Robust pressure control improves the performance of redundant fly-by-wire hydraulic actuators

Aircraft primary flight control surfaces are typically driven by two linear hydraulic actuators accepting the controlled flows from two independent hydraulic systems. When the actuators are part of a fly-by-wire system, actuators flows are controlled by valves, and a key issue is to ensure a pressure matching between the two actuators to prevent force fighting with a resulting loss of stiffness and dynamic performance. Different solutions have been used to minimize the pressure mismatch between two redundant electro hydraulic actuators, but limited use has been done of techniques based on generating compensation signals to reduce the difference between the pressure differentials of the two servo actuators. The advances in microprocessors computing power offer new possibilities that can be exploited to perform a good pressure matching between redundant actuators by using an appropriate control strategy without the need of a complex hydraulic design. This paper first presents a brief survey of the existing architectures of dual redundant electro hydraulic servo actuators, then describes a research activity performed on this subject. An optimal and robust equalization control strategy was devised, capable of minimizing the force fighting between servo actuators while using conventional electro hydraulic servo valves, thereby leading to appreciable weight and cost savings.