Patent GB-979501-A (GB)
Title: Improvements relating to interpolating arrangements especially for the automatic control of machine tools
Inventor: WEDGWOOD DAVID DEAN
IPC: G05B19/4105
Filed: 1960-04-13
Abstract: 979, 501. Interpolator arrangements. ELECTRIC & MUSICAL INDUSTRIES Ltd. April 5, 1961 [April 13,1960], No. 13111/60. Heading G4G. [Also in Division G3] An interpolating arrangement, comprising two interpolators arranged so that sets of input signals can be applied alternately to them and having a selector which moves so as to derive alternately from the two interpolators an output signal that varies to represent interpolated values of a function defined by the sets of input signals and by the interpolators, is provided with an arrangement which modifies the output signal to vary in accordance with a smooth curve from a value of the function interpolated by one interpolator to a value of the function interpolated by the other interpolator whilst the selector moves through a range of positions which includes the changeover from one interpolator to the other but is small compared with the total range of positions of the selector. An object of the invention is to allow the cutting speed of a machine tool to remain at a constant value whilst passing through a position of discontinuity in the locus to be cut. Such a locus may have its co-ordinates in one direction (say x) represented by points such as A, B... G (Fig. 1) where D is a reference point giving the location of a discontinuity and the other points represent interpolated values on two intersecting spans, where a span is the section of the locus lying between two reference points represented by input signals. In order to give a constant cutting speed dx/dt, it is necessary that dx/dT changes parabolically (where T is the interpolator parameter) so that the uniform acceleration of the interpolator speed [dT/dt] will give a constant dx/dt. The equation of the parabola section CJ is x = [d + p/4 (1 +T)](1 + T) - cT where C  is the point (-1, c),D is the point (0, and - 1 # T # 0 and where p = DC (resolved along OX) -ED (resolved along OX). This equation may be generated by the arrangement shown in Fig. 3, where alternating voltages are applied, one proportional to p/8 to the stator winding 10 of a variable ratio transformer 9, one proportional to d + p/8 to the terminal 14 and one proportional to c to the terminal 7. The rotor carrying the winding 12 is geared to perform a 90 degrees rotation during the whole sweep of the potentiometer 2. The voltage appearing at the terminal 8 is that required if # = 1 + T where # is the fractional position of the wiper of the potentiometer between the terminals 7 and 6. The arrangement shown in Fig. 4 comprises an interpolating device as described in Specification 830,462 modified in accordance with the present invention. Alternating voltages from five stores (described in Specification 846,381) representing values of one co-ordinate of five points of a locus are applied to the terminals 21, 22, 23, 24, 25. From these terminals the signals are applied to two interpolators comprising the autotransformers 26 and 28 corresponding to the two spans (as described in Specification 796, 994). The output signals from the interpolators are applied to a rotary switch 30, the wipers of which are connected to the ends of the autotransformers 31 and 32. These autotransformers have tappings taken to a rotary switch 33, which makes half a revolution as the wipers of the switch 30 move over one set of studs, and this system forms a linear sub-interpolator (as described in Specification 802,472). As the wipers of the switch 30 pass over the studs S1 and S3 and S2 and S5, the parabolic output previously explained with reference to Fig. 3 is obtained if the contacts A and C of the switches 50 and 51 are closed as shown. The correct voltages for this operation are obtained by means of suitable windings such as 35, 36, 48, 47 associated with the two interpolators 26 and 28. The variable linkage transformer 40 corresponds to the transformer 9 of Fig. 3, and performs one revolution for every two revolutions of the switch 33. In Fig. 5 an apparatus to produce the requisite change of dT/dt, used in conjunction with the apparatus of Fig. 4, is shown. The shaft 68 is linked to the switches 30 and 33 and the transformer 40 and has a tachometer 69 coupled to it, this being energized by a source of alternating reference voltage which is also applied to the inductive potentiometer 63. The output from the tachometer is applied to the autotransformer 70, which has taps 71, 72 which are set to be proportional to the lengths of span before and after the junction point D (Fig. 1), either manually or by programme tape. The wiper 74 on the inductive potentiometer 73 is connected to one side of a differential amplifier 65. The other side of this amplifier .is connected to the wiper 64 on the inductive potentiometer 63, which is set to a position proportional to the interpolator span. The output from the amplifier 65 is applied through an amplifier 66 and a motor 67 to drive the shaft 68. When the wiper 74 is set to the tapping 71, the shaft rotates at the speed required to produce the desired cutting speed for the span preceding the point D. At the point C however the gearbox and clutch 75 engages to move the wiper 74 across the potentiometer, so that the shaft speed will change. At the point E the wiper 74 reaches the tapping 72, the speed is that required for the span following the point D and the clutch 75 is disengaged. An alternative embodiment of the invention is illustrated in Fig. 7. Alternating voltages from five stores are applied via the terminals 101,102, 103,104,105 to two quadratic interpolators including the two auto-transformers 106 and 108. The output signals from these interpolators are applied to a rotary switch 110 (of the type described in Specification 796,994). The wipers W1,W2 are connected to the end terminals of a transformer winding 111 of 192 turns, and the wipers W3 and W4 are connected to the winding 112 of a second transformer which is not coupled with the winding 111. Tappings are taken from the winding 111 at 3 turn intervals and connected to the studs A1 to A25 of a 128 stud switch 113 whose wiper 114 scans the studs cyclically and applies the voltages therefrom to an output terminal 115. Four other transformers 117,118,119,120 are coupled to the windings 111, 112 by means of the link 116 in such a way that there are induced into single turn windings on the transformers 117, 118, 119, 120 voltages respectively equal to 1/3, 1/9, 1/27, 1/81 of the difference between the voltage induced into three turns on the transformer 111 and into three turns on the transformer 112. These transformers are coupled into single turns of three turn windings on transformers 111 and 112 so as to inject successively increasing fractions of the difference between the voltages across the windings 111 and 112 into the wires which lead to the studs A26 to A64 of the switch 113. The studs A65 to A105 are connected in a similar way to the transformer 112 to provide a successively decreasing fraction. The output will follow a quadratic curve between points similar to C and E of Fig. 1. This embodiment may be used in conjunction with the apparatus of Fig. 5 or with that of Fig. 8, where the apparatus of Fig. 5 has been modified by the addition of a further winding 73<SP>1</SP> to the switch having the wiper 74. This wiper is geared to rotate continuously, with the shaft 68, and each of the wipers 71 and 72 is set to a new position whilst the wiper 74 contacts the block of studs connected to the other. Fig. 8 shows the complete system for the control of a machine tool, where the block 76 represents the programme tape reader and control circuitry, blocks 71 and 78 represent the interpolators for the two co-ordinate directions and blocks 79, 80, 81 represent apparatus for setting up the positions of the wipers 64, 71,72 in accordance with programme instructions. The interpolators produce signals which are amplified by the differential amplifiers 82, 85 and are used to control the servomotors 83, 86 The servomotors control the rotation of the screws 88, 89. The feedback units 84, 87, produce output signals representing the shaft position and the signals are compared in the differential amplifiers 82, 85 with the interpolator signals, so that screw rotations occur according to the difference in the screw position and the interpolator output. Specifications 799,356,886,395 and 886,875 also are referred to.