Gain Scheduled Centralized PI Control Design for a Boiler-Turbine Unit

This paper considers the problem of designing centralized or multivariable proportional-integral (PI) controllers for a benchmark nonlinear model of boiler-turbine unit. To design the controller, first the PI design for the linearized plant is converted into a complete state feedback design problem for an augmented plant and then the state feedback gains are computed using the well-known optimal linear-quadratic-regulator (LQR) method. It is shown that when one-degree-of-freedom (1-DOF) PI controller is used, because of the proportional kick in presence of step reference input, it is extremely difficult to satisfy the rate constraint of the actuator. To solve the matter, a 2-DOF or I-P configuration of the controller structure is employed. Finally, as these controllers work well in the neighbourhood of an operating point only, a gain-scheduled 2-DOF PI controller is designed so that the controller works well for a wide range of operating points of the nonlinear model, even in presence of actuator constraints. Extensive simulation of the closed-loop nonlinear system is carried out to show the efficacy of the proposed controller.