Patent GB-967291-A (GB)
Title: Improvements relating to measuring apparatus
Inventor: WALKER DONALD FERGUSON
IPC: G01B21/00
Filed: 1961-09-07
Abstract: 967,291. Electric selective signalling. FERRANTI Ltd. Dec. 3, 1962 [Sept. 7, 1961], No. 32084/61. Heading G4H. Measuring apparatus for determing the extent and sense of the movement of an approximately sinusoidal cyclic wave pattern 10 includes two detecting devices 21, 22 producing signals proportional to the sine and cosine of an angle # representing the position of the pattern 10 which are used for energizing a tapped cosine potentiometer 19 and a tapped sine potentiometer 20 respectively, a bi-directional switching counter 35 for selecting tappings on the potentiometers 19, 20 being actuated by electric pulses so long as the difference between the output voltage from the selected tappings exceeds a predetermined value, to cause the tappings to be effectively traversed so as to tend to null said differences. In Fig. 1, the outputs from photocells 21, 22 detecting a MoirÚ fringe pattern 10 (90 degrees apart) are fed to the potentiometers 19, 20 through phase-splitters 17, 18. By means of switches CS, SS (details in Fig. 2, not shown) the counter 35 makes effective a tap on each potentiometer 19, 20, thus supplying to a difference amplifier 27 signals proportional to sin # cos Ï and cos # sin # respectively where # is the angle associated with the selected taps. The output of the amplifier 27, proportional to sin (#-#), opens a gate GA if # is more than 18 degrees greater than # and a gate GS if # is more than 18 degrees less than #,18 degrees being half the change in # between adjacent taps. The gates GA and GS are fed with pulses from a source 31, each pulse, if passed by either gate, being counted up or down by both counter 35 and on output counter 34, depending on which gate passed it. If # should lag behind # by significantly more than 18 degrees, the apparatus will operate in the wrong direction. Warning of this error condition is given (Fig. 4, not shown) by passing every pulse produced by source 31 to a two-stage binary counter, which it resets to zero unless the same pulse is passed by one of the gates GA, GS when it will also arrive at the binary counter by another route and will be counted. Consequently, the binary counter gives an output if more than three consecutive pulses from source 31 are passed to counters 34, 35. In order to allow for fluctuations in the light intensity illuminating pattern 10, the cosine and sine potentiometers 19, 20 may be provided with extra tappings to form in effect an extra sine and cosine potentiometer respectively (Fig. 3, not shown), these tappings being selected by the counter 35. The outputs from the selected extra taps are fed via a summing amplifier for comparison in a comparator with an adjustable reference voltage, the comparator output controlling the brightness of the lamp illuminating the pattern 10. Fig. 5 (not shown) shows a modification in which four photo-cells (90 degrees apart) are used to provide stabilization against shifts in circuit parameters, the outputs from photo-cells 1 and 3 being fed to a difference amplifier to energize potentiometer 19 and those from photo-cells 2 and 4 similarly supplying potentiometer 20. Fig. 6 (not shown) shows a stabilizing network and phase-converter for interposing between the photo-cells and the potentiometers 19, 20 when three photo-cells (at 120 degrees intervals) are used, as described in Specification 914, 190 which is referred to. A similar device may be used with more than four photo-cells. The pattern 10 may be magnetic instead of optical. The invention may be used for measuring movement in machine tools, measuring probes, gauges and extensometers. Specifications 760, 321 and 810, 478 also are referred to.