ErrorMovers

Invalid References

Objectives that return References to MoverLists or Movers have the potential to return invalid references. For example, a Station with no mover present can't possibly return a CurrentMover, because by definition no CurrentMover exists! To prevent this, it is important to ensure that all CurrentMover and CurrentMoverList output properties are only evaluated when a MoverInPosition, MoverPassedPosition, MoverInVelocity, etc. flag is true for the relevant Objective.

// There is no CurrentMover at the Station, so this code will not operate correctly!!
IF Station[1].MoverInPosition = FALSE THEN
    Station[1].CurrentMover.MoveToPosition( 200 );
END_IF

Typically, calling methods on these invalid references would result in a Pagefault and halt the XAR. However this outcome can be frustrating and slows development. Instead, an imposter ErrorMover object is returned in these circumstances as a quality-of-life improvement. ErrorMovers replace all the method functionality of standard Movers and will instead generate Errors in TwinCAT. For example:

Axis Linking

If ErrorMovers appear in the list of potentially linkable objects when connecting NC Axes to PLC axes, ensure that these objects are not selected.

By default, ErrorMovers are removed from the PLC Process Image to prevent them appearing in this list.