Design points of PLC system in automated production process - Database & Sql Blog Articles

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The design of the plc control system is very focused on steps and procedures. It is a system engineering, and it must be practiced repeatedly in order to be proficient in design. This paper introduces the design points of PLC system in the automatic production process. These designs have had good effects in practical application and are worthy of reference by designers.
1. Hardware Design The most important part of the PLC control system is the hardware design, which is related to the system operation safety, reliability and stability. It mainly includes two aspects of input and output.
(1) Input circuit design of PLC control system. Generally, the power supply is AC85-240V, which is suitable for a wide range of power supply. However, in order to prevent interference, a power supply purification component such as a 1:1 isolation transformer and a power supply filter should be installed. The isolation transformer can also choose to use a transformer primary and secondary coil shielding layer. Connected to the primary electrical neutral point, the secondary coil shield is connected to the PLC input circuit to the earth, etc. These double-layer isolation technology can reduce interference.
(2) Output circuit design of PLC control system. According to the requirements of the production process, the start and stop of various inverters, indicator lights and digital DC converters should use transistor output, mainly because it can be applied to high frequency operation and short response time; if the output of PLC control system The frequency is less than 6 times / 60s, the relay output is preferred, because the output circuit designed by this method is simple, and the anti-interference and load capacity are very strong. If the PLC output has an inductive load such as a solenoid coil, the PLC will have an impact on the PLC when the load is cut off. Therefore, the DC inductive load should be connected to the freewheeling diode, and the AC inductive load should be connected to the surge absorbing circuit. These two methods can be effective. Protection of the reliability and stability of the PLC control system (3) Anti-interference design of the PLC control system. With the rapid development of automation technology, the wide use of variable frequency speed control devices and thyristor controllable rectification has brought pollution to the AC power grid, and also brought a lot of interference to the PLC control system, so the anti-interference has become the PLC control system. Design Points. Generally, three methods are used: one, isolation. The distributed capacitive coupling between the primary and secondary windings creates high frequency interference in the grid. Therefore, a 1:1 superisolator transformer should be used and the neutral point should be connected to the ground via a capacitor. Second, shielded with a metal casing. This method is to place the PLC system in the metal cabinet, and the grounding of the metal shell can play a good electrostatic and magnetic field shielding effect to prevent radiation interference. Third, wiring. The strong electric power line and the weak electric signal line need to be separated and kept at a certain interval, and the analog signal transmission line is suitable to use the twisted pair shielded cable.
2. PLC control system finishing software design PLC control system software design work should start with the hardware design. Converting the process flow chart of PLC control system into ladder diagram is the key problem of PLC system, and it is also the most important task of software design of this system. The specific performance of software design is to write program. The main aspects of software design are as follows:
(1) The programming idea of ​​the PLC control system. Good software design is the key to controlling engineering applications. Excellent software design should be easy for engineers to understand, master, debug and routinely maintain. The program can be divided into basic programs and modular programs according to the complexity of the production process and the different structural forms. First, the basic program can not only control a simple production process and its process as an independent program, but also can be used as a unit program in a combined module structure. The basic program structure is divided into a sequential structure, a cyclic structure and a conditional branch structure. Second, the modular program divides the overall control target program into multiple program modules, and they have clear subtasks. These program modules are written and debugged separately, and finally form an overall program for completing the total task. Usually we adopt this design idea, mainly because the modules are relatively independent, and the relationship between them is simple, and the program transaction is modified, especially suitable for controlling more complicated production processes.
(2) Program design points of the PLC control system. PLC control system I / O allocation, according to the production line from front to back, I / O points from small to large; in order to facilitate maintenance, the I / O signal of the same system, equipment or components should be centralized for addressing, counter The timers must be numbered uniformly. In order to ensure the normal operation of the PLC system and ensure its reliability, the same number cannot be reused. The intermediate flag or internal relay used in the program should also be assigned a uniform number. After the assignment is completed, the intermediate flag or internal relay assignment table and I/O allocation table should be listed. For output devices related to each other, the output address should be arranged continuously, such as the motor's forward/reverse rotation.
(3) PLC control system programming principles. The basic principle of PCL programming is that the logical relationship should be simple and clear, occupy less memory, easy to program, and save scanning time. The main skills include: First, the various contacts in the PLC control system can be reused, there is no need to use complex procedures to reduce the number of contacts used; try to avoid using double coil output in the program, because the double coil output is easy to cause Error, if the output of the double coil is unavoidable, reset and set operation can be used; if the multiple output values ​​of the PLC are fixed value 1, it is recommended to use the digital transfer instruction to complete the task, you can use a command to hexadecimal The data 0A9H directly transmits QW2; for those devices that are not very important, it can be connected to the input of the PLC system by means of serial hardware contacts, or programmed to reduce the number of I/O points and save resources.