METHOD OF MEASURING THE DISPLACEMENT OF AN OBJECT AND ARRANGEMENT THEREFOR

• US 3,756,723 A
• Filed: 01/15/1971
• Issued: 09/04/1973
• Est. Priority Date: 01/27/1970
• Status: Expired due to Term

• Current Assignee: Fromund Hock
• Original Assignee: Fromund Hock
• Inventors: Hock, Fromund
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): Godwin, Paul K.
• Application Number:
  US05/106,735
• Time in Patent Office: 963 Days
• Field of Search: 356/114,169,170,110,113 250/237G
• US Class Current: 356/400
• CPC Class Codes: G01D 5/38 by diffraction gratings

Abstract

The method includes the steps of producing at least two bundles of light rays for measurement in one coordinate or at least four bundles of light rays for measurement in two coordinates by diffraction on a grating which is connected to the object to be measured or to a measuring head being provided with photoelectric detecting means. The bundles of light belong to different diffraction orders and pass through optical paths of substantially equal length and through a beam combiner wherein the polarized components are phase-shifted relative to one another. On leaving the beam combiner, the components are displaced in modulation phase by an angle unequal to n X 180* relative to one another and are detected by photoelectric means for providing signals from which the displacement of the object may be deduced.

Claims

1. A method for measuring the displacement of an object which is connected to a diffraction grating or to a measuring head of an optical measuring arrangement provided with photoelectric detecting means, wherein the grating and the measuring head are relatively displaced in accordance with the displacement to be measured, the method comprising the steps of producing at least two bundles of light rays diffracted by the grating, said bundles belonging to different diffraction orders;

of directing said bundles with approximately plane wave fronts through optical paths of substantially equal length and then for interference through a beam combiner having at least two outputs, wherein at least one of the interfering bundles of light rays passes through optical elements which are anisotropic in relation to the phaseshift of different vibration azimuths of the light, said optical elements being so arranged that the components of said one of the interfering bundles are polarized perpendicularly to one another and, on leaving the beam combiner, are displaced in modulation phase by an angle other than n X 180* in relation to one another, where n is a positive integer, and of detecting said modulation phase of said bundle components by said photoelectric means whereby electric signals are generated from which the displacement may be deduced.

2. An apparatus for measuring the displacement of an object which is connected to a diffraction grating or to a measuring head of an optical measuring arrangement provided with photoelectric detecting means, wherein the grating and the measuring head are relatively displaced in accordance with the displacement to be measured, comprising:

a. a diffraction grating connected to the object to be measured and being displaceable therewith;

b. a light source for illuminating said diffraction grating with substantially parallel light;

c. a composite prismatic optical element for receiving a plurality of beams of light from said grating, said element comprising a plurality of components of which at least two of said components are optical elements having different anisotropic properties to phase-shift said parallel light into differently phase-shifted beam components in relation to the different vibration azimuths of the light;

d. a partially reflecting mirror coating providing a beam combiner in said element;

e. at least two emergent surfaces being provided on said element of which surfaces at leasT one is followed by a polarizing splitter which splits into linear oscillation components said differently phase-shifted beam components; and

f. a plurality of photoelectric receivers whereto each of said linear oscillation components are respectively transmitted for generating electric signals.

3. The apparatus as defined in claim 2, wherein said parallel light is linearly polarized light and at an angle of 45* to the principal cuts of polarizing anisotropic elements.

4. The apparatus as defined in claim 2, wherein said parallel light contains light rays of all azimuths of vibration.

5. The apparatus as defined in claim 2, wherein said parallel light is circularly polarized light.

6. The apparatus as defined in claim 2, wherein an optical component assembly influencing the geometrical configuration of the optical path is provided in the optical path between the grating and the beam combiner.

7. The apparatus as defined in claim 2, and further comprising an electro-optical element inserted in the path of one of the bundles of rays for controlling the phase difference between differently polarized components.

8. The apparatus as defined in claim 2, and further comprising an elasto-optical element inserted in the path of one of the bundles of rays for controlling the phase difference between the differently polarized components of the bundle of light rays.

9. The apparatus as defined in claim 2, and further comprising a modulator in the optical path of said light source for varying the direction of polarization of the light, and switching means for modulator-dependent selecting of the electric signals generated by the photoelectric receivers.

10. The apparatus as defined in claim 2, wherein the diffraction grating comprises a cross-grating, and wherein at least two pairs of partial light ray bundes are produced in different planes, the composite prismatic element being symmetrically shaped in two directions corresponding to the coordinate directions of the diffraction grating, and pairs of photoelectric receivers being provided for producing the electric signals.

11. The apparatus as defined in claim 2, and further comprising push-pull amplifiers to the input of which, after the combining of the beams, said linear oscillation components are fed in pairs which are opposite in phase.

12. An apparatus for measuring the displacement of an object which is connected to a diffraction grating or to a measuring head of an optical measuring arrangement provided with photoelectric detecting means, wherein the grating and the measuring head are relatively displaced in accordance with the displacement to be measured, comprising:

a. a diffraction grating connected to the object to be measured and being displaceable therewith;

b. a light source for illuminating said diffraction grating with substantially parallel light;

c. at least two optically reflecting anisotropic surfaces for receiving two beams of light from the grating to produce partial bundles of rays;

d. a beam combiner whereto the partial bundles of rays forming the beams are directed by said reflecting surfaces, the two ray paths of the respective beams having different anisotropics in relation to the light polarization;

e. a phase grating being part of said beam combiner;

f. at least one polarizing beam splitter being arranged behind said phase grating in the direction of light travel, said beam splitter splitting differently polarized beam components into linear oscillation components; and

g. photoelectric receivers whereto said linear oscillation components are transmitted for generating electric signals.

13. The apparatus as defined in claim 12, wherein at least one of the optically reflecting surfaces is movably mounted.

14. The apparatus as defined in claim 12, wherein the grating has a structure which produces equal intensities in the + or - 1 and + or - 3 diffraction orders.

15. The apparatus as defined in claim 12, wherein The diffraction grating and the phase grating comprise a single grating.

16. The apparatus as defined in claim 12, wherein said parallel light is linearly polarized light and at an angle to the principal cuts of the polarizing anisotropic elements.

17. The apparatus as defined in claim 12, wherein said parallel light contains light rays of all azimuths of vibration.

18. The apparatus as defined in claim 12, wherein said parallel light is circularly polarized light.