Thursday, June 9, 2011
8.2 Trade Study Methodology
It is a good practice for a systems engineering organization to adopt a standard methodology for conducting trade studies. A standard methodology makes control and management easier and the discipline of following a standard methodology usually results in faster and better trade results and gives management more confidence in the results. Independent of the methodology used trades must be defined and planned. Key top level trades should be identified in the SEMP but not all trades can be listed in the SEMP. Trade Tree diagrams are useful for defining trades if there are only a few alternatives and a few levels under consideration. Trade Tree diagrams are decision trees without the chance values applied to each node. Trade Trees and Decision Trees are useful tools well defined in the NASA Systems Engineering Handbook. If there are many trades to be conducted for a single level then N-squared diagrams are more useful than Trade Trees. The value of the N-squared diagram is that it provides an easy way to determine the best order to conduct trades and conducting the trades in the best order saves time and money. An N-squared diagram for trades is developed following the same process as for defining and ordering tasks using N-squared diagrams as described in Chapter 4.
One standard trade study methodology is present here and it is defined for physical design trades. Experience shows that this methodology is useful for design trades at all levels of a system hierarchy. This methodology combines Design Trade Matrices with Pugh Concept Selection and is best described by the diagram shown in Figure 8-1.
Figure 8-1 An excellent trade study process combines Pugh Concept Selection with a Design Trade Matrix.
Brainstorming is a good way to identify alternatives and many alternatives should be defined. It is important to select people with a diversity of skills and experience for effective brainstorming. Otherwise the alternatives may not have the variety and innovativeness desired. If a large number of alternatives is identified the number of alternatives can be thinned to narrow the trade space. This is accomplished by point design analysis or by using engineering judgment to select a subset of the “best” concepts. The next step is to define selection criteria. These criteria are derived from the requirements. If a process like QFD is used there are a set of requirements that are more important to customers than others. These are often called cardinal requirements or key requirements and they are the ones most likely to be the important criteria that should be used in trade studies.
8.2.1 Pugh Concept Selection - Having defined selection criteria the next step is to refine the concepts. Pugh Concept Selection is the critical step that leads to improved concepts. If large or complex system designs are being traded then first conduct Pugh Concept Selection at a sub system level. Then combine the best results into a top level system design concept.
Pugh Concept Selection is conducted by choosing one alternative concept as the benchmark. Each of the other alternative concepts is then compared to the benchmark. The objective is to use the results of the comparison to suggest new concepts or even new criteria. A matrix diagram is used to compare the alternative concepts to the benchmark as shown in Figure 8-2.
Figure 8-2 A Pugh Concept Selection matrix for three alternative concepts compared to a benchmark concept.
For each criteria each concept is compared to the benchmark concept and a decision is made as to whether the concept is better (+1), worse (-1) or the same (S) as the benchmark. Weighting should not be used at this point in the trades. The score for each concept is obtained by summing the number of pluses and minuses, each counting as a plus one or minus one and the S counting as zero. The next step is to examine the concept that scores the best, assuming that one scores better than the benchmark. For example, in Figure 8-2 concept a scores better than the benchmark and the other two concepts. Note however that for criteria 3 concepts b and c are superior to both the benchmark and concept a. Examine the reasons for concepts b and c being better for criteria 3 and see if changes can be made to either concept a or to the benchmark that will turn concept a’s minus score for criteria 3 to a plus or a same. The idea is to consider new alternatives that combine the best features of the traded concepts to arrive at a better concept. Thus the objective of Pugh Concept Selection is not to select the best of the alternatives but to define new concepts that are better than any of the initial concepts.