JPS62200221A - Rotary encoder - Google Patents

Abstract

PURPOSE:To accurately measure the rotating condition of a rotating object to be inspected by removing the effect of a wave front aberration produced by the difference between the pitches outside and inside a radial grating by superposing the diffracted light beams of a specific order from the radial grating on each other so that the diffracting directions from the radial grating become the same as one another. CONSTITUTION:Optical members are arranged such that the light beam element of the diffracted light beam of a specific order diffracted from a point M1 on a radial grating 7, said light beam element being closer to the rotational center of a rotating object, and the light beam element of the diffracted light beam of the specific order diffracted from a point M2 on the radial grating 7, said light beam element being closer to the rotational center of the rotating object, are superposed on each other and, in the same manner, the light beams of the radial grating closer to the outside of the rotational center are superposed on each other. Thus, the effect of a wave front aberration, especially an astigmatism produced by the difference between the pitches outside and inside the radial grating is removed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a rotary encoder, in particular a rotary encoder having a plurality of diffraction gratings in a lattice pattern, for example, a transparent part and a reflective part, on the circumference.
A periodic radiation grating is attached to a rotating object, and the radiation grating is irradiated with a beam of light from a laser, for example, and the diffracted light from the radiation grating is used to determine the rotational speed or speed of the radiation grating or the rotating object. This invention relates to a rotary encoder that electrically detects the rotational state such as the amount of variation in the rotary encoder.

(Prior art) Conventionally, it has been used to measure the rotational speed and the amount of variation in rotational speed of computer equipment such as the drive of a floppy desk, office equipment such as a printer, or NC machine tools, as well as rotational mechanisms such as the carburetor stainless steel motor and rotating drum of a VTR. A rechargeable rotary encoder has been used as a means for detection.

For example, as shown in FIG. 3, a rechargeable rotary encoder has a so-called main scale 31 in which transparent parts and light-shielding parts are provided at equal intervals around a disc 35 connected to a rotating shaft 30, and a corresponding main scale. A so-called index scale system configuration is adopted in which a so-called fixed index scale 32 is provided with a light-transmitting part and a light-shielding part at equal intervals to the scale, and both scales are placed facing each other with the projection/tip means 33 and the light-receiving means 34 sandwiching them. It's full.

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. Therefore, the finer the scale interval between the light-transmitting part and the light-blocking part of both scales, the higher the detection accuracy can be. 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. As the thickness increases, the entire device becomes larger.
As a result, there are drawbacks such as an increase in the load on the rotating object to be tested.

(Problems to be Solved by the Invention) The present invention is characterized by providing a rotary encoder that has a small load on a rotating object to be inspected, can easily downsize the entire device, and can detect the rotational state with high precision. In particular, when two diffracted lights of a specific order from a radiation grating are superimposed, errors due to the grating pitch between the periphery and the inside of the radiation grating are corrected by aligning the directions of the diffracted beams emitted from the radiation grating and overlapping each other. The purpose is to provide a high-precision rotary encoder.

(Means for Solving the Problem) A plurality of coherent light beams are made incident on a diffraction grating on a disc connected to a rotating object at a plurality of different positions of the rotating object, and from the diffraction grating. Among the diffracted lights of a specific order, the light flux elements of one light flux near the center of the rotational axis of the rotating object and the light flux elements of the other light flux near the center of the rotational axis of the rotating object are made to overlap with each other, and then these The light flux is guided to a light receiving means, and the rotational state of the rotating object is determined using an output signal from the light receiving means.

Other features of the invention are described in the Examples.

(Embodiment) FIG. 1(A) is a schematic diagram of an optical system according to an embodiment of 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.
It is split into two linearly polarized beams. The reflected light flux passes through the wavelength plate 4, becomes circularly polarized light, passes through a prism 16 having two reflecting surfaces, and enters an optical member 18 made of a prism. The light beam is then made incident on a position M of a radiation grating 7 provided with a radial diffraction grating on a disk 6 connected to a rotating object to be measured via an optical member 18 . At this time, the light flux emitted perpendicularly to the radiation grating 7 from the prism 16 is transmitted to the optical member 1 as shown in FIG. 1(B).
By specifying the shape of the radiation grating 8, the diffracted light of a specific order by the radiation grating 7 is made to enter the radiation grating 7 so as to be emitted substantially perpendicularly to the radiation grating 7. Of the transmitted diffracted light that is incident on the radiation grating 7 and diffracted, the diffracted light of a specific order is guided to the optical means 8. The optical means 8 has, for example, a light condensing member and a reflecting mirror made of a plane mirror or a curved surface, and after the principal ray of the incident diffracted light directed toward the light condensing member passes through the light condensing member and is reflected by the reflecting mirror, It is configured to run substantially in the same direction as the incident optical path and in the opposite direction. Then, the diffracted light guided to the optical means 8 is made to re-enter the radiation grating 7 at substantially the same position M□ by traveling in the opposite direction to the incident optical path. Then, the diffracted light of a specific order re-diffracted by the radiation grating 7 is converted into linearly polarized light with a polarization direction different by 90 degrees from that when it is incident through the storm wave plate 4, and is made incident on the polarizing beam splitter 3.

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

FIG. 2 is an explanatory diagram of one embodiment of the optical means shown in FIG. 1.

In the figure, a reflecting mirror 40 made of a plane mirror is arranged approximately on the focal plane of a condensing lens 41 which is a condensing member, and only the diffracted light of a specific order that is incident parallel to the condensing lens 41 is reflected by a mask 42. After passing through the aperture 43 and being reflected by the reflecting mirror 40, the principal ray 44 at the condenser lens 41 returns along its original optical path.

The diffracted light of other orders is blocked by a mask 42. The reflecting means may be of any configuration, such as a cat's eye optical system or a plane mirror, as long as it has the same function as shown in FIG. 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, by applying a refractive index gradient lens, such as Selfoc Micro Lens (trade name) manufactured by Nippon Sheet Glass Co., Ltd., to the cat's eye optical system and providing a reflective film on one side, focusing on the fact that both ends of the lens are flat. , it can be effectively applied to the present invention as an optical element having a simple structure and high productivity.

Returning to Figure 1, the 2 beams split by the polarizing beam splitter 3
Of the two light beams, the transmitted light beam is converted into circularly polarized light through the wavelength plate 5, and is made incident on an optical member 19 made of a prism through a prism 17 having two reflective surfaces. Then, the position M1 on the radiation grating 7 on the disk 6 is transmitted through the optical member 19.
The light is made incident at a position M2 that is approximately point symmetrical with respect to the rotation axis 50. At this time, by specifying the shape of the optical member 19 for the light beam emitted perpendicularly to the radiation grating 7 from the prism 17 in the same manner as in the case of the reflected light beam described above, the diffracted light of a specific order by the radiation grating 7 is transmitted to the radiation grating 7. The beam is made incident on the radiation grating 7 so as to be emitted perpendicularly to the beam. Then, the diffracted light of a specific order among the transmitted diffracted light that is incident on the radiation grating 7 and diffracted is made to travel backward along the same optical path by an optical means 9 similar to the optical means 8 described above, and is returned to approximately the same position M2 of the radiation grating 7. It is incident. Then, the diffracted light of a specific order re-diffracted by the radiation grating 7 is converted into linearly polarized light with a polarization direction that is 90 degrees different from that when it is incident through the wavelength plate 5. The polarizing beam splitter 3
It is input to.

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 optical means 9 as in the above-mentioned reflected light beam.

Then, after being superimposed with the diffracted light that has entered through the optical means 8, it is made into circularly polarized light through the wavelength plate 10, and the beam splitter 1
1 is divided into two light beams, and each light beam is passed through polarizing plates 12 and 13 arranged with their polarization directions tilted by 45 degrees, and both light beams are converted into linearly polarized light with a phase difference of 90 degrees to each light receiving means. 14.15. and light receiving means 1
The intensity of the interference fringes of the two beams formed by 4.15 is detected.

At this time, in this embodiment, a light flux element near the rotation center of the rotating object of the diffracted light beam of a specific order diffracted from the point M of the radiation grating 7 and a rotation center of the rotating object of the diffracted light beam of the specific order diffracted from the point M2. This is caused by the difference in pitch between the outside and inside of the radiation grating by arranging each optical member so that the light flux elements closer to each other overlap with each other, and similarly, the light beam elements closer to the outside of the rotation center of the radiation grating overlap. The effects of wavefront aberration, especially astigmatism, are eliminated.

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 order diffracted light changes by 2mπ. Similarly, the phase of the n-th order diffracted light re-diffracted by the radiation grating 7 changes by 2nπ. As a result, (2m-2n) sine waveforms are obtained from the light receiving means as a whole. In this embodiment, the amount of rotation is measured by detecting the sine waveform at this time.

For example, if the pitch of the diffraction grating is 3.2 μm, the diffracted light is 1
If we use the next and -1st orders, the rotating object will have a pitch of 3
．． When rotated by 2 μm, four sine waveforms are obtained from the light receiving element. That is, the resolution per sine waveform is 3.2/4-OJB μm of one pitch of the diffraction grating.

In this example, the diffracted light that has been diffracted by the radiation grating is not made to enter the diffraction grating again, but the two diffracted lights are directly superimposed so as to satisfy the above-mentioned conditions, and the measurement accuracy is slightly reduced. Light may be guided.

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 11, the polarizing plates 12 and 13, and one of the light receiving means are unnecessary.

In this embodiment, when the optical members 18 and 19 are used to make the light beam incident on the radiation grating 7, the diffracted light of a specific order from the radiation grating 7 is emitted approximately perpendicularly to the radiation grating 7, so that the entire apparatus is Although the aim is to simplify and improve assembly accuracy, it is not always necessary to emit light substantially perpendicularly from the radiation grating 7, and the optical members 18 and 19 may be omitted and the light may be emitted at a certain angle.

In this embodiment, there are two positions M that are approximately symmetrical about the center of rotation.
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.

Although the configuration in this example uses diffracted light from two points that are approximately point symmetrical, it is possible to obtain approximately the same diffraction light by using diffracted light from multiple positions, not limited to approximately point symmetrical. You can get the effect. For example, it is also effective to use diffracted light from three points that are at an angle of 120 degrees to each other, or to use diffracted light from arbitrary two points that are not close to each other.

In this embodiment, from the polarizing beam splitter 3 to the reflecting means 8
.

In each of the above embodiments, the wavelength plates 4 and 5 may be placed anywhere between the polarizing beam splitter 3 and the reflecting means. Further, in each embodiment, reflected diffraction light may be used instead of transmitted diffraction light.

Furthermore, according to the present invention, not only the rotation angle but also the rotation speed can be detected.

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 parts having mutually different refractive indexes. In particular, phase-type diffraction gratings can be created, for example, by forming an uneven relief pattern on the circumference of a transparent disk, and can be mass-produced by processes such as embossing and stamping.

(Effects of the Invention) According to the present invention, when the diffracted lights of a specific order from the radiation grating are superimposed on each other, they are superimposed on each other so that the diffraction directions from the radiation grating are the same, and the pitch between the outside and the inside of the radiation grating is different. By eliminating the influence of wavefront aberration caused by this, it is possible to achieve a rotary encoder that can measure the rotational state of a rotating object to be detected with high precision.

[Brief explanation of drawings]

Figures 1 (8) and (B) are schematic diagrams of an optical system according to an embodiment of the present invention, Figure 2 is an explanatory diagram of a portion of Figure 1, and Figure 3 is an explanatory diagram of a conventional photoelectric rotary encoder. It is. In the figure, 1 is a laser, 2 is a collimator lens, 3 is a polarization splitter, 4°5.10 is a % wavelength plate, 6 is a disk, 7 is a radiation grating, 8.9 is an optical means, and 12. '1
3 are polarizing plates, and 14 and 15 are light receiving means.

Claims (1)

[Claims] A plurality of coherent light beams are incident on a diffraction grating on a disc connected to a rotating object at different positions of the rotating object, and one of the diffracted lights of a specific order from the diffraction grating is After the light flux element of the light flux near the center of the rotation axis of the rotating object and the light flux element of the other light flux near the center of the rotation axis of the rotating object are made to overlap with each other, these light fluxes are guided to a light receiving means, A rotary encoder characterized in that the rotational state of the rotating object is determined using an output signal from a light receiving means.