Design of a high-speed-force-stroke thermomechanical micro-actuator via geometric contouring and mechanical frequency multiplication

The aims of this research were to understand (1) why marked performance improvements are observed when one contours the geometry of micro–thermomechanical actuators (μTMAs), (2) how to parametrically model and optimize these improvements, (3) how to use transient electrical command signals to augment these improvements, and (4) how to design arrayed pairs of actuator teams that enable the realization of these improvements within small–scale precision machines. This work has extended the performance envelope of small–scale electromechanical systems to cover the needs of emerging positioning applications that were previously impractical. The results are important to, for example, small–scale machines that are increasingly needed within biological imaging equipment, equipment for nanomanufacturing, and instruments for nano–scale research. These positioning systems must be of small geometric scale in order to achieve viable bandwidth (kHz), resolution (nanometers), cost ($10s/device) and stability (Å/min) levels. Miniaturized machines require small–scale actuators, but unfortunately, state–of–the–art actuators are not capable of simultaneously satisfying the force (~10mN), stroke (~100μm) and bandwidth (~1kHz) requirements of the preceding applications. In the absence of a practical actuation technology, many small–scale devices were relegated to “demo” status, and they never realized the full promise that small–scale machines could deliver for the preceding applications. This work has generated two concepts – geometric contouring and mechanical frequency multiplication – that make μTMAs behave in a manner that is very different from how they have acted in the past: (1) Geometric contouring: The variation of a beam’s cross–sectional area along its length to achieve more favorable thermal characteristics, i.e. temperature profile, while simultaneously reducing the elastic energy storage within the beam, and (2) Mechanical frequency multiplication: The use of μTMAs pairs that cooperate to reduce their combined cycle time below their individual cycle times, thereby increasing their operating frequency. The utility and practical implementation of these techniques were illustrated via a case study on a three– axis optical scanner for a two–photon endomicroscope. The device consisted of three sub–systems: (i) an optical system (prism, graded index lens, and optical fiber) that was used to deliver/collect photons during imaging, (ii) a small–scale electromechanical scanner that could raster scan the focal point of the optics through a specimen and (iii) a silicon optical bench that connects the electromechanical and optical systems. The scanner was required to fit within a 7mm Ø endoscope port and scan at 1kHz throughout a 100x100x100 μm volume. The results of this thesis were used to engineer a scanner that was capable of 3.5kHz x 100Hz x 30Hz scanning throughout a 125 x 200 x 200 μm volume. Preceding μTMA technology could only scan over 12.5% of the required volume at 10% of the required frequency. This work forms a body of knowledge – design rules, principles and best practices – that may be used to realize similar benefits in other small–scale devices. Thesis Supervisor: Martin L. Culpepper Title: Associate Professor of Mechanical Engineering