PLC control system hardware design of PLC products to consider

With the popularization and application of PLC controller control, the amount and type of PLC products is increasing, and the function is perfect. In recent years, from the United States, Japan, Germany imported PLC products and products developed by the domestic manufacturer assembles or has dozens of series, hundreds of models. Mainly used in domestic PLC products mainly include: AB, GE, MODICON corporation in the United States, Germany's Siemens, Japan OMRON, mitsubishi PLC products. So PLC variety, its structure, performance, capacity, instruction system, programming method, price and so on various has its own characteristics, applicable occasions each have emphasize particularly on, too. Therefore, the rational selection of PLC, to improve the technical and economic indexes of PLC control system plays an important role. General choice models to meet the needs of the system functions as an objective, don't go in for grandiose projects blindly, lest cause investment in equipment and waste of resources. On the choice of models from the following several aspects to consider. Figure 2 figure 3 Siemens S7 - mitsubishi FX2N series PLC Series 200, figure 4 American company AB SLC500 PLC figure 5 domestic yonghong latest -based SoC ( 的边后卫) Micro computer ( 1) The selection of input/output point to find out in addition to the control system of the I/O points, and then according to the actual needs of 15 to 20% of the total points (set aside the spare quantity Leave room for system reform, etc) After determining the points of PLC. PLC output points can be divided into the common point, grouping and isolation of several kinds of connection. Isolation of each output point can be used between different types of voltage and voltage grade, but the price of this PLC on average every point is higher. If you don't need isolation between the output signal, should choose the way to the first two output of PLC. ( 2) Choice of storage capacity for user storage capacity can only make a rough estimate. In only the switch quantity control system, can be used to input total points by 10 words / + total output point by 5 words/point to estimate; According to the (counter/timer 3 ~ 5) Word/estimate; Have a processing according to the ( 5 ~ 10) Word/quantity estimation; In analog input/output system, can according to each input/( Or output) Analog about (all the way 80 ~ 100) Words about storage capacity to estimate; When there is communication processing according to the number of more than 200 words each interface as a rough estimate. Finally, according to estimate the capacity of 50 ~ 100% commonly left margin. For lack of experience of the designer, select the capacity left margin be larger. ( 3) Choice of I/O response time of PLC I/O response time includes scanning delay and input delay circuit, output circuit works caused by time delay ( Be in commonly 2 ~ 3 scan cycle) And so on. The switch quantity control system, PLC and I/O response time is generally can meet the requirements of practical engineering, can need not consider the I/O response problem. But for analog control system, especially the closed loop system to consider the problem. ( 4) According to the characteristics of the output load selection of different ways of load on the output of the PLC have corresponding requirements. Frequent on and off, for example, the inductive load, should choose transistor or thyristor output model, and should not choose relay output type. But the relay output type PLC has many advantages, such as conduction voltage drop is small, have isolation effect, the price is relatively cheap, strong ability to withstand transient overvoltage and overcurrent, flexible (the load voltage Can communicate, dc) And a large range of voltage grade, etc. So don't move frequently can choose ac, dc load relay output type PLC. ( 5) The selection of online and offline programming off-line programming is refers to the host and the programmer Shared a CPU, through programmer mode selector switch to choose PLC programming, monitoring, and operation condition. Programming state, CPU for programming services, without the field control. Online programming refers to a CPU in each host and programmer, the host CPU to complete control of the scene, and at the end of each one scan cycle programmer communication, programmer to modify the program host, host the next scan cycle will be effected according to the new program to control the scene. Computer aided programming can not only realize the off-line programming, also can realize the online programming. Need to buy computer, online programming and configuration software programming. What kind of programming method should be determined according to the need. ( 6) According to whether if networking communication type selection of PLC control system need to get the factory automation network, the PLC controller requires communication networking, requiring PLC should have to connect other PLC and upper computer, CRT interface, etc. Large and medium-sized machine has function of communication, at present most minicomputer also has the function of communication. ( 7) The selection of structure form of PLC I/O points in the same function and the same data, integral is lower than price of modular. But flexible modular has function expansion, convenient maintenance, In module) , the advantages of easy to judge fault, want to choose the structure of PLC according to the actual needs.
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