Figure 7.

Schematic of the motion process of torsion actuator when ${\theta }^{A_2}\le {\beta }_{cr}$ (a) and ${\theta }^{A_2}>{\beta }_{cr}$ (b). Ideal change is the torque of SMA torsion spring when it is heated and there is no friction torque, and its upper (lower) solid lines denote the torque of SMA torsion spring when τ^f is equals to τ^c and −τ^c, respectively. The motion process follows the sequence 1→2 → 3→4 →1.