Applications of feedback control system
The purpose of any control system is to control a set of system states. To make you understand Feed Back(FB) Control System, let me compare it with a control system without FB.
Well, a simple example that I can think of right now about a system without FB is a gas knob. Once the knob is set to a particular position, the heat generated will almost be constant, assuming the gas pressure and other parameters are not changing.
Let us try to apply the same sort of controller in a thermostat/air-conditioner for the house, we will set a knob and the air-conditioner(AC) will start to cool the house. If it continues to cool the house then we will wind up with an extremely cold situation in the house. Normally, we desire to set the temperature inside the house at some constant value. How can we achieve that ? By regulating the knob of the AC as a function of the temperature inside the house.
Similar is the case with constant speed drive, suppose we want to drive a car at constant speed, and we calibrated the thrust/power required to deliver the same for plane road and specific road conditions. If we will provide the same power for an inclined road situation then the desired speed will not be achieved. So that we need to do is provide the FB about the current speed(or any other parameter relating to speed) of the car, in turn, the car will provide more/less power till the desired speed is not met.