Design and experiment of a SMA-based continuous-stiffness-adjustment torsional elastic component for variable stiffness actuators

The variable stiffness actuator (VSA) is widely applied in robotics and human-robot interactions, generating controllable torque with sufficient compliance to ensure safety and robustness. At present, typical VSA design usually adopted pure mechanical structures that integrated an additional motor for stiffness adjustment, making it hard to meet the demand of lightweight joint actuation due to large actuator size and weight. As a new functional material, shape memory alloy (SMA) shows the prospect of lightweight actuation due to its high power-to-weight ratio. In this paper, a compact SMA-based torsional elastic component (SMA-TE) for the VSA is proposed. The SMA-TE with torsional stiffness is implemented by the confrontational arrangement of paired spiral SMA springs. The ceramic heating plate and semiconductor refrigeration plate are installed in the elastic component shell, and the thermally conductive medium is filled between the internal gap. The torsional stiffness of the SMA-TE can be continuously adjusted via a feed-forward cascade controller. Repeated experimental results show that the SMA-TE can precisely maintain constant stiffness and continuously adjust its stiffness between 2.6 Nm rad(-1) and 6.8 Nm rad(-1) when performing precise square wave stiffness tracking under variable frequencies.