A magnetic shape memory microactuator with intrinsic position sensing

This paper presents the design, fabrication and characterization of a linear actuator based on the magnetic shape memory (MSM) effect with intrinsic position sensing using the displacement-dependent change of electrical resistance. Active materials are Ni-Mn-Ga bulk single crystals with dimensions of 2.5 × 20 × 1 mm3 showing 10 M martensite with a dense parallel twin structure at room temperature. This structure gives rise to gradual strain characteristics when applying external stress or a magnetic field. In order to exploit this behavior for positioning applications, the correlation between magneto strain and electrical resistance is investigated as a function of external stress using either a compressive or tensile reset spring. Depending on spring constant, pre-stress as well as the modes of loading and sample fixation, reversible magneto strains up to 3.6% are achieved. The electrical resistance change shows linear correlation with magneto strain within 90% of actuation stroke allowing for position sensing with sensitivities of 2.1 and 2.7 μm/μΩ under compressive and tensile loading, respectively. This performance is simulated using a thermodynamics-based Gibbs free energy model taking into account a linear strain-resistance relationship. Near the end positions at large magnetic field, the magneto resistance effect gives rise to a nonlinear contribution to the magneto strain-resistance characteristics.