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Entry type: Application example Entry ID: 21331098, Entry date: 08/15/2019

Monitoring the feedback circuit in the safety program

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A machine executing dangerous movements is controlled via a fail-safe controller and switched by means of contactors. The correct functioning of the contactors shall be monitored in order to ensure a high diagnostic coverage and, thus, a high SIL or PL.

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

This is particularly required for a redundant setup. If one of the two contactors welds (without this being noticed), the two-channel system would become a single-channel system. Instead, the welding will be detected and it will be prevented that the system is switched on again until the error is eliminated.

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.

Documentation and example project
  Documentation (1,0 MB)
 Registrierung notwendig  Example project for TIA Portal V15.1 (7,1 MB)
 Registrierung notwendig  Evaluation of the safety function as SET project (34,3 KB)

Further information
For further details on monitoring emergency-stop control devices, please refer to 21064024

Last changes
Update TIA Portal V15.1
Solution with coupling relays

Additional keywords
Readback, feed back, mirror contact

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