High Accuracy and Fast Response Digital Speed Measurement for Control of Industrial Motor Drives

Abstract

This paper concerns digital speed control for high performance industrial drives (paper machines, rolling mills etc.) based on a novel principle for frequency measurement.In sections 1-2 the principle and theory for digital measurement of angular velocity by means of a low resolution pulse transmitter and the new frequency-to-digital convertor is presented. This system gives high measurement accuracy and resolution combined with short measuring time within a wide speed range at a low ripple content. Further, in section 3 it is shown how this F/D convertor, consisting of a binary rate multiplier, an up/down counter and a register can be built with a few digital integrated circuits.The next section describes the design of the speed control system for a thyristor controlled DC motor drive based on the new F/D convertor. Significantly better steady state accuracy and linearity (better than 0.01 %) compared with present analogue systems has been achieved. The dynamic control performance is also better than that of most analogue systems available (response time better than 30 ms for a 100 kW drive). These characteristics are documented by experimental performance curves.In the last section, two industrial applications of such drives are described - one for a paper machine and one for a wire rod mill. In both cases microprocessor-based program-controlled electronics is used for digital overriding (higher level) control.