Patent GB-966380-A (GB)
Title: Position signal generating apparatus
Inventor: DAVIES BEAUMONT JOHN
IPC: G05B19/07
Filed: 1959-08-07
Abstract: 966,380. Electrical position determination. GEORGE RICHARDS & CO. Ltd. July 29, 1960 [Aug. 7, 1959; Jan. 13, 1960], Nos. 27148/59 and 1288/60. Headings G1A and G1N. [Also in Divisions G4 and H4] Apparatus for generating a signal dependent on the relative angular or linear positions of two members includes means arranged to produce a space pattern of a cyclically varying quantity and a detector of the quantity, the relative positions of the pattern and the detector being dependent on the relative positions of the members, and the detector having n detector units so disposed in relation to the space pattern that they detect the value of the quantity at positions where the phase of the pattern is different, in combination with switching means cyclically operated to derive from the detector units an output which is a combination of the outputs representing the quantity detected by each of the n units, and a comparator for comparing the combined output with a signal related to the switching cycle. Alternatively a single detector unit may be used, the space pattern being shifted cyclically by the switching means into n discrete positions spaced along the direction of variation of the quantity, e.g. when the pattern is an optical pattern produced by adjacent gratings the pattern may be shifted by sequentially energizing n spaced light sources. The value of n may be 2 or 3 but an even number greater than 2 is preferred; e.g. as described n = 4. The spacing between adjacent detectors, or between adjacent discrete positions of the space pattern in the alternative embodiment, is (#/n + A#) where # is the wavelength of the space pattern and A is an integer which may be zero. The cyclically-varying space pattern may be an optical pattern or a pattern of electromagnetic radiation near the visible frequency range or produced as a standing wave on a transmission line or waveguide or the varying quantity may be electric, magnetic, acoustic, thermal or mechanical. When varying quantity is optical or magnetic the pattern may be recorded, e.g. as an optical grating or on a magnetic tape, the record being attached to one of the two relatively movable members. As described with reference to Fig. 1 the space pattern comprises a substantially sinusoidal optical fringe pattern 20 produced by two transmission gratings 11 and 13 having relatively skewed rulings and capable of a linear relative displacement; alternatively one of the gratings may be a reflection grating. As shown light from a source 14 is focused on to a fixed grating 11 and a grating 13 carried by a machine associated with adjustable shutters 17 and four photo-electric cells or transistors 16 with a total spacing corresponding to the wavelength of the wave-form 20, the four cells 16 being switched on in sequence by transistors 35 controlled by phase displaced square wave forms derived from a switching circuit 34. The outputs 37 of the four photo-transistors are applied via common load resistor 19 to an integrating circuit 40 and a Schmidt trigger circuit 41 to produce a square waveform 21 fed to a comparator circuit 49 which also receives a reference square waveform 22 of equal positive and negative portions from a circuit arrangement controlled by a primary oscillator 31, which also controls the circuit 34 producing the switching pulses for the transistors 35 and photo-transistors 16. If the phases of the waveforms 21, 22 are different, the comparator 49 produces a pulse, indicated at 23, which is applied to a servomotor 24 to move the machine member and grating 13 until phase equality is established. The normal reference waveform 22 is derived from a multivibrator 31 operating at 10 kc./s. whose output is converted by a monostable circuit 43 into corresponding 10 kc./s. pulses which are applied tnrough an "OR" circuit 45 and a NOT AND circuit 46 to a decade dividing circuit 47 controlling flip-flop 48 forming a further division by two so that the waveform 22 has equal mark and space portions corresponding to ten impulses applied to the dividing circuit 47. Pulses from one of the transistors 32 are also applied to two dividing circuits 33 producing in combination a division by ten for the pulses from the a or b terminal respectively, and the b output of the divider is applied to a flip-flop 71 producing two 180 degree phase displaced outputs at a frequency of 500 c.p.s. applied to the first and third transistors 35. The a output from the dividing circuit together with the lower output of the flip-flop 71 is applied to an AND circuit 73 and a flip-flop circuit 72 which produces from its lower and upper output terminals square waveforms which are 90 degrees displaced from those applied to the first and third transistors 35 so that the transistors 35 are controlled in sequence. In response to a command pulse applied to terminal 51 and synchronized with the next pulse from mono-stable vibrator 44 of which the pulses are intermediate those from the vibrator 43, an additional pulse passes through gates 64, 45 to add a pulse to the input of counter 47 so that the phase of the reference signal 22 is advanced by 1/20 of the signal cycle, and the comparator circuit 49 will then produce a signal to operate the servomotor 24 to move the machine member carrying the grating 13. For reverse movement of the machine member a command pulse applied to the terminal 52 is synchronized with a pulse from the circuit 43 and blocks said pulse in the NOT AND circuit 46 so that a pulse is deleted from the input to the counter 47 and the phase of the reference signal 22 is retarded with consequent operation of the machine member and grid 13 to restore phase coincidence. The synchronization of a command pulse at terminal 51 or 52 with the next pulse from circuit 44 or 43 respectively is effected, for a pulse at 51, by a flipflop circuit 61 in connection with AND circuit 62 producing a pulse when a pulse from circuit 43 appears and effective through flip-flop circuit 63 to produce a first input for AND circuit 64 which receives its second input from circuit 44 to pass to the additional pulse through circuits 45, 46 to the dividing circuit 47. The pulse from the AND circuit 64 is also fed to flip-flop circuit 61 and via an inverter 65 to flip-flop circuit 63 to restore these circuits to normal. The above described arrangement for phase modulating the reference waveform 22 in accordance with command pulses applied at terminal 51 or 52 forms the subjectmatter of Specification 966,379. A modified arrangement to produce the switching waves for the transistors 35 is described in which a square wave recorded on a magnetic tape is differentiated by a reading head to give alternate positive and negative pulses which are applied in push-pull to a first flip-flop 83, Fig. 3, producing outputs 84 and 85, the output 85 being also integrated at 86 and shaped at 87 to give outputs 88, 89 in phase quadrature with the outputs 84, 85. The reference wave 22 may be derived by applying the outputs of the flip-flops 71, 72 to the four corners of a resistance bridge having equal arms disposed around a cylindrical or other former, in conjunction with a rotatable wiper, the rotation of which can be utilized to produce desired phase shift of the reference signal to effect comparison with the signal derived from the photo-transistors 16. The integrator circuit 40 may comprise a suitable filter circuit and the inputs to the decade counters 33 and 47 may be changed over so that the photo-transistor switching voltages are phasemodulated by the command signal, the resultant error signal being compared with a signal of the same frequency of fixed phase.