Monitoring the feedback circuit in the safety program
Monitoring the actuators represents a diagnostic function and significantly contributes to the SILCL (SIL claim limit) or PL of the corresponding subsystem. For electromechanical components (e. g. relays or contactors), a positively driven auxiliary contact often is fed back to the controller and then evaluated. This process is referred to as monitoring of the feedback circuit or readback of the contactors.
Figure 1 Typical wiring of an actuator and its feedback circuit
In this application example, two machine parts are switched separately in order to illustrate the monitoring of the feedback circuit. Only the affected machine part shall be switched off via the local emergency-stop control devices. By means of the global emergency-stop control device, both machine parts are switched off safely.
Figure 2 Overview of the main components
Both contactors of a machine part are controlled in parallel via a failsafe output of the ET 200SP. The auxiliary contacts of both contactors of a machine part are connected in series and fed back to a DI of the ET 200SP. In the safety program, the signal of the feedback circuit is compared to the control signal of the contactors.
In addition the documentation shows a solution with coupling relays.
Documentation and example project
Documentation (1,2 MB)
Example project for TIA Portal V15.1 (7,1 MB)
Evaluation of the safety function (13,9 KB)
For further details on monitoring emergency-stop control devices, please refer to 21064024
Migration of Safety Evaluation Tool to Safety Evaluation with TIA Selection Tool
Readback, feed back, mirror contact
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