Section 6 — Motion Control Tutorial
6 - 3
To summarize, the
Following Error
is the instantaneous difference between
the actual position as reported by the position feedback device and the
ideal position, as seen by the controller. A negative following error means
that the load is trailing the ideal stage.
6.2.2 Error
Error
has the same definition as the
Following Error
with the exception that
the ideal trajectory is not compared to the position feedback device
(encoder) but to an external precision measuring device.
In other words, the
Following Error
is the instantaneous error perceived by
the controller while the
Error
is the one perceived by the user.
6.2.3 Accuracy
The
Accuracy
of a system is probably the most common parameter users
want to know. Unfortunately, due to its perceived simplicity, it is also the
easiest to misinterpret.
The
Accuracy
is a static measure of a point-to-point positioning error.
Starting from a reference point, we command the controller to move a certain
distance. When the motion is completed, we measure the actual distance
traveled with an external precision measuring device. The difference (the
Error
) represents the positioning
Accuracy
for that particular motion.
Because every application is different, we need to know the errors for all
possible motions. Since this is practically impossible, an acceptable com-
promise is to perform the following test.
Starting from one end of the travel, we make small incremental moves and
at every stop we record the position
Error
. We perform this operation for
the entire nominal travel. When finished, the error data is plotted on a
graph similar to Figure
6.2-1.
A
B
Position
max. error
Error
0
Figure 6.2-1 — Position Error Test
The difference between the highest and the lowest points on the graph is the
maximum possible
Error
that the motion device can have. This worst-case
number is reported as the positioning
Accuracy
. It guarantees the user that
for any application, the positioning error will not be greater than this value.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
