Watching the Watchman

As machines get more complicated, quick-acting and violent, they are more prone to self-destruction if something goes wrong. Some nuclear reactors, for instance, can turn into radioactive junk in a fraction of a second. To avoid such misadventures, most modern mechanical and electronic systems are equipped with built-in monitors that watch their operation and shut them down promptly at the first sign of trouble. But if a vacuum tube or relay in the monitor fails, the main machine is like a building whose night watchman has dropped dead. Trouble can start and get out of hand with no one to correct it or give the alarm.

In Control Engineering, W. G. Rowell of Scully Signal Co. and A. B. Van Rennes of M.I.T. describe a method that they have invented for "watching the watchman." The monitor as usual watches all operations of the machine, but when everything is going well, it does not merely sit back and give a "safe" signal. Instead it gives a rapid alternation of safe and unsafe signals. Unless this alternation continues, proving that the monitor is alert and on the job, the machine will shut itself off. If any part of the machine fails (including its readiness to shut itself off if the monitor fails), the wide-awake monitor steps in and stops everything.

Rowell and Van Rennes believe that this basic principle of "exercising" the monitor to prove that it is still alert can be applied to anything from airplane controls to chemical factories. It will not keep them from failing, but it should make them "fail safe," even when the electronic watchman has died on the job.

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