Polynomial solution of the 31/2-DOF H-2/H-infinity feedforward control problem

Feedforward controllers are more sensitive to modelling errors than feedbac k designs and an H-infinity solution of the feedforward problem, where robu stness can be improved, is therefore desirable. A novel optimisation proble m is required to provide the necessary independent tuning flexibility. The optimisation problem is based on an extended cost function which allows sep arate costing of the terms due to the feedback, reference and feedforward c ontrollers. This enables the amount of feedforward action to be varied rela tive to the feedback action and the robustness of the feedback loop to be s haped independently of the tracking performance. The use of separate cost t erms that enable the amount of feedforward control, relative to feedback ac tion, to be tuned is essential in some applications. The solution of a new 31/2-degree-of-freedom H-infinity/H-2 stochastic optimal control problem is discussed which may be either H-2 or H-infinity optimal. The system includ es a 21/2-degree-of-freedom tracking controller and a feedforward control a ction. The feedback controller must be robust and is therefore normally des igned to minimise an H-infinity criterion. However, the solution obtained i s sufficiently general that the feedback, tracking or feedforward control f unctions can be chosen to minimise either an H-2 or an H-infinity criterion .