A Tri-Stable Soft Robotic Finger Capable of Pinch and Wrap Grasps

Soft robotic pneumatic grippers have been shown to be versatile, robust to impacts, and safe for use on delicate objects. One type, fluidic elastomer grippers, are characterized by fingers with an inextensible gripping surface backed by extensible pneumatic chambers; when inflated, this mismatch in extensibility results in the finger curling. However, one drawback of these simple fingers is that they have one preprogrammed grasp, usually a simple constant-curvature wrap. While well-suited for finger-sized round objects, they do not grasp flat or small objects well. Here, we present an adaptable tri-stable soft robotic finger that can form either a pinch or wrap grasp based on the shape of the grasped object. We enable this by incorporating two bi-stable springs into the inextensible layer. The three stable positions are: i) open (unpressurized), ii) pinch (with only the proximal section bending), and iii) wrap (with the entire finger bending). We present a simple model of the behavior of our finger and experimental results verifying the model. Further, we apply forces and moments to grasped objects, and show that the tri-stable finger increases the grasping performance when compared to a control gripper with equal gripping force. Our work presents a novel design modification that is unobtrusive, simple, and passive. Our introduction of inexpensive programmable hardware advances the versatility and adaptability of soft grippers.