DESIGN OF FLOW AVERAGING LEVEL CONTROL-SY STEM USING IP CONTROLLER WITH GAP

Flow averaging level control (FALC) is employed in several chemical pr ocesses so that fluctuations in outflow from the process can be attenu ated against disturbances as well as load changes in inflow to the pro cess while the level is kept around the setpoint. In this paper, a pra ctical scheme of designing the FALC is proposed. Assuming that the pro cess has an integral element, the structure of a level control system becomes a second order system, which can be specified by the time cons tant as well as damping constant. The proposed scheme determines these two values so as to optimize two performance indexes : indexes of lev el controllability and attenuability of fluctuation in the outflow. Th e controllability index is defined by the integral of squared error of the level control and that of attenuability is defined by the integra l of squared derivative of the manipulated variable, outflow rate. The damping constant for optimizing performance is derived and it is 1/ro ot 2 (Relative Damping). The magnitudes of disturbance as well as allo wable ranges of level fluctuation in both nominal and load change oper ations are given as the design specifications. By giving specification s to the controller, an IP controller with a gap function is designed so as to maintain the optimal damping constant of the control system. The proposed scheme is applied to industrial plants to illustrate the validity of the scheme.