Patent GB-690426-A (GB)
Title: Improvements in and relating to automatic curve-following apparatus
Inventor: SCOTT WILFRED EMBERTON
IPC: B23Q35/128
Filed: 1949-07-26
Abstract: 690,426. Automatic curve-following apparatus, ENGLISH ELECTRIC CO., Ltd., Oct. 20, 195C [July 26, 1949], No. 19601/49. Class 40 (i) [Alsc in Group XL(b)]<SP>-</SP> Automatic curve-following apparatus comprises a scanning system arranged to scan, in a cyclic manner, a curve to be followed, photo-electric detection apparatus responsive to changes in light intensity as the line or boundary defining the curve is scanned and providing successive signals the aiming of which with respect to each other is an indication of any deviation between the curve and a reference print in the scanning system, and apparatus for connecting the deviation responsive to a comparison of the time intervals between successive components of the signal. Such apparatus may be employed .to follow a curve on a piece of paper and control an automatic flame cutter, profiling machine or computer through a servo system. Alternatively, in a paper cutting machine, for example, the scanning system may remain stationary and the servo system operate to maintain an edge or index line on the paper in correct relationship with the system. The scanning system scans the curve AB (Fig. 2) in a circular path F so that a short pulse appears in the output of the photo-cell 3 (Fig. 1) each time the scan traverses the curve, the scanning being effected by means of apertured disc 1 rotated by motor 2. If the centre of the scanning circle coincides with the curve, the time intervals between pulses are equal; any deviation produces a change in this timing. A mask 4 positioned by motor 5 is arranged to produce a dark segment CO in the path of the scan for the purpose of providing a time reference signal. The output of the cell is of the form shown in curve a (Fig. 3) and passes through amplifier 8 to a differentiating unit 9 from which the positive and negative pulses appear separately as shown in curves b and c. The positive pulses pass to a pulse lengthener circuit 10, the output, curve d, controlling a gate stage 11 to isolate the reference pulse D from the negative pulse train. The positive pulse train, together with reference pulse D, is then applied to a pair. of multivibrators 12, 13 which generate pulse outputs, curves hand j, representing the intervals D to B and A to C. Discriminator 20 compares the outputs and thus generates an output signal proportional to the angle between the mask CO and and the tangent to the curve, the signal being applied, through amplifier 21 to control motor 5 to maintain the angle at zero. The positive pulse train is also applied to a third multivibrator 14 which generates equal amplitude pulse trains f and g for application to a discriminator 14 which produces a D.C. signal proportional to the algebraic difference between the intervals A to B and B to A. Thus, whenever the head is displaced to the curve, an error signal is obtained proportional to the amount of displacement and changing in sign with direction. The error signal is converted to components resolved along the x and y axes in a sine-cosine unit 6 which is set mechanically in accordance with the angle between the curve and x axis by motor 5. At the same time, voltages proportional to the resolved co-ordinates of the required speed are derived in a second sine-cosine unit 7. Adding units 16 and 17 combine the speed and error signals and control servo systems 18 and 19, for x and y direction motors. Alternative features involve the generation of the marker pulse by means of a bright spot projected on the base on which the curve appears, or by means of a brush and commutator 22, 23. In applications where the follower scans a curve formed by the boundary between domains of differing reflectivity, the cell output consists of onoff pulses which must first be differentiated to produce narrow pulses which are then interpreted as described above for the case of a simple curve. Where the apparatus controls a cutting tool, a mechanical drive may be taken from motor 5 in order that the attitude of the tool may be maintained the same throughout the cutting. operation.