GB1382057A - Servomechanisms - Google Patents

Abstract

1382057 Control of servomotors RANK ORGANISATION Ltd 1 Nov 1972 [17 Dec 1971] 58670/71 Heading H2J A servomechanism for controlling the movement of a movable element in each of two opposite directions between two end positions comprises control means to produce a velocity demand signal for selecting a desired velocity of the element, means to produce a signal representing the position of the element, and a deceleration control circuit including a comparison device responsive to the velocity demand signal and the position signal and operative to cause the element to decelerate at a predetermined rate, which is independent of the selected velocity of the element prior to the commencement of deceleration, as it approaches one or other of the end positions, if the velocity of the element is above a predetermined value. If less than this value the element is allowed to approach the end position at the velocity determined by the control means. Fig. 2 shows a servomechanism including members 11, 13, 14, 19, 20 and 42 which are generally similar in nature and function to those described in Fig. 3 of Specification 1,350,084, the members 10, 18, 21-24 being added to produce deceleration of the servo-actuator 14 as the element controlled thereby approaches an end position, the rate of deceleration and the position at which it commences being respectively independent of and dependent on the velocity of the element. In a typical operation a desired velocity signal, selected by control means 11 and passed by amplifier 33 to servo-actuator 14, is substantially unmodified when the element is in a centre range of its travel, since both diodes 40, 41 of a comparison device 18 are reverse biased as a result of the two signals applied to the device 18 from the tapping of a potentiometer 42 movable with the element or actuator 14 and from the output of an amplifier 22. The latter acts as a voltage follower responsive to the selected velocity signal transmitted through a linear or non-linear impedance network 10 and maintains transistors 36, 37 non-conductive whereby the signal is unmodified. When the element position is such that one of the diodes 40, 41 becomes forward-biased, the output of amplifier 22 inereases to switch on one of the transistors 36, 37 so that the signal supplied to the actuator 14 decreases in a manner determined by the degree of attenuation of member 19 or 20, the subsequent saturation of the amplifier 33 producing an impedance change which is followed by the network 10 to decrease the velocity signal and so produce a predetermined linear or non-linear rate of deceleration. Typical velocity/position characteristics are discussed, Fig. 3 (not shown), together with various modifications to members 19, 20 and 18, 22 to produce a desired final speed and a less abrupt change from the selected speed to deceleration. The element may be controlled to reach one or both of the end positions travelling at a speed respectively not greater than or equal to the predetermined value of velocity, thereby producing equal or unequal speeds at the end positions.