TK457V050 3BSE004394R1 relies on feedback from the process

Basically, there are two types of control loops: open-loop control and closed-loop (feedback) control.

In open-loop control, the control action of the controller is independent of the “process output” (or “controlled process variable” -PV). A good example is the central heating boiler, which is controlled by only one timer, so that heat can be supplied continuously regardless of the temperature of the building. The control action is the timing switch of the boiler, and the process variable is the building temperature, but neither is related.

In closed loop control, the control action from the controller depends on feedback from the process in the form of the value of the process variable (PV). In the case of a boiler simulation, a closed loop will include a thermostat that compares the building temperature (PV) to the temperature set on the thermostat (set point – SP). This will produce a controller output that maintains the building at the desired temperature by turning the boiler on and off. Thus, a closed-loop controller has a feedback loop that ensures that the controller applies a control action to manipulate the process variables as the “reference input” or “set point”. Therefore, the closed-loop controller is also called the feedback controller. [11]

According to the British Standards Institute, a closed-loop control system is defined as “a control system with monitoring feedback, the deviation signal formed as a result of this feedback is used to control the action of the final control element, which tends to reduce the deviation to zero.” [12]