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maersk
Member 3
Joined: 17 Aug 2014
Posts: 891
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 Posted: 06/06/2016 16:54 Post subject: Monitoring the feedback circuit in the safety program |
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Monitoring the feedback circuit in the safety program
https://support.industry.siemens.com/cs/document/21331098/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.
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.
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.
https://support.industry.siemens.com/cs/attachments/21331098/21331098_Feedback_DOC_V30_en.pdf
https://support.industry.siemens.com/cs/attachments/21331098/21331098_Feedback_PROJ_V30.zip
https://support.industry.siemens.com/cs/attachments/21331098/21331098_Feedback_SET_V10.zip |
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