User manual

Rockwell Automation Publication MOTION-UM003F-EN-P - March 2015 183
Home an Axis Chapter 8
Absolute Position Recovery
Functionality
APR provides support for maintaining absolute position referenced to a specific
machine, commonly called the machine referenced absolute position or just
absolute position, after a power loss, program download, or firmware update.
Absolute position is established by a homing procedure initiated by successful
execution of an MAH instruction. Once the homing procedure has successfully
established a machine reference, the Axis Homed bit is set in the Motion Status
attribute, indicating that actual position and command position now have
meaning with respect to the associated machine.
It is good application programming practice to qualify dynamic machine
operation with the Axis Homed bit being set. Otherwise, absolute moves to a
specific position may not have any relationship to the position of the axis on the
actual machine.
Because the homing procedure usually requires the machine to be taken offline
and placed in a manual operating mode, for example, not making product,
anything that would require you to rehome one or more axes on the machine is
undesirable. This is downtime and costs money. The APR feature maintains the
machine reference or absolute position through power cycles, program
downloads, and even firmware updates under certain conditions.
See APR Fault Conditions on page 184
and Absolute Position Recovery
Scenarios on page 187 for more details.
Absolute Feedback Device
The absolute feedback device lets absolute position be retained through a power
cycle. These devices take various forms, but they all are capable of maintaining
absolute feedback position while power to the drive and feedback device is off.
When power is turned back on, the drive reads the feedback referenced absolute
position from the feedback device and, by applying a saved absolute offset to this
absolute feedback position, the motion control system can recover the machine
referenced absolute position.
Most drive products provide this capability. However, Absolute Position is
generally lost if the drive is swapped out or drive firmware is updated. Integrated
Motion on the EtherNet/IP network lets you recover Absolute Position through
power cycles, program downloads, and firmware updates.
Sercos Versus CIP
For a SERCOS axis with absolute feedback, the drive scaling function and
absolute position is maintained in the drive and therefore may be easily restored
in the control after a power cycle or download of a new project by simply reading
the position from the drive.