= Unidirectional positioning repeatability
<Unidirectional positioning repeatability> describes the scatter of repeated returns to one commanded position when the final approach direction is held fixed. Specify the complete reset-and-return procedure so that each trial approaches under comparable conditions. PI distinguishes this directional scatter from its summaries that include reversal effects.<How to Consistently Verify the Performance of Nanopositioning Motion Control Equipment by Physik Instrumente>[Sections3.1 and4.4, approach direction and unidirectional repeatability]{ref}
For an original example, always approach from below. At a 2 mm target, suppose the position errors are $-0.4,-0.2,0.0\,\mathrm{\mu m}$; at an 8 mm target they are $1.6,1.8,2.0\,\mathrm{\mu m}$. The means are $-0.2$ and $1.8\,\mathrm{\mu m}$, respectively, while both <sample standard deviations> are
$$
s=\sqrt{\frac{0.2^2+0^2+0.2^2}{3-1}}=0.2\,\mathrm{\mu m}.
$$
The same repeatability statistic accompanies quite different mean errors along the travel. A common shift of every reading at a target changes its mean but not its within-target spread.
For illustration, an explicitly declared specification taking the largest tested directional sample deviation would report $0.2\,\mathrm{\mu m}$ for these two data sets. A specification using a full width or a multiple of that deviation would have a different numerical value. PI's own published convention states which directional spreads and target aggregation it uses; compare like definitions rather than only their units.<How to Consistently Verify the Performance of Nanopositioning Motion Control Equipment by Physik Instrumente>[Section4.4, stated maximum/average unidirectional conventions]{ref}
Observe final platform motion with an independent position readout, not just the motor's <rotary encoder>. Allow the chosen <settling time> and keep load and environment controlled. Consistently approaching from one side does not establish <bidirectional positioning repeatability>, and a small return scatter does not show that a small incremental command can be executed: that is the separate question of <minimum incremental motion>.
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