Patent GB-805391-A (GB)
Title: Improvements relating to apparatus for inspecting the dimensional accuracy of a workpiece
Inventor: BOOTH RICHARD HERBERT
IPC: G05B19/4105
Filed: 1954-03-16
Abstract: 805,391. Electric measurement of workpiece dimensions. ELECTRIC & MUSICAL INDUSTRIES Ltd. March 8, 1955 [March 16, 1954], No. 7543/54. Class 40 (1). An apparatus for checking one dimension of a workpiece against the desired dimension represented in the form of electrical signals representing values of the latter at a series of spaced points on the workpiece comprises a means for interpolating between the signals to produce further signals representing values of the desired dimension at closely spaced intermediate points on the workpiece, means for sensing the actual dimension of the workpiece in timed relationship with the deviation of the further signals, converting its output to a signal representing the dimension on the same scale as the signal from the interpolator, and comparing the two signals to derive a difference signal. In the Fig. 1 arrangement an alternating voltage of amplitude proportional to the radius vector at successive points on the surface of cam 1 is derived as follows. The cam engages a feeler 2 to produce a signal from a piezo-crystal device 3 mounted on the movable nut 5. The signal is amplified at 8 and energizes a servomotor 7 to traverse the nut in the appropriate sense until the output of the amplifier is reduced to zero. Motor 7 also drives the tapping point 10 of inductive potentiometer 11, which has provision for continuous interpolation between tapping points as described in Specification 802,472, [Group XXXVI] to feed an alternating voltage signal representatve of the position of nut 5 and hence of the radius vector to comparator 12. A predetermined pressure is maintained at feeler 2 either by spring biasing it away from the crystal to give a zero signal for a fixed pressure or by biasing the amplifier to respond only to signals above a certain amplitude. Signals representative of the desired radius vector at a series of angularly spaced points # 1  to # 5  represented in binary code form on a punched tape 15 or the like are picked up by reproducing head 14 and fed via rotary switch 16 geared to shaft 1a to one of six stores S1 to S6 of the type described in Specification 796,995 which produce from the digital code signals, and store, representative alternating voltages. The stored voltages are applied to respective angularly spaced contacts (represented linearly) divided into two groups B1, B3, B5 and B2, B4, B6 scanned by rotating brushes C1, C4, C5; C2, C3, C6 respectively. The voltages on contacts B1 ... B6 thus represent desired radius vectors at six angular positions. At the instant shown, voltages from the first three B1, B2, B3 are fed to a quadratic interpolator 18 of the type described in Specification 796,994, [Group XXXVI], which produces a series of voltages corresponding to 10 angular positions in the range from midway between # 1  and # 2 , to midway between # 2  and # 3 . Voltages from B2, B3, B4 are fed to a similar quadratic interpolator 19 which in the situation shown covers the range from midway between # 2  and # 3  to midway between # 3  and # 4 . The outputs of the interpolators are fed to contacts E1 to E10 and E<SP>1</SP>1 to E<SP>1</SP>10 respectively of the rotary switch 20 geared to switch 16. The rates of rotation of the various switches and the cam carrier are so arranged that as the brush of switch 20 which is connected to comparator 12 and is of the make before break type, moves through half a revolution, the brushes C1 to C6 move a distance equal to the separation between centres of successive contacts B1 to B6. In the position shown the brush of switch 20 is moving on to the first of the E<SP>1</SP> series of contacts. Before it reaches E1 the brushes C1, C3, C4 will be in contact with B3, B4 and B5 respectively so that the voltages on contacts of the E series cover a range of angular positions following those represented by the E<SP>1</SP> series in the illustrated position. Similarly the voltages on the El series are altered as the brush scans the E series. In similar manner after the brushes C leave successive contacts B1 ... new code signals from tape 15 are delivered by brush 16 to the stores S to cancel and replace the voltages stored therein by voltages corresponding to the radius vector at a further series of angularlyspaced points. Consequently a signal changing by discrete increments is fed by the brush of switch 20 to comparator circuit 12. The output voltage from 12 representing deviation of the radius vector from the desired value is applied to a recording galvanometer or a warning device operating if the deviation exceeds a set tolerance. The interpolators 18, 19 may alternatively provide linear or nth power interpolation. The piezo crystal 3 may be replaced by a differential transformer device (Fig. 2) or the feeler may vary a capacitor or resistor. The relative displacement between feeler 2 and nut 5 may alternatively tilt a mirror to differentially illuminate a pair of light cells coupled to give an output dependent on displacement which is fed to amplifier 8. In a further modification the feeler is a roller on the ends of a pivoted arm held tangential to the cam profile by a spring. The arm operates the moving contact of an A.C. potentiometer delivering a signal which is amplified and then fed to the comparator circuit. The invention is particularly adaptable for use in arrangements in which the interpolating means (as represented within the broken line in Fig. 1) forms part of a control mechanism for a machine for automatically producing the workpiece. Such machines are described in Specifications 797,503 and 802,206, [Group XXII].