Bistable Spring Steel Grippers for Passive Grasping

Aerial grasping using flying robots necessitates gripper designs that can accommodate position/orientation errors to grasp different objects. This study investigates two bistable gripper designs that can automatically close upon contacting an object. Both grippers leverage pre-stressed spring steel bands (PSSB), with one having a single PSSB and the other having two PSSBs in a cross-shape. Both grippers can be opened with a single motor to drive a cable system routed through flexible joints on the bands. The same cable system, together with a friction adjustment mechanism, also ensures sequential closing of the gripper. We conduct experiments to evaluate and compare the gripping performance of both designs. On average, the single-band gripper can grasp within 0.14 s and open in 3 s, while the cross-shaped gripper takes 0.24 s and 4 s, respectively. Both designs can still grasp even when a cylinder makes off-center contact with the PSSB, up to a displacement of 50 mm. Further tests demonstrate both grippers can grasp cylinders with different diameters that approach the gripper with different orientations. Finally, we demonstrate both grippers can grasp real-world objects (e.g., water bottles, video game controllers, etc.). Our grippers, with their compact, lightweight design and passive grasping capabilities, hold the potential to advance aerial grasping for diverse applications.