A large-stroke shape memory alloy spring actuator using double-coil configuration

One way to increase the range of motion of shape memory alloy (SMA) actuators is to create displacements of the SMA associated with not only the deformation from straining but also rigid-body motion from translation and rotation. Rigid-body motion allows the SMA to create larger displacements without exceeding the maximum recovery strain so that the SMA actuators can have a larger shape recovery ratio. To improve the linear actuation stroke of SMA wire actuators, a novel SMA spring actuator is proposed that employs a double-coil geometry that allows the displacement of the SMA to be mainly induced by rigid-body motion. A double-coil SMA spring actuator is fabricated by coiling an SMA wire twice so that the double coiling results in a reduction of the initial length of the double-coil SMA spring actuator. The effects of the geometric parameters on the actuation characteristic of a double-coil SMA spring actuator are verified numerically by finite element analysis and experimentally according to a parametric study of the geometric parameters. The displacement-to-force profile of the double-coil SMA spring actuator is nonlinear, and the spring stiffness changes when the actuator transforms its configuration from a double-coil shape to a single-coil shape. According to the results of the parametric study, increasing the wire diameter increases both primary and secondary coil stiffness, and increasing the primary inner coil diameter decreases both primary and secondary coil stiffness, whereas increasing the secondary inner coil diameter decreases only the secondary coil stiffness. The result shows that one of the double-coil SMA spring actuators with an initial length of 8 mm has a recovery ratio of 1250%, while the recovery ratio of the single-coil SMA spring actuator with the same geometric parameters is 432%.