Double Helix Structure for Stable Flexural Actuation in Soft Robotics

Soft robotic actuators capable of bidirectional bending commonly feature axisymmetric geometries that are compliant in both flexure and torsion. While such designs offer a high degree of mechanical compliance, they can lead to inadvertent twisting and loss of mechanical stability. This work demonstrates how a double helix limb structure improves the twist‐bend ratio of a shape memory alloy (SMA) actuated soft limb. Drawbacks in previously reported SMA limb configurations include twisting upon contact with an external force, resulting in undesired loss in force transmission to the surface. The proposed novel double helix design optimized for helical density over length is compared to these past SMA soft limb structures. Experimental results show a 5× improvement in twist resistance and a 50% increase in maximum output (applied) force. These performance enhancements are further validated through demonstrations in a soft gripper and a locomoting soft robot, where the helical design uniquely enables stable and adaptive interaction with objects and obstacle negotiation.