Patent GB-1254911-A (GB)
Title: Method and apparatus for interferometric measurement
IPC: G01B11/00
Filed: 1969-01-13
Abstract: 1,254,911. Photo-electric position determination. ITEK CORP. Jan. 13, 1969 [Jan. 18, 1968], No.1855/69. Heading G1A. [Also in Divisions G2 and G3] An interferometer comprises a source of coherent light 10, a beam-splitter 12 for dividing the light into two beams 13, 14, mirrors 15, 16 and 19, 20, 21 defining two optical paths 28, a rigid body 18, e.g. a workpiece having at least one pair of reflecting surfaces 17, 22, each reflecting surface intersecting one of the optical paths 28, the beam-splitter 12 and the optical paths being so arranged that at least a portion 13a, 14a of the light directed along each path is normal to each reflecting surface 17, 22 and independantly of the angle of intersection of each reflecting surface to its corresponding light path, said portion of light being reflected back along its path to a detector 26. A reference point O is provided equidistant along the light paths from beam-splitter 12. This arrangement is sensitive to movement of the body 18 away from the reference point O only in either direction along paths 28. Movement of the body 18 in any other direction has an equal effect upon both path lengths. A prepared embodiment measures change in position of a body in two directions, X, Y, Fig. 5, at right angles. The optical system is coupled rigidly together. A stage plate 101 having an aperture 116 and two orthogonal pairs of reflecting surfaces 102, 103 and 104, 105 is mounted for movement relative to the fixed optical system in the X direction along rail 114 and in the Y direction along rail 111. An object requiring measurement is clamped to plate 101 by means 117. Coherent light from a source is split into two beams each of which is further split into a pair of beams one pair following the X-axis optical system through a pair of arms 118, 119 and one pair following the Y-axis optical system through a pair of arms 120, 121. The X and Y optical paths intersect at a fixed reference point for the interferometer. An object, e.g. a workpiece, is located on plate 101 and a fiducial mark in viewing system 122, 123 is imaged on to the object through aperture 116. The interferometer is then operated to measure the distance along the orthogonal co-ordinates from a desired point to the fixed reference point. The reflected light in each X and Y light paths recombines and interferes; and this recombined beam for each axis then is transmitted to a Wollaston prism which separates orthogonal polarization components and sends them along separate paths to fringe counters. Thus there a two counters for the X axis sensing movement in either direction along that axis, and similarly for the Y axis. It is preferable that the coherent light beam has spherical wavefronts, and that the beams emanate from a common source. The polarization components are introduced by placing a metallic phase shifting mirror into one of the beams of a pair. In another embodiment the apparatus is used to locate a workpiece relative to a drill but in a programmed sequence. A tape program 204 Fig. 11 supplies indicia for such hole to be drilled. The interferometer 200 has X and Y axis servomotors 201, 202. The outputs from the interferometer are X and Y signals from photo-electric detectors coupled to phase-logic circuits 203, 211. These determine direction of motion as + or - and feed signals to respective counters 208, 212. These counts are compared to the tape program 204 through registers 206, 207 and subtractors 209, 213. The subtractors are coupled to digitalto-analog converters 210, 214 which produce error signals to drive to appropriate servomotors. In use with e.g. a milking machine a thirst co-ordinate can be added. The reflecting surfaces on the object may be effectively back-to-back; or alternatively the light source may be located between the reflecting surfaces which face each other.