Patent GB-858002-A (GB)
Title: Improvements in an electronic curve follower
IPC: G06V30/144
Filed: 1957-10-24
Abstract: 858,002. Electronic curve followers. GENERAL ELECTRIC CO. Oct. 24, 1957 [Oct. 26, 1956], No. 33226/57. Class 40(3). 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 second harmonic sinusoidal component; a carrier voltage generator is provided having a frequency equal to the frequency of rotation of the spot around the search circle, and the phase of the carrier voltage generator is controlled by the phase of the second harmonic component to produce a signal which can vectorally represent the velocity of the search circle when its centre is moving along the curve and which is processed by an analog computor to derive voltages which are applied to the deflection system of the cathode-ray tube to cause the centre of the search circle to move along the curve. In 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 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 Q. To cause the search circle to initially intersect the curve 13 a search sweep generator as disclosed in Specification 858,003 may be provided, or potentiometers 20a, 20b may be used. The search circle Q intersects the curve 13 at two points G, H, Fig. 5 which result in a train of pairs of pulses G, H, G<SP>1</SP>, H<SP>1</SP> . . . , Fig. 6, in the output of the photo-cell 15 which are applied to an amplifier 16 the output of which feeds filters 18, 19, which, respectively, pass the first and second sinusoidal harmonic components of the pulse train. The output of the second harmonic filter 19 is used to phase synchronize an oscillator 29 operating at the frequency of rotation of the spot around the search circle Q. Because the output of the filter 19 is double the operating frequency of the oscillator 29, an ambiguity of 180 degrees in the phase of the oscillator 29 could occur, and to avoid this the first harmonic sinusoidal voltage filtered from the pulse train by the filter 18 selectively displaced in phase by 90 degrees by phase shift element 34 is also applied to the control terminals of the oscillator 29. Under the conditions when the search circle first meets the curve 13 the first harmonic voltage is at a maximum and by means of potentiometers 27, 28 and 33 it can be arranged that the first harmonic signal applied to the oscillator 29 under these conditions is greater than the second harmonic signal. The phase of the oscillator is thereby initially determined by the first harmonic signal in accordance with the setting of the phase shift element 34 which serves to control the direction in which the search circle will travel along the curve 13. As the centre of the search circle Q approaches the curve 13 the amplitude of the first harmonic falls to zero and the amplitude of the second harmonic rises to a broad maximum so that under stable conditions the second harmonic continues to control the phase of the oscillator 29. The phase of the second harmonic voltage (with respect to the phase of the output of the generator 22a) and thus the phase of the oscillator 29, represents the angle with respect to the direction OF of the curve 13 at the centre of the search circle when the search circle is centred on the curve. Thus the vector representation of the output voltage of the oscillator 29 represents the velocity of the centre of the search circle when it is moving along the curve 13. The output of the filter 18 represents the velocity acting at right angles to the curve necessary to return the centre of the search circle on to the curve when it is not perfectly centred. The outputs of the filter 18 and oscillator 29 are added at 35 and fed to an amplifier or clipper 49 having a constant output voltage which in turn feeds resolving circuits 52, 53 which determine the orthogonal co-ordinates of the output of the amplifier 49 and apply them to integrators 54, 55 which derive voltages representing distance orthogonal coordinates which are applied to the deflection circuits of the cathode-ray tube 10 to cause the centre of the search circle to follow along the curve 13. Phase shift elements 30, 31 are provided for the alignment of the circuit. The outputs of the integrators 54, 55 and the initial position voltages on the deflection circuits of the cathoderay tube 10 may be applied to adders 60, 61, which feed the deflection system of a cathode-ray tube 62 on which the curve 13 will then be reproduced. The outputs of the adders 60, 61 may be fed into a storage medium by analog to digital converters to be used as input data to a computor or to program or control machinery. Specification 837,341 also is referred to.