Novel underactuated grippers with torque-driven bistable mechanism for alternating grasping and unlimited twisting modes

This paper presents the design and prototyping of a novel class of underactuated robotic grippers capable of alternating between stable grasping and continuous bidirectional twisting, named Mode-Retentive Underactuated Mechanisms (MRUM). The core of the design integrates a two-degree-of-freedom mechanical transmission with a bistable elastic element, enabling automatic transition between grasping and twisting modes. The bistable element transitions between stable states are triggered through torque limiters, allowing the gripper to autonomously switch states when a torque overload occurs without sensing, control logic, or additional actuators. This dual functionality makes the gripper particularly suitable for manipulation tasks that involve both holding and rotary motion, such as operating manual valves or performing screwing and unscrewing operations. Two prototypes were developed and tested to validate the design principles and demonstrate practical feasibility. The proposed solutions offer a compact approach for complex in-hand manipulation with a single actuator.