We have used two mechanical designs for the arm.
1. ChArm‘s Mechanical design:-
For our prototype, string actuated fingers made up of three sections (See figure 1) were used. These sections were composed of Styrofoam affixed to cardboard sheets. Strings guided by thin plastic cylinder sections (attached to the underside of the cardboard sheets) actuate finger movement by pulling on section I. This, in turn causes sections II and III to curl towards the root of the finger. Finger retraction is assisted by an elastic string affixed to the back of each section. When the finger curls inwards, the elastic string tends to pull it back to place.
We used servos to activate finger movement by pulling the connecting string. Elastic strings attached to the back of the fingers allow them to return to their original positions with ease. The thumb and the index finger were made separately while the middle, ring and little finger form a single unit. These three units are connected to three separate motors. The entire structure is mounted on a wooden base and support assembly, which makes it highly compact.
Depending on the combination of motors activated, different fingers move to enable the user to perform a number of functions. An electrode sensor setup, as described earlier, senses muscular impulses at the bicep, trapezius(shoulder blade) and pectoralis major (chest). Each muscle movement controls a servo, thus moving the corresponding finger. Different combinations of muscle movements lead to seven different functional finger movements, as shown below:
2:- Inmoov Design
Currently we are trying to print a prosthetic arm using a 3-D printer. The design files are taken from InMoov. We wish to incorporate our EMG circuits with the InMoov arm and validate the results.
Resources on stl files and videos are available here.
We believe such 3-D printed arm can reduce the cost of prosthetic arms, and people can access it easily.