301 Review Paper A survey of design philosophies, models, methods and systems ....; F 0 Evbuomwan, * BEng, MSc, PhD, DIC, ...
:    302                                          N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB       The foregoing definit...
A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS                                      303 thought, as they mov...
304                                         N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB    might be conflicting, and the ...
A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS                                     305tory to practical usef...
306                                         N F 0 EVBUOMWAN, S SIVALOGANA THAN AND A JEBB     prepared for each of the sol...
A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS                                                       307    ...
308                                              N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB                             ...
A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS                                                              ...
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312                                         N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB                                  ...
A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS                                          313       and (d) ch...
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
A survey of design philosophies, models, methods and systems
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A survey of design philosophies, models, methods and systems

  1. 1. 301 Review Paper A survey of design philosophies, models, methods and systems ....; F 0 Evbuomwan, * BEng, MSc, PhD, DIC, CEng, MIStructE, S Sivaloganathan, BSc, MSc, PhD, CEng, FIMechE, MIEE, !IE(Sri Lanka) and A Jebb, BSc, PhD, DIC, CEng, MIMechE, MIEE · Engineering Design Centre, City University, London The study of the design process, design theory and methodology has been a preoccupation of engineers, designers and researchers over 1he fast four to jive decades. As the end of this millenium is approached and with the renewed inrerest around the world in engineeringdesign. it is fitting to examine the state of the art and currenr status of issues relating to design philosophies, theory and methodology.(Jeer the last 40 years, many approaches to design have been put forward by various researchers, designers and engineers, both in, 1cademia and industry, on how design ought to and might be carried out. These proposals on design have tended towards what has, orne to be regarded as design philosophies, design models and design methods. The thesis of !his paper is to discuss various aspects of,1eneric research in design, within the above classifications in the light of the work that has been done in the last four decades.Discussions will focus on various definitions of design, design theory and methodology, the nature and variety of design problems, designclassifications, philosophies, models, methods and systems.f ev words: design philosophies, design theory, design methodology, design models, design methods, design systems, design process,rrciduct design, computer aided design 1 INTRODUCTION theory and methodology. Further discussions will be on The design activity, although performed for many cen- the nature and features of the design process, the nature turies, did not, however, have any structure or organiz- and stages of thought in design, types of design prob-;.ttion to it. It was only just after the middle of this lems, product design classification and design goals.century that efforts began to give some formalism to theway design was done. What is design? Why is it done? How is it or can it be done? These questions have been 2.1 Definitions of designthe subject of discussions at various conferences on Several designers, engineers and researchers, fromengineering design and design methodology. In these observation and experience, have expressed their viewsconferences, which were held in the United Kingdom on the definition of design or what they consider designil-5), Europe (~10) and North America (11-18), a to be. Some of these viewpoints are expressed below:number of ideas were put forward on design method-ology. These ideas were mostly associated with design Feilden (19): Engineering Design is the use of scientificmodels, philosophies and methods or techniques as well principles, technical information and imagination inas applications, and they represented several schools of the definition of a mechanical structure, machine orthought on design and design methodologies. More system to perform prespecified functions with therecently, other researchers have started to report on maximum economy and efficiency.computer-based design systems . Finkelstein and Finkelstein (20): Design is the creative . The main focus of this paper is to give a detailed elu- process which starts from a requirement and defines acidatiOn of design philosophies, models, methods and contrivance or system and the methods of its realis-systems that have been proposed and developed over ation or implementation, so as to satisfy the require-the years. Discussions will centre on: definitions of ment. It is a primary human activity and is central todesign and design methodologies, the nature and fea- engineering and the applied arts.tures of design problems and the design process, as well Luckman (21): Design is a mans first step towards the~s the stages of thought in design and product classi- mastering of his environment . . . The process oftJcati?n. The nature and control of design goals will also design is the translation of information in the form ofbe discussed, including an extensive review of many requirements, constraints, and experience into poten-design models, methods and systems. This paper focuses tial solutions which are considered by the designer toon completely general aspects of engineering design meet required performance characteristics ... someresearch, and it should be noted that there is a large creativity or originality must enter into the processamount of other work in this area. for it to be called design. Archer (22): ·. . . design involves a prescription or 2 DESIGN, DESIGN THEORY AND DESIGN model, the intention of embodiment as hardware, and METHODOLOGY the presence of a creative step.The discussions in this section focus on definitions of Caldecote (23): . . . the basic design function ... todesign, as well as definitions and viewpoints on design design a product which will meet the specification, to design it so that it will last and be both reliable and; ~e liS was received on 3 June 1995 and was accepted for publication on easy to maintain, to design it so that it can be eco-· nrember /995.: Prnenr address: Deparrmenr of Civil Engineering, University of Newcastle. nomically manufactured and will be pleasing to the· •{ a.stle upon Tyne. eye. Proc Instn Mech Engrs Vol 210
  2. 2. : 302 N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB The foregoing definitions of design reflect the various refinements) are proposed to the current design in viewpoints of the proponents. In general, certain key- order to move to a better design. words and phrases can be noted which have a strong 3. Design as an exploratory activity (29) involves an bearing on design. These include: needs, requirements, exploration-based model of design and describes the solutions, specifications, creativity, constraints, scientific design process as a knowledge-based exploration principles, technical information, functions, mapping, task. transformation, manufacture and economics. The word 4. Design as an investigative (research) process involves customer, although absent, seemed to be implicitly rep- an enquiry into the clients needs and expectations, resented by the words needs, requirements or available design techniques, previous similar design market. Taking account of these key words, design can solutions, past failures and successes, etc. be described as: 5. Design as a creative process (art) involves creating a design solution with the help of know-how, ingenu- The process of establishing requirements based on human ity, good memory, pattern recognition abilities, needs, transforming them into performance specification and functions, which are then mapped and converted random search in the solution space, lateral thinking, (subject to constraints) into design solutions (using cre- brainstorming, analogies, etc. ativity, scientific principles and technical knowledge) that 6. Design as a rational process (logic based) relates to can be economically manufactured and produced. checking and testing of proposed solutions, involving logical reasoning, mathematical analysis, computer simulation, laboratory experiments and field trials, 2.2 Viewpoints on design theory and methodology etc. The subjects of design theory and design methodology, 7. Design as a decision-making process (value based). although well discussed by researchers, have not been In the design process, designers usually make a lot of fully explicated. Some definitions have, however, been value judgements in adopting alternative courses of given to them by various designers and researchers, and action or choosing between competing design solu- are reported below. The American Society of Mechani- tions. Such judgements and evaluation are usually cal Engineers (ASME) (24) defines the field of design based on experience and criteria derived from the theory and methodology as . . . an engineering disci- customers or clients requirements. pline concerned with process understanding and 8. Design as an iterative process. The iterative activity organised procedures for creating, restructuring and is the most common process in design. Proposed pre- optimising artifacts and systems. Design theory is taken liminary designs are usually analysed with respect to as a collection of principles that are useful for explain- constraints and, if unsatisfactory, are revised based ing the design process and provide a foundation for the on experience and the results of the analysis. basic understanding required to propose useful method- 9. Design as an interactive process. Interactive design ologies. Design theory is about design; it explains what brings the designer directly into the process by design is or what is being done when desi~ning. On the forcing him or her to be an integral part of it. This is other hand, design methodology is a collection of pro- necessitated in situations where: (a) the design cedures, tools and techniques for designers to use when problem is ill-defined, (b) there are insufficient ana- designing. Design methodology is prescriptive as it indi- lytical tools developed to enable quantitative analysis cates how to do design, while design theory is descrip- and (c) there is little or no experience available or tive as it indicates what design is. Rabins et al. (25) state associated with the design problem. that ... design theory refers to systematic statements of The above views on the nature and features of the principles and experientially verified relationships that design process represent different facets of the overall explain the design process and provide the fundamental design process. They are dependent on the engineering understanding necessary to create a useful methodology or design domain from which the particular viewpoint for design. is expressed as well as the nature, type, variety and com- These viewpoints represent the first steps towards plexity of the particular artefact/process or system being defining what might be regarded as design theory and designed. Most of the viewpoints are, however, comple- design methodology. The definitions by ASME are par- mentary to each other. A comprehensive design system ticularly encompassing and are worth noting. must therefore be able to support these various facets of design involving: (a) a top-down and bottom-up approach, (b) the evolutionary process of design, (c) the 2.3 The nature and features of the design process knowledge-based/exploratory aspects of design, (d) the The design process for any design model usually investigative and search aspects of the design process. exhibits certain properties and features which represent (e) the creative process in design, (f) the logical reason- various associated viewpoints and philosophies, activ- ing process involved in design, (g) the iterative as well as ities and processes that occur during the process. These the interactive process involved in design, (h) the features as highlighted by several researchers (26-28) are making of decisions based on value judgements and (i) discussed below: the mathematical analysis and computational simula- tion processes performed during design. 1. Design as an opportunistic activity represents the case where both top-down and bottom-up approaches are used by the designer in an opportun- 2.4 The nature and stages of thought in design istic manner. 2. Design as an incremental activity involves an evolu- In the process of design, most designers tend to go tionary process. where changes (improvements or through certain stages, referred to here as stages of Part B: Journal of Engineering Manufacture © !MechE 1996
  3. 3. A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 303 thought, as they move from an abstract problem to a tures are introduced, which still bear some resem- realizable product. These are the divergent, transform- blance to existing variables or features, and the. ation and convergent stages of design: decomposition of the problem is known. but the sub-problems and various alternatives to their1. Divergence. This is the act of extending the boundary solution must be synthesized. In other situations, of a design situation in order to have a large enough alternative recombination of the sub-problems solution search space. The divergent search approach may yield new designs. It is also considered that aims to de-structure the original design brief, while solving the same problem in different ways, or identifying the features of the design situation that different problems in the same way (by analogy), will permit a valuable and feasible degree of change. would fall under this class. Divergent search is most productive in the initial (b) Creative designs. In this case new variables or stages of design. features are introduced which bear no similarity2. Transformation. This is the stage of pattern making, to variables or features in the previous prototype high-level creativity, flashes of insight, changes of set and the resulting design has very little resem- and inspired guesswork. The objective here is to blance to existing designs. For creative designs impose upon the results of the divergent search a no design plan is known, a priori, for the problem pattern that is precise enough to permit convergence under consideration. to a single design.3. Convergence. The main objective of the convergent Sriram et al. (32), in the light of the foregoing, stage is to progressively reduce secondary uncer- describes the creative-routine spectrum of design as tainties as fast as possible as well as ruling out alter- follows: At the creative end of the spectrum, the design natives. The end result of this stage should be the process might be nebulous (hazy), spontaneous, chaotic, reduction of the range of options to a single chosen and imaginative, whereas at the routine end, the design design as quickly and as cheaply as can be managed is precise, predetermined, systematic, and mathematical. and without the need for unforeseen retreats or recursion. 2.6 Product design classification The end result of any design process is a product or2.5 The variety of design problems system. Such products, depending on the engineeringA design is strongly influenced by the lifestyle, training discipline or domain, vary in one way or the other.and experience of the designer, and the creativity and Product variation also arises depending on the marketeffort a designer puts into a design varies, depending on segment, knowledge available, the design process andthe type of design problem (30). Design problems that manufacturing capabilities. In the light of general con-confront engineers and designers can be classified under straints, products can be classified as either over-the following types (30-33): constrained or underconstrained, and depending on the customer demands and competition in the market, some1. Routine designs. These are considered to be derived products are considered as static or dynamic. These from common prototypes with the same set of vari- various forms or classifications are discussed below (34, ables or features and the structure does not change. 35): Here a design plan exists, with sub-problem decom- position, alternatives and prototypical solutions 1. Static product designs. Static products are those known in advance. whose market share is undiminishing and no changes2. Redesigns. This involves modifying an existing design are being demanded in the product. The design to satisfy new requirements or improve its per- concept is already known from existing products, formance under current requirements. The end result and hence such products are considered to be con- of redesigns may also exhibit some form of creative, ceptually static (also referred to as dominant design). innovative or routine design content. Redesigns will 2. Dynamic product designs. Dynamic products have a be discussed under adaptive designs and variant limited life before the next generation supersedes designs. them. Here, development is focused on the product, (a) Adaptive, configurative or transitional designs. and the design process involves the development of These forms of design involve adapting a known new, radical and alternative designs. In discussing the system (solution principle remaining the same) to dynamic-static spectrum of products, Clausing (34) a changed task. They also involve improvements highlights the following types of products sand- on a basic design by a series of detail refine- wiched between the two extremes: (a) genesis ments. product, (b) radical product, (c) new product, (d) (b) Variant, extensional or parametric designs. This clean sheet (generational) product, (e) market- follows an extrapolative or interpolative pro- segment entry (new) product, (f) market-segment cedure. The design technique involves using a entry (generational) product, (g) associated product, proven design as a basis for generating further (h) variant product and (i) customized product. geometrically similar designs of differing capa- 3. Overconstrained product designs. These products cities. tend to exist in the high-technology markets. Here,3. Non-routine designs, original or new designs. These the design process evolves around analysing alterna- forms of design are also known as original designs tive proposals until the <;orrect (or most acceptable) and are classified into innovative and creative solution is found. Overconstrained products are designs. usually subjected to several constraints of function, (a) Innovative designs. Here new variables or fea- materials, manufacturing processes, some of which:g IMechE 1996 Proc Instn Mech Engrs Vol 210
  4. 4. 304 N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB might be conflicting, and the product undergoes Cross (39), reporting on Lawsons work (40), whi< severaJ·analysis and trade-off situations. compared the ways in which designers and scientis 4. Underconstrained product designs (ideas centred). In solved the same problem, states that: The scientis the case of underconstrained designs, the design tended to use a strategy of systematically exploring tt activity is centred around bringing products into the problem, in order to look for underlying rules whic market to satisfy market demands. There are usually would enable them to generate the correct or optimur not very many constraints, and the designer has solution. In contrast, the designers tended to suggest ample room for innovation. The focus here is usually variety of possible solutions until they found one tha on the product concept, and materials and tech- was good or satisfactory. The evidence from the experi niques are chosen to satisfy the required function and ments suggested that scientists "problem-solve b; recognizable market style. Most industrial designs analysis", whereas designers "problem-solve by synthe fall into this category, and development is on aes- sis". Scientists use "problem-focused" strategies anc thetics, ergonomics and functionality. designers use "solution-focused" strategies. Thi5 5. Underconstrained product designs (skill based). This phenomenon was also observed in a creative design form of design focuses on the manufacturing aspects workshop organized at City University, London (41). of product development. Efforts are usually concen- Yoshikawa (42), in his paper on design philosophy, trated on the capabilities and skills available in the also discusses design from some philosophical view- company. points attributed to various designers who belong to the semantics, syntactics and past experience schools of thought. These viewpoints constitute the platform for2.7 Design goals most of the controversies in the design community. Some of them, however, complement each other, whileDesign goals can be defined as the purposes for design others are completely contradictory.actions and decisions taken in each design step. Theyguide the choice of what to do at each point during thedesign process (36). Design goals represent one or more 3.1 Semantics schooldecision points from a problem-solving point of view, This school of thought is attributed to Rodenacker (43).and they define some of the dimensions of the design The central dogma of this school is that any machine, asspace. Design goals do exhibit one kind of interaction an object of design, is something that transforms threeor the other in the form of: (a) goal conflicts, involving forms of inputs, viz. substance, energy and information,non-simultaneous achievement of two goals, (b) goal into three outputs respective to each input, but havingsharing, achieving a sub-goal helps achieve a goal other different states from the inputs. The differences betweenthan its ancestors in the goal tree, and (c) goal prerequi-sites, where one goal must be achieved before another the inputs and outputs are called functionality. Thegoal in a different part of the goal tree. Typical types of initial requirements are usually given in terms of thedesign goals include: (a) functionality goals, (b) per- functionality, which has to be analysed into a logicalformance goals, (c) knowledge goals and (d) design structure, which gives connections between sub-process goals. In the control and management of the functionalities. On decomposing the initial functionalitydesign process, there is need to explicate strategies for into finer sub-functionalities, these resulting sub-how to handle interacting design goals. functionalities are substituted with particular physical phenomena that realize the transformations respec- tively. 3 DESIGN PHILOSOPHIESThere have been various schools of thought expressed 3.2 Syntax schoolby designers and researchers as regards how design is, This school is associated with the effort made to givemight be or should be done. This undoubtedly has some formalism to the design process, and attention isresulted in controversy. Three schools of thought within paid to the procedural aspects of the design activitythe British design community were expressed by Broad- rather than on the design object itself. Here attemptsbent in the book Design: science: method (37). The first are made to abstract the dynamical or temporarygroup believed that the design process should be aspects from the design, neglecting the static aspects ofchaotic and creative, the second group believed that design as emphasized in the semantics school. Thedesign should be organized and disciplined, while the process of abstraction is considered as the premise forthird group argued that no design process should be improving the universality of design model~ belongingimposed on a designer (38). Support for the first view to this school, which are usually regarded as prescrip-point is usually based on the argument that the design tive models. This philosophy, which emphasizes thefunction is an art, and hence cannot be taught, which dynamical aspects of design, can be combined with theseems to imply that designers are born and not made. semantics one, which emphasizes the static aspects ofArcher (22), in support of the second viewpoint, com- design to achieve a more sophisticated design method-ments that: Systematic methods come into their own, ology.under one or more of three conditions: when the conse-quences of being wrong are grave; when the probabilityof being wrong is high (e.g. due to lack of prior 3.3 Past experience schoolexperience); and/or when the number of interacting Arguments put forward by those belonging to thisvariables is so great that the break-even point of man- school of thought are usually that universality, which ishour cost versus machine-hour cost is passed. the target of most design methodologists, is contradic-Part B: Journal of Engineering Manufacture © !MechE 1996
  5. 5. A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 305tory to practical usefulness and that the creativity of good number of these models emphasize the need todesigners can be hampered and may deteriorate if perform more analytical work, prior to the generationdesign methodologies are adopted. In this school of solution concepts (39). Models put forward by pro-emphasis is placed on the significance of case historie~ ponents of prescriptive models are discussed below.of design, including all necessary knowledge to be learntfor improving design ability. This school of thought is 4.1.1 Model by J. C. Jonesclosely associated with the view that the design abilitycannot be acquired efficiently in a theoretical manner This model by Jones (1) is principally made up of threebut by experience. stages, viz. analysis, synthesis and evaluation. At the analysis stage, the first activity involves producing a random list of factors related to the problem to be3.4 Summary solved and/or to its solution. These factors are thenThe above schools of thought, although they stand their classified into workable categories and sub-categories,ground in arguments, are relevant in one way or the after which the interactions between them are investi-other with respect to design. In todays world, it is gated. The final step then involves rewriting all theincreas!ngly becoming evident that design approaches design requirements into solution neutral performancebelongmg to the syntax (prescriptive models) school of specifications.thought are more likely to stand the test of time. In the synthesis stage, creative techniques like brain-Wallace (44) in his article points out that the engineer- storming (45, 46) are used to generate ideas and solu-ing design process cannot be carried out efficiently if it tions to the performance specifications. Limits are thenis left entirely to chance .. . and ... the aim of a sys- set for each partial solution within a range of dimen-tematic approach is to make the design process more sions, shape and variations in material properties thatvisible and comprehensible so that all those providing will satisfy any performance specifications. The nextinputs to the process can appreciate where their contri- step then involves combining compatible partial solu-butions fit in. Furthermore, the need to equip and train tions into combined solutions.young engineers as well as support collaborative design The last stage of this model is the evaluation stageteams will necessitate the adoption of a structured and which involves mainly two activities. These are (a)systematic approach to design. methods of evaluation and (b) evaluation for operation, manufacture and sales. Under methods of evaluation, Jones advocates the use of evaluation methods to detect 4 DESIGN MODELS errors at the stage when they can be most cheaply cor-Design models are the representations of philosophies rected. Such methods include evaluation by per-or strategies proposed to show how design is and may formance specifications and evaluation by use of precisebe done. Often, they are drawn as flow diagrams, judgements.showing the iterative nature of the design process by a This model emphasizes the need to establish specifi-feedback link. cations in a solution neutral form as well as investigat- In the past, design models that arose from various ing interactions between design factors. The synthesisphilosophical viewpoints have tended to belong to two stage does exhibit a bottom-up approach in developingmain classes, namely prescriptive and descriptive the overall design. The idea of evaluating the designs bymodels. The prescriptive models are associated with the the pre-operation, pre-production and pre-sales team issyntactics school of thought and tend to look at the a late occurrence in this model. These teams in adesign process from a global perspective, covering the modern manufacturing industry should be involvedprocedural steps (that is suggesting the best way some- right from the start of the design process. In this model,thing should be done). The descriptive models, on the they should be involved at the analysis stage.other hand, are concerned with designers actions andactivities during the design process (that is what is 4.1.2 Model by Asimowinvolved in designing and/or how it is done). More In representing the design activity, Asimow (47) showsrecently, another group of models known here as com- the process of design in three phases that bear on theputational models have started to emanate. These com- solution of the design project, while the part that dealsputational models place emphasis on the use of with the solution of subordinate problems is repre-numerical and qualitative computational techniques, sented as a sequence of operations as every step of theartificial intelligence techniques, combined with modern process proceeds. The three phases of design repre-computing technologies. Each of these design models, sented are the feasibility study phase, preliminary designalthough discussed under one of the above classes, share phase and detailed design phase:some characteristics of the other classes. 1. Feasibility study phase. In the feasibility study phase, the need for the project is established, after which the4.1 Prescriptive models based on the design process design problem is explored and the design param-These models in general tend to prescribe how the eters, constraints and major criteria identified. Plaus-design process ought to proceed and in some cases ible solutions are generated and then analysed forappear to suggest how best to carry out design. They their physical realizability, economic worthwhilenessalso attempt to encourage designers to adopt improved and financial feasibility.ways of working. They usually offer a more algorithmic 2. Preliminary design phase. In the preliminary designand systematic procedure to follow, and are often phase, the best design concept from among the viableregarded as providing a particular methodology. A solutions is selected. Mathematical models are then:Q !~hE 1996 Proc lnstn Mech Engrs Vol 210
  6. 6. 306 N F 0 EVBUOMWAN, S SIVALOGANA THAN AND A JEBB prepared for each of the solutions, upon which sensi- inary layouts and (f) the ·development of definitive tivity analysis is performed to establish the fineness layouts and final documentation. of the range within which the design parameters must be controlled, compatibility analysis is per- 4.1.5 Model by Watts formed to investigate the tolerances in the character- istics of major components and stability analysis is Watts (48) in his paper represents the design process by performed to examine the extent to which pertur- an iconic model of a designer or design team in bations of environmental or internal forces affect the dynamic relationship with an environment. The design design. The chosen concept finally goes through an process is described as consisting of three processes of optimization process, an evaluation process, a pre- analysis, synthesis and evaluation, as also proposed by diction process as well as an experimental design Jones (1). These processes are performed cyclicly from a process. The third, fourth and fifth steps in this lower (more abstract) level to a higher (more concrete) phase, if considered in todays terms, are somewhat level (representing design phases), as represented by the synonymous to Taguchis system, parameter and tol- helical path in Fig. 1. In moving from the abstract level erance design. to the concrete one, the designer or design team during 3. Detailed design phase. In this stage, capital budgets the design process frequently reiterate at one or more and time schedules are prepared for the design. The levels, and decisions are made along the way as shown sub-systems, components and parts of the product on the surface of the cylinder. A state function D of the are then completely designed. Assembly drawings are design is associated with the process path and can be then prepared for the components and sub-systems, externalized as a set of statements at intersections of the after which the prototype is constructed and tested path and the decision line. Various states of the design respectively. Further analysis of the prototype is then thus relate to the different levels. The design states (Dm, performed, before making minor revisions as con- Dn, etc.) give a vertical structure to the process and vergence is made towards the final design. proceed through analysis, synthesis of design concepts, evaluation of feasibility, optimization, revision and The design process as discussed by Asimow (47) is in communication.steps of analysis, synthesis, evaluation, decision, optim- The process can be considered complete when theization and revision. The important aspect here is that designer releases into E (a particular environment) athese six steps are repeated at each of the process communication P, being a set of prescriptions for thephases. embodiment of the design. The end to which P is a means is an artefact A. This possesses several functional4.1.3 Model by Pahl and Beitz attributes, some of which fulfil the need implied by N; others enhance the profits and reputation of thePahl and Beitz (33) represent their model of the design designer and the company while others could haveprocess in four main phases, which are: (a) clarification effects that are far reaching into the socioeconomicof the task, (b) conceptual design, (c) embodimentdesign and (d) detail design. The first phase of clari- environmentfication of the task involves the collection of informa-tion about the requirements in a solution neutral form. 4.1.6 Model by Marples (49)The second phase, which is the conceptual design phase, This model represents an attempt to abstract theinvolves the establishment of function structures, the process of design, as a result of design case studiessearch for suitable solution principles and their com-bination into concept variants. At the embodimentdesign phase, the designer starting from the concept CONCRETEdetermines the layout and forms and develops a techni-cal product or system in accordance with technical andeconomic considerations. At the last phase of the detaildesign, the arrangement, form, dimensions and surfaceproperties of all the individual parts are finally laid DECISIONdown, the materials specified, the technical and eco-nomic feasibility rechecked and all the drawings andother production documents produced. (E)4.1.4 Model by VDI 2221 (39)This model was produced by Germanys professionalengineers body, Verein Deutscher Ingenieure (VDI), intheir guidelines VDI 2221, Systematic approach to thedesign of technical systems and products. The VDI D2221 model expresses the design process in seven stages.These stages involve (a) the clarification and definitionof the design task, (b) the determination of the requiredfunctions, (c) the search for solution principles for allsub-functions and combination into principal solutions, N(d) the division of the solution into realizable modules,(e) the developmem of key modules into a set of prelim- Fig. 1 The design model by WattsPart 8: Journal of Engineering Manufacture © !MechE 1996
  7. 7. A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 307 carried out. These studies were used to illustrate design- 1. Programming: establishment of crucial issues and ing as a sequence of decisions leading from the original proposal of course of action statement of the requirements to the specification of the 2. Data collection: collection, classification and storing details of the hardware to be manufactured. The start- of data ing point in this model is a statement of the main 3. Analysis: identification of sub-problems, preparationy problem to be solved. This represents the starting node of design specifications, reappraisal of proposed pro-~ in the Marples tree. gramme and estimation1 From this node, sub-problems are derived that must 4. Synthesis: preparation of outline design proposalsf be solved before a solution to the main problem is pos- 5. Development: development of prototype design(s), sible. This involves a cyclic process of analysis of the preparation and execution of validation studies problem, theorizing solutions, delineating these solu- 6. Communication: preparation of manufacturing tions and modifying them (49, 50). Figure 2 shows a documents general representation of a typical sequence of the The above six stages were further classified and grouped design process. In this figure, the final solution is the into three phases, namely analytic, creative and execu- sum of the solutions a(21211), a(22211), a(22221) and tive. In describing his model, Archer comments that: a(232). If, for example, a(2) is preferred to a(l) or a(3), all ... the special features of the process of designing is the sub-problems p(21), p(22) and p(23) must be solved. that the analytic phase with which it begins requires Similarly, if a(222) is accepted as a solution to p(22), objective observation and inductive reasoning, while the then sub-sub-problems p(2221) and p(2222) must be creative phase at the heart of it requires involvement, solved. In the figure, a vertical line denotes a problem, subjective judgement, and deductive reasoning. Once while a slanting line denotes a solution. Eder (50) the crucial decisions are made, the design process con- further proposes that all precedent solutions to the tinues with the execution of working drawings, sched- main problem, such as competitors models, should also ules, etc., again in an objective and descriptive mood. appear on the design tree. This is analogous to competi- The design process is thus a creative sandwich. The tive assessment in quality function deployment (51). bread of objective and systematic analysis may be thick The model by Marples involves three principal or thin, but the creative act is always there in the phases of synthesis, evaluation and decision. At the syn- thesis phase, two activities are involved, that is the middle. Figure 3 shows the stages and phases of the design process as well as their interrelationships. search for possible solutions and the examination of proposed solutions. This phase is then followed by the evaluation of the viable solutions against certain cri- teria, before a final decision is made in choosing a par- 4.1.8 Model by Krick ticular solution. Krick (52) in his model describes the design process in five stages of problem formulation, problem analysis, search, decision and specification. The first step of 4.1.7 Model by Archer problem formulation involves defining clearly the design Archer (22) defines the nature of design methodology in problem to be solved. The next step involves analysing his model in six stages, viz.: the design problem and arriving at a detailed definition 0 POINT OF PROBLEM FORMULA noN a331 a332 a321 p3211 p3311 p3321 I p2111 i p222221 SOLUnON ACCEPnED SOLUnON REJECTED PROPOSED SOLUnON - AN AL nERNA TIVE A SUB-PROBLEM ARISING FROM ANY AL nERNA TIVE Fig. 2 The design model by Marples ~ I~1echE 1996 Proc Instn Mech Engrs Vol 210
  8. 8. 308 N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB TRAINING BRIEF PROGRAMMING PROGRAMMING OBSERVATION EXPERIENCE MEASUREMENT j ANALYTICAL PHASE j INDUCTIVE REASONING DATA COillCTION DATA COLLECTION j j ANALYSIS ANALYSIS j CREATIVE j EVALUATION JUDGEMENT DEDUCTIVE SYNTIHESIS SYNTIHESIS PHASE REASONING DECISION j j DEVELOPMENT DEVELOPMENT j j DESCRIP110N SOLUTION COMMUNICATION EXECUTIVE COMMUNICATION TRANSLATION TRANSMISSION PHASE (c) MOOEL Of THE DESIGN PROCESS (b) MAJN PHASES Of DESIGN Fig. 3 The design model by Archerof the specifications, constraints and criteria. In the objective in that stage. In the first stage of clarifyingthird step, the search for and generation of alternative objectives, the objectives tree method is used to clarifysolutions is performed through inquiry, invention and design objectives and sub-objectives and the relation-research. The decision stage, which is the fourth step, ship between them. The function analysis method isinvolves the evaluation, comparison and screening of then used to establish the function required and thealternative solutions until the best solution evolves. system boundary of a new design at the second stage. InFinally, the fifth step, which is the specification stage, is the third stage involving setting of requirements, anperformed. This involves a detailed documentation of accurate specification of the performance required of athe chosen design with engineering drawings, reports design solution is done using the performance specifi-and possibly iconic models being the resulting output. cations method. The morphology chart method is then used at the fourth stage to generate the complete range of alternative design solutions for a product. In the fifth4.1.9 Model by Nigel Cross stage the design alternatives are evaluated using theIn representing his model, Cross (39) expresses the weighted objectives method to compare the utilitydesign process in six stages within a symmetrical values of alternative design proposals on the basis ofproblem-solution model, as shown in Fig. 4. The six performance against differently weighted objectives. Thestages are clarification of objectives, establishing func- sixth and final stage of improving details involves usingtions, setting requirements, generating alternatives, the value engineering method to increase or maintainevaluating alternatives and improving details. For each the value of a product to its purchaser while reducingof the stages, a design method is used to achieve the its cost to its producer. OVERALL PROBLEM • OVERALL SOLUTION (SUB - PROBLEMS ) SUB - SOLUTIONS Fig. 4 The design model by CrossPart 8: Journal of Engineering Manufacture © !MechE 1996
  9. 9. A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 309 4.1.10 Model by Hubka Phase 4. Elaboration The model by Hubka (53) represents the design process Step 6. Detailing and elaboration in four phases and six stages or steps. These phases and steps in a procedural model, as shown in Fig. 5, are: Phase 1. Elaboration of assigned problem 4.1.11 Model by French Step 1. Elaborate or clarify assigned specifi- This model, as shown in Fig. 6, is based on the follow- cation ing activities of design (54): Phase 2. Conceptual design Step 2. Establish functional structures and 1. The analysis of the problem phase involving the Step 3. Establish concept identification of the need to be satisfied as precisely Phase 3. Laying out as possible or desirable Step 4. Establish preliminary layout and 2. The conceptual design phase involving the gener- Step 5. Establish dimensional layout ation of broad solutions in the form of schemes " a: wz~ - STATE OF MACHINE SYSTEM tJoe (CIRCLED STATEMENTS) - gs ~ 2 8 w ~--------------------------------------------,_--------------------~ ~ < 5 ESTABUSHED DESIGN u.. ...J c.:> CHARACTERISllCS OF THE DESIGN DOCUMENT ~ §~ MACHINE SYSTEM " :::E f- 1. DESIGN SPECIFlCA TlON 2.1 BLACK BOX DIAGRAM ESTABUSH TE~OI.OGICAL PRINCIPLE 2.2 ESTABUSH SEQUENCE ~ OPERATICN 2.3 2.4 NPLY liS IN TP AND BOUNDAAJES ~ liS 2.5 E51ABUSH GROUP1NG ~ FUNCTIONS 5 FUNCTION 2.6 STRUC11JRE Z ( FUNCTlDNAL STRUCTIJRE SCHEMAllC 2.7 ( OPTIMAL FUNCTlON STRUCTURE AI~ 3.1 INPUTS TO MS. IIOOE ~ ACTION MORPHOLOGICAL MATRIX 3.2 FAIIIUES ~ FUNCTION-CARRIERS 3.3, 3.4 CDNCEPTIJAL SCHEMA TlC 3.5 CONCEPT SKETCH 4.2 PARTs. ARRANCEIIENT. ROUGH FORM, SOME Dt.£NSIONS 4.3, 4.4 TYPE OF WATER1AL MD MANUFACTURlNG METHODS 4.5 4.6 ( OPTIIr.tAL PREUhiiNARY LAYOUT AI~ 5.2 OEF1N111£ ARRANG04ENT. FORM, ALL DIMENSIONS 5.3, 5.4 MATCR1AL AND MANUFACTURING METHODS, PARTIAL TOL£RANCES 5.5 DIMENSIONAL DIIIENSIONAL TRUE- TO- SCALE DIMENSIONAL! LAYOUT 1 LAYOUT 2 LAYOUT 5.6 ( OPTIMAL DIIIENSIONAL LAYOUT ) - I 6.1 - MACHINE ELEMENTS 0 RELEASE I 6.5 5.6. 5.7 ~ $§>$ (IdS REPRESENTATION ) DETAIL DRAWING ASSEMBLY DRAWING I Fig. 5 The design model by Hubkac; l!ect:E 1996 Proc Instn Mech Engrs Vol 210
  10. 10. :JlO N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB Proposed schemes are critically examined to see if they satisfy the needs, can be constructed, and how economic they are both in first cost and in function throughout their working life. Preliminary structural analyses are also carried out to check the broad ade- quacy of schemes. 4. Decision. After successive operations of conception and appraisal, it then becomes necessary to decide on a particular design scheme. Criteria for decision making may include both simplicity and distinction of the design, as well as constructability. · 5. Checking and elaboration. This is the stage where the designer makes sure of the adequacy of what is proposed and performs elaboration of necessary details. Here, models can be built and tested. Power- ful analytical techniques can also be employed in (a) defining the actions on the structure such as load, temperature difference, corrosion, etc., (b) analysing the effects of these actions and (c) comparing these SELECTED effects with a criterion of adequacy. The end result of SCHEMES the design process is the communication of the detailed design both in the form of drawings and text. 4.1.13 Total design activity model by Pugh (56) Pugh regards total design as the systematic activity necessary from the identification of the market/user need, to the selling of the successful product to satisfy that need-an activity that encompasses product, process, people and organization. The total des~gn activity model consists principally of a central design core, which in turn consists of market (user need), product design specification, conceptual design, detail design, manufacture and sales. The design process in this model proceeds, firstly, by identifying a need which, Fig. 6 The design model by French when satisfied, fits into an existing or a new market. From the statement of the need, the product design spe- cification (PDS), representing the specification of the3. The embodiment of schemes phase involving the product to be designed, is then formulated. The estab- development of generated schemes into greater lished PDS then acts as a mantle that envelops all the details subsequent stages in the design core, thus acting as the4. The detailing phase. where the selected scheme is control for the total design activity. Within this model, worked into finer details the design processes flow from market to sales, is an iterative one and recourse can be made to any of the4.1.12 Model by Sir Alan Harris (55) earlier stages, as new ideas and information emerge. This causes interactions between the different stages ofThis model is based on proposals regarding the teach- the design core. This model also recognizes the fact that,ing of design within the civil engineering discipline. The for effective and efficient design to be carried out, it ismodel consists of five stages, viz.: appreciation of the necessary to utilize various design techniques, to enabletask, conception, appraisal of concepts, decision, check- the designer/design team to operate the core activity.ing and elaboration: These design techniques or methods include:1. Appreciation of the task. This means discovering (a) discipline-independent ones which relate directly to what is needed and ascertaining what resources are the design core and can be applied to any product needed and from where. It involves finding out what or technology, such as tools for performing analysis, a client wants-regarded as the total function. synthesis, decision making, modelling, etc.;2. Conception. In this stage, based on the full digestion (b) specific discipline-dependent technique~ and tech~o­ of the facts generated from the previous stage, ideas logical knowledge such as stress analysis. hydraulics, of solutions should begin to emanate. Here the thermal analysis, thermodynamic analysis, elec- designer is putting together what is known of the tronics, etc. function of the work with tentative ideas of form, material and method of construction. This model also takes into account, within the overall3. Appraisal of concepts. This stage is where the search- product development process. the framework of plan- ing eye based on experience becomes invaluable. ning and organization. thus gaining insight into the wayPart B: Journal of Engineering :!anufacture ~!MechE 1996
  11. 11. ,, A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 311 if products should be designed within a business structure. 2221, the other models also contained in a moreJW The total design activity model is shown in Fig. 7. detailed form within each of their design phases/stages,)n the design activities that characterized a majority of the·a! 4.1.14 The BS 7000 design model (57) other models. The Watts model showed only the two e- ends of the design phase, that is abstract and concrete, This model commences with a feasibility study stage with the interval between represented by a cyclic and proceeds through conceptual design, embodiment (iterative, refining and progressive) process. design, detail design and design for manufacture stages. The models that were based on design acttv1ttes It also shows the output of each design stage in the included those by Jones, Marples, Archer, Krick, Cross form of design brief, concept drawings, layout drawings, and Harris. It can also be observed that in all of the detailed product definition and manufacturing instruc- models, three key activities were predominant, that is tions respectively. The model ends with a post-design·e analysis, synthesis and evaluation. Analysis was mostly support stage. It can be observed that this modelIS related to analysing the design problem, requirements derives from other models by Pahl and Beitz (33) andy and specifications. Synthesis was concerned with gener- French (54), with design for manufacture included as an·- ating ideas, proposing solutions to large or small design additional stage. This model is shown schematically inl) problems as well as exploring the design soiution space, Fig. 8.i, while evaluation involved the appraisal of design solu-g tions in order to establish whether they satisfied the 4.2 A critical appraisal of prescriptive modelse requirements and specifications and set corporate cri-,f An in-depth review of the prescriptive models on the teria. The sequence in general also tended to be analysis design process shows that a majority of them based the first, followed by synthesis and then evaluation. In thei procedural steps of their models on what can be re- model by Krick, synthesis was replaced by search and garded as design activities (that is analysis, synthesis, evaluation by decision. The model by Harris represent- evaluation, decisions, etc.), while others based their pro- ed analysis, synthesis and evaluation by appraisal of the cedural steps on what can be regarded as the phases/ task, conception and appraisal of concepts respectively. stages of design (that is conceptual design, embodiment It is not surprising that the three activities of analysis, design and detailed design). The models that were based synthesis and evaluation were predominant as they rep- on the phases of the design process include those of resent the core of the design process. If proper analysis Asimow, Pahl and Beitz, VDI 2221, Watts, Hubka and of the problem or requirements is not carried out, syn- French. With the exception of that of French and VDI thesizing solutions will be difficult and inappropriate Technology Technique TOTAL ACTIVITY UD~DHS CT S:PCClfJCAT!(l-j SYNTHESIS I CONCEPTUAL DESIGN Equa.t~s to Spec. J .~ECi-.ANISMS I DETAIL DESIGN Equ.:a.tPS to S:pN:. I OPTIMISATION I <C_______~DA~T~A_H~AN~D~L~IN~Gi 1 Df~~~~L~~~~L~w~, SPECIFICATION C::.~GANISED Fig. 7 The total design activity model by Pugh i; 1!echE 1996 Proc lnstn Mech Engrs Vol 210
  12. 12. 312 N F 0 EVBUOMWAN, S SIVALOGANATHAN AND A JEBB FEASIBILITY STUDY DESIGN FOR MANUFACTURE POST DESIGN SUPPORT Fig. 8 The BS 7000 design modelsolutions might be the result. Once plausible solutions In the above equation, the matrix [A] represents theare created, there is then a need to evaluate, test and design relationship. In furthering his principles ofassess their fidelity to the originating requirements and design, Suh defines two axioms, which are:specifications as well as set criteria. Besides the three activities, there are, however, othernecessary activities that should be performed during the Axiom 1: the independent axiomdesign process, such as optimization, revision, data col- Maintain the independence of functional requirementslection, documentation, communication, selection, deci- (FRs).sion making, modelling, etc. Some of these activities Alternative statement 1: an optimal design alwayswere included in some of the models. maintains the independence of FRs. Alternative statement 2: in an acceptable design, the4.3 Prescriptive models based on product attributes DPs and FRs are related in such a way that a specific DP can be adjusted to satisfy its corresponding FRA majority of product or systems failures can be attrib- without affecting other functional requirements.uted to either or a combination of the following: (a)incorrect or excessive functional requirements, (b) con-tinuing alterations to functional requirements, (c) wrong Axiom 2: the information axiomdesign decisions and (d) the inability to recognize faultydecisions early enough to rectify them. Thus the exis- Minimize the information content of the design.tence of unacceptable designs as well as good designs Alternative statement: the best design is a functionallylends weight to the argument that there should be some uncoupled design that has the minimum informationfeatures or attributes that can distinguish between good content.and unacceptable designs. The foregoing reasoning Associated with these axioms are eight corollariesformed the basis of Suhs axiomatic approach to design (having a flavour of design rules) and sixteen theoremsbased on attributes of the design produced (58). Taguchi (propositions that follow from the axioms or other(59) also argues that the total costs at the point of pro- propositions). Suh also classifies designs into three cate-duction and at the point of consumption should be gories, namely uncoupled, coupled and decoupledminimum for good designs and this should be the goal designs. An uncoupled design is a design that obeys theof product development. He introduces a loss function independent axiom and any specific DP can be ~djustedas an attribute of the product designed which has to be to satisfy a corresponding FR. A coupled design hasminimized to achieve robust designs. some of the FRs dependent on other functions. When the coupling is due to an insufficient number of DPs4.3.1 Suh s axiomatic design model when compared to the number of FRs, they may be decoupled by adding additional DPs. A decoupledThe basic premise of Suhs (58) axiomatic approach to design may have more information content. ~n t~edesign is that there are basic principles that govern deci- axiomatic approach, the design model (process) IS sphtsion making in design, just as the laws of nature govern into four main aspects of: (a) problem definition, whichthe physics and chemistry of nature. He describes the results in the definition of FRs and constraints, (b) ide-design process as a mapping process between the func- ation or creation of ideas, which is the creative processtional requirements (FRs) in the functional domain and of conceptualizing and devising a solution, (c) analysisthe design parameters (DPs) in the physical domain. of the proposed solution, which involve~ the proc~ss ofMathematically this can be expressed as: determining whether the proposed solutiOn ts a rat~~nal {FR} = [A]{DP} solution that is consistent with the problem defirutton,Part 8: Journal of Engineering Manufacture © !MechE 1996
  13. 13. A SURVEY OF DESIGN PHILOSOPHIES, MODELS, METHODS AND SYSTEMS 313 and (d) checking the fidelity of the final solution to the To achieve robustness, Taguchi suggests the follow- original needs. ing. sequence of events in .his design model: (a) systenr· design, (b) par~meter des~gn and (c) tolerance desi~ 43.2 Taguchis quality loss function model System design IS the physical embodiment of the func- tional requirements of the product, where special engin- The recent past has witnessed the shift in focus from eering and scientific knowledge is applied. Parameter on-line quality control to off-line quality. This has led design is the process of identifying the optimal settings to increasing focus on the integration of quality into the of various parameters under the control of the designer early design stage of product development Ensuring to limit variation. Tolerance design involves the control quality by design thus involves the use of structured off- of the variation in critical parameters when everything line methods to determine the design configurations else has failed to control the variation of performance that meet the customers needs and are robust, where within the required limit . robustness means that product performance character- istics are insensitive to variation in the manufacturing and operating environments (60). One of the main pro- 4.4 Descriptive models ponents of off-line quality control is the renowned Japanese, Professor Genichi Taguchi, who introduced Descriptive models emanated both from experience of the concept of quality loss or loss to society. individual designers and from studies carried out on Taguchis methodology is based on the precept that the how designs were created, that is what processes, strat- lowest cost to society represents the product with the egies and problem-solving methods designers used. highest quality, which is achieved by reducing variation These models usually emphasize the importance of gen- in product characteristics. This approach is expressed erating one solution concept early in the process, thus by what is called the loss function. The loss function is reflecting the solution focused nature of design think-he ing (39). The original solution goes through a process ofof a mathematical way of qualifying cost as a function of product variation. This loss function allows a determi- analysis, evaluation, refinement (patching and repair) nation to be made as to whether further reduction in and development (39, 62, 63). the variation will continue to reduce costs. The loss In their paper, Finger and Dixon (38) discuss descrip- function includes production costs as well as costs tive models from a different perspective and have identi- incurred by the customer during use (61). The simplest fied the research work in this area along two main lines:ItS form of the loss function is expressed by a quadratic 1. Research based on techniques from artificial intelli- relationship obtained from a Taylor series expansion, gence such as protocol analysis, involving systematicys and can be approximated by: gathering of data on how designers design.he L(Y) = k(Y- M) 2 2. Research based on modelling the cognitive process.fie where The aim of this research is to build computer-based"R L = loss associated with a particular performance cognitive models, which describe, simulate and characteristic Y emulate the mental processes and skills used by M = the performance target value designers while creating a design. k =loss parameter= LJD6 where 4.4.1 Model by March Lc = average loss to the customer when the per-ly formance characteristic is not within the limit The model of the design process proposed by Marchm Do (64) draws on the work of the American philosopher D 0 = customer tolerance limit Peirce on the three modes of reasoning, which are deduction, induction and abduction (production). Ines The loss function L(Y), which is shown graphically in rephrasing Peirces remarks, rational designing is con-ns Fig. 9, can thus be defined as the average of the finan- ceived as having three tasks:er cial loss due to deviations of the product characteristice- Y from the target function M over all customer condi- 1. The creation of a novel composition-accomplished!d tions up to the time required for the product life. by productive reasoningJ.e ? LCYl = k(Y-M)~!das!n:JsJe!d1ecit Lm _Lc~, l _ "·~ ~ ~ -i I II I;he- /1ss IisJi .~+Do:lin. Fig. 9 Taguchis quality loss function ~ 1le<:hE 1996 Proc lnstn Mech Engrs Vol 210