JPS62201313A - Rotary encoder - Google Patents

Abstract

PURPOSE:To obtain a device which satisfies a required measurement accuracy all the time by demanding the parallelism of a body to satisfy a specific expression in an encoder which detects interference fringes to determine the rotation state of the body. CONSTITUTION:Luminous flux emitted by a laser 1 is made incident on positions M1 and M2 of a radiation grating 7 provided with a diffraction grating on a disk 6 coupled with the rotating body to be measured. Then, diffracted light beams of specific degree among transmitted and diffracted light beams are superposed on each other to form interference fringes, which are detected by light receiving means 15 and 17 to detect the rotation state of the body. At this time, when the parallelism (a) of the body is equal to or larger than tan<-1>((pi(m-n)lambda/360n0P)XthetaK) where (m) and (n) are the degrees of the diffracted light beams, n0 the refractive index of the body, P the pitch of the diffraction grating, lambda the wavelength of coherent luminous flux, and thetaK the permissible value of an angle error, the rotation state of the rotating body is measured within a desired angle error.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field> The present invention relates to a rotary encoder, and in particular to a rotary encoder, in which a radially arranged diffraction grating is attached to a rotating object, the diffraction grating is irradiated with a beam of light from, for example, a laser, and the diffraction from the diffraction grating is detected. The present invention relates to a rotary encoder that uses light to photoelectrically detect the rotational state of a diffraction grating or a rotating object, such as the rotational speed and the amount of variation in rotational speed.

<Prior art> Conventionally, the rotation speed and the amount of variation in rotation speed of rotating mechanisms such as computer equipment such as floppy desk drives, office equipment such as printers, NC machine tools, and VTR cabbage stainless steel motors and rotary drums has been measured. Photoelectric rotary encoders have been used as a means of detection.

For example, as shown in FIG. 5, a photoelectric rotary encoder has a so-called main scale 31 in which transparent parts and light-shielding parts are provided at equal intervals around a disk 35 connected to a rotating shaft 30, and a corresponding main scale. A so-called fixed index scale 32 in which a light-transmitting part and a light-blocking part are provided at intervals equal to one scale.
A so-called index scale system configuration is adopted in which both scales are placed facing each other and sandwiched between the light projecting means 33 and the light receiving bristles stage 34. In this method, as the main scale rotates, a signal synchronized with the interval between the light-transmitting part and the light-blocking part of both scales is obtained, and this signal is frequency-analyzed to detect fluctuations in the rotational speed of the rotating shaft. For this reason, the finer the scale interval between the light-transmitting part and the light-blocking part of both scales, the more
Detection accuracy can be improved. However, when the scale interval is narrowed, the S/N ratio of the output signal from the light receiving means decreases due to the influence of the diffracted light, resulting in a decrease in detection accuracy. For this reason, if you fix the total number of gratings in the light-transmitting part and light-blocking part of the main scale, and try to increase the distance between the light-transmitting part and the light-blocking part to the extent that it is not affected by diffracted light, the diameter of the main scale disc will increase. This increases the size and thickness of the neck, making the entire neckpiece larger, which has the disadvantage of increasing the load on the rotating object to be tested.

- Conventionally, in linear encoders, a coherent light beam is made incident on a diffraction grating attached to a moving object, and interference fringes obtained by superimposing the diffracted lights emitted from the diffraction grating are detected by a light receiving means. However, a method has been proposed in which the amount of movement of a moving object is detected by photoelectrically converting the brightness and darkness of interference fringes on the light-receiving surface that occur when the moving object moves to obtain an electrical signal (pulse).

Therefore, if this interference fringe detection method used in a linear encoder is applied to a rotary encoder, it is considered that all of the above-mentioned drawbacks of the conventional rotary encoder can be eliminated. However, when this method is used in a rotary encoder, a radiation grating is formed on a rotating object such as a disk as one diffraction grating, and measurement is performed by making a coherent beam incident on this radiation grating. Since the center of the rotation object does not exactly match the center of rotation of the rotating object, the 1111 timing is often affected by eccentricity errors.

Therefore, in order to reduce the influence of eccentricity errors, it is effective to adopt a method in which the diffracted lights emitted from multiple positions of the radiation grating interfere with each other, but the f-compatibility (parallelism) of a rotating object such as a disk Bad and mutual.

A difference in optical path length occurs between the diffracted lights that should be caused to interfere with each other.
There was a possibility that a 1+11 constant error would occur.

<Summary of the Invention> In view of the above-mentioned problems, an object of the present invention is to focus on the parallelism of a rotating object in a rotary encoder, and to develop a rotating object with a parallelism that satisfies a desired tolerance at the A11l constant time. The objective is to provide a rotary encoder with

In order to achieve this goal, the rotary encoder according to the present invention makes coherent light beams enter different positions of a diffraction grating provided on a rotating object, and mutually separates different diffracted lights emitted from the nuclear diffraction grating. In a rotary encoder that detects the rotational state of the object by combining the interference fringes to form 1'-# fringes and detecting the interference fringes with a light receiving means,
+iii The parallelism a of the object is set to satisfy the following formula.

a≦tarrl(” ” 0< )60noP Here, m and n are the orders of the diffracted light, nQ is the refractive index of the object, P is the pitch of the diffraction grating, λ is the spread of the coherent light beam, and θ
K indicates the allowable value of angular error.

Further features of the present invention will be understood from the examples shown below.

<Embodiment> FIG. 1 is a schematic diagram of an optical system of an embodiment of a rotary encoder according to the present invention.

In this embodiment, the light beam emitted from the laser 1 is made into a parallel light beam by the collimator lens 2 and is made incident on the polarizing beam splitter 3.
The luminous flux of one linearly polarized light has 13 minutes. Among these, the reflected light flux passes through the storm wavelength plate 4 and the reflecting mirror 5, and becomes circularly polarized light.
The light is incident on the radiation grating 7, which is provided with a radial diffraction grating on the disk 6 connected to the rotating object to be measured. Then, among the transmitted diffracted light that is incident on the radiation grating 7 and diffracted, a specific order, for example, the +m-order diffracted light, is reflected by the reflecting means 8, and is caused to travel backward along the same optical path and re-enter the radiation grating 7 at approximately the same position M1. ing. and radiation grating 7
The diffracted light of a specific order that is re-diffracted by the reflector 5 and the radio wave plate 4 is converted into linearly polarized light having a polarization direction 90 degrees different from that when it is incident through the reflecting mirror 5 and the radio wave plate 4, and is made incident on the polarizing beam splitter 3.

In this embodiment, from the polarizing beam splitter 3 to the reflecting means 8
The round trip optical path of the diffracted light of a specific order is the same.

Although it is possible to use an ordinary reflecting mirror as the reflecting means 8, it is preferable to use an optical element such as a corner cube so that the light beam emitted from the person is always parallel to the optical element.

For example, a single reflecting mirror is placed approximately on the focal plane of the condensing lens, and only the diffracted light of a specific order that is incident parallel to the condensing lens passes through the aperture of the mask and is reflected by the reflecting mirror. It is preferable to use an optical element configured to reverse the optical path. At this time, diffracted light of other orders is blocked by the mask. The reflective film 1) may have any configuration, such as a cat's eye optical system, as long as it has the same function as this type of optical element. If such an optical system is used, for example, even if the oscillation wavelength of the laser changes and the diffraction angle changes somewhat, the light can be returned along substantially the same optical path.

In addition, a gradient index lens, such as Selfoc Micro Lens (trade name) manufactured by Nippon Sheet Glass Co., Ltd., is applied to the cat's eye optical system, and a reflective film is placed on one side, focusing on the flat points at both ends. Therefore, it can be effectively applied to the present invention as an optical element having a simple configuration and high productivity.

Returning to Figure 1, the 2 beams split by the polarizing beam splitter 3
The transmitted light beam among the two light beams is converted into circularly polarized light through the electromagnetic wave plate 9, and is further transmitted through the reflecting mirror lO to a position iM2 approximately symmetrical to the position M1 on the radiation grating 7 on the disc 6 with respect to the rotation axis O. It is input to. Of the transmitted diffracted light incident on the radiation grating 7 and diffracted, the diffracted light of a specific order, for example, -m order, is caused to travel backward along the same optical path by a reflecting means 11 similar to the reflecting means 8 described above. The light is re-injected into 1fi M2 at approximately the same level. When the diffracted light of a specific order re-diffracted by the radiation grating 7 is incident through the reflecting mirror 10 and the radio wave plate 9, it is converted into linearly polarized light with a 90 degree different polarization direction and is made incident on the polarizing beam splitter 3.

At this time, the transmitted light beam also has the same round-trip optical path of the diffracted light of a specific order from the polarizing beam splitter 3 to the reflecting means 11, as in the case of the above-mentioned reflected light beam. After combining the diffracted light incident through the reflection means 8, it is made into circularly polarized light through the electromagnetic wave plate 12, and split into two light beams by the light splitter 13. Each light beam is received as linearly polarized light with a 90 degree phase difference through polarizing plates 14 and 16 arranged at an angle of 15.17 degrees.
It is input to. Then, the intensity of the two-light east interference fringes formed by the light receiving stages 15 and 17 is detected.

In this embodiment, when the rotating object to be measured rotates by one pitch of the radiation grating 7, the phase of the m-th re-diffracted light changes by 2mπ. Similarly, the phase of the n-th order diffracted light that is 11 times diffracted by the radiation grating 7 is increased by 2nπ. As a result, (2m-2n) sine waveforms are obtained from the light receiving stage as a whole.

In this embodiment, the rotation e is measured by detecting the sine waveform at this time.

FIG. 2 is a sectional view of the rotary encoder shown in FIG. 1, with the optical system above the disk 6 omitted. Note that the same reference numerals as those in Figure 1 indicate the same members or positions.

Also, look at position M1 on the radiation grid. The distance between M2, tt and t2, is a notation indicating the thickness of the disk on the M1 and M2 sides, respectively.

In a rotary encoder using transmitted diffracted light as shown in FIG. 1, a radiation grating 7 as shown in the cross-sectional view of FIG.
Consider the case where the disk 6 forming the . In this case, it is incident on Ml shown by the solid line and diffracted,
A difference occurs between the optical path length of the m-order diffracted light that causes re-diffraction and the optical path length of the 1-m-order diffracted light that enters M2, which is shown by the broken line, and causes diffraction and re-diffraction, resulting in a measurement error. I end up.

That is, since the light beam incident on Ml and M2 is made to enter the radiation grating 7 back and forth, the difference in thickness between Ml and M2 can be reduced to Δ.
= t 1 - t2, when the refractive index of the disc 6 on which the radiation grating 7 is formed is nQ, and the oscillation wavelength of the laser I is λ, an optical path length difference of n□XΔ=λ/2 will occur. Therefore, an error of ±1 pulse (+lE sinusoidal waveform) occurs every time the disk 6 rotates once.

Therefore, Δ
For Δ<=, λ/2nO−−−−−−−−−−−−−−−
−−−−−−(1) is given.

Therefore, the parallelism a of the disk 6 on which the radiation path f-7 is formed is a = jan-1(Δto/text) = tan-1(kx
/2no) -------(2).

On the other hand, if the total number of radiation gratings 7 of the disk 5-1- is N, then if the angular resolution 0 per pulse of the rotary encoder shown in Fig. 1 is expressed in seconds, the diffracted light of order ±m is To use, 0=360°X602/4mN (sec) =324,0
00/mN (sec) -=-, (3).

In addition, the incident points M1 and M2 are approximately symmetrical with respect to the center O of the radiation grating, and if the grating pitch at each incident point is P, then P=πi/N, so the equation (3) Angular resolution θ is 0=324, 0OOP/πm! L-=-------
(4) Therefore, the error per pulse in ± becomes the error in seconds in θ shown in the following equation.

θ=324,000kP/πmfL−=−−−−−−
--------(5) From the above equations (2) and (5), the first
The relationship between the angle error θ in the rotary encoder shown in the figure in seconds and the parallelism a of the disk 6 on which the radiation grating 7 is formed is as follows: πm included tana (4, Oo no 0K):
-----(6).

Therefore, in order to obtain a measurement accuracy of OK seconds, the parallelism a of the disk 6 must be πmλ a≦jan' (4,00noP θK )
--(7), and in the case of a rotary encoder as shown in Fig. 1, it is sufficient to use this equation (7) to create a disk 6 to obtain the desired angular accuracy. The embodiment is configured based on equation (7). However, in the detailed design of the actual sawing, the measurement accuracy will vary depending on various factors other than the flatness of the disk 6, so the final specifications should be determined by taking into account the al constant error caused by each factor. should be determined.

Next, a numerical example using equation (7) will be shown.

In the rotary encoder shown in Figure 1, the order of the diffracted light used is m = ±1, and the oscillation wavelength of laser 1 is λ =
The pitch of the 0.78 lm radiation grating is P=2.85 pm,
If the refractive index of the disk 6 is no=1.5, the parallelism a of the disk 6 with respect to the required angular accuracy θK is a≦jan-1 (8,84X l □-7θ),
11111 To guarantee constant accuracy of 0K=lO (seconds),
The parallelism of the disk 6 must be determined by substituting =10 for θ in the above equation, so that a≦5.07X10−4 (degrees) Hl, 8 (seconds).

3 and 4 are schematic diagrams showing a form in which reflected and diffracted light is utilized in the rotary encoder of FIG. 1, and all reference numbers in FIG. 1 refer to FIG. 1.

The same member as the member in FIG. 2 is shown. Ai as shown here? Even in the case of a reflective rotary encoder, as can be seen from the figure, the allowable value of the parallelism a of the disk 6 for acceptable measurement accuracy can be determined using the above-mentioned equation (7).

Further, the rotary encoder shown in FIG. 1 has a configuration in which the diffracted light is re-guided to Ml and M2 using the reflecting means 8 and 10 to generate the diffracted light again. , M1. There is a method in which interference fringes are obtained by directly superimposing the diffracted lights generated by M2 using a predetermined means.

At this time, when the rotating object to be measured rotates by one pitch of the radiation grating 7, the phase of the m-th diffracted light changes by m=, and similarly, the phase of the n-th diffracted light diffracted by different positions of the radiation grating 7 changes. The phase of the light changes by nπ, and as a whole, (m−n) sine waveforms are obtained from the light receiving means.

Furthermore, in the explanation regarding FIG. 1, formula (7) is derived by taking as an example the case where ±m-order diffracted lights of the same order interfere with each other, but the diffracted lights that can be caused to interfere with each other are limited to diffracted lights of the same order. It's not something you can do. Therefore, formula (7) is not a general formula representing the present invention, and the general formula will be introduced below.

First, if the number of times that the 41st-order diffracted light used to form interference fringes is diffracted by the diffraction grating, that is, the number of times it is incident on the radiation grating, is χ7, then the optical path of nOXΔ=input/χ Due to the difference in length, an error of ±1 pulse occurs every time the disk 6 rotates once. Therefore, the above formula (1) is expressed as Δ=λ/χn o −−−−−−−−−−−−−
−−−−−−−−−−(1) It is transformed as '.

Therefore, equation (2) indicating the parallelism of the disk 6 is as follows: a = ta + r + (χno statement) −−−−−−−
−−−−−−−−−(2) '.

On the other hand, when the angular resolution θ per pulse of the rotary encoder according to the present invention is expressed in degrees, it is assumed that 1 m-order and n-order diffraction lights are used.

0 =360/χ(m −n) N −−1−−−=
--------(3) It will be expressed as '.

Therefore, if we rewrite equation (3)' using the lattice pitches P, M, and the distance between M2, we get 0 = 360P/πlχ(m-n) −−−−−−−−
−−−−−(4) ′.

Therefore, an error per pulse in ± causes an error in degrees in θ shown in the linear equation.

OK= 360 kP/πlχ(m-n) -=-=-
−−−−(5) 'From the above equations (2)' and (5)',
Angular error in the rotary encoder according to the present invention is 0
KCI■) and the rotation degree a of the disk 6 on which the radiation grating 7 is formed
The relationship with (degree) is tan aH strip ÷ T LOK-1.
---(6)'.

Therefore, in order to obtain a device with an angular error within OK (degrees),
The f-row degree a (degrees) of the disk 6 is a≦”-1(+0K)−−−−−−(7)′.

Therefore, in the rotary encoder according to the present invention,
By making a coherent light beam incident on an object having a diffraction grating that satisfies the equation (7)', the rotation state of the rotating object can be measured within a desired angle; ill difference range. In other words, (7)
'Using the formula, set the allowable angular error value from the device specifications, and design and manufacture the rotating object so that the parallelism of the rotating object that makes up the rotary encoder can be obtained so as to satisfy this allowable angular error. Also, needless to say, (7)
The lower limit of the equation ' is O, and it is clearly desirable to form the diffraction grating on a rotating object that maintains complete parallelism.

Moreover, the rotary encoder shown in FIG. 1 has a position jj! that is approximately symmetrical about the center of the radiation grating. I M1! Although the method uses the diffracted light obtained from M2, Equation (7)' is sufficient even when using the diffracted light obtained from two positions forming an asymmetric angle of 120' T degrees, for example. Can be applied.

Returning to the rotary encoder shown in Figure 1, in this rotary encoder, for example, if the pitch of the diffraction grating is 2.851 Lm and the 1st and -1st orders are used as diffracted light, the rotating object is 2.85 gm of the pitch. When the light receiving element rotates by the same amount, four sine waveforms are obtained from the light receiving element. That is, the resolution per sine waveform is 2.85/4=0.71 gm of one pitch of the diffraction grating.

In this embodiment, the light beam is divided into two by the light splitter 11, and a phase difference of 90 degrees is created between each beam, so that the direction of rotation of the rotating object can also be determined.

Note that if only the amount of rotation is to be measured, the light splitter 13, the deflection plates 14 and 16, and one of the light receiving means are unnecessary.

Furthermore, as described above, two positions M1. By using the diffracted light from M2, measurement errors due to eccentricity between the center of rotation of the rotating object and the center of the radiation grating are reduced.

Furthermore, the luminous flux element of one luminous flux near the rotation axis center and the luminous flux element of the other luminous flux incident at a substantially point-symmetrical position near the rotation axis center are interlocked with each other, and similarly the luminous flux element towards the outside of the rotation center is ; By superimposing them on each other, the influence of wavefront aberration caused by the difference in pitch between the outside and inside of the radiation grating is removed.

Furthermore, by making the round trip optical path of the diffracted light beams from the polarizing beam splitter 3 to the reflecting means 8.11 the same, the two diffracted beams at the polarizing beam splitter 3 are This makes the hair curling easier and improves the assembly accuracy of the entire device.

Furthermore, the wave plates 4, 9 may be placed anywhere between the polarizing beam splitter 3 and the reflecting means 8.11.

Further, in each embodiment, reflected diffraction light may be used instead of transmitted diffraction light.

The diffraction grating used in the present invention is a so-called amplitude type diffraction grating consisting of a light transmitting part and a light shielding part, and a phase type diffraction grating consisting of a part having different refractive indexes at IF. In particular, a phase-type diffraction grating can be created by forming a relief pattern of convexes and convexes on the circumference of a transparent disk, for example, and can be mass-produced by processes such as embossing and stamping.

<Effects of the Invention> North of this, the rotary encoder according to the present invention is 411
This rotary encoder has a diffraction path T provided on a rotating object having parallelism that satisfies a desired tolerance at one fixed time, and can always satisfy the required measurement accuracy.

[Brief explanation of drawings]

1 and 2 are diagrams showing an example of a rotary encoder according to the present invention, in which FIG. 1 is a schematic diagram of an optical system, and FIG. 2 is a sectional view of a disk. FIGS. 3 and 4 are schematic diagrams showing other examples of the rotary encoder according to the present invention. FIG. 5 is a schematic diagram showing a conventional rotary encoder.

Claims (1)

[Claims] (1) Coherent light beams are incident on different positions of a diffraction grating provided on a rotating object, and the diffracted lights of specific orders emitted from the different positions are superimposed on each other to form interference fringes. A rotary encoder for detecting the rotational state of the object by detecting the rotational state of the object, characterized in that the parallelism a of the object satisfies the following equation. a≦tan^-^1([π(m-n)λ]/360n_
0Pθ_K) Here, m and n are the orders of the diffracted light, and n_
0 is the refractive index of the object, P is the pitch of the diffraction grating, λ is the wavelength of the coherent light beam, and θ_K is the allowable value of the angular error.