Tuesday March 3, 2015

The roots of CAD, part 2: The design process

Steven Coons

Steven Coons, in front of the MIT Lincoln Laboratory TX-2 computer

In 1959, Douglas Ross, Steven Coons, and John Ward, along with their colleagues, started the MIT Computer-Aided Design Project, and planted the seeds that grew to define modern CAD.

At the 1963 Spring Joint Computer Conference, Steven Coons delivered a seminal paper on CAD: An Outline for the Requirements for a Computer-Aided Design System. A few days ago, in part one of this article, I published an except from that paper, where Coons described the original thinking at MIT about CAD. Today, I’m going to except an even more interesting section on the design process.

The distinctions that Coons makes in this paper are still relevant and valuable 50 years later, and point to way to today’s model-based engineering and design methodologies.

The Design Process

The design process begins with a graphical description of a proposed device or system to satisfy a human need. To say that the description is graphical is to assert that at the very inception of an idea the designer’s understanding of his creation is almost visceral instead of intellectual. He perceives his idea at first not in the perfection of a well-turned English word description, nor in the precision of a mathematical formula, but in some nebulous assembly of building blocks of structure, vaguely beheld; he “feels” his creation. The sketch forms the natural bridge between these vague stirrings of the imagination and the subsequent precise statement of the refined details of the concept.

At this early stage, decisions to keep, to modify, or to discard part or all of the original concept are made in a qualitative way, based upon qualitative criteria. The modified concept leads to further qualitative decision making, and to further modification of the concept. While this is going on, the concept which was at first nebulous and incomplete begins to assume a more concrete solid character; it becomes better defined, until at some stage it is well enough defined to permit more precise analytical tools to be applied.

In the design process, the designer is concerned with a large set of variables, some continuous (like the weight of a part) some belonging to discrete “point sets” (like the material: steel, brass, lead, plastic.) Moreover, these variables are interrelated, or cross coupled, in a very complex way. Some of the cross couplings are weak, some are strong. If the relationships happen to be linear, the cross couplings are constant in strength, but usually the relationships are non-linear, and the mutual influences of the various variables change with their values.

The designer structures such relationships so that he can thread through them, taking advantage of the loose couplings where possible, to obtain hopefully an exact, but more usually a first, or second, or closer approximation to the values of the variables. It is not at all unusual for this structuring to be done graphically, in the form of block diagrams or linear graphs or information flow charts. Thus he uses a graphical form for both the topological and geometric description of the design, and also for its abstract description in terms of physical function.

At the conclusion of the design process, the final result must be carefully defined so that it can be built. This is the function of layout draftsmen and detail draftsmen. If automatically controlled machines are involved in the fabrication processes, programmers are also a part of the system.

When we look at such a design sequence we see a few engineers performing highly creative tasks at the beginning, coupled with a very large number of draftsmen and technicians who perform relatively uncreative tasks over a fairly long period of time. Some of these tasks require high degrees of intellectual effort, such as stress analysis or aerodynamic analysis, but they are none-the-less not in themselves of a creative nature (except in those cases where new mathematical techniques are designed and put to use). Other tasks are obviously of a purely mechanical nature; for example, a detail draftsman does nothing creative whatever. At the worst, he merely traces the outline of a part from the layout drawing, and adds the dimensions. Usually this drawing goes directly to some machinist or patternmaker in the shop, but sometimes it is used by a part programmer and converted by him into symbolic information for use by a computer to prepare punched tape for automatic fabricating machinery. These are all essentially mechanical operations, however, and it is quite clear that at least in principle, the computer can be made to deal with them all.

Excerpted from: Coons, Steven, “An Outline for the Requirements for a Computer-Aided Design System,” Proceedings of the Spring Joint Computer Conference, Detroit, Michigan, May 21-23, 1963.

2 Responses to “The roots of CAD, part 2: The design process”
  1. Strabilla says:

    Coon’s arrogant assertions about the downstream tasks being mechanical are the sort of attitude that allowed the Japanese, and then the rest of East Asia to take over the world’s manufacture. The downstream engineer has the designer as his customer, and needs to be just as creative in fulfilling the needs of that customer.

  2. RogerDodger says:

    Now, Strabilla: Be nice. Understand that Coons is one of many PhD’s that don’t really have a clue of what it takes to produce a finished product. I have worked with a couple that did, but, after 40 years, they certainly are the exception.

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