256 Rockwell Automation Publication MOTION-UM003K-EN-P - January 2019
Chapter 12 Homing
Guidelines for Homing
To configure the homing procedure, you specify the mode (active or passive)
and sequence. Based on those selections, you can also choose the home
position, an offset for the home position, the direction, and speed. For switch-
based sequences, you can also select whether the limit switch is normally open
or normally closed.
Table 55
describes guidelines for homing procedures.
Active Homing
When the axis Homing mode is configured as Active, the physical axis is first
activated for servo operation. As part of this process, all other motion in
process is canceled and appropriate status bits cleared. The axis is then homed
by using the configured Home Sequence, which can be Immediate, Switch,
Marker, or Switch-Marker. The latter three Home Sequences result in the axis
being jogged in the configured Home direction. Then, after the homing
sequence is complete, the position is redefined. Based on detection of the home
event, the axis is automatically moved to the configured Home Position.
Table 55 - Guidelines for the Homing Procedures
Guideline Description
To move an axis to the home position, use Active
homing.
Active homing turns on the servo loop and moves the axis to the home position. Active homing also does the following:
• Stops any other motion.
• Uses a trapezoidal profile.
For a Feedback-only device, use Passive homing. Passive homing does not move the axis:
• Use passive homing to calibrate a Feedback-only axis to its marker.
• If you use passive homing on a servo axis, turn on the servo loop and use a move instruction to move the axis.
For single-turn equipment, consider homing to a
marker.
The marker homing sequence is useful for single-turn rotary and linear encoder applications because these applications
have only one encoder marker for full axis travel.
For multi-turn equipment, home to a switch or switch
and marker.
These homing sequences use a home limit switch to define the home position:
• You need a home limit switch if the axis moves multiple revolutions when it runs. Otherwise, the controller cannot
tell which marker pulse to use.
• For the most precise homing, use both the switch and marker.
If your equipment can’t back up, use unidirectional
homing.
With unidirectional homing, the axis doesn’t reverse direction to move to the Home Position. For greater accuracy,
consider using an offset:
• Use a Home Offset that is in the same direction as the Home Direction.
• Use a Home Offset that is greater than the deceleration distance.
• If the Home Offset is less than the deceleration distance does the following:
– The axis simply slows to a stop. The axis doesn’t reverse direction to move to the Home Position. In this case, the
MAH instruction doesn’t set the process compete bit.
– On a rotary axis, the controller adds one or more revolutions to the move distance. This addition makes sure that
the move to the Home Position is unidirectional.
Choose a starting direction for the homing sequence. Decide which direction you want to start the homing sequence in:
• Positive direction—choose a Forward direction.
• Negative direction—choose a Negative direction.
IMPORTANT The control moves the axis to the unwind position of zero. This movement
occurs only when unidirectional active homing is performed on a rotary axis
and the Home Offset value is less than the deceleration distance when the
home event is detected. This process helps make sure that the resulting
move to the Home Position is unidirectional.
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