I came up with an improvement that can get rid of the the big spring. I was thinking about the arrangement above with strings in tension along a and b. Ideally a+b would be a constant but that only works if c/d is infinity. However, I did happen to notice that as c/d increased the amount that a+b would vary over the range of motion would decrease. This got me thinking that maybe I would set a+b as a constant and then spring load c with small compression springs. As just a test I picked a+b=8" and d=2". In this case c would range from 4" to 4.5". That is hardly a difference at all. It will be easy to pick a spring that can produce virtually a constant force over the range of motion. This spring can be used to produce a 11-15lb force over the range of motion. I am sure I can find better. That was just the first workable one I saw. 15lbs is my max. With a .5" drive pulley and a 4 string block and tackle setup along a and b, I can generate 30lbs at most so 15lbs per c.
The beauty of this system is that you can adjust a+b. This will allow me to pick a perfect preload for the springs. I don't have to get the calculations perfect now.
I know this is one more joint and a complex string routing system but let me know what you think. I personally think this is the winner so far. I will CAD it up so better judgement can take place.
Thanks,
Nicholas Seward