Robust force control in a hydraulic workbench for flight actuators

The work deals with the design of the force control in a hydraulic workbench for primary flight actuators, to be used for hardware-in-the-loop simulations of a modern Fly-By-Wire Flight Control System. For this application, a high-bandwidth force response is needed in order to simulate aerodynamic loads on the control surfaces, but plant uncertainties can imply significant limitations. The variation of structural stiffness, due to hinge play and hinge local deformation, the uncertainties related to the flight actuator stiffness and the ones related to the hydraulic plant parameters lead to the necessity of a robust approach to the design of the force control. In the paper, a nominal LTI model of the plant is developed and the closed-loop force control is designed by means of a loop-shaping approach for different values of the bandwidth. The stability of the closed-loop force-controlled system is then verified by a robustness analysis, assuming the structural stiffness, the flight actuator stiffness, and some of the hydraulic plant characteristics as uncertain parameters. The bandwidth of the force control is determined by finding an optimal compromise between dynamic performance and stability margin. Finally, in order to overcome the additional problems related to the flight actuator movements due to aircraft manoeuvres, a compensation feedback based on the flight actuator acceleration is proposed.