Using the nominal operating point as a design variable for gain scheduled controllers

One approach to designing gain scheduled controllers is to decompose them into an inner loop scheduled so that the closed inner loop response is as close as possible to some nominal plant response and an unscheduled outer loop designed using the nominal plant. The choice of nominal plant/operating point is traditionally based on engineering judgment and experience. Rather than the usual approach of designing controllers for several operating points and interpolating between them, the authors have proposed a different procedure in which the parametric form of the controller is specified and parameter values are directly computed to minimize deviation from nominal over the entire operating range. This work presents an extension in which the nominal operating point is allowed to vary as an additional design variable in the optimization. This approach is applied to design of the F-18 longitudinal controller inner equalization loop. The resulting controller has significantly better consistency as compared to an existing design created using the old approach.