Variable Structure Control of a Ferromagnetic Shape Memory Alloy Actuator

Ferromagnetic Shape Memory Alloys (FSMAs) like Ni-Mn-Ga have attracted significant attention over the last few years. As actuators, these materials offer high energy density, large stroke, and high bandwidth. These properties make FSMAs potential candidates for the new generation of actuators. The preliminary dynamic characterization of Ni-Mn-Ga illustrates nonlinear behaviors including hysteresis, saturation, first cycle effect, and dead zone. In this paper, the positioning control of a FSMA actuator is investigated. To this end, a dynamic model is presented for a Ni-Mn-Ga linear actuator. The actuator model consists of the following subsystems: a field-induced strain model of Ni-Mn-Ga, an elastic deformation model of the Ni-Mn-Ga element, a dynamics/kinematics model, and an electromagnetic dynamics model. The material damping effect is also included in the model by the Kelvin-Voigt model. A Proportional-Integral-Derivative (PID) position control algorithm is developed based on the model. Simulation result indicates poor performance of the PID controller. This is mainly due to nonlinearities in the actuator behavior. To address this problem, a variable structure controller is developed. Simulations with the variable structure controller show that this algorithm can improve the performance of the actuator with respect to rising time and positioning accuracy.