In the design of the plc system, the system solution should be determined first, and the next step is the design 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 to choose PLC specifically? I think there should be the following aspects. 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. The author personally believes that, in general, for the control of independent equipment or a simple control system, supporting Japanese PLC products, the relative cost performance has certain advantages. For the system with large-scale network communication function requirements, open and distributed control system, remote I / O system, PLC produced in Europe and America has more advantages in network communication functions. 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 determination of the number of I/O points should be based on the sum of all I/O points required to control the equipment. 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. In the design phase, since the user application has not yet been compiled, the program capacity is unknown at the design stage and needs to be known after the program is debugged. In order to estimate the program capacity when designing the selection, it is usually replaced by an estimate of the memory capacity. There is no fixed formula for estimating the memory capacity of PLC memory. 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 number is the total number of words in the memory (16 bits is a word), and the remaining amount is considered by 25% of the number. (1) Computing function The operation functions of ordinary PLC also include data shifting, comparison and other arithmetic functions; more complex arithmetic functions include algebraic operations and data transfer; large-scale PLCs also have analog PID operations and other advanced arithmetic functions. With the advent of open systems, communication functions are now available in PLCs. Some products have communication with the lower computer. Some products have communication with the same or upper computer. Some products also have data communication with the factory or enterprise network. The function. When designing and selecting, we should proceed from the requirements of practical application and reasonably select the required computing functions. In most applications, only logic operations and timing counting functions are required. Some applications require data transfer and comparison. When used for analog detection and control, algebraic operations, numerical conversions, and PID operations are used. In addition, operations such as decoding and encoding are required to display data. (two) control function Control functions include PID control operations, feedforward compensation control operations, ratio control operations, etc., which should be determined according to control requirements. PLC is mainly used for sequential logic control. Therefore, in most occasions, single-loop or multi-loop controllers are often used to solve analog control. Sometimes, dedicated intelligent input/output units are used to complete the required control functions, and the processing speed of the PLC is improved. Save memory capacity. For example, a PID control unit, a high-speed counter, an analog unit with speed compensation, an ASC code conversion unit, and the like are employed. (3) Communication function Large and medium-sized PLC systems should support a variety of fieldbus and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when needed. The communication protocol shall comply with the ISO/IEEE communication standard and shall be an open communication network. The communication interface of the PLC system shall include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication ports, industrial Ethernet, common dcs interfaces, etc.; large and medium-sized PLC communication buses (including interface devices and cables) shall be 1 :1 Redundant configuration, communication bus should comply with international standards, communication distance should meet the actual requirements of the device. In the communication network of the PLC system, the upper-level network communication rate should be greater than 1 Mbps, and the communication load should be no more than 60%. 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 master 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 CPU communication tasks, PLC should select communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) according to the actual needs of the network. (four) programming function Offline programming mode: PLC and programmer share a CPU. When the programmer is in programming mode, the CPU only provides services for the programmer, and does not control the field devices. After programming is completed, the programmer switches to the run mode and the CPU controls the field device and cannot be programmed. Offline programming reduces system cost but is inconvenient to use and debug. Online programming mode: CPU and programmer have their own CPU. The host CPU is responsible for field control and exchanges data with the programmer in one scan cycle. The programmer sends the program or data programmed online to the host, the next scan cycle. The host runs according to the newly received program. This method is costly, but the system is easy to debug and operate, and is often used in large and medium-sized PLCs. Five standardized programming languages: sequential function diagram (SFC), ladder diagram (LD), function block diagram (FBD) three graphical languages ​​and statement table (IL), structure text (ST) two text languages. The selected programming language should comply with its standard (IEC6113123), and should also support multi-language programming forms, such as C, Basic, Pascal, etc., to meet the control requirements of special control occasions. (5) Diagnostic function Diagnostic functions of the PLC include hardware and software diagnostics. The hardware diagnosis determines the fault location of the hardware through the logic judgment of the hardware, and the software diagnoses the internal diagnosis and the external diagnosis. Diagnosing the performance and functions of the PLC through software is an internal diagnosis. Diagnosing the information exchange function of the CPU and external input and output of the PLC through software is an external diagnosis. The strength of the diagnostic function of the PLC directly affects the requirements of the technical capabilities of the operation and maintenance personnel and affects the average maintenance time. (6) Processing speed The PLC works in a scanning mode. From the perspective of real-time requirements, the processing speed should be as fast as possible. If the signal duration is less than the scan time, the PLC will not scan the signal, resulting in loss of signal data. The processing speed is related to the length of the user program, CPU processing speed, software quality, and the like. At present, the response of the PLC contacts is fast and the speed is high. The execution time of each binary instruction is about 0.2~0.4μs, so it can adapt to the application requirements with high control requirements and corresponding requirements. The scan period (processor scan period) should be such that the scan time of the small PLC is not more than 0.5ms/K; the scan time of the large and medium PLC is not more than 0.2ms/K. 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. Six, various module selection (a) digital I / O module 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 the surge absorbing circuit when used for an inductive load; the response time of the bidirectional thyristor output type module is fast. It is suitable for frequent switching, inductive low power factor load applications, but the price is more 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) Analog I/O module 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. (three) function module 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. (1) Control unit redundancy 1. Important process unit: CPU (including memory) and power supply should be 1B1 redundant. 2. If necessary, you can also choose the hot standby redundancy system composed of PLC hardware and hot standby software, 2 heavy or triple redundant fault tolerant system. (2) I/O interface unit redundancy 1. The multi-point I/O card of the control loop should be redundantly configured. 2. Multi-point I/O cards for important detection points can be redundantly configured. 3) Optional 2 or 3 I/O interface units for important I/O signals as needed. General principle 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. When selecting a module model The following principles should be followed. (1) Economics When choosing a PLC, you should consider the performance price ratio. 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. (2) Convenience Generally speaking, as a PLC, there are many types 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. (3) Universality 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. (4) 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. Repeater Triple Play,Wireless Device,Wifi port,Router Shenzhen GL-COM Technology CO.,LTD. , https://www.szglcom.com
Fourth, the control function This choice includes the choice of computing functions, control functions, communication functions, programming functions, diagnostic functions and processing speed. 
Because of signal attenuation and delay, the Ethernet segment has a distance limit according to different media. For example, 10BASE5 coaxial cable has a maximum distance of 500 meters (1,640 feet). The maximum distance can be achieved by an Ethernet repeater, which can amplify the signal in the cable and transmit it to the next section. The repeater can connect to 5 network segments at most, but there can only be 4 devices (that is, a network segment can connect to 4 repeaters at most). This can alleviate the problems caused by cable breaks: when a section of coaxial cable is disconnected, all devices on this section cannot communicate, and the repeater can ensure that other network sections work normally.
Similar to other high-speed buses, the Ethernet network segment must be terminated with resistors at both ends. For coaxial cables, the terminals at both ends of the cable must be connected to 50 ohm resistors and heat sinks called "terminators". If this is not done, a situation similar to cable disconnection will occur: the AC signal on the bus arrives It will be reflected when it is terminal, but not dissipated. The reflected signal will be considered as a conflict, making communication impossible. The repeater can electrically isolate the two network segments connected to it, strengthen and synchronize the signal. Most repeaters have a function called "automatic isolation", which can isolate network segments that have too many conflicts or conflicts that last too long, so that other network segments will not be affected by the damaged part. The repeater can restore the connection of the network segment after detecting that the conflict disappears.
Hub
Although the Ethernet network with a hub is a star-shaped structure physically, it is still a bus-type logically. The half-duplex communication method uses the CSMA/CD conflict detection method. The hub plays a very important role in reducing data packet conflicts. small. Every data packet is sent to every port of the hub, so bandwidth and security issues have not been resolved. The total transmission volume of the hub is limited by the speed of a single connection (10 or 100 Mbit/s). This is still considering the minimum cost of preamble, transmission interval, header, file tail, and packaging. When the network is overloaded, conflicts often reduce the transmission volume. The worst case is that when many hosts composed of long cables transmit many very short frames, the load of the network may be full at only about 50% due to excessive collisions. In order to increase the network load as much as possible before the conflict seriously reduces the transmission volume, some settings are usually made to avoid similar situations.
January 08, 2023