Optimization design of passive elastic element parameter for underactuated gripper

The underactuated gripper is adaptive to different objects, and is expected to satisfy diverse operation tasks. Its underactuated characteristic determines that the design of passive elastic elements has a great impact on stability and adaptability of the gripper. If the spring becomes compliant, stability of the underactuated gripper will be reduced. On the other hand, if the spring becomes too stiff, adaptability of the gripper will decrease. This paper proposes a metric to design the spring stiffness for the underactuated gripper based on the grasp-state plane method. The equation for the equilibrium between spring stiffness and the driving force is established based on statics analysis. Through the transition between various grasp types, the different contact states between the underactuated finger and the object are obtained, and the matching relationship between the spring stiffness and the driving torque is considered to ensure the stable grab of different grasp types. A stable region is defined based on the grasp-state plane method, and spring stiffness is chosen by maximizing the area of the stable region. Several grasp models are given to demonstrate the effectiveness of the proposed method, providing foundation for the design and control of the underactuated gripper in future study.