A Variable Stiffness Gripper with Antagonistic Magnetic Springs for Enhancing Manipulation

Robot grasping of objects based on variable stiffness actuation not only improves the safety and robustness of the grasp but also enhances dynamic manipulation. In this paper, we present the design aspects of a variable stiffness gripper and demonstrate how the controllable compliance of the fingers can improve the performance in dynamic manipulation tasks such as hammering/hitting. The proposed gripper consists of two parallel fingers and repulsive magnets are used as the nonlinear springs between gripper actuators and fingers. The position and force-stiffness characteristics of the fingers are adjusted simultaneously, by controlling the air-gaps between magnets. Finally, the application of the gripper in a nail hammering task is studied as an example of dynamic manipulation. For this purpose, an optimal stiffness control problem is solved to maximize the impact force of the hammering task through maximizing the kinetic energy of the grasped object at the hitting instance. Despite the simplicity of the design, experimental results indicate the effectiveness of the gripper for dynamic manipulation.