Closed Loop Step Servo Systems

Abstract

Stepping motors are motors which receive input in the form of pulses and which move through discrete angular steps in response to them. They also develop a detenting torque which holds the rotor in position after cessation of pulses. On account of these characteristics, stepping motors are found suitable for use as digital actuators in numerical machine tool controls, analog-to-digital converters, etc.

Stepping motors can be used in either open loop or closed loop servo systems which are known as step servo systems. The paper describes the design, construction and testing of a closed loop step servo system consisting of the following elements: (i) a bifilar wound permanent magnet stepping motor, (ii) a step discriminator, (iii) memory logic and translator, and (iv) power drivers. The step discriminator is an aluminium disc mounted on the motor shaft and contains 50 holes drilled 7·2° and ¼ in. apart. Four photosensors, spaced 23·4° apart, sense the rotor position and set the levels of four flip-flops accordingly. Logic gates pick off appropriate signals from the flip-flops in accordance with one of the four possible input commands, viz. (1) stop, (ii) high speed, (iii) clockwise, and (iv) counterclockwise, and apply them to power drivers. These, in turn, energize the motor windings. Once an input command is given, the motor pulses itself and keeps on running continuously. The closed loop step servo system can operate either in the single phase mode in which only one motor winding is energized at a time or in the two-phase mode in which two windings are energized simultaneously.

Transient response tests on the system show that the motor responds to an input command in one sampling period. Steady state torque versus pluse rate tests show that the motor which could barely run at 150 pulses/sec. with open loop control can run satisfactorily at 500 pulses/sec. in the single phase mode and at 800 pulses/sec. in two-phase mode. Since the motor pulses itself in closed loop control, it does not miss a step even when it runs at 500 and 800 pulses/sec. This is the greatest advantage of closing the digital loop around the stepping motor.