Control system for a process

A perturbed feedback control system for a process. Inputs to the process include a feed stream and a manipulated variable. The manipulated variable is controlled by a feedback controller. Measurements of a parameter measured "downstream" of the process are taken periodically and fed back to the controller. The feedback controller generates a control response on the basis of a comparison of the two most recent values of the downstream parameter. The comparison is made by calculating a perturbed set point which includes the values of the downstream parameter taken at the two most recent sampling periods. In a specific embodiment, the perturbed set point also includes measurements of the most recent and preceding values of the manipulated variable. A control response is applied to the manipulated variable by the feedback controller based on the value of the perturbed set point. The control system has been applied to optimize the feed rate of chemicals for sludge dewatering on a twin belt filter press.