1. A control system for a single input-single output process, said process including i) a controlled variable having an output magnitude and ii) an input receiving an operating signal in which is encoded a value responsive to which a process parameter changes, said parameter change causing the controlled variable's output magnitude to change in a predictable manner dependent on the value encoded in the operating signal, said control system comprising: sensing means which detects the controlled variable's output magnitude at predetermined intervals and provides a sensor signal encoding the output magnitude; error signal generating means receiving the sensor signal, for generating an error signal with a magnitude proportional to the difference between the sensor signal output magnitude and a predetermined set point, and a polarity indicative of error direction; a look up table receiving the error signal, said look up table having a plurality of recorded entries, wherein each of the entries comprises a preselected control value magnitude and an error value specifying a range of error signal magnitudes, wherein the look up table provides a control signal encoding the control value magnitude recorded in the entry in which is also recorded the error value specifying a range of error signal magnitudes within which falls the error signal magnitude in the current error signal from the error signal generating means; and control means receiving the error signal and the control signal from the look up table and responsive thereto, for providing the operating signal in which is encoded a value proportional to the magnitude of the look up table output and having a polarity dependent on the polarity of the error signal.