Application of computed phase transformation power to control shape memory alloy actuators

This paper presents a control law for shape memory alloy (SMA) wire actuators. Unlike most laws reported in the literature, this control law includes both a feedback and an open-loop part. The open-loop part takes into account the stress - strain - temperature behaviour of SMA wires, as well as the dynamics of the heating and cooling process. A constitutive model for a NiTi wire is developed. This model is a compromise between accurate description of the real behaviour, and simplicity, since it must be used on-line for the determination of the open-loop part of the control law. The method is tested both in simulations and experiments on a system with one degree of freedom. The tracking performance of the controlled system was investigated for a number of trajectory-following tasks. The tracking error obtained with the proposed control method is considerably smaller than with feedback control only.