Selection of sampling frequency for digital process control systems

Abstract

A simple method is proposed for estimating the performance degradation caused by repeated sampling at intervals in the digital, proportional-plus-integral control of process systems. Linear, single-loop, -lag systems with and without dead time are considered. The method is based on the use of Harriott's empirical index of performance extended from continuous to discrete systems. Experimental performance data are correlated against the index. The sharp minima reported by Roberts in plots of integral time-weighted absolute error against sampling interval are not found when the reset rate is unconstrained.