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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. Figure 6.2-6 shows an example of such a plot.
Relative error
100%
80%
60%
40%
20%
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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 6
resolution (encoder) increments.
6.2.7 Repeatability
Repeatability is the positioning variation when executing the same motion
profile. Assuming that we have a motion sequence that stops at a number
of different locations, the Repeatability is the maximum variation in
positioning all targets when the same motion sequence is repeated a large
number of times. It is a relative, not absolute, error between identical
motions.
6.2.8 Backlash (Hysteresis)
For all practical purposes, Hysteresis and Backlash have the same meaning
for typical motion control systems. The term Hysteresis has an electro-
magnetic origin while Backlash comes from mechanical engineering. Both
describe the same phenomenon: the error caused by approaching a point
from a different direction.
All parameters discussed up to now that involve the positioning Error
assumed that all motions were performed in the same direction. If we try to
measure the positioning error of a certain target (destination), approaching
the destination from different directions could make a significant differ-
ence.
In generating the plot in Figure 6.2-1 we said that the motion device will
make a large number of incremental moves, from one end of travel to the
other. If we command the motion device to move back and stop at the
same locations to take a position error measurement, we would expect to
get an identical plot, superimposed on the first one. In reality, the result
could be similar to Figure 6.2-7.
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Motion step size (in resolution increments)
Figure 6.2-6 — Error vs. Motion Step Size
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