Patent GB-836049-A (GB)
Title: Improvements in or relating to machine tool control systems
IPC: G05B19/39
Filed: 1957-05-22
Abstract: 836,049. Electric correspondence control; programmed electric control systems. SOC. D'ELECTRONIQUE ET D'AUTOMATISME. May 22, 1957 [June 16, 1956], No. 16250/57. Class 40 (1). In a control system for a machine-tool in which the positions of a cutting-tool and the work-carrier are to be controlled from a digital record bearing the values of tool and workcarrier co-ordinates at spaced points, an interpolation device is provided for each co-ordinate which gives an approximation of intermediate points such that there shall be a smooth continuity between points of given value, the interpolation device receiving and operating on the sets of data previously computed and stored on the tape which comprise predetermined functions of the first, second and third order derivatives of the variable co-ordinate. The interpolators indicate in a digital code the correct or approximately correct relative position of the tool and work-piece and this is compared with a digitally-coded indication of the actual relative positions, the differences being converted to analogue voltages and used to control driving motors to correct the relative position of the tool and workpiece. The interpolating device consists merely of a group of four registers interconnected through adding units so that the precalculated functions read from the input tape can be stored and added selectively to the initial co-ordinate value at each interpolation step to provide the approximations required. In Fig. 1 the invention is shown applied to a milling- machine of which the table 1 is controlled by a motor 9. The position of the table is indicated by an encoder 10 on the motor-shaft providing a rough value of the table position and by a sensing device 11 which co-operates with a scale 5 providing a fine value of the table position. The cutting tool is displaceable transversely and vertically by motors, not shown, and its position is indicated by scales 3 and 4. An interpolation and control sircuit is provided for each direction of movement but only the circuit for the table control is shown in Fig. 1. The coarse encoder comprises a pair of stator windings in quadrature to which A.C. in phase quadrature is applied., The rotor, 12 has a single winding, the phase of voltage induced being dependent upon its position. The phase shift produced by the encoder is converted in circuit 23 to an output pulse, the length of which indicates the phase-shift. This is done by applying one of the stator voltages or a reference voltage and the rotor voltage each to clipping amplifiers, differentiating the result and using the pulses produced to switch a trigger from one state to another. The sensing device .11 for the fine indication is similar in principle but the primary windings are laid out along the length of the table and the secondary winding 11 is laid out adjacent thereto. The output of this winding is also converted in circuit 22 to a pulse whose length indicates the phase shift. These pulses are applied respectively to gates 16 and 17 to gate a stream of pulses from a time-base circuit 14 into binary counters 18 for the fine value and 19 for the coarse value. The values in the counters therefore represent the fine and coarse positions of the table 1. The highest value digit of counter 18 should be the same as the lowest value digit of counter 19. If they are not, a checking circuit 24 detects the difference and adds a " 1 " to the lowest stage of counter 19. The'values in counters 18 to 19 are read out in serial form at the initiation of a read-out pulse from the time-base circuit at terminal 26. The data read out is serialized by converters 28-27 and made in complement form for application digit-by-digit to a comparison - circuit 29. The input tape 41 bearing values of 'the co-ordinates at spaced points along the cut to be made and also the functions mentioned above to be used for deriving intermediate approximations, is sensed at 40 and the sensed signals routed by a circuit 38 to the interpolation device 32 where they are stored. The number of interpolations to be made between given coordinates is set by special instruction from the record tape applied through store 44 to the timebase. Control signals generated by the time base are transmitted to the interpolator over line 33. Terminal 42 emits signals stepping on the record tape to the next position and preparing the interpolator for the entry of new data. At each step the values arrived at by the interpolator are read out in serial form to the comparator 29 which is in effect an adder. Since one of the coded pulse trains is in complement form the output of the adder 29 gives the difference between the position set by the record or interpolator and the actual position. This difference is applied to a register 34 and used in a decoder 35 to generate an analogue voltage controlling the motor 9, a reference voltage applied from the record tape over line 47 determining the speed at which the motor 9 is driven to make adjustments. The time-base consists of a high-frequency oscillator and a series of frequency dividers to give outputs of various control frequencies as required.