Influence of Materials on the Performance Limits of Microactuators
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1052-DD07-03
Influence of Materials on the Performance Limits of Microactuators Prasanna Srinivasan, and S. Mark Spearing School of Engineering Sciences, University of Southampton, Southampton, SO171BJ, United Kingdom ABSTRACT The selection of actuators at the micro-scale requires an understanding of the performance limits of different actuation mechanisms governed by the optimal selection of materials. This paper presents the results of analyses for elastic bi-material actuators based on simple beam theory and lumped parameter thermal models. Comparisons are made among commonly employed actuation schemes (electro-thermal, piezoelectric and shape memory) at micro scales and promising candidate materials are identified. Polymeric films on Si subjected to electro-thermal heating are optimal candidates for high displacement, low frequency devices while ferroelectric thin films of Pb-based ceramics on Si/ DLC are optimal for high force, high frequency devices. The ability to achieve ~10 kHz at scales < 100µm make electro-thermal actuators competitive with piezoelectric actuators considering the low work/volume obtained in piezoelectric actuation (~ 10−8J.m−3.mV−2). Although shape memory alloy (SMA) actuators such as Ni-Ti on Si deliver larger work (~ 1 J.m−3K−2) than electro-thermal actuators at relatively low frequencies (~ 1 kHz), the critical scale associated with the cessation of the shape memory effect forms the bounding limit for the actuator design. The built-in compressive stress levels (~ 1GPa) in thin films of Si and DLC could be exploited for realizing a high performance actuator by electro-thermal buckling. INTRODUCTION The choice of materials has a strong influence on the performance of Micro-ElectroMechanical-Systems (MEMS). The use of traditional MEMS materials is predominantly driven by their compatibility with existing micro-fabrication processes [1]. As a result, these materials are not often optimal from the performance point of view. This paper presents a comparative analysis of the performance limits of commonly employed MEMS actuation schemes (electrothermal/ piezoelectric/ shape memory). The results presented herein serve as a guide for the selection of actuation principle and the relevant candidate materials for various applications. The structural response of the actuators is assumed to follow Euler-Bernoulli beam theory and the thermal response is described by a lumped heat capacity formulation. The critical performance metrics which impact the actuator design are tip slope (deflection), blocked moment (force), work per volume and actuation frequency.
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Figure 1 Schematic of a cantilever bending actuator in bi-material architecture MECHANICS OF A BI-MATERIAL ACTUATOR STRUCTURE Bi-material actuators are very common in MEMS due to the simplicity of their design and their ability to achieve out of plane actuation. The capability of present micromachining processes to grow thin films of arbitr
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