In order to get the hand to work as a gripper, we need to have some way of combining the currently isolated fingers into a claw of sorts that hooks up to servos on the other end. In our case, we’re hoping to drive the hand off of one continuous rotation servo to decrease weight and complexity. However, we still want a gripper that will adapt to most object shapes, so we had a bit of an under-actuation problem: how do we let each finger close around an object independently using one motor? Thankfully, Google Scholar found this paper , which describes a mechanism designed for exactly this problem. Another goal for the mechanical design of the forearm aligns with that of the finger: no hardware (except fishing line as a “tendon”). The purpose of this goal is to force simplicity into the assembly process. Complex design is okay (if it works) because 3D printers can print crazy splines as easily as they can rectangles. With a mechanism and some design goals, the sketches started to take shape.
Okay, my sketches have never been accused of being too easy to follow.
Eventually, the Solidworks CAD of the forearm was complete. The sliding interface used in V7.6 (with the pink servo interface) was used to slide the fingers into one “palm” (in red below). The two halves of the “forearm” (in yellow below) are held together by a sliding door bolt-esque feature. Finally, the forearm clips into the palm somewhat like a bike helmet clip.
Miraculously, these mechanisms worked when printed. However, the assembly process doesn’t take into account the fishing wire very well, so bits of it should be re-designed for easier threading.
The inside of the forearm mechanism, featuring the sliding pulleys from the paper mentioned above
The forearm “plugged in” to the palm
More importantly, the system almost worked. With two fingers attached (the other two knuckles simply tied in place), the single fishing line coming out the back of the forearm pulled the fingers closed, but it took a lot of force. In fact, it took a carabiner attached to my door and the full strength of one arm to pull it closed while I took this picture:
This will definitely be too much force for the servo, so I will need to go back and reduce friction in the fingers. As a note, I am pretty certain that the problem is friction, not the springiness of the torsion bushings; it is very easy to hold the fingers in place with the fishing line if they are closed by other means, but it is difficult to pull them into that position with the fishing line. In light of the many bits that need improving, I have made a little cheat-sheet of the improvements needed to (hopefully) fix the problems at hand.
Well, time to get back into the CAD. We have a week to integrate this claw with the electronics and make everything work together. Wish us luck, and thanks for tuning in.
 Gosselin, C.; Pelletier, F.; Laliberte, T., “An anthropomorphic underactuated robotic hand with 15 dofs and a single actuator,” Robotics and Automation, 2008. ICRA 2008. IEEE International Conference on , vol., no., pp.749,754, 19-23 May 2008