A Generalized Control Model and Its Digital Algorithm for Aerospace Electrohydraulic Actuators

It is difficult to describe precisely and thus control satisfactorily the dynamics of an electrohydraulic actuator to drive a high thrust liquid launcher engine, whose structural resonant frequency is usually low due to its heavy inertia and its complicated mass distribution. A generalized model is therefore put forward for maximum simplification and sufficient approximation, where a second-order transfer function is used to model the heavy mass-spring nature of the large engine body outside of the rod position loop, another second-order transfer function with two zeros and two poles representing the hydro-mechanical composite resonance effect in the closed rod position loop. A combined control strategy is applied to meet the stringent specification of static and dynamic performances, including a notch filter, a piecewise or nonlinear PID and a feed-forward compensation. The control algorithm is implemented in digital signal processors with a same software structure but different parameters for different aerospace actuators. Compared to other approaches, it is easier this way to grasp the system resonance nature, and most importantly, the traditional dynamic pressure feedback is replaced with the convenient digital algorithm, bringing prominent benefits of simplified design, reduced hardware cost and inherent higher reliability. The approach has been validated by simulation, experiments and successful flights.