Patent GB-1363411-A (GB)
Title: Position indicating or measuring devices
IPC: G01D5/38
Filed: 1970-07-01
Abstract: 1363411 Measuring position photoelectrically NATIONAL RESEARCH DEVELOPMENT CORP 29 Sept 1971 [1 July 1970) 31794/70 Heading G1A In an apparatus for indicating or measuring the relative position of two relatively movable members carrying respectively first and second optical gratings which co-act to produce a fringe pattern when illuminated by a light source, the fringe pattern is monitored by an array of at least eight photo-cells 22(I)... 22(L) whose outputs are selected sequentially in cyclic order so as to provide a composite output having a cyclic waveform whose phase relative to a reference signal is indicative of the relative position of the two members. As shown, the photo-cell array 18 contains fifty photo-cells 22 and is part of an integrated circuit on a silicon chip. The photo-cells are of the storage mode type. Sequencing of the  photo-cell outputs is achieved by applying a train of scan pulses to a fifty-stage shift register 32. Each pulse causes a "bit" to move along the register so that in the fourth stage shown photocell 22(III) is charged to the line 26 voltage V while photo-cell 22(IV) discharges its remanent voltage into the video output line 30. As the "bit" leaves stage fifty, a synchronization pulse is fed into stage one to discharge photo-cell 22(I) and charge photo-cell 22(L), thereby restarting the cyclic sequence. The scan and synchronization pulses on lines 31 and 29 respectively- Fig. 4 - are derived from a clockpulse source 34 via a divide by two frequency divider 36 and a divide by fifty divider 38. Between successive synchronization pulses, the output from integrated circuit 21 is a train of fifty pulses whose heights vary according to the intensity of the fringe pattern. This output is filtered at 40 and squared by a threshold device 42, to give a squarewave into phase comparator 44 which may be of the flip-flop type. The output may be integrated before filtering at 40. The other input to comparator 44 is a squarewave reference signal derived from generator 34 via a divide-frequency-byone-hundred and squarer unit 46. The combined frequency division produced by dividers 36, 38 is equal to that produced by divider 46. Any phase difference at 44 causes an error signal to be generated in the comparator. Data pulses from an external source, e.g. a punched tape reader and interpolator, are fed into the system at 48. The receipt of a pulse at shift unit 50 causes either an additional pulse to be added into divider 36 or a pulse to be withheld from the divider. Thus a + or - error signal can be generated at comparator 44. This signal is amplified at 52 and fed to a motor 54, which moves the movable member so as to eliminate the error. In this way the movable member can be moved according to a predetermined pattern. The divider and squarer 46 may supply more than one axis. The squarewave outputs from 42 and 46 are also supplied to differentiators 58, 62 and combined in a counter 60. This counter is started by a pulse from 58 and stopped by a pulse from 62, hence its display corresponds to the phase difference between the two squarewave inputs to 44. This count is stored and displayed in unit 64, and is updated with each signal from video line 30. To set up a datum position and reading before starting a measurement, the motor 54 is switched off and push button 69c operated. Clock pulses are fed into the shift unit 50, thereby altering the phase of the signal from integrated circuit 21, until a digital zero is reached in store 64, indicating phase agreement, and a zero occurs in zero detector 68, which actuates inhibit gate 70 to open switches 69a, b. A point-to-point control system Fig. 6 (not shown) uses the integrated circuit photo-cell array (102) to make a "fine" position measurement and control in an apparatus in which the "intermediate" and "coarse" measurement and control are effected by respective synchro's (104, 106). The two gratings may both be transmission ones placed in a collimated beam from a filament lamp, the fringe pattern being projected on to the photo-cell array by a cylindrical lens Fig.2 (not shown). Alternatively, the movable grating may be a reflecting one Figs.7-9 (not shown), all the other optical components, which may include a beam-splitter (148) being contained in a protective enclosure. The gratings may be crossed (to produce MoirÚ fringes) or parallel (Vernier fringes).