If the level drops below the halfway mark, we can pour water in to increase the level. If the level rises above halfway, we can let water flow out to decrease the level. It is also possible to design a device to do this job automatically, by measuring the level of the tank and taking appropriate action depending on the measurement. Designing such automatic controllers is the focus of control engineering. Control engineering is about controlling systems.
Controllers adjust system inputs to produce desired system outputs.
To design controllers, control engineers use mathematical models of the system to be controlled. These models extract information about the essential behaviour of the system and discard irrelevant detail. In the water tank example all that is important is that letting water flow into the tank increases the level and that letting water flow out decreases the level. If A is equal to B , C remains constant. If A is less than B , C decreases.