The limitations of electrical resistance for accurate positioning of shape-memory actuators: The case of well-oriented CuZnAl single-crystals under uniaxial loading

Abstract

The possibility of using the electrical resistance as a feedback signal for precise positioning in the design of compact shape-memory actuators was revised. The analysis was performed on well-oriented CuZnAl single crystalline shape-memory alloys subjected to uniaxial tensile loadings, combining direct strain measurements determined with an extensometer and the four-lead electrical resistance technique. In that way the relationship between strain and electrical resistance could be evaluated by formulating a physics based model that can be easily contrasted with the experimental characterization of a simple benchmark system. It was found that the model here developed provides general guidelines for the selection of a shape-memory material for positioning actuators and that the correlation between strain and electrical resistance is of rather limited precision, even for the simplest material case and loading condition analyzed. The effects of temperature, deformation rate, temperature dependence of the electrical resistivity of austenite and martensite phases and temperature dependence of the strain associated with the stress induced transformation were carefully characterized. Results suggest that the direct use of feedback from electrical resistance measurements for precise positioning is arguable unless materials with low hysteresis and low latent heat of transformation are used in combination with quasi-static operating conditions.