HEIDENHAIN PT 855 for Milling - Manual (Page 13)

I - 1 Fundamentals of Positioning
Fundamentals of Positioning
POSITIP 855 Operating Instructions 11
Fig. 9: Linear scales: with distance-coded ref-
erence marks (upper illustration) and
one reference mark (lower illustration)
Fig. 8: Linear position encoder, here for the X
axis
Position feedback
The position feedback encoders convert the movement of the ma-
chine axes into electrical signals. The POSITIP constantly evaluates
these signals and calculates the actual positions of the machine
axes, which it displays as a numerical value on the screen.
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's reference mark
evaluation feature (REF).
Reference marks
The scales of the position encoders contain one or more reference
marks. When a reference mark is passed over, it generates a signal
which identifies that position as the reference point (scale reference
point = machine reference point). With the aid of this reference
mark the POSITIP's REF feature re-establishes the assignment of
displayed positions to machine axis positions which you last de-
fined by setting the datum.
If the position encoders feature distance-coded reference marks,
each axis needs only move a maximum of 20 mm (0.8 in.) for linear
encoders, and 20° for angle encoders.
Y
X
Z
Angle reference axis
For angular positions, the following reference axes are defined:
Plane Angle reference axis
X Y +X
Y Z +Y
Z X +Z
Positive direction of rotation is counterclockwise if the working
plane is viewed in negative tool axis direction (see fig. 10).
Example: Angle in the working plane X / Y
Angle Corresponds to the...
+ 45° ... bisecting line between +X and +Y
+/– 180° ... negative X axis
– 270° ... positive Y axis
Y
X
+45°
+180°
–180°
–270°
Fig. 10: Angle and the angle reference axis, e.g.
in the X / Y plane