Fundamentally, there are two types of control loops: open loop control and closed loop (feedback) control. Control theory is concerned only with closed loop control.
In open loop control, the control action from the controller is independent of the “process output” (or “controlled process variable” - PV). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. The control action is the timed switching on/off of the boiler, the process variable is the building temperature, but neither is linked.
In closed loop control, the control action from the controller is dependent on feed back from the process in the form of the value of the process variable (PV). In the case of the boiler analogy, a closed loop would include a thermostat to compare the building temperature (PV) with the temperature set on the thermostat (the set point - SP). This generates a controller output to maintain the building at the desired temperature by switching the boiler on and off.
A closed loop controller, therefore, has a feedback loop which ensures the controller exerts a control action to manipulate the process variable to be the same as the “Reference input” or “set point”. For this reason, closed loop controllers are also called feedback controllers.
The definition of a closed loop control system according to the British Standard Institution is ‘a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero.’ "
Likewise; "A Feedback Control System is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control.’
Fig : A block diagram of a negative feedback control system using a feedback loop to control the process variable by comparing its with a desired value, and applying the difference as an error signal to generate a control output to reduce or eliminate the error.