Implicit and explicit model-following helicopter flight controllers

This paper describes two approaches for design of a flight control system for a high performance helicopter. Both methods use H, and µ techniques to provide inner-loop control laws which decouple the roll, pitch, and yaw rates and vertical velocity, and provide desired bandwidth in those channels. The first design is an implicit model following approach in which the weighted H, norm between the actual system outputs and outputs of desired handling quality models is minimized. p-synthesis is used to improve the robustness of the H, design. The second design is an H_∞ explicit model following approach in which the controller shapes the open loop response and provides direct feedback of the error to the controller. As in the implicit model follower, p-synthesis is used to improve the robustness of the H, design. Performance of all the controllers is evaluated by nonlinear simulations. In both the implicit and explicit model following designs, the H, controllers provide excellent performance when the control inputs are small and the system is essentially linear. Performance is degraded for large inputs. The µ controllers provide improved response for large inputs in both designs. The explicit model following design provides better tracking than the implicit model following design.