Controlling processes with long transport lags

This paper describes the control of the wet-lap forming process in a hardboard mill. Pearce and Self in a previous paper indicate that the thickness of finished hardboard is affected by fibre type, structure, density, steam press cycle conditions and wet-lap thickness. They also indicate that the main cause of variability in dry board thickness is due to inadequate control over pre-press wet-lap thickness. The wetlap forming process was not previously controlled due to the long transport lag between the flow valve and the measurement transducer. The purpose of this work is to evaluate a variety of wet-lap thickness control strategies and implement as appropriate. The following strategies are simulated: predictive control; cascade IPD control; and fuzzy logic rule base control.