Robotic graspers historically employ actuators at each of their various joints. This allows them a great deal of freedom of movement, but also tremendously increases the weight of the robot, leading to negative effects. This thesis aims to alleviate this weight problem by employing a system of pulleys and cables which drive the joints via a remote actuator. Utilizing this method closely approximates the inter-finger coordinated synergies of the human hand.
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Each finger segment contains a circular arch running through the interior of the linkage. This arch acts as a pulley which drives the finger via a DC motor at the base of the hand. Each linkage also contains a hollow center. Cables that drive the pulley run through these cavities as they travel to their connection with the driving shaft. The diagram at left protrays the arch that acts as a pulley and the cable which runs through the subsequent linkage. |
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I designed and built this hand to perform a grasping motion. A separate cable drives each finger segment. Each cable runs to its own pulley on a single shaft. Unlike the pulleys on the finger segments, these pulleys vary in radius. As the control shaft turns at a constant velocity, the linear velocities of the seperate cables vary due to differences in radii. Thus, when making a fist, each finger joint closes at a different velocity and achieves a different end position; just like a human hand as it makes a fist around an object. |
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This strategy limits the range of motions of the robot by coupling the joints together; producing only one inter-coordinated motion. This is ideal for a simple grasper like this hand. However, the lack of variety could pose a problem for any robot which requires a more complex range of motions.
I believe that with further work it would be possible to “switch gears” between multiple shafts to produce motions. By changing the shape of each pulley to those of different oblong cams, a single shaft rotating at a constant velocity could still acheive various non-constant velocities at the desired joint.