“The important thing is not to stop questioning.” Albert Einstein
I always encourage readers to ask questions, and include a link in my article to make it easy. Asking questions is a great way to learn.
A reader recently asked, “When performing an FMEA, should the effect consider the mitigated case or unmitigated case? In this article we will look at different aspects that can be considered in order to answer the question.
Begin with an example
I’ll use an example from SAE J1739:2021 to illustrate the areas for consideration. In this example, a Design FMEA is being performed on a Windshield Wiper Motor. The FMEA team is addressing a failure mode of “Insufficient power to overcome static friction in system.” The Effect is “Wiper arm stops . . . with potential for loss of wiping and non-compliance with safety regulations. If the Effect happens, it has a Severity of 9 on a 1-10 scale, which of course is very serious. However, the wiper system design includes a feature which can transition the wiper system to intermittent operation, if the motor current exceeds a threshold. This response has the potential to mitigate the severity of the Effect to performance degradation rather than safety or regulatory.
Consideration 1
Where is the FMEA item on the system hierarchy, and what is the FMEA scope?
In the example from SAE J1739, the DFMEA is being done at the level of Wiper Motor, which is part of the Wiper System. The question becomes, does the scope of the FMEA include the parameters that are being monitored by a diagnostic feature that can mitigate the severity of the effect? In the wiper motor example, the scope of the FMEA includes the motor grounding, the cause is “insufficient motor current due to weak ground path due to insufficient tightening torque specified.” The diagnostic monitoring is the motor current. Therefore the scope of the FMEA includes the parameter (motor current) that is being monitored.
Consideration 2
Is a supplement to Design FMEA called FMEA-MSR needed?
The second consideration determines if FMEA-MSR (Monitoring and System Response) should be used as a supplement to the Design FMEA. FMEA-MSR will determine if the diagnostic feature is sufficiently robust to warrant a reduction in severity. Note, the procedure for FMEA-MSR is covered on SAE J1729:2021 and will also be the focus of a future Inside FMEA article.
For the purpose of this discussion, I’ll summarize three conditions that determine if FMEA-MSR is needed.
1. Does the item under consideration include potential Effect(s) of Failure in the DFMEA that may be harmful to persons, involve regulatory noncompliance, or disable the system?
2. Does the system architecture include direct or logical sensing, decision-making logic, and actuation to be able to monitor activity during system use, and respond to fault behavior?
3. Does the system have diagnostic capability to monitor and respond to the Cause or Failure Mode during system operation for the item being analyzed?
If the answer to all three questions is positive, FMEA-MSR can be used to help determine if mitigation warrants a reduction in severity.
Note, the full example of mitigated vs unmitigated effect in the wiper motor example is in SAE J1739:2021, including FMEA-MSR excerpts and explanations.
Consideration 3
If FMEA-MSR is not used, what then?
Before there was a technique called FMEA-MSR, FMEAs could (and still can) be used to analyze a system that includes monitoring and response. In the case of the wiper system that includes a diagnostic feature to mitigate wiper arm stoppage, FMEA can be done at the wiper system level. By performing FMEA at the system level, the scope of the FMEA can include both the wiper motor and the diagnostics. Therefore, the FMEA team can consider various failures of the wiper system to clean the windshield and the resulting consequence (if the diagnostic system works according to requirements). An FMEA can also be done on the diagnostic system to ensure it is working properly.
You can also supplement the System FMEA with a Fault Tree Analysis (FTA). FTA can analyze an unwanted event (such as unclean windshield) to determine causes that can occur in tandem. This can help determine the likelihood of an unclean windshield resulting from tandem failures (for example, motor overheating along with a failure of the diagnostic feature). In the event of tandem failures, the severity is not mitigated.
Readers who are interested in the question of mitigated vs unmitigated effects, are encouraged to study SAE J1739:2021 to learn more about FMEA-MSR, and when it is appropriate to warrant reduction in severity rating.
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