Patent GB-1181538-A (GB)
Title: Improvements in or relating to Photoelectric MOnitoring Devices.
IPC: G01D5/36
Filed: 1967-07-14
Abstract: 1,181,538. Digital measurement of position. ERNST LEITZ G.m.b.H. 14 July, 1967 [16 July, 1966], No. 32519/67. Heading G4H. [Also in Division G2] In a photo-electric displacement measuring system in which a first diffraction grating is imaged on to a second diffraction grating or, as in Fig. 1, a first portion of a grating 8 is imaged on a second portion of the same grating, and the imaging beam is split up into two differently polarized beams (by a Wollaston prism 20) which are directed by a beam splitter 11 on the one hand, back through the grating where further diffraction occurs, to be detected by a first pair of photocells 16, 17, and on the other hand, through an index grating 22 for detection by a second pair of photo-cells 24, 25, the optical arrangement being such that the frequency of the signals (representing the passage of diffraction fringes as the grating 8 moves) from the second pair of photo-cells 24, 25 is half the frequency of the signals from the first pair of photo-cells 17, 18. Polarizing beam splitting cubes 13, 23 are used to direct the two mutually perpendicularly polarized beam parts to respective detectors in each pair 16, 17 and 24, 25, the outputs of the detectors of each pair being phase-displaced relatively to one another in accordance with the splitting angle of the Wollaston prism 20. The outputs of detectors 17, 18 are fed to a counter 18. Detectors 24, 25 may also be connected to a counter and used to check the indication of counter 18. The grating shown in Fig. 1 is a linear one having reflecting markings 7, the index grating 22 having alternate opaque and transparent lines. In the arrangement of Fig. 2 (not shown) the rotation of a radial grating (38) with respect to an appropriate index grating (22a) is measured. Besides radial markings (37) on the outer edge of the disc-shaped grating (38), the disc has inner binary-coded tracks, Fig. 2a (not shown). The images of these tracks are viewed through an index grating provided with corresponding tracks by means of a first pair of detectors (44) through light (43) and by a second pair of detectors through a polarizing beam splitting cube (45). Using these tracks binarycoded signals representing displacement are obtained. Modifications of the embodiments of Figs. 1 and 2 are possible. For example, radial gratings may be used in Fig. 1 and linear ones in Fig. 2. The index grating of Fig. 2 may comprise two parts, one carrying the radial markings and the other the code tracks. The single, main grating of Figs. 1 and 2, may be replaced by two separate gratings, one imaged on the other. An embodiment using two separate phase gratings (30, 31) is described, Fig. 3 (not shown). The optical set-up differs from Fig. 1, in that the Wollaston prism 20 is placed in front of the pentagonal prism 10. The polarizing beam splitter 13 is in this embodiment associated with two arrays of detectors designed to detect fringes of different orders of diffraction. Similarly, a phase index grating (35) receiving light from a beam splitting cube (11) is associated with a similar pair of detector arrays. A third beam comprising two partial beams of different polarizations, is obtained from the pentagonal prism (10<SP>1</SP>) which is adapted by addition of a wedge (10a). This is directed via a retardation plate (60) through an amplitude index grating to a polarizing beam splitter (23) a detector pair 24, 25. The frequencies of signals from the three arrangements detectors (24, 25; 51, 52 and 32, 33) are in the ratio 1 : 2 : 4. This embodiment may be modified by using a single grating imaged on to itself.