Siemens SINAMICS S120 Function Manual page 51

Within a single-encoder system:
a) a single electrical fault in the encoder or
b) an encoder shaft breakage (or loose encoder shaft coupling), or a loose encoder housing
will cause a static state of the encoder signals (that is, they no longer follow a movement
while still returning a correct level), and prevent fault detection while the drive is in stop
state (for example, drive in SOS state).
Generally, the drive is held by the active closed-loop control. Especially for drives with
suspended load, from a closed-loop control perspective, it is conceivable that drives such
as these move without this being detected.
The risk of an electrical fault in the encoder as described under a) is only given for few
encoder types with specific function principle (for example, encoders with microprocessor
controlled signal generation such as the Heidenheim EQI, Hübner HEAG 159/160, or AMO
measuring systems with sin/cos signals).
The risk analysis of the machine manufacturer must include all of the faults described
above. Additional safety measures have to be taken for drives with suspended/vertical or
pulling loads - e.g. in order to exclude faults under a):
• Use of an encoder with analog signal generation
• Use of a two-encoder system
In order to exclude the fault described in b), for example:
• An FMEA regarding encoder shaft breakage (or slip of the encoder shaft coupling), and
• Implementation of a two-encoder system (the encoders must not be mounted on the
Safety Integrated
Function Manual, (FHS), 11/2009, 6SL3097-4AR00-0BP0
WARNING
a solution to prevent loose encoder housings, integration of a fault exclusion process to
CDV IEC 61800-5-2, or
same shaft).
System features
3.5 Residual risk
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