JPH0428014Y2 - - Google Patents

Description

[Detailed description of the invention] This invention makes it possible to accurately measure displacement regardless of the distance between the reading head and the scale by making the light irradiated from the reading head to the scale a parallel beam of light. The present invention relates to an optical scale reading device.

2. Description of the Related Art Optical scale reading devices that utilize light interference are conventionally known. This type of device usually uses a scale on which a reflective film is deposited in a grid pattern on a glass substrate. FIG. 1 is a block diagram showing an embodiment of an optical scale reading device already proposed by the applicant. In the figure, 1 is a coherent light source. For example, a semiconductor laser is used as the light source. L ₁ is a first lens that focuses the output light of the light source. 2 is a polarizing cube prism that receives the light passing through the lens. 3 is 1/4 which receives the light transmitted through the prism
It is a wave plate. L ₂ is a second condenser lens that receives the light transmitted through the wave plate. 4 is a scale that receives the light passing through the lens. The scale is composed of a diffraction grating in which reflecting parts and transmitting parts are regularly arranged.

The light incident on the scale 4 generates multimode diffracted light when reflected. The reflected and diffracted light is reflected through the lens
L ₂ , passes through the quarter-wave plate 3, enters the cube prism 2, and is reflected. A is an imaging part of this reflected diffraction light. Reference numeral 5 denotes a stopper placed in the imaging section A for removing 0th order diffracted light. S is an interference fringe generated behind the imaging section A. 6 is a light receiving element that receives the interference fringes S. The operation of the device configured as described above will be explained as follows.

The light emitted from the light source 1 is condensed by the subsequent lens L ₁ and enters the polarizing cube prism 2 . Of the light incident on the Cube prism, only the component whose polarization angle matches that of the prism passes through the prism. When a semiconductor laser is used as the light source 1, most of the light is linearly polarized and can pass through the prism 2. The light that has passed through the Cube prism 2 enters the 1/4 wavelength plate 3. The light incident on the wave plate 3 becomes circularly polarized light at the wave plate. The light that has passed through the 1/4 wavelength plate 3 is focused by the lens L ₂ , and is focused on the scale 4.
is irradiated. The light incident on the scale 4 generates multimode diffracted light when reflected. Here, the first imaging point of the 0th-order diffracted light is _O1 , and the imaging points of the +1st-order diffracted light and the -1st-order diffracted light are _P1 , _Q1 .

The diffracted light reflected by the scale 4 is the first
The lens L ₂ moves forward as if coming out from the imaging points O ₁ , P ₁ , Q ₁
The light is focused. The light that has passed through the lens L ₂ enters the quarter-wave plate 3 again. Here, the reflected light is returned to linearly polarized light. And since its polarization angle is 90° different from the incident linearly polarized light, the cube prism 2
All reflected light that enters is reflected. The reflected diffracted light is again imaged at the imaging section A. In the figure, _O2
is the imaging point of the 0th-order diffracted light, P ₂ is the +1st-order diffracted light, and Q ₂ is the -1st-order diffracted light. The 0th-order diffracted light is removed by a stopper 5 provided in the imaging section A, and the ±1st-order diffracted lights become divergent lights and interfere with each other, as is clear from FIG. 1, resulting in interference fringes S. . The light receiving element 6 that receives the interference fringes S is composed of multi-divided photodiodes, and each photodiode generates an electric signal depending on the brightness of the light.

Now, assume that the scale 4 is moved in a certain direction while being irradiated with coherent light from the light source 1. At this time, the light input to the light receiving element 6 causes periodic brightness and darkness according to the movement of the scale 4.
If the photodiodes are set to have a phase difference of 90 degrees, each of these photodiodes will output a sine wave with a phase difference of 90 degrees. By processing the output of each of these photodiodes in a control circuit (not shown), the scale 4
The displacement of can be found.

The above-mentioned device uses a cube prism and a quarter-wave plate in the optical system, and also places a stopper just in front of the light receiving element to prevent a decrease in the amount of light.
This is an excellent feature that can improve S/N. However, the device shown in the figure has the following drawbacks. Reading head (lens)
When the distance h between L ₁ , L ₂ , the cube prism 2, etc.) and the scale 4 changes, the position of the imaging section A changes to A'. Therefore, the stopper 5 disposed at the imaging section A is no longer able to block the 0th order diffracted light generated at the A' section.
Therefore, the interference fringes are a mixture of ±1st-order light and 0th-order light, making it impossible to accurately measure the amount of displacement. For these reasons, it is necessary to keep the distance between the reading head and the scale precisely constant.

The present invention was developed in view of these points, and it is possible to make accurate displacement regardless of the distance between the reading head and the scale by making the light irradiated from the reading head to the scale a parallel beam of light. This is an optical scale reading device that can perform measurements. Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 2 is a configuration diagram showing an embodiment of the present invention. Components that are the same as those in FIG. 1 are designated with the same numbers. As is clear from the figure, the configuration is almost the same as the device shown in FIG. The difference is that the optical system is arranged so that the image point of the lens L1 of the light source ₁ is at the focal point B of the lens _L2 . With this configuration, the light that has passed through the lens _L1 enters the lens _L2 as if it were coming out of the focal point of the lens _L2 . As a result, the light passing through the lens _L2 becomes parallel light and is projected onto the scale 4. Therefore, the 0th-order and ±1st-order diffracted lights reflected from the scale 4 are parallel to each other. These parallel lights enter the cube prism 2 through the lens L ₂ and the quarter-wave plate 3. The incident diffracted light is reflected to the imaging area A.
image is formed. As is clear from FIG. 2, the ±1st-order diffracted lights that are not blocked by the stopper 5 provided in the imaging section A become divergent lights and interfere with each other, resulting in interference fringes S.
occurs. According to the device of the present invention, in addition to preventing a decrease in the amount of light and improving the S/N ratio as in the case of the device shown in FIG. 1, the following unique effects are produced. That is, even if the distance between the reading head and the scale 4 changes, the reflected and diffracted light is still parallel light, so the position of the imaging section A does not change. Therefore, stopper 5
can always remove the 0th order diffracted light even if the distance between the reading head and the scale changes. This allows accurate displacement measurement. In the figure, if the interval between the image forming points P ₂ and Q ₂ is d, and the distance from P ₂ or Q ₂ to the light receiving element 6 is L, the pitch P± ₁ of the interference fringe is expressed by the following equation.

P± ₁ =λL/d λ: Wavelength From the above equation, it can be seen that the interference fringe pitch does not depend on the distance h between the reading head and the scale. Also,
The phase of the output signal is shifted by the arrangement of the photodetector, and the distance between the interference fringes is expanded by the lens relative to the pitch of the scale grating on the scale, achieving an accurate 90° phase shift. It is easy to It is also easy to make the slits for the screen.

Furthermore, since the magnification of the above-mentioned lens is large, a large interference fringe pitch can be obtained even if the distance between the lens and the photodiode is relatively small, making it easy to downsize the entire device.

Furthermore, since irradiation and interference can be performed with a single lens L ₂ , the number of parts can be reduced and the configuration can be simplified. Therefore, a small and simple device can be realized. Since the configuration is simple, adjustment is also easy.

In addition, the parallel light irradiated to the scale is transmitted through lens L ₂
It has a wide luminous flux corresponding to the size of the
Furthermore, since interference fringes have redundancy in principle, measurement errors are unlikely to occur even if there are defects in the scale, uneven pitch, dust, dirt, etc. on the scale.

Further, since the blocking means blocks light at a point where the light is focused, the stopper etc. can be small and light blocking is easy.

FIG. 3 is a block diagram showing another embodiment of the present invention. The device shown at the same time has a similar effect by providing a third lens _L3 at the position shown in the figure when a short focal length lens _L2 is used. In the above description, the case where the 0th order diffracted light is removed using a stopper has been described, but the present invention is not limited to this. Either one of the ±1st-order diffracted lights may be removed. In this case, interference fringes occur between the 0th-order light and the +1st-order light, or between the 0th-order light and the -1st-order light. An aperture is used to remove the ±1st order light.

As explained above in detail, according to the present invention, the light irradiated from the reading head to the scale becomes a parallel beam of light, so that accurate displacement can be measured regardless of the distance between the reading head and the scale. It is possible to realize an optical scale reading device that can perform

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of a conventional device. FIG. 2 is a block diagram showing one embodiment of the present invention, and FIG. 3 is a block diagram showing another embodiment. 1... Light source, 2... Cube prism, 3...
1/4 wavelength plate, 4...scale, 5...stopper,
6... Light receiving element, L ₁ , L ₂ , L ₃ ... Lens.

Claims (1)

[Claims for Utility Model Registration] A coherent light source, a first lens that condenses the output light of this light source, a polarizing prism that receives the output light of this first lens, and light transmitted through this polarizing prism. a 1/4 wavelength plate into which the light enters, a second lens that converts the output light of the 1/4 wavelength plate into parallel light, and a diffraction grating shape, on which the parallel light flux output from the second lens is irradiated. A scale, and the reflected diffracted light of this scale is converged by the second lens, and after passing through the 1/4 wavelength plate, the diffracted light of a specific mode among the light reflected by the polarizing prism is focused near its imaging area. and at least two light beams arranged with a phase shift of 90° relative to the interference fringes that are generated when the light that passes through this blocking means becomes divergent light and interferes behind the imaging section. An optical scale reading device comprising the above light receiving element.