International Journal of Control, Vol.72, No.4, 315-320, 1999
A delta operator approach to discrete-time H-infinity control
To increase the performance of digitally implemented control laws, it is desirable to directly synthesize the digital control law. Hence, this paper considers the direct synthesis of discrete-time, H-infinity control laws. Previously, two approaches have been proposed to this problem. The most common method uses the bilinear transformation to convert the discrete-time problem into an equivalent continuous-time problem, allowing the controller to be synthesized with software developed for continuous-time systems. This method is often effective but requires unnecessary steps and is not applicable in all circumstances. The second approach is to synthesize the discrete-time, H-infinity control law by solving discrete-time, H-infinity Riccati equations. Due to numerical ill-conditioning, this approach can fail at sufficiently high sample rates. This numerical ill-conditioning can be eliminated by representing the discrete-time system and H-infinity controller using the delta operator. Hence, the H-infinity design equations for direct synthesis using the delta operator are developed. These equations reduce to standard continuous-time H-infinity design equations when the sample period is set to zero. The results are illustrated with a numerical example which also shows that the central continuous-time H-infinity controller of the continuous-time system obtained by a bilinear transformation of the original discrete-time system does not necessarily correspond to the central discrete-time controller.