JPH0358043B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a reflective scale to measure the amount of movement of the scale by utilizing the interference of diffracted light of coherent light, and measures the amount of movement of the scale by using the phase delay of the light caused by the retardation plate. The present invention relates to an optical scale reading device that can measure the direction of movement of a scale.

Various types of optical scale reading devices that utilize light interference have been known. FIG. 1 is a diagram showing the configuration of a conventional device of this type. The laser beam l emitted from the laser is reflected by the mirror M
1 and lenses L1 and L2 to scale 1.
is irradiated. The +1st-order diffracted light diffracted by the scale 1 enters the mirror M2, and the 0th-order diffracted light enters the mirror M3.
The +1st-order diffracted light from mirror M2 goes directly to lens L3.
Also, the 0th order diffracted light from the mirror M3 is diffracted on the scale 1 and enters the lens L3 as -1st order diffracted light. At this time, the +1st-order and -1st-order diffracted lights become linearly polarized lights with polarization planes shifted by 90 degrees from each other by polarizers P1 and P2, respectively. The light focused by the lens L3 is divided into three directions by the spectroscope 2 and sent to the photoelectric conversion element D.
1 to D3. Here, the output of D3 is used for automatic gain control to keep the laser light constant. Also, the light entering D1 and D2 is 1/
A phase difference of 90° is provided by the 4-wave plate 13, and the plane of polarization is
The ±1st-order diffracted lights are mixed by analyzers P3 and P4 arranged at an angle of 45°. The light that has produced interference fringes as a result is converted into electrical signals by photoelectric conversion elements D1 and D2. The outputs of these photoelectric conversion elements are converted into scale movement values through predetermined processing.

FIG. 2 is a block diagram showing an embodiment of an optical scale reading device already proposed by the applicant. In the figure, 11 is a coherent light source, L11 is a first lens that condenses the output light of the light source, 12 is a polarizing cube prism that receives light passing through the lens, and 13 is a polarizing cube prism that receives light that has passed through the prism. 1/4 wavelength plate, L1
2 is a second condenser lens that receives the light transmitted through the wavelength plate; 14 is a scale that receives the light that passes through the lens;
A is an imaging section where the diffracted light reflected from the scale 14 forms an image. Reference numeral 15 denotes a stopper placed in the imaging section A to remove the 0th order diffracted light, S denotes interference fringes generated at the rear of the imaging section A, and 16 denotes a light receiving element that receives the interference fringes S.

The light emitted from the light source 11 passes through the following lens L1.
1 and enters the polarizing cube prism 12. 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 11, most of the light is linearly polarized and can pass through the prism 12. The light that has passed through the Cube prism enters the 1/4 wavelength plate 13. 1/4 wavelength plate 1
The light that has passed through the lens L12 becomes circularly polarized light, is focused by the lens L12, and is irradiated onto the scale 14. The light incident on the scale 14 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 and Q1, respectively. scale 1
The diffracted light reflected by 4 is directed to the first imaging point O
1, P1, advance as if it came out from Q1 Lens L12
The light is focused. The light that has passed through the lens L12 enters the quarter-wave plate 13 again. Here, the reflected light is converted back into linearly polarized light. Moreover, since its polarization angle differs by 90 degrees from the incident linearly polarized light, all of the reflected light that enters the cube prism 12 is reflected.
The reflected diffracted light is again imaged at the imaging section A.
In the figure, O2 is 0th order, P2 is +1st order, and Q2 is -
Each of the first-order diffracted lights is an imaging point. The 0th-order diffracted light is removed by a stopper 15 provided in the imaging section A, resulting in interference fringes between the ±1st-order diffracted lights. The light receiving element 16 that receives the interference fringes S is composed of a multi-segmented photodiode,
Each photodiode generates an electrical signal depending on the brightness of the light.

Now, assume that the scale is moved in a certain direction while being irradiated with coherent light from the light source 11. At this time, the interference fringes S on the light receiving element 16 move in accordance with the movement of the scale 14. If you arrange the photodiodes so that their phases differ by 90 degrees,
Each of these photodiodes outputs a sine wave with a phase difference of 90 degrees. The displacement of the scale 14 can be determined by performing arithmetic processing on the output of each of these photodiodes using a control circuit (not shown). In both examples, the output is a sine wave with a phase difference of 90 degrees, and by counting the peak value of the sine wave, you can measure the amount of movement of the scale, and by determining the phase relationship between the two sine waves, you can measure the direction of movement. can. Here, in order to obtain even higher resolution, it is conceivable to divide the phase voltage of the sine wave in an analog manner. Therefore, it is necessary that the shape of the sine wave and the 90° phase difference be as accurate as possible.

The above-mentioned devices all use transparent scales, so it is difficult to fix the scales.
In the case of Figure 2, if the pitch of the scale 14 is small, the diffraction angle becomes large, so L12 has a high NA.
This has the disadvantage that the area on the scale 14 where the light reaching the light receiving element 16 is reflected becomes very small, making it susceptible to dust and dirt on the scale.

The present invention has been made in view of these points, and uses a reflective scale as the scale, increases the diameter of the light beam irradiated to the scale, reduces the influence of dust, etc., and uses a retardation plate. By utilizing this to create a 90° phase difference, we have realized an ultra-high resolution optical scale reading device that is simple in construction and easy to operate.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a configuration diagram showing an embodiment of the present invention. In the figure, 20 is a coherent light source, L21 is a lens, M21 is a half mirror that guides multiple diffracted lights in the same direction, 21 is a scale, 22 is a first retardation plate, and M22 is a mirror that directs reflected diffracted lights in two directions. A beam splitter for separating, 23 a first polarizing plate, 24 a second retardation plate, 25 a second polarizing plate, PD1,
PD2 is a light receiving element, 26 and 27 are amplifiers mainly composed of operational amplifiers U1 and U2, respectively;
8 is a waveform shaping circuit that receives the output of each amplifier; 29
Reference numeral 30 indicates an arithmetic circuit that receives the output of the waveform shaping circuit, performs arithmetic processing on it, and divides it into a desired resolution, and 30 a display unit that displays the output of the arithmetic circuit. Coherent light source 2
For example, a semiconductor laser is used as the beam splitter M22, a half mirror is used as the retardation plates 22 and 24, and a half mirror is used as the beam splitter M22. The operation of the device configured as described above will be explained as follows.

The output of the semiconductor laser 20 is made into an angle (or parallel light) so that the light is focused onto the light receiving element by the lens L21. At this time, the plane of polarization is perpendicular to the plane of the paper as shown in the figure. Half mirror M21
and separate it into two directions. Half mirror M
The transmitted light l ₁ of 21 is incident on the scale 21 at an angle of θ ₁ . The reflected light l ₂ of the half mirror M21 is the wavelength plate 2
2 and then enters the scale 21 at an incident angle θ ₁ . Here, if the angle of the quarter-wave plate 22 is set so that the fast axis and the slow axis are at 45 degrees with the polarization plane of the incident light, as shown in Figure 4a, the light passes through the quarter-wave plate. The light becomes circularly polarized. FIG. 4 is a diagram showing the state of the polarization plane of each part. As mentioned above, a is the first 1/4 wavelength plate 22, b is the second 1/4 wavelength plate 24, and c is the first 1/4 wavelength plate 22.
In the change plate 23, d indicates the state of the second polarizing plate 25, respectively.

Here, the wavelength of the semiconductor laser 20 is λ, the scale pitch is d, and ±1
In order for the diffracted lights l ₃ and l ₄ to be reflected in the same direction as the incident lights l ₂ and l ₁ , respectively, the incident angle θ ₁ must satisfy the following equation.

sinθ ₁ +ginθ ₁ =λ/d From now on, θ ₁ can be expressed by the following equation.

θ ₁ =sin ⁻¹ λ/2d The 0th order diffracted lights l ₅ and l ₆ are each reflected in the directions shown in the figure and are automatically removed. The +1st-order diffracted light l ₃ enters the 1/4 wavelength plate 22 again, becomes a polarization plane that is 90° different from the polarization plane of the incident light l ₂ (that is, parallel to the plane of the paper), and passes through the half mirror M21. The -1st-order diffracted light _l4 is directly reflected from the scale 21 by the half mirror 21 and proceeds in the same direction as the +1st-order diffracted light _l3 . At this time, ±
Since the polarization planes of the first-order diffracted lights l ₃ and l ₄ differ by 90°, they do not interfere with each other. This ±1st-order diffracted light is divided into two by a second half mirror M22. The light reflected by the half mirror passes through the polarizing plate 23 and reaches the light receiving element PD.
Reach 1. At this time, if the polarization plane of the polarizing plate 23 is set in the direction of 45 degrees for each of the ±1st-order diffracted lights l ₃ and l ₄ as shown in FIG. The components interfere with each other, and the output of the light receiving element PD1 becomes a sine wave corresponding to the movement of the scale 21.

On the other hand, the light passing through half mirror M22 is 1/4
It passes through the wave plate 24. At this time, if the slow axis of the wave plate is aligned with the +1st order (or -1st order), +
1st order (or -1st order) becomes -1st order (or +1st order)
The phase is delayed by 90°. This is transferred to the second polarizing plate 2.
Similarly to the first polarizing plate 23, +1st order and −
When the first-order diffracted light is interfered with, the output of the second light receiving element PD2 that receives the light passing through the polarizing plate 25 also becomes scale 2.
According to the movement of 1, the output becomes a sine wave. , PD1
The phase will be delayed (or advanced) by 90° compared to the output of . This output with a phase difference of 90° is amplified by amplifiers 26 and 27, as is generally done in this type of scale reading device, and then sent to a waveform shaping circuit 28.
The waveform is shaped by the arithmetic circuit 29, divided into a desired resolution by the arithmetic circuit 29, and the direction is discriminated and counted by the counter 30 and displayed as the moving distance of the scale or fed back and used for controlling the position of the scale.

The features of the device of the present invention explained above are as follows:
It is as follows.

(1) Since a reflective scale is used, the scale and head can be easily installed and made smaller.

(2) The spacing between the scale and head can be changed freely.

(3) The light beam that irradiates the scale is wide, and no spatial filter (slit) is used on the light receiving element, so there is little influence from dust or dirt on the scale.

In the above explanation, the half mirror M22 was used to separate the reflected and diffracted light into two directions, but the invention is not limited to this, and for example, a grating type beam splitter as shown in FIG. 5 may be used. . In the figure, 40 is a grating type beam splitter. If necessary, a condensing lens L40 may be placed in front of it, as shown in the figure. Also. In the above description, a case where two light beams are projected onto a scale using a half mirror from an obliquely incident coherent light source has been described as an example, but the present invention is not limited to this. FIG. 6 is a diagram showing various embodiments in which a coherent light source that enters obliquely is projected onto a scale by using a mirror. In the figure, LD is a semiconductor laser, HM is a half mirror, M is a mirror, G is a grating, and λ/
2, λ/4 is a retardation plate, and PD is a light receiving element.

As explained in detail above, according to the present invention, a reflective scale is used, the diameter of the light beam irradiated to the scale is increased to reduce the influence of dust, etc., and the reflection of the half mirror is further reduced. By creating a 90° phase difference by utilizing the phase lag of light due to the phase shift of the light, it is possible to realize an optical scale reading device with a simple configuration and good operability.

[Brief explanation of drawings]

1 and 2 are diagrams showing an embodiment of a conventional device of this type, FIG. 3 is a configuration diagram showing an embodiment of the present invention, FIG. 4 is a diagram showing the state of the polarization plane, and FIG. This figure shows another embodiment of the beam splitter, and FIG. 6 shows another embodiment of the present invention. 1, 14... Transmission type scale, 2... Spectrometer,
L1~L3, L11, L12, L21, L40...
...Lens, M1-M3...Mirror, P1, P2...Polarizer, P3, P4...Analyzer, 13, 22, 24
...1/4 wavelength plate, D1 to D3...photoelectric conversion element,
11, 20... Semiconductor laser, 12... Polarizing cube prism, 16, PD1, PD2... Light receiving element, 21... Reflective scale, 23, 25... Polarizing plate, 26, 27... Amplifier, 28... ... Waveform shaping circuit, 29 ... Arithmetic circuit, 30 ... Display section, M2
1... Half mirror, M22, 40... Beam splitter.

Claims (1)

[Claims] 1. A coherent light source, a reflective scale, and a first device that makes one of the reflected and diffracted lights perpendicular to the other with a polarization plane.
a retardation plate, a half mirror that guides both diffracted lights in the same direction, a beam splitter that separates the light into two or three directions, a polarizing plate that causes the diffracted lights to interfere with each other in each direction, and these a second retardation plate that provides a phase difference to the interference light; two or three light receiving elements that receive each interference light that has a phase difference;
An optical scale reading device that measures the moving distance of the scale by processing the outputs of these light receiving elements. 2. The optical scale reading device according to claim 1, wherein the obliquely incident coherent light source uses a half mirror to project two light beams onto the scale. 3. The optical scale reading device according to claim 1, wherein a coherent light source that enters obliquely projects one beam of light onto the scale using a mirror.