This is just an Excerpt from a larger document, click here to view the entire document.Allocate Lower Level Requirements
Allocations provide a means to assign reliability requirements for complex products to lower levels. Product-level requirements are often insufficient to scope the design effort. For example, a requirement that a truck have an MTBF of 1,000 hours doesn't help the designers of the transmission, engine, and other components. How reliable must these components be?
Allocation addresses these questions. The allocation process is often iterative, requiring several attempts to satisfy all requirements. In other cases, the requirements can't be satisfied (to meet the product-level requirement, components are needed with unachievable levels of reliability) and dialogue with the customer and trade-offs are required to resolve the problem. Allocation of product-level reliability requirements to lower levels of indenture makes it easier to manage and track requirements. It enables the tracking of progress toward meeting the product requirements, provides a means of making a sanity check of product-level requirements, and facilitates trade-off studies.
Allocation of product reliability to lower levels of indenture begins as soon as the product-level design requirements have been derived from performance reliability requirements during the C/P phase. An initial allocation should be completed before design begins at each level of indenture, usually before a functional design review during the D/D phase. Updates are normally made before any major design reviews.
For large, complex products, it is difficult for a supplier to depend only on product-level reliability requirements. Productlevel requirements should not be imposed on the designers of each of the different components, subsystems, etc. When outside suppliers are involved, the problem is even more difficult. Some way of assigning a portion of the product-level requirement to designers and to outside suppliers is necessary. Allocation is the method of apportioning requirements.
If only product-level requirements were used, analysis would be the sole means of tracking progress until the entire product was built and tested. Testing of lower indenture items, however, can begin very early in a product development program. By tracking the progress made on each of these items, and then analytically "combining" the results, a good idea of the progress being made toward the product-level requirements can be gained. Problems, and solutions to problems, can be identified earlier than would otherwise be possible.
Even carefully developed product-level requirements may be unachievable. A way to check the realism of product-level requirements is through the allocation process. Of the many methods of allocating reliability, the five most common are: 1) Equal Distribution, 2) Complexity Based, 3) Feasibility of Objectives, 4) Minimization of Effort, and 5) Similarity.
Equal Distribution Method allocates the same value of reliability to each lower indenture item. It is most useful when the components are similar. It is defined as follows.
λia = (1 / n) * λpr
where,
λ
=
Lambda, the failure rate
i
=
Subscript for each lower indenture "item"
a
=
Subscript for "allocated"
N
=
Number of lower indenture items
p
=
Subscript for "product"
r
=
Subscript for "requirement"
It is often appropriate to allocate reliability "with reserve" where a certain fraction of the product requirement is held in "reserve" (is unallocated). By its very nature, this method provides conservative design goals for the lower indenture items. This modification can be used with any of the allocation methods described.
Other complexity based methods similar to the Equal Distribution method exist that attempt to account for the complexity of the product by weighting the allocations. Three common complexity based methods of allocation are the ARINC, AGREE, and parts count methods. Descriptions of the ARINC and parts count methods follow.
ARINC Method Complexity is assumed to be measured by the relative failure rates of the items (i.e., the higher the failure rate, the more complex the item). The method is described by:
λia = λpr (λie / λpe) = 1
/ MTBFia
where,
λ
=
Lambda, the failure rate
i
=
Subscript for each lower indenture "item"
a
=
Subscript for "allocated"
p
=
Subscript for "product"
r
=
Subscript for "requirement"
e
=
Estimated (or predicted)
The component with the highest predicted reliability is allocated the highest requirement, the component with the lowest prediction is allocated the lowest requirement, and so forth. For the example, no component has an estimated reliability equal to, or greater than, the allocated value. The designers can improve the reliability of the components, use redundancy, or select different components to meet the product reliability requirement.
Parts Count Method This method implicitly uses parts count as a measure of complexity. Allocations of reliability, as a failure rate, are made in proportion to the number of parts.
Other Methods More sophisticated allocation methods are described in the literature, including the minimization-of-effort, the feasibility-of-objectives, and the similarity method.
Minimization-of-Effort Method attempts to allocate requirements in a way that minimizes the effort needed to achieve the allocated requirements. Effort is a function of the number of tests, amount of analysis, and number of trades made, etc.
The Feasibility-of-Objectives Method was originally developed for repairable electromechanical products. Allocations to lower indenture items are based on numerical ratings of the design maturity (state of the art), intricacy, mission operating time, and conditions for each item to which the product reliability will be allocated. Ratings are assigned by a lead design engineer based on experience and judgment, or by a group of engineers using the Delphi technique. Ratings for each factor range from a low of 1 to a high of 10. Definitions of the factors and the meanings of the ratings are as follows:
Design Maturity An indication of the level of technology and degree of proven design approaches used in an item. Items with the most highly developed, mature design are assigned a rating of 1, those with the least mature design a 10.
Intricacy An indication of the number of parts and sophistication of the architecture. The least intricate items are assigned a rating of 1, the most intricate a 10.
Mission Operating Time An indication of the percentage of mission time during which an item operates. Items that operate for a small percentage of the mission time are assigned a rating of 1; those that operate continuously are assigned a 10.
Environment An indication of the severity of the environment experienced by the item during product operation. Items that operate in the least severe environment are given a rating of 1; those operating in the most severe are assigned a 10.
Just as the product-level requirement can be based on the achieved reliability of a similar product, allocations can be made based on how previous allocations were made for products with similar architectures.
