Rockwell Automation GMLC Reference Manual - Manual (Page 519)

Publication GMLC-5.2 - November 1999
482 Motion Variables
Note that the calculated uncertainty is the maximum error in the direction
in which the axis is moving. The error after any individual registration
event is somewhere (essentially random) between 0 and the calculated
soft registration position uncertainty. In other words,
Marker Distance
Marker_Distance is the incremental distance (in axis position units)
between the location of the home limit switch, and the encoder marker of
a servo, master only or virtual axis as measured during the last (most
recent) homing sequence. It is updated whenever the axis is homed to a
limit switch and marker using an active homing sequence, or when the
axis is passively homed (in this case, the limit switch position is assumed
to be 0). Use marker distance when you need to accurately position the
encoder marker relative to the home limit switch.
In the Tag Explorer, select both Axis System Variables and the desired
axis, then select Marker_Distance in the Tag Window. See the Setup
section of the Installation and Setup manual for your motion controller
for more information on axis homing.
You can use the Marker_Distance variable to calculate how much a
passive homing operation changes the actual position of the axis. The
home limit switch position is assumed to be 0 for passive homing,
therefore, Marker_Distance is the position of the axis when the marker
occurs—but before it is set to the Home Position. Thus, the change in axis
position due to passive homing can be calculated as the Home Position
minus the Marker_Distance of the axis. Use a Control Settings block to
read the Home Position parameter of the axis.
The soft registration position is: If:
Greater than the true position of the axis at the
occurrence of the registration event The axis is moving in the positive
direction.
Less than the true position The axis is moving in the negative
direction.