ACU-RITE CNC 3500i - Manual (Page 52)

26 2 Machining Fundamentals
2.1 F
u
ndamentals of P
o
sitioning
2.1 Fundamentals of Positioning
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 CNC 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 CNC when they are
crossed over. From that signal the CNC 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.