Rockwell Automation GMLC Reference Manual - Manual (Page 166)

Publication GMLC-5.2 - November 1999
Imaginary Axis Applications 129
• This technique works with trapezoidal profile moves, and with S-
curve profile moves using version 3.6 (or higher) firmware. You
cannot change the endpoint of a parabolic move, or an S-Curve move
using version 3.5 (or lower) firmware while the axis is decelerating.
• The position of the feed axis is polluted by the corrections, and the
actual (and command) position of the axis at the end of the feed is
different from its true physical position by an amount equal to the
compensated slip. This makes back-to-back absolute feeds virtually
impossible.
Using the imaginary axis to superimpose two moves on the feed axis
overcomes these disadvantages.
To implement continuous slip compensation using the imaginary axis,
gear the feed axis to the imaginary axis at a 1:1 ratio and move the feed
axis indirectly by moving the imaginary axis. You can now make
corrections as incremental moves to the real (physical) feed axis based on
the position of the material encoder and they are superimposed on the
gearing (really another move) motion from the imaginary axis, as shown
in the following diagram.