Design and Performance of a Microengine Realized with Arrays of Asymmetrical Electrothermal Polysilicon Surface Micromac
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Design and Performance of a Microengine Realized with Arrays of Asymmetrical Electrothermal Polysilicon Surface Micromachined Microactuators Edward S. Kolesar, Matthew D. Ruff, William E. Odom, Joseph A. Jayachandran, Justin B. McAllister, Simon Y. Ko, Jeffery T. Howard, Peter B. Allen, Josh M. Wilken, Noah C. Boydston, Jorge E. Bosch and Richard J. Wilks Texas Christian University, Department of Engineering TCU Box 298640, 2800 South University Drive Fort Worth, TX 76129 U.S.A. ABSTRACT This research focuses on the design and experimental characterization of two types of MEMS asymmetrical electrothermal microactuators. Both microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. Deflection and force measurements of both microactuators as a function of applied electrical power are presented. Also described is the practical integration of the electrothermal microactuators in a monolithic microengine that is capable of rotating a set of gears. INTRODUCTION The seamless integration of conventional microelectronics with three-dimensional, microdynamic, mechanical components can readily be accomplished using microelectromechanical systems (MEMS) technology. Numerous electrically-driven microactuators have been investigated for positioning individual elements in microelectromechanical systems (MEMS). The most common modes of actuation are electrostatic, magnetostatic, piezoelectric and thermal expansion [1]. Unfortunately, the forces produced by electrostatic and magnetostatic actuators tend to be small, and to achieve large displacements, it is necessary to either apply a large voltage or operate the devices in a resonant mode. On the other hand, piezoelectric and thermal expansion actuators can be configured to produce large forces and large displacements. Unfortunately, piezoelectric materials are not routinely supported in the fabrication processes offered by commercial MEMS foundries. As a result, these limitations have focused attention on thermally-actuated devices for generating large forces and displacements [2]. This research focuses on improving the design and experimental performance of the MEMS electrothermal microactuator [3-8]. As depicted in Figure 1, the conventional MEMS polysilicon electrothermal microactuator uses resistive (Joule) heating to generate thermal expansion and movement [8]. When current is passed through the actuator from anchor-to-anchor, the larger current density in the narrower “hot” arm causes it to heat and expand along its length more than the “cold” arm. Since both arms are joined at their free (released) ends, the difference in length of the two arms causes the microactuator tip to move in an arc-like pattern about the flexure element incorporated at the anchor end of the “cold” arm. Removing the current from the device allows it to return to its equilibrium state. The design of the flexure used in an electrothermal microactuator is an important functional element [7]. Ideally, the flexure element should be as narrow as possi
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