HEIDENHAIN ND 920 - Manual (Page 8)

8
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.
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
which identifies that position as a reference point (scale datum =
machine datum).
When this reference mark is crossed over, the ND's reference mark
evaluation feature (REF) restores the relationship between axis slide
positions and display values which you last defined by setting the
datum. If the linear encoders have distance-coded reference marks,
you only need to move the machine axes a maximum of 20 mm to do
this.
Y
X
Z
Workpiece
Scale in Distance-coded
linear encoder reference marks
Reference mark
Position
encoder
Fundamentals