Patent GB-858003-A (GB)
Title: Improvements in electronic curve follower and analog computer
IPC: G06V30/144
Filed: 1957-10-24
Abstract: 858,003. Electric correspondence control systems. GENERAL ELECTRIC CO. Oct. 24, 1957 [Oct. 26, 1956], No. 33227/57. Class 38 (6). [Also in Groups XXXVI, XL (b) and XL (c)] In an electronic curve follower, the spot of a cathode-ray tube is caused to describe a small search circle intersecting the curve whereby the spot produces, by a photo-electric &c. arrangement, a train of pulses which contains information determined by the relationship between the search circle and the curve and which is filtered to obtain the fundamental sinusoidal component which is processed by an analogue computer having vector resolving and synthesising circuits to produce voltages representing horizontal and vertical orthogonal components of a correction vector, which voltages are applied to the deflection system of the cathode-ray tube to cause the centre of the search circle to be servo-controlled to follow around a line spaced at a predetermined distance from the curve. In the embodiment of Fig. 1, an image of the spot on the screen of a cathode-ray tube 10 is projected by a lens 11 on to a stencil 12 on which there is drawn a curve 13. The light passing through the stencil 12 is focused by a lens 14 on to a photo-cell 15. Voltages in the form E cos wt, E sin wt produced by generators 22a, 22b are applied to x and y deflection amplifiers 25, 26 of the tube 10 to cause the spot to trace a small search circle. Sawtooth waveforms from generators 20a, 20b are also applied to the deflection amplifiers 25, 26 to cause the centre of the search circle to trace out a television type raster over the screen. At the first intersection of the search circle with the curve 13, a pulse in the output of the photo-cell 15, via a pulse detector 18 and flip-flops 19, causes the outputs of the amplifiers 20a, 20b to be clamped to the values which they have at that instant whereby the search circle remains in contact with the curve 13. The search circle Q, Fig. 6, intersects the curve 13 at two points G, H, which results in a train of pairs of pulses G, H, G<SP>1</SP>, H<SP>1</SP> ... , Fig. 7, in the output of the photo-cell 15, which are fed to a filter 27 which transmits only the fundamental component having the rotational frequency of the spot around the search circle. This fundamental component has an amplitude proportional to the distance of the centre O of the search circle from the curve 13 and a phase angle (relative to the direction OF which represents the phase of the output of generator 22a) which depends upon the direction of the normal ON to the curve 13 and which therefore carries information relating to the direction in which the centre O of the search circle should move if it is to follow the curve 13. A comparator circuit 34 produces a D.C. error signal A<SP>11</SP> which is proportional to the difference between the distance of the centre O of the search circle from the curve 13 and a predetermined distance at which it is desired that the centre O should be spaced from the curve 13 under stable operating conditions. A phase detector 33 produces a D.C. error signal A<SP>1</SP> of amplitude proportional to the sine or cosine of the angle between the normal to the curve 13 (which is related to the direction in which the centre O should move) and a voltage vector representing the actual velocity of the centre O which is fed back from a potentiometer 33a. The error signals A<SP>1</SP>, A<SP>11</SP> are added together by a circuit 35 to produce a D.C. output A which represents the curvature of the curve 13 and which is equal to the acceleration necessary to cause the search circle to follow the curve 13. The signal A is fed to a balanced modulator 36 which produces an A.C. output of amplitude A and of such phase relative to the output of generator 22a) that the vectoral representation of the modulator output is parallel to the direction in which the centre O of the search circle Q is actually moving. To obtain this phase angle the balanced modulator 36 is controlled by feedback of a velocity voltage vector from a later stage in the apparatus. The output of the modulator is given a 90 degrees phase lead by a network 37 (since the acceleration is required at right angles to the direction of movement of the centre O of the search circle Q along the curve 13) and is then applied to a pair of phase detectors 38, 39 which act as resolving circuits to determine the orthogonal components of the acceleration vector. The output of the phase detectors 38, 39 are integrated at 40, 41 to obtain velocity voltages to which are added, in networks 42, 43, small voltages applied by flip-flops 44, 45 when the search circle first intersects the curve 13, to give an arbitrary movement to the centre of the search circle which is corrected by the feedback through the potentiometer 33a. The outputs of adders 42, 43 are applied to modulators 46, 47 which produce A.C. outputs in quadrature which are added at 48 to produce a resultant A.C. voltage, the phase angle of which (relative to the direction of OF, Fig. 6) represents the actual direction of movement of the search circle. The output of the adder 48, amplified to a constant and predetermined amplitude in an A.G.C. amplifier 49 is resolved by phase detectors 52, 53, into its orthogonal components which are intergrated at 54, 55, to produce positional orthogonal components which are fed to the deflection amplifiers of the cathode-ray tube to cause the centre O of the search circle Q to move with constant velocity along a path spaced at a predetermined distance from the curve 13. By adding the outputs of integrators 54, 55 to the clamped outputs of the amplifiers 20a, 20b in adders 56, 57, voltages representing the x and y co-ordinates of the curve 13 as a function of its arc length are obtained, which may be applied as inputs to analogue to digital converters as analogue voltages sampled at equal time intervals. The outputs of the converters which are digitally encoded representations of the curve as a fuction of its arc length may then be applied to a storage medium, e.g. magnetic tape or punched cards which may be used, e.g. for programming an automatic machine tool to reproduce a part shaped like the curve 13. The arrangement of Fig. 1 is suitable for following closed curves, but a modified circuit, Fig. 13 (not shown), is described for following open curves. Specification 837,341 is referred to.