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.

[[!img step_backlash.png
  alt="Data from 17 counterclockwise approach-retreat cycles"
  title="Data from 17 counterclockwise approach-retreat cycles"]]

Conclusions
===========

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.

[backlash]: http://en.wikipedia.org/wiki/Backlash_%28engineering%29

[[!tag tags/hardware]]