1. Stand-alone control system
The stand-alone control system is controlled by one PLC to control one device or one simple production line, as shown in Figure 7-1. The composition of the stand-alone control system is simple, the number of I/O points required is less, the storage capacity is small, when choosing the model of PLC, regardless of whether there is a requirement of communication networking at present, the PLC with communication function should be selected, so as to meet the requirements of future system function expansion.
2. Centralized control system
The centralized control system is to control multiple devices or several simple production lines by one PLC, as shown in Figure 7-2. This control system is characterized by the close proximity of the positions of multiple controlled objects, and there is a certain relationship between their actions. Since multiple controlled objects are controlled by the same PLC, there is no need to set up a special communication line for the change of data and status between each controlled object.
The biggest disadvantage of a centralized control system is that if the control program of a controlled object needs to be changed or the PLC fails, the whole system will stop working. For large centralized control systems, redundant systems can be used to overcome this shortcoming, which requires a large margin of I/O points and memory capacity of the PLC.
Stand-alone control system
Centralized control system
3. Remote I/O control system
The remote I/O control system is that the I/O module is not placed together with the PLC, but is placed near the controlled object at a distance. Information is transmitted between the remote I/O channel and the PLC via a coaxial cable connection. The length of the coaxial cable should be selected according to the needs of the system. Figure 7-3 shows the configuration of the remote I/O control system. Three remote I/O channels (A, B, D) and one local I/O channel (C) are used.
Remote I/O control system
4. Distributed control system
The distributed control system has multiple plant objects, and each plant is controlled by a PLC with communication function, as shown in Figure 7-4.
Distributed control systems
The characteristics of the distributed control system are that the distribution area of a plurality of controlled objects is larger, and the distance between each other is far, and each PLC can communicate with the host computer through the data communication bus, and can also exchange information with other PLCs through the communication line. The biggest benefit of a distributed control system is that if one plant or PLC fails, it does not affect the other PLCs.
The development of PLC control system is very fast, after the stand-alone control system, centralized control system and distributed control system, the EIC integrated control system of PLC is proposed, that is, electrical control (Electric), instrument control (Instrumentation) and computer (Computer) control are integrated into one to form an advanced EIC control system. Based on this control idea, when carrying out the overall design of PLC control system, it is necessary to consider how to adapt to this advanced nature, and is conducive to the further expansion of system functions.
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