There is a good deal of drive-train backlash in the stepper mechanism.

Detection procedure

  1. Step towards surface until there is plenty of surface contact.
  2. Step away until you leave the surface.
  3. Repeat until you have too much data :p.

Data from 17 counterclockwise approach-retreat cycles


About 47 steps of backlash. Since we never touch the stepper while any critcal surface interactions are going on, simply replace step back 1 step with step back 60 steps, then forward 59.

Supporting evidence

Noise on reversal

When we reversing the stepper (by any means), the movement is “clearly audible” for “a bit” and then becomes “almost silent”.

To give a rough idea, I usually have keep my finger on the drive-shaft to tell if the motor is moving in “almost silent” mode, but can hear the motor from around 5 ft away in “clearly audible” mode. “A bit” is about 0.5 s when controlling the motor using the manual Up/Down switch.