Failure Mode Effects Testability Analysis (FMETA) : testability, diagnosability, prognosability from an FMEA

Abstract

Testability analysis should be performed during the design stage of a new product, because Design for Testability (DFT) can require design changes. If left to a later stage, changes may no longer be economical or feasible. Diagnosability and prognostic health management (prognosability) also require early consideration. With that in mind, we need to understand what information we can use early in design to assist in improving these properties. A Failure Mode Effects Analysis (FMEA), while not intended specifically to be a testability analysis document, can be instrumental because it too should be developed early in the design process. It contains information we can use to improve test, testability, diagnosability and prognostic attributes of the design. For example, a failure mode (FM) that results in no observable effect is undetectable in any test and requires that the design be revised to provide an observable effect. Similarly, if two or more failure modes result in the same effect, diagnosability will be limited so design changes should be made to differentiate the root causes. Additionally, sensors may be required in the design to help calculate remaining useful life for prognosability. Information contained in the FMEA can be supplemented with additional data to generate a document we call a Failure Mode Effects Testability Analysis (FMETA). The FMETA highlights diagnosability factors by adding a column to identify the replaceable unit(s) responsible for the failure. This allows calculation of how many replaceable units could cause the failure mode. Finally, for prognostics, we need to identify imminent (but not yet manifested) failure modes. The FMETA also incorporates a Risk Testability Number (RTN) that indicates the severity of each testability issue in the design. Issues with high RTNs are of higher priority to resolve. The FMETA identifies the parameters that we need to assess the design's testability, diagnosability and prognosability.