JPS63309817A - Linear encoder - Google Patents

Abstract

PURPOSE:To prevent output variation due to variation in the wavelength of laser light with temperature and to obtain a stable output regardless of the temperature variation by converging laser light from a diffraction grating on the focus position of a correcting lens and then generating parallel light. CONSTITUTION:The laser light emitted by a laser diode 22 is so adjusted by a lens 21 that the light is reflected by a reflecting mirror 26 and then converged on the diffraction grating 23. Then the light is diffracted spectrally by a beam splitter 20 and reflected light beams 34 and 35 from those reflecting mirrors 24a and 24b are incident on the diffraction grating at a predetermined angle. Those light beams diffracted twice by passing the grating 23 and glass substrate 25 before and after being reflected by a reflecting plate 26, projected while diverged, and collimated by a convex lens 27. The light is polarized circularly by a wavelength plate 28 and two component laser light beams interfere with each other and are split into transmitted light and reflected light by a beam splitter 29. Then those light beams reach photodetectors 31 and 33 through polarizing plates 30 and 32 and are converted photoelectrically. Those polarizing plates 30 and 32 are given a phase difference so as to discriminate the moving direction of the grating 23.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a linear encoder that utilizes light diffraction and interference.

<Prior art> As a conventional linear encoder, for example, JP-A-61-1
30816, and is configured as shown in FIGS. 7 to 9.

In FIG. 7, 1 is a laser, 2 is a collimator lens, and 3 is a diffraction grating in which reflective parts and transmitting parts are provided at equal intervals, which are attached to the object to be inspected. 4 and 4' are m-order positive and negative diffracted lights, 5 and 5' are corner cube reflectors, 7 and 7' are beam splitters, and 18 and 8' are polarizing plates provided so that the polarization directions are 45 degrees to each other. It is.

Laser light 4 reflected and diffracted at a predetermined angle from a diffraction grating 3 disposed orthogonally to the output optical path of the beam splitter 7
Polarizing plates 6 and 6' are arranged for optical paths 6 and 4' (arranged so that their polarization directions are orthogonal to each other). 10
is a concave mirror, and the laser beam 4. from the polarizing plate 6.6' is reflected.
4' is reflected back to the same optical path. In the above configuration, the laser beam emitted from the laser 1 becomes a parallel beam by the collimator lens X2, passes through the aperture on the optical path of the concave mirror 10 via the beam splitter 7, and is then reflected and diffracted by the diffraction grating 3. . The reflected and diffracted laser beams 4 and 4' pass through the polarizing plates 6 and 6', are reflected by the concave mirror 10, return to the optical path of the laser beam C, and reirradiate the diffraction grating 3. The irradiation position at this time is the same as the laser beam incident position on the diffraction grating 3. Then, the beams are again diffracted by the diffraction grating 3, overlap each other, pass through the aperture on the optical path of the concave mirror, and are split by the beam splitters 7 and 7'. The divided lights are polarized by polarizing plates 8 and 8', respectively.
and enters the photodetectors 9 and 9'.

The laser beams overlapped on the same detection surfaces of the photodetectors 9 and 9' interfere, causing a change in brightness and darkness as the diffraction grating 3 moves. Also, by combining the polarizing plates 6.6' and 8.8', there is a 90° difference between the output signals of the photodetectors VfP9 and 9'.
A phase difference of .degree. is provided, and the moving direction of the diffraction grating 3 can be discriminated.

With such a configuration, even if the oscillation wavelength of the laser 1 changes, an error in measuring the amount of movement of the diffraction grating 3 cannot occur.

Also, due to fluctuations in the oscillation wavelength of the laser 1, the reflected diffracted light 4
Even if the reflection diffraction angles of 4' and 4' change, the photodetectors 9 and 9 ensure that the light returns to the original path after being reflected by the concave mirror 10.
There is no reduction in the S/N ratio of the change in brightness of interference occurring on the detection surface of '.

Further, FIGS. 8 and 9 are modifications of the configuration shown in FIG. 7. The configuration shown in FIG. 8 utilizes transmitted diffracted light, and the configuration shown in FIG. 9 is provided with a condenser lens 11 that condenses laser light diverged by a concave mirror 10. The operating principle in both cases is the same as that shown in FIG. 7, and the explanation thereof will be omitted.

However, in the conventional linear scale, the concave mirror 10
Since the reflected light is a parallel beam, all the light that enters the photodetector becomes diverging light. Therefore, it is necessary to shorten the optical path length from the concave mirror to the photoelectric element (1, which will result in a decrease in output. Furthermore, it is necessary to arrange a polarizing plate facing the concave mirror, so a large concave mirror is used. The equipment inevitably becomes larger.

Furthermore, although the configuration shown in FIG. 9 solves the problem of decreased output, it has the disadvantage of increasing the number of parts because it requires the use of a convex lens.

Purpose of the present invention: The present invention eliminates the drawbacks of the prior art, and provides a linear encoder with a simple configuration that prevents fluctuations in laser oscillation wavelength caused by temperature changes and provides stable output. The purpose is to

In order to achieve the above object, the present invention has a diffraction grating r-, which is configured by forming grooves at predetermined intervals on the surface of a moving object, perpendicular to the moving direction, and a a light irradiation means that divides the coherent light into different directions and focuses each of the coherent lights on the same point on the surface of the diffraction grating to cause interference; and interference light that the irradiation light from the means emits after being diffracted by the diffraction grating. The present invention is characterized in that it includes a lens that converts the interference light into parallel light, and a photodetector that photoelectrically converts the interference light based on the output light of the lens. Next, embodiments of the present invention will be described with reference to the drawings.

A laser as a light source that irradiates the lens 21 and the lens 21 with laser light in alignment with the optical path of the transmitted light for the polarizing beam splitter 20 that splits the incident light into transmitted light and reflected light according to the polarization direction of the light. A diode 22 is provided. Reflection mirrors 24a and 24b are arranged symmetrically with respect to the polarizing beam splitter 20 in order to reflect each of the laser beams output from the polarizing beam splitter 20 to the same point on the surface of the diffraction grating 23.

The diffraction grating 23 is formed on one side of a glass substrate 25, and a reflection plate 26 is provided on the other side of the glass substrate 25 to reflect light from the diffraction grating 23 side toward the diffraction grating 23 side.

The light from the reflection plate 26 is further diffracted and then emitted in the vertical direction, and a convex lens 27 is disposed in its forward path, and a wavelength plate 28 and a beam splitter 29 are further disposed in this order. On the straight optical path of the beam splitter 29,
A polarizing plate 30 and a photodetector 31 are arranged in this order, and a polarizing plate 3 is also arranged on the reflected optical path of the beam splitter 29.
2 and a photodetector 33 are arranged in this order. Next, the operation in the above configuration will be explained.

After the laser light generated by the laser diode 22 is reflected by the lens 21 and 26 reflective mirrors,
After the light is adjusted to be focused by the diffraction grating 23, it enters the polarizing beam splitter 20. The light split into P and S components by the beam splitter 20 is output to each of reflection mirrors 24a and 24b. The reflected lights 34 and 35 from these reflecting mirrors 24 IL and 24b are set to be incident on the diffraction grating 23 at a predetermined angle. That is, the angle of incidence α with respect to the diffraction grating 23 is determined by equation (1).

sinγ=sir+a-2n^/P ・・・・・・(1
) (where γ is the emission angle, n is the diffraction order, P is the lattice constant,
λ is the laser light wavelength. ) As shown in FIG. 2, the reflected light 34 that enters the diffraction grating 23 enters the glass substrate 25, and when it is reflected by the reflection plate 26, passes through the glass substrate 25, and passes through the diffraction grating 23 again. It undergoes two diffraction steps and exits while diverging.

The emitted laser light is made into parallel light by a convex lens 27, and further made into circularly polarized light by a wavelength plate 28 (for example, a λ/4 wavelength plate). At this time, the P component and S component laser beams interfere with each other, and this interference light is transmitted to the beam splitter 29.
It can be divided into jul light and reflected light. Passing light passes through the polarizing plate 30
The reflected light reaches the photodetector 31 via the polarizing plate 32, and is charged and converted. The polarizing plates 30 and 32 function to provide a phase difference (for example, 90 degrees) necessary for discrimination of the movement direction of the diffraction grating 23 and signal processing.

In the configuration shown in FIG. 1, since the focal position of the convex lens 27 is placed on the surface of the diffraction grating 23, the diffraction angle changes in response to wavelength fluctuations due to temperature changes, as shown by the broken line in FIG. Even if this occurs, all the beams of laser light emitted from the lens 27 will be parallel. In this way, since each light beam is kept parallel, even if there is a deviation in the degree of overlapping of each light beam, visibility is hard to change over a relatively wide temperature range, and the S/N ratio is also hard to be affected.

FIG. 4 shows γ° when the overlap and conjugation of parallel light beams are shifted from each other due to a change in the diffraction angle, and the ■ value characteristic of ID shifted in parallel. In the figure, characteristic I indicates a characteristic due to a deviation in the reflection and diffraction angle γ° of a parallel beam of light, and characteristic ■ indicates a characteristic due to a deviation D of a parallel beam of light. As is clear from Figure 4, when the deviation is parallel, the change in V value is more gradual than when the deviation is due to a change in angle, and up to D = 0.4 mm,
The value is almost constant. This degree of deviation corresponds to a temperature change of ±30°C (provided that the distance from the reflection diffraction point to the convex lens is 20 mm), and it can be said that the influence of temperature change can be almost prevented in actual use.

Note that due to changes in the diffraction angle, -C1 reflection 2α shifts from the focal point of the lens as shown in diagram vJ3, but this shift δ is (
2) It is determined by the formula.

・・・・・・・・・(2) (However, Z is the thickness of the glass substrate 25) Here, n
= 1, P ” 2 pnl, Z = 2 mm
, α=51°-9=26°, λ1=780nm (at 25°C), λ2=7
When it is 86 nm (50°C), δ=0.
It becomes 015mm. This amount of deviation is a value that poses no problem in practice.

FIG. 5 is a configuration diagram of main parts showing another embodiment of the present invention. The difference between this embodiment and the embodiment shown in FIG. 1 is that the convex lens 2
7 is replaced by a concave lens 36. The other configurations are completely the same, so redundant explanation will be omitted. The laser beam is adjusted by a lens 21 so that it is focused on the focal point of the concave lens 36, and the diffracted light from the diffraction grating 23 is made into parallel light as in the previous embodiment. Moreover, FIG. 6 is a block diagram of the main part showing another embodiment of the photodetection method. This embodiment differs from the embodiment shown in FIG. 1 in that the wave plate 28 is moved after the beam splitter 29. By doing this, there will be a gap between the P component and the S component of the light that has passed through the beam splitter 29.
The wave plate 28 provides the necessary phase difference. The above-mentioned passing light and reflected light (same phase) from the beam splitter 29, and their respective P and S components interfere with each other by polarizing plates 30 and 32, and are photoelectrically converted by photodetectors 31 and 33, respectively.

<Effects> As explained above, according to the present invention, since the laser beam emitted from the diffraction grating is focused at the focal position of the correction lens and the parallel beam forming means is provided, the laser beam emitted from the diffraction grating is It is possible to prevent output fluctuations due to changes in the wavelength of the generated laser light, and to obtain stable output regardless of temperature changes. Furthermore, since the number of parts can be reduced, it is possible to simplify maintenance and adjustment, improve reliability, and reduce costs.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the diffraction situation in the diffraction grating, and FIG. 3 is an explanatory diagram showing the angular setting of the diffraction angle and reflection angle in the diffraction grating.
Figure S4 is a relationship diagram with the V value when the overlap of parallel light beams is shifted by an angle due to a change in the diffraction angle, and when it is shifted parallel by a correction lens, and Figure 5 shows the main part of another embodiment of the present invention. 7 is a block diagram showing an example of a conventional linear encoder, and FIGS. 8 and 9 are block diagrams showing two modified examples of the conventional device shown in FIG. 7. 20.29... Beam splitter 21... Collimating lens 22... Laser diode 24a,
24b... Reflection mirror 23... Diffraction grating 25.
...Glass substrate 26...Reflector plate 27...Plano-convex lens'X 28...λ/4 plate 30.32...
・Polarizing plate 31.33... Photodetector 36...
double convex lens

Claims (1)

[Claims] A diffraction grating is constructed by forming grating grooves at predetermined intervals on the surface of a moving object perpendicular to the moving direction, and a diffraction grating that divides coherent light into two directions, and divides each of the coherent lights into two directions on the surface of the diffraction grating. A light irradiation means for condensing light to the same point and causing interference, and the light irradiated by the means is arranged so that the focal point coincides with the emission point of the interference light emitted while being diffracted by the diffraction grating and diverged. , a convex lens that converts the interference light into parallel light,
A linear encoder comprising a photodetector for photoelectrically converting interference light based on the output light of the lens.