JPS63215917A - Optical type displacement detector - Google Patents

Abstract

PURPOSE:To obtain an optically dividable displacement detector regardless of a larger set grating pitch, by multiplying a specific length by a specified natural number to obtain a length as the pitch of the grating on a reference scale. CONSTITUTION:A reference scale 18 or the like is provided with a grating 20 containing a high frequency which is formed with a grating pitch Q as length nMr (M=(u+v)/u) obtained by multiplying a length (u+v)r/u (u: distance from diffusion light source, r: fundamental pitch) by a natural number (n) differing from a natural number (m) and positioned at the grating interval (v) from a grating 16. Then, by utilizing a geometric image of the grating 16, a better S/N (amplitude component/DC component) of the detection signal is obtained when the grating interval (v) is nearly integer k multiples of a value (u+v)r<2>(lambdau) (lambda: wavelength of diffusion light source).

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an optical displacement detector, and in particular, the relative position of two members is determined by the relative displacement between a main scale on which an optical grating is formed and a reference scale on which a corresponding optical grating is formed. This invention relates to improvements in optical displacement detectors that detect changes in photoelectric conversion signals that occur.

[Conventional technology]

When measuring the feed rate of a machine tool stage, etc.
A main scale provided with a first grating is fixed on one of the relatively moving members, and a slider including a reference scale provided with a second grating, an illumination system, and a light receiving element is fixed on the other member. Then, the change in the amount of light from the illumination system caused by the relative movement of the first and second gratings is photoelectrically converted, and the resulting signal is converted into pulses and counted by an attached counting circuit, thereby measuring the amount of relative movement. This is contradictory to the optical displacement measuring device. In such a measuring device, it is required to improve the processing accuracy and to further subdivide the measurement resolution. In the past, much research has been focused on increasing the number of interpolations in the attached counting circuit, but another solution is to increase the number of interpolations in the optical displacement detector that generates the detection signal. Detectors that use so-called optical division, which output detection signals with pitches obtained by optically subdividing the grating pitch of the main scale, are being researched. The present applicant also has Japanese Patent Application No. 61-208554 and Japanese Patent Application No. 61-20855.
-208555, proposes a detector that performs optical splitting. As shown in FIG. 9, there is a point light source (laser diode) 10 as a diffused light source that generates coherent illumination light, the distance from the diffused light source is U, and the basic pitch r is a natural number m. The length obtained by multiplying 1
The first grating 1 containing harmonic components, where r is the grating pitch P.
6 is formed on the main scale 14, and the first grating 1
The lattice spacing from 6 is at the position of V, and the length is u + v
) When the reference scale 18 on which the second grating 20 with a grating pitch Q of r/u (=Mr) is formed and both scales 14 and 18 are moved relative to each other in the X direction, the first and second It includes a light receiving element 22 for detecting a change in the amount of illumination light due to the overlapping of the gratings 16 and 20, and a preamplifier 24 thereof, and is capable of generating a detection signal with a pitch equal to the basic pitch r (=P/n). is being used. Therefore, the first grating 16 bits P on the main scale 14
can be optically divided into ■. Furthermore, since the image of the first grating 16 is enlarged with a diffused light source, the pitch Q of the second grating 20 on the reference scale 18 can also be set larger than the basic pitch.

[Problems to be solved by the invention]

However, in the conventional detector shown in FIG. 9, if distance 1 = grating interval V, then the pitch Q of the second grating 20 on the reference scale 18 is
However, if you want to further increase the number of optical divisions, you must also reduce the grating pitch Q of the second grating 2° on the reference scale 18, which allows for finer resolution. There was a problem in that a processing technique was required, making it difficult to manufacture the second lattice 2o. This is not an essential problem for the reference scale 18, which does not need to have a length that spans the entire measurement range like the main scale 14, and only needs a few millimeters at most, but as the number of divisions increases, Yield deteriorates. Further, since there are problems such as an increase in production cost due to the need for direct drawing with an electron beam or the like, it is desirable that the grating pitch Q of the second grating 20 is also as large as possible.

[Purpose of the invention]

The present invention has been made to solve the above-mentioned conventional problems, and in an optical displacement detector using a diffused light source, even if the grating pitch of the second grating on the reference scale is set larger, the optical The object of the present invention is to provide a divisible optical displacement detector.

[Means to solve the problem]

The present invention provides an optical displacement detector including a diffused light source that generates coherent illumination light, a distance U from the diffused light source, and a length obtained by multiplying the basic pitch by a natural number. is the grating pitch, the main scale on which the first grating containing harmonic components is formed, and the grating interval from the first grating is located at V, and the length is u+v)r/u and the natural number is A second grating containing harmonic components is formed with a length obtained by multiplying different natural numbers as a grating pitch, and a reference scale and the overlap of the first and second gratings when both scales move relative to each other. The object is achieved by generating a detection signal with a pitch equal to the basic pitch. Both the first and second gratings containing the harmonic components have a ratio of bright areas to dark areas of approximately 1:1.
It is a value grid. Further, in another embodiment of the present invention, the second grating including the harmonic component has a length of the bright part or the dark part of approximately (u + v) r
/(2u) binary lattice. Further, in another embodiment of the present invention, the lattice spacing V is set to (U + V )r2/(u
λ) is approximately an integer multiple of λ). In another embodiment of the present invention, the grating pitch of the first grating is an even multiple of the basic pitch.Furthermore, in another embodiment of the present invention, the diffused light source is In another embodiment of the present invention, the diffused light source is a line light source.

[Effect]

The basic principle of the present invention will be explained below with reference to FIGS. 1 to 3. Now, as shown in Fig. 1, the first grating 16 (Fig. 1 (A)
) is irradiated with a parallel beam of wavelength λ. Then, the intensity distribution of light immediately after this first grating 16 F (x
) becomes a binary function as shown in FIG. 1(B). When Fourier analysis is performed on this interval number F(x), the following equation is obtained excluding the constant. F(x)+1/2+0.65in(2rx/P
)+0.25in(3-2πx/P) +0.1 sin<5-2xx/P)+...
......(1) Therefore, the function F (x
) contains, in addition to the fundamental wave (Fig. 1 (C)), the 3rd order wave (Fig. 1 (D)), the 5th order wave (Fig. 1 (E)), etc. . In this case, even-order waves are not included, but since the light-transmitting part and the light-blocking part are generally not exactly one-to-one, even-order waves are also included as harmonics. You can think about it. In the applicant's previous application, optical division was achieved by selectively using a second grating to selectively use the P-wave of an enlarged image of one of these harmonics from a diffused light source, but the grating pitch of the second grating teeth,
It was exactly the same as the pitch of the enlarged image. In contrast, in the present invention, harmonic components are also used in the second grating. That is, taking the case of optical division into five as an example, as shown in FIG.
iIlage) pitch Mr (M is the magnification M-
(u + v )/u ) matches the pitch of the third harmonic, not the first harmonic, of the second grating 20, for example.
If the grating pitch Q of the grating 20 and the grating pitch P of the first grating 16 are set as shown in the following equation, optical division into five can be performed. P=5r, Q=3Mr −・−−−−−−<
2) This can be done by considering the geometrically enlarged image of the fifth-order Fourier component of the first grating 16 as a P wave with the third-order Fourier component of the second grating 20. In this case, the first grating 16
The enlarged image of the 10th harmonic and the 6th harmonic of the second grating 20 also match, but the signal is so small that it can be ignored. Further, the conditions for the grating interval V in which the S/N ratio of the detection signal in this case is good, that is, the harmonic enlarged image of the first grating 16 is a signal with good contrast, are as follows. vcMv(k)=Mr2/λ(k=1.2
．．・ ・ ・ ) ・・・・・・・・・ (3
) This can be explained by the theory of Fresnel diffraction. FIG. 3 illustrates the principle of another embodiment of the invention. In the embodiment shown in FIG. 2, a geometric image is used as the enlarged image, but in this embodiment, a diffraction effect image (diHracti) with a pitch of 1/2 is superimposed on the enlarged image.
ve il'Iage). For example, when performing optical division into 10 using this diffraction effect image, the distance from the diffused light source 10 is U, as shown in FIG. For pitch 2'', the pitch of the diffraction effect image (+ 2 (x)) at the position where the grating interval from the first grating 16 is V becomes the pitch of the third-order wave of the second grating 20, using the following formula. Set the grating pitches P and Q according to the relationship: P=5. 2r = 10r −−・−
(4)Q=3-Mr=3(u+v)r/v
・(5) In this case, if we consider that the enlarged image (pitch is 1/2) due to diffraction of the 5th-order Fourier component of the first grating 16 is a P wave with the 3rd-order Fourier component of the second grating 20. Good. In this case, the geometrically enlarged image is also superimposed, but the detection signal only increases the DC component, so there is no problem. Furthermore, in the range where the grating spacing V is larger than r2/λ, diffraction occurs. Since the effect image exists almost constantly, the S/N ratio of the obtained detection signal is also almost constant regardless of the grating spacing (2), and theoretically the grating spacing V is not limited by the wavelength λ. As explained above, in the present invention, the second grating 20 on the reference scale 18 also uses harmonic components to convert the enlarged image of the harmonic components of the first grating 16 into P waves. The grating pitch Q can be set larger than that previously proposed by the applicant. Moreover, when both the first grating and the second grating containing the harmonic components are binary gratings with a ratio of bright portions to dark portions of approximately 1:1, the gratings can be manufactured easily. Further, when the second grating containing the harmonic components is a binary grating with a bright part or dark part length of approximately (u+v)r/(2u), the S/N ratio of the signal is good. be. Furthermore, the S/N ratio of the signal is good even when the lattice spacing V is approximately an integer multiple of (u 10v )r ''/(tlλ), where λ is the effective wavelength of the illumination light. Also, the grating pitch of the first grating is the basic pitch r
When it is set to an even multiple of , the restriction on the grid spacing is relaxed. Moreover, when the diffused light source is a point light source, it is inexpensive. Furthermore, when the diffused light source is a line light source, the level of the detection signal can be increased.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. As shown in FIG. 4, the first embodiment of the present invention includes a laser diode 12 as a diffused light source (point light source) that generates coherent illumination light, and a distance U from the laser diode 12, The length l" obtained by multiplying the basic pitch r by a natural number 1 is set as the grating pitch P, and the grating interval from the main scale 14 where the first grating 16 containing harmonic components is formed and the first grating 16 is Located in the V position,
The length nMr (M
-(u + v a light receiving element 22 that detects a change in the amount of illumination light from the laser diode 12 due to the overlapping of the first and second gratings 16 and 20; and a preamplifier 2 thereof.
4. In both the first grating 16 and the second grating 20, the ratio of the light transmitting part to the light blocking part is not exactly 1:1, but approximately 1:1,
It is a binary lattice formed to include all harmonic components. In this first embodiment, the expanded geometrical image of the first harmonic of the first grating 16 and the zeroth harmonic of the second grating 20 interact to obtain a detection signal of pitch r. . Therefore, the pitch P (
= l r ) can be optically divided into one, and the pitch Q (= n Mr ) of the second grating 20 is n
It has doubled. In this first embodiment, since the geometrical image of the first grating 16 is used, the grating interval V is an integer (u+v)r2/(λU), as shown by the solid line A in FIG. k ) times, the S/N ratio (amplitude component/DC component ratio) of the detection signal becomes good. Next, a second embodiment of the present invention will be described in detail with reference to FIG. This second embodiment has the same grating pitches P and Q as the first embodiment.
The main scale 30 is a reflection type scale in which a first grating 32 consisting of a light transmission part or a light absorption part and a reflection part is formed. This is different from the first embodiment. The other configurations and effects of the present invention are the same as those of the first embodiment, so their explanation will be omitted. Next, a third embodiment of the present invention will be described in detail with reference to FIG. The arrangement of the third embodiment is the same as that of the first embodiment, but in the first grating 16 containing harmonic components, the length P2 of the light transmitting part and the length Pl of the light shielding part are changed significantly (1 ≠P2)
, the harmonic components are made stronger, and the length R of the light transmitting part of the second grating 20 containing the harmonic components is "/2".
(u + v)/IJ (=M) times, 2
This embodiment differs from the first embodiment in that the value lattice is set so that the n-th order 7:S harmonic component is maximized. In this third embodiment, the primary wave (g
The enlarged image of eoIletric image ) and the n-th order wave of the second grating 20 act to change the pitch r (=P/n
) detection signal is obtained. In particular, the second grating 20 has the largest n-th harmonic component, and the S/N ratio of the detection signal is good. In this third embodiment as well, since the geometrical image of the first grating 16 is utilized, the grating spacing V is close to an integral multiple of M2/λ, as shown by the solid line A in FIG. When , the S/N ratio of the detection signal becomes good. Next, a fourth embodiment of the present invention will be explained in detail with reference to FIG. This is the same as in the first embodiment, but the pitch P of the first grating 16 is 211 (
(even number) times, and the pitch Q of the second grating 20 is
The difference from the first embodiment is that the fundamental pitch is n (u + r) 7u (=nM) times. In this fourth embodiment, the first harmonic of the first grating 16 is expanded. Diffraction effect image (actually an enlarged image of the 2n+ order wave)
and the n-th order wave of the second grating 20, the pitch r
(=P/21) detection signal is obtained, so the optical 2
One division is performed, and the pitch of the second grating 20 may be n times that of the previous application. The S/N ratio of the detection signal with respect to the grid spacing V is as shown by the broken line B in FIG.
If it is larger than 2/λ, there is almost no limit. In each of the above embodiments, binary gratings consisting of only bright and dark areas were used as the first grating 16, 32 and the second grating 20 containing harmonic components, so the fabrication of the gratings was simplified. It's easy. Note that the configuration of the grating including harmonic components is not limited to this, and a grating having intermediate brightness or a phase grating may also be used. Further, in this embodiment, since the laser diode 12, which is a point light source, is used as the diffused light source 10, the light source can be configured simply and at low cost. The types of diffused light sources are:
The present invention is not limited to this, and for example, a light source that emits light in a linear manner may be arranged in the width direction of the grating. In this case, the level of the detection signal increases and subsequent processing becomes easier. Further, in each of the above embodiments, only one second grating 20 is illustrated, but when actually configuring a detector, the second grating is set at a pitch of 90° with a basic pitch of 360°. By providing phase-shifted gratings and adding corresponding light-receiving elements, two-phase detection signals with a 90° phase difference can be obtained and used for determining the direction of relative movement, etc.

【Effect of the invention】

As explained above, according to the present invention, in an optical displacement detector using a diffused light source, optical division can be performed even if the grating pitch of the second grating on the reference scale is set larger. . Therefore, there is no need to make the grating pitch of the second grating on the reference scale very small, and there are excellent effects such as improved yield and lower production costs.

[Brief explanation of the drawing]

1 to 3 are diagrams for explaining the principle of the optical displacement detector according to the present invention, and FIG. 4 is a diagram showing the configuration of the first embodiment of the optical displacement detector according to the present invention. Cross-sectional view shown, No. 5
The figure is a diagram showing an example of the relationship between the grid spacing and the S/N ratio of the detection signal in the embodiment, FIG. 6 is a sectional view showing the configuration of the second embodiment of the present invention, and FIG. 8 is a cross-sectional view showing the structure of the fourth embodiment of the invention, and FIG. 9 is a sectional view showing the structure of the third embodiment of the invention.
532 is a cross-sectional view showing the configuration of an optical displacement detector proposed in No. 532. 10... Diffuse light source, 12... Laser diode (point light source), λ... Effective wavelength, 14.30... Main school, 16.32... First grating, Su... Distance, P, Q...Grating pitch, ``...Basic pitch, 1, n...Natural number, 18...Reference scale, 20...Second grating, ■...Grid spacing, 22... Light receiving element.

Claims (7)

[Claims] (1) A diffused light source that generates coherent illumination light, and a distance u from the diffused light source, and a basic pitch r
With the length obtained by multiplying by a natural number as the grating pitch, there is a main scale on which a first grating containing harmonic components is formed, a grating interval from the first grating is at a position v, and the length ( u+v)r/u multiplied by a natural number different from the natural number described above, and a second grating containing a harmonic component is formed, with the grating pitch being the length obtained by multiplying r/u by a natural number different from the above natural number, and when the reference scale and both scales move relative to each other, An optical displacement detector, comprising: a light receiving element that detects a change in the amount of illumination light due to the overlapping of the first and second gratings, and generates a detection signal with a pitch equal to the basic pitch r. (2) The optical displacement detection according to claim 1, wherein the first and second gratings containing harmonic components are both binary gratings with a ratio of bright areas to dark areas of approximately 1:1. vessel. (3) The optical displacement according to claim 1, wherein the second grating including the harmonic component is a binary grating with a length of a bright portion or a dark portion of approximately (u+v)r/(2u). Detector. (4) Any one of claims 1 to 3, wherein the lattice spacing v is approximately an integral multiple of (u+v)r^2/(uλ), where λ is the effective wavelength of illumination light. The optical displacement detector described in . (5) Claims 1 to 3, wherein the grating pitch of the first grating is an even multiple of the basic pitch r.
The optical displacement detector according to any one of paragraphs. (6) The optical displacement detector according to any one of claims 1 to 5, wherein the diffused light source is a point light source. (7) The optical displacement detector according to any one of claims 1 to 5, wherein the diffused light source is a line light source.