have you thought about head control instead. Control dynamics were going to do a head position sensing system although I am not sure if it came out onto the market. There is also a system which uses a sort of syrrup at the back of the head. I can't remember who does this but I can find out if you are interested.
Another long shot is to have a 4 switch (probably linear) array proximal or distal to the communication aid. This will not be proportional of course.
Clive Thursfield
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