OVERVIEW

Control loops are said to interact when movement of the final control element of one loop affects not only its own process variable but the process variable of one or more additional control loops as well. If the corrective action by the controllers in the other loops then affects the process variable of the initial loop, an endless cycle of interaction will occur between the loops.

Numerous instances of control loop interaction can be found in industrial process control. Suppose that both the overhead and bottom composition of a distillation column are to be controlled by manipulating the reflux and reboiler steam flow rates. An increase in reflux rate will lower the impurity of the overhead composition and at the same time will increase the amount of heavy component recovered at the bottom of the tower. An increase in reboiler steam rate will lower the impurity of the bottom composition. At the same time, by forcing more of the lighter component to the top of the tower, it will increase the recovery of the lighter component in the overhead stream. The composition control loops in this application, whether using analytical instruments or inferred (e.g., temperature) measurements, may experience severe interaction.

In the heating, ventilating and air conditioning (HVAC) industry, simultaneous control of temperature and humidity is often desired. It is especially critical in clean rooms and other highprecision environmental control applications. If a heating coil is used to control temperature and a cooling coil for dehumidification, severe interaction may...