NONLINEAR CONTROL STRATEGIES FOR CONTINUOUS FERMENTERS

Nonlinear controllers based on exact linearization are designed and evaluated for continuous fermenters. The dilution rate and feed substrate concentration are considered as manipulated inputs in single-input/single-output strategies for productivity control. The resulting controllers are compared theoretically and via simulation. It is shown that state-space linearization techniques are not appropriate for this class of fermenters. Exact input-output-linearizing control employing the dilution rate as the manipulated input is shown to provide excellent regulatory behavior. Conversely, input-output linearization with the feed substrate concentration as the manipulated input is problematic. The exact approach yields unreasonably large control moves while an approximate technique recently proposed results in very sluggish responses. A modified approach to exact input-output linearization is proposed that results in satisfactory control.