Siemens SINAMICS S120 Function Manual page 27

Within a single-encoder system:
a) a single electrical fault in the encoder
b) a break of the encoder shaft (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. Under the aspect of closed-
loop control it is conceivable that particularly vertical (suspended) drives move downward
and that this movement is not 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 Heidenhain 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 at suspended/vertical drives or for
handling dragging loads in order to exclude faults as described in a). For example:
• 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 may not be mounted on the
Safety Integrated
Function Manual, (FHS) , 07/2007 Edition, 6SL3097-2AR00-0BP0
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
2.4 Residual risk
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