Patent GB-1379457-A (GB)
Title: Numerical control system
IPC: G05B19/25
Filed: 1972-03-01
Abstract: 1379457 Automatic control TEKTRONIX Inc 1 March 1972 [5March 1971] 9453/72 Heading G3N Movement along a segmental path is effected at a desired feedrate by using a digital differential analyzer to establish the actual feedrates in the co-ordinate ones. As shown, Figs. 1, 1A segmental path movement instructions for a cutting tool of a milling machine recorded on a punched tape is B.C.D. blocks are read by a reader 14, and set into co-ordinate command registers 18, 20 in binary form through a converter 16. The actual positions of motors 10, 12 driving the tool are determined by pulse transducers 64, 66 and fed through interface circuits 66, 78 to actual position registers 26, 30. The co-ordinate position errors #X, #Y are derived by subtracters 28, 32 and fed to D.D.A. interpolators which comprise for the X axis register 38, 40 and an adder 42 providing an overflow on a lead 46 to an error counter 58, receiving position feedback from interface 66, and providing a rate control signal through a D to A converter 60 for an analog servo 62, having tachometric feedback, not shown, for the motor 10. When the position error approaches zero subtractor 28 provides a detect zero output on lead 68 which inhibits a gate 56 to stop pulses on lead 46 and jump the counter 58 to a given value for holding the servo. At the same time the gain of servo 62 is enhanced. The Y axis interpolator and servo-mechanism as similar to those for the X axis. The control system has two modes of operation selected by a junction register 84, Figs. 2, 2A. The normal mode A applies acceleration and deceleration for each block, while the anticipatory mode B applies acceleration in the first block only and deceleration in the last block. The mode is selected by an instruction read with each block and outputted on a line 86 from the convertor 16, Figs. 1A. In mode A a register 116 is pre-set by a device 118 in accordance with an acceleration command from a selector circuit 132. The count in the register 116 is increased during acceleration by successive pulses from a circuit 126 and the circulated contents of the register through an adder 120 and register 122, forming thereby a D.D.A. providing an overflow to a regulator 136 which compares it with the overflow from a register 406 representative of the calculated rector feedrate. The acceleration is exporential in nature. At a value of feedrate equal to that set from by a command from the converter, Fig. 1A, on a line 87 in a register 404 the selector circuit 122 switches so that the overflow 124 is determined by the commanded feedrate. When the larger position error, as determined by a detector 138, is below a set value the cycling of the register is inhibited and the contents of detector 138 entered through an AND - gate 140. When the contents of register 116 falls below that of register 404 the selector circuit switches so that the overflow 124 decreases with the contents of the register 116. A facility is provided for starting deceleration during the acceleration phase for short path segments. Mode B a command from converter 16 is set in register 84 and selector circuit 13.2 over-rides detector 138. A rapid checking mode for the perforated is provided by disabling the servo-mechanism, Fig. 1, and coupling the register 40 to the input of registers 26, 30 as indicated by dashed lines 144, 148 A bi-stable cathode ray storage tube, not shown, is connected through D to A converters 152, 154 to present the segmental instructions. The control of movement of the tool in dependence on the resolute of the axis velocities is determined by feeding the incremental positions from registers 38, 48 together with the vector sum of the command velocities from a register 447, Fig. 2, to gates 434, 436 of inter-connected summing registers which, in association with adders 438, 440, provide a #X<SP>2</SP> or #Y<SP>2</SP> overflow on a line 454. The overflow is fed to a discriminator 456 providing add or subtract signals on  lines 458, 460 to the register 446. The register 446 with an adder 409 and a register 406 from a D.D.A. the overflow from which on a line 408 provides the comparison for the overflow 124 in the regulator 130.