Tuesday, April 19, 2011
6.6.4 The Design Loop
As the physical design is created many alternatives should be considered. One task is to check that the grouping and sequencing of functions defined during the functional analysis task leads to an effective physical partitioning, i.e. a modular design as described in the previous section. If reasonable physical designs don’t result in subsystems being allocated to single functions or single groups of functions then recheck the grouping of functions to see if alternative grouping lead to cleaner physical partitioning and more modularity. It is important to seek clean partitioning of subsystems and their associated functions because the cleaner the partitioning the easier systems are to integrate and test, maintain and upgrade. A function that is implemented in two or more subsystems results in system designs that are more difficult to maintain and upgrade and are often more difficult to test. Envision the design loop as iteration between functional and physical design until both result in a modular physical architecture.
A second task during design synthesis is conducting trade studies, described in a later chapter, to select between design alternatives. Again when evaluating alternative architectures consider the partitioning for each design alternative and examine the possibility that modifying the functional architecture might lead to a better functional to physical allocation and partitioning.
220.127.116.11 Functional to Physical Allocation Matrices
A simple tool that is helpful in refining the functional and physical architectures is a functional to physical allocation matrix. An example matrix for the toaster functional architecture shown in Figure 6-24 and the design concept architecture shown in Figure 6-31 is shown in Figure 6-33. The functional to physical allocation matrix is particularly helpful in examining the partitioning of a design concept for modularity. Typically the more diagonal this matrix the better the modularity. However, opportunities for one physical entity to perform two or more functions are highly desirable and readily apparent in the matrix. Similarly, when the matrix shows a function spread across several physical entities the matrix provides a visual means of examining if the physical design concept is sound or if it should be changed to allow cleaner partitioning. Sometimes the nature of a function causes it to be spread across several physical entities without complicating the design in ways that cause manufacturing, testing or upgrade problems. For example, in Figure 6-32 the “apply heat” function is allocated to three entities and this is probably a reasonable design approach because it likely reduces the parts count and makes operation simpler.
Figure 6-33 A functional to physical allocation matrix for one candidate toaster design concept.