HEIDENHAIN ND 221 B v.2 - Manual (Page 6)

6
Scale in Distance-coded
linear encoder reference marks
Position Encoders and Reference Marks
The ND 221 B display unit is designed for use with
photoelectrical linear encoders with 11 µA
pp
sinusoidal
signals: primarily for HEIDENHAIN MT length gauges with
11 µA
pp
.
The MT length gauges have one reference mark. The scales
of other photoelectric linear encoders (see "Linear 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 (signal period: 20 µm).
Reference mark
Reference marks on linear encoders
Position Encoders and Reference Marks