Hollysys PLC - What are the selection methods of PLC?

1. Budget for input and output (I/O) points

When budgeting I/O points, the appropriate margin should be considered, usually based on the calculated number of input and output points, and then add 10%-20% of the expandable, and after the margin, it will be used as the budget data of input and output points. When ordering in practice, it is also necessary to round the number of input and output points according to the product characteristics of the manufacturer's PLC.

2. Budget for memory capacity

The memory capacity is the size of the hardware storage unit that can be supplied by the programmable controller itself, and the program capacity is the size of the storage unit used by the user in the memory, so the program capacity is smaller than the memory capacity. In the planning stage, because the user has not yet prepared the program, the program capacity is not known in the planning stage, and it needs to be known after the program is debugged. In order to have a certain budget for program capacity when planning and selection, the budget of memory capacity is usually used instead.

There is no fixed formula for the budget of memory capacity, and many literature materials give different formulas, which are generally based on 10-15 times the number of digital I/O points, plus 100 times the number of imitation I/O points, and then use this number as the total number of words in memory (16 bits are a word), and then think about the margin according to 25% of this number.

Third, the selection of control functions

The selection includes the selection of features such as computing function, control function, communication function, programming function, diagnosis function, and processing speed.

(1) Computing function

The operation functions of a simple PLC include logic operation, timing and counting functions; Generally, the computing function of PLC also includes data shift, comparison and other computing functions; The more chaotic operation functions include algebraic operations, data transmission, etc.; Large PLCs also have PID operations that mimic quantities and other high-end computing functions. With the presentation of the open system, now in the PLC has the communication function, some goods have the communication with the lower computer, some goods have the communication with the same computer or the upper computer, and some goods also have the function of data communication with the factory or enterprise network. When planning and selecting, we should start from the requirements of practical application and reasonably select the required computing functions. In most applications, only logic operations and timing and counting functions are required, some applications require data transmission and comparison, and algebraic operations, numerical transformations and PID operations are used when they are used for imitation quantity checking and control. Operations such as decoding and encoding are required to display data.

(2) Control function

The control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., which should be determined according to the control request. PLC is mainly used for sequential logic control, therefore, in most occasions, a single-loop or multi-loop controller is often used to deal with the control of analog quantities, and sometimes a special intelligent input and output unit is also selected to complete the control function required to improve the processing speed of PLC and save memory capacity. For example, PID control units, high-speed counters, imitation units with speed compensation, ASC code conversion units, etc.

(3) 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 connect to the plant management network (TCP/IP) when required. The communication protocol should conform to the ISO/IEEE communication specification and should be an open communication network.

The communication interface of the PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication interfaces, industrial Ethernet, common DCS interfaces, etc.; Large and medium-sized PLC communication buses (including interface equipment and cables) should be 1:1 redundant equipment, communication buses should conform to international specifications, and communication spacing should meet the practical requirements of equipment.

In the communication network of the PLC system, the communication rate of the superior network should be greater than 1Mbps, and the communication load should not be greater than 60%. The primary methods of the communication network of the PLC system are as follows: 1) PC is the master station, and a plurality of PLCs of the same type are slaves to form a simple PLC network; 2) 1 PLC is the master station, and other PLCs of the same type are slave stations, forming a master-slave PLC network; 3) The PLC network is connected to the subnet of the DCS in the large DCS through a specific network interface; 4) Dedicated PLC network (dedicated PLC communication network of each manufacturer).

In order to alleviate the mission of CPU communication, communication processors with different communication functions (such as point-to-point, fieldbus, and industrial Ethernet) should be selected according to the practical requirements of network composition.

(4) Programming function

Offline programming method: PLC and programmer share a CPU, and when the programmer is in the programming form, the CPU only provides services for the programmer, and does not control the field equipment. After the programming is completed, the programmer switches to the operation mode, and the CPU controls the field device, but cannot be programmed. The offline programming method can reduce the cost of the system, but it is inconvenient to use and debug. Online programming method: CPU and programmer have their own CPU, the host CPU is responsible for on-site control, and in a scanning cycle with the programmer for data exchange, the programmer will send the online program or data to the host, the next scanning cycle, the host will run according to the newly received program. This method has a high cost, but the system is easy to debug and operate, and is often used in large and medium-sized PLCs.

Five normalized programming languages: Sequence Function Diagram (SFC), Ladder Diagram (LD), Function Module Diagram (FBD) and Sentence Table (IL) and Structural Text (ST). The selected programming language should adhere to its specifications (IEC6113123), and together with it, it should also support a variety of language programming methods, such as C, Basic, etc., to meet the control requirements of special control occasions.

(5) Diagnostic function

The diagnostic function of PLC includes the diagnosis of hardware and software. Hardware diagnosis determines the fault location of the hardware through the logical judgment of the hardware, and the software diagnosis is divided into internal diagnosis and external diagnosis. The internal performance and function of the PLC are diagnosed by the software, and the information exchange function between the CPU and the external input and output components of the PLC is confirmed by the software.

The strength of the diagnosis function of the PLC directly affects the request for the skills and abilities of the operation and maintenance personnel, and affects the uniform repair time.

(6) Processing speed

The PLC selects the scanning method to work. From the point of view of real-time request, the processing speed should be as fast as possible, if the signal duration is less than the scanning time, the PLC will not be able to scan the signal, resulting in the loss of signal data.

The processing speed is related to the length of the user's program, the CPU processing speed, the quality of the software, etc. At present, the response of PLC contacts is fast and high, and the execution time of each binary instruction is about 0.2——, so that it can get used to the application needs of high control requests and fast corresponding requests. The scanning cycle (processor scanning cycle) should meet: the scanning time of the small PLC is not greater than; The scanning moment of large and medium-sized PLCs is not greater than.

Fourth, the selection of models

(a) The type of PLC

PLC is divided into two categories according to the structure: integral type and module type, and is divided into two categories according to the application environment: field device and control room device; According to the CPU word length, it is divided into 1 bit, 4 bits, 8 bits, 16 bits, 32 bits, 64 bits, etc. Starting from the perspective of operation, it is usually possible to select the type according to the control function or the number of input and output points.

The I/O points of the whole PLC are fixed, so the user selects a small number of points, and is used for small control systems; Modular PLC provides a variety of I/O cards or plug-in cards, so users can reasonably select and equip the I/O points of the control system, and the function expansion is convenient and sensitive, usually used in large and medium-sized control systems.

(2) Selection of input and output modules

The selection of input and output modules should be considered in the unity of application requirements. For example, for input modules, you should consider the application requests such as signal level, signal transmission interval, signal blocking, and signal power supply method. For the output module, the type of output module should be considered, usually the relay output module has the characteristics of low quotation, wide application voltage, short life, long response time, etc.; The thyristor output module is suitable for frequent switching and electrorational low power factor load occasions, but the quotation is more expensive and the overload can be poor. The output module also has DC output, AC output, and analog output, etc., which should be common to the application request.

According to the application request, the intelligent input and output module can be reasonably selected, so as to improve the control level and reduce the cost of use.

Think about whether you need expansion racks or remote I/O racks, etc.

(3) Selection of power supply

The power supply of PLC, in addition to the introduction of equipment together with the introduction of PLC should be planned and selected according to the product manual, usually the power supply of PLC should be planned and selected 220VAC power supply, which is common with the domestic power grid voltage. For the main application occasions, uninterruptible power supply or stabilized power supply should be used for 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 planned. In order to prevent the introduction of PLC due to misoperation of the external high-voltage power supply, it is necessary to block the input and output signals, and sometimes a simple diode or fuse tube can be used.

(4) Selection of memory

Due to the development of computer integration chip skills, 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 usually requested, and at least 8K memory is selected according to 256 I/O points. When cluttered control functions are required, a larger, higher-level memory should be chosen.

(5) Selection of redundant functions

1. Redundancy of the control unit

(1) The main process unit: CPU (including memory) and power supply should be 1B1 redundant.

(2) When required, the hot standby redundant system composed of PLC hardware and hot standby software, the redundant fault-tolerant system of 2-fold or 3-fold redundancy, etc. can also be selected.

2. Redundancy of I/O interface units

(1) The multi-point I/O card of the control loop should be redundant.

(2) The multi-point I/O card of the main checkpoint can be redundantly equipped. 3) For the main I/O signal according to the requirements, the 2-plex or triple-based I/O interface unit can be selected.

(6) Economic thinking

When choosing a PLC, you should consider the performance-to-offer ratio. When thinking about economy, we should think about the scalability, operability, input-output ratio and other factors of use, compare and take into account, and finally select the more satisfactory products.

The number of input and output points has a direct impact on the quote. Each additional I/O card requires a certain fee. When the number of points is added to a certain value, the corresponding memory capacity, rack, motherboard, etc. must also be added accordingly, so the addition of points has an impact on the selection of CPU, memory capacity, control function scale, etc. In the budget and selection, full consideration should be given to make the whole control system have a more reasonable performance quotation ratio.

Hollysys PLC - What are the selection methods of PLC?

Hollysys PLC - What are the selection methods of PLC?

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