The Closed Control Loop

The closed control loop and it's behavior are one of the basic concepts in control engineering. On Picture 1 is shown one basic example of the closed control loop.

Picture 1: The closed control loop

As we can see from the picture, in this control loop we have one process and one controller. The process need to be controlled by the controller. Because here we talk about the basic concepts and principles of control, we do not specify the process itself, is it fast or slow process, and so on.
The controller in this control loop has two input parameters, which values are used in calculations and processing made by the controller, and as a result of those calculations the controller sends the output parameter to the process. This output parameter actually controls the process. On the picture, the input parameters are signed as x and s. Here x is the process variable that need to be held constant, and s is the setpoint that has been defined for the controller. The value of the process variable is actually the signal that we get from the sensor. For example, if the process variable is a temperature, then as sensor which will measure that temperature can be used a resistance thermometer or a thermocouple. The sensor is connected to an input on the controller which is proper for that kind of sensor.

Changing its output level, the controller regulate the value of process variable x to meet the setpoint s. The difference between the setpoint and the process value is known as the control deviation e ( e = s - x ). So, in a closed control loop, the value of the process variable can be influenced by the output level y, which on the other hand is controlled by the output level of the controller c. In most of the cases, the controller can not alter the output level y directly, and therefore actuators are used. Actuators are control devices that are operated by the controller, through its controller output level c. For example, actuator can be a valve which fed the process with energy, if we have a gas-fired oven, then actuator is the gas valve which will be fed into the process (oven) the amount of gas depending on the controllers output level.

An undesirable effect on the process variable have a disturbance z, especially when it alters its value.