The level of system design will directly affect the performance of the control system and the reliability of the equipment. This PLC selection is very important, how to choose the appropriate PLC according to the different control requirements, design a stable operation, reliable action, safe and practical, convenient debugging, easy to maintain the control system?

        In the PLC system design, first should determine the system program, the next step is the PLC design and selection. The choice of PLC is to determine the PLC manufacturer and the specific model of PLC. For the system scheme requires a distributed system, remote I/O system, but also need to consider the requirements of network communication. So how to choose PLC specifically? The author thinks should have the content of following a few respects.

To determine the PLC manufacturer, we should mainly consider the requirements of equipment users, designers for different manufacturers of PLC familiar degree and design habits, the consistency of supporting products and technical services and other factors. Considering from the reliability of PLC itself, in principle, as long as the products of large companies, there should be no problem of poor reliability.

  In addition, for some special industries (such as metallurgy, tobacco, etc.) should be selected in the relevant industries have operational performance, mature and reliable PLC system.

       PLC input/output points is one of the basic parameters of PLC. I/O points shall be determined on the basis of the sum of all input/output points required to control the device. In general, the PLC I/O point should have the appropriate margin.

      Usually according to the statistical input and output points, and then add 10% ~ 20% of the expandable margin, as the input and output points estimate data. The actual order, but also according to the manufacturer's PLC product characteristics, the input and output points are adjusted.

  Memory capacity refers to the size of the hardware storage unit that can be provided by the PLC itself. The memory capacity of all kinds of PLC can be found in the basic parameter table of the PLC.

      How do you estimate the program capacity?

      Many literature sources give different formulas, which are generally 10 ~ 15 times the number of digital I/O points, plus 100 times the number of analog I/O points, which is the total number of words in memory (16 bits for a word), and then 25% of this number to consider the allowance.

Now the communication function of PLC is more and more powerful, many PLC support a variety of communication protocols (some need to be equipped with the corresponding communication module), the choice should be based on the actual need to choose the right way of communication.

The communication network of PLC system mainly has the following forms:

In order to reduce the communication task of CPU, communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet, etc.) should be selected according to the actual needs of network composition.

         PLC is divided into two categories according to the structure: integral type and modular type;

         Integral PLC I/0 points are less and relatively fixed, so the user has less room to choose, usually used for small control systems. This kind of PLC is represented by: WINMO DPH series, Siemens S7-200 series, Mitsubishi FX series, OMRON CPM1A series and so on.

Modular PLC provides a variety of I/O modules can be inserted on the PLC substrate, convenient users according to the need to reasonably select and configure the control system I/O points. Therefore, the modular PLC configuration is more flexible, generally used in large and medium-sized control systems. For example, Siemens S7-300 series and S7-400 series, Mitsubishi Q series, Omron CVM1 series, etc.

(a) the selection of digital input and output modules:

       The selection of input and output modules for digital quantities should be considered for application requirements. For example, the input module, should consider the input signal level, transmission distance and other application requirements. There are many types of output modules, such as relay contact output type, AC120V/23V bidirectional thyristor output type, DC24V transistor driver type, DC48V transistor driver type and so on.

       Generally, the output/output module of relay has the advantages of low price and wide voltage range, but it has short service life and long response time, and it needs to add surge absorbing circuit when it is used for inductive load. Bidirectional thyristor output module with faster response time is suitable for frequent switching, inductive low power factor load occasions, but the price is more expensive, the overload capacity is poor.

       In addition, the input and output module according to the input and output points can be divided into: 8 points, 16 points, 32 points and other specifications, the choice should also be reasonably equipped according to the actual needs.

(2) Selection of analog input and output modules:

        Analog output module is also divided into voltage type output module and current type output module, the current output signal is usually 0 ~ 20mA, 4 ~ 20mA. The voltage type output signal usually has 0 ~ 10V, -10V~+10V, etc.

        According to the number of input and output channels, the analog input and output modules can be divided into 2 channels, 4 channels, 8 channels and other specifications.

        Function modules include communication module, pulse output module, high-speed counting module, PID control module, temperature control module, etc. The choice of PLC test rate to the possibility of functional module matching, the choice of functional module involves hardware and software two aspects.

       Winmo DPH series PLC with these modules, there are weighing and other special modules.

       In terms of hardware, first of all, it should be considered that the functional modules can be conveniently connected with PLC. PLC should have the relevant connection, installation location and interface, connection cable and other accessories. In the software, PLC should have the corresponding control function, can be convenient to program the function module. For example, Winmo DPH series PLC through the "FROM" and "TO" instructions can be convenient TO control the corresponding functional modules.

  After the PLC model and specifications are generally determined, the basic specifications and parameters of each component of PLC can be determined one by one according to the control requirements, and the model of each component module can be selected. When choosing the module type, the following principles should be followed.

(1) Convenience:Generally speaking, as a PLC, can meet the control requirements of the module often have many kinds, the choice should be to simplify the line design, easy to use, as far as possible to reduce the external control devices for the principle.

(2) universality: selection, to take into account the PLC components of the module of the unity and general, to avoid too many types of modules. This is not only conducive to procurement, reduce spare parts, but also can increase the interchangeability of each component of the system, providing convenience for design, debugging and maintenance.

(3) Compatibility:When choosing each module of PLC system, the compatibility should be fully taken into account. In order to avoid the problem of bad compatibility, the main components of the PLC system should not be too many manufacturers. If possible, try to choose products from the same manufacturer.