HEIDENHAIN TNC 640, User Manual, Page: 146 / 948

Fundamentals, File Management | Fundamentals
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HEIDENHAIN | TNC 640 | Conversational Programming User's Manual | 10/2017
4.1 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 control 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 assignment, 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 control when they are crossed over. This enables the control
to re-establish the assignment of the displayed position to the
current machine position. 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.