HEIDENHAIN ND 281 B v.2 - Manual (Page 7)

7
Scale in Distance-coded
linear encoder reference marks
Reference Marks
The MT length gauges have one reference mark. The scales of
other photoelectric linear or angular encoders can contain one
reference mark or many
distance-coded reference marks.
If there is an interruption of power, the relationship between the
position of the length gauge and the displayed position value is
lost. The reference marks on the position encoders and the REF
reference mark evaluation feature enable the display unit to
quickly reestablish this relationship again when the power is
restored.
When a reference mark is crossed over, a signal is generated
which identifies that position as a reference point. At the same
time, the display unit restores the relationship between length
gauge position and display values which you last defined by
setting the datum.
To restore the datum on scales with distance-coded reference
marks, you only need to traverse a maximum of 20 mm for
linear encoders, and 10° or 20° for angle encoders, depending
on the model.
Reference mark
Reference marks on linear encoders
Reference Marks