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).



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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).

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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.

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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.

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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.

art1-fig5

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

 

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