A model that accurately describes the transient and steady-state response of thermal microactuators is desirable to provide guidance for design and operation. However, modeling the full response of thermal actuators is challenging due to the temperature-dependent material properties and nonlinear deformations that must be included to obtain accurate results. To meet these challenges a three-dimensional multi-physics nonlinear finite-element model was developed using commercial code. The Thermomechanical Inplane Microactuator (TIM) was chosen as a candidate application to validate the model. TIMs were fabricated using the SUMMiT V™ process and their response was measured using a high-speed camera. The TIMs were modeled and the model output was compared to the experimental data. The finite-element model predicts the steady-state response to within 0.74 percent and the transient response, as described by the time constant, to within 42 percent. The usefulness of the model was further demonstrated by its predicting that response time and energy consumption can be reduced by actuating thermal microactuators with short-duration high-voltage pulses. This behavior was verified through testing.
Feedback control has proven useful in improving reliability and performance for a variety of systems. However there has been limited success implementing feedback control on surface micromachined MEMS devices. The inherent difficulties in sensing microscale phenomena complicate the development of an economical transducer that can accurately monitor the states of a surface micromachined system. We have demonstrated a simple and effective sensing strategy that uses the piezoresistive property of the polysilicon thin film of which surface micromachined MEMS devices are fabricated. The states of the device are monitored by measuring the change in resistance of flexible members which deflect as the device moves. Measurement of the output displacement of an in-plane thermal actuator is presented as a candidate application. The thermal actuator is constructed of angled pairs of expansion legs that are connected to a center shuttle. As current flows through the legs they heat up and expand. The expansion causes the center shuttle to displace in the direction the legs are angled. The center shuttle is also connected to a pair of sensing legs. Theses legs are identical to the expansion legs except that they are angled in the opposite direction. Three other leg pairs are electrically connected to the sensing legs in a Wheatstone bridge configuration. An excitation voltage is applied to the bridge, and as the sensing legs deflect with the center shuttle displacement, the resistance change across the legs can be determined by measuring the voltage across the bridge. While there still is a noise issue to be dealt with, this setup provides adequate signal strength to implement feedback control using off-chip analog circuitry. Implementation of proportional/integral control on the system is successfully demonstrated.
College and Department
Ira A. Fulton College of Engineering and Technology; Mechanical Engineering
BYU ScholarsArchive Citation
Messenger, Robert K., "Modeling and Control of Surface Micromachined Thermal Actuators" (2004). All Theses and Dissertations. 38.
MEMS, dynamics, feedback control, thermal actuator, FEA, finite element, piezoresistivity, TIM, response time