Efforts are underway to implement an improved design of the device described in "Normally Closed, Piezoelectrically Actuated Microvalve" (NPO-20782), NASA Tech Briefs, Vol. 25, No. 1 (January 2001), page 39. To recapitulate: This valve is being developed as a prototype of valves in microfluidic systems and other microelectromechanical systems (MEMS). The version of the valve reported in the cited previous article (see Figure 1) included a base (which contained a seat, an inlet, and an outlet), a diaphragm, and an actuator. With the exception of the actuator, the parts were micromachined from silicon. The actuator consisted of a stack of piezoelectric disks in a rigid housing. To make the diaphragm apply a large sealing force on the inlet and outlet, the piezoelectric stack was compressed into a slightly contracted condition during assembly of the valve. Application of a voltage across the stack caused the stack to contract into an even more compressed condition, lifting the diaphragm away from the seat, thereby creating a narrow channel between the inlet and outlet.
The improvements are being pursued because of the following deficiencies of the previous version of the valve:
- The valve-seat design was marginal in that dirt particles easily became stuck between the diaphragm and the tops of sealing rings, contributing to leakage.
- By virtue of the placement of the inlet orifice under the actuator, the inlet flow and pressure opposed the sealing force, thereby reducing the ability to seal against high pressure with low leakage.
- The piezoelectric actuator stack could not be machined as precisely as could the silicon parts. As a consequence, if the valve cap (the item designated the actuator housing in Figure 1) was flexible and the piezoelectric stack was thicker than the actuator housing, then the valve could not be actively opened. If the piezoelectric stack was thinner than the actuator housing, then the valve would always be open.
Figure 2 depicts some aspects of the improved version of the valve. The inlet is repositioned from the previous version, such that now the inlet flow and pressure contribute to sealing and thus to the desired normally-closed mode of operation. The piezoelectric actuator stack and the cap have been redesigned to conform to this pressure-aided-sealing design. The valve seat has been redesigned to replace the former blunt-cross- section sealing rings with knife-edge sealing rings that would be less susceptible to trapping of particles between the rings and the diaphragm. The micromachined parts of the improved design are assembled by roomtemperature indium hermetic bonding.
This work was done by Eui-Hyeok Yang and David Bame of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at ">www.nasatech.com/tsp under the Mechanics category. NPO-30158
This Brief includes a Technical Support Package (TSP).
Improved Piezoelectrically Actuated Microvalve
(reference NPO-30158) is currently available for download from the TSP library.
Don't have an account? Sign up here.