New phase diagram for low hysteresis response of a coiled shape memory alloy actuator

Abstract

We demonstrate a new phase diagram describing the responses of shape memory alloy (SMA) spring actuators. The SMA spring actuator has a wide range of temperatures for phase transitions and responses with low hysteresis. To describe these specific responses of the SMA spring actuator, a new phase diagram with an overlapping layer is proposed. In the overlapping layer, even if it changes from a heating phase to a cooling phase or vice versa, a phase transition occurs whenever the temperature of the SMA changes. Because there is no region in which only the temperature changes without a force changes, the hysteresis is reduced. The overlapping layer is formed by changing the properties of the SMA when fabricating the spring actuator. One of the properties that change in the SMA is the amount of latent heat when the phase transition occurs. The latent heat of the SMA spring is significantly low compared to that of the SMA wire due to the plasticity produced by the stress applied to form the spring shape. Therefore, there will be a tradeoff between the amount of the phase transition and the hysteresis of the SMA actuator.