Ratio control: Theoretical basis and practical implementation

Despite its widespread use in the process industry, a theoretical basis for ratio control has been lacking. It is sometimes described as a special case of feedforward control, but this interpretation is misleading. Feedforward control requires an explicit process model, whereas ratio control does not. Instead, ratio control relies on physical insight: For systems that satisfy the scaling assumption, maintaining constant ratios between extensive variables when there are throughput changes, results in constant intensive variables. Moreover, the ratio setpoint can be generated by an outer feedback loop, again without requiring a process model. The paper further discusses practical aspects of ratio control implementation, including more advanced schemes, such as dual ratio control for handling actuator saturation and cross-limiting control. • Ratio control provides feedforward action for throughput disturbances without an explicit process model • First paper to discuss its theoretical basis. • First paper to give a comprehensive overview of its practical use and implementation.