Siemens SINUMERIK 840C Installation Instructions Manual page 738

10 Axis and Spindle Installation
10.4.5 Distance-coded reference marks
The absolute offset is the offset between machine zero and the 1st reference mark on the
linear scale; at any one point (any axis position) the absolute offset corresponds to the
difference between the position to be measured in the machine system (e.g. measured with a
laser interferometer) and the current position on the linear scale (1st actual value display when
MD 396* = 0).
The absolute offset (MD 396*) is derived as follows:
• Machine data MD 396* for the absolute offset must be set to 0.
• Reference point approach (manually in REFPOINT mode)
When two reference marks have been crossed, the axis is decelerated to rest.
The absolute position in the reference system on the linear scale (scale system) is now
available as the "actual value".
• By deriving the current axis position relative to the machine system, i.e. the offset relative
to the machine zero or another known accessible point in the machine system (e.g. by
measuring it with a laser interferometer).
• By calculating the absolute offset from the measured positional value in the machine
system and the value shown as the actual value in the scale system (acc. to the formula).
• The calculated absolute offset must be entered in MD 396* and declared as valid with
"Absolute offset valid", MD 1808*, bit 3.
Now the axis can be referenced in the usual way (manually or automatically).
If the absolute offset is not declared valid for the referencing, i.e. when "Absolute offset valid"
(MD 1808*, bit 3) has not been set, the PLC interfacer interface signal, "Reference point
reached" is not set and no part program can be started. The software limit switches are not
enabled and no values are displayed in the reference point display.
10–48
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SINUMERIK 840C (IA)