JPS63115012A - Displacement measuring instrument - Google Patents

Abstract

PURPOSE:To measure displacement with high accuracy at any time by correcting an error signal contained in an interference signal by utilizing a reference signal based upon the intensity of diffracted light generated by a diffraction grating provided to a body to be measured. CONSTITUTION:Luminous flux from a laser light source 1 is diffracted twice by a diffraction grating 2 and its positive and negative diffracted light beams are made incident on a beam splitter 51 one over the other and divided into two. The transmitted light between the two light beams is photodetected by a photodetecting element 73 without interfering and a reference signal is outputted. The reflected luminous flux, on the other hand, is split by a beam splitter 52 into two beams, which are made 90 deg. difference with each other through polarizing plates 61 and 62 and photodetected by photodetecting elements 71 and 72, thereby outputting an interference signal. Then, a computing element divides the respective interference signals by a reference signal for corrections, thereby obtaining a corrected signal with a constant amplitude. Further, processing based upon a specific slice level is performed to obtain a stable rectangular wave signal. Thus, the invariably stable signal is obtained to facilitate the electric division of a succeeding stage. Consequently, the displacement is measured with high accuracy and high resolution.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a displacement measuring device for measuring the displacement of an object to be measured in its rotational state, moving state, etc. A coherent light beam is incident on a periodic or predetermined pattern of diffraction gratings, the diffracted lights generated from the diffraction gratings are made to interfere with each other to form interference fringes, and by counting the brightness and darkness of these interference fringes, the object to be measured can be detected. This invention relates to a displacement measuring device for determining displacement.

(Prior art) Displacement measuring devices have conventionally been used in industrial machine tools to detect the amount of movement of a moving object, the rotation of a robot arm, one position of movement, etc., and the amount of rotation (one rotational speed) of a rotating mechanism. Photoelectric rotary encoders and near encoders are often used.

Among these, various diffraction-type displacement measuring devices have been proposed, in which a diffraction grating is provided on the object to be measured, and the diffraction light generated by the diffraction grating is used to measure the displacement 9 of the object to be measured, such as the amount of movement or rotation. There is. Since this displacement measuring device is relatively easy to measure with high precision, it is often used for precision machines such as NCC honorable mention machine semiconductor printing equipment.

In this diffraction type displacement measuring device, diffracted lights generated from a diffraction grating are caused to interfere with each other to form interference fringes, and the brightness and darkness of these interference fringes are counted by a light receiving means to obtain an interference signal regarding displacement.

If the output of the secondary light source fluctuates due to environmental changes such as temperature changes, the transmittance of the diffraction grating (reflectance in the case of a reflective diffraction grating) is uneven, or if an amplitude type diffraction grating is used. If the line width of the transmitting part or the reflecting part is not uniform, the output value E related to interference fringes from the light receiving means will be outputted as an unstable waveform as shown in FIG. 5(A), for example. .

In particular, diffraction gratings are prone to uneven etching during manufacturing.
It is very difficult to improve the non-uniformity of the line width (in the case of a phase-type diffraction grating, the shape of steps etc.) over the entire measurement area, and this tendency becomes particularly noticeable.

Due to the above reasons, the output value from the light receiving means fluctuates as shown in Figure 5 (8), and when it falls below the slice level of the comparator in the subsequent counting circuit, the output waveform cannot be counted accurately. It's going to disappear. In addition, even if the output value is higher than the slice level, the center level of the amplitude is unstable, so the “H” output of the comparator as shown in the same figure (B)
” level and “L” level width become unstable.

This makes it difficult to divide electricity in subsequent electrical circuits.
It becomes very difficult to measure displacement with high precision and high resolution.

(Problems to be Solved by the Invention) The present invention uses a light receiving means to detect the brightness and darkness of interference fringes formed by making diffracted lights interfere with each other, even if the diffraction efficiency fluctuates due to fluctuations in the output of the light source or manufacturing errors in the diffraction grating. An object of the present invention is to provide a diffraction-type displacement measuring device that can always measure displacement with high precision by keeping the output value from a light receiving means constant during counting.

(Means for solving the problem) Diffraction lights generated from a diffraction grating connected to an object to be measured are caused to interfere with each other to form interference fringes, and an interference signal corresponding to the brightness and darkness of the interference fringes is transmitted to the object to be measured. Displacement 4 extracted as a signal related to displacement! The present invention is further characterized in that a correction means is provided in the measurement device for correcting an error signal included in the interference signal using a reference signal based on the intensity of the diffracted light generated from the diffraction grating.

(Embodiment) FIG. 1 is a schematic diagram of an optical system of an embodiment when the present invention is applied to a linear encoder. In the figure, 1 is a monochromatic light source emitting a coherent light beam such as a semiconductor laser, 2 is a diffraction grating connected to an object to be measured (not shown) moving in the direction of arrow 21, and 31 and 32 are corners. Cube, 41° 42 is a daylight wavelength plate, 51.52 is a non-polarizing beam splitter, 161, 62 is a polarizing plate, 71, 72° 7
3 is a light receiving element.

The light beam from the light R1 is diffracted by the diffraction grating 2. At this time, the positive and negative order diffracted lights are each
1.32, and enters the diffraction grating 2 again via the % wavelength plate 41.42. Here, the positive and negative diffracted lights diffracted again are superimposed, enter the beam splitter 51, and are split into two light fluxes: a reflected light flux and a transmitted light flux.

Of these, the transmitted light beams do not interfere with each other and are simply received as intensities by the light receiving element 73, and the light receiving element 73 outputs a reference signal. On the other hand, the reflected light beam is split again into two light beams, a reflected light beam and a transmitted light beam, by a beam splitter 52, and each is split by a polarizing plate 6.
1.62, it becomes an interferable light beam and reaches the light receiving element 71.
．． 72. At this time, the light beams received by the light receiving elements 71 and 72 correspond to the intensity of the brightness and darkness of interference fringes that interfered with each other, and the light receiving elements 71 and 72 output an interference signal.

That is, if the pitch of the diffraction grating 2 is P, and the orders of positive and negative diffracted lights are m, then the light receiving elements 71 and 72 will move MP of the diffraction grating 2.
/ Outputs one sinusoidal waveform signal every 4 m.

In this embodiment, the linearly polarized light of the light source 1, the % wavelength plate 41.42,
By adjusting the combination of polarization states of the polarizing plates 61 and 62, a phase difference of 90 degrees is established between the output signals of the light receiving elements 71 and 72, so that the moving direction of the diffraction grating 2 can be determined.

In the embodiment shown in FIG. 1, the light receiving element 73 side and the light receiving elements 71 and 72 side may be replaced.

Next, a method of processing output signals from each of the light receiving elements 71, 72 and 73 in this embodiment will be explained.

Now, the linearly polarized light of the light source l is expressed as (a sinw t , 0
) (a: amplitude, W: frequency), the diffraction grating 2
The positive and negative diffracted lights me are diffracted by the % wavelength plate 41, 42, and then diffracted again by the diffraction grating 2.
l me is each expressed by the following formula.

m@-ag/('i (sin(wL+45')
, cos(wt+450) ) m6-ag/,
/T (s in (wt+45°+δ), cos
(wt+45°◆ δ)) Here, δ is the phase difference between the positive and negative diffracted lights brought about by the movement of the diffraction grating 2, g
is the 10mth order diffraction efficiency of the diffraction grating 2. Therefore, the luminous flux obtained by superimposing the positive and negative diffracted lights is tntB+ me-, /Tag sin(wt+4
5'+ δ/2)x (cos δ/2, s
inδ/2). The intensity of the luminous flux passing through the non-polarized beam splitter 51 and entering the light receiving element 73 (3) is I 3 - IH + m012 = 2a2g2sin2 (wt
+45°+δ/2)x (cos2δ/2 + 5i
n2δ/2)・2a2g2 sin'(wt+45°+
δ/2) The output E3 of the reference signal of the light receiving element 73 is E3 =2a2g2·-== (1).

If the polarization azimuth angle of the polarizing plate 61 is φ1, then the light receiving element 71
The incident light flux is J”iag sin(wt+45′+δ/2)(co
sφ, -δ/2)x (cosφ, , sinφ1
) The strength work 1 is I = 2a2g” cos2(φ1-δ/2) s
It is expressed as in2(wt, +45°+δ/2). The output E1 of the interference signal of the light receiving element 71 is E I = 2a''g2cos2(φ1-δ/2)-a2
g2(1+cos(2φ,-δ)) ・----(
2).

Similarly, the output E2 of the interference signal of the light receiving element 72 is E2=a2g2(1+cos(2φ2−δ)
) -----(3).

If the azimuth angles of the polarizing plates 61 and 62 are set to 450 degrees to each other, φbina is 2.45 degrees, so equations (2) and (3)
), a phase difference of 900 is obtained between the outputs of the light receiving elements 71 and 72.

The output E3 of the light receiving element 73 from Bunmu (1) is the output El
, E2, and has a value that depends on the output change a2 of the light source 1, the efficiency change g2 of the diffraction grating 2, etc.

Now, the output value of the light source 1 fluctuates, the efficiency of the diffraction grating changes due to manufacturing errors of the diffraction grating, etc., and the amplitude of the output signal E1 from the light receiving element 71 fluctuates as shown in FIG. 2(A) due to the error signal. Suppose we did. At this time, the output signal from the light receiving element 73 becomes as shown in FIG. In this embodiment, the output signal E
1 is divided by the output signal E3 obtained at that time by the arithmetic unit, and this value El - El /E3 is corrected by dividing it by the output signal E3 obtained at that time by the arithmetic unit.
I am looking for the output signal from. As a result, the same figure (C
), 1 is obtained for the output signal with constant amplitude.

Then, the output signal El is processed at a predetermined slice level to obtain a stable rectangular wave signal of "H" level and "L" level as shown in FIG. 4(D).

In this way, in this embodiment, the output signal E from the light receiving element 73 is
3', even if the output signal E1 from the light-receiving element 71 includes an error signal caused by fluctuations in the output of the light source 1, manufacturing errors in the diffraction grating, etc., as shown in FIGS. 2(C) and 2(D).
As shown in the figure, a stable output signal is always obtained.

Obtaining such a stable signal facilitates electrical division in the subsequent stage and enables highly accurate and high resolution measurements.

A stable output signal is obtained for the output E2 from the light receiving element 72 in exactly the same manner as for the output E1.

FIG. 3 is a block diagram of an electric circuit for obtaining the output signal shown in FIG. 2. In the figure, the output signals El and E2 from the light receiving elements 71.72 are divided by the output signal E3 from the light receiving element 73 by the divider 74.75, and the output signal El-El is divided by the output signal E3 from the light receiving element 73.
/E: ] , ] We are looking for the output 102-E2/E3.

These output signals El and E2 are then sent to the counting circuit 76 at the subsequent stage.
The signal shown in Figure 2 (D) is obtained manually.

FIG. 4 is an explanatory diagram of an embodiment in which the present invention is applied to a rotary encoder. In FIG. 4, members that perform the same functions as in FIG. 1 are given the same numbers. In the figure, 10 is a collimator lens, and 11 is a parallel glass plate, which are arranged at an angle of 45 degrees with respect to the optical axis. 1
2 is a polarizing prism, 13 is a radiation grating, 14 is a light receiving element,
15 is a rotating glaze to be connected to the rotating object to be tested, 16 is a non-polarizing prism, 17 is a cylindrical lens, 81° 82 is a reflecting means, 91.92 is a reflecting prism, 41.42.4
3 is a % wave plate. In FIG. 4, a light beam emitted from a light source l such as a laser is turned into a substantially parallel light beam by a collimator lens 10. In FIG. This light beam enters the polarizing prism 12 via the parallel glass plate 11. Polarizing prism 12
The light beam incident on the light beam is reflected inside, guided to a polarizing beam splitter on the bonding surface, and split into a reflected light beam and a transmitted light beam.

Of the light beams split into two by the polarizing beam splitter, the reflected light beam undergoes internal reflection repeatedly at the polarizing prism 12 and exits from the polarizing prism 12 in a direction parallel to the direction of incidence. Then, this reflected light beam is reflected by the reflection prism 92 and enters a position M on the radiation grating 13 at a predetermined angle. Here, the diffracted light of a specific order among the transmitted diffracted light that is incident on the radiation grating 13 and diffracted is reflected by the reflecting means 82 via the % wavelength plate 42, travels the same optical path backwards, and is emitted again via the % wavelength plate 42. The light is re-injected at approximately the same position M on the grating 13.

Therefore, the diffracted light of a specific order re-diffracted by the radiation grating 13 is made into linearly polarized light having a polarization direction 90° different from that when it was incident by reciprocating through the % wavelength plate 42 and is directed toward the reflecting prism 92.

The diffracted light reflected by the reflecting prism 92 travels back along its original optical path, enters the polarizing prism 12 again, and reaches the polarizing beam splitter.

In this embodiment, the round trip optical path of the diffracted light of a specific order from the polarizing beam splitter of the polarizing prism 12 to the reflecting means 82 is the same.

As the reflecting means 82.81, a rust-tight optical element such as a general plane mirror or a corner cube may be used, or a reflecting mirror may be placed approximately on the focal plane of the condensing lens, so that the specific light incident parallel to the condensing lens may be used. A configuration may be adopted in which only the diffracted light of the order passes through the opening of the mask, is reflected by the reflecting mirror, and then returns to the original optical path, and the diffracted light of the other orders is blocked by the mask. The reflecting means may have any other configuration, such as a cat's eye optical system.

If such an optical system is used, for example, the oscillation wavelength of the laser will change, and the angle of diffraction will change to 4. It has the characteristic that even if there is a slight change, it can be returned to approximately the same optical path.

Returning to FIG. 4, among the two beams split by the polarizing beam splitter, the transmitted beam repeatedly undergoes internal reflection at the polarizing prism 12, then exits from the polarizing prism 12, passes through the reflecting prism 91, and passes through the radiation grating 13. The light is made incident at a position M2 that is approximately symmetrical with respect to the upper position M1 and rotation #115. Then, out of the transmitted diffracted light that is incident on the radiation grating 13 and diffracted, the diffracted light of a specific order is reversed along the same optical path by a reflecting means 81 similar to the reflecting means 82 described in 01.
1 to substantially the same position M2 on the radiation grating 13. Therefore, the diffracted light of the specific order re-diffracted by the radiation grating 13 is made to re-enter the reflecting prism 91 so as to become linearly polarized light with a polarization direction different by 900 from when it was incident.

The diffracted light reflected by the reflecting prism 91 travels back along its original optical path, enters the polarizing prism 12 again, and reaches the polarizing beam splitter.

At this time, the transmitted light beam also has the same round-trip optical path of the diffracted light of a specific order from the polarizing beam splitter to the reflecting means 81 as in the case of the discontinuously reflected light beam. Then, after being superimposed with the diffracted light that has entered through the reflection means 82, it is emitted from the polarizing prism 2, and is converted into circularly polarized light through the electric wave plate 43.
The light is made incident on the non-polarizing prism 16.

A portion of the light beam incident on the non-polarizing prism 16 is transmitted and enters the light receiving element 73 to obtain a reference signal that does not interfere. The non-polarizing prism 16 is provided with two non-polarizing beam splitters, and the light beam reflected here is sent to the light receiving element 71, 72 via polarizing plates 61, 62 whose polarization directions are shifted by 45 degrees from each other. incident and obtain an interference signal. The light receiving element is similar to the embodiment shown in FIG. By providing an electric circuit as shown in FIG. 3 after 1, 72, and 73, a signal with stable amplitude can be obtained.

(Effects of the Invention) According to the present invention, as described above, by using a reference signal based on a non-interfering light beam, the brightness and darkness of interference fringes can be counted even when there are fluctuations in the output of the light source, fluctuations in the diffraction efficiency of the diffraction grating, etc. Since the interference signal at the time of displacement does not become unstable and can be kept constant, it is possible to achieve a displacement measuring device that can always perform highly accurate measurements.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of a method of processing output signals from each light-receiving element in FIG. 1, and FIG. 3 is a diagram showing each light-receiving element in FIG. 1. 4 is an explanatory diagram of an embodiment of the present invention applied to a rotary encoder, and FIG. 5 is a block diagram of an electric circuit showing processing of output signals obtained from a conventional displacement measuring device. It is an explanatory diagram. In the figure, 1 is a light source, 2 is a diffraction grating, 31.32 is a corner cube, 41.42 is a % wave plate, 51.52
is a beam splitter, 161, 62 is a polarizing plate, 71, 7
2.73 is a light receiving element. Patent applicant Canon Co., Ltd. 1st page 2nd page 3rd session

Claims (2)

[Claims] (1) Displacement in which diffracted lights generated from a diffraction grating connected to an object to be measured interfere with each other to form interference fringes, and an interference signal corresponding to the brightness and darkness of the interference fringes is extracted as a signal related to the displacement of the object to be measured. A displacement measuring device characterized in that the measuring device further comprises a correction means for correcting an error signal included in the interference signal using a reference signal based on the intensity of diffracted light generated from the diffraction grating. (2) The displacement measuring device according to claim 1, wherein the reference signal and the interference signal are obtained using diffracted light generated from the same position of the diffraction grating.