Absolute Position Recovery

Absolute Position Recovery (APR) supports the establishing and maintaining of absolute position referenced to a specific machine, commonly called the machine referenced absolute position or just absolute position.

A homing procedure that is initiated by successful execution of an MAH instruction establishes Absolute position. 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.

Since the homing procedure usually requires the machine to be taken offline and placed in a manual operating mode, for example, not making a 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.

Axis Test Mode

Axes with the Test Mode attribute set to Enabled and that are configured for Controller Loop Back do not execute Absolute Position Recovery.

Certain scenarios, such as power cycles or firmware downloads do not generate APR-related faults. They do however, clear, that is reset, the homed status bit.

Absolute Feedback Device

The absolute feedback device lets absolute position be retained through a power cycle. These devices take various forms, but they all can maintain absolute feedback position while power to the drive and to the 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. But what happens if the drive is swapped out, or the drive firmware is updated? Absolute Position is lost.

CIP Motion lets you recover the absolute position not only through power cycles, but also program downloads, and even 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.

By contrast, a CIP Motion axis supports controller-based scaling where absolute position is maintained in the controller’s firmware. Without the work of the APR feature, absolute position would be lost after a power cycle or project download.