Rockwell Automation GMLC Reference Manual - Manual (Page 515)

Publication GMLC-5.2 - November 1999
478 Motion Variables
Command Position
Command_Position is the desired or commanded position of a physical
axis or the imaginary axis (in the position units of that axis) as generated
by any previous motion blocks. In the Tag Explorer, select both Axis
System Variables and the desired axis, then select Command_Position
from the Tag Window scrolling list.
Actual position is the current position of the axis as measured by the
encoder or other feedback device. Position error is the difference between
the command and actual positions of a servo axis, and is used to drive the
motor to make the actual position equal to the command position.
See your motion controller’s Installation and Setup manual, for more
information about the Nested Digital Servo Loop used in motion
controllers.
Command position is useful when performing motion calculations and
incremental moves based on the current position of the axis while the axis
is moving. Using command position rather than actual position avoids the
introduction of cumulative errors due to the position error of the axis at
the time the calculation is performed.
Strobed Position
Whenever motion begins for a servo axis (for example, using a Move
Axis block), the command position of the axis—at the precise instant the
move begins—is stored as the Strobed_Position in the position units of
the axis.