Position Encoders; Reference Marks - HEIDENHAIN POSITIP 880 User Manual

Position Encoders

The position feedback encoders convert the movement of the
machine axes into electrical signals. The POSITIP 880 constantly
evaluates these signals and calculates the actual positions of the
machine axes, which it displays as a numerical value on the screen.
See Fig. I.9.
If there is an interruption in power, the calculated position will no
longer correspond to the actual position. When power is restored, you
can re-establish this relationship with the aid of the reference marks
on the position encoders and the POSITIP 880's reference mark
evaluation feature (REF).

Reference Marks

The scales of the position encoders contain one or more reference
marks. When reference marks are crossed they can be used to define
an absolute position in an incremental system. If power is interrupted
this absolute position is lost and the relationship between the
reference mark and scale position is lost. The reference marks on the
position encoders and the POSITIP 880's reference mark evaluation
feature allows the unit to quickly re-establish this relationship again
when power is restored. See Fig. I.10.
When a reference mark is crossed over, it generates a signal which
identifies that position as the reference point. The POSITIP 880 uses
this reference point to restore the relationship between the scale
position and the display value which was last defined by setting the
datum.
If the position encoders feature distance-coded reference marks,
there are reference marks uniquely spaced along the length of the
scale. Crossing any two reference marks will restore the datum. Each
axis need only move a limited distance for linear encoders, and angle
for rotary encoders.
The datum setting cannot be restored from one power
cycle to the next if the reference marks were not crossed
before the datum was set.
POSITIP 880
Fig. I.9 Linear position encoder, here for the X axis
Fig. I.10 Linear scales: with distance-coded
reference marks (upper illustration) and one
reference mark (lower illustration)
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