In its simplest form, a conrtol system provides an output or response (or controlled variable) for a given input or stimulus (or command signal) as shown in the figure.
Some example of control systems
1. Think about the situation when you are driving a car. What happens when you apply force on the accelerator? You know the car will speed up. The force applied to an accelerator pedal causes the speed of the vehicle to increase. In this case!
Input or command signal = force applied
Output or Controlled variable = speed
2. Air conditioner control system = automatically regulating the temperature of a room or an enclosure.
Input or commend signal = reference temperature by setting a thermostst
Output or Controlled variable = actual temperature of the room or enclosure
When the thermostat detects that the output is greater than the input, the cooler provides cooled air until the temperature of the room equals to the reference input. Then the cooler automatically turn off. When the temperature rises above the reference then again the cooler is turned on.
3. Pointing at an object with a finger
Input or commend signal - precise direction of the object (moving or not) with respect to some reference.
Output or Controlled variable - actual pointed direction with respect to the same reference.