Siemens SINUMERIK 840D sl Function Manual page 561

Note
Note regarding Point 3.
The error at a reference point approach is no more than one revolution of the motor (difference
between two zero marks). This offset is usually not critical for the mechanical parts of the
machine. If problems arise with the traversing limits because of the type of reference point
approach, then for example, set the offset values in MD34080/34090 to non-critical values.
Alarms 27000/C01797 "Axis not safely referenced" are still active and indicate that the
functions "safe limit positions" and "Safe Cams" are not active in this state. For example, if the
"safe limit positions" are to be used as a substitute for hardware limit switches, then they are
not functioning at this time!
After completion of the reference point approach, the axis goes into the "referenced" status.
However, because of the zero mark offset between the encoders, the reference position still
has to be calibrated. This means that the position reference with respect to the mechanical
system must be re-established. The system is calibrated after measuring the difference -
usually in MD34080 REFP_MOVE_DIST or MD34090 REFP_MOVE_DIST_CORR.
Comments about 2-encoder systems
Case A
1st measuring system: Incremental motor measuring system
2. Measuring system: Absolute direct measuring system
As active measuring system via the axis interface, the 2nd position measuring system (DBAx
1.5 = 0, DBAx 1.6 =1) is statically selected
In this case, motor replacement is straightforward because the NC reference point position is
only supplied with values from the 2nd measuring system (DMS). This means that in this case,
it is not necessary to recalibrate the motor measuring system.
Case B
1st measuring system: Absolute motor measuring system
2. Measuring system: Incremental, optical measuring system
As active measuring system via the axis interface, when booting, for monitoring purposes the
1st position measuring system (DBAx1.5 = 1, DBX 1.6 =0) is selected and then subsequently
the 2nd position measuring system is selected (DBAx 1.5 = 0, DBX 1.6 =1).
In this case, the motor must be replaced carefully observing the Description, motor with
absolute value encoder. This is because it is necessary to recalibrate the absolute value
encoder. When re-calibrating the system, we recommend that you permanently select the 1st
position measuring system and the axis is only traversed using the motor measuring system.
Replacing the encoder modules
When replacing the encoder modules (SMC, SME, DRIVE-CLiQ encoders) or when replacing
motors with integrated encoders (motor with DRIVE-CLiQ), a change to the configuration of
the safety-related components is detected, and a request is made that a service person
acknowledges this.
Safety Integrated
Function Manual, 12/2017, 6FC5397-4BP40-6BA1
Commissioning
10.7 Replacing a motor or encoder
561