User manual

176 Rockwell Automation Publication MOTION-UM003F-EN-P - March 2015
Chapter 8 Home an Axis
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 cancelled and appropriate status bits cleared. The axis is then homed by using
the configured Home Sequence, which may be Immediate, Switch, Marker, or
Switch-Marker. The later three Home Sequences result in the axis being jogged in
the configured Home Direction and then after the position is redefined. Based
on detection of the home event, the axis is automatically moved to the configured
Home Position.
Passive Homing
When the axis Homing mode is configured as Passive, the MAH instruction
redefines the actual position of a physical axis on the next occurrence of the
encoder marker. Passive homing is most commonly used to calibrate Feedback
Only axes to their markers, but can also be used on Servo axes. Passive homing is
identical to active homing to an encoder marker except that the motion
controller does not command any axis motion.
After initiating passive homing, the axis must be moved past the encoder marker
for the homing sequence to complete properly. For closed-loop Servo axes, this
may be accomplished with a MAM or MAJ instruction. For physical Feedback
Only axes, motion cannot be commanded directly by the motion controller, and
must be accomplished via other means.
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 PC bit.
– On a rotary axis, the controller adds one or more revolutions to the move distance. This 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.
Table 21 - Guidelines for the Homing Procedures (Continued)
Guideline Description
IMPORTANT
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, the control moves the axis to the unwind position of zero.
This helps ensure that the resulting move to the Home Position is
unidirectional.