An experimental investigation of the feasibility of "self- sensing" shape memory alloy based actuators

Shape memory alloys (SMAs) change their crystallographic structure and shape during heating/cooling and, as a consequence, their electrical resistance also changes. This allows the determination of the location of a SMA-based structure in space without separate sensors by suitably measuring this change. In this paper, this "self-sensing" concept is explored in SMA wire-type actuators. Step responses, expressed in terms of resistance (voltage drop) across the wire, and the corresponding displacement changes during heating/cooling, were measured. It was shown that the relationship between the displacement and the voltage drop can be approximated by a linear regression with a correlation coefficient close to 1. System identification has shown that SMA wire actuator performance can be best approximated by first or by second order system response depending on the thermal insulation condition of the actuator. The resolution and the sensitivity of the self-sensing method were evaluated based on experimental data and it was shown that their minimal values were less than 1.7 μm and 0.7 μm, respectively, thus supporting the feasibility of the "self-sensing concept." Both values exponentially increase with the increase in the range of the measured displacements whose magnitudes vary under different working conditions.