JP3030905B2 - Fixed point detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixed point detecting device suitable for installation in a linear encoder, a rotary encoder, etc., whose position is optically detected.

[0002]

2. Description of the Related Art As this type of device, an origin position detecting device used for a linear encoder or the like is known.

In this device, a main scale (movable) in which a large number of light-transmitting portions and light-shielding portions are formed in a random lattice shape.
And an index scale (fixed) on which a corresponding pattern is formed, and by detecting the overlap of the lattice-like patterns, the origin position is detected, or the main scale and the index scale are respectively provided. An origin position is detected by forming one slit and detecting its overlap.

[0004]

However, in the conventional detection method, the distance between the main scale and the index cannot be widened, and when the detection sensitivity is increased, light diffraction occurs. It does not have the characteristics, and it is difficult to detect with high sensitivity.

In addition, when the distance between the main scale and the index scale fluctuates, and when the amount of light from the light source fluctuates, the detection position of the origin changes, and high-precision detection cannot be performed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fixed-point position detection apparatus capable of detecting a fixed-point position with high sensitivity by taking a large distance between a scale and a detection head and performing a high-precision fixed-point position detection even when the light amount of a light source fluctuates. It is to provide a device.

[0007]

According to the fixed point detecting device of the present invention, for example, as shown in FIG. 1, the diffraction efficiency of incident light can be changed along the measuring direction X, and the light of the same light source 3A can be changed. A pair of diffraction gratings 5A, 5B to be split into two, light / electric converters 7A, 7B for converting the light diffracted by the respective diffraction gratings 5A, 5B into electric signals, and the converted electric signals And a detecting means 9 for detecting that the level values of the two match.

[0008]

In the fixed point detecting device according to the present invention, the diffracted lights obtained from the pair of diffraction gratings whose diffraction efficiencies change along the measuring direction are respectively converted into electric signals, and the level values of the electric signals coincide with each other. Sometimes a desired fixed point is specified.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a fixed point detecting device according to the present invention will be described below with reference to the drawings. FIG. 1 shows a fixed point detection apparatus 1 to which the present invention is applied. The apparatus 1 includes a light source system 3 and a diffraction grating 5 whose diffraction efficiency changes along a measurement direction.
And optical / electrical converters (O / I converters) 7A and 7B, and an electrical processing circuit 9 (detection means).

The light source system 3 includes a light source 3A such as a semiconductor laser.
Is emitted to the diffraction grating 5 via the collimator lens 3B and the condenser lens 3C. The diffraction grating 5 is a pair of diffraction gratings 5A and 5B, and the incident laser light is split by the diffraction gratings 5A and 5B, and the diffracted light enters the O / I converters 7A and 7B, respectively. Is done.

In the O / I converters 7A and 7B, the incident diffracted light is converted into an electric signal (current I) and inputted to the differential amplifier 9C and the comparator 9D via the amplifiers 9A and 9B of the circuit 9. You.

In this case, the laser light from the light source system 3 is stopped down by the lenses 3B and 3C so as to have an appropriate size and shape on the diffraction gratings 5A and 5B, and is incident perpendicularly to the grating surface. .

The configuration of the light source system 3 and the like is as follows.
/ I converters 5A and 5B (see FIG. 2). In a reflection type diffraction grating, the mirror 2
Becomes unnecessary (see FIG. 3).

On the other hand, the diffraction gratings 5A and 5B partially overlap each other as understood from FIG.
(Shown by oblique lines), whereby the laser light is incident on both diffraction gratings 5A and 5B.

In this case, the laser light is transmitted through the diffraction gratings 5A and 5B.
Incident perpendicularly to the lattice plane of.

The diffraction efficiencies of both diffraction gratings 5A and 5B are set to be symmetrical along the measurement direction X about a position P where the diffraction efficiencies are equal (see FIG. 5).

Therefore, the current I obtained by O / I conversion of the diffracted light from the diffraction gratings 5A and 5B is supplied to the amplifier 9A,
When the current-voltage conversion is performed at 9B and input to the differential amplifier circuit 9C, as can be understood from FIG. 6, an XV (voltage) curve having a maximum value and a minimum value is obtained. The zero cross point S corresponds to the fixed point to be obtained. The diffraction grating 5
A and 5B may all be overlapped (see FIG. 7), and a configuration in which they are arranged adjacent to each other is also preferable, as understood from FIGS.

When the diffraction gratings 5A and 5B are adjacent to each other, as will be understood from FIG. 10, the influence in the Y direction other than the X direction (the hatched portion in the figure) is not detected. (A curve shown by a solid line in FIG. 12).

That is, as understood from FIG.
If the portion irradiated with the laser beam (the portion shown by oblique lines in the drawing) is small, the XI curve shown by the dotted line in FIG. 11 is obtained, and the position P is shifted. It is necessary to increase it.

As described above, in this embodiment, the current I obtained by O / I conversion of the diffracted light from the diffraction gratings 5A and 5B is converted into a voltage change by the electric processing circuit 9, and its XV The zero cross point S of the curve is detected as a point corresponding to the origin position.

Therefore, since the detection signal of the detection point is clear (sharp), the fixed point can be detected with high sensitivity.

Further, since the laser beam is perpendicularly incident on the lattice plane, even when the lattice plane moves along its own plane direction, the detection point S does not change and high-precision detection can be performed. .

In this case, when the laser beam is incident obliquely to the lattice plane, as understood from FIG.
The detection point S changes.

Further, since the diffraction efficiency changes symmetrically with respect to the point P, when the light amount of the light source 3A changes (FIG. 1).
4) Even when the light wavelength of the light source 3A changes (FIG. 15) and when the focal position of the laser light changes (FIG. 16), the detection point S does not change and is stable and high in only one direction. Accurate detection is possible.

In addition, by using a volume hologram as the diffraction gratings 5A and 5B, the origin (fixed point) can be created in the same manufacturing process as a scale for a linear encoder equipped with a volume hologram. In addition, an origin pattern can be easily formed on a linear encoder using a volume hologram, and higher sensitivity (S / N ratio) can be obtained by using a volume phase hologram.

When the origin of the linear encoder is specified, the distance between the scale and the detection head can be increased, so that the degree of freedom in designing the scale device is improved.

Next, the principle on which the diffraction gratings 5A and 5B are manufactured by the fixed point detecting device 1 of the above embodiment will be described.

The apparatus 1 includes a coherent light source 3A, a pair of diffraction gratings 5A and 5B, and O / I converters 7A and 7B. The diffraction gratings 5A and 5B are arranged along the measurement direction X. The diffraction efficiency changes.

Therefore, a case where a transmission type volume hologram is used as an example of such a diffraction grating will be described.

H. Kogelnik, Bellsys
t. Tech. J. 48, 2909 (1969), Kogellink's paper describes that for this type of hologram, the diffraction efficiency η is given by

[0031]

Η = Sin ² ｛(υ ² + ξ ² ) ^1/2 ｝ / (1+
ξ ² / υ ² )

Here, υ and ξ are used as parameters, and are given by the following equations, respectively.

[Number 2] _{υ = πn 1 d / λ (} C R C S) 1/2 ξ = ΔθKdSin (φ-θ O) / 2C S = -ΔλK 2 d / 8πnC S

Where C _R = Cos θ C _S = Cos θ- (K / β) Cos φ K = 2π / Λ β = 2πn / λ

Here, Λ: grating pitch, λ: wave of incident light
Length, Δθ: deviation from the incident angle that satisfies the Bragg condition,
Δλ: deviation from the wavelength of incident light that satisfies the Bragg condition
And θ _O: Incident angle satisfying the Bragg condition, n: Diffraction pattern
The refractive index of the child, n₁: The amount of change in the refractive index of the diffraction grating.

Therefore, it can be seen from the above equations 1 and 2 that when the parameter と or ξ is changed, the diffraction efficiency η is changed.

In FIG. 17, when the change of the exposure power when exposing the hologram, the thickness d of the recording medium α, the inclination φ of the lattice plane, and the like are changed, the parameter υ is changed.

On the other hand, when the grating pitch Λ, the inclination of the grating, etc. are changed, the parameter ξ is changed.

Therefore, if the wavelength and the incident angle of the incident light are constant, the diffraction grating whose diffraction efficiency changes along the measurement direction has the gradual parameters υ, ξ along the measurement direction.
Is understood to change.

Therefore, as can be understood from FIG. 18, when the light beam A having a cylindrical wavefront and the parallel beam B having a plane wavefront interfere with each other and are recorded as a hologram, they are moved in the X direction (measurement direction). A diffraction grating along which the grating pitch and the gradient of the grating change (the direction of the grating vector K changes) is obtained (see FIG. 19).

Therefore, as can be understood from FIG. 20, when the light beam is irradiated, the diffraction efficiency is changed by the change of the grating pitch and the inclination of the grating. As shown in FIG. A voltage V that varies along the direction is obtained.

It is also preferable to use a reflection type volume hologram instead of the transmission type one (see FIG. 22). In this case, the parallel beam B is incident from the side opposite to the recording surface. You.

The wavefront to be interfered may be two cylindrical wavefronts.

[0043]

As will be understood from the above description, in the fixed point detecting device according to the present invention, the diffracted light obtained from each of the pair of diffraction gratings whose diffraction efficiency changes along the measurement direction is converted into an electric signal. The conversion is performed, and when the level values of the electric signals match, a desired fixed point is specified. Therefore, the fixed point position detection signal has sharp characteristics, so that the fixed point position detection can be performed with high sensitivity. In addition, a fixed point position change due to a light amount or a light wavelength change from the light source or a change in a direction other than the detection direction is extremely small, and highly accurate position detection is possible.

[Brief description of the drawings]

FIG. 1 is an overall schematic configuration diagram of a fixed point detection device according to the present invention.

FIG. 2 is a configuration diagram showing another example of the light source system.

FIG. 3 is a configuration diagram showing another example of the light source system.

FIG. 4 is an explanatory view showing an example of the arrangement of diffraction gratings.

FIG. 5 is a characteristic diagram showing an XI curve.

FIG. 6 is a characteristic diagram showing an XV curve.

FIG. 7 is an explanatory diagram showing an example of the arrangement of diffraction gratings.

FIG. 8 is an explanatory diagram showing an example of the arrangement of diffraction gratings.

FIG. 9 is an explanatory diagram showing an example of the arrangement of diffraction gratings.

FIG. 10 is an explanatory diagram of a light irradiation range.

FIG. 11 is a characteristic diagram showing an XI curve.

FIG. 12 is an explanatory diagram of a light irradiation range.

FIG. 13 is an explanatory diagram showing a state where a fixed point has changed.

FIG. 14 is an XI curve when the amount of light changes.

FIG. 15 is an XI curve when the light wavelength changes.

FIG. 16 is an XI curve when the focus changes.

FIG. 17 is an explanatory diagram of parameters.

FIG. 18 is an explanatory diagram showing light interference in a transmission type volume hologram.

FIG. 19 is an explanatory diagram of a change in diffraction efficiency.

FIG. 20 is an explanatory diagram of a change in diffraction efficiency and the like.

FIG. 21 is an explanatory diagram showing an XV curve.

FIG. 22 is an explanatory view showing light interference in a reflection type volume hologram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fixed point detection device 3 Light source system 3A Light source 5A, 5B Diffraction grating 7A, 7B Light / electricity converter 9 Electric processing circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-49-76427 (JP, A) JP-A-3-261821 (JP, A) JP-A-63-277926 (JP, A) JP-A-1- 280215 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01D 5/26-5/38 G01B 11/00-11/30

Claims (1)

(57) [Claims] The diffraction efficiency of incident light can be varied along the measurement direction. A pair of diffraction gratings for splitting light of the same light source into two, and the light diffracted by each of the diffraction gratings is electrically operated. A fixed-point detection device comprising: an optical / electrical converter that is converted into a signal; and detection means for detecting that the level values of the converted electric signal match.