Thermal actuators are devices that utilize constrained thermal expansion of thin members to achieve an amplified motion. Typically, the thin members are heated by passing a current through them. The thermal actuator design used in this work is the thermomechanical in-plane microactuator (TIM). The thin legs in this design are constrained from freely expanding, and are fabricated at some small initial angle such that when heated, they expand and displace the movable shuttle. This actuator design only requires that there be 2 legs per side for stable operation, however, the number of legs can be increased to increase the output force of the actuator. The leg lengths can also be modified to control the maximum output displacement.
Actuators based on thermal expansion offer advantages over electrostatically actuated devices because they can produce a much greater force output per unit area at significantly lower voltages, while also being capable of relatively large displacements. The disadvantage of thermal actuators when compared to other methods is that they require a greater amount of power in their operation. The higher energy requirements are due to the energy loss from the heated legs to the surrounding air and into the substrate. However, this disadvantage can be minimized through proper actuator design. Operating in a vacuum has been shown to reduce the power requirements of the actuators used in this study by over an order of magnitude due to the absence of a medium for heat conduction from the heated legs. It has also been found that applying a rapid current pulse to the actuators, where a higher current is passed through the actuator legs for a short time interval, increases the deflection amplitude and improves the operating efficiency when compared to a steady-state operating mode.
Multiple-leg thermomechanical in-plane actuators were used in this work because of their relatively large force and displacement output. However, they were modified slightly from the published work in an attempt to decrease the power requirements, and match the actuator output force and displacement to that required by the bistable mechanisms being actuated. Specifically, the actuator legs were grouped closer together to reduce the heat loss due to conduction in the surrounding air. In the SUMMiT process the legs were also elevated higher off the substrate to increase the thermal insulation between the legs and the substrate. Both of these changes were found to improve the actuator efficiency.
A second modification was also made to the thermomechanical in-plane microactuator to increase its maximum displacement. While increasing the length of the heated legs does increase the output displacement, there is a practical limit to how much the legs can be lengthened. As the length is increased, the critical buckling force is decreased and eventually the legs will simply buckle when heated instead of expanding and moving the shuttle. The cross-sectional area of the heated legs cannot be increased to overcome this limitation because the current requirements to heat a larger leg would be prohibitive. To increase the maximum displacement the TIM was staged with a center piece which is not heated. In this configuration, two small-displacement, high-force TIMs each push on a constrained amplifier similar in function to the TIM legs themselves. However, because the amplifier legs are not heated, they can be made wider to overcome the critical buckling limitation.