We have just seen that asking increasingly detailed questions exposed several design tasks. We will now formalize such design tasks into a design process. Many design process models are descriptive: they describe the elements of the design process. Other models are prescriptive: they prescribe what must be done during the design process. We will first briefly present some descriptive models, and then introduce an extended set of our design tasks to convert one simple descriptive model into a more detailed prescrip- tive model.
The simplest descriptive model of the design process defines three phases:
1. Generation: the designer generates or creates various design concepts.
2. Evaluation: the designer tests the chosen design against metrics that reflect the client’s objectives and against specifications that stipulate how the design must function.
3. Communication: the designer communicates the final design to the client and to manufacturers or fabricators.
Another three-stage model splits up the design process differently: Conceive, design, and implement a final design, with the context providing meanings for these three steps. These two models are simple, but they are very abstract and provide no useful advice on how to actually generate designs.
We show a more extensive prescriptive model of the design process in Figure 2.1. It has five phases, shown in boxes with rounded corners, starting with an initial problem statement and ending with final design documentation. Figure 2.1 also shows, in ovals, the output of each design phase that also serves as input to the next design phase, and it displays the links between the five stages of this design process. We can also delineate the model and its design tasks in charts that describe each phase, showing the input(s) to that phase, the design tasks to be performed, and the output(s) or product(s) of that phase that in turn work as input to the next phase:
Problem definition: We frame the problem by delineating the customer requirements, which means clarifying the client’s objectives, identifying constraints, and establish- ing functions before we begin conceptual design.
Objectives Analysis
• Develop metrics for objectives
Design Communication
• Document final design
Detailed Design
• Refine and optimize chosen design
• Assign and specify design details
Problem Definition: Detailing Customer Requirements
• Revise original problem statement
• Clarify objectives
• Identify constraints
• Establish principal functions Customer Requirements:
Revised Problem Statement Initial Lists of:
Objectives Constraints Principal Functions
Designed Details of Chosen Design
FINAL REPORTS (WRITTEN, ORAL) TO CLIENT: (1) Description of Design Process (2) Drawings and Design Details (3) Fabrication Specifications
• Generate design alternatives
• Refine and apply metrics to design alternatives
• Estimate major attributes of design alternatives
• Choose a design concept
Conceptual Design: Translating Customer Requirements into Engineering Specifications Constraints Analysis
• Set limits or boundaries
Functional Analysis
• Establish functional specifications
• Establish means for functions
Preliminary Design
• Model and analyze chosen design
• Test and evaluate chosen design Chosen Design
Requirements and Function Specifications
ORIGINAL PROBLEM STATEMENT Analysis, Test and Evaluation
Results for Chosen Design
Figure 2.1 A five-stage prescriptive model of the design process, presented as a spiral to convey the idea that design is not a simple linear sequence of tasks to be done. The design stages are in rectangles, and each stage’s outputs are in ovals.
3GCH02 08/29/2013 22:50:23 Page 21
1. During problem definition we frame the problem by clarifying objectives,
identifying constraints, establishing functions, and gathering the other information needed to develop an unambiguous statement of a client’s wishes, needs, and limits, that is, the customer requirements.
Input: original problem statement Tasks: revise client’s problem statement
clarify objectives identify constraints establish principal functions Outputs: customer requirements:
revised problem statement initial list of final objectives initial list of constraints initial list of principal functions
Conceptual design: We generate different concepts or schemes to achieve a client’s objectives, satisfy constraints, and perform functions. Enough details (e.g., the spatial and structural relationships of the principal components) are worked out to estimate costs, weights, overall dimensions, and so on. Ladder concepts might be an extension ladder, a stepladder, or a rope ladder. We evaluate these concepts first translating the customer requirements (i.e., objectives, con- straints, and functions) into engineering specifications that we use to articulate and benchmark our design.
2. In the conceptual design stage of the design process we translate the customer requirements into engineering specifications to generate concepts or schemes of design alternatives or feasible (i.e., acceptable) designs.
Input: customer requirements revised problem statement initial list of final objectives initial list of constraints initial list of principal functions Tasks: establish functional specifications
establish means for functions
write limits or boundaries of constraints develop metrics for objectives
generate design alternatives
refine and apply metrics to design alternatives estimate design alternatives’ major attributes choose a design concept
Output: a chosen design
analysis, test, and evaluation results for chosen design
With its focus on trade-offs between high-level objectives, conceptual design is clearly the most abstract and open-ended part of the design process. Its output may include several competing concepts. Some argue that conceptual design should produce two or more schemes since early commitment to or fixation on a single design choice may be a mistake. This tendency is so well known among designers that it has produced a saying: “Don’t marry your first design idea.”
Preliminary design or embodiment of schemes: Here we flesh out our proposed concepts, that is, we embody or endow design schemes with preliminary versions of their most important attributes. We select and size the major subsystems, based on lower-level concerns that take into account the performance and operating require- ments. For a stepladder, for example, we size the side rails and the steps, and perhaps decide how to fasten the steps to the side rails.
3. In the preliminary design phase we identify and preliminarily size/estimate the principal attributes of the chosen design concept or scheme.
Input: a chosen design specifications
Tasks: model and analyze chosen design test and evaluate chosen design Output: analysis, testing, evaluation of chosen
design
Preliminary design is definitely more “technical”: We might do back-of-the- envelope or computer calculations. We make extensive use of rules of thumb about size, efficiency, and so on, that reflect our design experience.
Detailed design: We now articulate our final design in much greater detail, refining the choices we made in preliminary design down to specific part types and dimensions. We use detailed design knowledge and procedures expressed in specific rules, formulas, and algorithms that are found in design codes (e.g., the ASME Pressure Vessel and Piping Code, the Universal Building Code), handbooks, data- bases, and catalogs.
4. During detailed design we refine and optimize the final design and assign and fix the design details.
Input: the analyzed, tested, evaluated design Tasks: refine, optimize the chosen design
assign and specify the design details Output: proposed design and design details
3GCH02 08/29/2013 22:50:24 Page 23
Design communication: We now spell out and present our design process, the resulting final design, and its fabrication specifications. In practice, the designer will usually have already developed much of the documentation along the way, and this communication phase will be more about tracking and organizing prior work products than writing a “new” report from “scratch.”
5. Finally, during the design communication phase we document the fabrication specifications and their justification.
Input: proposed design and design details Task: document the final design
Outputs: final written, oral reports to client containing: (1) description of design process
(2) drawings and design details (3) fabrication specifications
We have depicted the design process in Figure 2.1 as a spiral for several reasons. Design processes are often described as a linear sequence that seems to imply: do task 1, then do task 2, then do task 3. In practice, we actually keep completed phases and tasks in our minds as our design unfolds, and we refer back to them regularly. We may wonder while deep in a design project, “Why are we doing this?” By looking back at a project’s objectives or constraints, or at a design decision we’ve already made, we can answer this question.
There are two other important elements that we hope the spiral depiction will help reinforce: feedback and iteration. Feedback occurs in two notable ways in the design process. First, internal feedback comes during the design process as test and evaluation results are used to verify that the design performs as intended. This feedback may come from the client and from internal customers, such as manufacturing (e.g., can it be made?) and maintenance (e.g., can it be fixed?). Second, external feedback comes after a design reaches its market and user feedback validates (or not) a successful design.
Iteration occurs when we repeatedly apply a common method or technique at different points in a design process. For example, we might write equilibrium equations for an entire structure, and then at a lower level of abstraction (i.e., on a different scale) we write equilibrium equations for structural components). Similarly, as we fix more details in a design and become less abstract, and we might review and reestablish means for our functions. Again, we always want to keep in mind the original objectives, constraints, and functions as we get closer to our final design. This may also mean that we have to do some redesign, in which case we will certainly repeat tasks such as analyzing the design or testing and evaluating the design.
Given that there are feedback loops and that we will reiterate some tasks, why didn’t we include them in Figure 2.1? As important as feedback and iteration are, it is also important not to be overly distracted by these adaptive characteristics when learning about and doing design for the first time.
We now have a “checklist” we can use to ensure that we have done all of the “required” steps. Lists like this are often used by design organizations to specify and propagate approaches to design within their firms. However, we should keep in mind that this and other detailed elaborations add to our understanding of the design process only in a limited way. At the heart of the matter is our ability to model the tasks done within each phase of the design process. With this in mind, we will present some means and formal methods for doing these design tasks in Section 2.3.