PLC is not difficult to learn. The PLC programming language complies with the IEC 61131 standard. The more mainstream language is the modular programming language, which is the IEC 61131-3 language.
Since all PLC manufacturers must follow the IEC 61131 standard, after learning a certain PLC in depth, other PLCs are generally similar. T-shaped diagrams are generally used when explaining the school PLC textbook. This is a very backward programming method. After going to work, be sure to use modular programming. The reason why you want to abandon the programming method of T-shaped graphs is because T-shaped graphs have many limitations. To give a simple example: When three switches form a triangle on a T-shaped graph, the T-shaped graph cannot be executed and must be converted to a star structure using a triangle-star. This problem does not occur at all in the modular programming language. Due to the limitations of the T-shaped diagram, the module diagram is also started in the pure T-shaped diagram program.
The following picture is part of the program I wrote for the dual-channel automatic power-off system for the ring-controlled power distribution room of a station in Beijing Metro: This program is written in the modular programming language IEC 61131-3.
It can be seen that the modular PLC program is somewhat similar to the form of integrated circuits, and even the PID measurement and control has corresponding modules. Let's take a look at the communication management part of the same program:
There is a communication interface initialization program, a MODBUS-RTU master definition program, and a MODBUS-RTU slave data acquisition and exchange cycle program.
It can be seen that the modular PLC programming language is very different from the T-shaped programming language learned in the school. Since the PLC programming language supports program debugging at the software level, there is not much problem with the simulation.
How to learn PLC?
In addition to having a programming environment, an engineering paradigm is required. In fact, the engineering examples are very easy to obtain. We can find any book describing the motor control, and all the various ways to control the motor, of course, are made up of relays and contactors. We write all these electromechanical control methods into PLCs. control method. After a few examples have been compiled, I basically understand.
After the examples are compiled, add difficulty to yourself. For example, we envision a four-motor, which is used as a central air-conditioning chiller, hot water unit, cooling tower motor, and a fan of the terminal air-conditioning unit, which constitutes a complete central air-conditioning system. The central air conditioner adopts the hot enthalpy control mode and adopts PID adjustment. Try to use PLC as the central control system to write a program. After this example is completed, finally learn the communication technology of PLC.
Pay special attention to the MODBUS-RTU module in the PLC modular programming language and understand its contents and essentials. Then write a data acquisition and forwarding system. The data source is the peripheral switching quantity, temperature quantity, enthalpy value, current and voltage parameters, motor running status, etc. of all motors in the previous example. These parameters are written into a communication protocol, ie a data point table, which is then sent to the ACCESS database on the computer. Using ACCESS's VBA, write a number of control interfaces, and then implement information exchange and control on the software. After all these are completed, you can graduate. We are now qualified for any PLC programming work.
The application of PLC is very extensive. In an industrial environment, PLC is almost omnipotent. Know why?
The first is the high reliability of the PLC: because its programs are executed sequentially, they do not enter an infinite loop and hardly crash. It is very important that the PLC will not crash. We originally used the industrial computer as the communication management machine for the measurement and control and distribution system. As a result, it was found that the hard disk of the industrial computer would malfunction under high temperature, and once the power of the industrial computer stopped, the power supply quickly stopped due to overheating (dead), which caused communication to block. The industrial computer implements data acquisition and control functions through various card components, and these card components have a high failure rate. It is very difficult for the industrial computer to realize hardware redundancy, and the CPU hardware redundancy of the PLC is very convenient. After we replaced the communication management machine with PLC in the measurement and control project, these problems were completely solved once and for all.
The second is that PLC has a high level of EMC (electromagnetic disturbance), that is, anti-interference ability. The so-called EMC refers to the ability of electrical components to withstand environmental electromagnetic interference during operation, and the extent to which electromagnetic interference occurs to the environment. The IEC and national standards impose mandatory EMC test requirements on electrical components and control cabinets. High-end PLCs generally need to pass the 2nd to 3rd stage EMC group pulse test and grounding current impact test. I have tested that the board of a multi-function power meter is only slightly damaged because of the power supply filter capacitor welding. The whole board is destroyed in the ground current impact test. It can be seen that EMC testing is very cruel to electronic instruments, and it is very difficult to pass the test.
Bottom line: Anything that a single-chip computer can do, PLC can do it; in turn, what PLC can do, the microcontroller can't do it, and even the side can't touch it. Contrast: It is difficult for general household appliances to pass the first-level EMC test. Because the PLC application is so wide, a senior programmer of PLC is also a generalist of an industrial system. He is familiar with the performance parameters of components and sensors under various industrial conditions, is familiar with the measurement and control principles and automatic control principles under various industrial conditions, clarifies the mechanical principle and various industrial processes, and clarifies how to select components in a strong electromagnetic environment. And switching appliances. Of course, this person should be very familiar with the PLC module. In short, excellent PLC programmers must also be technical leaders. ==================
At the beginning of the study, don't always think about making money, and sink your heart to learn the true ability. Where is the world, where is there no grass? Why is there nowhere to work? The money earned by the PLC salesman is far less than the money earned by the PLC programmers; after the construction is over, the user is remembering the programmer, not the supplier of the PLC; in the follow-up project, the programmer will continue to participate, and What about PLC salesmen? At most, it is only for a few PLCs. Conclusion: The key is to be your own programming skills and skill level.
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