A novel piezohydraulic aerospace servovalve. Part I: Design and modelling

Servovalves are compact, accurate, high bandwidth modulating valves widely used in aerospace, defence, industrial and marine applications. However, manufacturing costs are high due to high part count and tight tolerances required, particularly in the first stage of the valve, and due to manual adjustments required as part of the setup process. In this research, a novel servovalve concept is investigated which has the potential to be more cost effective. In particular, for the first time a piezoelectric first stage actuator is developed to move a servovalve spool using the deflector jet principle; this is especially suited to aerospace actuation requirements.
In the new valve the conventional electromagnetic torque motor is replaced by a multi-layer bimorph piezo-actuator. The bimorph deflects a jet of fluid to create a pressure differential across the valve spool; hence the spool moves. A feedback wire is used to facilitate proportional spool position control via mechanical feedback. The bimorph is directly coupled to the feedback wire and is immersed in hydraulic fluid.
A high order nonlinear model of the valve has been developed and used to predict valve static and dynamic characteristics, and is described in this paper. This makes use of stiffness constants derived analytically for the bimorph-feedback wire assembly and cross-referenced to finite element (FE) analysis predictions. The model of the flow force acting on the deflector is an approximation of the force characteristic found from computation fluid dynamic (CFD) analysis. The measured characteristics of the prototype valve are in good agreement with the simulation results, and prove that the operational concept is viable.