34 4 Programming: Fundamentals of NC, File Management, Programming Aids, Pallet Management
4.1 Fundamentals
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. 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 an interruption of power, the calculated position will no
longer correspond to the actual position of the machine slide. The TNC
can re-establish this relationship with the aid of reference marks when
power is returned. 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 the signal the TNC identifies that position as the
machine-axis reference point and can re-establish the assignment of
displayed positions to machine axis positions.
Linear encoders are generally used for linear axes. Rotary tables and
tilt axes have angle encoders. If the position encoders feature
distance-coded reference marks, you only need to move each axis a
maximum of 20 mm (0.8 in.) for linear encoders, and 20° for angle
encoders, to re-establish the assignment of the displayed positions to
machine axis positions.
Reference system
A reference system is required to define positions in a plane or in
space. The position data are always referenced to a predetermined
point and are described through coordinates.
The Cartesian coordinate system (a rectangular coordinate system) is
based on the three coordinate axes X, Y and Z. The axes are mutually
perpendicular and intersect at one point called the datum. A
coordinate identifies the distance from the datum in one of these
directions. A position in a plane is thus described through two
coordinates, and a position in space through three coordinates.
Coordinates that are referenced to the datum are referred to as
absolute coordinates. Relative coordinates are referenced to any other
known position (datum) you define within the coordinate system.
Relative coordinate values are also referred to as incremental
coordinate values.
X (Z,Y)
X
MP
Y
X
Z
Y
X
Z
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