GB1056661A - Method and apparatus for compensating machine feed drive servomechanisms - Google Patents

Abstract

1,056,661. Programmed control of machine tools. GIDDINGS & LEWIS MACHINE TOOL CO. Jan. 13, 1964 [Jan. 14, 1963], No.1354/64. Heading G3N. Relates to a servo control system for positioning a movable element of a machine tool along a path responsive to a continuously applied command signal (e.g. using programmed magnetic tape) whereby a servomotor moves the movable element and a feedback resolver rotor follows the movement of the element and transmits a feedback signal the phase of which is compared with the phase of the command signal and the resultant error signal operates the servomotor until there is no phase difference, whereupon the servomotor stops and the element is positioned as commanded. A second correction feedback loop is provided to account for inaccuracies of the drive means between the motor and element. MAIN FEEDBACK LOOP (FIG. 1) A numerical control equipment (not described) feeds a programmed modulated square wave command signal to a phase discriminator 18 and a reference square wave signal to a wave shaper and phase splitter. A servomotor 12 drives through gear 14 and rack and pinion 46 a movable column 10; and is coupled through differential gearing 28 to a rotor 26 of a main feedback resolver 16, the rotor 26 thereby rotating as the controlled column 10 moves. The differential 28 is arranged so that in the absence of rotation of an input shaft 32, by a correction servomotor 72, the rotor shaft 39 and the resolver rotor 26 rotate exactly half the amount of the servomotor shaft 38, thus the phase of the signal induced in the rotor 26 represents by a factor of one-half the angular position of the servomotor shaft 38 and will change by 360 degrees for each tenth of an inch, for example, of the column movement. The resolver field windings 24a, 24b, are fed with sine and cosine reference signal voltages from the phase splitter, and the phase splitter output is also fed through leads 22a-d to control channels for the various other axes of control Fig. 2, (not shown). The output from resolver rotor 26 represents the column position, with whatever positioning inaccuracies that may be present, and is fed back to the discriminator 18 wherein its phase is compared with the phase of the command signal. The discriminator produces a D. C, error signal proportional in magnitude to the phase difference between the two input signals and of a polarity corresponding to the sense of the phase difference. The servomotor thereby is rotated until the phase difference is zero and no error voltage appears at the discriminator output. CORRECTION FEEDBACK LOOP A position measuring transformer or linear resolver 50 is provided to measure errors (e. g. rack and pinion backlash) in the feed drive and is excited from the output of a correction feedback resolver 51, the rotor 55 of which is coupled to the rotor 24. The linear resolver 50 has relatively movable parts, a slider 52 and stator scale 54, fixed respectively to the column 10 and the side of the worktable Fig. 2 (not shown) and provides a signal which is an amplitude modulated, phase reversible sinusoidal wave representing by its amplitude and phase the magnitude and direction of the error between the actual position of column 10 as sensed by the linear resolver 50 and the position as sensed by resolvers 51, 16. The rotor 55 is excited by a sinusoidal voltage from an oscillator 68 and quadrature output voltages from stator windings 56a, 56b are fed to slider windings 52a, 52b which are in space quadrature, i.e. 90 degrees apart where 360 degrees = 0À1 inch linear separation along the stator scale. The linear resolver 50 output is a measure of the difference between the resolver rotor 55 motion and the slider 52 motion (column) and is fed through an amplifier 64 to a discriminator 66 which is also fed with a signal from the oscillator 68 identical in phase to that fed to the rotor 55. The D.C. output from the discriminator 66 is amplified at 70 and fed to the correction servomotor 72 which corrects the shaft input to the main and correction resolvers 16, 51 and is driven in a direction to reduce the difference to zero. Since resolver rotor 26 and rotor 55 are coupled the rotor 26 is also rotated to a position accurately representing the position of the column 10 and the main feedback signal to the discriminator 18 accounts for inaccuracies of the feed drive 14 and the column 10 will therefore be accurately moved in accordance with the command signal. The above description relates to movement along one axis only (A axis in Fig. 2 not shown) and additional similar servo control arrangements control the column movement along other axes. CASCADE ARRANGEMENT OF RESOLVER AND MANUAL ADJUSTMENT OF COLUMN. In an alternative embodiment Fig. 3 a correction, resolver rotor 90 is energized by a sinusoidal reference signal via a wave shaper from the numerical control and the output from its stator windings 94a, 94b energizes stator windings 82a, 82b of a main feedback resolver 80. A rotor 112 of a hand adjustable resolver 110, a rotor 84 of the resolver 80, the slider windings of the linear resolver 50 and stator windings 100a, 100b of a read out resolver 102 are cascade connected. The output voltages from stator windings 111a, 111b of the hand adjust resolver are sine and cosine voltages which thereby represent the algebraic sum of the angles of the rotors 90, 84, 112 in the cascade string, and are fed back to the discriminator 18 through a phase shift circuit 88 the output of which is a phase shifted sinusoidal voltage representing the actual position of the column 10. The linear resolver output is a measure of the difference between the position of the column 10 as detected by the linear resolver and the position as represented by sine and cosine voltages from the main resolver rotor windings 84a, 84b, and is fed through a phase discriminator 66 through amplifier 70 to rotate a correction servomotor 72. The servo 72 thus rotates the rotor 90 and thereby adjusts and corrects the output of rotor windings 84a, 84b until the output from the linear resolver 50 is a fundamental null when the output of the rotor 84 will accurately represent the column 10 position. The main feedback loop is generally similar to the Fig. 1 embodiment but includes the band adjust resolver 110. When the rotor 112 is hand adjusted a shift in the phase of the signal in line 89 occurs and since the two inputs to discriminator 88 no longer match in phase the main servomotor 12 operates to reposition the main feedback resolver 88 and the column 10 to reestablish matching. READ OUT UNIT (FIG. 3) The output of rotor 104 will be a fundamental null when the rotor angle indicated by dial 106 matches the equivalent input angle represented by the input to stator windings 100a, 100b. The servomotor 108 will therefore be caused to run by a discriminator 105 until a fundamental null output from the rotor 104 is obtained when the dial 106 will accurately indicate the position of the column. The command signal Fig. 3 may be developed by a stylus riding on a template instead of from a numerical control equipment.