Patent GB-828526-A (GB)
Title: Improvements in and relating to machine tool control systems
Inventor: WORTHY WILLIAM DIGBY
IPC: G05B19/4105
Filed: 1955-07-08
Abstract: 828,526. Programming for machine-tools. ENGLISH ELECTRIC CO. Ltd. July 2, 1956 [July 8, 1955], No. 19823/55. Class 83 (3). [Also in Group XXIII] A machine-tool control system comprises means 14, Fig. 3, for modifying the control so that an object may be machined along any of a number of contours displaced in a normal direction from one another. Thus a control programme based on a particular tool size may be adapted for tools of different size, and for adjusting for wear of a grinding-wheel, or for taking a series of roughing and finishing cuts. The programme may be on a tape record or in the form of a turbine - blade pattern cooperating with a follower, which may control several machine-tools having tools with differing degrees of wear, or taking different roughing and finishing cuts. If the programme has been calculated to produce a path PS, Figs. 1 and 2, and the tool 10 is required to follow a path QT, displaced by the amount PQ, then new x, y values must be computed such that To achieve this computation, a control unit 12, Fig. 3, supplies information in terms of x 0 , y 0  to a computer 13 which affords information which is a measure of the functions A control system 14 influences a means 15 to multiply the outputs of the computer 13 by " PQ ". The output signals are finally supplied, together with signals from the unit 12, to units 16, where they are appropriately added or subtracted. It is convenient to arrange the recorded programme so that the velocity of the point P along the curve is constant, that is, so that there is a fairly constant rate of cutting and the outputs of the computer 13 are simply Fig. 4 illustrates a modified system in which information is supplied in a series of pulses specifying increments of distance along the x or y axis. The pulses are supplied from a digital interpolator, or a magnetic tape record, to gating units 17a, 17b forming pulses which have a constant voltage-time area, the output signals thus being proportional to the rates of change of x 0  and y 0 . Smoothing-filters 18a, 18b produce output signals, proportional to x 0  and y 0 , which are supplied to bridge-modulators 19a, 19b, which are fed with A.C. signals 90 degrees out of phase with each other. The modulator outputs, kx 0  sin wt and ky 0  cos wt are mixed to give a signal of amplitude which is demodulated by a demodulator 20 to give a steady signal. A unit 22 then compares the signal with a signal, representing PQ, from a potentiometer system 21. The resulting error signal, through an amplifier 23, controls the gating units 17a, 17b through feed-back action. The control signals required are the inputs of the gating-units 17a, 17b (x 0  and y 0  in digital form) and the outputs of the smoothing-filters 18a, 18b (kx 0  and ky 0  in analogue form). The latter signals are converted to digital form by analogue-digital converters 24. The adding or differencing units 25 produce the necessary signals representing the values of x and y which are used to control a machine-tool, as described in Specification 828,525. Polar co-ordinates may be used. Thus, Fig. 5 shows a grinding- wheel 31 machining a turbine blade 30, the path 32 being defined in terms of Ro, # 0 . To calculate a path displaced by a distance PQ, simplified equations may be obtained if PQ is small compared with R 0 . Although the velocity along the path may be constant it is preferred to make # 0  constant. Another approximate calculation may be made by retaining the values of # 0 , shifting the cutter-centre to Q<SP>1</SP> and calculating the values of OQ<SP>1</SP>. In the latter examples, multiplication and division may be performed by electro-mechanical devices such as potentiometers, or electronically by mark-space ratio techniques applied to trains of rectangular pulses, or by recording information relating to the actual values of the velocity components. For interpolating between values, an arm 50c, Fig. 6, rotates around a potentiometer 50 to interpolate linearly between different potentials supplied successively to tappings 50a, 50b. If this system is applied to that of Fig. 3, a simplification results. A further simplification consists in recording with the data relating to successive control values, the first and/or second differences between these quantities. For a parabolic interpolation, a current is supplied to the arm 50c, the value of the current being computed automatically in terms of the values of key potentials, or recorded in the data from which the machine-tool is controlled. The current may also be controlled continuously by a second potentiometer 51, a buffer amplifier 52 being provided so that a cubic interpolation may be performed as described in Specification 828,525. The system may be modified so that, by supplying data relating to key-values of x 0 , a continuously-varying output quantity x 0  may be obtained. Such outputs for x 0  and y 0  can be supplied to modified potentiometer systems to produce the x, y values, the required offset being set-in on one of the potentiometers. Besides using a simplified polar co-ordinate system as in Fig. 5, a simplified cartesian co-ordinate system may be used as in Fig. 11, where x=x 0  and A system for machining in three dimensions may also be provided. The tool may then proceed in a helical path, as described in Specification 828,525, one co-ordinate of which path is uncorrected. The six mathematical cases of how the computers may operate are set out at the end of the Specification.