GB703329A - Improvements in or relating to automatic control systems such as servo systems - Google Patents

Abstract

703,329. Control of A.C. motors. HEMING- WAY, A. V. Jan. 3, 1950 [Jan. 3. 1949], No. 114/49. Class 38(3) [Also in Group XXXVIII] In an automatic follow-up control system wherein an input shaft is rotatable through discrete intervals to generate trains of discrete electrical pulses representing its angular position which are compared with similar trains of discrete pulses representing the angular position of the output shaft to derive an error signal controlling a servomotor to drive the output shaft through discrete intervals into positional correspondence with the input shaft, there is provided a device which, when the error signal reaches zero, generates a direct current to magnetize one of the operative motor windings to saturation, thereby generating eddy currents in the rotor which damp oscillation or hunting of the output shaft about the position of correspondence. In Fig. 1 trains of discontinuous discrete electrical pulses digitally representing the angular positions # 1 and # 0 of an input shaft (not shown) and an output shaft 2 at discrete intervals of time (unit change in the angular positions of each shaft being represented by a unit digital change in the coded pulse signal) are subtracted in known manner in a unit 1 to produce a digital error signal e representing (# 1 -# 0 ,) which is converted in known manner to a unidirectional voltage analogous in sign and magnitude to the digital error signal, changing in fixed steps with variations in the value of e but remaining constant over the intervals between successive digital error signals. Such voltage is fed to a mains energized magnetic amplifier 4 producing an A.C. representing in magnitude and phase sense the magnitude and polarity of the unidirectional voltage to energize winding 5 of servomotor 3 having its winding 6 energized in quadrature through transformer 7 and condenser 6a, so as to drive the output shaft into positional correspondence with the input shaft. The digital error signal # is bridge rectified at 10, Fig. 2, to a negative voltage which normally biases a triode V, to cut-off, but releases the triode when the error signal falls to zero, and the resultant unidirectional anode voltage drop, after amplification at 8 is injected across condenser 6a to saturate the quadrature winding 6 of the motor at zero error signal, whereby the eddy currents generated in the rotor effectively damp the motor rotation and disable amplifier 4, so that the output shaft is arrested in correspondence with the input shaft and hunting is prevented. Rotation of the input shaft out of correspondence develops an error signal which biases the triode to cut-off and removes the damping until the servomotor once more reduces the error to zero.