Lead In the plc system design, the system plan should be determined first, and the next step is to design the PLC. Select PLC, mainly to determine the PLC manufacturer and PLC specific models. For a system solution that requires distributed systems and remote I/O systems, the requirements for networked communications must also be considered. So how should choose PLC? I think there should be the following aspects. First, PLC manufacturer Determine the manufacturer of the PLC, mainly should consider the requirements of the equipment user, the designer's familiarity with the different manufacturers of PLC and design habits, the consistency of supporting products and technical services and other factors. Considering the reliability of the PLC itself, as long as it is a product of a large foreign company, there should be no problem of poor reliability. I personally believe that, in general, for the control of independent equipment or a relatively simple control system occasions, supporting Japan's PLC products, there is a certain relative advantage of cost-effective. For large-scale network communication functions, open and distributed control systems, and remote I/O systems, PLCs manufactured in Europe and the United States have more advantages in network communication functions. In addition, for some special industries (such as: metallurgy, tobacco, etc.) should choose to put into operation in the relevant industry sectors, mature and reliable PLC system. Simple PLC computing functions include logic operations, timing and counting functions; ordinary PLC computing functions also include data shift, comparison and other computing functions; more complex computing functions have algebraic operations, data transmission, etc.; large-scale PLC also has analog PID operations and other advanced computing functions. With the advent of open systems, there are communication functions in PLCs. Some products have communication with lower computers. Some products have communication with peer computers or upper computers. Some products also have data communication with factories or enterprise networks. The function. Design selection should proceed from the requirements of practical application, reasonable selection of the required computing functions. For most applications, only logic operations and time counting functions are needed. Some applications require data transfer and comparison. When used for analog detection and control, algebraic operations, numerical conversions, and PID operations are used. Others require decoding and encoding operations to display data. (b) Control functions The control function includes PID control operation, feedforward compensation control operation, ratio control operation, etc. It should be determined according to the control requirements. PLC is mainly used for sequential logic control. Therefore, in most cases, single-loop or multi-loop controllers are often used to solve analog control, and sometimes dedicated intelligent input and output units are used to complete the required control functions to improve the processing speed of PLC. 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 used. (III) Communication Functions Large and medium-sized PLC systems should support a variety of fieldbuses and standard communication protocols (such as TCP/IP) and should be able to interface with the plant management network (TCP/IP) when needed. The communication protocol should conform to the ISO/IEEE communication standard and should be an open communication network. PLC system communication interface should include serial and parallel communication interface (RS2232C/422A/423/485), RIO communication port, industrial Ethernet, commonly used dcs interface, etc.; large and medium PLC communication bus (including interface equipment and cables) should be 1 : 1 redundant configuration, communication bus should meet international standards, communication distance should meet the actual requirements of the device. In the communication network of the PLC system, the upper network communication rate should be greater than 1 Mbps, and the communication load should not be greater than 60%. The main forms of the PLC system's communication network are the following: 1) The PC is the master station, and multiple PLCs of the same model are slave stations to form a simple PLC network; 2) One PLC is the main station, and the other PLCs of the same model are slaves, forming a master-slave PLC network; 3) The PLC network is connected to a large DCS as a subnetwork of the DCS through a specific network interface; 4) Dedicated PLC network (dedicated PLC communication network for each manufacturer). In order to relieve CPU communication tasks, PLCs should select communication processors with different communication functions (such as point-to-point, fieldbus, and industrial Ethernet) according to the actual needs of the network. (four) programming function Off-line programming mode: PLC and the programmer use a common CPU. When the programmer is in programming mode, the CPU only provides service for the programmer and does not control the field device. After programming is completed, the programmer switches to operating mode and the CPU controls the field devices and cannot be programmed. Off-line programming reduces system costs, but it is inconvenient to use and debug. Online programming: CPU and programmer have their own CPU, the host CPU is responsible for on-site control, and data exchange with the programmer in a scan cycle, the programmer sends the online programming or data 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. It is often used in large and medium-sized PLCs. Five kinds of standardized programming languages: SFC, LD, and FBD. They are three kinds of graphical languages, IL (statement table), and text (ST). The chosen programming language should abide by its standard (IEC 6113123) and should also support multiple language programming forms such as C, Basic, Pascal, etc. to meet the control requirements for special control applications. (five) diagnostic function The diagnostic functions of the PLC include hardware and software diagnostics. Hardware diagnosis determines the fault location of the hardware through logic judgment of the hardware, and the software diagnosis includes internal diagnosis and external diagnosis. The internal diagnosis is performed through the software to diagnose the performance and functions of the PLC. The external diagnosis is performed through the software to diagnose the PLC's CPU and external input and output parts. The strength of the diagnostic function of the PLC directly affects the technical capabilities of the operating and maintenance personnel and affects the average maintenance time. (6) Processing speed PLC uses scan mode to work. From the point of view 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 the 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 PLC contact responds fast and has high speed. Each binary instruction execution time is about 0.2-0.4μs, so it can meet the application requirements of high control requirements and fast response requirements. The scan cycle (processor scan cycle) 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. Fifth, the PLC type PLC type: PLC divides into the integral type and the modular type according to the structure two kinds. The integral PLC has fewer I/0 points and is relatively fixed, so the user has less room to choose and is usually used for small control systems. Representatives of this type of PLC are: Siemens' S7-200 series, Mitsubishi's FX series, and Omron's CPM1A series. The modular PLC provides a variety of I/O modules that can be plugged into the PLC base to facilitate the user's selection and configuration of the I/O points of the control system as needed. Therefore, the configuration of the modular PLC is more 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, Omron's CVM1 series. Six, various module choices (a) digital I/O module The selection of digital input and output modules should take into account the application requirements. For example, for the input module, application requirements such as input signal level and transmission distance should be considered. There are many types of output modules, such as relay contact output type, AC120V/23V bidirectional thyristor output type, DC24V transistor-driven type, and DC48V transistor-driven type. Usually the relay output output module has the advantages of low cost, wide range of use voltage, but the service life is shorter, the response time is longer, and the surge absorption circuit needs to be added when used for inductive loads; the response time of the bidirectional thyristor output type module is faster. Applicable to frequent switching, inductive low power factor load occasions, but the price is more expensive, less overload capacity. In addition, according to the input and output points, the input and output modules can be further divided into 8 points, 16 points, 32 points, etc., and they should be properly equipped according to actual needs when selecting. (II) Analog I/O Modules Analog input modules, according to the analog input signal types can be divided into: current input type, voltage input type, thermocouple input type. The current input type usually has a signal level of 4 to 20 mA or 0 to 20 mA. The voltage 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. Current output signals usually have 0-20 mA and 4-20 mA. Voltage type output signals usually have 0 to 10V, -10V to +10V, and so on. 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. (III) Function modules Function modules include communication module, positioning module, pulse output module, high-speed counting module, PID control module, temperature control module and so on. When selecting a PLC, the possibility of matching to the functional module should be checked. Selecting a functional 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 accessories, installation locations and interfaces, and connection cables. In software, the PLC should have corresponding control functions, which can facilitate the programming of function modules. For example, Mitsubishi's FX series PLCs can conveniently control corresponding function modules through the "FROM" and "TO" commands. VII. Redundancy function 2. Hot standby redundancy system consisting of PLC hardware and hot standby software can also be used when necessary. It can also be used for redundancy or redundancy. (II) Redundancy of I/O interface unit 1. Multipoint I/O card of control loop should be redundantly configured. 2. The multi-point I/O card of the important detection point can be configured redundantly. 3) According to the needs of the important I / O signals, you can choose to double or triple I / O interface unit. General principles After the PLC model and specifications are roughly 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 number of each component module is selected. When selecting a module model, follow these guidelines. (A) Economical When choosing a PLC, you should consider the performance-cost ratio. When considering economics, factors such as scalability, operability, and input-output ratio should be considered at the same time to compare and take into account, and eventually select more satisfactory products. The number of input and output points has a direct effect on the price. Each additional input and output card requires a certain increase in costs. When the number of points is increased to a certain value, the corresponding memory capacity, rack, motherboard, etc. must be increased accordingly. Therefore, the increase in the number of points affects the selection of the CPU, the memory capacity, and the range of control functions. In the estimation and selection should be fully considered so that the entire control system has a more reasonable cost performance. (b) Convenience In general, 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 the circuit design, facilitating the use, and minimizing external control devices. For example: For the input module, the input form that can be directly connected with the external detection element should be selected preferentially to avoid using the interface circuit. For the output module, priority should be given to the output module that can directly drive the load, minimizing intermediate relays and other components. (three) versatility When selecting a type, it is necessary to consider the unity and common use of the various PLC modules and avoid excessive types of modules. This will not only be conducive to procurement, reduce spare parts, but also increase the interchangeability of the various components of the system, to facilitate the design, commissioning and maintenance. (d) Compatibility When selecting the PLC system components, full consideration should be given to compatibility. In order to avoid the problem of poor compatibility, the manufacturers of the main components that make up the PLC system should not be excessive. If possible, try to choose the same manufacturer's product.
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Second, the input and output (I/O) points
The number of PLC input/output points is one of the basic parameters of the PLC. The number of I/O points should be determined based on the sum of all I/O points required to control the equipment. In general, the PLC I/O point should have a proper margin. Usually based on the statistics of the number of input and output points, and then increased by 10% to 20% of the expandable margin, as the input and output points estimated data. In the actual ordering, the input and output points need to be adjusted according to the product features of the manufacturer's PLC.
Third, the storage capacity
The memory capacity is the size of the hardware memory unit that the programmable controller itself can provide. The program capacity is the size of the memory unit used by the user application project in the memory, and therefore the program capacity is smaller than the memory capacity. In the design stage, since the user application is not yet programmed, the program capacity is unknown at the design stage and needs to be known after the program is debugged. In order to design a model to have a certain estimate of program capacity, an estimate of memory capacity is often used instead. There is no fixed formula for estimating the memory capacity of PLC memory. Many formulas are given in many literatures, 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 for a word), and then consider the margin by 25% of this number. Fourth, the control function The choice includes the calculation function, the control function, the communication function, the programming function, the diagnosis function and the processing speed and so on the characteristic choice.
(A) computing function 
(I) Control unit redundancy 1. Important process units: The CPU (including memory) and power supply should be 1B1 redundant.