Type of 1.plc
PLC is divided into two types according to the structure: integral type and modular type. It is divided into two types according to the application environment: field installation and control room installation. According to the CPU word length, it is divided into 1 bit, 4 bits, 8 bits, 16 bits, 32 bits, 64 bits. Wait. From the application point of view, it is usually possible to select the control function or the number of input and output points.
The integral PLC has fixed I/O points, so the user has less room for small control systems. The modular PLC provides multiple I/O cards or cards, so the user can select and configure the control system reasonably. The number of I/O points, the function expansion is convenient and flexible, and is generally used for large and medium-sized control systems.
2. Selection of input and output modules The selection of input and output modules should be considered in line with the application requirements. For example, for the input module, 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. Usually, the relay output module has the characteristics of low price, wide operating voltage range, short life, long response time, etc. The thyristor output module is suitable for frequent switching, inductive low power factor. Load occasions, but the price is more expensive, the overload capacity is poor. The output module also has DC output, AC output and analog output, which should be consistent with the application requirements.
According to the application requirements, intelligent input and output modules can be reasonably selected to improve the control level and reduce the application cost.
Consider whether you need to expand a rack or remote I/O rack, and so on.
3. Power supply selection
PLC power supply, in addition to the introduction of equipment at the same time the introduction of PLC should be designed and selected according to product specifications, the general PLC power supply should be designed to use 220VAC power supply, consistent with the domestic grid voltage. For important applications, it should be powered by an uninterruptible power supply or a regulated power supply.
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, isolation of the input and output signals is necessary, and sometimes a simple diode or fuse tube isolation can be used.
4. Memory selection Due to the development of computer integrated chip technology, the price of memory has been reduced. Therefore, in order to ensure the normal operation of the application project, the memory capacity of the PLC is generally required, and at least 8K memory selection is selected according to 256 I/O points. When complex control functions are required, you should select a memory with a larger capacity and a higher grade.
5. Selection of redundant functions
1. Control unit redundancy
(1) Important process units: CPU (including memory) and power supply should be 1B1 redundant.
(2) Hot standby redundancy system, 2-fold or triple-redundant redundant fault-tolerant system composed of PLC hardware and hot standby software can also be selected when needed.
2. Redundancy of I/O interface units
(1) The multipoint I/O card of the control loop should be redundantly configured.
(2) Multi-point I/O cards at important detection points can be redundantly configured. 3) Optional 2 or 3 I/O interface units for important I/O signals as needed.
6. Economic considerations When choosing a PLC, the performance price ratio should be considered. When considering economics, we should consider the scalability, operability, input-output ratio and other factors of the application, compare and consider, 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 requires a certain increase. When the number of points is increased to a certain value, the corresponding memory capacity, rack, motherboard, etc. should also be increased accordingly. Therefore, the increase of the number of points has an influence on the selection of the CPU, the memory capacity, and the range of the control functions. It should be fully considered in the estimation and selection, so that the entire control system has a reasonable performance price ratio.
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