Advantages and Disadvantages of the PLC

Figure 1: A sample of the programmable logic controller produced by Allen-Bradley.

The programmable logic controller (PLC) is a digital operating electronic system specially designed for application in industrial environment. Its advantages and disadvantages are shown as follows:

Advantages of the PLC

1) High reliability. Since most PLCs use single-chip microcomputers, they have a high degree of integration. And it has corresponding protection circuit and self-diagnosis function, which improves the reliability of the system.
2) Easy programming. PLC programming mostly uses relay control ladder diagrams and command statements, the number of which is much less than that of a microcomputer. Except for medium and high-grade PLCs, small PLCs have only about 16 instructions in general. Because the ladder diagram is vivid and simple, it is easy to master, easy to use, and can be programmed even without computer expertise.
3) Flexible configuration. Since the PLC adopts a building block structure, users can flexibly change the function and scale of the control system by simply combining them. Therefore, it can be applied to any control system.
4) Complete input/output function modules. One of the biggest advantages of PLC is: for different field signals (such as direct or alternating current, digital or analog, voltage or current, etc.), there are corresponding templates that can be directly connected with devices in the industrial field (such as buttons, switches, sensing current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through the bus.
5) Easy to install. Compared with computer systems, PLC installation does not require a server room nor strict shielding measures. When in use, it is only necessary to connect the detection device correctly with the I/O interface terminals of the actuator and the PLC, and then it can work normally.
6) Fast running. Since the control of PLC is executed by program control, its reliability and running speed are superior to relay logic control.

Figure 2: A sample of a programmable logic controller.

Disadvantages of the PLC

Although the PLC has plenty of advantages, it still has some disadvantages:
1) unable to provide high-speed implementation, which limits its application;
2) requiring a programmer, which brings about extra costs;
3) not as efficient as an industrial computer when it comes to large amounts of complicated math work.
Despite these disadvantages, the PLC still plays an indispensable role in industry and daily life.

Related Info