S3Hand: Design of a Variable Stiffness Soft Hand With Self-Locking Mechanism

Compared with rigid robotic hands, soft hands demonstrate better grasping ability and interaction safety, but their low stiffness and weak output force limit their application. In this article, we propose a variable stiffness soft finger design with self-locking mechanism (S3Hand). Variable stiffness is achieved via a shape memory alloy (SMA) spring as a switch, unidirectionally locking the ratchet in self-locking mechanism. This design enhances stiffness of soft finger at multiple deformation positions while preserving dexterity in high-stiffness mode. To enable the soft finger and the self-locking mechanism to complement each other, an adaptive control strategy is proposed, ensuring both flexibility for precise tasks and rigidity for handling heavier loads. The designed S3Hand offers significant stiffness enhancement while maintaining the soft finger's adaptive deformation capability. Each S3Hand finger weighs only 26.8 g and can support a load of 1 kg, yielding an load ratio of 37. The proposed current-driven strategy based on model prediction and SMA switch overcome challenges of low output force, slow response, and temperature control, achieving a response time of 1.5 s. S3Hand integrates the advantages of both flexible and rigid grippers, making it well-suited for grasping fragile, complex-shaped, and heavy objects, with potential applications from industry to prosthetics.