JPH02231526A - Encoder - Google Patents

Abstract

PURPOSE:To easily obtain high resolution and to enable detection with high accuracy without making the grating pitch of a diffraction grating so fine by utilizing a diffracting means where plural diffraction gratings which are equal or different in pitch are laminated. CONSTITUTION:The luminous flux from a laser 1 is passed through a collimator lens 2 and split by a polarization beam splitter(PBS) 3 into transmitted P- polarized light and reflected S-polarized light, which are made incident on the diffracting means 100 coupled with a moving body. The P-polarized or S-polarized light is diffracted by the diffraction grating 102a to generate diffracted light beams of +1st order and -1st order, which are polarized circularly through lambda/4 plates 42 and 41 and reflected by reflecting members 52 and 51 to travel backward; and they are put one over the other by a PBS 3, passed through a lambda/4 plate 6 and split into two by a BS 7, and made incident on photodetectors 91 and 92 through polarizing plates 81 and 82. The movement state of a body coupled with the diffracting means 100 is detected from their detection signals.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an encoder, and in particular to an encoder, in which a coherent light beam such as a laser beam is incident on a diffraction grating attached to a moving object, and diffracted light of a predetermined order is emitted from the diffraction grating. This relates to encoders such as rotary encoders and linear encoders that measure the amount of movement of a diffraction grating, that is, the amount of movement of a moving object, by interfering with each other to form interference fringes and counting the bright and dark fringes of the interference fringes. ．． (Prior Art) Encoders have long been used as measuring instruments that can measure the amount and direction of movement of moving objects with high precision, for example in submicron units, and are used in various fields. In particular, a coherent beam of light such as a laser is incident on a diffraction grating provided on a moving object, the diffracted lights of a predetermined order generated from the diffraction grating are made to interfere with each other, and the brightness and darkness of the interference fringes are counted to determine the amount of movement of the moving object. Encoders that determine the state of movement, such as direction and movement direction, are well known. The present applicant has proposed such an encoder, for example, in Japanese Patent Application Laid-open No. 62
This method has been proposed in Japanese Patent Application Laid-Open No. 163926, Japanese Patent Application Laid-Open No. 163924-1982, and Japanese Patent Application Laid-Open No. 200225-1982. (Problems to be Solved by the Invention) When the wavelength used in such an encoder is in the visible or infrared region, one way to improve measurement accuracy is to use a diffraction grating with a fine grating pitch. However, it is generally very difficult to form a diffraction grating with a fine grating pitch, and for example, when using an electron beam lithography system, there is a problem that a stable line width cannot be obtained if the line width is set to 1 μm or less. ．． Furthermore, if holography is used, it is relatively easy to obtain a grating pitch with a line width of 1 μm or less, but there is a problem in that it is difficult to form the grating pitch with high precision. In particular, there was a problem in that it was extremely difficult to form a fine radial diffraction grating on a disc with high precision in rotary encoders and the like. The present invention provides an encoder that uses a diffraction means in which a plurality of diffraction gratings having a predetermined grating pitch are laminated, whereby high resolution can be easily obtained without making the grating pitch of the diffraction gratings too fine, and high-precision detection can be performed. The purpose is to provide. (Means for Solving the Problems) The encoder according to the present invention makes a light beam incident on a diffraction means connected to a moving object, forms an interference light from diffracted light of a predetermined order from the diffraction means, and forms an interference light of the interference light. When detecting the moving state of the moving object by detecting brightness and darkness with a detection means,
The diffraction means is characterized by being constructed by laminating a plurality of diffraction gratings with a predetermined grating pitch. (Embodiment) FIG. 1(A) is a schematic diagram of the main part of a first embodiment when the present invention is applied to a linear encoder. In the same figure, 1
is a laser, and 2 is a collimator lens, which converts the light beam from laser 1 into a parallel light beam. 3 is a polarizing beam splitter which converts the parallel light beam from the collimator lens 2 into P
It is split into two beams: polarized light and S-polarized light. +00 is a diffraction means connected to a moving object (not shown). The diffraction means 100 includes a diffraction grating 1 having a predetermined grating pitch.
02a. A plurality of diffraction scale plates forming 102b are constructed by laminating two diffraction scale plates +01a and IOlb in the figure, and are moving together with a moving object at a speed V in the direction of the arrow, for example. In this example, there are two diffraction scales tentatively +01a. The grating bit 4 of the diffraction grating formed in lolb is the same. 4+. 4m is a 1/4 wavelength plate that converts linearly polarized light into circularly polarized light or vice versa. 5. ．． 5. is a reflective member, and 1/4 wavelength plate 4=. The light flux from 4t is returned to its original optical path. 6 is a quarter-wave plate, which converts linearly polarized light into circularly polarized light. 7 is a beam splitter that splits the incident light beam into two light beams. 8. ．． 8. are polarizing plates, g, . 9g
is the photodetector. In this embodiment, the light beam from the laser 1 is made into a substantially parallel light beam by the collimator lens 2, the P-polarized light is passed through the polarizing beam splitter 3, and the S-polarized light is reflected to be split into two light beams, and each of these light beams is divided into two light beams. The beam is made incident on the diffraction means 100 at an angle. The +1st-order diffraction of the P-polarized light is performed by the diffraction grating 102a, and the +1st-order diffraction is performed by the diffraction grating 102b.The light beam is passed through the 1/4 wavelength plate 48 and reflected by the reflection member 5 as circularly polarized light to return the original light. Return to the optical path and return to the 174 wavelength plate 4. As a linearly polarized light whose polarization direction is 90 degrees different from the initial linearly polarized state, +1st order diffraction is performed again by the diffraction grating 102b, and +1st order is diffracted by the diffraction grating 102b.
The light beam that has undergone first-order diffraction (this means that it has undergone +1st-order diffraction four times in total during the round trip) is sent to polarizing beam splitter 3.
It is input to. This time, since the polarization direction is 90 degrees different from before, it is reflected by the polarizing beam splitter 3 and guided into a 174-wavelength beam 6. On the other hand, the polarized beam splitter 3 is reflected, and the diffraction means 10
Of the S-polarized light incident on the zero beam, the -1st order diffracted by the diffraction grating 102a and the -1st order diffracted by the diffraction grating 102b are treated as 1/4 wavelength in the same way as in the case of the P-polarized light described above4. Return to the original optical path via the reflection member 51 and redirect to the diffraction grating 10
2b and 102a, the polarized light is converted into linearly polarized light whose polarization direction is 90 degrees different from the initial linearly polarized state, and then passed through the polarization beam splitter 3 to be transferred to the 1/4 wavelength plate. ．． The light is guided to 6. Since each of the two beams guided to the quarter-wave plate 6 undergoes first-order diffraction four times, the phase of the wavefront of 2 x 4 radians corresponds to the movement of the diffraction means lOO by one pitch of the diffraction grating. is shifted. In other words, the light beam that undergoes +1st-order diffraction four times is +2π
×4=+81, and the phase advances by 8 times. On the other hand, one! The light beam that undergoes the next diffraction four times becomes -2πX4=-8π and becomes 8π
The phase of is delayed. Therefore, the phase difference between the two beams is determined by the diffraction means 1
16 per 00 move l bitch? It becomes C. In the figure, since the two beams guided to the quarter-wave plate 6 are linearly polarized beams that are perpendicular to each other, they do not interfere with each other and no bright/dark signal can be obtained. Therefore, circularly polarized light that rotates in opposite directions is superimposed on each other via a quarter-wave plate 6, resulting in linearly polarized light whose linear polarization direction changes depending on the phase difference between the two light beams. And polarizing plate 8+. The light and dark signals based on the interference of the two overlapping lights are detected by the light receiving elements 9- and 9-9 through the 8m distance, and the light and dark signals are detected by the light receiving elements 9- and 9-9. In this embodiment, eight cycles of brightness and darkness signals are obtained while the phase difference between the two beams is 167t. In this way, in this embodiment, the moving state of the diffraction means l00 is detected using the output signal from the detection means. In this embodiment, instead of using two diffraction scale plates, marks shown in FIG.
B), each of the diffraction gratings 102a. A similar effect can be obtained by using a material formed with l02b. In this example, the case where two diffraction scale plates are used is shown, but if more diffraction scale plates are used, the phase difference between the two light beams with respect to the movement of the diffraction means +00 per pitch will increase accordingly, so the detection accuracy will be improved. Towards. FIG. 2 is a schematic diagram of a main part of a second practical example in which the diffraction means 100 of FIG. 1 is constructed by laminating five diffraction gratings. The measurement principle as a linear encoder is the same as that shown in Figure 1. Elements that are the same as those shown in Figure 1 are given the same numbers. In general, is the light similar to that shown in Figure 1 obtained by stacking n diffraction gratings? When using this device, the phase shift between the two beams due to the movement of the diffraction scale plate is 8πX 2 I+ − 1 per l bit.
radian, 4 x 2 m per pitch of the diffraction grating
− 1 = 2 M − 1 cycle of bright and dark signals appears. For example, if you stack 5 diffraction gratings with 1 pitch 12.8 μm, you will get a bright/dark signal with 2"' = 64 periods per 1 pitch 12.8 μm. Therefore, the movement position of the diffraction grating per period is 12.8 /64=0.05μm.Increasing the number of diffraction cycles generally reduces the amount of light.For this purpose, for example, a phase grating or a holographic grating with a rectangular or triangular cross-sectional shape of the diffraction grating can be used to obtain the desired amount of light. This is preferable because the diffracted light of the order can be obtained efficiently. Figure 3 is a schematic diagram of the main parts of the third embodiment when the present invention is applied to a rotary encoder. In the same figure, the elements are the same as those in Figure 1. The elements are given the same reference numerals. In the figure, 300 is a diffraction means connected to a rotating object (not shown), and a diffraction grating with a predetermined grating pitch is placed around the disk on its surface. Two diffraction scale plates 30 formed
1a. It consists of 30lb. In this example, the diffraction scale f<301a, 30
The grating pitch of the diffraction grating on the lb is the same. 33 are two trapezoidal prisms 33. ．． The optical component 331 is bonded together, and 34 is a light splitting surface, which is made up of the bonded surface of the optical component 33, and performs light splitting with the same function as a polarizing beam splitter. Reference numerals 35 and 37 each have a prism reflecting mirror that reflects the light beam in a predetermined direction. M.I. M imaginary indicates the point of incidence of the light beam onto the diffraction grating provided around the diffraction scale plate 301a. In this embodiment, the light beam emitted from the light source 1 is made into a parallel light beam by the collimator lens 2, and the trapezoidal prism 33.
After being reflected by the slope of the beam, the beam is directed so that it is incident on the beam splitting surface 34 at a predetermined angle. The light beam incident on the light splitting surface 34 is split into two linearly polarized light beams, a reflected light beam and a transmitted light beam, at a ratio of approximately I.1. The light beam from the light source 1 is set to be linearly polarized in a predetermined direction (usually 45''') with respect to the orthogonal polarization plane of the light splitting surface 34. It is reflected twice, exits the optical component 33, and enters the predetermined positions m and M of the temporary diffraction scale 301a at a predetermined angle of incidence by the prism reflection fJ135 or fJ37. Among them, the diffracted light of a specific order is further transmitted to the diffraction grating 302b.
After diffracting the light of a specific order, a 174-wavelength plate 4l
or 4, the light is reflected by the reflection stage 5l or 58, and the same optical path is made to travel backwards. And a diffraction grating 302b,
The diffracted light of a specific order that has been sequentially diffracted by 302a is reflected by a prism reflecting mirror 35 or 37 to reverse the same optical path,
The light is repeatedly internally reflected by the light component 33 and guided to the light splitter 34. Since the light flux here passes through the 174 wavelength plate twice before and after being reflected by the reflection means 51 or 5, the polarization direction differs by 90 degrees from that before entering the diffraction means 300. Therefore, the light beam that was previously on the reflection side is now transmitted through the light splitting surface 34, and the light beam that was on the transmission side is now reflected and overlapped to form interference fringes, which are internally reflected by the optical component 33 and reflected at 1/4 The light is incident on the wave plate 6. The light beams passing through the quarter-wave plate 6 have the same polarization. Then, as in the first embodiment shown in FIG. 1, the light beam passing through the 1/4 wavelength temporary 6 is divided into two parts by the light splitter "a7", and is linearly polarized through the polarizing plates 8 or 8, each of which is arranged with a different polarization direction. As a result, the light receiving element 9. or 9t receives the intensity of the interference of the two light beams, and obtains a two-phase signal having a phase difference according to the orientation of the polarizing plates 8I and 8. As in the first embodiment shown in Fig. 1, when the rotating object to be measured rotates by one pitch of the grating pitch of the diffraction grating, the phase of the m-th order diffracted light changes by 8mπ.Similarly, the phase of the -m-th order diffracted light changes by 8mπ. The phase changes by -8mπ.As a result, 8m sine waveforms are obtained as a whole from the light receiving elements 9..9.In this embodiment, the rotational speed of the diffraction means 300 is determined by detecting the sine waveforms at this time. In this embodiment, the light beam is divided into two by the light splitter 7, 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.In the present invention If the pitches P of the diffraction gratings provided on the n diffraction scale plates used in the diffraction means are all equal, a periodic signal of 1 pitch pi92'' and 1 period will be extracted as described above, but if the pitch of each diffraction grating is different. It may be configured more. For example, using two diffraction scale plates, the diffraction grating 102 of the diffraction scale plate 101a on which the light beam first enters
P+, then input diffraction scale plate 10l
If the pitch of the diffraction grating 102b in b is 22, the amount of movement of the diffraction hand is . The first diffraction grating 102a causes the phase of the first-order diffracted light to fluctuate by 2π x sentence/p + (in the case of one-way transmission). In the second diffraction grating 102b, further:
2π×sentence/P Changes by 2 (in case of one-way trip). If you go back and forth, each will be doubled, so a total of 4πχ sentences / (
The phase changes by 1/p+ +1/ps).
-1st order diffraction light is -2π by the first diffraction grating]02a
The phase fluctuates by X l / p + t, and the phase further fluctuates by -2π
The phase changes by 1/p+ + 1/pt). Therefore, the difference between the 1st order diffracted light and the comrades! ) The signal is a periodic brightness signal obtained by the moving intersecting of the diffraction scale plate (diffraction means). For example, pitch p,=4#Lm. If a diffraction grating of p* -2 m is used, it is possible to obtain a periodic signal with a pitch that cannot be expressed as a decimal number or the amount of movement of the diffraction means per period. Generally, the number of diffraction scale plates is n and the pitch of each diffraction grating is P+. P.O. ．． ．． ．． p. Then, for the movement i sentence of the diffraction means, the sentence (2.2ya.,.ya2) 1'+ 9m I'
A sinusoidal signal with +1 period is obtained. Such a diffraction means combining a plurality of diffraction scale plates can be obtained by forming diffraction gratings with various grating pitches on both sides of a glass plate, or by combining diffraction scale plates consisting of a plurality of diffraction gratings with different grating pitches into one piece. be able to. Furthermore, the present invention is based on the principle of accumulating the phase shifts that occur when a light beam passes through a diffraction grating multiple times and being diffracted by each diffraction grating to obtain a large total phase shift.
If the optical configuration is such that a signal is obtained by interfering with two types of light (for example, ±1st-order diffracted light) among the diffracted lights obtained by passing through a plurality of diffraction gratings, the encoder that is the object of the present invention can be achieved. can. FIGS. 4 to 7 are schematic diagrams of main parts of fourth to seventh embodiments when the present invention is applied to a linear encoder, respectively.

In each of these embodiments, the same elements as those shown in FIGS. 1 to 3 are given the same reference numerals, and the method of detecting a moving object as a linear encoder is basically the same as the first embodiment shown in FIG. are essentially the same. In the fourth embodiment shown in FIG.
The light beam from the 174-wave plate 4, . Diffraction gratings 402a, 402 formed on the front and back surfaces of the diffraction scale plate 401a of the diffraction means 400 by passing through the diffraction gratings 402a, 402
The beams are sequentially incident on b. Then, the ±1st order diffraction points diffracted by the 1,000 diffraction stages 400 are emitted vertically and overlappingly from the diffraction means 400, are reflected by the common reflection member 5, return to the original optical path, and return to the diffraction grating 402b. After diffraction at 402a, quarter wavelength plates 4, . 4. ．． 17 via optical component 33
The light is guided to a four-wavelength plate 6. Then, as in the embodiment shown in FIG. 1, the beam splitting means 7 divides the beam into two beams, and these interference beams are sent to the light receiving elements 9, . The light is being received by 'l. and light receiving element 9+. The movement of the diffraction means 400 is detected using the output signal from the 9th foot. In the fifth embodiment shown in FIG. 5, compared to the embodiment shown in FIG.
It is constructed by stacking two diffraction scale plates 501a to 501e, and transmits two diffracted lights of a predetermined order from the diffraction means 500 to a dedicated 174-wavelength plate 41. 4ffi and reflective member 5
+. The difference is that the optical path is returned to the original optical path via 5g, but the other configurations are the same. In the sixth embodiment shown in FIG. 6, two diffraction gratings 102a. The ±1st-order diffracted light beams diffracted by L02b are superimposed using the half mirror surface 67a of the prism-type light splitter 67, and the overlapping light beams are passed through a polarizing plate 88 having a polarization plane in the 45° direction. It becomes an interference signal and enters the light receiving element 9, and the other overlapping beam becomes 1
/4 wavelength plate 6 changes the wavefront of one of the light beams to l/
8. A polarizing plate with a polarization plane in the 45° direction after being shifted by 4λ. The signal becomes an interference signal and enters the light receiving element 9l. In this example, the phase difference between the two beams detected by the light receiving element is 8π for one pitch movement of the diffraction grating. In FIG. 7, a beam from a laser I is passed through a collimator lens 2 to three diffraction scale plates 101a, 10lb. 101
10a. Interference light is obtained by superimposing two beams using the half mirror surface 77a of the rear beam splitter 77 via the lob. Then, in the same manner as in the embodiment shown in FIG. 6, polarization provisional 8+. 81
．． The interference signal is detected by the light receiving element 91.9m via the 1/4 wavelength plate 6. (Effects of the Invention) According to the present invention, by using a diffraction means in which a plurality of diffraction gratings with the same or different grating pitches are laminated as described above, movement can be made without using a diffraction grating with a fine grating pitch. An encoder that can effectively obtain a good periodic signal even for small displacements of an object and easily obtain high resolution can be achieved.

[Brief explanation of the drawing]

FIG. 1(A) and FIG. 2 are schematic diagrams of the main parts of the first and second embodiments when the present invention is applied to a linear encoder, and FIG.
B) is an explanatory diagram of a partial modification of FIG. 1(A), and FIG. 3 is a third diagram when the present invention is applied to a rotary encoder.
FIGS. 4 to 7 are schematic diagrams of the main parts of the fourth to seventh embodiments, respectively, when the present invention is applied to a linear encoder. In the figure, 1 is a laser, 2 is a collimator lens, 3 is a polarizing beam splitter +00.300, 40
0.500, 700 are diffraction means, 4, 4, . 4. ．． 6 is a quarter wavelength plate 25, 5+. 5m is a reflecting member, 7 is a beam splitter (light splitter). 8. ．． 8. is a polarizing plate, 9+
．． 9m is a light receiving element.

Claims (4)

[Claims] (1) Make the light beam incident on a diffraction means connected to a moving object,
When detecting the moving state of the moving object by forming interference light from diffracted light of a predetermined order from the diffraction means and detecting the brightness and darkness of the interference light with the detection means, the diffraction means is arranged at a predetermined grating pitch. An encoder characterized by comprising a plurality of laminated diffraction gratings. (2) The encoder according to claim 1, wherein the plurality of diffraction gratings have different grating pitches. (3) A light beam is made incident on a diffraction means constructed by laminating a plurality of diffraction gratings with different grating pitches, and interference occurs from the diffracted light of a predetermined order obtained when passing through the plurality of diffraction gratings of the diffraction means. An encoder characterized in that a moving state of a moving object connected to the diffraction means is detected by forming light and detecting the number of brightness and darkness of the interference light by a detection means. (4) The encoder according to claim 1 or 3, wherein the diffraction means has at least one diffraction scale plate having a diffraction grating on each of the front and back sides of a transparent substrate.