Assume that the user has a motion device with a 1 µm resolution. If
every time the user commands a 1 µm motion the measured error is
never greater than 2%, the user will probably be very satisfied and
declare that the Minimum Incremental Motion is better than 1 µm.
If, on the other hand, the measured motion is sometimes as small as
0.1 µm (a 90% error), the user could not say that 1 µm is a reliable
motion step. The difficulty is in drawing the line between acceptable
and unacceptable errors when performing a small motion step. The
most common value for the maximum acceptable error for small
motions is 20%, but each application ultimately has its own
standards.
One way to solve the problem is to take a large number of
measurements (a few hundred at minimum) for each motion step size
and present them in a format that an operator can use to determine the
Minimum Incremental Motion by its own standards.
Motion step size (in resolution increments)
Relative error
100%
80%
60%
40%
20%
1 2 3 4 5 6 7 8 9
10 11
Figure 5.6: Error vs. Motion Step Size
Figure 5.6 shows an example of such a plot. The graph represents the
maximum relative error for different motion step sizes. In this
example, the Minimum Incremental Motion that can be reliably
performed with a maximum of 20% error is one equivalent to 4
resolution (encoder) increments.
5.2.7 Repeatability
Repeatability is the positioning variation when executing the same
motion profile. Assuming that the user has a motion sequence that
stops at a number of different locations, the Repeatability is the
maximum position variation of all targets when the same motion
sequence is repeated a large number of times. It is a relative, not
absolute, error between identical motions.
Section 5 – Motion Control Tutorial 5-7
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