Control Systems

Interconnections of components forming system configurations which will provide a desired system response as time progresses.

Open- and closed-loop control

The basis for analysis of a control system is the foundation provided by linear system theory, which assumes a cause-effect relationship for the components of a system.
A component or process to be controlled can be represented by a block. Each block possesses an input (cause) and output (effect).
The input-output relation represents the cause-and-effect relationship of the process, which in turn represents a processing of the input signal to provide an output signal variable, often with power amplification. An open-loop control system utilizes a controller or control actuator in order to obtain the desired response (Fig. 1).


Open-loop control system. (Fig. 1)


In contrast to an open-loop control system, a closed-loop control system utilizes an additional measure of the actual output in order to compare the actual output with the desired output response (Fig. 2).


Closed-loop control system. (Fig. 2)

Comparison of open and closed loop control systems

The terms open-loop control and closed-loop control are often not clearly distinguished. Therefore, the difference between open-loop control and closed-loop control is demonstrated in the following example of a room heating system. In the case of open-loop control of the room temperature according to Figure 3 the outdoor.

Open-loop control of a room heating system. (Fig. 3)


Temperature will be measured by a temperature sensor and fed into a control device. In the case of changes in the outdoor temperature ( disturbance ) the control device adjusts the heating flow according to the characteristic of Figure 4 using the motor M and the valve V. The slope of this characteristic can be tuned at the control device.
If the room temperature is changed by opening a window ( disturbance ) this will not influence the position of the valve, because only the outdoor temperature will influence the heating flow. This control principle will not compensate the effects of all disturbances.

Characteristic of a heating control device for three
different tuning sets (1, 2, 3) (Fig. 4)


In the case of closed-loop control of the room temperature as shown in Figure 5 the room temperature is measured and compared with the set-point value , (e.g. ). If the room temperature deviates from the given set-point value, A controller (C) alters the heat flow . All changes of the room temperature , e.g. caused by opening the window or by solar radiation, are detected by the controller and removed.


Closed-loop control of a room heating system (Fig. 5)


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