Thanks for the explanation. The induction sensors work by detecting changes in the magnetic field created by the coil, right? The field is changed, if a magnetic object enters the field, and that results in a change in the current through the coil. That's the theory behind inductive sensors, right? Hopefully I'm not too far off in left field here!
Then, the distance would depend on the size of the coil and the sensitivity of the equipment used to measure the current. I don't quite understand why the distance would only be able to change in one axis.