In terms of positioning method, the conventional SMA actuators can be simply classified into two types; on-off type, continuous type. The on-off type SMA actuator has two controlled positions, i.e., fully elongated position and fully contracted position. An open loop control is used to actuate it in a way that electrical input power turns on or off. While the on-off type actuator has a disadvantage that it has only two controlled positions, its control system is very simple by virtue of sensor-less and on/off control scheme. For the continuous type SMA actuator, the close loop control is used to control it at an arbitrary position through a position-sensor feedback. The overall control system of the continuous type actuator has disadvantages which a bulky position sensor such as a potentiometer or an optical encoder is needed and the control method is usually complex to deal with SMA's inherent hysteresis with minor loops.

To compromise between control complexity and number of controlled positions, a novel multistep SMA actuator is proposed in this paper. The multistep SMA actuator has multiple controlled positions and it can be actuated by sensor-less and open loop control. A multistep SMA actuator can be constructed in a way that several electrical taps are connected on one SMA wire and each section between two taps has same length. The controlled positions vary stepwise according to the number of actuated sections. To control a multistep SMA actuator, a PWM(pulse width modulated) voltage with an appropriate duty ratio should be applied to designated sections in consideration of resistance value of the sections as well as a recommended current of the given SMA wire.

To verify effectiveness of the proposed concept of the multistep SMA actuator, experiments was performed using an actuator mechanism having one nitinol wire and a bias spring and an electronic control system with a micro-controller(TMS320F2812) and eight power transistors(D882-Y).

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