JPH0620969Y2 - Grating interference displacement detector - Google Patents

Description

[Detailed description of the device] [Industrial applications]

The present invention relates to a grating interference type displacement detector, and particularly to a scale having a diffraction grating formed thereon, which is suitable for use when a semiconductor laser is used as a light source in an XY table or the like, and a light beam to the diffraction grating. A light source that emits light, and a detector that includes a light receiving element that photoelectrically converts a mixed wave of a plurality of light fluxes generated by the diffraction grating, and periodically according to relative displacement between the scale and the detector. The present invention relates to an improvement of a grating interference type displacement detection device that generates a changing detection signal.

[Prior art]

2. Description of the Related Art Photoelectric encoders that generate a periodic detection signal by using a scale having a fixed pitch of optical scales are widely used. The resolution of this photoelectric encoder is determined by the width and pitch of the optical grating and the characteristics of the electronic circuit that divides the photoelectrically converted signal. In general, since the optical grating is manufactured by the etching method, the optical grating of about 4 μm is close to the limit of the accuracy of the final measurement, and if the electronic circuit is used within a range without a large cost up, the final resolution is It was around 1 μm, and it was difficult to further improve the precision. On the other hand, with the spread of photoelectric encoders, there is a demand for those that generate detection signals with higher resolution and higher accuracy. As one of the efforts to improve the resolution of the photoelectric encoder, a fine pitch (usually about 1 μm) scale is formed on the scale using holographic technology, and the scale is used as a diffraction grating to actively generate diffraction. There has been proposed a grating interference type displacement detection device that obtains a detection signal by using the above method. FIG. 3 shows a grating interference type displacement detection device proposed in Japanese Patent Laid-Open No. 47-10034. This grating interference type displacement detection device includes a scale 10 on which a diffraction grating of pitch d is formed and a laser beam 14 as a light beam on the diffraction grating.
He-Ne laser light source 12 for irradiating (wavelength λ), mirrors 16 and 18 for respectively reflecting 0th-order and 1st-order diffracted light (light flux) generated by the diffraction grating, and mirror 1.
A beam splitter (coarse diffraction grating) 20 that divides a mixed wave of the 0th-order light of the 1st-order light reflected by 8 and the 1st-order light of the 0th-order light reflected by the mirror 16 into three, and the beam splitter 20
Light receiving element 2 for photoelectrically converting each of the mixed waves divided into three
2A, 22B, and 22C. Here, each element except the scale 10 constitutes a detector. In FIG. 3, the polarization directions of the polarizers 24 and 26 inserted in the optical paths of the 0th-order light and the 1st-order light are set to be orthogonal to each other, and the central mixed wave divided into three parts is divided. The light receiving element 22A that receives the light does not generate interference fringes. Therefore, in the light receiving element 22A, not a fringe pattern but a mere sum signal is obtained, and therefore the reference signal v _r is used. In addition, immediately before the light receiving element 22B, the analyzer 2 for generating the interference fringes is formed.
8B is arranged, and the light receiving element 22B generates a first detection signal a by interference fringes. Further, a 1/4 wavelength plate 30 and an analyzer 28C are arranged immediately in front of the light receiving element 22C, and from this light receiving element 22C, a second detection signal b which is 90 ° out of phase with the first detection signal a.
Is generated. Here, the incident angle θ of the laser beam 14 and the diffraction angle φ of the primary light are related by the following equation. d (sin θ + sin φ) = λ (1) According to such a grating interference type displacement detection device, for example, by manufacturing the scale 10 by a hologram method,
Since an optical grating of 1 μm or less can be formed,
It is also possible to achieve a resolution of 0.01 μm.

[Problems to be solved by the device]

However, the central mixed wave, which is photoelectrically exchanged by the central light receiving element 22A, originally has a polarization direction orthogonal to each other.
The reference signal v _r should be at the DC level because there is no interference signal in the mixed wave of the light flux, but (1) since the scale 10 and the beam splitter 20 have some effect of an analyzer,
(2) Since the polarization directions are not orthogonal to each other, there is a problem that an interference signal is superposed even if there is no analyzer and an AC component is included in the reference signal v _r .

[The purpose of the device]

The present invention has been made to solve the above-mentioned conventional problems, and the reference signal does not include an AC component due to the interference of mixed waves, and accordingly, the grating interference type displacement detection that can obtain a stable detection signal. The purpose is to provide a device.

[Means for solving problems]

The present invention provides a detector including a scale having a diffraction grating, a light source for irradiating the diffraction grating with a light beam, and a light receiving element for photoelectrically converting a mixed wave of a plurality of light beams generated by the diffraction grating. In a grating interference type displacement detection device that generates a detection signal that periodically changes according to relative displacement between the scale and the detector, before mixing a plurality of diffracted light beams generated by the diffraction grating. , An optical element that extracts or separates a part of each diffracted light beam, two light receiving elements that photoelectrically convert the extracted or separated diffracted light beams, and a reference signal by adding the outputs of the two light receiving elements The above-mentioned object is achieved by including an adder for obtaining the above and a circuit for compensating the level fluctuation of the detection signal with the reference signal. Further, the optical element for extracting a part of the light beam generated by the diffraction grating is the diffraction grating itself, and the two light receiving elements detect the 0th order diffracted light.

[Action]

The present invention is, in the above-mentioned grating interference type displacement detection device, before mixing a plurality of diffracted light beams generated by a diffraction grating, after extracting or separating a part of each diffracted light beam, and after photoelectrically converting each part. The reference signal is obtained by addition. Therefore, the reference signal does not include an AC component due to the interference of the mixed waves, and a stable detection signal can be obtained.

【Example】

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, the first embodiment of the present invention is a light source comprising a scale 10 having a diffraction grating and a laser diode 32 and a collimator lens 34 for irradiating the diffraction grating with a laser beam 14 having a wavelength λ. 31 and light receiving elements 22B and 22C for photoelectrically converting a mixed wave of a plurality of light fluxes generated by the diffraction grating, and analyzers 28B and 28.
C, a detector including a 1/4 wavelength plate 30, and a grating interference type displacement detection device that generates detection signals a and b that periodically change according to the relative displacement between the scale 10 and the detector, The laser beam 14 from the light source 31 is divided into two,
After the polarization directions are made orthogonal to each other by the polarizers 42A and 42B, the half mirror 40 as an optical element that is incident on two diffraction points A and B on the diffraction grating substantially symmetrically (incident angle θ ₁ θ ₂ ), The diffraction grating (scale 10) as an optical element for extracting a part of the diffracted light beams (0th order light of the incident beam a and 0th order of the incident beam b) generated at different diffraction points A and B, and Two light receiving elements 44A, 44B and an adder 45 for photoelectrically converting the extracted diffracted light fluxes and adding them to obtain a reference signal v _r , and level fluctuations of the detection signals a, b with the reference signal v _r The difference calculators 46A and 46B are provided as circuits for compensating the above. The mixed wave of the two first-order (diffraction) lights a and b generated substantially symmetrically at the two diffraction points A and B is analyzed by the half mirror 48 to the analyzer 28B or the analyzer 28C and the quarter-wave plate, respectively. The light receiving elements 22B and 22 similar to the conventional example
It is incident on C. In the figure, 50 is the light receiving elements 44A, 44B, 2
It is a preamplifier for amplifying the outputs of 2B and 22C, respectively. The half mirror 40 above the scale 10 and the two polarizers 42A and 42B can be replaced by one polarization beam splitter. The other configurations are similar to those of the conventional example, and thus the description thereof is omitted. The operation of the first embodiment will be described below. The light beams extracted by the diffraction grating itself on the scale 10 are the 0th order (diffraction) light a and the 0th order (diffraction) light b, and the outputs of the light receiving elements 44A and 44B are amplified by the preamplifier 50, The adder 45 performs a sum operation to obtain the reference signal v _r . On the other hand, the detection signals a and b which are obtained by the light receiving elements 22B and 22C and the preamplifier 50 and have a phase difference of 90 ° are
A difference calculator 46A as a circuit for compensating for level fluctuations,
46B, the reference signal v _r is subtracted, and the detection signal a ′,
b'is generated. With respect to the output fluctuation of the laser diode 14 and the attenuation of the intensity of the laser beam due to the contamination of the scale 10, the detection signals a and b and the reference signal v _r are attenuated with the same tendency.
The final DC levels of the signals a'and b'are stable, and good counting processing can be performed. At this time, before mixing the plurality of diffracted light beams generated by the diffraction grating, a part of each diffracted light beam is extracted and the reference signal v _r is extracted.
Therefore, the reference signal does not include an AC component due to the interference signal of the mixed wave, and the stable reference signal v _r , and accordingly, the detection signals a ′ and b ′ can be obtained. In the present embodiment, the 0th-order lights a and b, which are originally wasted, are utilized, which is efficient. Further, since the optical element for extracting a part of the diffracted light flux is the diffraction grating itself, the configuration is simple. Further, even if the wavelength λ of the laser diode 32 changes, the diffraction angles φ ₁ and φ _{2 of} both the primary light a and the primary light b become symmetrically small, so that they are incident on the light receiving elements 22B and 22C. The position is almost constant, and the detection signal is stable. or,
There is no back talk back to the laser diode 32. Further, the reflected light on the surface of the scale 10 does not directly enter the light receiving element. Next, a second embodiment of the present invention will be described in detail with reference to FIG. The second embodiment is similar to the above-mentioned conventional example in that the scale 10
A light source 31 including a laser diode 32 and a collimator lens 34, light receiving elements 22B and 22C, and an analyzer 2.
8B, 28C, a detector including a quarter wave plate 30,
A half-mirror 60 as an optical element that divides the laser beam 14 from the light source 31 into two in a grating interference type displacement detection device that generates a detection signal that periodically changes according to relative displacement between the scale 10 and the detector. , Which is an optical element that causes the divided light beams to enter the same diffraction point C on the diffraction grating symmetrically at the same incident angle θ. The pair of mirrors 62A and 62B and the first-order (diffracted) lights b and a are made to have polarization directions orthogonal to each other by the polarizers 64A and 64B, respectively, are reflected by the pair of mirrors 66A and 66B, and are further separated. Half mirror 68A as an optical element,
68B and a light receiving element 4 similar to that of the first embodiment for photoelectrically converting the separated diffracted light to obtain a reference signal v _r.
4A and 44B and an adder 45, and difference calculators 46A and 46B similar to those in the first embodiment for compensating the level fluctuation of the detection signals a and b with the reference signal v _r . It is a thing. Regarding the incident angle θ and the diffraction angle φ, for example, the pitch d of the diffraction grating is 0.5 μm, and the wavelength λ of the laser beam 14 is 0.78.
In the case of μm, incident angle θ = 58.5 °, diffraction angle φ = 45 °
Is set to a value that is significantly different from. This is the incident angle θ
This is because when the diffraction angle φ is close to 0, the 0th-order light and the 1st-order light are superposed and the S / N ratio of the detection signal deteriorates. The rest of the configuration and operation are the same as those of the first embodiment, so description will be omitted. In the present embodiment, since the reference signal v _r is created from a part of the diffracted light separated by the half mirrors 68A and 68B, the output fluctuation of the laser diode 32 and the intensity of the laser beam due to the contamination of the scale 10 are reduced. In addition to the attenuation, even if there is a change in the intensity of the laser beam due to a change in the diffraction efficiency of the scale 10 depending on the location, a part of the diffracted light itself is converted into the reference signal v _r , so the detection signal a ′, The DC level of b'is kept stable. Further, even if the wavelength λ of the laser diode 32 changes, both the primary light a and the primary light b have the same diffraction angle φ.
The diffraction angle φ decreases symmetrically. Therefore, each light receiving element 22
The incident directions and positions on B and 22C are almost constant, and the detection signals a and b are stable. Also, the reflected light on the surface of the scale 10 does not directly enter the light receiving element. Further, since the diffraction points C coincide with each other, an error due to the inclination of the diffraction grating surface does not occur.

[Effect of device]

As described above, according to the present invention, the reference signal does not include an AC component regardless of whether or not the interference signal is superimposed on the mixed wave. Therefore, it has an excellent effect that a stable detection signal can be obtained.

[Brief description of drawings]

FIG. 1 is a first view of a grating interference type displacement detection device according to the present invention.
FIG. 2 is a sectional view showing the structure of the embodiment, FIG. 2 is a sectional view showing the structure of the second embodiment, and FIG. 3 is a sectional view showing the structure of an example of a conventional grating interference type displacement detection device. 10 ... Scale, A, B, C ... Diffraction point, 14 ... Laser beam, 22B, 22C, 44A, 44B ... Light receiving element, 31 ... Light source, 42A, 42B, 62A, 62B ... Polarizer, 45 ... Adder, 46A, 46B ... Difference calculator, 60, 68A, 68B ... Half mirror, 62A, 62B ... Mirror, a, b ... Incident beam.

Claims (2)

[Scope of utility model registration request] 1. A detection including a scale having a diffraction grating, a light source for irradiating the diffraction grating with a light beam, and a light receiving element for photoelectrically converting a mixed wave of a plurality of light beams generated by the diffraction grating. And a grating interference type displacement detection device that generates a detection signal that periodically changes according to relative displacement between the scale and the detector, before mixing a plurality of diffracted light beams generated by the diffraction grating. And an optical element for extracting or separating a part of each diffracted light beam, and photoelectrically converting the extracted or separated diffracted light respectively 2.
One light receiving element, an adder for adding the outputs of the two light receiving elements to obtain a reference signal, and a circuit for compensating the level fluctuation of the detection signal with the reference signal, Displacement detection device of grating interference type. 2. A utility model in which the optical element for extracting a part of the light beam generated by the diffraction grating is the diffraction grating itself, and the two light receiving elements are adapted to detect 0th-order diffracted light. The grating interference type displacement detection device according to claim 1.