Feedback design for robust tracking and robust stiffness in flight controlactuators using a modified QFT technique

The problem of dynamic stiffness of hydraulic servomechanisms has often bee n recognized as a significant performance issue in a variety of application s, the most notable of which includes flight control actuation. When a hydr aulic actuator such as this is operated in position control, an aerodynamic flutter load on the control surface manifests itself as a force disturbanc e on the system. Although this would appear to be a standard disturbance re jection problem, the disturbance does not enter the system as in the classi cal sense (i.e, at the plant output) and hence, this problem must be consid ered in a modified formulation. A hydraulic servomechanism is said to be 's tiff' if it exhibits acceptable rejection of force disturbances within the control bandwidth. In this paper, an approach to feedback design for robust tracking and robust disturbance rejection is developed via the quantitativ e feedback theory (QFT) technique. As a result, it is shown that reasonable tracking and disturbance rejection specifications can be met by means of a fixed (i.e. non-adaptive), single loop controller. The methodology employe d in this development is the sensitivity-based QFT formulation. As a result , robust tracking and robust disturbance rejection specifications are mappe d into equivalent bounds on the (parametrically uncertain) sensitivity func tion; hence, the frequency ranges in which tracking or disturbance rejectio n specifications dominate become immediately obvious. In this paper, a real istic non-linear differential equation model of the hydraulic servomechanis m is developed, the linear parametric frequency response properties of the open loop system are analysed, and the aforementioned QFT design procedure is carried out. Analysis of the closed loop system characteristics shows th at the tracking and disturbance rejection specifications are indeed met.