With the popularization of PLC, there are more and more types and models of PLC products, and their functions are becoming more and more perfect. At present, there are dozens of series of PLC products imported from abroad and assembled by domestic manufacturers or developed by themselves. Hundreds of models. Its structure, performance, capacity, instruction system, programming method, price, etc. are different, and the applicable occasions are also different. Therefore, a reasonable choice of PLC products plays an important role in improving the technical and economic indicators of the PLC control system. How to choose to meet their own needs when the products produced by various manufacturers can meet their own needs in terms of quality, which is economical and does not waste machine capacity. The selection of PLC mainly includes several aspects such as model selection, capacity selection, input and output module selection, and power supply module selection.
(1) PLC type PLC is divided into two types according to the structure of integral type and module type, according to the application environment, it is divided into two types: on-site installation and control room installation; according to the CPU word length, it is divided into 1 bit, 4 bit, 8 bit, and 16 bit , 32-bit, 64-bit, etc. From the application point of view, it can usually be selected according to the control function or the number of input and output points. The integral type PLC has a fixed number of I/O points, so users have less room for choice and are used in small control systems; modular PLCs provide a variety of I/O cards or plug-in cards, so users can choose and configure the control system more reasonably The number of I/O points, the function expansion is convenient and flexible, and it is generally used in large and medium-sized control systems.
(2) Selection of input and output modules The selection of input and output modules should consider the unity of application requirements. For example, for input modules, application requirements such as signal level, signal transmission distance, signal isolation, and signal power supply mode should be considered. For the output module, the type of output module should be considered. Generally, the relay output module has the characteristics of low price, wide voltage range, short life, long response time, etc.; the SCR output module is suitable for frequent switching, low inductance and low power factor. Load occasions, but the price is more expensive and the overload capacity is poor. The output module also has DC output, AC output and analog output, etc., which should be consistent with the application requirements. According to application requirements, intelligent input and output modules can be selected reasonably in order to improve the control level and reduce the application cost. Consider whether you need an expansion rack or a remote I/O rack, etc.
(3) Selection of power supply The power supply of PLC should be designed and selected according to the requirements of the product manual when the equipment is introduced at the same time. The power supply of general PLC should be designed and selected 220VAC power supply, which is consistent with the domestic power grid voltage. For important applications, uninterruptible power supply or regulated power supply should be used. If the PLC itself has a usable power supply, it should be checked whether the supplied current meets the application requirements, otherwise, an external power supply should be designed. In order to prevent the external high-voltage power supply from being introduced into the PLC due to misoperation, it is necessary to isolate the input and output signals, and sometimes simple diodes or fuse tubes can be used for isolation.
(4) The choice of memory Due to the development of computer integrated chip technology, the price of memory has dropped. Therefore, in order to ensure the normal operation of the application project, the memory capacity of PLC is generally required, and at least 8K memory is selected according to 256 I/O points. . When complex control functions are required, a memory with a larger capacity and a higher grade should be selected.
(5) The choice of redundant function 1． Redundancy of control unit
(1) Important process units: CPU (including memory) and power supply should be 1B1 redundant.
(2) When needed, a hot backup redundancy system composed of PLC hardware and hot backup software, a double or triple redundancy fault-tolerant system, etc. can also be selected.
2. Redundancy of I/O interface unit
(1) The multipoint I/O card of the control loop should be redundantly configured.
(2) The multi-point I/O card of important detection points can be redundantly configured.
(3) For important I/O signals, 2 or 3 heavy I/O interface units can be selected according to needs.
(6) Economic considerations When choosing a PLC, the performance-price ratio should be considered.
When considering economy, factors such as application scalability, operability, and input-output ratio should be considered at the same time to compare and take into account, and finally select a more satisfactory product. The number of input and output points has a direct impact on the price. Each additional input and output card will need to increase a certain amount of cost. When the number of points increases to a certain value, the corresponding memory capacity, rack, motherboard, etc. will also increase accordingly. Therefore, the increase in points has an impact on the selection of CPU, memory capacity, and control function range. In the estimation and selection, full consideration should be given to make the entire control system have a more reasonable cost-performance ratio.
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