Smart force control system for real time loading of primary flight control actuators

Primary flight control actuators of fixed and rotary wing aircraft need to be tested while they are subjected to loads representative of those that will be encounterd in service. Loading actuators are thus used, that develop a controlled force on the flight control actuators under test, while these actuators are commanded to move replicating typical flight profiles. A critical issue with these force control systems is the ability to maintain a good accuracy for the load while the flight control actuator is moving with high speed and acceleration. Speed and acceleration act as a disturbance for the force control system and large errors can easily arise during fast movements of the flight control actuator. This paper describes an innovative force control system architecture able to ensure a good accuracy of the controlled force for all operating conditions. This control technique uses the speed and acceleration signals provided by relevant sensors placed on the force control actuator to generate appropriate corrections to the commands to the servovalve controlling the actuator flow and pressure. The force control law includes a predictor algorithm that uses a moving time history of speed and acceleration to estimate the values of these quantities at a time ahead of an amount equal to the servovalve time delay, thereby allowing an actual real-time compensation of the disturbances acting on the force control system.