Newport MM4006, User Manual

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MM4006 M o t i o n Control Tutorial
4.2.2 E r r o r
Error has the same definition as the Following Error with the exception that
the ideal trajectory is not compared t o 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.
4.2.3 A c c u r a c y
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 cer-
tain distance. When the motion is completed, we measure the actual dis-
tance 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 Fig. 4.2.
Error
Max. Error
Position
Fig. 4.2 — 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 guaranties the user
that for any application, the positioning error will not be greater than this
value.
(%)
M e w p o r t . 4 . 5
EDI10181En1020 — 03/03