Defining and developing a product's diagnostic capability depends on a number of factors such as:
- The product's performance and usage requirements
- Maintenance support requirements (e.g., levels of maintenance)
- Technology available to: improve diagnostics in terms of test effectiveness; reduce the need for test equipment, test manuals, personnel, training, and skill levels; and reduce cost
- The amount of testability designed into the product
- Previously known diagnostic problems on similar systems
Each of these factors will play a role in determining the approach to detecting and isolating faults. A typical approach to diagnostics includes the use of BIT. BIT is a design response to the need to reduce maintenance manpower and external test equipment. Other approaches may consider the use of automatic or semi-automatic test equipment, manual testing using benchtop test equipment, or visual inspection procedures. In all cases,
tradeoffs are required among system performance, cost, and test effectiveness.
It is important to remember that the effectiveness of the diagnostic capability, and the cost of development, is greatly influenced by the amount of testability that has been designed into the system. Should there be a lack of test points available to external test equipment, for example, then the ability to isolate failures to smaller ambiguity group sizes may be adversely affected. The result is higher costs to locate the failure to a single replaceable item. The cost of test development may also increase. BIT design should be supported by the results of a failure modes and effects analysis (FMEA). An FMEA (see 220.127.116.11.3) should be used to define those failures that are critical to system performance, and to identify when the effects of a failure can be detected using BIT. Without such information, BIT tests can be developed based only on the test engineer's knowledge of how the system works, and not on whether a test needs to be developed for a particular fault. Finally BIT must be a part of the product design or the risks and consequences shown in Table VII can ensue. Further information on BIT design can be found in Appendix C.
TABLE VII. Risks and Consequences of Not Making BIT Part of Product Design.
|BIT is designed independently of the product
||BIT fails to support operational and maintenance needs|
|BIT is designed after the fact
||BIT's MTBF is less than that of the product|
|Production personnel are not consulted on BIT
||BIT is not effective in the factory|