We are designing an arm with three joints that move in a 2D plane.  Neither of the joints can pass over each other.  My question is what is the best method to set an initial zero degree offset?  From this offset, the arm's position is then calibrated over a known location.  The arm must be able to locate to a defined position with no more than a 0.10 inch error.  From past experiences, limit switches and hard stops have not been given us the desired accuracy.