Q: Siemens plc, such as s7200s7300s7400, has a new s71200. What are the main criteria for selecting masters for these plcs in practical applications? Answer: When designing the PLC system, the control plan should be determined first. The next step is the PLC engineering design selection. The characteristics and application requirements of the process are the main basis for design selection. PLC and related equipment should be integrated and standard. According to the principle of easy to integrate with industrial control system, it is easy to expand its function. The selected PLC should be put into operation in the relevant industrial field, mature and reliable system, PLC The system hardware, software configuration and functions should be adapted to the device size and control requirements. Familiar with programmable controllers, function table diagrams and related programming languages ​​will help to shorten the programming time. Therefore, when selecting and estimating engineering design, the characteristics of the process and control requirements should be analyzed in detail, and the control tasks and scope should be determined. The operation and action, then according to the control requirements, estimate the number of input and output points, the required memory capacity, determine the function of the PLC, external device characteristics, etc., finally select the PLC with higher performance and price ratio and design the corresponding control system. The appropriate margin should be taken into account when estimating the number of I/O points. Usually, according to the number of input and output points, 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 rounded according to the product characteristics of the manufacturer's PLC. The memory capacity is the size of the hardware storage unit that the programmable controller itself can provide. The program capacity is the size of the storage unit used by the user application in the memory, so the program capacity is smaller than the memory capacity. 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 the estimation of memory memory 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. The 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. This selection includes selection of features such as arithmetic functions, control functions, communication functions, programming functions, diagnostic functions, and processing speed. (1) Computing function The operation functions of simple PLC include logic operation, timing and counting functions; the operation functions of ordinary PLC also include arithmetic functions such as data shifting and comparison; algebraic operations and data transfer are required for more complex arithmetic functions; analog quantities are also available in large PLCs. PID arithmetic and other advanced computing 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. 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 communication system of PLC system are as follows: 1) PC is the main station, multiple PLCs of the same type are slave stations, which constitute a simple PLC network; 2) 1 PLC is the main station, and other PLCs of the same type are slave stations. , constitute a master-slave PLC network; 3) PLC network connected to a large DCS as a DCS subnet through a specific network interface; 4) dedicated PLC network (special PLC communication network of each manufacturer). (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, 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. (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 PLC contact has fast response and high speed. The execution time of each binary instruction is about 0.2~0.4Ls, 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. Fourth, the choice of model (1) Type of PLC PLC is divided into two types according to the structure: integral type and modular type. It is divided into two types according to the application environment: field installation and control room installation. According to the CPU word length, it is divided into 1 bit, 4 bits, 8 bits, 16 bits, 32 bits, 64 bits. Wait. From the application point of view, it is usually possible to select the control function or the number of input and output points. The integral PLC has fixed I/O points, so the user has less room for small control systems. The modular PLC provides multiple I/O cards or cards, so the user can select and configure the control system reasonably. The number of I/O points, the function expansion is convenient and flexible, and is generally used for large and medium-sized control systems. (two) the choice of input and output modules The choice of input and output modules should be considered in line with the application requirements. For example, for the input module, application requirements such as signal level, signal transmission distance, signal isolation, and signal power supply mode should be considered. For the output module, the type of output module should be considered. Usually, the relay output module has the characteristics of low price, wide operating voltage range, short life, long response time, etc. The thyristor output module is suitable for frequent switching, inductive low power factor. Load occasions, but the price is more expensive, the overload capacity is poor. The output module also has DC output, AC output and analog output, which should be consistent with the application requirements. According to the application requirements, intelligent input and output modules can be reasonably selected to improve the control level and reduce the application cost. Consider whether you need to expand a rack or remote I/O rack, and so on. (three) the choice of power supply PLC power supply, in addition to the introduction of equipment at the same time the introduction of PLC should be designed and selected according to product specifications, the general PLC power supply should be designed to use 220VAC power supply, consistent with the domestic grid voltage. For important applications, it should be powered by an uninterruptible power supply or a regulated power supply. If the PLC itself has a usable power supply, it should be checked whether the supplied current meets the application requirements, otherwise an external power supply should be designed. In order to prevent the external high-voltage power supply from being introduced into the PLC due to misoperation, isolation of the input and output signals is necessary, and sometimes a simple diode or fuse tube isolation can be used. (four) the choice of memory Due to the development of computer integrated chip technology, the price of memory has been reduced. Therefore, in order to ensure the normal operation of the application project, the memory capacity of the PLC is generally required, and at least 8K memory selection is selected according to 256 I/O points. When complex control functions are required, you should select a memory with a larger capacity and a higher grade. (5) Choice of redundant functions 1. Control unit redundancy 2. Redundancy of I/O interface units (6) Economic considerations 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. Then according to the technical specifications of Siemens plc can be selected.
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banann Bossgoo(China)Tecgnology , https://www.cn-gangdao.com
First, the estimation of input and output (I / O) points
Second, the estimation of memory capacity
Third, the choice of control functions
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) should be selected.
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.
(1) Important process units: CPU (including memory) and power supply should be 1B1 redundant.
(2) Hot standby redundancy system, 2-fold or triple-redundant redundant fault-tolerant system composed of PLC hardware and hot standby software can also be selected when needed.
(1) The multipoint I/O card of the control loop should be redundantly configured.
(2) Multi-point I/O cards at important detection points can be redundantly configured. 3) Optional 2 or 3 I/O interface units for important I/O signals as needed.
October 08, 2023