A force equalization controller for active/active redundant actuation system involving servo-hydraulic and electro-mechanical technologies

Abstract

The force equalization of a hybrid actuation system combining one servo-hydraulic actuator and one electro-mechanical actuator operated in position control and in active/active mode is addressed for safety critical applications such as primary flight controls. In a first step, an accurate virtual test bench is built to facilitate the analysis of force fighting and the assessment of the performance and robustness of the proposed force equalization strategies. It is validated from real experiments performed for the aileron actuator of a single-aisle commercial aircraft. Static force equalization is achieved first by adding equalization offsets in the position control loops as a function of the integral of the force difference between actuators. In order to keep a high level of segregation, the authority for this action is limited to 4% of the total actuator stroke. The dynamic force equalization is performed by forcing the two actuators to follow the same path. Thus, a trajectory generator is introduced to output the required position, velocity and acceleration from the position set point with realistic reproduction of the actuator power limits. Feedforward actions are used to compensate the major and invariant effects such as servo-hydraulic actuators functional flow and electro-mechanical actuator inertial torque. In this way, the pursuit errors are significantly reduced without decreasing robustness. Then, the accurate virtual test bench is used to assess the robustness of the force equalization strategy by analyzing the sensitivity of performance indicators to parameters and operating conditions. It is shown that the proposed force equalization scheme meets all the requirements, including segregation, robustness and simplicity.