Prototypes of microelectromechanical system (MEMS) rotational actuators based on a principle of torsion induced by thermal expansion of electrically heated offset beams have been designed, fabricated, and tested. It is envisioned that after further development, these actuators might be used to satisfy stringent competing requirements for smaller, larger-force, largerdisplacement actuators in increasingly complex MEMS systems. Until now, MEMS thermal actuators have been regarded as inefficient and capable of producing, variously, either large forces and small displacements or small forces and large displacements. The actuators of the present type are intended to overcome some of the deficiencies heretofore attributed to MEMS thermal actuators by producing medium displacements and medium forces.

Figure 1. Rotation of the Vertical Beam about the center of the yoke is produced when an electric current heats the two horizontal beams, causing them to expand against the yoke at offset positions.
An actuator of this type (see Figure 1) includes three beams connected to each other at a yoke. Two of the beams are parallel and offset at the yoke. The third beam is perpendicular to the other two beams. The two parallel beams are electrically conductive (the perpendicular beam may be, but need not be, electrically conductive). The outer ends of the two parallel beams are anchored at fixed positions with electrical contacts. The outer end of the perpendicular beam is not anchored and, hence, this beam is free to rotate if the yoke rotates. When the two parallel, offset beams are heated by driving an electric current through them, they expand against the yoke; this gives rise to torsion and, thus, rotation of the yoke and the third beam. In principle, the tip of the third beam could be connected to a mechanism to produce a force or displacement.

The efficiency of such an actuator and its force and displacement capabilities, and, thus, its suitability for a given application, depend on its electrothermal and thermomechanical characteristics as well as such purely geometrical and mechanical characteristics as its dimensions and its linear and torsional spring stiffnesses. A theory taking these considerations into account has been developed for use in designing such actuators and evaluating results of experiments in which their forces and displacements are measured at different levels of applied electric power.

Figure 2. An Actuator Tipped With a Vernier Scale is shown at rest and with an applied electric current causing it to push against a cantilever latch.
In preparation for initial experiments, several actuators tipped with vernier scales for measuring their tip displacements and accompanied with cantilever latches for measuring their tip forces were fabricated on silicon-on-insulator wafers by use of standard microfabrication processes, including photolithography and etching. Figure 2 shows one such device both as undeflected (zero applied current) and as deflected by application of a current. As an example of an experimental result, in one test, a displacement of 12.7 μm at a force of 637 mN was produced at an applied power of 250 mW. This and other results have been compared with theoretical predictions and used for comparisons with other actuators and to provide a basis for optimal design of future actuators of this type of this type for specific applications.

This work was done by Danny Gee and Luke Currano of the Army Research Laboratory.

This Brief includes a Technical Support Package (TSP).
MEMS Offset-Beam Torsional Electrothermal Actuators

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This article first appeared in the June, 2008 issue of Defense Tech Briefs Magazine.

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