Optical displacement detector including a displacement member's surface having a diffractive pattern and a holographic lens pattern

• US 5,073,710 A
• Filed: 09/19/1990
• Issued: 12/17/1991
• Est. Priority Date: 09/21/1989
• Status: Expired due to Term

• Current Assignee: Copal Company Limited (Nidec Corporation)
• Original Assignee: Copal Company Limited (Nidec Corporation)
• Inventors: Takagi, Masaaki, Omura, Atsushi, Nakamura, Shigekazu
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): LE, QUE TAN
• Application Number:
  US07/584,786
• Time in Patent Office: 454 Days
• Field of Search: 250/237 G, 250/231.14, 250/231.16, 356/374, 356/375
• US Class Current: 250/231.14
• CPC Class Codes: G01D 5/38 by diffraction gratings

Abstract

An apparatus for optically detecting displacement. The light source is provided for emitting a coherent primary light. A displacement member is disposed to undergo a displacement transversely of the primary light to receive the primary light on its face. The displacement member has on its face a diffractive pattern effective to diffract the primary light and a holographic lens pattern effective to converge the primary light to thereby convert the primary light into a secondary light. A detector is receptive of the secondary light to detect optically the displacement of the displacement member.

Claims

1. An apparatus for optically detecting displacement, comprising:

light source means for emitting a coherent primary light;

a displacement member undergoing a displacement transversely of the primary light to receive the primary light on its face, the displacement member having on its face a diffractive pattern effective to diffract the primary light and a holographic lens pattern effective to converge the primary light, thereby converting the primary light into a secondary light; and

detection means receptive of the secondary light to detect optically the displacement of the displacement member.

2. An apparatus according to claim 1;

wherein the diffractive pattern comprises a one-dimensional diffractive pattern arranged along a direction of the displacement so as to continuously diffract the primary light to form an enlarged diffraction image which shifts in the displacement direction, and the holographic lens pattern comprises a local holographic lens pattern disposed at a reference position of the displacement member and operative when traversing the primary light to converge a part of the primary light to form a reference spot image.

3. An apparatus according to claim 2;

wherein the light source means includes a point light source for emitting a beam of coherent primary light having spherical wavefront, and the local holographic lens pattern has a size and shape corresponding to an effective cross section of the primary light beam.

4. An apparatus according to claim 2;

wherein the displacement member comprises a rotational disc, the one-dimensional diffractive pattern is comprised of radial slits formed along an annular periphery of the disc, and the local holographic lens pattern is comprised of coaxial circular slits formed adjacent to the diffractive pattern radially inward of the annular periphery.

5. An apparatus according to claim 1;

wherein the diffractive pattern comprises a one-dimensional diffractive pattern arranged along a direction of the displacement so as to continuously diffract the primary light to form an enlarged diffraction image which shifts in the displacement direction, and the holographic lens pattern comprises a planar holographic lens pattern arranged orthogonally in superposed relation to the diffractive pattern so as to contract the enlarged diffraction image in a direction normal to the displacement direction.

6. An apparatus according to claim 5;

wherein the displacement member comprises a rotational disc, the one-dimensional diffractive pattern is comprised of radial slits formed along an annular periphery of the disc, and the planar holographic lens pattern is comprised of coaxial circular slits formed on the annular periphery.

7. An apparatus according to claim 1;

wherein the diffractive pattern and the holographic lens pattern are comprised of etched slits formed on the same face of the displacement member.

8. An apparatus according to claim 1;

wherein the displacement member comprises an elongated member undergoing linear displacement.

9. An apparatus according to claim 1;

wherein the displacement member comprises an encoder plate disposed a given axial distance from the light source means along its optical axis to undergo displacement transversely of the primary light, the diffractive pattern comprises a one-dimensional diffractive pattern operative to diffract the primary light to form an enlarged diffraction image depending on a wavelength of the primary light in a direction of the optical axis, and the light source means includes a light source for emitting along the optical axis the coherent primary light having a varying wavelength dependent on an ambient temperature, a base for mounting the light source, and a compensative member interposed between the light source and the base to undergo thermal expansion and contraction along the optical axis in response to the ambient temperature so as to adjust the axial distance between the light source and the encoder plate to thereby effect compensation for the varying wavelength of the primary light dependent on the ambient temperature.

10. An apparatus according to claim 9;

wherein the one-dimensional diffractive pattern comprises a type operative to form an enlarged diffraction image according to a first relation of ##EQU15## where M denotes a distance between the encoder plate and the enlarged diffraction image, L denotes the axial distance between the light source and the encoder plate,

T denotes a pitch of the one-dimensional diffractive pattern, λ

denotes the wavelength of the coherent primary light, and G and H denote an integer,

the light source comprises a type operative to emit the coherent primary light according to a second relation of λ

=λ

₀ -α

·

Δ

t where λ

₀ denotes a reference wavelength, Δ

t denotes a temperature change and α

denotes a wavelength variation coefficient,

the encoder plate comprises a type adjustable to set the axial distance L according to a third relation of L= -Aλ

+B where A and B are constants in order to satisfy the first relation, and

the compensative member has an axial thickness S determined by a fourth relation of S= α

·

A/ β

where β

denotes a linear thermal expansion coefficient of the compensative member in order to satisfy the third relation.

11. An apparatus according to claim 9;

wherein the compensative member is composed of a thermally expandable and contractable plastic material.

12. An apparatus according to claim 2;

wherein the detection means includes a mask plate disposed to selectively pass the reference spot image and formed thereon with a pair of a wide slot having a relatively wide effective opening width and a narrow slot having a relatively narrow effective opening width, one photodetector receptive of the reference spot image through the wide slot to produce a comparative pulse signal having a wide pulse width, another photodetector receptive of the reference spot image concurrently to said one photodetector to produce a detection pulse signal having a narrow pulse width, and a detection circuit operative to process the detection pulse signal with the comparative pulse signal to output a reference pulse signal indicative of passage of the reference point on the displacement member.

13. An apparatus according to claim 12;

wherein the detection circuit includes a comparator operative to threshold the comparative pulse signal to form a gate pulse signal, and a gate circuit for gating the reference pulse signal in response to the gate pulse signal.

14. An apparatus according to claim 2;

wherein the detection means includes a stationary mask plate arranged to selectively pass the secondary light, a first photodetector receptive of the shifting diffraction image through the stationary mask plate to produce a corresponding AC signal, a second photodetector receptive of the reference spot image superposed with the diffraction image through the stationary mask plate to produce a detection pulse signal containing a detection pulse component due to the spot image and an AC noise component due to the diffraction image, and a detection circuit operative to process the detection pulse signal with the AC signal to remove the AC noise component to output a reference pulse signal in synchronization with the detection pulse signal to thereby detect the reference point of the displacement member.

15. An apparatus according to claim 14;

wherein the stationary mask plate has an effective slot for passing the reference spot image and a part of the diffraction image and a dummy slot arranged in registration with the effective slot along a direction normal to the displacement direction so as to pass only a part of the diffraction image, and wherein the first photodetector is disposed behind the dummy slot to thereby produce a background AC signal free of the detection pulse component and the second photodetector is disposed behind the effective slot to produce a detection pulse signal containing an AC background noise component and a detection pulse component.

16. An apparatus according to claim 14;

wherein the stationary mask plate has an effective slot disposed to pass the reference spot image and a part of the diffraction image and a plurality of stationary slot groups operative to pass the shifting diffraction image with phase difference of 90°

or 180°

among the slot groups, and wherein the second photodetector is disposed behind the effective slot to produce a detection pulse signal containing an AC noise component having a given phase, and the first photodetector is disposed behind a slot group selected from the plurality of stationary slot groups to produce an AC signal having the same phase as that of the AC noise component.

17. An apparatus according to claim 2;

wherein the detection means includes three photodetectors disposed adjacently to each other along the displacement direction for sequentially receiving the reference spot image while the local holographic lens pattern is traversing the primary light to thereby output successively a preceding detection pulse, an intermediate detection pulse and a succeeding detection pulse, and a detection circuit connected to the three photodetectors and operative to logically process with each other one comparison result between the preceding and intermediate detection pulses and another comparison result between the intermediate and succeeding detection pulses so as to produce a reference pulse signal indicative of the reference point of the displacement member.

18. An apparatus according to claim 17;

wherein the detection circuit includes a comparator for thresholding an added output of the three successive detection pulses to output a gate signal, and a gate circuit for outputting the reference pulse signal in response to the gate signal.

19. An apparatus according to claim 17;

wherein the three photodetectors are arranged a given interval corresponding to a pitch of the enlarged diffraction image.