JPH07117426B2 - Optical encoder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical encoder, and more particularly, to rotating a radiation grating in which a plurality of grating patterns of a light transmitting portion and a reflecting portion are periodically engraved on a circumference. Attached to an object, irradiating the radiation grating with a light beam from a laser, for example, and using the diffracted light from the radiation grating, a rotary encoder for photoelectrically detecting rotation information such as the rotation angle of the radiation grating or the rotating object. It can be applied well.

(Prior Art) Conventionally, computer equipment such as drive of floppy disk, office equipment such as printer, or NC machine tool,
Furthermore, a photoelectric rotary encoder has been used as a means for detecting the rotational speed of a rotating mechanism such as a VTR capstain motor or a rotating drum and the amount of fluctuation of the rotational speed.

The method using a photoelectric rotary encoder is a so-called main scale in which a light-transmitting part and a light-shielding part are provided at equal intervals around a disk connected to a rotary shaft, and correspondingly, a light-transmitting part is provided at the same interval as the main scale. The structure employs a so-called index scale system in which both scales of a so-called fixed index scale provided with a light-shielding portion and a light-shielding portion are opposed to each other by sandwiching them between the light projecting means and the light receiving means. With this method, a signal synchronized with the distance between the light-transmitting portion and the light-shielding portion of both scales is obtained with the rotation of the main scale, and the frequency analysis of this signal is performed to detect the fluctuation of the rotation speed of the rotating shaft. Therefore, the finer the scale interval between the light-transmitting portion and the light-shielding portion of both scales, the higher the detection accuracy can be. However, if the scale interval is made fine, there is a drawback that the S / N ratio of the output signal from the light receiving means is lowered due to the influence of the diffracted light and the detection accuracy is lowered. Therefore, if the total number of gratings of the light-transmitting part and the light-shielding part of the main scale is fixed and the distance between the light-transmitting part and the light-shielding part is expanded to the extent that it is not affected by the diffracted light, the diameter of the disc of the main scale will change. There is a drawback in that the load is increased, the thickness is increased, the size of the entire device is increased, and as a result, the load on the rotated object is increased.

This is not limited to the rotary encoder, but there is a problem that the load on the subject is increased not only by the optical encoder.

(Object of the Present Invention) An object of the present invention is to provide an optical encoder that reduces the load on the subject.

The present invention is particularly applicable to an optical encoder for detecting displacement by interference between two-time diffracted light, as far as possible for an optical system for two-time diffraction, which requires a high degree of adjustment for stabilizing accuracy or a special optical element. It is an object to provide an optical encoder capable of giving a degree of freedom.

(Main Features of the Present Invention) An optical encoder of the present invention includes a scale in which a diffraction grating is formed along a displacement detection direction, a light source that generates a coherent linearly polarized light beam, and a straight line of the light beam from the light source. The azimuth is 45 ° with respect to the polarization direction, and the luminous flux from the light source is 2
A polarization beam splitter for splitting the two light beams split produced by the polarization beam splitter, is incident to the two positions M _1, M ₂ on the diffraction grating of the scale, occurring at the position M ₁ The + m-th order diffracted light is made to substantially go backward in the optical path and is made incident again to the position M ₁ on the diffraction grating of the scale to generate + m-th order diffracted light twice, and −m generated at the position M _2. An optical system for causing the next diffracted light to travel substantially backward in the optical path and to enter the diffracted light at approximately the position M ₂ of the diffraction grating of the scale again to generate diffracted light of the −m th order twice.
First and second quarter-wave plates provided in the optical system corresponding to each of the two light beams and arranged so that a light beam traveling through the optical system passes twice in a reciprocating manner; A third quarter-wave plate provided at a position through which the diffracted light obtained by combining the ± m-order two-time diffracted lights generated by the optical system with each other by the polarization beam splitter, and the third 1/4
It is characterized in that it has a light receiving means arranged so that a plurality of different polarization azimuth components are selected from the diffracted light that has passed through the wave plate and are detected by a plurality of light receiving elements.

In particular, the plurality of light receiving elements are respectively arranged at positions where the diffracted light that has passed through the third quarter-wave plate is branched and passed through the plurality of polarizing plates whose polarization directions are shifted from each other. Is characterized by.

(Example) FIG. 1 is a schematic view of an example of the present invention.

In the figure, 1 is a coherent light source such as a laser, 2
Is a collimator lens, 3 is a polarization beam splitter, and its polarization axis is
It is arranged at 45 °. 4 ₁ , 4 ₂ and 4 ₃ are 1/4 each
The wavelength plates are arranged so that their respective polarization axes are 45 ° with respect to the linearly polarized light of the reflected and transmitted light beams of the polarization beam splitter 3. That is, the 1/4 wavelength plate 4 ₁ is arranged so that its polarization axis is 45 ° with respect to the linear polarization direction of the reflected light beam of the polarization beam splitter 3, and the 1/4 wavelength plate 4 ₂ is It is arranged so that its polarization axis is 45 ° with respect to the linear polarization direction of the transmitted light flux. The quarter-wave plate 4 ₃ is arranged so that its polarization axis with respect to either the polarization direction of the transmitted or reflected light beam of the polarization beam splitter 3 is 45 °. Reference numeral 5 is a reflecting mirror. 6 ₁ , 6 ₂ , 6 ₁ ′ and 6 ₂ ′ are cylindrical lenses, and 7 and 7 ′ are reflecting mirrors. Reference numeral 8 denotes a radiation grating forming a scale in which, for example, a grid pattern of a light transmitting portion and a reflecting portion is provided at equal angles on a disc, and 9 is a rotation axis of a rotating object to be inspected (not shown). 10 is a beam splitter, 11 and 11 'are polarizing plates,
It is arranged so that the polarization directions are 45 ° to each other. 1
2, 12 'are light receiving elements.

The light beam emitted from the laser 1 is made into a substantially parallel light beam by the collimator lens 2 and is incident on the polarization beam splitter 3. The polarization beam splitter 3 is arranged so that its azimuth is 45 ° with respect to the linear polarization azimuth of the laser 1, and splits the light flux from the laser 1 into a reflected light flux and a transmitted light flux having substantially equal light amounts. After passing through the divided two light beams each quarter-wave plate 4 ₁ and 4 _2, a circularly polarized light.
Among them, the transmitted light beam via a cylindrical lens _61, is linear irradiance on position M ₁ on the radiation grid 8. on the other hand,
The reflected light flux is linearly irradiated onto the position M ₂ on the radiation grating 8 via the reflecting mirror 5 and the cylindrical lens 6 ₁ ′. Here, the cylindrical lenses 6 ₁ , 6 ₁ ′ are arranged as necessary so as to linearly irradiate the light flux in a direction orthogonal to the radiation direction of the radiation grating 8. By performing the linear irradiation in this way, it is possible to reduce the pitch error of the lattice pattern of the light transmitting portion and the reflecting portion, which corresponds to the irradiation portion of the light flux on the radiation grating 8. Further, the irradiation positions M ₁ and M ₂ on the radiation grating 8 are set so as to have a substantially point-symmetrical positional relationship with respect to the rotation center of the rotating object to be inspected (not shown).

The light beam incident on the radiation grating 8 is reflected and diffracted by the grating pattern of the radiation grating 8. Now, if the pitch of the lattice pattern at the irradiation position of the luminous flux is p, the reflected diffracted light L of the m-th order,
The diffraction angle θ _m of L ′ is represented by the following formula (I).

sin θ _m = mλ / p (1) where λ is the wavelength of the light beam.

Now, assuming that the radiation grating 8 is rotating at an angular velocity ω, the distance from the rotation center of the radiation grating to the irradiation positions M ₁ and M ₂ is r.
Then, the peripheral velocity at the irradiation positions M ₁ and M ₂ is v = rω.

Here, the incident light fluxes to M ₁ and M ₂ are represented by i in the wave number vector representation, the reflected diffracted lights L and L ′ are represented as s and ′ s in the wave number vector representation, and the radiation at the positions M ₁ and M ₂ The peripheral velocity of the grating 8 is represented by a vector display and the relationship is shown in FIG. Then, the frequencies of the reflected diffracted lights L and L ′ undergo so-called Doppler shift by the amounts Δf and Δf ′ expressed by the following equations.

Here, · represents the dot product of the vector.

Then, the positions M ₁ and M ₂ are re-irradiated by the reflecting mirrors 7 and 7 ′ through the cylindrical lenses 6 ₂ and 6 ₂ ′. Then position M ₁ , M
Reflected diffracted light beam of the m-th order diffracted this time again ₂ is back to the original optical path, it undergoes a Doppler shift amount expressed again during re-diffraction (2). Therefore, the light beam re-diffracted at the position M ₁ and returning to the original optical path undergoes the Doppler shift of frequency 2Δf, and the light beam re-diffracted at the position M ₂ and returning to the original optical path undergoes the Doppler shift of frequency −2Δf.

Here, the light beam that is re-diffracted at the position M ₁ and returns to the original optical path is 1 /
4 When again transmitted through the wavelength plate 4 _2, its orientation as the incoming 90
Polarized beam splitter 3
Is reflected by. The light beam is again diffracted back to its original light path at a location M ₂ also, when again passing through the 1/4-wave plate 4 _1, becomes linearly polarized light whose direction is from the incoming rotated 90 degrees, transmitted through the polarization beam splitter 3 To do. The light flux re-diffracted by the position M _1, M ₂ may overlap, 1/4 through the wavelength plate 4 _3, the light receiving element 12 is transmitted through the divided by the beam splitter 10 into two beams polarizing plates 11, 11 ', It is incident on 12 '.

In this way, the two light beams subjected to the Doppler shift of the frequency 2Δf and the frequency −2Δf overlap each other, so that the frequency of the output signals of the light receiving elements 12 and 12 ′ is 2Δf − (− 2Δf
f) = 4Δf. That is, the frequency F of the output signals of the light receiving elements 12 and 12 'is F = 4Δf = 4rωsin θ _m / λ, and F = 4mrω / p from the equation of the diffraction condition of the equation (1). Assuming that the total number of the lattice patterns of the radiation grating 8 is N and the equiangular pitch is Δφ, p = rΔφ and Δφ = 2π / N, and thus F = 2mNω / π (3). Now, if the wave number of the output signal of the light receiving element during the time Δt is n and the rotation angle of the radiation grating 8 between the times Δt is θ, then n = FΔT, θ = ωΔt, then n = 2Nθ / π ... (4) The rotation angle θ of the radiation grating 8 can be obtained by the equation (4) by counting the wave number n of the output signal waveform of the light receiving element.

By the way, when detecting the rotation angle, it is more preferable if the rotation direction can be detected. Therefore, in this embodiment, as is known in conventional photoelectric rotary encoders, etc., a plurality of light receiving elements are prepared, and the signals are arranged so that their phases are shifted by 90 degrees. A method of extracting a signal indicating the rotation direction from the phase difference signal is used.

In this embodiment, a 90 ° phase shift is provided between the output signals obtained by the two light receiving elements 12 and 12 ', which is used as a polarization beam splitter, a 1/4 wavelength plate 43 and polarizing plates 11 and 1.
It is created by combining 1 '.

FIG. 6 is a schematic view showing the optical action of each element after the polarization beam splitter 3 in FIG.

That is, as shown in FIG. 6, the positions M ₁ , M on the radiation grating 8 are
_The light beams LM ₁ and LM _{2 that} are re-diffracted by ₂ and return to the original optical path are reflected and transmitted by the polarization beam splitter 3 to overlap with each other, and pass through the 1/4 wavelength plate 4 ₃ to transmit clockwise circularly polarized light TM ₁ And the left-handed circularly polarized light TM ₂ .

When the two circularly polarized light TM ₁ and TM ₂ are overlapped with each other, the polarized light components are partially canceled to result in a linearly polarized light TL having a polarization plane in a predetermined direction.

The polarization direction of the linearly polarized light TL at this time changes as the radiation grating 8 rotates. And this linearly polarized light TL is a half mirror
The light beam is divided into _two light beams TL ₁ and TL ₂ at 10 and guided to the polarizing plates 11 and 11 ′, respectively.

Then, the two divided light beams TL ₁ and TL ₂ are passed through the polarization plates 11 and 11 ′, and only the component in the constant vibration direction is received by the light receiving elements 12 and 12 ′. At this time, the light receiving element 12,1
For the first time from 2 ', the bright and dark signals S ₁ , S ₂ accompanying the rotation of the radiation grating 8
Is obtained.

In this embodiment, the orientations of the two polarizing plates 11 and 11 'are shifted from each other by 45 °, so that the light receiving elements 12 and 12' are separated from each other by 9 °.
Bright and dark signals S ₁ and S ₂ with a 0 ° phase shift are obtained.

Then, as shown in FIG. 1, for example, the output signals of the light-receiving elements 12 and 12 'are waveform-shaped, and after detecting the rotation direction, they are integrated in the counter to obtain the rotation angle.

FIG. 3 shows the radiation grating 8 of FIG. 1 and the radiation grating 8 of two light beams.
FIG. 3 is an explanatory diagram of upper irradiation positions M ₁ and M ₂ and a rotation center of a rotating object to be inspected.

In this embodiment, two points M ₁ and M ₂ that are substantially point-symmetric with respect to the center of the radiation grating 8 and the center of rotation of the rotating object to be measured are used as measurement points, so that the center of the radiation grating 8 and the rotating body to be measured are measured. The effect of eccentricity with the rotation center of is reduced. That is, the radiation grating 8
It is mechanically difficult to completely match the center of with the center of rotation, and eccentricity of both is inevitable. For example, as shown in FIG. 3, when the amount of eccentricity is a between the center O of the radiation grating 8 and the rotation center O ′, at the measurement point M ₁ located at the distance r from the rotation center. The Doppler frequency shift of changes from r / (r + a) to r / (r−a) compared to when there is no eccentricity. On the other hand, at this time position M ₁
, And the frequency shift at the measurement point M ₂ that is point-symmetric with respect to the center of rotation changes from r / r−a to r / r + a, which is the reverse of the change at position M ₁ , The effect of eccentricity is reduced by simultaneously measuring two points, ₁ and M ₂ .

Further, in the present embodiment, for example, in the index scale type photoelectric rotary encoder which has been conventionally used, the wave number n of the output signal from the light receiving element and the total number N of the main scales corresponding to the above formula (4) are used. And the rotation angle θ is n = Nθ / 2π (5), the rotation angle Δθ per wave number is Δθ = 2π / N (radian) (6) is there. On the other hand, in the present embodiment, from the equation (4), Δθ = π / 2mN (radian) (7)

Therefore, according to the present embodiment, it is possible to detect the rotation angle with accuracy of 4 m times as compared with the conventional example even if the scale with the same division number is used.

Further, in the conventional photoelectric rotary encoder, the distance between the light transmitting portion and the light shielding portion is limited to about 10 μm in consideration of the influence of light diffraction.

Now, in order to obtain the rotation angle detection accuracy of, for example, 30 seconds, in the conventional example, only N = 360 × 60 × 60/30 = 43,200 is required as the number of divisions of the main scale from the above equation (6). . Therefore, if the distance between the light-transmitting portion and the light-shielding portion at the outermost periphery of the main scale is 10 μm, the diameter of the main scale needs to be 0.01 mm × 43,200 / π = 137.5 mm.

However, according to the present embodiment, in order to obtain the same rotation angle detection accuracy as in the conventional example, the number of divisions of the radiation grating may be 1/4 m. When m = 1 using the ± 1st order diffracted light, the number of divisions of the radiation grating 8 for obtaining the rotation angle detection accuracy of 30 seconds may be 43,200 / 4 = 10,800. In the present embodiment, if the diffracted light of the laser is used, the distance between the light transmitting portion and the reflecting portion may be narrow. Therefore, for example, if this is 4 μm, the diameter of the radiation grating is 0.004 mm × 10,800 / π = 13.75 mm. It will be good. That is, according to the present embodiment, as a shape for obtaining the rotation angle detection accuracy equivalent to that of the conventional index scale type photoelectric rotary encoder,
The following size will suffice. Therefore, the load on the rotating object to be inspected is much smaller than that of the conventional example,
Accurate measurement is possible.

FIG. 4 is a schematic view of a part of another embodiment of the present invention, and shows the vicinity of the part where the luminous flux is incident on the radiation grating 8 of FIG. In the figure, the numbers given to the respective elements indicate the same elements as those shown in FIG. The transmission / diffracted light of the ± mth order of the luminous flux incident on the positions M ₁ and M ₂ of the radiation grating 8 is transmitted through the cylindrical lenses 6 ₂ , 6 ₂ ′ and the reflecting mirrors 7, 7 ′ to the radiation grating 8
The same effect as that of the embodiment shown in FIG. 1 is obtained by re-entering the beam and returning it to the original optical path.

FIG. 5 is a schematic view similar to FIG. 4 of still another embodiment of the present invention.

In FIG. 5, 13 and 13 'are corner cube reflectors, respectively. The ± mth-order reflected (or transmitted as shown in FIG. 4) diffracted light of the light beam incident on the positions M ₁ and M ₂ of the radiation grating 8 is passed through the cylindrical lenses 6 ₂ and 6 ₂ ′ to the corners. Return to the original optical path of the cube reflector, and first
The same effect as the embodiment shown in the figure is obtained. Radiant grating 8
When the center of the eccentricity and the center of rotation of the rotating object to be inspected are eccentric, the distance between the light transmitting portion and the reflecting portion of the radiation grating 8 changes depending on the location, so the diffraction angles of the diffracted lights L and L'change. It will be. Further, in the embodiment of FIG. 1, the light source 1
For this reason, semiconductor lasers are the most desirable because of their small size, low cost, and high output.

However, it is known that the oscillation wavelength of the semiconductor laser changes depending on the ambient temperature change. First
In the illustrated embodiment, when the wavelength of the light source 1 changes, the diffraction angles of the diffracted lights L and L'change. At this time, the diffracted light L
If a plane mirror such as 7 and 7'in FIG. 1 is used as the reflecting mirrors for L and L ', the reflecting mirror becomes a so-called tilted state, and a large number of interference fringes are generated on the front surface of the light receiving elements 12, 12'. The S / N ratio of the output signals of the light receiving elements 12 and 12 'may decrease. Therefore, as shown in FIG. 5, by using the corner cube reflecting mirrors 13 and 13 'as the reflecting mirrors, even if the diffraction angles of the diffracted lights L and L'diffracted by the radiation grating 8 change for the above reason, the corner cube The light beam reflected by the reflecting mirrors 13 and 13 'can be returned to the original optical path. By this, the light receiving element
A large number of interference fringes are prevented from being generated on the front surfaces of 12, 12 ', and the S / N ratio of the output signals of the light receiving elements 12, 12' is improved.

In each of the above-described embodiments, two m-th order diffracted lights are used, but ± m-th order diffracted lights or two diffracted lights of different orders may be used. Further, in this embodiment, the deviation error between the center of the radiation grating and the center of rotation of the rotating object to be inspected can be neglected, and if it is used as a tachometer, the irradiation point on the radiation grating should be one and one light receiving element should be provided. Good.

(Effects of the Present Invention) According to the present invention, it is possible to achieve a compact and high-precision optical encoder with a small load on the subject.

In particular, according to the present invention, the light beam from the light source is split into two by a polarization beam splitter having an azimuth angle of 45 ° with respect to the linear polarization direction of the light beam from the light source. By making it diffract twice and changing the linear polarization azimuth at the time of separation by passing it through a quarter-wave plate twice in this optical system and combining by the above-mentioned polarization beam splitter, the diffraction of twice occurs. In order to separate and combine the light fluxes, there is a simple structure that uses few members, and theoretically, there is no light quantity loss except for diffraction, and the separation and combination is performed.

In addition, the two light fluxes thus combined by the polarization beam splitter are further converted to circularly polarized light of opposite directions by passing through a 1/4 wavelength plate, and the linearly polarized light component obtained by the combined state of the circularly polarized light beams. Rotates with the displacement of the diffraction grating, and the light receiving element obtains a plurality of signals having different phase difference in the polarization direction component at this time only by the difference between the half mirror and the polarizing plate.

That is, according to the present invention, when the polarization azimuth component is selected from the combined diffracted light of linearly polarized light that rotates with the displacement of the diffraction grating in order to obtain a plurality of signals having different phases with a simpler configuration, the polarized beam By such arrangement of the splitter and the quarter-wave plate, the polarization state of each light beam is appropriately converted while it is traveling, and finally, the linearly polarized light 2 which rotates with the displacement of the diffraction grating with no loss of light quantity with a small number of members 2 By obtaining the diffracted light, an optical encoder that can obtain the rotation information of the rotating object under test with high accuracy is achieved.

[Brief description of drawings]

FIG. 1 is a schematic view of an embodiment of the present invention, FIG. 2 is an explanatory view of vector display of incident light flux and reflected diffracted light on the radiation grating according to the invention, and FIG. 3 is a center of the radiation grating of the invention. 4 and 5 are partial explanatory views of another embodiment of the present invention, and FIG. 6 is a partial explanatory view of FIG. 1. In the figure, 1 is a light source, 2 is a collimator lens, 3 is a polarization beam splitter, 4 ₁ , 4 ₂ and 4 ₃ are quarter-wave plates, 5, 7 and 7'are reflecting mirrors, and 6 ₁ , 6 ₂ and 6 ₂ 'cylindrical lens, the radiation grid 8, the rotation axis of the test rotation object 9, 10
Is a beam splitter, 11 and 11 'are polarizing plates, 12 and 12' are light receiving elements, and 13 and 13 'are corner cube reflecting mirrors.

Claims (2)

[Claims] 1. A scale on which a diffraction grating is formed along a displacement detection direction, a light source for generating a coherent linearly polarized light beam, and an azimuth with respect to the linearly polarized direction of the light beam from the light source.
A polarization beam splitter for dividing the light beam from the light source, which forms 45 °, into two and two light beams split and generated by the polarization beam splitter are respectively divided into two positions M ₁ and M on the diffraction grating of the scale. ₂ and the position M ₁
The + m-th order diffracted light generated in step (3) is made to travel substantially backward in the optical path and is made incident on the position M ₁ on the diffraction grating of the scale again.
The m-th order diffracted light is generated at the same time, and the -m-th order diffracted light generated at the position M ₂ is made to substantially go backward in the optical path and is again incident on the position M ₂ of the diffraction grating of the scale. An optical system for generating the next two-time diffracted light, and a first optical system provided in the optical system corresponding to each of the two light fluxes and arranged so that the light flux traveling through the optical system passes twice in a reciprocating manner. The first and second quarter-wave plates and the ± m-order two-times diffracted light generated by the optical system are combined by the polarization beam splitter, and the diffracted light is provided at a position where the diffracted light passes. 3 1/4 wavelength plate, and a plurality of light receiving elements arranged to select a plurality of different polarization azimuth components from the diffracted light that has passed through the third 1/4 wavelength plate and detect them with a plurality of light receiving elements. An optical encoder having means. 2. The plurality of light receiving elements are respectively arranged at positions where the diffracted light passing through the third quarter-wave plate is branched and passed through a plurality of polarizing plates whose polarization directions are shifted from each other. The optical encoder according to claim 1, wherein: