Means by which a set of variable quantities is held constant or caused to vary in a prescribed way.
Control systems are intimately related to the concept of automation but have an ancient history. Roman engineers maintained water levels in aqueducts by means of floating valves that opened and closed at appropriate levels. James Watt 's flyball governor (1769) regulated steam flow to a steam engine to maintain constant engine speed despite a changing load. In World War II, control-system theory was applied to anti-aircraft batteries and fire-control systems. The introduction of analog and digital computers opened the way for much greater complexity in automatic control theory . See also Jacquard loom , pneumatic device , servomechanism .