An Internal Model Control Approach to Mid-Ranging Control

Sandira Gayadeen and William Heath
The University of Manchester


Abstract

Mid-ranging control typically refers to the class of control problems where two control inputs are manipulated to control one output with the condition that one input should return to a setpoint. Furthermore the inputs usually differ in their dynamic effect on the output. Existing mid-ranging control strategies such as Valve Position Control (VPC) are sub-optimal and can be improved. Many mid-ranging designs have been proposed, of them Modified VPC (MVPC) and Direct Synthesis are most appropriate. In this paper a novel strategy for mid-ranging control based on Internal Model Control (IMC) principles is presented. This IMC design reformulates the mid-ranging control specifications in terms of classical bandwidth and sensitivity requirements. This design focuses on disturbance responses rather than shaping step responses as with MVPC and Direct Synthesis. The performance of IMC and Direct Synthesis for setpoint changes and unmeasured disturbances are compared through simulation studies. MVPC allows the use of a simple conventional VPC structure and PID controls but both IMC and Direct Synthesis give better performance than MVPC by using a more general structure and is not restricted to PID controls. The mid-ranging design in this paper exploits both the structure and tuning methodology of IMC which makes the design trade-offs transparent and in some cases achieves better performance than Direct Synthesis. Additionally the IMC design provides a natural structure for anti-windup. Simulation studies are done to demonstrate the performance of the classical IMC structure and a modified IMC structure when the inputs are saturated. Other mid-ranging control schemes such as MVPC require additional control blocks foacceptable performance during saturating whereas IMC is already a natural structure for anti-windup.