JPH0545164B2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention relates to an encoder, and in particular to an encoder that makes a coherent light beam incident on a diffraction grating attached to a rotating object, and uses the diffracted light from the diffraction grating to determine the rotational state of the object. Concerning the encoder to be detected.

<Prior Art> In recent years, precision machines such as NC machine tools and semiconductor printing equipment have required precision measuring instruments that can measure in units of 1 μm or less (submicron).

Conventionally, a linear encoder that uses a coherent light beam such as a laser to form interference fringes from diffracted light from a moving object is well-known as a measuring instrument capable of measuring in submicron units. It is being

On the other hand, computer equipment such as floppy desk drives, office equipment such as printers, or
Photoelectric rotary encoders have been used as a means to detect the rotational speed and variation in rotational speed of rotating mechanisms such as NC machine tools and VTR capsten motors and rotating drums.

For example, a photoelectric rotary encoder
As shown in the figure, a so-called main scale 31 has light-transmitting parts and light-shielding parts provided at equal intervals around a disk 35 connected to a rotating shaft 30, and correspondingly, a so-called main scale 31 has light-transmitting parts and light-shielding parts provided at equal intervals to the main scale. A so-called fixed index scale 32 is provided with a light shielding part, and a so-called index scale type structure is adopted in which both scales are placed opposite to each other with a light projecting means 33 and a light receiving means 34 sandwiching the scales. In this method, as the main scale rotates, a signal is obtained that is synchronized with the interval between the light-transmitting part and the light-blocking part of both scales, 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. This increases the size and thickness of the device, making the entire device larger, resulting in disadvantages such as an increase in the load on the rotating object to be tested.

As one means to eliminate such drawbacks of the conventional rotary encoder, it is conceivable to directly apply the measurement principle of the above-mentioned rear-air encoder using interference fringes to the rotary encoder.

One of the configurations of an encoder that uses this type of interference fringes is to separate the coherent light beam emitted from a light source such as a laser and make multiple light beams enter the same position on the diffraction grating, or to make them enter different positions on the diffraction grating. There is a method of obtaining interference fringes by superimposing diffracted lights emitted from the same position, or by superimposing diffracted lights emitted from different positions. In this case, the conventional method uses a semiconductor laser to miniaturize the device, and the linearly polarized laser light emitted from the semiconductor laser is converted into circularly polarized light by a 1/4 wavelength plate and incident on the polarization beam splitter, and the P-polarized light is and S-polarized light, which are divided into light beams of equal intensity and utilized.

However, along with the miniaturization of the device, reducing the number of parts and lowering costs is required not only for this type of device but also for various devices, and in that sense, the 1/2 It was not desirable to use special optical components such as a four-wave plate.

<Summary of the Invention> In view of the above-mentioned conventional problems, an object of the present invention is to provide an encoder that is required to be smaller and lower in price without using special optical parts such as a 1/4 wavelength plate. The object of the present invention is to provide an encoder with high resolution capable of dividing a luminous flux.

In order to achieve the above object, an encoder according to the present invention includes a semiconductor laser that emits linearly polarized light with a wavelength λ and a polarizing beam splitter in an encoder that detects the rotational state of a disk on which a radial diffraction grating is formed. and directing means for directing the linearly polarized light emitted by the semiconductor laser toward the polarizing beam splitter, the semiconductor laser and the directing means directing the linearly polarized light toward the polarizing beam splitter. arranged such that the polarization direction of the polarized light forms an angle of 45 degrees with respect to the orthogonal axis of the orthogonal polarization plane of the polarization beam splitter,
Furthermore, in order to cause the pair of polarized lights generated by splitting the linearly polarized light by the polarizing beam splitter to enter two substantially symmetrical locations with respect to the center of the disc of the radial diffraction grating, each polarization of the pair of polarized lights is An input means provided for each light and each of the first-order diffracted lights generated from the two locations of the radial diffraction grating is returned to the location where the first-order diffraction light was generated to be re-diffracted, and the first-order re-diffraction generated from the two locations is a cat's-eye optical system provided for each of the first-order diffracted lights so that the light enters the polarizing beam splitter via the incident means, and each of the incident means or each cat's-eye optical system is configured to input light from the polarizing beam splitter to the polarizing beam splitter. a λ/4 plate that makes the polarization direction of the first-order re-diffraction light incident on the polarization beam splitter perpendicular to the polarization direction of the polarized light;
The method further includes means for forming interference light by causing the respective first-order re-diffracted lights combined by the polarization beam splitter to interfere with each other, and converting the interference light into a signal indicating the rotational state of the disk. Features.

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

<Embodiment> FIG. 1 is a diagram showing an embodiment of an encoder according to the present invention, and shows a rotary encoder. In the figure, 1 is a laser, 2 is a collimator lens that converts the coherent beam emitted from the laser 1 into a parallel beam, 3 is an optical component made by bonding two trapezoidal prisms, and 4 is the bonded surface of the optical component 3. , which constitutes a polarizing beam splitter. 5 and 7 are reflecting mirrors, and 6 is a radiation grating mounted on a rotating object, and the rotation center of the rotating object and the center 0 of the radiation grating approximately coincide with each other. 8 and 10 are 1/4 wavelength plates that change the polarization direction of the light beam entering and exiting the radiation grating 6. 9
and 10 are reflecting means for redirecting the diffracted light of a specific order emitted from the radiation grating 6 to the radiation grating 6, which is comprised of a cat's-eye optical system. Reference numeral 12 denotes a 1/4 wavelength plate, which changes the polarization direction of the light beams superimposed via the optical component 3 into circularly polarized light. 13 is a light splitter, 14
and 16 are polarizing plates, and 15 and 17 are light receiving means consisting of photoelectric conversion elements and the like. Further, 0 indicates the center of the radiation grating 6, and M ₁ and M ₂ indicate arbitrary points on the radiation grating 6.

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 optical component 3. After being reflected by the slope of the trapezoidal prism forming the optical component 3, it is directed to the light splitting surface 4 at a predetermined angle. Orient it so that it is incident at an angle of . The parallel light beam incident on the light splitting surface 4 is split into two linearly polarized light beams, a reflected light beam and a transmitted light beam, at an intensity ratio of 1:1.
Incidentally, since the laser 1 in this embodiment uses a semiconductor laser, the light beam is linearly polarized in advance in a predetermined direction. Of the two light beams split by the light splitting surface 4, the reflected light beam undergoes internal reflection repeatedly on the light beam entrance/exit surface and the slope of the optical component 3, and exits from the optical component 3 in a state parallel to the time of incidence. The emitted reflected light flux is incident on a predetermined position _M1 of the radiation grating 6 at a predetermined angle of incidence by the reflecting mirror 5, but at this time, the diffracted light of a specific order, for example, the m order, from the radiation grating 6 is approximately The light beam is incident so that it radiates vertically. Then, the diffracted light of a specific order among the transmitted diffracted light incident on the radiation grating 6 and diffracted is reflected by the reflecting means 9 via the quarter-wave plate 8, and is caused to travel backward along the same optical path to approximately the same position M on the radiation grating 6. ₁ is re-injected. That is, here, the diffracted light vertically emitted by the radiation grating 6 is once changed in its polarization direction to circularly polarized light by the quarter-wave plate 8, reflected by the reflecting means 9, and passes through the quarter-wave plate again. By this,
The diffracted light of a specific order re-diffracted by the radiation grating 6 is directed toward the reflecting mirror 5 as linearly polarized light with a polarization direction that is 90 degrees different from that when it was incident. Then, the diffracted light of the specific order reflected by the reflecting mirror 5 travels back along the same optical path again, enters the optical component 3, repeats internal reflection, and reaches the light splitting surface 4.

In this embodiment, as described above, the round trip optical path of the diffracted light of a specific order from the light splitting surface 4 to the reflecting means 9 is the same. In addition, the cat's eye optics used as the reflecting means 9 places a reflecting mirror 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 is reflected by the reflecting mirror. After that, the light path is reversed. Then, the diffracted light of other orders is blocked by a predetermined means, and compared to reflecting the diffracted light using a normal reflecting mirror, for example, the oscillation wavelength of the laser changes, and the diffraction angle changes somewhat. It has the characteristic that it can be returned using almost the same optical path even if the

In addition, a gradient index lens such as the reflecting means 9 shown in FIG. 1 is added to the cat's eye optical system, such as a self-occurring micro lens (trade name) manufactured by Nippon Sheet Glass Co., Ltd.
By applying the above, and providing a reflective film on one side by paying attention to the fact that both ends thereof are flat, it is possible to effectively apply the present invention as an optical element having a simple structure and high productivity.

On the other hand, the light beam transmitted by the light splitting surface 4 out of the two divided light beams is reflected by the light beam incident surface of the optical component 3 and the slope of the other trapezoidal prism forming the optical component 3, and is emitted from the optical component 3. The light is incident on a predetermined position M ₂ on the radiation grating 6 by the reflecting mirror 7 . Here, as in the case of the transmitted light flux, the transmitted diffracted light of a specific order, for example, the n-th order, emitted from the radiation grating 6 is
The light is made incident on M 2 at a certain angle of incidence by the reflecting mirror 7 so that the light is emitted perpendicularly to _M2 . The diffracted light of a predetermined order among the transmitted diffracted light incident on M ₂ and diffracted travels backward along the same optical path via a quarter-wave plate 10 by a reflecting means 11 similar to the above-mentioned reflecting means 9, and then passes through a radiation grating 6.
The beam re-enters at approximately the same position _M2 . Therefore, here as well, the diffracted light of a specific order re-diffracted by the radiation grating 6 is directed to the reflecting mirror 7 as linearly polarized light having a polarization direction different by 90° from that when it was incident on the radiation grating 6. Then, the diffracted light of the specific order reflected by the reflecting mirror 7 travels back along the same optical path again, enters the optical component 3, repeats internal reflection, and reaches the light splitting surface 4.

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 light splitting surface 4 to the reflecting means 11, as in the case of the reflected light beam described above. Then, after overlapping the diffracted lights that have passed through the reflection means 9, 1/
The light is circularly polarized through a four-wavelength plate 12, split into two beams by a light splitter 13, and each beam is polarized by 90 degrees through polarizing plates 14 and 16, which are arranged with their polarization directions tilted at 45 degrees. The light is input to each of the light receiving means 15 and 17 as linearly polarized light with a phase difference of .
Then, the intensity of the interference fringes of the two beams formed by the light receiving means 15 and 17 is detected.

Now, in the encoder of this embodiment, the light beam emitted from the laser 1 is divided into a transmitted light beam and a reflected light beam at the same light quantity ratio by the light splitting surface 4 of the optical component 3 without using any special optical element. ing.
The reason for making the light intensity ratio the same is to obtain the brightness ratio (visibility) of the interference fringes that is the best when detecting the intensity of the interference fringes mentioned above.
This is to improve the radiation grating 6.
The structure is such that the intensities of the diffracted light of a specific order obtained at positions M ₁ and M ₂ are substantially the same. Hereinafter, the function of the light splitting surface 4 in this embodiment will be explained in detail.

Fig. 2 is a functional explanatory diagram of the light splitting plane of this embodiment, and 4 indicates the light splitting plane of Fig. 1, and for ease of explanation, it is shown in place of a normal polarizing beam splitter. ing. Further, two types of arrows L and A in the figure indicate the traveling direction L and the polarization direction A of the light beam.

In general, a polarizing beam splitter has the function of transmitting a P wave component of a light beam having an arbitrary polarization direction through its light splitting surface 4 and reflecting an S wave component. Therefore, when a light beam having a polarization plane forming an angle θ as shown in the figure is incident on the orthogonal polarization plane of the light splitting surface 4, the amplitude of the P wave component and the S wave component split by the light splitting surface 4 will be The ratio is sinθ:cosθ.
Therefore, if the light source is installed so that the polarization direction of the light beam emitted from the light source forms an angle of 45 degrees with respect to the orthogonal polarization plane of the light splitting surface 4, the P wave component and the S wave split by the light splitting surface 4 can be separated. The amplitude ratio of the components, that is, the transmitted light beam and the reflected light beam, is sin45°:cos45°=1:1, and they are ultimately divided at the same intensity.

In this embodiment, a semiconductor laser is used as the laser 1, and the laser 1 is aligned with the optical axis of the collimator lens 2 so that the plane of polarization determined by the structure of the semiconductor laser is at a predetermined angle of 45 degrees with respect to the light splitting plane 4 of the optical component 3. It is installed tilted (rotated). With such a configuration, the intensity ratio between the transmitted light beam and the reflected light beam can be made 1:1 at the light splitting surface 4 without using any other optical element. Therefore, it greatly contributes to reduction in size and cost reduction by reducing the number of optical components required for this type of measuring device, as well as to adjustment of assembly and optical accuracy.

In this embodiment, the rotating object to be measured is the radiation grating 6.
When rotated by 1 pitch, 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 6 changes by 2nπ. As a result, the distance from the light receiving means as a whole is (2m
−2n) sine waveforms are obtained. In this embodiment, the amount of rotation is measured by detecting the sine waveform at this time.

For example, the pitch of the diffraction grating is 3.2 μm, M ₁ and M ₂
If the first-order and -1st-order diffracted lights are used as the diffracted light obtained from the light, four sine waveforms will be obtained from the light receiving element when the rotating object rotates by the pitch of 3.2 μm. That is, the resolution per sine waveform is 3.2/4=0.8 μm, which is 1/4 of one pitch of the diffraction grating.

In this embodiment, the light beam is divided into two parts by the light splitter 13, and a phase difference of 90 degrees is created between the two light beams, 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 splitting surface 13, the polarizing plates 14 and 16, and one of the light receiving means are unnecessary. Furthermore, the rotational speed of a rotating object can be easily determined by measuring the sine waveform frequency.

In this example, measurement errors due to eccentricity between the rotation center of the rotating object and the center 0 of the radiation grating are reduced by using diffracted light from two positions M ₁ and M ₂ that are approximately symmetrical about the rotation center. I'm letting you do it.

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 these, the influence of wavefront aberration caused by the difference in pitch between the outside and inside of the radiation grating, which is unique to rotary encoders, is removed.

In this embodiment, from the light splitting surface 4 to the reflecting means 9, 11
By making the reciprocating optical path of the diffracted light of a specific order the same, the degree of overlapping of the two diffracted light beams at the light splitting surface 4 is facilitated, and the assembly precision of the entire device is improved.

In this embodiment, the quarter-wave plate 8 may be placed between the optical component 3 and the radiation grating 6. other 1/
The same applies to the four-wavelength plate 10. In addition, this embodiment uses an optical component 3 having a quantity function of a polarizing beam splitter and an internal reflection type prism, and by using this type of polarizing prism with a specific shape, the number of optical components can be reduced and each The aim is to improve the assembly precision of optical components and to downsize the entire device.

Furthermore, in this embodiment, in order to obtain diffracted lights of different orders from positions M ₁ and M ₂ on the radiation grating 6, the positions of the reflecting mirrors 5 and 7 and the inclinations of the reflecting surfaces with respect to the light beam are devised. There is. Here, the diffracted lights of different orders in M ₁ and M ₂ are configured so that they both exit perpendicularly from the radiation grating, but the reflecting mirrors 5 and 7 are tilted at the same angle to split the light. The reflected light beam and the transmitted light beam divided by the surface 4 are incident perpendicularly to the radiation gratings _M1 and _M2 , and reflecting means are appropriately arranged in the direction of emission of the diffracted light of a predetermined order from each position. Also good.

The configuration of this type of device and the selection of the orders of the diffracted lights that should mutually form interference fringes are diverse, and it goes without saying that various encoders can be configured based on the idea of the present invention. For example, in this embodiment, the number of parts is reduced by using the optical member 3, but it may be configured by combining multiple mirrors, prisms, light splitters, etc., and the specifications, cost, ease of manufacturing, etc. of the device may be improved. The configuration of this encoder can be determined by taking these into consideration.

In addition, in the configuration shown in Figure 1, the semiconductor laser is arranged so that the polarization direction of the light beam emitted from the laser is at a predetermined desired angle of 45 degrees with respect to a predetermined light splitting plane. On the contrary, the orthogonal polarization plane of the light splitting plane is determined by considering the known polarization direction of the light beam, i.e.
Optical components may be configured with an angle of 45°.

Note that the reflection 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.

Furthermore, although the above explanation mainly refers to an encoder that uses transmitted diffraction light, the present invention applies encoders that use reflected diffraction light or both reflected diffraction light and transmitted diffraction light. I can do it. In particular, the method using reflected diffraction light allows all optical elements to be placed on one side of the diffraction grating or the moving or rotating object, which has advantages in terms of device configuration depending on the use of the encoder.

<Effects of the Invention> As described above, the encoder according to the present invention is a high-resolution encoder that requires only a small number of device parts, which makes a particularly large contribution to cost reduction, and achieves desired light splitting using a simple method. It is a device that can

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of an encoder according to the present invention. FIG. 2 is a functional explanatory diagram of the light splitting plane in FIG. 1. FIG. 3 is a schematic diagram showing a conventional example of an encoder. 1...Laser, 2...Collimator lens, 3...
...Optical component, 4... Light splitting surface, 5, 7... Reflector, 6... Radiation grating, 8, 10, 12... 1/4 wavelength plate, 9, 11... Cat's eye optical system, 13...
...Light splitter, 14, 16...Polarizing plate, 15, 17
... Light receiving means, M ₁ , M ₂ ... Light beam incidence position on the radiation grating.

Claims (1)

[Scope of Claims] 1. In an encoder that detects the rotational state of a disk on which a radial diffraction grating is formed, a semiconductor laser that emits linearly polarized light with a wavelength λ, a polarization beam splitter, and a straight line emitted by the semiconductor laser directing means for directing the polarized light toward the polarized beam splitter, the semiconductor laser and the directing means are configured so that the polarization direction of the linearly polarized light when the linearly polarized light is incident on the polarized beam splitter is the polarized light. arranged at an angle of 45 degrees to the orthogonal axis of the orthogonal polarization plane of the beam splitter;
A pair of polarized lights generated by splitting the linearly polarized light by the polarizing beam splitter are split into two polarized lights that are substantially symmetrical about the center of the disc of the radial diffraction grating.
An input means provided for each polarized light of the pair of polarized lights to make each of the first-order diffracted lights from the two locations of the radial diffraction grating enter the said one.
A cat's-eye optical system is provided for each of the first-order diffracted lights so that the second-order diffracted lights are returned to the locations where the next-order diffracted lights are generated, and the first-order re-diffracted lights generated from the two locations are incident on the polarizing beam splitter via the input means. each of the input means or each of the cat's eye optical systems determines the polarization direction of the first-order re-diffracted light to be incident on the polarization beam splitter with respect to the polarization direction of the polarized light from the polarization beam splitter. λ/4 plates arranged orthogonally to each other are provided;
An encoder comprising means for forming interference light by causing subsequent re-diffracted lights to interfere with each other and converting the interference light into a signal indicating the rotational state of the disc. 2. The encoder according to claim 1, wherein the cat's eye optical system includes a lens and a reflecting mirror provided approximately at the focal point of the lens. 3. The encoder according to claim 2, wherein the cat's-eye optical system includes a gradient index lens whose both ends are flat, and a reflective film provided on one end surface of the lens.