You should just put hinge to limit movement, and motor to move. Put high damp and high speed on motor to make it stand still in position. That is how I make my models, which you can see on my site: http://forsa07.wix.com/dl-design#!design/cc9

Maybe it is best sp setup, but pieces (legs, arms ...) dont colide as they look like. You can see my models similar to this on my site: http://forsa07.wix.com/dl-design#!design/cc9