Tuesday, February 22, 2011
126.96.36.199 Spider Diagrams - Another useful diagram is the spider diagram; named from its shape. Spider charts are useful for tracking progress of requirements that are interrelated so that careful attention is needed to avoid improving one at the expense of others. An “ility” diagram is a good example of a spider chart that tracks progress toward goals or requirements for selected ilities; i.e. reliability, maintainability, reparability, etc. An ility diagram treats the system or product as a single entity. It is constructed by selecting and ranking in importance eight ilities as described by Bart Huthwaite in his book “Strategic Design” cited earlier. Initially measures and goals are established for each ility. Later, as the system design takes form initial estimates or calculations of each ility are made and plotted on the diagram. As system design trades progress and the design is refined the progress toward goals in tracked. By including the most important eight ilities and tracking their measures on a chart the design team has a clear picture of how well the design is progressing and if any ility is being sacrificed for the benefit of some other ility or performance parameter. An example ility chart as it might appear in the middle of system trades is shown in Figure 6-18. Here the eight ilities are numbered in rank order of importance so that if tradeoffs cannot be avoided the most important ilities are favored over less important one. The line connecting the eight legs is the CBE for each ility. Trends can be indicated by keeping the previous three or four CBEs.
Figure 6-18 An example of the form of an “ility” chart that tracks progress toward goals for eight ilities ranked in importance from 1 to 8.
6.3.4 Balancing Customer Needs
A major goal of requirement analysis is to ensure the set of requirements are complete, accurate, and non-redundant. An additional activity during the requirement analysis is to identify opportunities to optimize the customer’s desires and needs. This task is considered part of the Technical Management task shown in Figure 6-4 in a previous post. Working with the customer to adjust requirements often presents an opportunity to save cost or schedule with little impact on the customer. This may also provide significant improvements in a highly desired performance parameter with little impact on a lesser desired parameter. These changes are best identified during requirement analysis. As the program moves into the architecture, then design phase, changes become more costly providing less opportunity for savings. The understanding that a very large percentage of the program cost and schedule are established very early in the program is an important responsibility of the system engineer. Requirements drive the system architecture trades, and once the architecture is defined, the opportunity for savings is a small percentage of the opportunity early in the program. Pugh diagrams (to be described in Chapter 8) and QFD tables (defined in Chapter 7) are valuable tools for assessing these opportunities.