Patent GB-1210653-A (GB)
Title: Improvements in or relating to control systems for machine tools
Inventor: RUSSELL ALEXANDER
IPC: G05B19/39
Filed: 1967-01-04
Abstract: 1,210,653. Programmed control of machine tools. NATIONAL RESEARCH DEVELOPMENT CORP. Dec.29, 1967 [Jan. 4, 1967], No.499/67. Heading G3N. A point-to-point position control, for an element of a machine tool, is responsive to a digital form path instruction signal and an actual digital position signal in a comparison means to derive an error signal causing operation of a reversible motor so as to tend to reduce the error, at a near-zero value a balance detector means transfers control of the motor to a derived analogue holding signal which maintains the element at the signalled position. As shown, a machine cutting tool T is movable along the X-axis by a screw-shaft 50 rotated, through gearing 51, by a reversible motor XM and along the Y-axis by screw-shaft 52, gearing 53 and a motor YM. A reader TPR is responsive in parallel-mode to a punched paper tape having twenty-one channels coded whereby a hole in channel 1 denotes the X axis and absence of a hole the Y axis. Channels 2-4 denote by a 3 binary digit the limiting tool speed. Channels 5-20 as four, four binary digit groups, denote four decimal digits specifying the desired tool destination. Channel 21, by presence or absence of a hole indicates whether a further operation step is to be performed. A signal in channel 1 sets a trigger circuit XTR, which enables gates G10X-G13X and inhibits a trigger circuit YTR. Gate 10X allows the speed signals from channel 2-4 to pass to a speed limiter SLX connected to a control circuit MCX for the X axis motor XM. Gate 11X allows digital signals, from a read-out ROX of X-axis digital position transducer DTX, to pass to a subtractor STR in which they are compared with a desired position from channels 5-20. The digital difference value from the subtractor is fed to a digital-to-analogue converter DAC. A step determining signal on channel 21 is delayed in circuit DL sets a trigger circuit GTR which provide a signal on line 47 which enables a gate G14 to allow the analogue positionerror signal from converter DAC to pass through to the open gate G12X and thence to the motor control circuit MCX. The motor XM now operates, subject to speed limitation by circuit SLX, to bring the tool T towards the desired position at a speed which falls as the position is approached. At the desired position, the output from subtractor STR is zero and sets a balance detector BTR the output of which resets the trigger circuit GTR. The gate G14 closes and removes the analogue signal from the motor control circuit MCX whereupon it is replaced by an intermediate analogue waveform output wHS form the digital read-out ROX through a gate G15X. This signal retroactively holds the tool at the desired position whilst the tape reader TPR is released in readiness for being stepped on by a mechanism 41 responsive to the reset output of trigger circuit GTR. In a modification, Fig. 7 (not shown), the balance detector BTR comprises a Schmitt device having a limited amount of back-lash between its change-over levels. The solution of the intermediate analogue value wHS is controlled by the least significant digit of the position instruction.