Design, modeling, fabrication and testing of a piezoelectric microvalve for high pressure, high frequency hydraulic applications

Abstract

A piezoelectrically-driven hydraulic amplification microvalve for use in high specific power hydraulic pumping applications was designed, fabricated, and experimentally characterized. High frequency, high force actuation capabilities were enabled through the incorporation of one or more bulk piezoelectric material elements beneath a micromachined annular tethered-piston structure. An hydraulic amplification mechanism was employed to amplify the limited stroke of this piezoelectrically-driven piston structure to a significantly larger motion (40-50x) of a micromachined valve membrane with attached valve cap. This valve cap was actuated through its stroke to open and close against a fluid orifice. These design features enabled the valve device to simultaneously meet a set of high frequency (1-10kHz), high pressure(0.1-IMPa), and large stroke (15-40,um) requirements that had not previously been satisfied by other microvalves presented in the literature. This research was carried out through a series of modeling, design, fabrication, assembly, and experimental testing tasks. Linear and non-linear modeling tools characterizing the structural deformations of the active valve sub-systems were developed. These tools enabled accurate prediction of real-time stresses along the micromachined valve membrane structure during deflection into its non-linear large-deflection regime. A systematic design procedure was developed to generate an active valve geometry to satisfy membrane stress limitations and valve power consumption requirements set forth by external hydraulic system performance goals.

(cont.) Fabrication challenges, such as deep-reactive ion etching (DRIE) of the drive element and valve membrane structures, wafer-level silicon-to-silicon fusion bonding and silicon-to-glass anodic bonding operations, preparation and integration of piezoelectric material elements within the micromachined tethered piston structure, die-level assembly and bonding of silicon and glass dies, and filling of degassed fluid within the hydraulic amplification chamber were overcome. The active valve structural behavior and flow regulation capabilities were evaluated over a range of applied piezoelectric voltages, actuation frequencies, and differential pressures across the valve. For applied piezoelectric voltages up to 500Vpp at lkHz, the valve devices demonstrated amplification ratios of drive element deflection to valve cap deflection of 40-50x. These amplification ratios correlated within 5 - 10% of the model expectations. Flow regulation experiments proved that a peak average flow rate through the device of 0.21mL/s under a lkHz sinusoidal drive voltage of 500Vpp, with valve opening of 17pm, against a differential pressure of 260kPa could be obtained. Tests revealed that fluid-structural interactions between the valve cap and membrane components and flow instabilities (due to transition between the laminar and turbulent flow regimes through the valve orifice) limited the valve performance capabilities.