8Control Systems

Control system is a device (or set of devices), which manages, commands and controls behaviour of other devices or systems.

Control system may represent simple control of a machine that allows a worker to perform simple operations. An example could be a machine that is manually operated, but it is equipped with a safety logic ensuring that if a worker is in the workspace, the machine cannot be put into operation.

Automatic sequential control system may trigger a series of mechanical actuators in the correct order to perform the required task. An example of this may be increasing of the air flow when CO2 limit is exceeded in a room. Use of PID controllers in such cases is very common and efficient. Control systems that include scanning and evaluation of the results to achieve the desired goal can adapt to constantly changing circumstances.

Nowadays, control systems use IP-based networks in buildings to communicate with each other and with supervisory systems, and to provide data and information to SCADA systems. The communication infrastructure is very often shared with other systems, such as VoIP (voice over IP), office network, etc. Due to this fact, the importance of network control safety is very high.

Two types of control systems are used in common – control systems with open or closed loop. In the open loop system, the output of the system is based on the input information.

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Fig. 8.1: Direct control systems

In the closed loop system, the output is based partly or solely on the information obtained by feedback. This type of control system is therefore called feedback control system.

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Fig. 8.2: Feedback control systems

Washing machine is an example of a control system with open loop. When the start button is pressed, a predefined and fixed program is executed. Human body is a typical example of a feedback control system.

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Fig. 8.3: PID regulator scheme