My team and I built a machine which simulates reduced moon-level gravity to test a moon rover.
We built a prototype sensing mechanism for this gravity simulator. The gravity offloading cord must always be on top of the object that you are simulating moon level gravity on in order to pull it up and "offload" a portion of the force of gravity acting upon the object. Thus, if the object moves, the cord must move at the same time.
Our sensing mechanism detects the angle offset of the object and sends commands to the tether cord motor controller to move it back on top of the object.