Energy-saving control strategies for a ferromagnetic shape memory alloy based actuator

Two different strategies are presented for the control of a positioning actuator based on a ferromagnetic shape memory alloy (FSMA). The actuator consists of two orthogonal pairs of coils that generate a pulsed magnetic field when a bank of capacitors is discharged through one of the coil pairs. The FSMA element is located between the coils and expands when a field perpendicular to the crystal's long axis is applied, and contracts in response to a parallel one. In each case, the deformation of the element remains until a driving force in the opposite direction is applied, therefore operating in a “set-and-forget” mode. The current experimental set-up is capable of achieving submicrometer resolutions. Two discrete control strategies are proposed and compared based on their ability to achieve a position with a given precision and transient behaviour. Taking into account that the energy consumption of the actuator is directly related to the number of control actions, two control strategies aimed to minimize such consumption by reducing the number of actions are presented and their experimental performance described. The first of the proposed strategies has two control modules, one for each pair of coils, while the second approach is based on an event-based PI control algorithm.