Hi John.

Yes fair point. I was also thinking similar lines at one point. An electromagnetic brake certainly seems to resolve the back drive issue but pushes up cost and control complexity. (my budget suits basic range components ;-))

My other concern is the stepper sees a high load trying to lift the weight of the knee and a much lower load lowering it.
I am therefore tending toward the counter balance solution as it means the stepper sees more equal loads in both directions. This possibly means a smaller motor, or at least less maximum load on the motor.

A brake will be plan B if plan A doesn't come off.......may come down to actual weight and stroke details...that's my mission to find out today.

Cheers for your post....

andy