Читать книгу Design for Excellence in Electronics Manufacturing - Cheryl Tulkoff - Страница 48

Block diagrams are useful for performing system‐reliability models and calculations and may be simple, parallel, or redundant. A series reliability system model is used when one failure of one component results in the failure of the system. Let's calculate the reliability of a simple fuel system, Rfs, as shown in Figure 2.2.

 Rfs(t) = Rfp(t) Rfl(t) Rfi(t) Recu(t) Rfw(t)

 Rfs(t) = 0.980 0.998 0.985 0.975 0.964 = 0.905 90.5% reliability

 F(t) = 1 Rfs(t) = 1 .905 = .0945 9.45% failure

For every 100,000 vehicles built, be prepared for 100,000 0.0945 = 9453 fuel system repairs by time (t).

Parallel or redundant reliability system models are used where all the items in a parallel branch must fail in order for the system to fail. They can model backup systems used to maintain system availability of critical functions in case the primary system fails. A repair of the failed unit is still required, but critical functionality is maintained: for example, separate brake channels where the loss of one brake circuit degrades brake performance, but reduced braking capability remains. Let's calculate the reliability of a parallel brake system, as shown in Figure 2.3.

Figure 2.2 Block diagram for a simple fuel system.

Figure 2.3 Parallel brake system.

 Rbt = 1 [1 Rbc1(t)] [1 Rbc2(t)]

 = 1 [ 1 0.990] [ 1 0.990]

 = 1 [ 0.010] [ 0.010]

 = 1 0.0001 = 0.9999 or 99.99%,

 then Fbc = 1 [1 0.990] = 0.02 and Fbt =.0001

For one failure out of two, the calculation is . For two failures out of two, the calculation is . So, the probability of a single brake circuit failure is , or 2000 incidents for every 100,000 vehicles built. The probability of both brake circuits failing is 0.0001, or 10 incidents for every 100,000 vehicles built: i.e. 200 times less likely to fail due to the dual design.