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Design by Inversion

Inversion is a classic technique to solve design problems that involves exchanges functions between conventional systems or components.

Butcher's Meat Saw

Consider a simple hacksaw for wood (which was invented before the meat saw). In operation, the wood is held still, and the saw is passed over it to cut it. In a butcher's meat saw, on the other hand, the meat is held on a tray and passed by a fast-spinning, but stationary, rotary saw. That is, for the hacksaw, the saw moves through the wood which remains stationary; whereas for the meat saw, the meat moves while the blade remains stationary.

We have inverted the problem in the meat saw by requiring the meat to move against the blade rather than having the blade move through the meat.

Exercise for the reader: Why is this done? What is the problem that we are dealing with by inversion?

Computer Printer

The original computer printer was a typewriter, with a robot attached to it that would strike keys on actuation by the computer. The problem was that (a) the robot was working so fast to keep up with the output from the computer, that the inertia of the typewriter keys themselves made the hammers strike one-another and get stuck; and (b) the carriage which carried the paper could not perform a “carriage return” quickly enough to match the speed with which the computer was operated.

In this case, inversion was applied by making small, light print heads (a concept originated by the IBM Selectric Typewriter) that have little inertia, and that can be made to move across the paper very quickly. That is, the carriage no longer does carriage returns; it's the print head that provides this function. The problem was inverted by making that which moved stationary, and that which was stationary move.

Heavy Vehicle Brakes

A long time ago, brakes for trucks and other heavy vehicles were normally released and depressing the brake pedal engaged them. This is obvious and intuitive. However, there was a problem: when the brakes failed, the vehicles would lose control (imagine a truck on a highway being unable to stop for a traffic jam as a result of brake failure).

The solution, still in use today, was to invert the problem. In modern braking systems, the brakes are engaged unless the engine is running and all pressure sensors indicate nominal operation. In this case, when the vehicle is operating normally, the brakes are forced to release by a hydraulic or pneumatic mechanism. If there is a failure of the braking system, then the brakes will automatically engage, stopping the vehicle.

In this case, the inversion is functional. The function of the old braking system was to be released unless engaged by the driver; the function of the new braking system is to be engaged unless released by the driver (in combination with the brake sensors).

It is interesting to note that the human brain may work in a similar way. This article at Scientific American describes research suggesting that the brain is always active, and that some parts of the brain dampen other parts of the brain (like the truck's engine dampens the brakes by forcing them off), and that turning off the dampening with psychadelic drugs is what enables hallucinations. This is inverted from conventional wisdom, in which the psychadelic drugs “turn on” parts of your brain that are normally “turned off.”

Inkless Printing

Zink Imaging is developing a way of making high-quality full-colour digital printouts without ink in the conventional sense.

The idea is to put special heat-activated chemicals into the paper instead of the printer. In this scheme, the printer itself is little more than a high-tech heating unit, that can activate the chemical crystals in the paper, which then change colour.

This greatly simplifies the printer and opens the possibility of turning nearly any electronic product into a printer.

See Also

design/design_by_inversion.txt · Last modified: 2020.03.12 13:30 (external edit)