Polynomial solution of the 31/2-DOF H2/H∞ feedforward control problem

The use of separate cost terms that enable the amount of feedforward control, relative to feedback action, to be tuned is essential in some applications. The solution of a new 31/2-degree-of-freedom H_∞/H₂ stochastic optimal control problem is discussed which may be either H₂ or H_∞ optimal. The system includes a 21/2-degree-of-freedom tracking controller and a feedforward control action. The feedback controller must be robust and is therefore normally designed to minimise an H_∞ criterion. However, the solution obtained is sufficiently general that the feedback, tracking or feedforward control functions can be chosen to minimise either an H₂ or an H_∞ criterion