In this paper, we propose the addition of passive hydraulic mechanisms to simple parallel robotic grippers for improving disturbance rejection while maintaining the low cost of an industry standard gripper design. Each adaptive jaw on our gripper consists of three parallel hydraulic cylinders that are connected to a common local reservoir. The resultant passive hydraulic system is fully encased in the finger and moves independently of the actuator that closes the fingers. Such a design eliminates the need to engineer a complex cable or linkage system to allow for finger adaptability as many underactuated grippers do. Specifically, hydraulic cylinders need only be selected and connected together. As with other underactuated devices, the unconstrained freedoms of this design allow the gripper to adapt to unknown objects instead of creating a custom gripper shape for each new object the robot needs to grasp. In this paper, we analyze the ability of this gripper to maximize contact points over various sized objects and object placements while creating immobilizing form closure grasps. We than tested these improvements on a physical robot and found that grasp performance increased by up to 30% over a gripper lacking underactuation.

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