The design of a robust linear controller is proposed for the control o
f an unstable bioreactor. The performance of the proposed controller i
s compared with that of a Ziegler-Nichols type controller. The robustn
ess of the controllers for perturbations in maximum specific growth ra
te and yield factor (affecting the process gain and time constant) is
evaluated by simulating the non-linear model equations. For uncertaint
y in the process parameter values the PI controller gives a sustained
oscillation in the servo response, whereas the proposed controller giv
es a robust response.