Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 301)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 293
Active Bi-directional Home with Marker
This active homing sequence is useful for single turn rotary and linear encoder
applications since these have only one encoder marker for full axis travel.
When this sequence is performed, the axis moves in the specified Home
Direction at the specified Home Speed until the marker is detected. The Home
Position is then assigned to the axis position corresponding to the marker
location, and the axis decelerates to a stop. If Home Offset is non-zero, then
the Home Position will be offset from the point where the marker is detected
by this value. The controller then moves the axis back to the Home Position at
the specified Home Return Speed using a trapezoidal move profile. If the axis
is configured as a Rotary Axis, the move back to the Home Position takes the
shortest path (i.e., no more than ½ revolution). The axis behavior for this
homing sequence is shown below.
Figure 13.4 Active Bi-Directional Home with Marker
The accuracy of this homing sequence depends only on the time delay in
detecting the marker transition. The position uncertainty of the home position
is the product of the maximum delay for the control to detect the marker pulse
(~1 microsecond) and the specified Home Speed.
For example, if a Home Speed of 1 inches per second (60 IPM) is specified,
the uncertainty of the home position is calculated as shown below:
Uncertainty = 1 Inch/Sec * 0.000001 Sec = 0.000001 Inch.
Clearly, using the marker pulse as part of the homing sequence results in a
tremendous increase in absolute homing accuracy over just employing
mechanical limit switches.