One spring is tied-off so that a nonlinear spring constant is produced when the tied-off spring extends to a predetermined length.
Pulleys attached to the potentiometers provide a simple zeroing of the potentiometers during set up. The cord passes through a hole in the flanges of the pulleys and is secured by a wire tie.
A capstan drive, where a tension cord is wrapped around the shaft of the motors, provides for a simple mechanical coupling that releases when if string breaks.