Siemens Sinamics S110 Function Manual page 353

Within a single-encoder system:
a) a single electrical fault in the encoder
(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. With closed loop control, it is
conceivable that vertical (suspended) drives in particular can move without their motion
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
dragging loads in order to exclude the faults described in a). For example:
 Use of an encoder with analog signal generation
In order to exclude the fault described in b), for example:
 Perform an FMEA regarding encoder shaft breakage (or slip of the encoder shaft
Function Manual
Function Manual, 01/2011, 6SL3097-4AB10-0BP3
WARNING
coupling), and implement a solution to prevent loose encoder housings, integration of a
fault exclusion process - for example according to IEC 61800-5-2
Safety Integrated Functions
8.3 System features
351