2008

Miniature Latching Valve

This valve remains either open or closed when power is not supplied.

A miniature latching valve has been invented to satisfy a need for an electrically controllable on/off pneumatic valve that is lightweight and compact and remains in the most recently commanded open or closed state when power is not supplied. The valve (see figure) includes a poppet that is moved into or out of contact with a seat to effect closure or opening, respectively, of the flow path. Motion of the poppet is initiated by electrical heating of one of two opposing pairs of nickel/titanium shape-memory alloy (SMA) wires above their transition temperature: heated wires contract to their “remembered” length, applying tension to pull the poppet toward or away from the seat. A latch consisting mainly of a bistable Belleville washer (a conical spring) made of a hardened stainless steel operates between two stable positions corresponding to the fully closed or fully open state, holding the poppet in one of these positions when power is not applied to either pair of SMA wires.

The reason for using SMA wires is that in comparison with other linear actuators of the same mass and size, SMA wires produce more work output. The light weight and compactness of the SMA-wire actuators and the Belleville-washer latch make it possible for this valve to be smaller and less massive than are prior valves of comparable performance.

To obtain maximum actuation force and displacement, the SMA wires must be kept in tension. The mounting fixtures at the ends of the wires must support large tensile stresses without creating stress concentrations that would limit the fatigue lives of the wires. An earlier design provided for each wire to be crimped in a conical opening with a conical steel ferrule that was swaged into the opening to produce a large, uniformly distributed holding force. In a subsequent design, the conical ferrule was replaced with a larger crimped cylindrical ferrule depicted in the figure.

A major problem in designing the valve was to protect the SMA wires from a bakeout temperature of 300 °C. The problem was solved by incorporating the SMA wires into an actuator module that is inserted into a barrel of the valve body and is held in place by miniature clip rings.

This work was done by A. David Johnson of TiNi Alloy Co. and Glendon M. Benson of Aker Industries for Goddard Space Flight Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

TiNi Alloy Company
1619 Neptune Drive
San Leandro, CA 97577

Refer to GSC-14881-1, volume and number of this NASA Tech Briefs issue, and the page number.