An H∞ based controller for a gas turbine clearance control system

In order to achieve high turbine efficiencies in turbojet engines, turbine clearance control systems are used to maintain a minimum clearance, without rubbing, between the turbine blade tips and the turbine shroud. These systems function by modulating the flow of air over the shroud, which either expands or contracts, depending on the temperature of the air. Typically, shroud temperature, along with other operating conditions, are used as the sensed variables for this control. In this paper an H_∞ based design method is used to develop a controller which explicitly deals with the uncertainty associated with the shroud thermal dynamics, and hysteresis associated with the turbine cooling valve. The resulting control system is shown to have good closed loop performance characteristics