Grasp force optimization in the design of an underactuated robotic hand

For people without the ability to use their hands, gripper-equipped robotic aids may provide partial rehabilitation. This article discusses the optimization of an underactuated mechanism for a new prototype gripper on robotic aid ARM. When the underactuated mechanism of the tentacle-like Soft Gripper is translated to a three-phalange equivalent for the prototype, increasing forces on the proximal phalanges tend to drive objects out of the grasp when the finger flexes. To obtain equal grasp forces on all phalanges, the pulley radii in the mechanism were optimized. This could however only decrease the ratio between highest and lowest force from 4.0 to 2.9. An extra bi-articular tendon was added to the mechanism, which further improved this ratio to 1.5. The prototype gripper that was fitted with the bi-articular mechanism stably grasps and holds objects. Preliminary test with disturbance forces and torques on the grasped objects show a performance increase with respect to a parallel gripper mounted on ARM. However, some improvements on the bi-articular mechanism are necessary to make the force in the bi-articular tendon linearly dependent from the actuation force of the gripper.