In PLC system design, the first step should be to determine the system program, the next step is the PLC design selection. Selection of PLC, mainly to determine the manufacturer of PLC and PLC specific models. For the system program requires a distributed system, re
I think there should be the following aspects:
I. the PLC manufacturer
Determine the manufacturer of PLC, should mainly consider the requirements of equipment users, designers for different manufacturers of PLC familiarity and design habits, the consistency of supporting products and technical services and other factors. From the reliability of the PLC itself to consider, in principle, as long as the products of large foreign companies, there should not be a problem of poor reliability.
I personally believe that, in general, for the control of independent equipment or simpler control system occasions, supporting Japan's PLC products, relatively speaking, cost-effective have certain advantages. For a larger system size network communication function requires high, open distributed control system, remote I / O system, Europe and the United States production of PLC in the network communication function is more advantageous. In addition, for some special industries (eg: metallurgy, tobacco, etc.) should be selected in the relevant industry sectors have a track record of operation, mature and reliable PLC system.
II. the number of input and output (I / O) points
The number of input/output points of PLC is one of the basic parameters of PLC.The number of I/O points should be determined on the basis of the sum of all I/O points required for the control device. In general, the PLC should have an appropriate margin of I/O points. Usually based on the number of statistical input and output points, and then add 10% to 20% of the expandable margin, as the input and output points estimated data. The actual order, but also according to the characteristics of the manufacturer's PLC products, the number of input and output points to adjust.
III. the memory capacity
Memory capacity is the programmable controller itself can provide the size of the hardware storage unit, the program capacity is the size of the storage unit used by the user application in the memory, so the program capacity is less than the memory capacity. Design phase, because the user application has not been prepared, therefore, microsoft PLC engineers can get information can increase knowledge can improve skills program capacity in the design phase is unknown, need to know after the program debugging. In order to design the selection of the program capacity can have a certain estimate, usually used to replace the estimate of the memory capacity.PLC memory memory capacity estimate is not a fixed formula, many literature gives a different formula, generally by the number of digital I / O points of 10 to 15 times, plus 100 times the number of analog I / O points, the number of memory for the total number of words (16-bit word), and then by 25% of this number to consider the total number of words (16-bit word), and in addition 25% of this number is then considered as a margin.
IV. Control Functions
The selection includes the selection of characteristics such as arithmetic function, control function, communication function, programming function, diagnostic function and processing speed.
(i) Arithmetic function
Simple PLC arithmetic functions include logic operations, timing and counting functions; ordinary PLC arithmetic functions also include data shifting, comparison and other arithmetic functions; more complex arithmetic functions such as algebraic operations, data transfer, etc.; large-scale PLC there are analog PID operations and other advanced arithmetic functions. With the emergence of open systems, now in the PLC has a communication function, some products have a communication with the lower computer, some products have a communication with the same machine or the upper computer, some products also have a factory or enterprise network data communication function. Design selection should be based on the requirements of the actual application, a reasonable selection of the required computing functions. Most applications, only logical operations and timing and counting functions, some applications require data transfer and comparison, when used for analog detection and control, the use of algebraic operations, numerical conversion and PID operations. Others require decoding and encoding operations when displaying data.
(ii) control function
Control functions, including PID control operations, feed-forward compensation control operations, ratio control operations, etc., should be determined according to the control requirements. PLC is mainly used for sequential logic control, therefore, most of the occasions often use a single-loop or multi-loop controllers to solve the control of analog, and sometimes also use a dedicated intelligent input and output microblogging PLC engineers can get the information to enhance the knowledge and improve skills to complete the required control functions, improve the processing of PLC, and improve the control functions. control functions, improve the processing speed of the PLC and save memory capacity. For example, the use of PID control units, high-speed counters, analog units with speed compensation, ASC code conversion unit.
(iii) communication function
Large and medium-sized PLC system should support a variety of field buses and standard communication protocols (such as TCP / IP), when needed, should be able to connect with the factory management network (TCP / IP). Communication protocols should be in line with ISO/IEEE communication standards, should be an open communication network.
PLC system communication interfaces should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication port, industrial Ethernet, commonly used DCS interfaces, etc.; medium and large-sized PLC communication bus (including interface devices and cables) should be 1:1 redundant configuration, the communication bus should be in line with international standards, and the communication distance should meet the actual requirements of the device.
In the communication network of PLC system, the network communication rate of the upper level should be more than 1Mbps, and the communication load is not more than 60%.The main forms of communication network of PLC system are as follows:
(1) PC as the master, multiple PLCs of the same model as the slave, forming a simple PLC network;
(2) 1 PLC master, other PLC of the same type as the slave, constituting a master-slave PLC network;
(3) The PLC network is connected to a large DCS as a sub-network of the DCS through a specific network interface;
(4) Dedicated PLC network (dedicated PLC communication network of each manufacturer).
PLC in order to alleviate the CPU communication tasks, according to the actual needs of the network composition, should be selected with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) communication processor.
(iv) Programming function
Offline programming mode: PLC and programmer share a common CPU, when the programmer is in programming mode, the CPU only provides services for the programmer and does not control the field devices. After completing the programming, the programmer switches to the operation mode, and the CPU controls the field devices without programming. Offline programming mode can reduce system cost, but it is not convenient to use and debug.
Online programming method: CPU and programmer have their own CPUs, the host CPU is responsible for field control and exchanges data with the programmer during a scan cycle, the programmer sends the program or data prepared online to the host, and the host runs according to the newly received program in the next scan cycle. This approach is more 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 Chart (SFC), Ladder Diagram (LD), Function Block Diagram (FBD) three kinds of graphical language and Statement List (IL), Structured Text (ST) two kinds of text language. Selected programming language should comply with its standards (IEC6113123), but also should support a variety of language programming forms, such as C, Basic, Pascal, etc., in order to meet the control requirements of special control occasions.
(E) processing speed
PLC works by scanning. From the point of view of real-time requirements, processing speed should be as fast as possible, if the signal duration is less than the scanning 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 so on. At present, the PLC contacts fast response, high speed, each binary instruction execution time of about 0.2 to 0.4 μs, so it can adapt to the high control requirements, the corresponding requirements of the application needs of fast. The scanning cycle (processor scanning cycle) should meet: the scanning time of small PLC is not more than 0.5ms/K; the scanning time of large and medium-sized PLC is not more than 0.2ms/K.
V. PLC models
PLC type: PLC is divided into two categories according to the structure of the whole type and module type.
Integral PLC I / 0 points less and relatively fixed, so the user has less room to choose, usually used for small control systems. Representatives of this type of PLC are: Siemens S7-200 series, Mitsubishi's FX series, Omron's CPM1A series.
Module-type PLC provides a variety of I / O module can be plugged in the PLC substrate, to facilitate the user according to the need to reasonably select and configure the control system I / O points. Therefore, the configuration of the module type PLC is more flexible, generally used for medium and large control systems. For example, Siemens S7-300 series and S7-400 series, Mitsubishi's Q series, Omron's CVM1 series.
VI. Various module selection
(A) digital I / O module
The selection of digital input and output modules should consider the 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-driven, DC48V transistor-driven and so on.
Usually relay output module has a low price, the use of a wide range of voltage and other advantages, but the service life is shorter, the response time is longer, when used for inductive loads need to increase the surge absorber circuit; bidirectional thyristor output module response time is faster for switching frequently, inductive low power factor load occasions, but the price is more expensive, the overload capacity is poor.
In addition, the input and output modules in accordance with the number of input and output points can be divided into: 8 points, 16 points, 32 points and other specifications, the choice should be reasonably equipped according to the actual needs.
(B) analog I / O module
Analog input module, according to the analog input signal type can be divided into: current input type, voltage input type, thermocouple input type. Current input type usually signal level for 4 ~ 20mA or 0 ~ 20mA; voltage type input module usually signal level for 0 ~ 10V, -5V ~ +5V and so on. Some analog input modules are compatible with voltage or current input signals.
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; voltage-type output signal is usually 0 ~ 0V, -10V ~ +10V and so on.
Analog input and output modules, according to the number of input and output channels can be divided into 2 channels, 4 channels, 8 channels and other specifications.
(C) Function Module
Function modules include communication module, positioning module, pulse output module, high-speed counting module, PID control module, temperature control module and so on. Selection of PLC should take into account the possibility of matching function modules, the choice of function modules involves two aspects of hardware and software.
In terms of hardware, the first should consider the function module can be easily connected to the PLC, PLC should have the relevant connections, installation locations and interfaces, connecting cables and other accessories. In software, PLC should have the corresponding control function, can be convenient to the function module programming. For example, Mitsubishi's FX series PLC through the "FROM" and "TO" commands can easily control the corresponding function module.
VII. Redundancy Functions
(A) control unit redundancy
1, the important process unit: CPU (including memory) and power supply should be 1B1 redundant.
2, in the need for PLC hardware and hot standby software can also be used to form a hot standby redundancy system, 2 redundancy or 3 redundancy redundancy fault-tolerant system.
(B) I / O interface unit redundancy
1, the control circuit of the multi-point I / O card should be redundant configuration.
2,Multi-point I/O cards of important detection points can be redundantly configured. 3)According to the need for important I/O signals, 2-revision or 3-revision I/O interface units can be selected.