Resolution,
Accuracy, and Repeatability
Accuracy
-
For a specific point of interest in three-dimensional space,
accuracy is the difference between the actual position in
space and the position as measured by a measurement device.
Stage accuracy is influenced by the feedback mechanism (linear
encoder, rotary encoder, laser interferometer), drive mechanism
(ball screw, lead screw, linear motor), and trueness of
bearing ways. The measurement reference for Aerotech linear
products is a laser interferometer.
Repeatability
- Repeatability is defined as the range of
positions attained when the system is repeatedly commanded
to one location under identical conditions. Uni-directional
repeatability is measured by approaching the point from
one direction, and ignores the effects of backlash or hysteresis
within the system. Bi-directional repeatability measures
the ability to return to the point from both directions.
Many vendors specify repeatability as ± (resolution). This
is the repeatability of any digital servo system as measured
at the encoder. All of Aerotech's specifications, which
include the effects of Abbe error, friction, etc. are based
on actual operating conditions and usage - not on theoretical,
unachievable values.
Resolution
- The smallest possible movement of a system. Also known
as step size, resolution is determined by the feedback device
and capabilities of the motion system. Theoretical resolution
may exceed practical resolution. For example, in a ball-screw-based
positioning system, a theoretical resolution of 4 nm can
be obtained by combining a 4 mm/rev screw, 1000-line encoder,
and an x1000 multiplier. The actual motion system will never
be able to make a single 4 nm step due to friction, windup,
and mechanical compliance. Therefore, the practical resolution
is actually less. All of Aerotech's specifications are based
on practical resolution. |
Low Accuracy
Low Repeatability
Low Accuracy
High Repeatability
High Accuracy
High Repeatability
Fine Resolution
Coarse Resolution
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