Digital robust control of throttled variable displacement hydraulic motors in aircraft power drive units

The introduction of variable displacement hydraulic motors to secondary and primary flight actuation offers a considerable potential for power optimization and saving in aircraft hydraulic systems. The application in aircraft systems requires high safety, reliability and availability at the smallest expense possible. Therefore the combination of a variable displacement hydraulic motor with a fixed orifice guarantees fail-passive system behavior. The integration in fly-by-wire flight control architecture leads to digital control structures. Robust controller design is forced because of the influence of several uncertain physical parameters during flight mission. Moreover the significant nonlinear characteristic of the throttled VDHM affects dynamic behavior. An integral discrete-time robust controller design methodology is presented considering specified requirements as dynamic bandwidth, static position accuracy and stationary power drive performance. Here the parameter space approach is used for direct sampled-data controller synthesis. A static linear state feedback controller is chosen. All combinations of uncertain and linearized parameters leads to a multi-model problem which is solved by simultaneous stabilization. Finally simulated and experimental results show typical operation cases and verify expected system dynamics in time- and frequency-domain