A programmable logic controller (abbreviated as "PLC") is a computer used to solve problems in a specific assembly process. These devices come in a wide variety of shapes and sizes, and have many alternative options, can be used for computerized and simple I/O, and have the function of preventing high temperature, vibration and electrical noise. The invention of PLC allowed to simplify the computer into an industrial automation process.
The PLC can be a separate device that determines and execute operations, or it can be a rack for various modules that meet the requirements of the automation system. Part of it includes processors, power supplies, other IOs, interfaces, etc. Each part collaborates to select open-loop or closed-loop activities to run fast or high-precision evaluations. Take CNC machine tools as an example; PLC will be used to control positioning, motion and torque control. These devices are popular because they are cheap relative to the power and longevity they possess. The PLC can run continuously for several hours.
The following figure shows the process of the programmable logic controller system.
History of PLC
Programmable logic controllers (PLC) first appeared in the late 1960s. The main purpose of planning this equipment is to eliminate the high cost of replacing the complex relay-based control systems required by major US automakers. There is a major problem, and that is that they are mechanical. This means that they wear out and must be replaced from time to time. In addition, the relay takes up too much space. These and different considerations prompted the development of PLC. In the 1970s, more improvements were made to the PLC. In 1973, communication took place between PLCs. This allows the control circuit to be executed at a certain distance from the control machine. In some cases, the lack of institutionalization of PLC led to different problems. This was improved in the 1980s. The size of the PLC is further reduced, which means that the factory can use space more efficiently. In the 1990s, the way to modify the PLC was expanded, such as block programs and manual lists. They also observed that PLC was replaced by PC in some cases. Nevertheless, PLC is still used in a wide range of businesses and will remain in this state for the foreseeable future.
Working principle "three tasks"
The working method of PLC is very simple: PLC receives data from associated sensors or information equipment, processes the information, and triggers output according to predefined parameters.
According to the input and output, the PLC can monitor and record runtime data, such as the machine's productivity or working temperature, automatically start and stop the process, and generate an alarm if the machine fails, and this is just the beginning. Programmable logic controller is a universal and powerful control device suitable for almost any application.
The PLC essentially performs three tasks: the PLC checks the information input, traverses the program and changes the output. Then, it returns to the top and starts again. This seems very simple, but it tends to become very complicated due to various I/O sources. Scan time is the time it takes for the PLC to go through the three basic tasks. This time is important because it affects the reading speed of information input. The opening or closing time of the information source should be long enough for the PLC to read them. Once the chances of them not being there are so long, problems start to appear. Fortunately, there are some ways to solve this problem. Perhaps the most ideal method is to use interrupts when the input goes high. This will ensure that the PLC will not lose changes.
Input and output (I/O)
So far, as we have seen, input and output are very important for PLC activities. The two key elements to consider when choosing a privileged PLC are the number of I/Os and their location. Since PLC control has to go through a large process, you need to make sure that it can handle various I/Os. The number of analog devices and discrete devices the system has will also affect this choice. Remember, the number of I/Os will also determine the size of the PLC body. The location of the I/O will also affect your choice. Does your framework require local I/O, or does it require both local and remote I/O? Subsystems are needed to adequately answer these questions. Remember, the speed and distance at which the PLC runs are important for this.
