Reliability design in the US military


14.5.6 Section 7: Reliability Engineering Design Guidelines

Reliability engineering is the technical discipline of estimating, controlling and managing theprobability of failure in devices, equipment and systems. Design principles and tools which should be utilized by the designer include

(1)  Part Selection and Control

(2)  Part Derating

(3)  Reliable Circuit Design

(4)  Redundancy

(5)  Environmental Design

(6)  Human Factors Design

(7)  Failure Modes, Effects and Criticality Analysis (FMECA)

(8)  Fault Tree Analysis (FTA)

(9)  Sneak Circuit Analysis

(10)  Design Reviews

Items (1) and (2) are addressed in Section 7 largely by reference to MIL-HDBK- 338, Volume II.

Discussion of reliable circuit design includes design simplification, use of standard circuits, transient and overstress protection, parameter degradation and analysis, minimizing design errors and fundamental design limitations. Redundancy techniques addressed include simple parallel, bimodal, majority vote and standby, plus examples of redundant systems used in sophisticated aircraft and space vehicles. Appendix A to Section 7 gives multiple examples of these techniques.

Designing for the environment considers measures of protection against high and low temperatures, shock and vibration, moisture, sand and dust, explosion, electromagnetic and nuclear radiation. Table 14.1 demonstrates the relationship among stresses, their effects, andreliability improvement techniques. Appendix B to Section 7 details environmental effects, including air-launched weapon environmental criteria.

Discussion of human factors active in the design of electronic equipment addresses the motor responses and physical capabilities of operators, human performance reliability, the relationship

between human factors and reliability, the three factors affecting human behavior, i.e., stimulus- input (S), internal reaction (0) and output response ®, and man-machine interaction and trade- offs.

Failure Modes, Effects and Criticality Analysis (FMECA) is discussed in detail which includes a step-by-step procedure, demonstration requirements, failure mode distribution, determination ofcriticality, use of computer analysis and its limitations. Note: FMECA is also addressed in Chapter 12.0 of this Primer.

Fault Tree Analysis (FTA) the “top-down” corollary to the FMEA “bottom-up” reliability risk analysis technique is thoroughly investigated. Step-by-step procedures for the performance of an FTA are detailed, including the three basic methods for solving fault trees, i.e., (1) direct simulation (2) Monte Carlo and (3) direct analysis.

A sneak circuit is defined as an unexpected path or logic flow within a system, which, under certain conditions, can initiate an undesired function or inhibit a desired function. Sneak Circuit Analysis (SCA) is the term applied to analytical techniques used to detect and identify sneak circuits in a system. The point is made that unlike other reliability analyses, SCA concentrates onthe interconnections, interrelationships and interactions of system components rather than the components themselves….