It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.
- Louis Sullivan, 1896
Let me ask you a question. If I were to ask you to describe the design of something—say, for example, a coffee cup on your desk—how would you respond? Would you describe its form, its function, or both?
If you told me the coffee cup was 4” tall and 3.5” in diameter, you’d be describing form. If you told me it had a handle and a rim, and could hold 8 ounces of liquid, you’d be describing function.
You can’t really talk about design (at least, in the context of useful artifacts) without considering both form and function.
CAD systems provide tools to derive form from function for many common engineering problems. Even the most basic CAD systems have bolt circles, for example. High-end CAD systems do even more. Want to design an airplane wing spar? CATIA’s likely got you covered. (On second thought, it might be a little bit more involved than I thought.)
Yet, no CAD system provides a general solution to deriving form from function.
Consider this example: A reconfigurable clamp, used in an automotive factory to hold workpieces during manufacturing operations.
One of its most important functional requirements for this sort of clamp is that it not interfere with the workpiece, or with any tooling operating on the workpiece. It may touch the workpiece only at the clamping points, and may not touch the tooling at all.
Determining interference is a conceptually straightforward process:
- You compute the envelopes of allowable motions for the workpiece, tooling, and clamp, for all configurations.
- So long as those envelopes don’t intersect at any point in time (except for where they are supposed to), the clamp meets the funcional requirement of non-interference.
Piece of cake.
Except I don’t know of any CAD system that can do it.
(Technomatix or Delmia might be able to pull it off, but they’re not actually CAD systems. And, in any event, I’m not even sure they can do it. More than likely, they can alter the motion of the tooling to avoid interfering with the clamp, but I’d be surprised if they could derive the allowable envelope of the clamp from the envelopes of the tooling and workpiece.)
This automotive clamp is an example of a configuration problem. (The term configuration means, loosely, the six-dimensional space of possible positions and orientations of a rigid body) It is a difficult design problem because the clamp, the workpiece, and the tooling all have multiple configurations (the latter two being time variant.)
There is a reason that modern CAD systems have a really hard time with configuration problems. It is because they are optimized for dealing with 3D geometric objects, and treat configurations and motion as second-class objects.
As much as I’d like to be able to make fun of CAD vendors for not supporting configuration modeling, it wouldn’t really be fair. Here’s what someone who really knows what he’s talking about says:
While configuration space is a widely used theoretical concept, it has not been utilized directly in computer aided design systems for the purposes of modeling, analysis, or synthesis of mechanical systems: missing unifying theoretical foundations and lack of formulation for non-trivial problems are reinforced by a general belief that computations in configuration space are impractical.
Tomorrow, I’ll give you the name of the person who said that, and tell you about the work he’s doing to try and make configuration modeling more practical. (If you were at COFES 2011, you might have already met him.)
CAD vendors do have tools for solving some configuration related problems, but they’re limited. They don’t handle anything but special cases. In most cases, users need to either use simplified requirements, design around what their software can do, or punt and hope for the best.
For CAD vendors, there is still much to be done.