Design and Experimentation of a Variable Stiffness Bistable Gripper

Grasping and manipulating objects is an integral part of many robotic systems. Both soft and rigid grippers have been investigated for manipulating objects in a multitude of different roles. Rigid grippers can hold heavy objects and apply large amounts of force, while soft grippers can conform to the size and shape of objects as well as protect fragile objects from excess stress. However, grippers that possess the qualities of both rigid and soft grippers are under-explored. In this paper, we present a novel gripper with two distinct properties: 1) it can vary its stiffness to become either a soft gripper that can conform its shape to fit complex objects or a rigid gripper that can hold a large weight; 2) when the gripper is soft, it has two stable states (i.e., bistable): open and closed: allowing it to be closed without an actuator but through contact force with a target object. The variable stiffness is accomplished by heating a shape memory polymer (SMP) material through its glass transition temperature. The bi-stability is achieved by shaping the gripper's energy landscape through two elastic elements. This paper details the design and fabrication process of this gripper, as well as quantifies the influence of temperature variations on this gripper. The capability of the gripper is experimentally verified by grasping different objects with various shapes and weights. We expect such a gripper to be suitable for many applications that traditionally require either a rigid or a soft gripper.