HEIDENHAIN TNC 620 Programming Station, User Manual, Page: 112 / 695

Fundamentals, file management
3.1 Fundamentals
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HEIDENHAIN | TNC 620 | ISO Programming User's Manual | 9/2016
3.1 Fundamentals
Position encoders and reference marks
The machine axes are equipped with position encoders that
register the positions of the machine table or tool. Linear axes are
usually equipped with linear encoders, rotary tables and tilting axes
with angle encoders.
When a machine axis moves, the corresponding position encoder
generates an electrical signal. The TNC evaluates this signal and
calculates the precise actual position of the machine axis.
If there is a power interruption, the calculated position will no
longer correspond to the actual position of the machine slide.
To recover this association, incremental position encoders are
provided with reference marks. The scales of the position encoders
contain one or more reference marks that transmit a signal to the
TNC when they are crossed over. From that signal the TNC can
re-establish the assignment of displayed positions to machine
positions. For linear encoders with distance-coded reference
marks, the machine axes need to move by no more than 20 mm,
for angle encoders by no more than 20°.
With absolute encoders, an absolute position value is transmitted
to the control immediately upon switch-on. In this way the
assignment of the actual position to the machine slide position is
re-established directly after switch-on.