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Design science is a collection of many different logically connected knowledge and disciplines. Even though no single model can provide a perfect definition of the design process, design models can enable increased understanding and provide explanations for different phenomena in the design process (Braha & Maimon, 1997). Design as problem solving is a natural and the most ubiquitous of human activities. Needs and dissatisfaction with the current state combined with a determination that some action must be taken to solve the problem is the start of a design process. In this view, scientists have been designing and acting as designers throughout their lives however sometimes not being aware of or recognizing that they are performing in a design process. The field of design theory is relatively new (Braha & Maimon, 1997). Three computer-related technological advances have particularly stimulated the field of design theory. These are computer-aided design (CAD), knowledge-based expert systems (KBES), and concurrent engineering (CE). The main source of the slow development and confusion about design theory is, according to Braha & Maimon (1997), that engineering lacks the sufficient scientific foundations. A design method (within a design paradigm) does not constitute a theory. Theory emerges when there is a testable explanation for why the method behaves as it does. Two interesting design theories are (1) the axiomatic theory of design and (2) design as scientific problem solving. The former theory is grounded more in the "real" environment of design while the latter theory is abstract, speculative, or philosophical.
Viewed at a high level of abstraction the design process is a rather generic process. The designer, design team, or designing organization modifies (due to bounded rationality) either the tentative or current design or the requirements and specifications, based on new information that has become available Braha & Reich (2003). This ongoing process of modifications is performed in order to remove discrepancies and eventually establish a fit between the problem space (expressed through requirements and specifications) and the proposed design solution. The design process shows the following characteristics (Braha & Reich, 2003):
Braha & Reich (2003) propose a mathematical framework that can be used to analyze design processes. The basic mechanisms of the model can be described in the following way. In the model, the design process starts from abstract specifications such as customer needs or functional requirements in the function space. These specifications are iteratively refined by moving to a better "proximal" specification list. At some point, the designer is able to match partial structural information, in the structure space, with the current refined specifications. The process then continues in the structure space by refining the partial structural information until a design solution is obtained.
As an entry point to further reasoning regarding different design theories a reference to Kikuchi & Nagasaka (2002) is provided here for convenience. Several other references to different flavors of design theories are discussed and commented by Kikuchi & Nagasaka.
Design methodology is the science of methods that are or can be applied in designing (Roozenburg & Eekels, 1995). There are two important questions in design methodology: (1) what is the essential structure of designing, and (2) how should the design process be approached to make it effective and efficient? The constructing of new methods is therefore an important issue in design methodology. In the construction of new methods there is often a transition from a descriptive statement (an observation that something is found to be this or that) to a prescriptive statement (that is something should be this or that). This transition is in philosophy called an is-ought transition. Transitions of this kind are common in design methodology since they cannot be avoided. However, they should be treated with care and caution because in a formal logical sense this transition is a reversed implication, and drawing a conclusion about the premise based on the observation of the consequence may or may not be true. This is the reason for the focus on verification and validation discussed earlier (see section 3.2) and the step called a leap of faith by Pedersen et al (2000). This is-ought transition can be found in the design methodology framework described in Figure 7 (Blessing et. al. 1998) in the transition from description I to prescription. The second step of observation and analysis in description II in Figure 7 giving feedback to the first description and prescription can provide for the necessary care and caution required for the is-ought transition.
Design methodology aims at providing conceptual tools for designers to organize the design process effectively and efficiently. Among the most important such tools are models of the structure of design and development processes, and methodics, i.e. the body of rules and methods for (part of) the design process; these include not only descriptions of rules and methods, but especially recommendations for their meaningful application, and concepts, that is, the system of concepts and terminology referring to it are a by-product of design methodology, but therefore not less important for the thinking about, and studying of, design processes, and for the communication between different experts contributing to product development.
The structure of reasoning is studied in formal logic. Thus, it should be possible to use formal logic to illustrate the reasoning in design. In reasoning something is done with knowledge. Something is added to knowledge, something is derived from it, or it is applied. Much knowledge can be put into the form of the so-called material implication. The material implication is a compound statement in which two statements are connected with one another by the logical connective "if then ". Atomic statements are statements that cannot be composed from more simple statements. The statements that are combined by the connective "if then " need not necessarily be atomic statements. They may themselves be compound statements as well, even material implications. The material implication can illustrate two basic forms of reasoning: deductive reasoning and reductive reasoning. Deduction and three types of reductive reasoning patterns based on the material implication are shown in Figure 10.
Figure 10: Modes of reasoning (Roozenburg and Eekels, 1995).
According to Roozenburg & Eekels (1995) innoduction is the key mode of reasoning in design. The core of designing is the transition of a functional description of a product to the description of its form. The reasoning from function to form is a form of reasoning in which very little is given and very much is asked. It is an open process that allows for many good solutions. Because innoduction is inherently non-deductive it cannot be grasped in an algorithm. It is here that the creative capacity of the designer plays an indispensable and irreplaceable role. This applies to product design as well as to design as reasoning from ends to means in general.
Even though an area may have a dominating form of reasoning this does not mean that the other forms are not used or are unimportant (Roozenburg & Eekels, 1995). To the contrary, they often serve a supporting role to the characteristic mode of reasoning. In deductive reasoning the pattern of reasoning is from general to particular. Deductive reasoning is characteristic for mathematics and logic. Three forms of reductive reasoning are illustrated in Figure 10. In inductive reasoning the pattern of reasoning is from particular to general. Inductive reasoning is characteristic to natural sciences and social sciences. In abductive reasoning the pattern of reasoning is from particular to particular. Abductive reasoning is characteristic for legal, historical, and medical sciences. In innoductive reasoning the pattern is from general to general. Innoductive reasoning is characteristic to technology and pedagogy.
Potter et al. (2003) provide an additional contribution that discusses using experience driven heuristics to support automated conceptual design and that also includes a discussion of reasoning and knowledge in design activities. Kikuchi & Nagasaka (2003) also provide interesting discussions on the topic of reasoning in design. In their introduction they refer back to Peirce1) who in the late 19th century wrote that "logic is the art of reasoning", and "reasoning is the process by which we attain a belief which we regard as the result of previous knowledge". Peirce continued to state that inference is "the conscious and controlled adoption of a belief as a consequence of other knowledge". Peirce classified inferences in two categories, deductive or analytic inferences and synthetic inferences, and Peirce distinguished synthetic inferences into induction and hypothesis, where hypothesis was later renamed abduction or retroduction. Kikuchi & Nagasaka (2003) further remark that, although each of these inferences should be a subject of logic, mathematical logic in the 20th century was devoted exclusively to deductive inferences. Investigations of non-deductive inferences are inevitable for understanding human reasoning and, actually, they are currently important topics in AI. However, in spite of a great deal of effort, it is not easy to capture such non-deductive inferences.
Function is a key notion for a theory of engineering design (Kikuchi & Nagasaka, 2003b). In their formulation, a function is represented by a pair of the situation of the use and the situation of the outer system. An artifact is also defined by these two situations. With such a formulation the naïve idea that a function is something intrinsic to a device is changed. Kikuchi & Nagasaki state that their proposed formulation of an artifact agrees with Simons view that an artifact is an interface between its inner and outer environment. They further state that it is controversial as to what an artificial thing is and claim that it is almost equivalent to asking what design is. Kikuchi & Nagasaki refer to Simon2) who listed four features of artificial things: (1) artificial things are synthesized (though not always or usually with full forethought) by human beings; (2) artificial things may imitate appearances in natural things while lacking, in one or many respects, the reality of the latter; (3) artificial things can be characterized in terms of functions, goals, and adaptation; and (4) artificial things are often discussed, particularly when they are being designed, in terms of imperatives as well as descriptives. Kikuchi & Nagasaki continue to state that engineers design an artifact to achieve a function they have in their minds. Therefore, function is one of the key notions for the understanding of artificial things. A clear definition of function is further necessary for a formal theory of design. Although the notion of function is of primary concern in designing artifacts, it has been used without rigorous definition in many existing design theories and methodologies.
Some additional perspectives are compiled and described by Andersson (2003) who concludes that the term function is used differently, and that each field has its own specific meaning of the term. However, common to all perspectives is that in the function of a product the reason for its existence is embodied, that is the purpose of the product (Warell, 2001). With a perspective based more upon a mechanical engineering view Andreasen (1980) states that "a (purpose) function is the capability of a machine to create a usable effect". This definition is further refined by Jensen (1999) who states that "function is intended and purposeful behavior, i.e. the sub-set of behavior that subjectively is considered purposeful by a human being". A further perspective on function is given by Rosenman & Gero (1998) where function is a clarification/ concretization of purpose (or "the task of the designer"). In addition, Andreasen makes a distinction between transformation functions (e.g. transform electricity to rotation) and purpose functions (e.g. create rotation).
An interesting extension of function as being not only technical but also interactive is proposed by Warell (2001) in proposing form functionality. Interactive functions designate those functions that are associated with the interaction between product and user (i.e. functions that are there to enhance the usability and attractiveness of the product). Warell further classifies these functions as ergonomic (i.e. functions aiding physical and cognitive ergonomics) and communicative (i.e. semantic or syntactic functions).
Figure 11: Classes of product functions (Andersson, 2003).
With reflections on different perspectives on product functions Andersson (2003) proposes that a function can be classified using a taxonomy composed of six general function classes (see Figure 11). In the taxonomy active functions describe the activities required by the technical system to deliver the effects necessary to bring about a transformation. Passive functions refer to functions that are static in nature (i.e. they do not bring about a transformation). Internal functions designate functions associated with internal actions of the product, while external functions are associated with interaction between the technical system and its surrounding systems (e.g. humans and lifecycle systems). Furthermore, functions can be seen as primary (i.e. they are designated to the purpose of the design) or secondary (i.e. they assist in the realization of the primary functions).
An approach to the concept of function that is related more closely to the design artifact itself, but is still to some extent in agreement with the discussion and definitions provided in Kikuchi & Nagasaka (2003b) as well as the compiled views by Andersson (2003), is provided by Roozenburg & Eekels. Roozenburg & Eekels (1995) view the function of a product as the intended and deliberately caused ability of its design to bring about a transformation of some part of the products environment. To realize a goal (a desired state) something must be changed in our environment. The natural process of change that affects this environment (including ourselves) should be adjusted by the product in a desired direction. Some process should run differently than it would without the product. Unlike statements about properties, statements about functions are normative. A product has certain properties or does not have them, irrespective of the purposes of a user. Functions, however, are imposed on products they must be fulfilled, otherwise the intended goal will not be reached. Function is a general concept. It refers to the purpose of a product, which is usually many-sided. We can therefore talk about the technical; the ergonomic, the aesthetic; the semantic; the business economic; the social; and other functions of a product. The detailed description of the function of a product in all its aspects leads to the design specification this is the list of all properties that the product should possess to achieve its purpose.
Roozenburg & Eekels (1995) defined how function is achieved in a product using material implications from logic.
Each product has many extensive properties, and each extensive property, or group of properties, represents a possibility to function. However, a product must also be used in a certain manner. Usually, we only observe and notice a few of the many properties that any product possesses. Properties only become visible when we do something with the product. Properties are hypothetical statements, and even if such a statement is true, the consequence only becomes evident when we actualize the antecedent. To do so, we actually have to bring the object into certain conditions. A product with the required properties therefore functions in the intended manner only if it is used in an environment and in a way that the designer has thought up and prescribed. The instructions for use are not given facts for the designer, like the function, but are thought up together with the form of the product and thus form an essential part of the design.
Figure 12 provides an overview of Roozenburg & Eekels (1995) view on product function. A product is a material system that is made by people for its properties. Because of its properties it can fulfill one or more functions. By fulfilling functions a product satisfies needs, and this gives people (e.g. the customer) the possibility to realize one or more values (see Figure 12). In general, however, the development of a new product proceeds in the opposite direction. The more to the right we start in Figure 12, the more open-ended the product development will be. The product development process can be divided into two parts product planning and strict development. In product planning, reasoning back from the goals (values) to statements on functions that are worth fulfilling forms the kernel of this part of the product development process. The actual designing of products takes place in the strict development phase. The kernel of the strict development phase is reasoning back from statements on functions to statements on the form of the product.
Figure 12: Product functioning (Roozenburg & Eekels, 1995).
In manufacturing processes an in-going material is transformed into an out-going material. In doing so, energy and information are transformed. What is usually immediately evident is that the geometrical form of the in-going material changes in the manufacturing process, for example when machining a part. There are, however, also manufacturing processes that change the physico-chemical form (the properties of the material) of an object. In manufacturing, changes of a geometrical and physico-chemical form always go together, but these changes are usually not both wanted at the same time. The design of the product is the geometrical and physico-chemical form that a product must have after the manufacturing process.
Because of its form, a product has certain properties, such as mass, strength, hardness, color, etc. Although we usually describe properties categorically, we actually claim that some corresponding hypothetical statements are true for the object concerned. For example, if we categorically state that the stiffness of this construction is so and so great, we thus claim that the following hypothetical statement about that construction is true: if this construction is loaded in manner y, then it will be deformed in manner z. The first part of the hypothetical statement is called the antecedent and the second part the consequence. An object has as many properties as true hypothetical statements can be made about it, and these are quite a lot. Each property tells us something about the reaction the object will show if we bring it into a certain environment and use it in a certain way. The total of all properties describes the behavior to be expected under certain conditions.
Intensive properties depend on the physico-chemical form only, such as specific gravity. Extensive properties, or thing properties, are a result of intensive properties plus the geometrical form, for example, the mass of an object. The designer is especially interested in the extensive properties, as these directly determine the functioning of the product. By choosing a certain material, the designer immediately sets many intensive properties for a product, both good and less desirable. The art of designing is to give the product such a form that it has the desired extensive properties, given the intensive ones.
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