So, now that I've some time over Christmas break, I should start
filling this blog up with my projects from the last few years and from my time
in Boston so far!
First project is my masters thesis project.
For my masters thesis, I designed and manufactured a prosthetic
hand that could be controlled by the residual muscles of the forearm. Why? Well
currently there are commercially available hands that retail for in excess of
$15,000 to give someone a hand that functions in a relatively realistic manner.
This to me seemed like rather steep price so I wanted to see if it was
possible to make a prosthetic hand with similar functionality to the commercial
ones, but for less than $1,000. My hand would not be as polished as the commercial
ones but would at least feature a similar control methodology with an
anatomically similar hand with individually controllable fingers and various
grip patterns. CAD files and design files are available here.
The project consisted of two main chunks, the mechanical hand and
the control electronics. For the hand, I was going the 3d print the hand on my
reprap for the easy, flexibility and speed of prototyping. For the electronics,
I was going to need some EMG sensors to read the muscle activity, a
microcontroller for all the control and then some bidirectional motor drivers
for the output. I also decided to incorporate force feedback for tactile
feedback. Instead of force sensors, I decided to use current feedback to give
me approximate force data. My design splurge on the hand was to include a clock
face of RBG led's on the wrist to act as a watch but also a battery level
monitor.
For the mechanical, I started with the design of the finger and
worked back from there. I took some measurements of my hand and digits and
averaged my index, ring and middle to make an "average finger". I
wanted to use and average finger so that I could just copy and paste once I got
one finger designed. I decided to use a capstan system which extends and flexes
the finger. I used a stainless steel nylon coated wire from (link), motors from fingertech (link) and a tensioner mechanism to keep the wire from slipping on the capstan. This
capstan system is similar to the one used in the open hand project though it was developed independently. It also allowed
for a reduced number of spares. The shape of the fingers was quickly defined so
the only thing that had to be done was find the correct internal profile to make
the finger curl in the right manner (Start proximally to the palm then move
distally). This took a number of iterations to get the correct as many of the
initial profiles led to the distal segment curling first and moving inward. Anyway, after a number or prototypes I got it working satisfactorily.
Next I designed the palm to have an anatomical shape. The fingers were splayed by degrees and offset to match the anatomy of my hand. Small clips and nubs are used the limit any motion. The palm consists of two plates, one which holds the four fingers while the second acts as the palm and is the mount for the thumb. The thumb can adduct and abduct thanks to a servo mounted in the palm. The thumb itself is a modified finger, just being slightly wider than then the other fingers and a section shorter.
The stump mount or sock is based on an article I'd seen here where old plastic bottles were thermoformed around a model of the stump. These socks are only suited for light to medium duty but that should be sufficient for the duty it would see. To mount the bottle to the hand, I made a split adapter that bolts together with 4 M4 bolts. One side interfaces with the bottle, the other with the PCB's.
The last part of the mechanical design was the wrist.Initially I wanted to have this joint actuated but due to limited time, I had to got with a fixed position wrist. The orientation can be changed by removing a M3 bolt and rotating it to one of 8 locating holes. A central M8 bolt acts to transfer forces through the wrist joint.
