H∞ and H2 Controllers

In the LQG controller design we assumed that the control inputs are collocated with disturbances, and that the control output was collocated with the performance. This assumption imposes significant limits on the LQG controller possibilities and applications. The locations of control inputs do not always coincide with the disturbance locations, and the locations of controlled output are not necessarily collocated with the location where the system performance is evaluated. The H2 and H∞ controllers address the controller design problem in its general configuration of noncollocated disturbance and control inputs, and noncollocated performance and control outputs. Many books and papers have been published addressing different aspects of H∞ controller design, and [24], [81], [73], [78], [12], [77], [95], and [102] explain the basic issues of the method. The H∞ method addresses wide range of the control problems, combining the frequency and time-domain approaches. The design is an optimal one in the sense of minimization of the H∞ norm of the closed-loop transfer function. The H∞ model includes colored measurement and process noise. It also addresses the issues of robustness due to model uncertainties, and is applicable to the singleinput-single—output systems as well as to the multiple-input—multiple-output systems.