JP3013467B2 - Laser interference encoder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser interference encoder, and more particularly, to an optical encoder capable of obtaining a high-precision output signal without being affected by a position change of an optical scale in a direction of an optical path length.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a conventional laser interference encoder. This encoder has already been disclosed in Japanese Patent Laid-Open No. 1-176.
No. 914, and includes a laser diode 8, a collimating lens 9, an optical scale 10, a prism 11, detectors 12a and 12b, and the like.

In the apparatus shown in FIG. 4, a laser diode 8 is disposed at a focal position of a collimator lens 9 and irradiates an optical scale 10 with coherent parallel light.
The diffracted light from the diffraction grating surface A of the optical scale 10 is bent twice in the perpendicular direction by the prism 11 and is projected again on the diffraction grating surface B of the optical scale 10. The optical scale 10 also serves as an index scale, and the optical path length g from the point A to the point B from the detectors 12a and 12b, that is, the general interval between the main scale and the index scale is g = (P ² /
When λ) (n ± 1/4) is satisfied, outputs having a 90 ° phase difference are obtained. That is, according to this configuration, a sine wave output having a phase difference of 90 ° can be obtained with a simple configuration, and the number of output pulses is doubled to obtain high resolution.

[0004]

However, in the laser interference type encoder shown in FIG. 4, fluctuations in the optical path length direction of the optical scale, that is, in the vertical direction in FIG.
The optical path length g changes, and the above-mentioned g = (P ² / λ) (n ± 1 /
The phase term 1/4 in the expression 4) fluctuates, and as a result, there is a disadvantage that the two detectors 12a and 12b cannot obtain an output that is exactly 90 ° out of phase. Therefore, in the above-described conventional apparatus, it is necessary to strictly suppress the vertical fluctuation of the optical scale.

An object of the present invention has been made in view of the above-mentioned problems in the conventional apparatus, and is intended to prevent the optical path length from being changed even if the optical scale is fluctuated in the optical path length direction. The object is to obtain a stable 90 ° phase difference output.

[0006]

A laser interference encoder according to the present invention has a diffraction grating in which a light transmitting portion and a light non-transmitting portion are repeated at a constant pitch, and also serves as an index scale. Formula scale, a coherent light source that transmits and illuminates the optical scale with coherent parallel light, and the optical formula again from the same direction as the direction in which light transmitted through the optical scale is illuminated from the coherent light source to the optical scale. An optical system for projecting the image on a scale and moving the projected image in a direction opposite to a measurement moving direction of the optical scale.

[0007]

In the above-described laser interference encoder, the parallel light emitted from the coherent light source to the optical scale is again irradiated by the optical system in the same direction as the coherent light source illuminates the optical scale. Irradiated from the direction. Accordingly, by detecting, with the detector, the light transmitted through the optical scale out of the light re-irradiated on the optical scale, an output signal similar to the conventional one can be obtained. In addition, the distance from the point at which the light transmitted through the optical scale from the coherent light source to the point at which the light is transmitted through the optical scale and projected onto the optical system again by the action of the optical system and enters the optical scale is as follows. It does not fluctuate due to position fluctuation in the optical path length direction of the optical scale. Therefore, the phase of the signal output from the detector becomes stable even if there is a position change in the optical path length direction of the optical scale.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a front view of a laser interference encoder according to one embodiment of the present invention, and FIG. 2 is a side view of the encoder of FIG. FIG. 3 shows FIG.
3 is a perspective view of the laser interference encoder shown in FIG. 2 and FIG. FIG. 1 corresponds to a diagram viewed from the direction of arrow A in FIG. 3, and FIG. 2 corresponds to a diagram viewed from the direction of arrow B in FIG.

As shown in these figures, one of the present inventions
The laser interference encoder according to the embodiment includes a laser diode 1, a collimating lens 2, an optical scale 3, triangular prisms 4, 5, a rhombic prism 6, a detector 7a,
7b and the like. The laser diode 1 is arranged at the focal position of the collimating lens 2, and is configured to irradiate the optical scale 3 with parallel light. The optical scale 3 can be constituted by a linear scale that also serves as an index scale.

The coherent monochromatic light emitted from the laser diode 1 is collimated by a collimator lens 2 and enters a linear scale 3. The diffracted light transmitted through the linear scale 3 is bent twice at right angles by the prism 4 and enters the prism 5. The light incident on the prism 5 is
The light is bent at a right angle twice in a direction perpendicular to the bending direction by the prism 4 and enters the prism 6. Prism 6
Incident on the optical scale 3 is bent twice by the prism 6 and again incident on the optical scale 3. In this case, the direction of light incident on the optical scale from the prism 6 is the same as the direction in which light from the laser diode 1 illuminates the optical scale 3. In this manner, the detectors 7a and 7b are configured to detect a change in brightness due to the relative movement between the diffracted projected image applied to the optical scale and the optical scale 3.

In the laser interference type encoder having the above-described configuration, the coherent light emitted from the laser diode 1 constituting the coherent light source is converted into parallel light by the collimator lens 2 and applied to the diffraction grating of the optical scale. You. Light diffracted by passing through the diffraction grating of the optical scale 3 is bent by the prism 4, the prism 5, and the prism 6 as shown in the drawing, and is incident on the optical scale 3 again. Since the optical path length g of the diffracted light from the diffraction grating surface 3a to the diffraction grating surface 3b of the optical scale 3 is set to satisfy g = (P ² / λ) (n ± ｎ), the diffraction Sinusoidal interference fringes having the same pitch as the linear scale of the optical scale 3 appear on the grating surface 3b. This is because the intensity distribution of the interference fringes of only the direct light and the first-order diffracted light I ⁺ (x, z) = {(1/4) + (1 / π ² )} − (1 / π) sin} (2π / P ^{) x- (λπ / P 2)} z} and intensity distribution of the interference fringes only direct light and the minus first-order diffracted light ^{I - (x, z) =} {(1/4) + (1 / π 2)} - ( 1 / π) sin {(2π / P) x− (λπ / P ² ) z}.

Also, due to the geometrical optical effect of the prism 5, the projected sinusoidal interference fringes having the same pitch as the linear scale 3 move in the direction opposite to the measurement moving direction of the linear scale 3. For this reason, when the linear scale 3 moves by one pitch, the change in brightness appears for two pitches. That is, the original number of pulses of the optical scale can be doubled.

The diffracted light corresponding to the sinusoidal interference fringes projected on the diffraction grating surface 3b in this manner passes through the diffraction grating surface 3b and then enters the detectors 7a and 7b. Each of the detectors 7a and 7b is arranged to detect a change in an interference region of only the direct light and the first-order diffracted light, and a change in an interference region of only the direct light and the minus first-order diffracted light, respectively. As a result, a complete sinusoidal output having a phase difference of 90 ° from each detector 7a, 7b is obtained. Moreover, in this case, the optical path length g does not change even if the optical scale 3 moves in the optical path length direction, so that an output without fluctuation in the phase difference is obtained from each of the detectors 7a and 7b.

[0014]

As described above, according to the present invention, by using a simple optical system, even if there is a variation in the optical path length direction of the optical scale, a stable 90 ° phase difference can be obtained. And an extremely high-precision encoder can be realized.

[Brief description of the drawings]

FIG. 1 is a front view showing a schematic configuration of a laser interference encoder according to one embodiment of the present invention.

FIG. 2 is a side view showing the laser interference encoder of FIG. 1 as viewed from the side.

FIG. 3 is a perspective view showing a configuration of the laser interference type encoder shown in FIG.

FIG. 4 is a front view showing a conventional laser interference type encoder.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser diode 2 Collimating lens 3 Optical scale 3a, 3b Diffraction grating surface 4, 5, 6 Prism 7a, 7b Detector

Claims (2)

(57) [Claims] 1. An optical scale having a diffraction grating in which a light transmitting portion and a light opaque portion are repeated at a constant pitch, and also serving as an index scale, and a coherent collimated optical scale of the optical scale. A coherent light source that transmits and illuminates with the parallel light, and light that has passed through the optical scale is projected onto the optical scale again from the same direction as the direction in which the coherent light source illuminates the optical scale, and An optical system in which the projected image moves in a direction opposite to a measurement moving direction of the optical scale. 2. The distance g between the optical scale and the index scale is converted to air, and approximately g = (P ² / λ) (n ± １ ／), where λ is the wavelength of the illumination light P is the scale pitch n The laser interference encoder according to claim 1, wherein is an arbitrary natural number.