PLC model selection principles ( How to select PLC)

Model selection is the basic principle of meet the function requirement and guarantee under the premise of reliable, convenient maintenance, strive for the best performance/cost ratio. Reasonable structure integral PLC every I/O points of the average price is cheaper than that of modular, and the volume is relatively small, relatively fixed so commonly used in system process small control system; Flexible and modular PLC function expansion, the I/O number, input point and the proportion of output points, the types of I/O module, etc. , choice is bigger. Should be replaced when maintenance module, determine the scope of the fault is also very convenient. As a result, the modular PLC controller generally applies to more complex systems and the environment ( Large amount of maintenance) The occasion. Installation choice according to the installation, PLC system can be divided into centralized and remote I/O type and distributed PLC connected to the Internet. Centralized don't need to set up drive remote I/O hardware, system response is fast, low cost. Large systems often use a remote I/O type, because their device distribution range is very wide, remote I/O to spread installed near the I/O device, the I/O attachment, shorter than centralized, but need to add drives and remote I/O power supply. Many sets of networked distributed applicable to multiple devices independently control, and to connect with each other, can choose small PLC, but must be additional communication module. The function of the fairly general small ( Low) PLC has the logic operation, timing, counting and other functions, the only need to switch quantity control equipment can be satisfied. In the switch quantity control is given priority to, with A small amount of analog control system, can choose to take A/D and D/A unit. Add and subtract arithmetic. Enhanced low-grade PLC data transfer function. For more complex control, realize the PID arithmetic, closed loop control, communications, networking, and other functions, visual control size and complexity, choose mid-range or high-end PLC. But the price is more expensive, medium and top grade PLC general mainframe is mainly used for large-scale process control and distributed control system, etc. The requirements of response speed of PLC scanning way cause delay of 2-3 scan cycle. For most applications, the response speed of the PLC controller can meet the requirements, not a major problem. For some individual cases, however, is to consider the response speed of the PLC. To reduce the number of PLC I/O response delay time, can choose high scanning speed of PLC, or choose PLC with functions of high speed I/O processing instructions, or have fast response selection module and interrupt input module of PLC. The reliability of the system requirements for general reliability of PLC controller can meet. The high requirement of reliability of system, should consider whether to adopt or hot standby redundancy control system. Unified model, an enterprise should try to be united PLC models. Consider the following three main problems: ( l） The same type of PLC, the programming method, the same was beneficial to the improvement of the technical strength training and technical level. ( 2) The same type of PLC, it can be as spare module and is advantageous for the spare parts purchasing and management. ( 3) The same type of PLC and its peripheral equipment and general, resources can be Shared, easy networking communication, match after the upper computer is easy to form a multi-level distributed control system.
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