Stability problems in the control of multipass processes

The general characteristics of multipass processes are discussed and a method of process modelling is proposed based on the single independent variable: total distance passed. The stability of a number of such processes, including longwall coal cutting, ploughing and metal rolling, is examined using this type of model analysed by the inverse Nyquist technique. Important stability problems, arising from the multipass nature of the processes, are exposed in all cases. Transport delays and resonance in the singlepass loops are shown to be particularly troublesome. A general approach to the dynamic analysis of bidirectional systems is outlined using discrete time-series observation.