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Failure Modes and Effects Analysis (FMEA)

In simple terms, the FMEA process attempts to list every failure mode of each component of a system, and to predict its effect on system operation. Failure effects can be considered at more than one level (e.g., effects at the subsystem or overall system levels).

FMEA can be accomplished using either a component or functional approach. In the component approach, actual failure modes are listed (e.g., resistor open, bearing seizure). The functional approach is used when the details of the design are not yet fully defined. In this approach, function failures are considered (e.g., no feedback, memory lost). FMEA can also be performed using a combination of component and functional approaches. The failure mode is the symptom of the failure, as distinct from the cause of the failure, which consists of the proved reasons for the existence of the symptoms. Reliability aspects of the components must be considered in this process. FMEA requires inputs from hardware, software, systems, customer service, and manufacturing in assessing component failures, effects at higher levels of the system, fault detection, and in evaluating failure compensating provisions in the design.

Procedure          

FMEA should be performed by a team of people having broad knowledge of the system's design and application. All available information on the design should be obtained: external and internal reference specifications, schematics, computer-aided design (CAD) data, stress analysis, reliability prediction data, test results, etc.

A system functional block diagram and reliability block diagram (Figure 1) should be prepared, as these are important for preparing the FMEA, and for understanding the completed analysis. All possible operating modes of the system and their functional relationships should be considered in the analysis. If the system has redundancy, it should also be considered by evaluating the effects of failure modes assuming the redundant system may or may not function.

Figure 1: Reliability block diagram examples.

FMEA can be performed from different points of view, such as failure detectability, safety, repair cost, availability, etc.  The customer's point of view should always be first, in order to improve customer satisfaction with the final product. The viewpoint being considered should be consistent throughout a particular analysis, in order to assign proper criticality values. A hierarchical approach works well with systems, using a component approach at the lowest assembly level, and a functional approach to combine the effects of various subsystems. This also allows the FMEA to be performed even when some subsystems are not yet completely designed.

If CAD has been used on parts of the system, the FMEA can utilize that capability to simulate the effects of various failure modes. Such an analysis can greatly enhance the accuracy and objectivity of the FMEA process.

FMEA Process

FMEA is an ongoing process which should be started as soon as initial design information is available. It should be updated and enhanced as the design evolves, so that the analysis can be used to improve the design. All possible design alternatives should be analyzed separately, so that the effect on system performance and reliability can be considered in deciding which option to implement. Test results should be used later to update the analysis.

Requirements to Perform FMEA

• A team of people with a commitment to improve the ability of the design to meet the customer's needs
• Schematics and block diagrams of each level of the system, from subassemblies to complete system
• Component specifications, parts lists and design data
• Functional specifications of modules, subassemblies, etc.
• Design manuals 
• Manufacturing requirements and details of the processes to be used
• FMEA forms and a list of any special considerations, such as safety or regulatory, that are applicable to this product

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