In eighth grade I was inspired by a person who came into the class to talk about bone replacement surgery and prosthetics. After the talk, I wanted to build my own robotic hand. I set out to built a hand and within a week or two I had a model where the fingers would curl up or extend in unison (not pictured). After graduating from grade school I decided to revisit the robotic hand. After a couple of prototypes I had the hand pictured below. Then not satisfied with just a hand I decided to start building a whole arm. Starting with the hand I worked my way up to the shoulder, including motors at each joint for the full range of human motion.
| I started building the arm during
the summer before I hit high school, and I didn't finish until sometime
my junior year of high school. There are a total of twelve motors
throughout the arm.
In most of the cases I used the lowest
gearing possible to get the most lifting force out of the motors. This
makes the arm extremely slow, but it allowed
me to build it with the limited resources and knowledge I had at the
Even with a the lowest gear and the strongest hobby motor I could buy, the motor was not enough to rotate the arm at the shoulder. To overcome this obstacle, I used lead fishing weights as counterweights to aid the motor.
The motor allocation goes as follows: one motor for each finger, two for the wrist, two for the elbow/bicep, and three for the shoulder/bicep. All of these motors have two directions and are operated via the control panel in the picture below. They operate off of 12 volts and have potentiometers to limit the speed of the motor should it be necessary.
| The motors that provided the rotation
for three of the joints, the shoulder, ricep, and forearm, needed a way
to interface with the arm. I did this by creating gears from old tin
drink bottles, plaster of Paris, and a chain and gear kit.
First I cut the bottom off of the tin bottle and then I drilled a hole in the center for the axle. I glued a nut over the axle hole so that the gear could be attached to a bolt that would serve as the axle. Once the glue for the nut set I filled the surrounding volume with plaster of paris to create a stronger gear and ensure that the glued nut would not break off. The teeth for the gear came from a hobby kit that consisted of black plastic chain links and red gears. I glued a chain onto the circumference of the tin bottle bottom to give it teeth.
The hand is my favorite part of the arm. Everything is compactly fit onto the back of the hand and it works well. I feel that I managed to elegantly incorporate seven motors (including the wrist) into such a small space and maintain good functionality. However, looking back on the design I know this could have been built better. There are a couple of things I would change if I were to go back and rebuild the hand.
First I would change how the fingers are moved by the motors. Currently the setup is such that the motors pull the fingers open and rubber bands pull them closed. This should be reversed so that the arm has some gripping strength. The motors should pull the fingers closed. Second I would use different motors arrangements to spool the wire that moves the fingers. The blue servos do not have the proper attachments, they are not designed to spool up wire. Otherwise I am pretty satisfied with the functionality of the wrist and thumb. The thumb does not operate exactly like a human thumb, but with one motor it does okay.
|This is another view of the arm. The shoulder is shown in better detail.|
Copyright, Brian De Vitis, 2011 Contact Me at firstname.lastname@example.org