Parts Count

Boolean equations can be used to model any system the system s reliability is calculated by factoring the equations into cutsets and substituting the probabilities for component fai lure 1 lus can be done for either success or failure models. Working directly with equations is not e erv one s ctip of tea many individuals prefer graphical to mathematical methods. I hus, symbols and appearance of the methods differ but they must represent the same Boolean equation for them to be eqni valent. 

Perhaps the simplest way to assess the reliability of a system is to count the active parts, flic 1C liability estimate is the product of the number of parts and some nominal failure rate for the parts. Ill the design phase, two competing designs may be compared on the basis of the numbei of parts but several cautions are in order. 

If designs are compared on the basis of count, then failure rates of each type of pint cither must be about the same, or be adjusted for the variations. For example, the parts count of vacuum-tube and solid-state television sets (using discrete components) are approximately the same, but their reliabilities are considerably different because of the better reliability of solid state components. 

We previously encountered failure modes and effects (FMEA) and failure modes effects and criticality analysis (FMECA) as qualitative methods for accident analysis. These tabular methods for reliability analysis may be made quantitative by associating failure rates with the parts in a systems model to estimate the system reliability. FMEA/FMECA may be applied in design or operational phases (ANSI/IEEE Std 352-1975, MIL-STD-1543 and MIL-STD-1629A). Typical headings in the F.Mld. A identify the system and component under analysis, failure modes, the ef fect i f failure, an estimale of how critical apart is, the estimated probability of the failure, mitigaturs and IHissihiy die support systems. The style and contents of a FMEA are flexible and depend upon the. ilitcLiives of the analyst. 

It is interesting that NASA in their review of WASH-1400 Draft (included in W.ASH-1400 Final Appendix II), indicated that they had discontinued the use of fault tree analysis in fas or of the FMEA. 

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I can see that this design is not iikeiy to be capabie, but my new director has said we are to use this design soiution because it has the iowest part count. 

Before setting about the task of developing such a model, the product development process requires definition along with an indication of its key stages, this is so the appropriate tools and techniques can be applied (Booker et al., 1997). In the approach presented here in Figure 5.11, the product development phases are activities generally defined in the automotive industry (Clark and Fujimoto, 1991). QFD Phase 1 is used to understand and quantify the importance of customer needs and requirements, and to support the definition of product and process requirements. The FMEA process is used to explore any potential failure modes, their likely Occurrence, Severity and Detectability. DFA/DFM techniques are used to minimize part count, facilitate ease of assembly and project component manufacturing and assembly costs, and are primarily aimed at cost reduction. 

FMEA is particularly suited for root cause analysis and is quite useful for environmental qualification and aging analysis. It is extensively used in the aerospace and nuclear ]iowei indiistrii-s but seldom used in PSAs, Possibly one reason for this is that FMEA, like parts count. ,s not chrectlv suita lundant systems such as those that occur in nuclear power plants Table i 4... 

The preceding shows the care needed to understand the system logic and redundancies to model a system. Neither the parts count nor FMEA methods aid the analyst in conceptualizing the logical structure... 

Labor cost in a structure is directly related to part count. If part count can be reduced, then labor costs (and inventory costs) wili decrease. Composite structures are generally composed of many fewer parts than are metal structures. Integral part design and fabrication techniques reduce fastener count and bonding operations. Thus, composite structures can have cost elements that are considerably lower than those for metal structures. 

Often, the manufacturing processes involved for composite structures fabrication are greatly simplified as compared to those for metal structures. Reduced part count results in a much lower assembly cost and overall reduction in the factory labor hours. 

Military Handbook 217E (MIL 217E) establishes uniform methods for predicting the reliability of military electronic equipment and systems. There are two methods of reliability predictions, namely parts count and parts stress analysis. 

The parts count method is suitable for early design phase reliability prediction. The method uses information on generic types, quality levels, and environment. The latter two effects are considered with the application of specified factors. The failure rates for both methods are calculated using the same generic expressions. 

I wound a new primary centre-tapped winding with 24 turns in total (12 + 12). I also wound two feedback windings of 8 turns each. I used two BD249C transistors that I turned on and off with the pulses induced to the feedback windings. The device outputs a nice "square-wave" at 2500V AC and about 75Hz. Parts count 8. It still needs some work, but as such I get nice blue plasma on the spark plug gap. 

An example of successful use of SPC comes from the injection molding industry where part counts are high and part values are generally low. The material in reject parts can sometimes be recycled. The process is very rapid, with few control variables and the major, often the only, criteria for quality is reproducible shape. For this situation, SPC is an excellent... 

Advantages Moderate parts count, flat response in the pass band Disadvantages Filter Q greater than that of other Alter types... 

Advantages Adds controlled delay to a given signal with moderate parts count Disadvantages Rounds off signal and places importance on detection device... 

Advantages Moderate part count, pulse delay and reshaper... 

Advantages Moderate parts count Disadvantages Underdamped response... 

Advantages Low parts count, inexpensive, good accuracy, good ripple rejection Disadvantages Excessive power dissipation at higher currents, not as efficient as other topologies (owing to headroom requirements)... 

Advantages Medium parts count, good output voltage line and load regulation, versatile, can provide step-up or step-down voltages Disadvantages No isolation from input to output... 

Advantages Good pulse amplitude range (100 mV to 5 V), runs from single supply Disadvantages High parts count when compared with other solutions... 

Advantage Low parts count Disadvantage Limited accuracy... 

Advantages Low parts count, adjustable range Disadvantages Small range, enlarging range decreases resolution... 

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