An electro-hydrostatic actuator for hybrid active-passive vibration isolation

The concept, design, modelling and testing of a novel electro-hydrostatic actuator (EHA) intended for active vibration isolation is described. The EHA consists of a brushless DC motor integrated with a bidirectional gear pump, controlling oil flow to a double-ended hydraulic cylinder. The permanent magnet rotor runs in oil, the motor and pump forming a pressure vessel pre-charged to a mean system pressure (about 80 bar). This arrangement means no pump case drain and refeeding circuit is required. The actuator is designed to cancel the force transmitted through a flexible vibration mount, with dominant vibration frequency of around 20Hz. The particular motivation is to provide isolation between a helicopter fuselage and the vibrating rotor hub, and would be integrated into the flexible strut connecting the two. The inertia of the EHA is tuned to provide a degree of passive vibration isolation at the dominant frequency, and hence is termed a Resonant EHA. Active control of the motor extends the isolation performance of the passive device by compensating for losses (which give damping), and extend the frequency of isolation away from the resonant frequency. A 7kN prototype has been built and extensively tested, demonstrating a reduction by a factor of four in the root-mean-square transmitted force and a near elimination at the fundamental frequency. The advantage of the Resonant EHA is a wider range of operating frequencies than a purely passive system, and a lower power consumption than a purely active system.