Tuesday, April 12, 2011
6.6.2 Decision Management during Design
The degrees of freedom of a design are greatest and the cost to make a design change is lowest during concept design. This is illustrated schematically in Figure 6- 32. The high degrees of design freedom means that design alternatives are relatively unconstrained as long as they map to the functional architecture and meet the functional requirements. In general, the greater the design degrees of freedom the greater the potential for influencing performance, life cycle cost and other important measures of design quality. Decisions made on top level architecture during concept design not only directly reduce the degrees of freedom but these decisions often constrain the design alternatives available at lower levels of the system hierarchy addressed in preliminary and detailed design. This argues strongly for conducting the most extensive exploration of design alternatives during concept design.
Figure 6-32 Design alternatives cost less to explore and have a greater potential influence during concept design.
The objective is to sufficiently explore alternative concepts that high confidence is achieved that the selected design concept is “best” from a number of measures. These measures include the obvious of high performance on high priority customer requirements, low life cycle cost, excellent “ility” measures (manufacturability, testability, reparability, etc.), and perhaps attractive features that might increase sales. Thus there is a tension between the need to make design decisions quickly and to explore a wide range of design alternatives. Once the desired concept design is established, i.e. a baseline design is defined and trade studies are conducted to select the best alternative for the final baseline, then the design freedom is reduced so the opportunities to significantly improve the design are also reduced.
A standard approach to achieving the desired characteristics in concept designs is to seek modular designs. Here the term module refers to design elements, i.e. subsystems, assemblies etc. Modular designs are achieved by refining the allocation of functions to physical modules and partitioning functions between the modules.
6.6.3 Partition for Modular Designs
The DoD SEF says modular designs have the three desirable attributes of low coupling, high cohesion, and low connectivity. Coupling is the amount of information shared between modules; the lower the amount of information that must flow between modules the more independent they are. Having low dependence lowers design risk and makes future upgrades or modifications easier. Cohesion is the similarity of tasks performed within a module. High cohesion leads to easier and less complex designs. A design for which a single component performs multiple functions has high cohesion. Connectivity is a measure of the internal interfaces between modules. A design that has multiple interconnections between the internal parts of one module and those of a neighboring module has undesirable high connectivity, which again complicates design, integration and testing as well as future upgrades.
Note that modularity is a measure of system complexity; the higher the modularity the lower the complexity. Risk is a measure of the complexity of the development program; the higher the risk or the more risks that a program has the more complex the development becomes due to the work necessary to mitigate risk. Modularity and risk are related. A system concept design with low modularity is usually higher risk than a design with high modularity. Therefore to achieve the lowest program risk design concepts should be traded to find the highest modularity. However, risk must be evaluated for each concept to ensure that in striving for higher modularity unnecessary risks haven’t been introduced.