ABSTRACT
The purposeful management of the state of an engineering system over time is called system control or, to emphasize that the details are carried out by machine rather than by a person, automatic control. In one scenario, the state is varied in some desired manner: a vehicle is accelerated or turned automatically, a machine tool is directed to follow a prescribed path, or a metal casting is cooled in such a manner that its properties achieve a desired standard. In the other most common scenario, the state is held constant despite buffeting from the environment: a boat is prevented from rolling in the waves, a rolling mill for sheet steel produces uniform thickness product despite thermal expansion and contraction of the rollers, or the temperature in your house is held at 72◦F despite outside temperature fluctuations. These two scenarios often coexist.
