In the design of the PLC system, the system solution should be determined first, and the next step is the design and selection of the PLC. Select PLC, mainly to determine the specific model of the PLC manufacturer and PLC. For system solutions, there are distributed systems and remote I/O systems, and network communication requirements need to be considered. So how do you choose a PLC? To determine the manufacturer of the PLC, the main considerations should be given to the requirements of the equipment user, the familiarity of the designer to the PLC of different manufacturers, the design habits, the consistency of the supporting products, and the technical services. From the reliability of the PLC itself, in principle, as long as it is a product of a large foreign company, there should be no problem of poor reliability. In addition, for some special industries (such as: metallurgy, tobacco, etc.), we should choose a proven and reliable PLC system in the relevant industry. The number of input/output points of the PLC is one of the basic parameters of the PLC. The determination of the number of I/O points should be based on the sum of all input/output points required to control the device. In general, the PLC's I/O points should have an appropriate margin. Usually, based on the number of input and output points of the statistics, an additional 10% to 20% of the expandable margin is added, and the data is estimated as the number of input and output points. In actual ordering, the number of input and output points must be adjusted according to the product characteristics of the manufacturer's PLC. The memory capacity refers to the size of the hardware storage unit that the programmable controller itself can provide. The memory capacity of various PLCs can be found from the basic parameter table of the PLC. For example, the user program storage capacity of Siemens S7-314PLC is 64KB, and the user program storage capacity of S7-315-2DPPLC is 128KB. The program capacity is the size of the memory unit used by the user program in the memory, so the memory capacity should be greater than the program capacity. In the design phase, since the user application has not yet been compiled, the program capacity needs to be estimated. How to estimate the program capacity? Many literatures have given different formulas, which are generally 10 to 15 times the number of digital I/O points, plus 100 times the number of analog I/O points. This is the total number of words in memory (16 bits is One word), and then consider the margin by 25% of this number. Now PLC's communication function is more and more powerful. Many PLCs support multiple communication protocols (some need to be equipped with corresponding communication modules). When selecting, you should select the appropriate communication method according to actual needs. The main forms of the communication network of the PLC system are as follows: (1) PC is the main station, and multiple PLCs of the same type are slave stations, which constitute a simple PLC network; (2) One PLC is the main station, and other PLCs of the same type are slave stations, which constitute the master-slave PLC network; (3) The PLC network is connected to the subnet of the DCS as a DCS through a specific network interface; (4) Dedicated PLC network (special PLC communication network of each manufacturer). In order to alleviate the CPU communication task, according to the actual needs of the network composition, communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet, etc.) should be selected. Type of PLC: PLC is divided into two types according to the structure: integral type and modular type; The overall PLC has fewer I/O points and is relatively fixed, so the user has a small choice and is usually used in small control systems. Representatives of this type of PLC are: Siemens S7-200 series, Mitsubishi FX series, Omron's CPM1A series. The modular PLC provides a variety of I/O modules that can be plugged into the PLC base, making it easy for the user to select and configure the I/O points of the control system as needed. Therefore, the configuration of the modular PLC is relatively flexible, and is generally used for large and medium-sized control systems. For example, Siemens' S7-300 series and S7-400 series, Mitsubishi's Q series, and Omron's CVM1 series. 1. Selection of digital input and output modules The choice of digital input and output modules should consider the application requirements. For example, for the input module, the application requirements such as the level of the input signal and the transmission distance should be considered. There are also many types of output modules, such as relay contact output type, AC120V/23V bidirectional thyristor output type, DC24V transistor drive type, DC48V transistor drive type, and so on. Generally, the relay output-output type module has the advantages of low price and wide application voltage range, but has a short service life and a long response time, and needs to increase a surge absorbing circuit when used for an inductive load; The response time of the bidirectional thyristor output module is fast applied to frequent switching and inductive low power factor load applications, but the price is relatively expensive and the overload capability is poor. In addition, the input and output modules can be divided into 8 points, 16 points, 32 points, etc. according to the number of input and output points. When selecting, they should be properly equipped according to actual needs. 2. Selection of analog input and output modules The analog input module can be divided into current input type, voltage input type, and thermocouple input type according to the analog input signal type. The current input type has a normal signal level of 4 to 20 mA or 0 to 20 mA; the voltage type input module usually has a signal level of 0 to 10 V, -5 V to +5 V, and the like. Some analog input modules are compatible with voltage or current input signals. The analog output module is also divided into a voltage output module and a current output module. The current output signal usually has 0 to 20 mA and 4 to 20 mA. The voltage type output signal usually has 0 to 10V, -10V to +10V, and the like. The analog input and output modules can be divided into 2 channels, 4 channels, and 8 channels according to the number of input and output channels. The function module includes a communication module, a positioning module, a pulse output module, a high-speed counting module, a PID control module, and a temperature control module. When selecting a PLC, the possibility of matching the function module should be taken. The selection of the function module involves both hardware and software. In terms of hardware, first of all, it should be considered that the function module can be easily connected to the PLC. The PLC should have related connections, installation locations and interfaces, and connection cables. In the software, the PLC should have a corresponding control function, which can be easily programmed for the function module. For example, Mitsubishi's FX series PLC can easily control the corresponding function modules through the "FROM" and "TO" commands. After the PLC model and specifications are generally determined, the basic specifications and parameters of each component of the PLC can be determined one by one according to the control requirements, and the model numbers of each component module are selected. The following guidelines should be followed when selecting a module model. 1. Convenience: Generally speaking, as a PLC, there are many kinds of modules that can meet the control requirements. The selection should be based on the principle of simplifying circuit design, convenient use, and minimizing external control devices. For example, for an input module, the input form that can be directly connected to an external detection component should be preferred to avoid the use of an interface circuit. For the output module, the output module that can directly drive the load should be preferred to minimize the components such as the intermediate relay. 2. Versatility: When selecting the type, it is necessary to consider the unity and generality of the PLC components, and avoid too many types of modules. This not only facilitates procurement, reduces spare parts, but also increases the interchangeability of various components of the system, providing convenience for design, commissioning and maintenance. 3. Compatibility: When selecting the components of the PLC system, compatibility should be fully considered. In order to avoid problems with poor compatibility, the manufacturers of the main components that make up the PLC system should not be too much. If possible, try to choose the same manufacturer's product. Switching Residual Current Transformer Switching Residual Current Transformer,Switching Transformer,Open Residual Current Transformer,Busbar Open Residual Current Transformer Zibo Tongyue Electronics Co., Ltd , https://www.tongyueelectron.com
