An analog/hybrid computer simulation of phase-locked-loops in motor speed control

Abstract

This paper describes the simulation of the precise control of speed in servo systems using the digltal phase-locked-loop (PLL) method and analog/hybrld computer techniques. The PLL for motor speed control is essentially a hybrid feedback control system in which a reference square-wave signal is used to command servo velocity and position. The system consists of a phase comparator, a lowpass filter, the motor under control, and a voltage-controlled-oscillator (VCO) in the feedback path. The mathematical model of the motor and the low-pass filter are simulated on the analog computer. The input reference is a frequency-modulated (FM) digital signal. An integrated circuit is used as the phase comparator which compares the perturbed VCO feedback frequency and the input signal.The method introduced here is simple to learn and practical to implement in undergraduate control courses.It introduces the student to the basic concepts of PLL control, without overwhelming him with its complex problems.