Newport ESP300 Series, User Manual

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For this reason, all factory measurements are made using a
number of high precision interferometers, most of them connected
to a computerized test station.
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To avoid unnecessary confusion and to more easily understand
and troubleshoot a problem, special attention must be paid to
avoid bundling discrete errors in one general term. Depending on
the application, some discrete errors are not significant. Grouping
them in one general parameter will only complicate the
understanding of the system performance in certain applications.
5.2.1 Following Error
The Following Error is not a specifications parameter but, because it
is at the heart of the servo algorithm calculations and of other
parameter definitions, it deserves our attention.
As will be described later in Section 5.3: Control Loops, a major part
of the servo controller's task is to make sure that the actual motion
device follows as close as possible an ideal trajectory in time. The
user can imagine having an imaginary (ideal) motion device that
executes exactly the motion profile they are requesting. In reality, the
real motion device will find itself deviating from this ideal trajectory.
Since most of the time the real motion device is trailing the ideal one,
the instantaneous error is called Following Error.
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 motion device.
5.2.2 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.
5.2.3 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.
Section 5 – Motion Control Tutorial 5-3