Siemens SINUMERIK 840C - Manual (Page 733)

11.92 10 Axis and Spindle Installation
10.4.5 Distance-coded reference marks
10.4.5 Distance-coded reference marks
Corresponding data
NC MD 240* Reference point value, reference point for leadscrew error
compensation
NC MD 284* Reference point creep speed
NC MD 396* Absolute encoder offset
NC MD 564* bit 0 Reference point approach direction (only required for G74)
NC MD 1300* Basic distance between coded reference point marks
NC MD 1304* External pulse multiplication
NC MD 1808* bit 2 Encoder absolute system opposite to machine system
NC MD 1808* bit 3 Absolute offset valid
NC MD 1808* bit 4 Measuring system 1 distance coded
DB32 DW (K) bit 12 Signal "REFERENCE POINT REACHED"
Machine data 240* (reference point value) is of no significance to the referencing movement,
but continues to described the reference point for the leadscrew error compensation table
(= position to which no offset is applied).
Description of function
In the case of linear scales with distance coded reference marks which have a reference mark
track and an incremental track running parallel to each other, neither a cam play has to be
evaluated, nor a certain point (reference point) approached when referencing the axes. The
distance between any two reference points is always different. The absolute position of the
axis in the machine system can thus be determined by crossing two reference marks and
measuring the distance between those two marks. The absolute position is sent to the NC: the
actual position has now been determined.
The direction and position from which the reference marks are crossed is of no significance. A
reference point cam is no longer necessary.
In operating mode REFPOINT, the travel direction (referred to the machine system) is
determined by the direction key (+/-) which is pressed on start.
In operating modes AUTOMATIC and MDI AUTOMATIC, a reference point can be taken from a
part program with the G74 command. As the G74 command contains no information regarding
direction, the reference point approach direction defined in machine data MD 564*, bit 0, is
taken (see also FUNCTIONAL DESCRIPTIONS Section).
Reference point approach is executed at creep speed (MD 284*).
If the reference point approach does not produce successful results within double the "Basic
distance between reference marks" (MD 1300*), because, for example, the zero mark tracks
are faulty, the axis is brought to a halt with alarm 132* ("Control loop hardware").
The function "Distance coded reference marks" does not affect the reference point approach
with position encoders already in use.
© Siemens AG 1992 All Rights Reserved 6FC5197- AA50 10–43
SINUMERIK 840C (IA)