Specification Definitions; Following Error - Newport ESP6000 User Manual

6.2

Specification Definitions

6 - 2
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The chances are that you are less interested in how the components look
or what their individual specifications are, but want to be sure that they
perform reliably together according to your needs.
We mentioned this to make a point: A component is only as good as the
system lets (or helps) it to be.
For this reason, when discussing a particular system performance specifi-
cation, we will also mention which components affect performance the
most and, if appropriate, which components improve it.
People mean different things when referring to the same parameter name.
To establish some common ground for motion control terminology, here
are some general guidelines for the interpretation of motion control terms
and specifications.
• As mentioned earlier, most motion control performance specifications
should be considered system specifications.
• When not otherwise specified, all error-related specifications refer to
the position error.
• The servo loop feedback is position-based. All other velocity, accelera-
tion, error, etc., parameters are derived from the position feedback and
the internal clock.
• To measure the absolute position, we need a reference, a measuring
device, that is significantly more accurate than the device tested. In our
case, dealing with fractions of microns (0.1µm and less), even a standard
laser interferometer becomes unsatisfactory. For this reason, all factory
measurements are made using a number of high precision interferom-
eters, most of them connected to a computerized test station.
• To avoid unnecessary confusion and to more easily understand and
troubleshoot a problem, special attention must be paid to avoid bun-
dling 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 perfor-
mance in certain applications.

6.2.1 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 the Control Loops paragraph, a major part of the
servo controller's task is to make sure that the actual stage follows as close
as possible an ideal trajectory in time. You can imagine having an imagi-
nary (ideal) stage that executes exactly the motion profile you are request-
ing. In reality, the real stage will find itself deviating from this ideal trajec-
tory. Since most of the time the real stage is trailing the ideal trajectory, the
instantaneous error is called Following Error.