Patent GB-974503-A (GB)
Title: Improvements in automatic control mechanism for machine tools
Inventor: WHITEMORE GERALD
IPC: G05B19/4105
Filed: 1960-02-23
Abstract: 974,503. Electric analogue calculating. ELECTRIC & MUSICAL INDUSTRIES Ltd. Feb. 21, 1961 [Feb. 23, 1960] No 6237/60. Heading G4G. [Also in Division G3] Automatic apparatus controlling the displacement of a machine tool cutter relative to a workpiece comprises a device producing output signals representing the values of at least one co-ordinate of successive points of the cutter locus relative to the workpiece corresponding to successive values of a parameter; the device including motive means responsive to a rate signal representing the rate of variation of the parameter for determining the rate of production of the output signals, a first signal source operative synchronously with the motive means for producing a first rate signal which is variable smoothly from one fixed limit to another, a second signal source which can be preset to produce one of a number of fixed rate signals each within such limits, a selector switch for applying the variable rate signals to the motive means as a rate signal therefor when the two rate signals have one relationship, and for applying the fixed rate signal to the motive means as a rate signal therefor when the two rate signals have a different relationship, together with means for disabling the selector switch and applying the fixed rate signal to the motive means, independently of the relationship between the rate signals. In an interpolating device for controlling the displacement of a cutter relatively to a workpiece of a machine tool, a reference alternating voltage energizes an autotransformer 4 having taps 12 engaged by wiper 13 and extremities 3, 9 energising a ring potentiometer 6 over resistance 10. Wiper 8 of the potentiometer is energized from extremity 3 over resistance 7, and the voltages on wipers 8, 13 are applied to a comparator 14 energizing relay 15 over normally closed switch 16 to operate moving contact 19 between contacts 17,18 to energize servo amplifier 20 controlling motor 21, together with a velocity feedback signal from tachometer 23 driven together with wiper 8 by the motor shaft 22, whose angular displacement represents the magnitude of an interpolation variable T driving quadratic interpolators 25, 26 of the kind described in Specification 830,462 to generate interpolated signals x(T), y(T) defining a locus to be machined in terms of parameter T, which control servo systems 27, 28, 30; 31, 32, 33 driving the work-table 34 of a machine tool in the x and y directions. Initially the control programme is arranged to close switch 16 only at the beginning of selected half-spans of the interpolators 25, 26 wherein it is required to accelerate or retard the rate of variation of parameter T, and to open the switch at the end of each such span, while the rotary switches of the interpolaters are synchronized with wiper 8 so that the latter lies on the connection of resistance 10 at the beginning of each interpolation span. The required speed of interpolation is set by wiper 13, so that the voltage on wiper 8 is less than that of wiper 13 and relay 15 is energized to close contacts 17, 19. A creep voltage over resistance 10 energizes servo amplifier 20 to drive motor 21, which rotates thereafter at an increasing speed dependent on the voltage on wiper 8, which moves clockwise over potentiometer 6 until the voltages applied to comparator 14 are equal and relay 14 releases contact 19 to close contact 18, so that shaft 22 rotates at a steady speed determined by the position of wiper 13, and interpolation is accelerated during the first half span from rest. When wiper 8 reaches point 5, the programme opens switch 16, so that shaft 22 continues to rotate at the determined speed. When interpolation is to be slowed, as when the cutter approaches a sharp angle of the work, switch 16 is closed by the programme at the middle of a selected span, so that when the voltage of wiper 13 exceeds that of wiper 8 relay 15 is energized to close contacts 17, 19, and the control voltage and the consequent speed of shaft 22 decrease progressively to the end of the span. Switch 24 and a brake (not shown) may be programmed to arrest shaft 22. Potentiometer 8 is linear and in the absence of resistance 7, gives exponential acceleration and retardation, but the latter are rendered constant by inclusion of an appropriately proportioned resistance. Alternatively the potentiometer may be non-linearly wound. The tappings of transformer 4 may be logarithmically spaced, and wiper 8 may be driven by a separate motor switchable by the programme.