Design Under Contradictory Requirements

This article presents a design method aimed at addressing contradictory requirements during the conceptual design activities or new product development. Of several methods aimed at developing a “good design” (not necessarily solving a contradiction), a general formal method was proposed in the Axiomatic Design Theory (ADT) by NP Suh [1]. ADT views design as a process that translates a set of functional requirements into a set of design parameters through a design matrix. The goal of the axiomatic design is to force a designer to start from scratch and explore the relationship between functions of the product and its design characteristics. Because the design characteristics in this approach are determined from scratch, the contradictions theoretically will be eliminated at a high level, before the design is developed in more detail. The ADT, however, does not offer specific tools to address contradictory requirements. Hegel’s Logic claims that “there is absolutely nothing whatever in which we cannot and must not point to contradictions“[2]. In this paper, we argue that with the right focus, contradictions can be leveraged to develop a stronger design solution. While contradictory requirements to product characteristics arise in almost every project, most often they are addressed by searching for a useful compromise in a highly iterative procedure. A more efficient approach, presented in this paper, satisfies both sides of a contradictory requirement (at different moments of time, or for different parts of the object, or at different sections of its non-linear characteristic). It is shown that in many cases, the most important step is reframing of the initial problem, which can be done by listing contradictory requirements and indicating to which parts of the object / moments of time/ stages of its life cycle they apply. Once it is done, the solution can often transpire from the reformulated problem statement, or can be generated using a very limited set of separation principles. An additional option, which has not been previously recommended for resolving contradictions, is separation of contradictory requirements in the space of material or object parameters, by selecting non-linear material or device characteristics. For instance, a medical device needs to meet different requirements in different tests: high elasticity (for the kink test) and at the same time high strength (for the burst test). This means that the target material should meet contradictory requirements to a single characteristic, its stress-deformation curve. The contradiction can be resolved using the fact that high elasticity (the kink test) is required at relatively low deformations whereas high strength (the burst test) is required at large deformations. Generally, for selection of a non-linear characteristic, it is proposed to use a morphological table with non-linear characteristics of a material or of similar devices based on different operation principles (such as different I-V curves of a current limiting device). Several case studies dealing with different subject matter applications illustrate the proposed method. The case studies include medical devices (peripherally inserted central catheter, vena cava filter), aerodynamic tractor-trailer devices, current limiting devices. The case studies are based on real life projects that resulted in patented designs.