JPS62200220A - Rotary encoder - Google Patents

Abstract

PURPOSE:To accurately measure the rotating condition of a rotating object to be inspected by coinciding the advancing and returning light paths for the diffracted light beam of a specific order from a diffraction grating with each other by using optical means. CONSTITUTION:A light beam emitted from a prism 17 to a diffraction grating 7 perpendicularly thereto is projected upon the grating 7 so that the light beam of a specific order diffracted by the grating 7 is emitted perpendicularly thereto by specifying the configuration of an optical member 19 as with the case of a reflected light beam. The diffracted light beam of a specific order in the transmitted and diffracted light beams that are incident upon and diffracted by the grating 7 is returned in the same light path as in advance by optical means 9 and again let be projected upon approximately the same position M2 of the grating 7. After superposed on the diffracted light beam incident via optical means 8, the diffracted light beam is made a circularly polarized light beam through a quarter wavelength plate 10, split in two light beams by a light beam splitter 11. The two beams are projected upon light receiving means 14 and 15 via polarizing plates 12 and 13, respectively, arranged with polarization bearings inclined by 45 degrees relative to each other as a linear polarization with phase differences of 90 degrees relative to the two light beams and the intensity of the interference fringes formed by the two light beams is detected.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a rotary encoder, and in particular to a radial diffraction grating having a plurality of lattice patterns of, for example, transparent parts and reflective parts on the circumference, periodically intersecting each other. is attached to a rotating object, the diffraction grating is irradiated with a beam of light from, for example, a laser, and the diffracted light from the diffraction grating is used to determine the rotational state of the diffraction grating or the rotating object, such as the rotational speed or the amount of variation in rotational speed. This relates to a rotary encoder that detects electrical charges.

(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 photoelectric rotary encoder has been used as a means for detection.

For example, as shown in FIG. 3, 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 31. 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 opposite to each other with the scale tip means 33 and light receiving means 34 sandwiched therebetween. There is.

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, an object of the present invention is to provide a rotary encoder in which the overall size of the device is reduced by controlling the diffraction angle and diffraction optical path of diffracted light of a specific order from a diffraction grating.

(Means for Solving the Problem) A coherent light beam is made incident on a diffraction grating on a disk connected to a rotating object at least at one position of the rotating object, and a specific order from the diffraction grating is After passing the diffracted light through an optical means that causes the principal ray of the diffracted light to travel backwards along substantially the same optical path as the incident optical path, the diffracted light is made to enter the diffraction grating at substantially the same position again, and a specific order of diffraction is caused by the diffraction grating. After guiding the light to a beam superimposing means, the diffracted light of the specific order is superimposed, and the light 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. That's what happened.

Other features of the invention are described in the Examples.

(Embodiment) FIG. 1(t,) 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 beam passes through a wavelength plate 4, becomes circularly polarized light, passes through a prism 6 having two reflecting surfaces, and enters an optical member 18 made of a prism. Then, the light beam is made incident on a position M1 of a diffraction grating 7, which is provided with a radial diffraction grating on a disk 6 connected to a rotating object to be measured via an optical member 8. At this time, the light beam emitted from the prism 16 perpendicularly to the diffraction grating 7 is transmitted to the optical member 1 as shown in FIG. 1(B).
By specifying the shape of the diffraction grating 8, the diffracted light of a specific order by the diffraction grating 7 is made to enter the diffraction grating 7 so as to be emitted substantially perpendicularly to the diffraction grating 7. Of the transmitted diffracted light that is incident on the diffraction grating 7 and diffracted, the diffracted light of a specific order is guided to the optical means 8. The optical means 8 includes, for example, a light condensing member and a reflecting mirror made of a plane mirror or a curved surface, and the principal ray of the incident diffracted light directed toward the light condensing member is reflected by the reflecting mirror via the light condensing member. It is configured to travel backward along substantially the same optical path as the optical path. Then, the diffracted light guided to the optical means 8 is made to travel backward along substantially the same optical path as the incident optical path, and is made to re-enter substantially the same position M1 on the diffraction grating 7. Then, the diffracted light of a specific order re-diffracted by the diffraction grating 7 is converted into linearly polarized light having a polarization direction different by 90 degrees from that when it is incident through the % wavelength 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 flat reflecting mirror 40 is arranged approximately on the focal plane of a condensing lens 41, which is a condensing member, and only 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.

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 a wavelength plate 5, and is incident on an optical member 19 made of a prism through a prism 7 having two reflecting surfaces. Then, a position M1 on the diffraction 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 diffraction 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 diffraction grating 7 is transmitted to the diffraction grating 7. The light is made incident on the diffraction 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 diffraction grating 7 and diffracted is reversed along the same optical path by an optical means 9 similar to the optical means 8 described above, and is re-injected into substantially the same position M2 of the diffraction grating 7. I'm letting you do it. Then, the diffracted light of a specific order re-diffracted by the diffraction 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.

In this embodiment, when the rotating object to be measured rotates by one pitch of the diffraction 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 diffraction 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 2/4-0.8 .mu.m, which is % of 1 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 11, the polarizing plates 1, 2, 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 diffraction grating 7, the diffraction light of a specific order from the diffraction grating 7 is emitted approximately perpendicularly to the diffraction 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 the 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.
, , M2 is used to reduce measurement errors due to eccentricity between the center of rotation of the rotating object and the center of the diffraction grating.

The configuration in this example uses diffracted light from two points that are approximately symmetrical, but by using diffracted light from a plurality of positions that are limited to approximately point symmetrical, it is possible to obtain approximately the same 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.

Furthermore, the luminous flux elements of one luminous flux near the center of the rotation axis and the luminous flux elements of the other luminous flux incident at a substantially point-symmetrical position near the center of the rotation axis are overlapped with each other, and similarly the luminous flux elements near the outside of the rotation center are overlapped with each other. By overlapping them, the influence of wavefront aberration caused by the difference in pitch between the outside and inside of the diffraction grating is removed.

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

4th. Fifth. 6A and 6B are explanatory diagrams of other embodiments of the optical means of the present invention shown in FIG. 2, respectively.

In FIG. 4, a concave mirror 45 is arranged so that the center of curvature is located at the exit pupil of the condensing member 41, and a principal ray 44 of the diffracted light of a specific order that enters the condensing member 41 at an arbitrary angle
is constructed so that it travels in substantially the same optical path as the incident optical path. This reduces assembly errors and improves measurement accuracy.

FIG. 5 shows the condenser lens 41 shown in FIG. This is an embodiment in which the mask 42 and the reflecting mirror 40 are integrated to simplify the optical means 8 as a whole. In the figure, 51 is a condensing lens surface, 52 is a reflective surface, and 53 is a mask.

Figure 6 shows a gradient index lens such as Selfoc Lens 61 (manufactured by Nippon Sheet Glass) used as a light-converging member.Using the fact that both ends of the lens are flat, a reflective film is applied to one of the converging surfaces to reflect the light. fi 62 is configured such that the principal ray 44 of the incident diffracted light travels in the same optical path as the incident optical path.

In addition to the embodiments shown above, the optical means in the present invention may be of any configuration as long as the diffracted light beam corresponding to the principal ray of the diffracted light emitted from the diffraction grating travels in substantially the same optical path as the incident optical path. good.

If such an optical means is used, 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.

The above describes a case in which the present invention is applied to a rotary encoder. However, by using the optical means of the present invention, the diffracted light beam corresponding to the principal ray of the diffracted light of a specific order is made to go back to the optical path that is substantially the same as the incident optical path, and the whole device is The technical idea of reducing the size and improving assembly accuracy can be applied to linear encoders as is.

In each of the above embodiments, the wavelength plate 4.5 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 stunburring.

(Effects of the Invention) According to the present invention, the rotational state of a rotating object to be tested can be measured with high precision by using optical means for diffracted light of a specific order from a diffraction grating and making the round trip optical path the same. Moreover, it is possible to achieve a rotary encoder whose entire device is miniaturized.

[Brief explanation of drawings]

1(8) and (B) are schematic diagrams of an optical system according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of a portion of FIG. 1, and FIG.
5 and 6 are schematic diagrams of optical systems of other embodiments of the optical means according to the present invention, and FIG. 3 is an explanatory diagram of a conventional photoelectric rotary encoder. In the figure, 1 is a laser, 2 is a collimator lens, and 3 is a polarizing beam splitter 14
, 5, 10 are storm wave plates, 6 is a disk, 7 is a diffraction grating, 8,
9 are optical means, 12.13 are polarizing plates, 14.1
5 are light receiving means.

Claims (1)

[Claims] A coherent light beam is made incident on a diffraction grating on a disc connected to a rotating object at at least one position of the rotating object, and diffracted light of a specific order from the diffraction grating is made into a chief ray of the diffracted light. After passing through an optical means such that the light travels in a reverse direction along substantially the same optical path as the incident light path, the light is made to enter the diffraction grating at substantially the same position again, and the diffracted light of a specific order from the diffraction grating is guided to the light beam superimposing means. After that, the diffracted lights of the specific order are superimposed and 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.