Rockwell Automation Publication 2080-UM002M-EN-E - April 2022 159
Chapter 8 Motion Control
Axis Error Scenarios
In most cases, when a movement function block instruction issued to an axis
results in a function block error, the axis is also usually flagged as being in
Error state. The corresponding ErrorID element is set on the axis_ref data for
the axis. However, there are exception scenarios where an axis error is not
flagged. The exception can be, but not limited to, the following scenarios:
• A movement function block instructs an axis, but the axis is in a state
where the function block could not be executed properly. For example,
the axis has no power, or is in Homing sequence, or in Error Stop state.
• A movement function block instructs an axis, but the axis is still
controlled by another movement function block. The axis cannot allow
the motion to be controlled by the new function block without going to a
full stop. For example, the new function block commands the axis to
change motion direction.
• When one movement function block tries to control an axis, but the axis
is still controlled by another movement function block, and the newly
defined motion profile cannot be realized by the controller. For example,
User Application issues an S-Curve MC_MoveAbsolute function block to
an axis with too short a distance given when the axis is moving.
• When one movement function block is issued to an axis, and the axis is
in the Stopping or Error Stopping sequence.
For the above exceptions, it is still possible for the user application to issue a
successful movement function block to the axis after the axis state changes.
CommandPos
REAL
(float)
(1)
On a moving axis, this is the current position the controller commands the axis
to go to.
TargetVel
REAL
(float)
(1)
The maximum target velocity issued to the axis by a move function block.
The value of TargetVel is same as the velocity setting in current function
block, or smaller, depending on other parameters in the same function
block. This element is a signed value indicating direction information.
See PTO Pulse Accuracy on page 172 for more information.
CommandVel
REAL
(float)
(1)
During motion, this element refers to the velocity the controller commands
the axis to use. This element is a signed value indicating direction
information.
(1) See Real Data Resolution on page 170 for more information on REAL data conversion and rounding.
IMPORTANT
• Once an axis is flagged with error, and the error ID is not zero, the user needs
to reset the axis (using MC_Reset) before issuing any other movement
function block.
• The update for axis status is performed at the end of one program scan
cycle, and the update is aligned with the update of Motion Axis status.
Data Elements for Axis_Ref (Continued)
Element Name Data Type Description
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