JPH0212017A - Detector for detecting position of averaged diffraction moire - Google Patents

Abstract

PURPOSE:To obtain a diffraction moire signal which is not influenced by the variation of intervals of each diffraction grating and sensible in lateral displacement so that highly accurate position detection can be performed by providing 1st and 2nd diffraction gratings and an adder. CONSTITUTION:The 1st diffraction grating 21 is set perpendicularly to laser beam LB and the 2nd diffraction grating 22 is obliquely arranged against the 1st grating 21. Rays of light which are diffracted to rays of light of plural orders by transmitting the gratings 21 and 22 are converged by means of a cylindrical lens 23 behind the grating 22. Then the converged 2nd-order diffracted rays of light L+2 and L-2 are respectively detected and converted into electric signals proportional to the light quantities by means of photoelectric conversion elements 24A and 24B. The converted electric signals I+2 and I-2 are added to each other and a displacement signals is obtained at an adder 25. Thus the error component of each signal is mutually compensated and an excellent displacement signal is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical linear encoder used for position measurement in machine tools and the like, and particularly to a position detector using moiré fringes formed by a diffraction grating.

(Prior art) Moiré fringes obtained by overlapping a set of two diffraction gratings have a relative change in the lateral direction of I'7Il, and the displacement can be counted and measured in delicate steps. It has been widely used as.

Two diffraction gratings (hereinafter, each will be referred to as a '4x grating).

Since the second grid (referred to as the second grid) is used by being attached to two relatively displaced parts of the machine, it is necessary to maintain an appropriate gap at all times. On the other hand, if the grating pitch of each of the above-mentioned diffraction gratings is decreased in order to increase the resolution of length measurement, the influence of the light diffraction effect increases. Therefore, the shadow of the first grating on the second grating becomes thinner due to the diffraction effect, making it impossible to obtain direct (area) fringes with high visibility. In other words, when the first grating is irradiated with a parallel light beam with a uniform phase, due to the diffraction effect of the light, a distance equal to twice the square of the pitch P of the grating divided by the wavelength is A brightness distribution of light with the same pitch as the grid is created at positions that are an integer multiple of the grid (a light distribution with an inverted brightness and darkness is created at positions that are several times the size), and this reproduced brightness distribution of light is called a Fourier image. If the second grating is placed at the position where this Fourier image is formed, the diffracted light from the second grating will have a clear contrast of period P with respect to the relative displacement of the two gratings in the lateral direction. This is called diffraction moiré.Using this principle,
The use of relatively short measuring distances, such as mask alignment in microfabrication in semiconductor manufacturing, is being studied (for example, J, VAC, SC1, TECIINOL,
15 (1978) (7) 984 pages TEC II
NOL 81 (1983), page 127δ).

On the other hand, if you try to increase the length measurement accuracy by increasing the measurement distance and decreasing the grating pitch P, the distance 2P27λ where the Fourier image is formed will rapidly decrease in proportion to the square of the grating pitch P, so it will be longer. It becomes difficult to accurately hold the two diffraction gratings in a gap where a Fourier image can be formed over a distance. If the grating gap deviates from the position where the Fourier image can be formed, the intensity of the diffracted light changes greatly, making it impossible to perform only "C" positioning.For example, if the grating pitch P is 1 μm and the wavelength λ is 0.633 μm, then , the grating gap Q is the diffraction grating gap G and the light wavelength λ
Divide the product by the square of the pitch P of the diffraction grating to give the Fresnel number (λ・G)/P'' -2 1.6μl
It must be kept within a sufficiently small variation. Therefore, diffraction moire cannot be used as a highly accurate length measurement method for general machine tools.

Considering these circumstances, there is a shadow on the change in the gap between the first and second gratings! A position detector that can perform highly accurate position detection by obtaining a diffractive Moray No. (See Publication No. 17016).

This detector obtains a signal corresponding to the average value of the diffraction moiré signal by changing the optical path length of the gap between each grating in each part of the effective facing area of the first grating and the second grating, and obtains a signal corresponding to the average value of the diffraction moiré signal. Position detection is performed using signal changes whose period is one-half of the pitch P of the diffraction grating that appears in the value.

4 to 6 are perspective views showing an example of the above-mentioned averaged diffraction moiré position detector, and the case where 0th order diffracted light is used will be described below.

In FIG. 4, first, the first grating 1 is irradiated with a laser beam 1.8, and a transparent plate 3 having stepped steps is attached on the second grating 2 placed behind the first grating 1. There is. The transparent plate 3 with steps has an optical gap G in the range G.
It is made by adding steps to a high refractive index material so that from o to G0+ 2P2/λ, and a transparent plate 3 with these steps.
This gives an optical path difference to each part of the laser beam LB.

The transparent plate 3 with steps shown in FIG. 4 divides the range of optical distance 2P2/λ into five parts, so it has a five-step stair-like structure. The l/lens group arranged one-dimensionally behind the second grating 2 converges the light beams that have passed through the iiQ regions having different optical distances divided into five parts in the second grating 2. The light condensed by the lens group 4 is separately detected by the group 5 to the first photo die. After that, an adder 7 composed of an operational amplifier etc.
The signals of the diode group 5 are added to obtain a displacement signal.

In FIG. 5, a first grating 1 and a second grating 2 are placed in parallel, and a random optical path difference plate 9 is attached to the second grating 2. This random optical path difference plate 9 is made of a transparent plate with unevenness so that the optical path difference of each portion of the laser beam LB is random within the range of 2P2/λ. Each portion of the laser beam 1B is separately focused on the diffuser plate 10 by the lens group 4, and the laser beams i4 are arranged so that the focal points of the lens group 4 are arranged in a line on the plate IO without overlapping. The condensed light flux of each portion becomes incoherent light by the diffuser plate lO. The light diffused by the diffusion plate 10 passes through a convex lens 11 and is detected by an optical sensor 12 such as a photodiode. Since the diffuser plate 10 is used, the light beams passing through different gap optical path lengths are averaged without mutually interfering with each other.

In FIG. 6, the first grating is placed perpendicular to the laser beam L8, and the second grating 2 is placed at an angle with respect to the first grating. 1.7, the gap between each diffraction grating 1 and 2 is 2p2/λ in the effective opposing area of each diffraction grating 1 and 2.
Adjust to include the range.

Among the lights transmitted through each diffraction grating 1 and 2, O-order diffracted light l,
. The light is detected by being incident on the light receiving surface of the photoelectric conversion element 13 located at the rear.

Moreover, FIG. 7 is a perspective view showing an example of an averaged diffraction moiré position detector using second-order diffraction light, corresponding to FIG. Adjust to include. In the case of the 2nd order diffracted light, unlike the 0th order diffracted light, the same brightness and darkness distribution occurs at positions that are integral multiples of p2/4λ, so the gap optical path length 2p2 is averaged by the 0th order diffracted light.
It is sufficient to average the gap optical path length of t7'a of /λ. Note that even if it is adjusted to include the same range of 2p2/λ as the O-order diffracted light, it corresponds to an integral multiple of p2/4λ, so the condition for the averaged gap optical path length using the second-order diffracted light is satisfied.

In this way, even if the second-order diffracted light (or diffracted light of other orders is fine) is used in a similar optical system, the first-order diffracted light
Highly accurate position detection can be performed without being affected by changes in the gap between the grating and the second grating.

(Problems to be Solved by the Invention) According to each of the above-mentioned averaged diffraction moiré position detectors, the first
Displacement where the amount of light I changes according to the relative displacement X of each diffraction grating without being affected by the change in the gap between the grating and the second grating, as shown in FIG. 8, and whose period is one half of the pitch P of the diffraction grating. I can get a signal. This displacement signal can be approximated by the following equation (1).

I (x)-8-cos (2yr・2x /
P) +8 -------・(1) However, A: Amplitude B: Offset componentHowever, the gap optical path length that should be averaged during mounting and assembly and operation, and the average path length during actual averaging. If an error occurs between the two or in other installation conditions, the resulting displacement signal may include an error component having a period equal to the pitch P of the diffraction grating or an odd-order error component. Soshi”C
1 When such an error component is included in the displacement signal, P/2
There is a problem in that the repeatability of the displacement signal is lost over a period, making it impossible to perform accurate position detection.

The present invention was made in view of the above-mentioned circumstances, and an object of the present invention is to obtain a diffraction moiré signal that is free from fP and snow due to gap changes in each diffraction grating and is sensitive to lateral displacement. It is an object of the present invention to provide an averaged diffraction moiré position detector which can perform highly accurate position detection by reducing the influence of errors occurring during mounting and assembly and during operation.

(Means for Solving the Problems) The present invention provides a first diffraction grating, a second diffraction grating that is laterally displaced with respect to the first diffraction grating, and a space between the two diffraction gratings. means for changing the optical path length of the gap between the two diffraction gratings over a range of optical path lengths corresponding to a Fresnel number of 2 or an integral multiple of 2 for each portion of the effective facing area of the two provided diffraction gratings; means for obtaining a signal corresponding to the average value of the diffraction moiré signal over a portion of the effective area of the two diffraction gratings, the signal having a period equal to one half of the pitch of the diffraction grating appearing at the 6N average value. The present invention relates to an averaged diffraction moiré position detector capable of detecting the relative displacement in the lateral direction of the diffraction grating with high precision by using the change, and the above object of the present invention is to The means for obtaining the signal to be
This is achieved by comprising an adding means that adds the respective light amounts of positive and negative diffracted light of the same order among the light transmitted through one diffraction grating and diffracted into a plurality of orders, or an electric signal proportional to each light amount, More specifically, the adding means photoelectrically converts the positive and negative diffracted lights of the same order together, or individually photoelectrically converts the positive and negative diffracted lights of the same order and then electrically adds them. .

(Function) The averaged diffraction moiré position detector of the present invention utilizes the fact that the phases of the respective error components included in the positive and negative diffracted lights of the same order are shifted, and the light amount of each diffracted light is added. By doing so, each error component cancels out each other, and a good displacement signal is output.

(Example) FIG. 1 is a perspective view showing an example of the averaged diffraction moire position detector of the present invention, and the case where second-order diffracted light is used will be described below.

In FIG. 1, the first grating 21 is arranged perpendicular to the laser beam LB, and the second grating 22 is arranged at an angle with respect to the first grating 21. The light transmitted through each of the diffraction gratings 21 and 22 and diffracted into a plurality of orders is converged by the cylindrical lens 23 arranged behind the second grating 22 (however,
In FIG. 1, only the ±2nd-order diffraction light weight t2 is shown). Converged 2
Next-order diffraction light, t, and -2 are detected by photoelectric conversion elements 248 and 24B, respectively, and converted into electrical signals proportional to the amount of light, and each converted electrical signal I. 2 and L2 to adder 2
5 to obtain a displacement signal.

In such a configuration, for example, the pitch P of the diffraction grating
If the error component having a period of is +2nd order diffracted light and is included in 2, the +22nd order diffracted light L42 changes in light 1, that is, the displacement signal (2). (x) has a waveform as shown in FIG. 2(A). Note that this waveform is approximated by the following equation (2).

! +z(x)-acos(2ytx/P)+Acos(
2π・2x/I')+8・・・・・・・・・(2) However, a is the amplitude of the error component whose period is P, while -2
The change in the light amount of the second-order diffracted light L-2 also includes an error component whose period is the pitch P of the diffraction grating, and the phase of the error component is the +second-order diffracted light 1. The phase is shifted by P/2 from the phase of the +2 error component. Therefore, −2nd order diffraction 1. -2 change in light amount, that is, the displacement signal [2(X) has a waveform as shown in FIG. 2(B). Note that this waveform is approximated by the following equation (3).

12(x)-acos(2π(x/P-172)l+A
cos(2rt ・2x/P)+8--acos(2m
x/P)+Acos(2a ・2X/P)+8...
......(3) Therefore, the electric signal (displacement signal) ■ which is proportional to the change in the amount of light of the +2nd order diffracted light 14, 2, (x) and the -2nd order diffracted light L-
An electrical signal (displacement signal) proportional to the change in the amount of light in 2 [2(
By adding X), the error component having a period equal to the pitch P of the diffraction grating is canceled out, and an accurate displacement signal having a period equal to P/2 can be obtained (FIG. 2(C)).

FIG. 3 is a perspective view showing another example of the averaged diffraction moiré position detector of the present invention, corresponding to FIG. 1, and the same components are given the same reference numerals and the description thereof will be omitted. In this way, among the diffracted lights of each order converged by the cylindrical lens 23, the O-order diffracted light and the ±1st-order diffracted light are shielded by the shielding plate 26, and only the ±2nd-order diffracted light L12 is transmitted to the diffracted light L12 disposed behind the shielding plate 26. If they are detected by two photoelectric conversion elements 24G and simultaneously converted into electrical signal No. 15, it becomes the same as adding two electrical signals, and the same effect as the above-described embodiment can be obtained.

In addition, although ±2nd-order diffraction light was used in each of the above-mentioned examples,
It is possible to use other positive and negative diffracted lights of the same order.

(Effects of the Invention) As described above, according to the averaged diffraction moiré position detection of the present invention, position detection can be performed with twice the amount of light compared to a detector that detects the position using the amount of light of either positive or negative diffracted light. At the same time, since the error components cancel each other out, highly accurate position detection can be performed, and, for example, highly precise machining can be easily performed with a machine tool, and production efficiency can be improved.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of the averaged diffraction moiré position detector of the present invention, FIGS. 2(A) to (C) are characteristic diagrams showing output waveforms of the present invention, and FIG. A perspective view showing another example of the averaged diffraction moiré position detector, FIG.
FIG. 7 is a perspective view showing an example of a conventional averaged diffraction moiré position detector, and FIG. 8 is a characteristic diagram showing an output waveform of the conventional example. 1.21... First diffraction grating, 2.22... Second diffraction grating, 3... Transparent plate with steps, 4... Lens group, 5... Photodiode group, 7 ... Adder, 9
... Random optical path difference plate, lO... Diffusion plate, 11...
・Convex lens, 12... Optical sensor, 13.24A, 24
B, 24C... Photoelectric conversion element, 23... Cylindrical lens, 25... Adder, 26... Shielding plate.

Claims (1)

[Claims] 1. A first diffraction grating, a second diffraction grating that is displaced laterally with respect to the first diffraction grating, and the second diffraction grating provided between the two diffraction gratings. means for changing the gap optical path length between the two diffraction gratings over a range of optical path lengths corresponding to a Fresnel number of 2 or an integral multiple of 2 for each portion of the effective facing area of the two diffraction gratings; means for obtaining a signal corresponding to an average value of the diffraction moiré signal over a portion of the effective area of the diffraction grating, using a signal change having a period of one half of the pitch of the diffraction grating appearing in the average value, In an averaging diffraction moiré position detector capable of detecting a relative displacement in the lateral direction of a diffraction grating with high precision, means for obtaining a signal corresponding to the average value of the diffraction moiré signal transmits a plurality of signals through the two diffraction gratings. An averaged diffraction moiré position detector comprising an adding means for adding respective amounts of positive and negative diffracted light of the same order or electric signals proportional to the respective amounts of light among the light diffracted to the order of . 2. The averaged diffraction moiré position detector according to claim 1, wherein the adding means photoelectrically converts the positive and negative diffracted lights of the same order together. 3. The averaged diffraction moiré position detector according to claim 1, wherein the adding means individually photoelectrically converts the positive and negative diffracted lights of the same order and then electrically adds them.