JPH01176914A - Laser interference type encoder - Google Patents

Abstract

PURPOSE:To obtain a complete sine wave with a simple constitution by irradiating an optical scale by transmission with parallel light which is projected by a coherent light source and collimated. CONSTITUTION:When the optical scale 3 is irradiated with the coherent parallel light from the laser light source 1 which is collimated by a collimator lens 2, its transmitted light contains much diffracted light is addition to direct light. When, however, a specific position is selected in a space, only the direct light and plus primary diffracted light interfere with each other to obtain interference fringes in a complete sine wave shape. Then, an index scale 4 is arranged at this part and then an output waveform becomes a complete sine wave. Thus, the complete sine wave can be obtained with the simple constitution.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to optical encoders.

[Conventional technology]

Conventional optical encoders usually do not use a laser light source, so in order to make the output waveform a perfect sine wave,
For example, as in US Pat. No. 3,674,372, special measures were required and the cost was high. Also, JP-A-61
There are some devices, such as No. 212728, which have an interferometer configuration by branching a laser beam, but they require polarizing prisms, wave plates, etc., making the configuration complicated and expensive.

On the other hand, Japanese Patent Laid-Open No. 61-10716 proposes an optical encoder that can obtain a perfect sine waveform with a simple configuration. This device includes a coherent light source, an illumination objective lens that converges the light beam from the coherent light source, a diffraction grating plate that is movable relative to the converged light beam from the illumination objective lens, and a diffraction light beam produced by the diffraction grating plate. It has a condensing objective lens that condenses light, and a two-split light-receiving element arranged near the exit pupil of the condensing objective lens, and the dividing line of the two-split light-receiving element is aligned with the optical axis of the condensing objective lens. , and further includes arithmetic means for outputting a difference signal and a sum signal of each output signal from the two-split light-receiving element.

Then, the intensities of the plus first-order and minus first-order diffracted lights incident on each of the two split light-receiving elements are detected, and the calculation means calculates the difference and sum of the output signals of both light-receiving elements, and the phase difference is 9.0° from each other. are getting different signals.

However, since this device makes a coherent beam of focused light incident on the diffraction grating plate, it is important that the grating pitch of the diffraction grating plate is accurate and that the distance between the diffraction grating plate and the objective lens is set correctly. This is a condition for obtaining highly accurate signals.

[Problem that the invention seeks to solve]

In the present invention, a perfect sine wave can be obtained with a simple configuration without significantly changing the configuration of a conventional optical encoder, and even if the distance between the diffraction grating plate and the objective lens is not set correctly. The purpose is to provide an optical encoder.

Means for Solving Problem c] The present invention provides an optical scale (3) having a diffraction grating repeated at a constant pitch using collimated parallel light emitted from a coherent light source (1) such as a laser. an area where only the positive first-order diffracted light of the illumination light after passing through the scale and the direct light (zero-order diffracted light) interfere with each other;
Alternatively, there is a laser interference encoder characterized in that index scales (3, 4) having the same pitch as the scale (3) are arranged in a region where only the negative first-order diffracted light and the direct light interfere.

[For production]

As shown in FIG. 1, when the optical scale 3 is irradiated with coherent parallel light from a laser light source 1 collimated by a collimator lens, the transmitted light includes many diffracted lights in addition to the direct light. However, if a specific position in space is selected, only the direct light and the positive first-order diffracted light (or the direct light and the negative first-order diffracted light) interfere, and a perfect sinusoidal interference pattern can be obtained. The first diagonally shaded position is such a position, and if the index scale is placed at this position, the output waveform will become a perfect sine wave. From Figure 1, the distance between the index scale and optical scale is d in the figure.
It can be seen that the vicinity of the value shown by is convenient. Here, the scale pitch is P, the illumination wavelength is λ, and half the width of the direct light is R1.
Then, dζPR/λ ・・・・・・・・・
(1) Here, interference fringes occurring in the shaded area along the coordinate system of FIG. 1 are formulated. First, consider the interference between direct light and positive first-order diffracted light.

The amplitude of direct light Uo(x, z) is expressed as Uo(x, z) = −exp (i k z
)2 ・・・・・・(2) The amplitude U, (x, z) of the first-order diffracted light is (J+(x, Z)= ・・・・・・(3) Therefore, the amplitude after interference U” ( x, z) is U” (
x,,z) -Uo(x,z)+U+(x,z)...
...(4) The intensity distribution M (x, z) of the interference fringes is (2), (3),
From (4), I' (xS z)-lU" (x, z)l"4
π2 π P P”・・・・・・
・ (5) Intensity distribution of interference fringes between direct light and minus first-order diffracted light (
Similarly, x, z) becomes...(6). From (5) and (6), it can be seen that the interference fringes are the same as the pitch of the optical scale, and when the observation plane is moved in the Z direction, the phases of the fringes move in opposite directions.

From equations (5) and (6), the phase difference between ■゛ and ■− is P! Since the encoder requires two outputs with a 90° phase difference for traveling direction discrimination, it is convenient that δ=±90″. In this case, 2 becomes λ 4 from equation (7).

〔Example〕

FIG. 2 shows a first embodiment of the present invention, in which coherent monochromatic light emitted from a laser diode 1 is turned into parallel light by a collimator lens 2, and then enters a linear scale 3. The linear scale 3 is a transmission type diffraction grating, for example, one in which slits are formed with a constant pinch by vapor deposition or the like. The light diffracted by the linear scale 3 enters the index scale 4, but the linear scale 3
Since the interval d of the index scale 4 satisfies , there are sinusoidal interference fringes on the index scale 4 with the same pinch as the linear scale 3, and there are interference fringes on the right half and left half of the index scale 4. The phases of are shifted by 90°. Therefore, a linear encoder whose direction can be determined by the arithmetic circuit 6 and the display device 7 from the outputs of the photodetector 5a that detects the transmitted light from the right half of the index scale 4 and the photodetector 5b that detects the transmitted light from the left half of the index scale 4 is configured. In this case, the index scale 4 may be a simple linear scale, but it is of course also possible to use a conventional index scale having a plurality of marked windows having different phases.

2λ 4 will vary depending on the phase difference between the scored windows of the index scale.

FIG. 3 shows a second embodiment of the present invention, and the functions of the laser diode 8 and collimator lens 9 are the same as in FIG. 2, but in FIG. The image is then projected onto the scale 10 again. In this case, the linear scale 10 also serves as an index scale, and the photodetector 12a on the right side of FIG.
From the photodetector 12b on the left side, the optical path length from point A to point B is λ 4 and outputs having different phases by 90° are obtained. Further, in the second embodiment, by using a projection type, the detectors 12a, 12
It is clear from geometrical optics that when b and the linear scale 10 are displaced, the pitch of the linear scale 10 is equivalently halved (the number of output pulses is doubled). From the above, with this system, it is possible to construct an encoder that can discriminate the direction of travel, and it is also possible to double the number of output pulses. Further, since the optical output signal has a perfectly sinusoidal waveform as described above, electrical interpolation within one pitch is theoretically possible infinitely.

〔Effect of the invention〕

As described above, according to the present invention, an encoder capable of outputting a sine wave can be provided with a simple configuration by simply using a coherent monochromatic light source such as a laser diode as a light source. Furthermore, since parallel light is incident on the scale, even if there are some pitch irregularities in the scale grating, they are averaged out.

Further, even if a simple linear scale is used as the index scale for the scale-index interval, two sine wave outputs having a phase difference of 90° can be obtained. As an application of this, a self-projection encoder can be realized as shown in FIG. 3, and the number of output pulses can be doubled.

[Brief explanation of the drawing]

Fig. 1 is an optical path diagram explaining the present invention in detail, Fig. 2 is a diagram showing an optical system and electric block showing a first embodiment of the invention, and Fig. 3 is an optical system according to a second embodiment of the invention. FIG. [Explanation of symbols of main parts] -1... Laser diode, 2... Collimator lens, 3... Linear scale, 4... Index scale, 5a, 5b... Photodetector. Applicant Nippon Kogaku Kogyo Co., Ltd.

Claims (4)

[Claims] (1) An optical scale having a diffraction grating repeated at a constant pitch is transmitted and illuminated with collimated parallel light emitted from a coherent light source such as a laser, and the positive first-order diffracted light of the illumination light after passing through the scale and direct light (
A laser interference encoder characterized in that an index scale having the same pitch as the scale is arranged in a region where only the zero-order diffracted light (zero-order diffracted light) interferes or a region where only the negative first-order diffracted light and direct light interfere. (2) A region where only the positive first-order diffracted light and the direct light interfere,
The laser interferometric encoder according to claim 1, characterized in that one index scale is disposed across a region where only the negative first-order diffracted light and the direct light interfere. (3) As the diffraction grating of the optical scale, a grating in which a light transmitting part and a light non-transmitting part are repeated at a constant pitch is used, and an optical system that projects the light that has passed through the optical scale onto the scale again. 3. The laser interferometric encoder according to claim 2, wherein a system is provided so that the optical scale also serves as an index scale. (4) The distance z between the optical scale and the index scale is approximately z=P^2/λ (n±1/4) when converted to air. However, λ is the wavelength of the illumination light P, and the scale pitch n is arbitrary. 4. A laser interference encoder according to claim 1, wherein the encoder is a natural number.