After the system has been drawn in block diagram form, subsystem and component reliability goals and targets are established. This is a common practice in the development of complex systems, particularly when different design teams or subcontractors are involved. Reliability allocation involves setting reliability objectives for components or subsystems in order to meet a system reliability objective and should occur in the initial stages of design or prior to designing major system upgrades. The simplest method for allocating reliability is to distribute the reliability objective uniformly among all the subsystems or components. While uniform allocation is easy to calculate, it is generally not the best way to allocate a reliability objective. The “best” allocation of reliability would take into account the cost or relative difficulty of improving the reliability of different subsystems or components.
The RBD model of a system can be used to optimize the allocation of reliability to different components in order to meet a reliability specification while considering the cost of the allocation. This allows the design team to focus their reliability efforts into the least expensive options for increasing the overall system reliability to meet the requirements. The allocation distributes reliability of subsystems and individual components into more critical blocks or bottlenecks in reliability which will help optimize the best combination of component reliability improvements that meet the intended goals and at sufficient allocated costs.
Reliability allocation usually starts from past experience and is first performed at a higher level of subsystems instead of lower levels such as components sourced from different vendors. This level of detail is more appropriate during the first stages of design. It is not efficient to develop a detailed design and then have to redesign and reallocate reliability if the initial allocation is not achievable. The assignment of reliability values between the components can be made based on the complexity, criticality, estimated achievable reliability, or whichever factor the engineering team performing the analysis deems important.
Many benefits exist from RBD analysis. It allows the maximization of costs and design benefits in allocation; it provides a realistic view of subsystem performance required to meet system objectives; it shows the most cost-effective areas for design improvements, and avoids putting design efforts into subsystems that may not gain any additional reliability by improvements. RBD analysis is an essential part of DFR and should be performed early in the design of every system.
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