Generating propulsion with small-scale devices is a major challenge due to both the domination of viscous forces at low Reynolds numbers as well as the small relative stroke length of traditional actuators. Ferromagnetic shape memory materials are good candidates for such devices as they exhibit a unique combination of large strains and fast responses, and can be remotely activated by magnetic fields. This paper presents the design, analysis, and realization of a novel NiMnGa shear actuation method, which is especially suitable for small-scale fluid propulsion. A fluid mechanics analysis shows that the two key parameters for powerful propulsion are the engineering shear strain and twin boundary velocity. Using high-speed photography, we directly measure both parameters under an alternating magnetic field. Reynolds numbers in the inertial flow regime (>700) are evaluated. Measurements of the transient thrust show values up to 40 mN, significantly higher than biological equivalents. This work paves the way for new remotely activated and controlled propulsion for untethered micro-scale robots.
Micro electro mechanical systems (MEMS) frequently require displacement measurements with high accuracy, a high sampling rate, and a long sensing travel. However, the available methods for measuring displacements of micro devices are typically limited in terms of at least one of these figures of merit. In this paper, we present a novel implementation of an optical encoding method for measuring displacements of micro devices that provides good capabilities in all of these figures of merit. The optical encoding system combines a commercial reading apparatus with a custom-made metal grating that can be easily produced during MEMS fabrication. Experimental tests demonstrate the ability of the system to measure displacements with a resolution of 25 nm and sampling rate of 1 MHz, under a variety of displacement rate functions.
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