JPH0389115A - Optical displacement detecting apparatus - Google Patents

Abstract

PURPOSE:To make it possible to detect the rotating direction of a rotary encoder accurately with a simple structure by inserting a correcting member between a point light source and a disk member. CONSTITUTION:Coherent light emitted from a point light source 1 is projected on the inner band and the outer band of a diffraction grating 3. An interference pattern is formed on a fixed pattern 4. When a correcting member 7 is inserted in the inner light path part between the light source 1 and the grating 3 at this time, the distance of the inner light path is corrected so as to offset the change in the pitch of the inner band of the grating 3. Thus the inner interference pattern having visibility is obtained. The clear interference pattern which is formed by using the member 7 is detected with a photodetector 5. The rotating direction of a rotary encoder is detected based on the relative position relationship between the two-phase detected signals A and B outputted from the photodetector 5. The rotating speed and the rotating amount of the rotary encoder are detected based on the frequencies of the detected signals and the number of waves.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical displacement detection device, and particularly to a laser rotary encoder using point source diffraction.

[Conventional technology]

In recent years, with the development of automation technology, rotary encoders have been attracting attention as rotation angle sensors due to the need for high-speed, high-precision positioning of industrial robots or numerically controlled machine tools, and smooth speed control from high to low speeds. .

However, conventionally known magnetic or optical rotary encoders cannot adequately meet the new needs mentioned above because the higher the resolution, the more susceptible they are to vibration and shock, the larger the size, and the higher the cost. is the current situation.

In view of this point, an optical rotary encoder has been proposed that utilizes the diffraction phenomenon caused by spherical waves from a coherent point light source. A diffraction image produced by a point light source has properties that are significantly different from those produced by a parallel light source. For one thing, when an object (e.g., a -dimensional grating) moves, its diffraction image moves, as in the case of a shadow puppet. In addition, the diffraction pattern in this case can be enlarged by changing the ratio of the distances between the light source and the diffraction grating, and between the diffraction grating and the photodetector, making it possible to detect minute movements of the grating very easily without the need for a magnifying optical system. can. Taking advantage of this fact, a high-performance and high-resolution optical rotary encoder can be obtained using a semiconductor laser and a several-pitch diffraction grating. This type of rotary encoder has a simple structure, and because the distance between the optical sensor section and the encoder plate can be maintained, it is resistant to shock and vibration.

FIG. 8 is a schematic diagram for explaining the principle of a laser encoder using point light source diffraction. Coherent light with a wavelength λ is emitted from the point light source O along the optical axis. A bidirectionally movable one-dimensional diffraction grating is arranged at a distance in front of the point light source O, as shown by the arrow. This diffraction grating is composed of a plurality of slits having a pitch T. When this moving diffraction grating is irradiated with coherent light, an interference pattern is projected at a distance M in front of the diffraction grating. The diffraction pattern consists of bright and dark striped patterns arranged at a predetermined spatial period P. This interference pattern is an enlarged projected image of the apparent diffraction grating and moves in accordance with the movement of the diffraction pattern.

By the way, in order to obtain a clear diffraction pattern, it is necessary to satisfy the following relational expression (1) according to the so-called Fresnel diffraction theory.

MLλ H If the parameters L, M, λ, and T of the rotary encoder are set to satisfy this relational expression (L), an interference pattern with high clarity can be obtained, and the spatial period or fringe interval P of the interference pattern at that time is It is expressed by the following relational expression (2>).

G As shown in relational expression (2>), the period P of the interference pattern is .

[Problem that the invention seeks to solve]

When constructing a rotary encoder based on the principle shown in FIG. 8, a rotating disk is used as the encoder plate.

Then, a radial diffraction grating consisting of a plurality of slits arranged radially at a predetermined pitch along the circumferential surface of the rotating disk is formed. However, since the plurality of slits are arranged radially, the pitch changes along the radial direction and is not constant. That is, if the radial diffraction grating is divided into two parts along the radial direction into an inner band and an outer band, the pitch of the inner band is smaller than the pitch of the outer band. Since an error occurs in the pitch T of the diffraction grating as described above, there is a problem that a clear diffraction pattern cannot be formed on the same surface over the entire radial direction of the radial diffraction grating.

In particular, in the case of a rotary encoder, in order to detect the direction of rotation, it is necessary to receive and detect each diffracted light that has passed through the inner and outer bands of the radial diffraction grating with a phase difference of 90''.At this time, the above-mentioned diffraction grating Since it is not possible to obtain a partially clear interference pattern due to pitch errors, there is a problem in that it becomes difficult to detect the direction of rotation.

[Means for solving problems]

In view of the above-mentioned problems, it is an object of the present invention to correct the radial pitch error of a radial diffraction grating and to realize detection of the rotational direction of an encoder plate with a simple structure.

To this end, the optical displacement detection device according to the present invention includes a point light source that emits coherent light along a predetermined optical path, and a disc member that is bidirectionally rotatable across the optical path. A plurality of slits arranged radially at a predetermined pitch are formed along the circumferential surface of the disc member, forming a radial diffraction grating.

A fixing member is arranged along the moving direction of the interference pattern formed by the diffracted light that has passed through the diffraction grating. On the fixed member, there is an inner spatial filter for transmitting the inner diffracted light that has passed through the radially inner band of the radial diffraction grating, and an inner spatial filter for transmitting the outer diffracted light that has passed through the radially outer band of the radial diffraction grating. An outer spatial filter is formed. The outer space filter is disposed radially outward than the inner space filter, and there is a spatial phase difference of 90'' between the rain space filters, and they are offset from each other in the circumferential direction. A photodetector is arranged to receive the diffracted light through the inner and outer spatial filters, and outputs two-phase detection signals according to the phase difference between the two filters.The phase of one detection signal is different from that of the other. The rotation direction of the rotary encoder is detected depending on whether the detected signal is advanced or delayed compared to the phase of the detection signal.

A correction member is inserted between the point light source and the disk member in order to form a clear interference pattern. This correction member has a function of optically correcting the difference in pitch between the inner and outer bands of the radial diffraction grating.

This correction element preferably consists of an optically transparent plate inserted only in the radially inner part of the optical path of the coherent light.

[For production]

In order to obtain a clear interference pattern, it is necessary to satisfy the above-mentioned relational expression (1). When this relational expression (1) is applied to a rotary encoder, the wavelength λ of coherent light and the distance M between the disk member fixing members are constant. A variable factor is the pitch T of the radial diffraction grating, and the pitch of the inner band is smaller than the pitch of the outer band. In order to compensate for this variation in pitch T, a correction member is used to partially correct the optical distance between the point light source disk members. That is, by adjusting the optical distance so as to offset the variation in the pitch T of the diffraction grating, the relational expression (L) is always satisfied over the entire radial direction of the radial diffraction grating.

〔Example〕

FIG. 1 is a perspective view showing an embodiment of a laser rotary encoder using point light source diffraction according to the present invention. The rotary encoder is equipped with a point light source 1 that emits coherent light having a wavelength λ-830n. Point light source 1
is composed of, for example, a semiconductor laser. A disc member 2 is arranged at a distance M L −6 m+* in front of the point light source 1 . The disk member 2 can be rotated in both directions so as to cross the coherent light. A radial diffraction grating 3 consisting of a plurality of slits arranged radially at a predetermined pitch is formed along the circumferential surface of the disc member 2. As is clear from the figure, the pitch of the radial diffraction grating 3 changes in the radial direction. The pitch of the diffraction grating 3 located at a radius of 29 mm from the center of the disc member 2 is T −2h! m is set. In the radial diffraction grating 3, the annular portion existing in the vicinity of the radius 29 degrees will be referred to as the inner band. Also radius 30
At the position of mm, the radial grating 3 has a pitch T-3
0 μs, and this portion will be referred to as the outer band hereinafter.

A fixing member 4 is arranged at a distance M -58, 25 mm in front of the disc member 2. Coherent light emitted from the point light source 1 is diffracted by the diffraction grating 3, and forms an interference pattern on the surface of the fixed member 4 with a spatial period determined by the relational expression (2>).

The fixed member 4 includes an inner spatial filter for selectively transmitting the diffracted light that has passed through the inner band of the diffraction grating 3, and the diffraction grating 3.
It has an outer spatial filter for selectively transmitting the diffracted light that has passed through the outer band of the filter. At a position near the front of the fixed member 4,
A photodetector 5 is arranged. The photodetector 5 receives the diffracted light that has passed through the inner and outer spatial filters, and outputs a detection signal according to the change in intensity.

A correction member 7 is inserted in the optical path between the point light source 1 and the disc member 2. For example, the correction member 7 has a plate thickness d = 1.28
+nm and a transparent parallel plane plate with a refractive index of n -1.5. In the case of this embodiment, the parallel plane plate is inserted only in the optical path portion of the coherent light passing through the inner band of the diffraction grating 3 (hereinafter referred to as the inner portion). Further, a mask 8 is inserted between the point light source 1 and the correction member 7.

The emission angle of the semiconductor laser constituting the point light source 1 varies considerably among individuals, and the interference pattern is influenced by the emission angle. Therefore, the mask 8 is used to define the exit angle.

FIG. 2 is a plan view showing the arrangement relationship between the fixing member 4 and the photodetector 5 in the laser encoder shown in FIG. An inner spatial filter 4A and an outer spatial filter 4B are formed on the fixed member 4 along the moving direction of the interference pattern. Although each spatial filter is composed of a plurality of slits in this embodiment, it may be composed of one slit.

Each spatial filter has a spatial period corresponding to the period of the interference pattern, and selectively filters the diffracted light. However, the inner spatial filter 4A and the outer spatial filter 4B
are arranged with a phase difference of 90@. Therefore, for example, considering the case where the interference pattern is moving in the direction shown by the arrow, the inner diffracted light that has passed through the inner band of the radial diffraction grating is filtered by the inner spatial filter 4A, and the outer light that has passed through the outer band of the radial diffraction grating is filtered by the inner spatial filter 4A. The diffracted light is filtered by the outer spatial filter 4B.

At this time, the inner spatial filter 4A and the outer spatial filter 4B
Since there is a phase difference of 90'' between them, the transmission timing of the outer diffracted light is delayed by a phase difference of 90° compared to the transmission timing of the inner diffracted light.On the contrary, when the interference pattern moves in the opposite direction, , the transmission timing of the inner diffracted light is delayed compared to the transmission timing of the outer diffracted light.The fixed member 4 further has an additional slit or a spatial filter 4z.The additional spatial filter 4Z is located at the rotation direction reference position of the disc member 2. This is for selectively transmitting diffracted light representing .That is, a predetermined diffracted light signal is transmitted every time the disk member 2 rotates once.

The light-receiving surface of the photodetector 5 placed behind the fixing member 4 is divided into three parts. The light receiving surface 5A is the inner spatial filter 4
A, and outputs a detection signal A in response to a change in the intensity of the inner diffracted light. The light-receiving surface 5B is disposed corresponding to the outer spatial filter 4B, and outputs a detection signal B in accordance with a change in the intensity of the outer diffracted light. Furthermore, the light receiving surface 5
Z is arranged corresponding to the additional spatial filter 4Z, and outputs the detection signal 2 every rotation of the disk member 2.

Next, the operation of the rotary encoder will be explained with reference to FIGS. 3 to 6. When the coherent light emitted from the point light source 1 is irradiated onto the inner and outer bands of the diffraction grating 3, an interference pattern is formed on the fixed member 4. Figure 3 shows
FIG. 3 is a diagram showing an interference pattern formed by outer diffracted light that has passed through an outer band. The vertical axis indicates the light intensity change corresponding to the striped pattern of the interference pattern as a relative value, and the horizontal axis indicates the relative position of the bright and dark striped pattern. As shown in the figure, for the outer diffracted light, each parameter is set in advance so as to satisfy the relational expression (1) (Lsm6+a+s).

M-56,25+n, λ-830nm, T-3
0m+), an extremely clear interference pattern can be obtained.

FIG. 4 shows the case where the correction member 7 is not used.
FIG. 3 is a diagram showing an interference pattern formed by inner diffracted light. In this case, the pitch of the inner bands of the diffraction grating is 30-
Since it changes from 29- to 29-, relational expression (1) is not satisfied. Therefore, it is not possible to obtain a clear interference pattern,
Noise components appear between each peak.

FIG. 5 is a diagram showing an interference pattern obtained when the correction member 7 is inserted between the point light source 1 and the diffraction grating 3. For example, if a parallel plane plate having a thickness d-1,28++u*% and a refractive index n-1,5 is inserted into the inner optical path as the correction member 7, the inner side will be adjusted so as to cancel out the pitch change of the inner band of the diffraction grating. The distance of the optical path is corrected so that the relational expression (1>) is satisfied.Therefore, as shown in the figure, an inner interference pattern having the same sharpness as that in FIG. 3 can be obtained.

When a parallel plane plate having a thickness d and a refractive index n is inserted between the point light source 1 and the diffraction grating 3, the relational expression (1) becomes the following relational expression
It is converted as shown in 3゛.

By setting the plate thickness d or refractive index n so as to offset the change in the pitch T of the diffraction grating, it is possible to maintain the left side of relational expression (3) at a constant value and obtain a clear interference pattern.

FIG. 6 is a diagram showing the waveforms of detection signals A and B output from the photodetector 5 after forming a clear interference pattern using the correction member 7 as described above.

The vertical axis shows the amplitude change of the detection signal, and the horizontal axis shows the rotation angle of the rotary encoder. As shown in the figure, when the rotary encoder is rotating clockwise, the detection signal B is in a phase-lag state with respect to the detection signal A, and conversely, when the rotary encoder is rotating counterclockwise, the detection signal B is in a phase-lag state with respect to the detection signal A. Detection signal B is in a phase-advanced state compared to detection signal A. In this way, the rotational direction of the rotary encoder is detected based on the relative phase relationship between the two-phase detection signals A and B. Also, the frequency of the detection signal corresponds to the rotation speed of the rotary encoder,
The wave number of the detection signal corresponds to the amount of rotation of the rotary encoder.

FIG. 7 is a perspective view showing another embodiment of the rotary encoder according to the present invention. The basic structure is the same as that of the embodiment shown in FIG.
, a fixing member 4, a photodetector 5, a correction member 7, and a mask 8. In the case of this embodiment, 1, correction member 7
A wedge plate is used as a Unlike the parallel plane plate used as the correction member of the rotary encoder shown in FIG. 1, the wedge plate has a tapered shape in which the thickness of the wedge plate changes continuously. When this wedge plate is inserted into the inner part of the optical path of coherent light, the inner diffracted light is deflected and separated from the outer diffracted light. Therefore, the inner interference pattern formed by the inner band of the diffraction grating and the outer interference pattern formed by the outer band are clearly separated along the radial direction of the disc member, making it easier to detect the rotation direction of the rotary encoder. It will be held in

Further, a cylindrical lens 6 is inserted into the optical path between the disc member 2 and the fixed member 4. The cylindrical axis of the cylindrical lens 6 is aligned with the circumferential direction of the disc member 2, and the inner and outer diffracted lights can be separately focused on the photodetector 5 without disturbing the period of the interference pattern.

〔Effect of the invention〕

As described above, according to the present invention, in a laser rotary encoder using point light source diffraction, in order to correct the pitch error of the inner and outer bands of the radial diffraction grating, Since the correction member 7 is inserted in the holder, an interference pattern having optimum clarity can be formed, and the rotational direction of the rotary encoder can be detected accurately with a simple structure.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an example of a laser rotary encoder using point source diffraction, Fig. 2 is a plan view showing the structure and arrangement of a fixing member and a photodetector used in the rotary encoder, and Fig. 3 Figures 5 to 5 are interference pattern diagrams to explain the operation of the rotary encoder, Figure 6 is a waveform diagram showing the two-phase detection signal of the rotary encoder, and Figure 7 is another diagram of the laser rotary encoder using point source diffraction. A perspective view showing an embodiment and FIG. 8 are schematic cross-sectional views for explaining the principle of a laser encoder using point light source diffraction. 1... Point light source 2... Disc member 3...
・Diffraction grating 4...Fixing member 5...Photodetector 6...Cylindrical lens 7...Correction member 8...Mask applicant Co., Ltd.
Kobal Fig. 2 Fig. 3 Light intensity (relative value) Light intensity (relative value) 1 Fig. 4

Claims (1)

[Claims] 1. A point light source that emits coherent light along a predetermined optical path; a disc member that is bidirectionally rotatable across the optical path; and a circumferential surface of the disc member. a radial diffraction grating made up of a plurality of slits arranged radially at a predetermined pitch along the diffraction grating; a fixing member disposed along the moving direction of an interference pattern formed by diffracted light that has passed through the diffraction grating; an inner spatial filter formed on the member for transmitting the inner diffracted light that has passed through the radial inner band of the radial diffraction grating; The outer spatial filter is a filter for transmitting the outer diffracted light that has passed through the outer band and has a spatial phase difference of 90° with respect to the inner spatial filter, and the inner and outer spatial filters receive the diffracted light. , a photodetector that outputs two-phase detection signals according to the phase difference of the filter, and a photodetector that is inserted between the point light source and the disk member, and the inner and outer bands of the diffraction grating to form a clear interference pattern. and a correction member that optically corrects a difference in pitch between the two. 2. The displacement detection device according to claim 1, wherein the correction member is an optically transparent plate inserted only in a radially inner portion of the optical path of the coherent light. 3. The displacement detection device according to claim 2, wherein the transparent plate is a parallel plane plate having a predetermined thickness and refractive index. 4. The displacement detection device according to claim 2, wherein the transparent plate is a wedge plate for deflecting the inner diffracted light relative to the outer diffracted light. 5. It has a cylindrical lens that is inserted into the optical path between the disc member and the fixed member, and the cylindrical axis of the cylindrical lens is aligned with the circumferential direction of the disc member, so that the inner and outer diffracted light is transmitted without disturbing the period of the interference pattern. 5. The displacement detection device according to claim 4, wherein the displacement detection device collects the two beams separately. 6. The displacement detection device according to claim 1, wherein the fixed member is formed with an additional spatial filter for selectively transmitting the diffracted light representing the rotational direction reference position of the disc member.