Axiomatic Design

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One of the attributes of a good designer is the ability to satisfy the perceived needs with a minimal set of independent functional requirements.

Nam P. Suh, "The Principles of Design"

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The goal of the axiomatic design methodology is to make the design process more rational by using the axiomatic method to prove design principles. Those axioms and the theorems are used to judge the quality of a solution and guide its design.

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The Axiomatic Design methodology was introduced in The Principles of Design by Nam P. Suh and is part of a larger effort to make a science of design.

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The two axioms used are:

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  1. The Independence Axiom: Maintain the independence of functional requirements.
  2. The Information Axiom: Minimize the information content.

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In the axiomatic design methodology, the design problem is represented as a tree of functional requirements whose root is a minimal set of independent functional requirements expressed in the problem domain. A solution of the design problem is defined as a set of design parameters in the solution domain. Designing a solution consists in defining the mapping between the functional requirements and the design parameters.

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A design solution is a mapping from functional requirement (FR) to design parameters (DP).
A design solution is a mapping from functional requirement (FR) to design parameters (DP).

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The first axiom dictates that the design parameters should not couple the requirements.

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The second axiom dictates that the solution should be as simple as possible. What Suh calls "information content" is the complexity of the design.

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It is important to distinguish functional coupling from physical coupling. In order to simplify the design (axiom 2) we might physically couple two parts. But this is acceptable only if we don't generate a functional coupling as well (axiom 1).

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We try to avoid functional coupling so that we can change a design parameter to adapt to a functional requirement without affecting any other requirement. Designing a system when the parts couple the requirements is like trying to solve a game of ball-maze puzzle.

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A coupled system.
A coupled system.

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A design that do not couple any of the requirement is called an uncoupled design. The converse is a coupled design. A coupled design can be decoupled if we can adjust the design parameters in a particular order to maintain functional independence.

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In Notes on the Synthesis of Form, Christopher Alexander presents a similar idea about the independent adaptation of the parts of a system.

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Contrary to Alexander, Suh distinguishes requirements from constraints. Constraints are either provided by the design specification or imposed by the system in which the solution will function. Constraints are different from functional requirements because they don't need to be independent from the functional requirements or other constraints.

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From those 2 axioms, several theorems and corollaries are proved. Those theorems will look familiar to many designers. They are design principles we usually find in the literature.

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The Axiomatic Design method follows a Top-Down Approach to design. You need to start from clear top-level functional requirements. But those requirements can be iterated upon.

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