Concepts for position and load control for hybrid actuation in primary flight controls

Moving towards the More Electric Aircraft, a hybrid actuator configuration, in which an electromechanical actuator (EMA) and an electrohydraulic servoactuator (EHSA) operate on the same control surface, provides an opportunity to introduce electromechanical actuators into primary flight controls. Besides the operation in active/passive or active/active mode, an “active/no-load” mode is promising. In this mode the EMA is controlled such that it actively follows the movement of the control surface without carrying external air loads, thereby reducing power dissipation compared to active/active mode and failure transients compared to active/passive mode. However, force fighting will occur if both actuators are actively controlled. In this paper, control concepts for a hybrid configuration, extending the original actuator control loops, are presented, enabling active/active as well as active/no-load operation. Nonlinear as well as linear models for an EMA, an EHSA, and a control surface structure are derived from technical data for an airworthy EHSA and combined to a model of the hybrid configuration. These models are used for matching of actuator dynamics and simulation of the developed control laws. For active/active mode, maximum force fighting between the actuators is reduced from about 500% to 7% of the stall load. For active/no-load mode, a force control loop is added to the EMA control, causing the EMA to follow movements of the control surface such that the external loads on the EMA are zero in steady-state. Force fighting is reduced to 30% of the stall load.