Analog Angle Article

When your problem is also part of the answer

Engineers like to bemoan the things that cause circuit and system misery, such as dissipation and noise. And they're right, of course. If you really drill down to the core of the design obstacles, internal and external noise is the root of much of it.

Think about it for a while: if there was no noise, virtually every aspect of design would be easier. The only “benefits” I know of to noise is when it is used to mask or encrypt a signal, or to get higher resolution from an A/D converter via dithering; however, in a zero-noise system, there are other, better techniques to achieve both these goals.

But engineers also have issues with fundamental forces such as gravity and friction. If not for friction or weight induced by gravity's pull, many electromechanical design problems would go away.

Or would they? Without friction and gravity, there would be lots of new problems, instead. Fundamental operations that count on these factors, usually without conscious thought, would be impractical or impossible. Try putting paper into a printer, or having the printer feed it through, in a zero-friction world.

Instead, smart designers look to turn these detriments to advantage, when possible. When engineers were installing Amtrak's overhead electrification system for the New York to Boston tracks a few years ago, they did not use springs to tension the guy wires of the overhead feeder catenary system. Instead, they used dead weights and a simple pulley, similar to an exercise machine (see photo here). I asked the construction foreman about this, and said there were many reasons.

First, they would need to have several spring sizes, since the desired tension varied along different stretches of the line; with the weight system, they simply changed the weight load. Spring coefficient and tension varies with temperature, while tension due to weights is only a function of gravity, and independent of the ambient temperature. And of course, the length of the wire being tensioned changes dramatically with temperature, adding to the spring challenge; a weight system doesn't care. The only drawback to the pulley and weight approach is that the hub of the pulley has to be greased periodically.

What these designers have done is take something we may complain about, but have in abundance, and use it beneficially. That's good design thinking.

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