HEIDENHAIN ND 730 v.1 - Manual (Page 9)

Fundamentals
9
Z
Position encoders
The position encoders on the machine convert the movements of the
machine axes into electrical signals. The ND display unit evaluates
these signals, determines the actual position of the machine axes and
displays the position as a numerical value.
If the power is interrupted, the relationship between the machine axis
positions and the calculated actual positions is lost. The reference
marks on the position encoders and the REF reference mark
evaluation feature enable the ND to quickly re-establish this
relationship again when the power is restored.
Workpiece
Reference marks
The scales of the position encoders contain one or more reference
marks. When a reference mark is crossed over, a signal is generated
identifying that position as a reference point (scale datum = machine
datum).
When this reference mark is crossed over, the ND's reference mark
evaluation feature restores the relationship between axis slide
positions and display values as you last defined it by setting the
datum. If the linear encoders have distance-coded reference marks,
you need only move the machine axes a maximum of 20 mm to
restore the datum.
Scale in linear
encoder
Reference mark
Distance-coded
reference marks
Encoder