Stability and Quality of Control

In the process industries issues of loop stability and controller performance are seldom investigated by classical methods such as frequency response and root locus. The root locus plot is a useful aid to understand the effect of increasing controller gain on the closed-loop performance. The open-loop stability was first considered because classical theories draw many inferences about the closed loop from the open loop. As in the case of root locus, the main features of Bode plots can be sketched by inspection. This chapter explains the usefulness of the Nyquist plot, representing an open-loop frequency response, for determining closed-loop stability. The Nyquist stability criterion does not have the limitation of the frequency response stability criterion. It is sometimes useful to use a ‘magnitude versus phase angle’ plot of the frequency response, as opposed to the Bode plot or Nyquist p.