Patent GB-1180829-A (GB)
Title: Improvements in or relating to Photoelectric Devices.
IPC: G01B11/00
Filed: 1967-07-20
Abstract: 1,180,829. Digital measurement and control of position. ERNST LEITZ G.m.b.H. 20 July, 1967 [21 July, 1966], No. 33376/67. Heading G4H. [Also in Divisions G1 and G2] Photo-electric apparatus for controlling or indicating the position of an object, attached to the shaft 21 of a grating 9, comprises means for imaging one portion of the grating 9 on to another portion of the grating, and at least two photo-electric receivers 16, 18 producing signals, representing the location of the object, which can be used to control the location of the object, or the location of an optical deflecting member in the light path, or both. Alternatively, two separate gratings may be used, one being imaged on to the other. The optical system comprises a lamp 13, light from which is directed through one edge of the radial grating by a prism 20 and at the other side is deflected by prism 8 towards a pentagonal prism 10. The light passing through prisms 22 is deflected in accordance with the angular setting of mirror 25 which is moved under the control of an electromagnetic coil arrangement 3, 4. After reflecting prism 23, the light is split up into two partial beams polarized at right angles to one another, by a Wollaston prism 26. The light is directed through the other edge of the grating by prism 28, 24 and is split up into its differently polarized partial beams by cube 15 for detection by cells 16, 18. Measurement of position. Fig. 2 (not shown). The outputs of cells 16, 18 (obtained at terminals 32) are shaped into square waves (33, 34) and directed to a counter (36) arranged to count forward or backward as determined by a direction discriminator (35). The registered count is the number of complete signal cycles (fringes passing the detectors). To determine fractions of a cycle the photo-cell outputs are directed through a differential amplifier (37) and the output of this combined with temperature, scale &c., correction signals, in summing amplifier (33) which has negative feedback stabilization. This amplifier feeds coil 4 controlling the position of the mirror 25 to null the output of the differential amplifier (37), at which time the indication on an analogue or digital indicator (10) gives the required fraction of a cycle. An apparatus for obtaining digital values of fractions of a cycle is described, Fig. 4 (not shown), and utilizes a stepping device halving the current supplied to coil 4 each step until the differential amplifier output is nulled. Control of position, Fig. 3 (not shown).- Movement of the object is by a motor (54) receiving the output of an amplifier 53. The position to be attained by the object is set into two devices for coarse (51) and fine (60) positioning of the object. Initially, for coarse positioning, the output of the photo-cells is counted by a device 50 and the count compared with the preset count in device (51) by comparator (52) to produce an analog signal for the amplifier (53). When the output of the comparator (52) is zero indicating that the coarse position has been attained, a blocking stage opens to allow photo-cell signals representing the fine position setting from a differential amplifier (55) to reach amplifier (53). The fine position setting is controlled by the position of the mirror 25, this being set by connecting a D.C. source to a selected one of a number of resistors connected to an amplifier supplying coils 4. The angled position of the mirror thereby produces an unbalance of the photo-cell outputs resulting in the fine position setting control signal from the differential amplifier 55. The intensity of the lamp 13 can be kept at a constant level by sensing the light beam intensity with photo-cell 11, mounted adjacent the pentagonal prism 10.