined position in front. This interference pattern is an enlarged image of the one-dimensional diffraction grating 3 that is displaced in response to the movement of the encoder plate 2. A reference optical element is formed at a reference position on the encoder plate 2. In this embodiment, the reference optical element consists of a holographic lens grating 4 placed adjacent to a one-dimensional diffraction grating 3. The holographic lens grating 4 is composed of a plurality of concentric slits, and each time an incident light beam passes through, the holographic lens grating 4 emits an output light beam and images a light spot at a predetermined position in front. By properly setting the slit pitch of the holographic lens grating 4 1f in advance, the incident light beam can be diffracted and a light spot with an extremely sharp peak can be formed by interference. The image plane of the light spot is on the same plane as the image plane of the interference pattern. In this way, the holographic lens grating 4 utilizes diffraction and interference of the light beam, so like the one-dimensional diffraction grating 3, there is a dependence on the wavelength of the incident light beam, and the peak intensity of the light spot changes as the wavelength changes. fluctuate.

A mask plate 5 is placed on the imaging plane of the interference pattern and the light spot. A plurality of spatial filters 6 having a spatial period corresponding to the pitch of the interference pattern are formed on the mask plate 5. Further, a pair of wide slits 7C and narrow slits 7D having wide and narrow effective aperture widths are formed at the imaging position of the light spot. The wide slit 7C and the narrow slit 7D are arranged adjacent to each other and can transmit the tip spot simultaneously and separately.

A plurality of light receiving elements 8 are arranged close to each other in front of the mask plate 5 so as to face each other. The plurality of light receiving elements 8 include a light receiving element that receives the emitted light flux through the wide slit 7C and outputs a reference pulse signal having a wide pulse width, and a light receiving element that receives the emitted light flux through the narrow slit 7D and outputs a reference pulse signal having a narrow pulse width. It includes a light receiving element that outputs a detection pulse signal having a detection pulse signal. In addition, it also includes a light receiving element that receives the output JJ light flux through the plurality of spatial filters 6 and outputs a corresponding AC detection signal. The plurality of light receiving elements 8 are built into a detection circuit 9, and the detection circuit 9 outputs a reference pulse signal by comparing and evaluating the detection pulse signal with a reference pulse signal, and processes the AC detection signal to generate an encoder. A displacement signal representing the displacement of the plate 2 is also output.

FIG. 2 is a schematic diagram showing the correspondence relationship between the mask plate 5 and the light receiving element 8 shown in FIG. 1. As shown in the figure, the light spot representing the reference position of the encoder plate has an extremely sharp peak intensity due to the focusing action of the holographic lens grating.

The shape of the light spot has a narrow width in the moving direction of the encoder plate, but has an extended flat shape in the orthogonal direction, and has a wide slit 7C and a narrow slit 7D.
It is possible to cover the longitudinal direction of. Narrow slit 7D
has an extremely narrow slit width to detect the reference position of the encoder plate with high resolution, and corresponds precisely to the imaging position of the light spot. Further, the wide slit 7C has a wide slit width because it is intended to obtain a reference signal. A light receiving element 8C is arranged behind the wide slit 7C, and another light receiving element 8D is arranged behind the narrow slit 7D. Since the pair of light receiving elements 8C and 8D are arranged corresponding to the wide slit 7C and the narrow slit 7D, respectively, it is possible to receive and detect the light spot simultaneously and almost equally divided. The light receiving element 8C outputs a reference pulse signal having a wide pulse width in order to receive the emitted light beam through the wide slit 7C, and the light receiving element 8D receives the emitted light beam through the wide slit 7C.
Since the emitted light beam is received through the sensor, a detection pulse signal having a narrow pulse width is output. These reference pulse signals and detection pulse signals are output simultaneously, and the reference pulse covers the detection pulse.

- The interference pattern formed by the dimensional diffraction grating has continuous peaks and moves in both directions as shown by the arrows.

A pair of spatial filters 6 along the moving direction of the interference pattern
A and 6 people are assigned. The spatial period of these spatial filters corresponds to the pitch of the interference pattern, and the filter 6A and the filter 6 have a phase difference of 180' from each other.

Also, a pair of spatial filters 6B and 6B are arranged in parallel with the spatial filter 6A and the six people. These spatial filters also have a spatial period corresponding to the pitch of the interference pattern, and the spatial filter 6B and the spatial filter 6B
have a phase difference of 180@ from each other. Furthermore, a pair of spatial filters 6A.

6 people and another pair of spatial filters 6B, 6m are each 9
It has a phase difference of 0°. The reason for providing a phase difference of 90° is to detect the moving direction of the interference pattern.

These four spatial filters 6A, 6X, 6B and 6B
4 light receiving elements 8A, 8A corresponding to.

8B and 8B are respectively arranged.

FIG. 3 is a block diagram showing the detailed circuit configuration of the detection circuit 9. As shown in FIG. As shown in the figure, an amplifier A1 is connected to the light receiving element 8B, an amplifier A2 is connected to the light receiving element 8B, an amplifier A3 is connected to the light receiving element 8A, and an amplifier 4A is connected to the light receiving element 8A. . Further, an amplifier A5 is connected to the light receiving element 8D, and an amplifier A5 whose amplification factor can be adjusted by a variable resistor VRI is connected to the light receiving element 8C.
6 is connected. A comparator C1 is connected to the output ends of the amplifiers A1 and A2, and a comparator C2 is connected to the output ends of the amplifiers A3 and A4. A differential amplifier A7 whose input level can be adjusted by a variable resistor VR2 is connected to the output terminals of the amplifiers A4 and A5.
Similarly, a differential amplifier A8 whose input level can be adjusted by a variable resistor VR3 is connected to the output terminals of A4 and A6. Further, a comparator C3 is connected to the output terminals of the amplifiers A7 and A8, and a comparator W is connected to the output terminal of the amplifier A8.
C4 is also connected. Comparator C4 is at reference voltage level V
A comparison operation is performed based on RP and F. Finally, comparator C
A gate circuit G consisting of an AND gate is connected to the output terminals of C.3 and C4.

FIG. 4 is a waveform diagram showing various signal waveforms appearing in the circuit shown in FIG. 3. The operation of the laser encoder according to the present invention will be explained with reference to FIG. As shown in the figure, the light-receiving cable 8A outputs an A-phase AC detection signal, and the eight light-receiving elements output inverse phase in-phase AC detection signals having a phase difference of 180'. This phase difference corresponds to the spatial phase difference to the spatial filters 6A and 6. Similarly, the light receiving element 8B outputs an n-phase AC detection signal, and the light receiving element 8B outputs an n-phase AC detection signal with an opposite phase. Furthermore, the light receiving element 8C outputs a reference pulse signal. The reference pulse signal includes a wide reference pulse as well as a noise component caused by an interference pattern. The light receiving element 8D outputs a detection pulse signal which has a narrow width and also contains a noise component. The peak positions of the reference pulse and the detection pulse coincide with each other, and as a result of receiving and detecting the light spot in equal parts, the pulse heights of the reference pulse and the detection pulse are substantially equal to each other. However, the width of the reference pulse is wider than the width of the detection pulse.

The A-phase AC detection signal and the human-phase AC detection signal are each amplified by amplifiers A3 and A4, and then compared with each other by a comparator C2 to output an A-phase displacement signal. The A-phase displacement signal consists of a rectangular pulse train, and the amount of movement of the encoder plate can be determined by the number of pulses. Similarly, the n-phase AC detection signal and the n-phase AC detection signal are each amplified by amplifiers A1 and A2, and then compared with each other by comparator C1 to output a B-phase color change signal. The B-phase color change signal similarly consists of a rectangular pulse train. However, corresponding to the 90° spatial phase difference between one pair of spatial filters 6A, 6λ and the other pair of spatial filters 6B, 6B, the B phase discoloration signal has a 90° phase difference with respect to the human phase displacement signal. has. This phase difference is advanced or delayed depending on the direction of movement of the encoder plate, and the direction of movement of the encoder plate can be determined from the relative phase relationship between the human phase displacement signal and the B-phase discoloration signal.

The detection pulse signal is amplified by an amplifier A5, and then noise components are removed by a differential amplifier A7 and output as a waveform-shaped detection pulse.

The reference pulse signal is amplified by a variable amplifier A6, and then a differential amplifier A8 removes noise components and outputs a waveform-shaped reference pulse. At this time, variable amplifier A6
The amplification factor of is set smaller than that of amplifier A5, and as a result, the shaped reference pulse that is the output of amplifier A8 has a relatively lower voltage level than the shaped detection pulse that is the output of amplifier WA7. . As a result, the shaped reference pulse can be used as a reference voltage level for directly comparing and evaluating the shaped detection pulse. That is, comparator C3
can output a reference pulse signal by directly inputting the shaped detection pulse and the shaped reference pulse and comparing the two.

However, as shown in the figure, the output waveform of the comparator C3 does not necessarily include only the reference pulse signal, and there is also a possibility that an indefinite noise component is output. That is, when the encoder plate is not rotating or when the reference position of the encoder plate does not pass the incident light beam, the amplifier A7
and the output of A8 is in an unstable state and therefore comparator C3
The operation of the system also becomes unstable. As a result, the comparator C3 may output a noise pulse due to disturbance or the like. In order to eliminate such unstable factors in the operation of the comparator C3, in this embodiment, the output signal of the amplifier A8 is compared with a predetermined reference voltage level VREP by the comparator C4 to generate a gate pulse signal. I have to. As shown in the figure, this gate pulse signal is composed of relatively wide rectangular pulses. By opening the gate circuit G in response to this gate pulse signal, only the reference pulse signal is selectively output and becomes the final Z-phase signal.

Finally, although not shown, the A-phase displacement signal, the B-phase discoloration signal, and the Z-phase signal are subjected to arithmetic processing using a computer or the like to obtain information such as the feed body rotation amount and rotation direction of the encoder plate.

Although the embodiments described above relate to laser rotary encoders, the present invention is not limited thereto and can also be applied to laser linear encoders.

Furthermore, in this embodiment, a semiconductor laser that emits a coherent light beam is used as a light source, and diffraction of a one-dimensional diffraction grating is utilized. is also applicable. Furthermore, the wide slit can be configured not only by a single slit as shown in this embodiment, but also by a set of a plurality of narrow slits arranged in parallel. Although a single holographic lens grating was used as the reference optical element in this example, a pair of closely spaced holographic lens gratings was used to separately image the light spot onto the wide and narrow slits. Also good. or,
An optical element that causes birefringence may be disposed between a single holographic lens grating and a mask plate, and the light spot may be divided and focused, and the light may be detected by the corresponding light receiving elements 8C and 8D. Furthermore, a cylindrical lens or a parallel flat plate tilted with respect to the optical axis is placed between the holographic lens grating and the mask plate to flatten the shape of the light spot by applying aberration to the light spot in a direction perpendicular to the moving direction of the encoder plate. You can also do it. Finally, although a holographic lens grating is used as the reference optical element in this embodiment, the reference optical element is not limited to this, and may be configured with a single slit, for example.

〔Effect of the invention〕

As described above, according to the present invention, even if the intensity of the light spot representing the reference position of the encoder plate changes due to fluctuations in the wavelength of the incident light flux, it is possible to stably detect the destination spot. As a result, there is an effect that the reference position of the encoder plate can always be detected without malfunction.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the configuration of a laser rotary encoder, Figure 2 is a schematic diagram showing the arrangement of the mask plate and light receiving element used in the laser rotary encoder, and Figure 3 is a detailed diagram of the detection circuit of the laser rotary encoder. A circuit block diagram, FIG. 4 is a waveform diagram to explain the operation of the detection circuit of the laser rotary encoder, and FIG. 5 is a schematic diagram to explain the operating principle of the optical displacement detection device using point light source diffraction. and FIG. 6 is a graph showing the interference pattern shown in FIG. 5 and the dependence of the peak intensity of the light spot on the wavelength of the incident light beam. 1...Fixed light source 2...Encoder plate 3
... One-dimensional diffraction grating 4 ... Holographic lens grating

Claims (1)

[Claims] 1. A fixed light source for generating an incident light beam, an encoder plate that moves across the incident light beam, and a fixed light source formed at a reference position of the encoder plate each time the incident light beam passes through the reference position. A reference optical element for emitting the emitted light beam, a mask plate in which a pair of wide slits and narrow slits are formed, which are arranged so as to block the emitted light beams and have different effective aperture widths; One light receiving element receives the emitted light flux and outputs a reference pulse signal having a wide pulse width, and the other light receiving element simultaneously receives the emitted light flux through a narrow slit and outputs a detection pulse signal having a narrow pulse width. , an optical detection device comprising a detection circuit for outputting a reference pulse signal by comparing and evaluating a detection pulse signal with a reference pulse signal and detecting passage of an encoder plate reference position. 2. The detection circuit includes a comparison circuit that forms a gate pulse signal by comparing a reference pulse signal with a predetermined reference level, and a gate circuit that outputs a reference pulse signal in response to the gate pulse signal. The optical detection device according to claim 1. 3. The optical detection device according to claim 1, wherein the wide slit comprises a set of a plurality of narrow slits arranged in parallel. 4. The fixed light source comprises a light source that emits a coherent incident beam, and the reference optical element comprises a holographic lens grating for diffracting the coherent incident beam and imaging the output beam into a light spot. The optical detection device described. 5. The encoder plate has a one-dimensional diffraction grating formed along its moving direction, and the one-dimensional diffraction grating diffracts the coherent incident light beam and images the output light beam into a moving interference pattern; a spatial filter having a spatial period corresponding to the pitch of the interference pattern; a light receiving element that receives the emitted light flux through the spatial filter and outputs a corresponding AC detection signal; 5. The optical detection device according to claim 4, further comprising a circuit for outputting a displacement signal representing displacement. 6. The optical detection device according to claim 1, wherein the reference optical element comprises a slit for transmitting the incident light beam.