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3D finite-element simulation of a shape memory microgripper

A shape memory alloy (SMA) microgripper for manipulation of microparts is presented, which is made of a monolithic SMA device of 2 × 5.8 × 0.1 mm3 size comprising various integrated functional units like two microactuators for active closing and opening, gripping jaws and an optical slit for position sensing. Recently, the design and fabrication technology of the SMA microgripper have been developed. The present work concentrates on the mechanical and thermal performance. A 3D macromodel and its implementation in a coupled finite element (FEM) routine is introduced, which allows simulation of mechanical, electrical and thermal fields in shape memory actuators of arbitrary shape. The mechanical behavior is described by a two-phase macromodel taking into account material nonlinearity and history effects. The spatial distribution of electrical heating power is determined from simulation of the electrical potential distribution. For simulation of temperature profiles a heat transfer model is used, which takes the distribution of electrical heating power and the effects of latent heat, of heat conduction and of heat convection into account. The simulation results are compared with experimentally determined characteristics. Mechanical tests reveal spring constants of the microactuators in austenitic condition of 1600 N/m, which are confirmed by the simulations. The stroke of the gripping jaws is between 250 and 300 μm depending on the maximum prestrain. The maximum gripping force is determined to 35 mN. Typical heat transfer times are about 100 ms upon heating and 150 ms upon cooling, which are in quantitative agreement with experimentally determined time constants.