Descriptive Models of the Design Process

I’ve often said, “reliability occurs at the point of decision.”

At the point of design during the design process. At each and every decision.

The design team of engineers establish the bulk of the reliability capability early in the design process. The teams decisions about materials or shape, concerning inventions or outsourcing, about how and where to build the product, and many more decision impact the final product’s reliability performance. Reliability is designed into the product right from the start.

As a reliability engineer, you cannot be present nor discuss every decision during the design process. You cannot highlight the merits of each decision often carried out alone by the designer. You can understand the designer’s process and work to influence the myriad of decisions. You can create guidelines and information that design teams can use to inform their decisions.

A Simple Descriptive Model

Nigel Cross in Engineering Design Methods: Strategies for Product Design explore how design engineers go about the task of creating a new product. He outlines various descriptive models of the design process, starting with a simple four stage model.

1. Exploration — grappling with the problem, working to fully understand the problem definition
2. Generation — starts with potential solution concepts, maybe many of them
3. Evaluation — sort and revise acceptable solutions to the problem, iterates with Generation step
4. Communication — drawings, plans, details, specifications, etc.

This is different than how a scientist or reliability engineer would approach a problem. See Understanding the Design Process for more information on the differences in problem solving approaches. Here we need to be aware that during a design process if the decisions made along the way include a well formed set of boundaries concerning product reliability, the decision are more likely to create a solutions that will meet the reliability objectives.

Michael Joseph French is his book, Conceptual Design for Engineers described a detailed model of the design process. In includes the following tasks:

• Analysis of Problem —articulating the Need into a Statement of Problem
• Conceptual design — transforming the Statement of Problem into Selected Schemes
• Embodiment of Schemes — exploring and refining the Selected Schemes to select solution for Detailing
• Detailing — create drawings, instructions, specifications for the solution to meet the Need

 

The process starts with the recognition of a need

The initial analysis is a refinement of the need and an important step in the overall process

The statement of the problem should include three elements: a formal statement of the problem (goal), limitations for the solution (constraints), and excellence criterion (criteria).

The conceptual design stage pushes designers to create solutions in the form of schemes. In this phase the designer makes the most important decisions concerning the solution and it’s eventual reliability performance. The designer balances engineering science and knowledge, production methods, and business requirements to form potential solutions.

The crafted schemes on the first iteration may be little more then concept sketches. With iteration of the evaluation steps the schemes become refined and one scheme eventually becomes the path a new product.

The embodiment stage includes greater detail for the various proposed solutions, plus an eventual selection of a scheme to craft the product upon. The results of this stage is a set of general arrangement drawings or a draft product requirements document.

Detailing as the name implies is the detailed design and engineering work to finalize the essential remaining points of the design. For the reliability engineer this is the step where design engineers make decisions concerning production methods and specifications, both impacting process capability.

The result of the design process is a set of instructions, drawings, schematics, etc., that permit the assembly and delivery of the product to fill the customer’s need.

Summary

The primary work is done by design engineers with the aide of many within the organization. The initial steps to define the problem include the questions concerning product operating environment and expected longevity. As the schemes take shape, the initial reliability evaluations influence the refinement of the reliability goals and constraints. During the final steps the reliability engineer works closely with the design team to identify and resolve potential/actual reliability issues with the design, supplied components, and production process.