Patent GB-1409339-A (GB)
Title: Interferometer system
IPC: G01B9/02
Filed: 1973-04-19
Abstract: 1409339 Photo-electric interferometers HEWLETT-PACKARD CO 19 April 1973 [8 May 1972] 19032/73 Heading G1A In an interferometric system a light source projects a beam along an optical path to a beamsplitter which produces two output beams deviating by equal and opposite angles from the direction of the incident beam, a pair of reflecting surfaces arranged to reflect the two beams back to the beam-splitter to form a recombined beam, and a detector responding to optical fringes produced in the recombined beam by path differences between the beam-splitter and the two reflecting surfaces. As shown, Fig. 1, a device for measuring the vertical deviation from a straight line during movement in the X-direction of a machine tool holder 27 mounted on a slide 28 utilizes a two frequency laser 11, a quarter wave plate 13 to produce a beam of light 17 in which the two frequency components f 1 , f 2  are orthogonally polarized. Part of this beam is diverted to a detector 15 which produces an output at the beat frequency between the two frequency components f 1  and f 2  and the remainder is incident on a Wollaston prism 23, mounted on the tool holder 27, which splits the two components of the beam 17 and deviates them by equal and opposite angles. The split beams are reflected from a pair of plane mirrors 24, 25 attached to a spindle element 26 of the machine (e.g. a lathe) with their joining line horizontal and in line with the axis of the spindle. The reflected beams are recombined by prism 23 and a beamsplitter 21 directs this beam to a second detector 31. The outputs of the two photo-detectors are applied to a reversible counter 16. Movement of the tool holder 27, and hence prism 23, along the X direction results in equal changes in the path lengths of the two beams but vertical movement changes the path length by opposite and equal amounts causing the generation of fringes which are accumulated by the reversible counter 16. The angle of deviation produced by the beamsplitter 23 may be selected so that the accumulated count gives a direct measure of the displacement from the straight line in suitable units. The apparatus may be adapted for measuring lateral displacements perpendicular to the slide movement by turning the prism 23 and mirrors 24, 25 through a right angle, Fig. 3 (not shown). The squareness of the movement of the tool holder may be measured by arranging the laser beam at right angles to the direction of movement of the slide and directed on to the prism carried on the tool holder which is offset from the centre of the slide at which is mounted a pentaprism for turning the split light beams through a right angle for reflection from the mirrors attached to the spindle member, Fig. 4 (not shown). Instead of the prism moving the system can be arranged so that the mirrors move relative to a fixed prism. In a system for measuring the roll of a member, Figs. 5A and B (not shown), the beam from the light source is first split into two parallel beams each of which is applied to a Wollaston prism the separated beams from which are reflected by a common pair of mirrors arranged with their joining line vertical. The reflected beams are recombined by the two prisms and directed to separate detectors feeding a counting system. As the distance between the prisms and the mirrors changes the path lengths of the separated beams change by equal amounts so that no net count is recorded. Vertical relative movement does not change the path length but horizontal relative movement causes identical path length changes in both systems so that the detector outputs changes equally and again no net count is recorded. Relative rotation (roll), however produces different path length changes in the two systems resulting in a count related to the degree of roll. The system may be modified by using a corner cube reflector instead of the mirrors, a pair of lenses being placed after the Wollaston prism to form a beam expander. Other monochromatic light sources arranged to create two differently polarized beams associated with the different measuring paths may be used. Reference has been directed by the Comptroller to Specification 1367886.