How Should We Select Functional Requirements?

Abstract

The preceding chapters provided broad outlines of the design of various systems based on Axiomatic Design (AD). Two axioms were the basis of making design decisions: the Independence Axiom and the Information Axiom. The process of AD consists of the transformation of the problem identified in the “customer domain” into a set of functional requirements (FRs) in the “functional domain,” which was in turn transformed as design parameters (DPs) in the “physical domain” that are chosen to satisfy FRs. DPs were, in turn, transformed into process variables (PVs) in the “process domain” to fulfill DPs. The mapping process was illustrated.

Once the highest FRs and DPs are finalized at the highest level of the system design hierarchy, we may have to decompose FRs and DPs, if the selected FRs and DPs lack sufficient details to complete the design. This decomposition process must continue until the design has enough details that can be implemented. To decompose, we should zigzag between FR and DP domains. Similarly, DP versus PV can be decomposed through zigzagging.

The resulting designs were classified as uncoupled, decoupled, and coupled designs. Coupled designs violate the Independence Axiom and, thus, should not be implemented. They are unreliable and require monitoring, resulting in the waste of resources, cost overruns, and delays in implementation. When a system design is uncoupled, it can readily be satisfied because each FR is a function of only one DP, irrespective of the total number of FRs and DPs that the system has to satisfy. The design matrix is constructed to identify coupling. Throughout these chapters, “real-life” examples were presented.

The quality of the design is determined by how well the problem is identified and how FRs are selected. Although every step of the transformation is essential, the selection of FRs ultimately determines the functional quality of the design output. It is helpful to have an in-depth understanding of basic sciences, engineering, and other relevant fields to be proficient in system design. It may also take different kinds of information, depending on the field. In designing commercial products, market information on the current state and prospect is critical in selecting the right set of FRs. In other fields, such as the economy, the set of information required to choose the right set of FRs would be entirely different. However, the basic system structure is similar. The domain-specific knowledge is field-specific.

In this chapter, the process of selecting FRs is elaborated further. One of the essential concepts to remember in selecting FRs is the idea of choosing them in a “solution-neutral environment,” that is, the designer should not think of a design solution first and then define FRs for the assumed solution. Given the critical importance of selecting the right set of FRs for the problem identified, the designer should have a broad knowledge base and also access to extensive “database for scientific facts and various technologies,” including the commercial database for existing products.