Siemens SINAMICS S120 Function Manual page 51

Two-encoder system
The failsafe actual values for a drive are provided by two separate encoders. The safety-
relevant actual values are generated either directly in the encoders or in the Sensor Modules
and are transferred without retroaction to the Control Unit by way of failsafe communication
via DRIVE-CLiQ. Each measuring system requires a separate connection or a separate
Sensor Module.
Figure 4-7
Figure 4-8
Safety Integrated
Function Manual, (FHS) , 07/2007 Edition, 6SL3097-2AR00-0BP0
Example of a two-encoder system on a linear axis via ball bearing spindle
Example of a two-encoder system on a rotary axis
Extended Functions
4.8 Safe actual value acquisition
51