Design and control of multi-step SMA actuator

To balance the control complexity and number of controlled positions, a novel multi-step SMA actuator is proposed that has multiple controlled positions and can be actuated by sensor-less and open-loop control. The proposed multi-step SMA actuator is constructed so that several electrical taps are connected to one SMA wire and the sections between the taps are all the same length. The controlled positions then vary stepwise according to the number of actuated sections. To control the multi-step SMA actuator, a PWM(pulse width modulated) voltage with an appropriate duty ratio is applied to designated sections while considering the resistance value of the sections, as well as the recommended current for the given SMA wire. To verify the effectiveness of the proposed multi-step SMA actuator, experiments were performed using a single nitinol wire actuator with a bias spring mechanism and electronic control system.