Unsteady Fluid Flow in Hybrid Hydraulic Actuators

The ability of smart materials like piezoelectrics to deliver large blocking forces in a small package while operating at high frequencies makes them extremely attractive for converting electrical energy to mechanical power. This led to the development of hybrid actuators consisting of co-located smart material actuated pumps and hydraulic cylinders that are connected by a set of fast-acting valves. The overall success of the hybrid concept hinges on the effectiveness of the coupling between the smart material and the fluid. This, in turn, is strongly dependent on the resistance to fluid flow in the device. This article presents results from 3D simulations of unsteady fluid flow in the pumping chamber of a prototype hybrid actuator powered by a piezoelectric stack. The results show that the forces associated with moving the fluid into and out of the pumping chamber exceed 10% of the piezo stack blocked force at relatively low frequencies near 100 Hz and approach 80% of the blocked force at 800 Hz. This reduces the amplitude of the piston motion in such a way that the volume flow rate remains approximately constant above operating frequencies of 600 Hz. As the driving frequency is increased beyond 800 Hz, the flow rate starts to decrease. This study also presents a decomposition of the pressure loss into its Fourier components and identifies the important harmonics.