HEIDENHAIN TNC 335 Technical Manual - Manual (Page 133)

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TNC 360 2 Reference Marks 8/95
2 Reference Marks
By setting a datum point, a definite positional value (coordinate) is assigned to each axis position for
the machining of the workpiece. Since the actual-position value is established incrementally by the
encoder, this defined assignment of values to positions must be re-established after every power
interruption.
The HEIDENHAIN linear encoders are therefore equipped with one or several reference marks.
When a reference mark is traversed, it generates a signal that identifies the particular position as a
reference point. By traversing the reference marks after a power interruption, the assignment of
positional values to axis positions (and, at the same time, the fixed machine references) that were
most recently determined by datum point setting are re-established.
Measuring system
Machine table
REF Value
– 44,985
10 20 30 40
Reference mark = Machine datum
+Z
+X 0
Workpiece
datum
Workpiece
REF Value
0
Since it is inconvenient to restore the reference points if large traverses are required to do so,
HEIDENHAIN recommends the use of encoders with distance-coded reference marks. On these
measuring systems the absolute position value is available after crossing two reference marks.
Scale with distance-coded reference marks
The graduation consists of an incremental grating and a parallel track for the reference marks. The
absolute position of each reference mark is encoded by varying the distance between adjacent
reference marks according to a mathematical algorithm.
Scale with distance-coded reference marks
Scale with one reference mark