Engineering models of product design

From WikID


How can you structure engineering design processes?
Figure 1: Example of a function structure (from student report)

Models of the design process have been developed since the early nineteen-sixties. In engineering design, this development has converged to what might be called a consensus model. Typical examples of this model are the model of Pahl and Beitz and the VDI-model (Verein Deutscher Ingenieure). These models are also called phase-models or procedural models.

The engineering models are fundamentally derived from the way in which engineering design problems are conventionally perceived and modelled. Products are seen as technical systems that transform energy, material and information. The functional behaviour of a technical system is fully determined by physical principles and can be described by physical laws. The engineering design problem is to find and define the geometry and materials of the system in such a way that the required prescribed physical behaviour is realised in the most effective and efficient way.

Engineering models are based on the idea that a design-in-the-making can exist in three different ways:

  1. As a function structure; this is a representation of the intended behaviour (the functions) of a product and its parts.
  2. As a solution principle; this defines the working principle, or mode of action, of a product or a part thereof. It specifies (in generic terms) the function carriers or ‘organs’ of which a product should be built up, to fulfil its internal and external functions.
  3. As an embodied design; this is a design in the more usual meaning of the word. It is a description, usually as a drawing, of the geometrical and physico-chemical form of a product and its parts.

The Function Structure

In a function structure (see figure 1), the product and its components and parts are represented by their functions. It is an abstract representation that does not refer to concrete shape and material of the physical parts of the system. The function structure is an important methodological tool; it provides an aid for thinking about the mode of action of a product, without enforcing premature decisions on its embodiment.

The Solution Principle

Figure 2: Example of a solution principle (from student report)

A function structure is a model of the intended behaviour of a material system; it shows what internal functions must be realised by (not yet concretely defined) elements, so that the system as a whole can fulfil its external overall function. Designers try to realise this behaviour by thinking up concrete parts and components for the internal functions. For each part its place in the whole is established, as well as its precise geometry and materials. A solution principle (see figure 2) is an idealised (schematic) representation of the structure of a system or a subsystem. The characteristics of the elements and the relations are qualitatively determined. Yet a solution principle already establishes essential characteristics of the form of the product. Just as the overall function of a system is the resultant of a number of sub-functions, a solution principle for a product as a whole arises from the combination of solution principles for its parts. The overall solution principle, which is chosen for further development, is called the principal solution.

The core of designing - reasoning from function to form - is especially evident in the creation of a principal solution, for the principal solution marks the transition of the abstract functional structure to the concrete material structure of the product to be developed. Reasoning from function to form does not lead to a unique answer. Any function can therefore be realised with different physical effects, and these can be worked out into different solution principles and an overall principal solution.

The Embodied Design

Figure 3: Example of embodied design (from student report)

A principal solution is already a first design proposal, because it embodies decisions on the geometry and material of the new product. It is, however, not more than an outline design proposal, which deals with physical feasibility only. It is a technical possibility that has to be worked out to some extent, before it can be evaluated against non-technical criteria as well. The development of a principal solution to a embodied design (see figure 3) can be seen as a process of establishing increasingly accurate, and more numerous characteristics of the new product, in particular: (1) the structure of the entire product (the arrangement of the parts) and (2) the shape; (3) the dimensions; (4) the material(s); (5) the surface quality and texture; (6) the tolerances and (7) the manufacturing method of all the parts.

A product design is ready for production once all the design properties have been specified definitively and in all required detail. Usually many properties have to be considered, and the relations among them are complex. Therefore the development of a principal solution into a detailed definitive design usually requires some stages in between. Typical intermediate stages are the design concept and the preliminary design (or sketch design).

In a design concept a solution principle has been worked out to the extent that important properties of the product - such as appearance, operation and use, manufacturability and costs – can be assessed, besides the technical-physical functioning. One should also have a broad idea of the shape and the kinds of materials of the product and its parts.

A preliminary design is the following stage and also the last stage before the definitive design. It is characteristic of this stage that the layout and shape and main dimensions have been established for at least the key parts and components of the product, and the materials and manufacturing techniques have been determined.

The modes of existence of a design proposal as described above, enable designers to explicate their thoughts about a design, and to judge and further develop them. Often there corresponds a more or less usual form of representation to each stage, such as flow diagrams for function structures, diagrams for solution principles, sketches for concepts, layout drawings for preliminary designs and standardised technical drawings for definitive designs. Such documents mark a stage in the development of the design and a phase in the design process.

The model of Pahl & Beitz

A typical example of this ‘consensus model’ is the model of Pahl & Beitz (figure 4). Their model has four phases:

  • clarification of the task (‘Aufgabe klären’)
  • conceptual design (‘konzipieren’)
  • embodiment design (‘entwerfen’)
  • detail design (‘ausarbeiten’)

Broadly speaking, the phases involve the following activities:

Clarification of the task

In this phase the problem, handed over to the designer by the product planning department or an external client, is analysed, and information on the problem is collected. Based upon that information a design specification (or programme of requirements) is drawn up. The specification defines the functions and properties that are required for the new product, as well as the constraints placed upon the solution and the design process itself, such as standards and date of completion.

The specification directs the work in all other phases of the design process. Work done in later phases may change ones understanding of the problem and new information may become available. Therefore modification and refinement of the initial specification should be undertaken regularly. This is indicated by the feedback loops in the models.

Conceptual design

Figure 4: Phase model of the Product Design Process by Pahl and Beitz (Roozenburg and Eekels, 1995)

Given the specification, broad solutions are to be generated and evaluated, that provide for a suitable point of departure for embodiment design and detail design. Such broad solutions are called concepts (Pahl & Beitz) or schemes (French). Normally they are documented as diagrams or sketches. The conceptual phase starts with determining the overall function and important sub functions to be fulfilled and establishing their interrelationships (function structure). Next solution principles (‘Lösungsprinzipien’), also called working principles (‘Wirkprinzipien’), for sub-functions or sub-problems are generated and integrated into overall solutions, in accordance with the function structure. Such a combination of solution principles has been called a principal solution (‘Prinzipielle Lösung’). A principal solution defines those physical-technical characteristics of a product, that are essential for its functioning.

However, the choice for a particular principal solution is not to be based upon technical criteria only. Criteria relating to use, appearance, production, costs and others, must also be taken into account. To that end principal solutions have to be worked up into concept variants that show already part of the embodiment of the principle. A concept, or scheme, should be carried to a point ‘where the means of performing each major function has been fixed, as have the spatial and structural relationships of the principal components. A scheme should have been sufficiently worked out in detail for it to be possible to supply approximate costs, weights and overall dimensions, and the feasibility should have been assured as far as circumstances allow.

A scheme should be relatively explicit about special features or components, but need not go into much detail over established practice. Conceptual design is commonly seen to be the most important phase of the design process, because the decisions made here, will strongly bear upon all subsequent phases of the design process. A weak concept can never be turned into an optimum detailed design, so to speak.

Embodiment design

In this phase the chosen concept is elaborated into a definitive design, also called definitive layout. The definitive design defines the arrangement (‘layout’) of assemblies, components and parts, as well as their geometrical shape, dimensions and materials (‘form designs’).

Contrary to what the phrase ‘definitive’ may suggest, the definitive design need not be completely worked out into full detail. The configuration of the product and the form of the parts are to be developed up to the point where the design of the product can be tested against all major requirements of the specification, preferably as a working model or prototype.

The decisions to be taken about the layout and form of the components and parts are strongly interrelated. Therefore, more than conceptual design, embodiment design involves corrective cycles in which analysis, synthesis, simulation and evaluation constantly alternate and complement each other. Embodiment design is essentially a process of continuously refining a concept, jumping from one sub-problem to another, anticipating decisions still to be taken and correcting earlier decisions in the light of the current state of the design proposal. It proves therefore difficult to draw up a detailed plan of action for this phase, that holds in general.

In Pahl and Beitz’ model embodiment design is subdivided into two stages. The first stage is leading to a preliminary design, in which the layout, form and material of the principal function carriers are provisionally determined. In this stage several alternative embodiments of a concept are often worked up in parallel in order to find the layout. In the second stage, then, the best preliminary design is elaborated, up to the point where all major decisions about the layout and form of the product are taken and tests of its functionality, operation and use, appearance, consumer preference, reliability, manufacturability and cost can be carried out. Normally at the end of this phase the design is represented by layout drawings, made to scale and showing important dimensions, and preliminary parts lists.

Detail design

In this final phase the geometrical shape, dimensions, tolerances, surface properties and materials of the product and all is individual parts are fully specified and laid down in assembly drawings, detail drawings and parts lists. Also instructions for production, assembly, testing, transport and operation, use, maintenance and the like, have to be worked out now. All these documents fall under the heading of the ‘product documents’.

The VDI Model (Verein Deutscher Ingenieure)

Figure 5: Phase model of the Product Design Process by VDI (Roozenburg and Eekels, 1995)

Of a more recent date than the model of Pahl and Beitz is the Guideline VDI 2221, Systematic Approach to the Design of Technical Systems and Products. This guideline aims for a general approach to design, which is applicable to a wide variety of tasks including product design, and transcends specific branches of industry. To demonstrate its potential, examples are given for mechanical engineering, process engineering, precision engineering (mechatronics) and software engineering. Yet, the ideas presented in the guideline seem to be more closely associated with mechanical engineering design.

The general approach is divided into seven stages, correspondingly producing seven results (figure 5). Either all or some of the stages are to be completed, depending on the task at hand. Individual stages can be combined into design phases, in order to assist the overall planning and management of the design process. It is stated that the way stages are grouped into phases can differ depending on the branch of industry or company. Apart from stage 4, in which a so-called module structure (‘modulare struktur’) is to be established, all stages and results can be recognised in the Pahl and Beitz model as well. The module structure takes more or less the place of the concept in the Pahl and Beitz model. The module structure specifies the division of a principal solution into realisable parts, components or assemblies, which has to be undertaken before starting the process of defining these modules in more concrete terms. Such a breakdown is particularly important for complex products, as it facilitates the distribution of design effort in the phase of embodiment design.

Some comments on phase-models

  • First, it is stressed by all authors of phase-models that sharp divisions between the phases cannot be drawn, and that the stages and phases do not necessarily follow rigidly one after the other. They are often carried out iteratively, returning to preceding ones, thus achieving a step-by-step optimisation.
  • Second, a phase-model does not show the problem-solving process, by which solutions for the design problem are generated and refined; in each phase the designer will go through the basic design cycle, often more than once.
  • Third, in each phase alternative solutions can be thought up. Working out all solution variants through all phases would lead to an explosion of the number of possibilities to be studied. On the other hand, restricting oneself to one track only within the network of possibilities is dangerous, because, then, the better or best alternatives may be overlooked. One is therefore urged to diverge and converge in each phase.
  • Fourth, the models have been developed with the designing of new, innovative technical systems in mind. Therefore they pay (too) much attention to the conceptual design phase, at the expense of the phases of embodiment design and detailed design. In practice many design projects can do without inventing new technical principles, and start from known, proven, concepts. However the phase models offer little procedural advice concerning embodiment and detail design. It has even been questioned whether more detailed procedural models for these phases may exist (but see the ‘Fish-Trap’ Model)
  • In phase-models the end of each phase can be taken as a decision point. Herein lies the importance of phase models. At the decision points you look back on the work performed, and you weigh the results obtained against the goals of the project. Phase models therefore urge a regular evaluation of the project: reject, do a step back, or continue to the following phase.

References and further Reading

  • Roozenburg, N. and Eekels, J. (1995) Product Design: Fundamentals and Methods, Chichester: Wiley, 1995, pp. 94-114.
  • Roozenburg, N. and Eekels, J. (1998, 2nd ed.), Productontwerpen: Structuur en Methoden, Utrecht: Lemma, pp. 104-129.
  • VDI 2221, Systematic Approach to the Design of Technical Systems and Products. Düsseldorf, VDI, 1987.
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