5.2.8 Backlash and Hysteresis
Hysteresis and Backlash contribute to error caused by approaching a
point from a different direction.
Backlash
The maximum magnitude of an input that produces no measurable
output upon reversing direction. It can be a result of insufficient axial
preloads or poor meshing between drivetrain components, i.e., gear
teeth in a gear-coupled drivetrain. Backlash is relatively repeatable
and can be compensated for by capable controllers.
Hysteresis
The difference in the absolute position of an object for a given
commanded input when approached from opposite directions (Figure
2). It is due to elastic forces accumulated in various drivetrain
components (leadscrew wind-up, for instance). This is often confused
with backlash which can be characterized and compensated for by
capable controllers.
All parameters discussed up to now that involve the positioning Error
assumed that all motions were performed in the same direction. If the
user tries to measure the positioning error of a certain target
(destination), approaching the destination from different directions
could make a significant difference.
In generating the plot in
Figure 5.2 we said that the motion device
will make a large number of incremental moves, from one end of
travel to the other. If the user commands the motion device to move
back and stop at the same locations to take a position error
measurement, the user would expect to get an identical plot,
superimposed on the first one. In reality, the result could be similar to
Figure 5.7.
Error
+
Position
0
Hysteresis
–
Figure 5.7: Hysteresis Plot
The error plot in reverse direction is identical with the first one but
seems to be shifted down by a constant error. This constant error is
the Hysteresis of the system.
5-8 Section 5 – Motion Control Tutorial
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