Simulation of a Ferromagnetic Shape Memory Actuator in a Magnetic Field
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Simulation of a Ferromagnetic Shape Memory Actuator in a Magnetic Field B. Krevet and M. Kohl
Forschungszentrum Karlsruhe, IMT, Postfach 3640, D-76021 Karlsruhe, Germany; Phone +49 7247 822759; FAX +49 7247 824331; [email protected]
ABSTRACT In this work, we present simulations of the thermo-magneto-mechanical performance of a ferromagnetic SMA microactuator in the inhomogeneous field of a CoSm permanent magnet and compare it with experimental results. For the simulations, a combination of different finite element (FEM) programs is used allowing the coupling of electrical, thermal, magnetic and mechanical properties. The field of a permanent magnet is calculated along the trajectory of the microactuator, from which the magnetic forces Fmag and their gradients are derived. Shape memory forces FSME are calculated based on a two-phase macromodel. Both forces, Fmag and FSME, determine the quasi-static behavior of a NiMnGa microscanner, which has been proposed recently. INTRODUCTION Smart materials like ferromagnetic shape memory alloys (SMAs) allow the development of novel kinds of actuators, which perform complex operations with remarkably simple construction. Recently, ferromagnetic thin films have been developed, which have a large potential for MEMS applications. So far, first prototypes of microvalves [1] and microscanners have been developed [2]. The microscanner consists of a double-beam cantilever of a NiMnGa thin film in an inhomogeneous magnetic field. Figure 1 shows a scheme of the microscanner. Depending on the heating conditions, the cantilever is either in martensitic and ferromagnetic condition (a) or an austenitic and paramagnetic condition (b). Thus, either ferromagnetic or shape recovery forces occur, respectively, which act in opposite directions creating a perfect antagonism in a single microcomponent. Operation of a scanner prototype is shown in Figure 2. For dimensions of a few millimeters, typical operation frequencies are in the range of 20-200 Hz. Corresponding optical scanning angles of more than 120° have been demonstrated [2]. A complete simulation of the frequency behavior of the microscanner is quite complex because of the coupling of the various physical properties involved. In addition, mechanical and magnetic hysteresis effects strongly influence the results. In order to cope with such problems a simulation tool has been developed, which couples different FEM programs for calculation of the hysteretic material behavior under multi-field loading conditions. In this tool, the execution of the involved FEM programs and coupling programs for data transfer and updating material data is controlled by an overlaying program [3]. A two-phase shape memory model including history effects is implemented in the mechanical solver. This tool has already been used to calculate the behavior of conventional SMA actuators loaded by heating with electrical currents [3]. In this work, we present simulations of actuator deflections in the inhomogeneous field of a permanent magnet for diff
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