Patent GB-1141851-A (GB)
Title: Digital-analogue positioning control system
IPC: G06F7/50
Filed: 1966-04-07
Abstract: 1,141,851. Programmed control of machine tools. GIDDINGS & LEWIS Inc. April 7, 1966 [April 12, 1965], No. 15640/66. Heading G3N. A system for moving an element to bring its actual position into agreement with desired position defined by a first multi-digit decimal number comprises a first storage means for representing and storing the first number in binary-coded decimal form, second storage means for a binarycoded decimal number representing the actual position of the element, element motivating means responsive to the stored numbers for driving the element until the difference is reduced to a, predetermined value, analogue means responsive to the representations of a predetermined least significant order digits of the first number for producing an analogue signal indicative within a fine distance span of the desired position of the element, the fine distance, span being greater than said predetermined value and a fine servo-loop responsive to the analogue signal for controlling the element motivating means for driving the element precisely to the desired position. As shown, an adder-subtractor 105, under control of multiplex gates 106, sequenced by timing signals from a generator 107, algebraically combine the X, Y command numbers with the correction numbers, stored in switches 95, and the resulting adjusted command numbers X' Y' are transferred to the stores 99, 100. The actual positions of the saddle and table are derived from reversible counters 102, 101 responsive to the increment pulses from generators PGY, PGX driven by lead screws. The adder-subtractor 105, after completion of the correction routine, then repeatedly subtracts the actual position numbers held in stores 101, 102 from the adjusted numbers in stores 99, 100 and the resulting numerical error is compared with switch point values held in stores 111, 112. Above a predetermined error the velocity of the saddle and the table, determined by motors 86, 68 and multi-ratio transmissions TY, TX is at a maximum. As the error falls to successive preselected switch point values, signals are applied to slowdown networks 114, 115 which change the energized pattern of clutches in the transmissions TX, TY thus reducing the velocities of the table 58 and saddle 55. To bring the saddle and table precisely to the adjusted positions, a fine analogue positioning system is enabled when the error falls to a value within the span of the analogue system. Information is fed from the final digits of adjusted command stores 99, 100 to BCD-to-decimal converters 120, 130 the outputs of which are transferred to digital to analogue converters 121, 131 which energize windings of sliders 124, 134. The detected errors from scales 125, 135 are fed to analogue error detectors 136, 137 controlling the slowdown networks, 114, 115 and the direction and stopping controllers 138, 139. The velocity of the table and saddle is reduced to a creep value within the fine analogue span and the motors are de-energized slightly before the desired position so that the stored kinetic energy is dissipated in coasting. In an arrangement, Fig.2 (not shown), backlash compensation is effected by the final approach to the desired position being unidirectional.