PLC selection summary

 In plc system design, the system should first determine the program, the next step is to PLC design selection. Select PLC, is to determine the specific PLC manufacturer and PLC models. System requirements for the distributed system, remote I / O systems, but also need to consider the requirements of networked communications. So how to choose a specific PLC? I think there should be the following aspects.

First, PLC manufacturers

PLC manufacturers to determine the main, should consider the requirements of equipment users, designers for different manufacturers PLC familiarity and design habits, matching product consistency and technical services and other factors. From the reliability of PLC itself, in principle, as long as the products of large foreign companies, there should be no reliability problem.

I personally think that, in general, for the control of independent equipment or simple control system of the occasion, supporting Japan's PLC products, relatively cost-effective have some advantages. For the large-scale network communication system requires a high, open distributed control system, remote I / O system, PLC production in Europe and the United States in the network communication more advantages. In addition, for some special industries (such as: metallurgy, tobacco, etc.) should choose to put into operation in the field of related industries, proven PLC system.

Second, the input and output (I / O) points

PLC input / output points is one of the basic parameters of PLC. The number of I / O points should be based on the sum of all the I / O points required to control the equipment. Under normal circumstances, PLC I / O points should have the appropriate margin. Usually based on the statistical input and output points, and then increase the scalable margin of 10% to 20%, as the input and output points estimate data. The actual order, but also according to the characteristics of manufacturers PLC products, input and output points to be adjusted.

Third, storage capacity

The memory size is the size of the hardware memory unit that the programmable controller itself can provide. The program size is the size of the memory location used by the user application in the memory. Therefore, the program size is less than the memory size. During the design phase, since the user application has not been programmed yet, the program capacity is unknown during the design phase and needs to be known after the program is debugged. In order to design the selection of program capacity can have a certain estimate, usually used to estimate the memory capacity to replace. There is no fixed formula to estimate the memory capacity of PLC memory. Many documents give different formulas, which are generally 10 to 15 times of digital I / O points plus 100 times of analog I / O points. This number is the total number of words in memory (16 words for one word), and then 25% of this number to consider margin.

Fourth, control functions

This selection includes the selection of features such as arithmetic functions, control functions, communication functions, programming functions, diagnostics functions and processing speed.

(A) computing function

Simple PLC computing functions include logic operations, timing and counting functions; ordinary PLC computing functions include data shift, comparison and other computing functions; more complex arithmetic functions algebraic operations, data transfer, etc .; large-scale PLC there are analog PID operations and other advanced computing functions. With the advent of open systems, communication functions are now available in PLCs, with some products having communications with the next crew, some having communications with a peer or a host computer, and some having data communication with a factory or corporate network Function Design selection should be based on the actual application requirements, reasonable selection of the required computing functions. For most applications, only logic and timer counting functions are required. Some applications require data transfer and comparison. Algebraic operations, numerical conversions, and PID operations are used for analog detection and control. Others need to decode and encode calculations to display the data.

(B) control functions

Control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., should be determined according to the control requirements. PLC is mainly used for sequential logic control, therefore, most occasions often use single-loop or multi-loop controller to solve the analog control, and sometimes also uses a dedicated intelligent input and output unit to complete the required control functions to improve PLC processing speed and Save memory capacity. For example, using PID control unit, high-speed counter, analog unit with speed compensation, ASC code conversion unit.

(C) communication function

Large and medium-sized PLC systems should support a wide range of fieldbus and standard communication protocols (such as TCP / IP) and should be able to interface with the plant management network (TCP / IP) when needed. Communication protocol should be consistent with ISO / IEEE communication standards, should be an open communication network.

PLC system's communication interface should include serial and parallel communication interface (RS2232C / 422A / 423/485), RIO communication port, industrial Ethernet, common dcs interface, etc .; medium and large PLC communication bus (including interface equipment and cables) should be 1 : 1 redundant configuration, the communication bus should comply with international standards, communication distance should meet the actual requirements of the device.

PLC system communication network, the higher the network communication rate should be greater than 1Mbps, communication load is not more than 60%. The main form of communication system of PLC system has the following forms:

1) PC as the main station, more than one with the same type of PLC from the station, composed of simple PLC network;

2) 1 PLC as the main station, the other with the same type of PLC from the station, constitute the master-slave PLC network;

3) PLC network through a specific network interface to connect to a large DCS as a DCS subnet;

4) Dedicated PLC network (each manufacturer's dedicated PLC communication network).

PLC to reduce CPU communication tasks, according to the actual needs of the network composition, should have different communication functions (such as point-to-point, field bus, industrial Ethernet) communications processor.

(Four) programming function

Offline programming: PLC and programmer common one CPU, programmer programming mode, the CPU only to provide programmer services, not on-site equipment control. After programming, the programmer switches to run mode, the CPU controls the field devices and can not be programmed. Off-line programming can reduce system costs, but not easy to use and debug.

Online programming: CPU and programmer have their own CPU, the host CPU is responsible for on-site control and in a scan cycle with the programmer for data exchange, the programmer to online programming or data sent to the host, the next scan cycle, The host is running on the newly received program. This method is costly, but the system debugging and easy to operate, often used in large and medium-sized PLC.

Five kinds of standard programming languages: Sequential Function Chart (SFC), Ladder Diagram (LD) and Functional Block Diagram (FBD) are three kinds of graphical language and statement list (IL) and structured text (ST). The programming language used 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.

(E) diagnostic function

PLC diagnostic functions include hardware and software diagnostics. Hardware diagnosis through the hardware logic to determine the hardware fault location, software diagnosis sub-diagnosis and out-diagnosis. The PLC diagnoses the performance and function of the PLC through the software. The software diagnoses the information exchange function of the CPU and external input / output of the PLC through the software.

The strength of PLC diagnostic functions directly affects the technical and operational requirements of operators and maintenance personnel and affects the average repair time.

(F) processing speed

PLC using scanning methods. From the 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.

Processing speed and the length of the user program, CPU processing speed, software quality and so on. At present, PLC contact response fast, high speed, each binary instruction execution time of about 0.2 ~ 0.4μs, so to adapt to the high control requirements, the corresponding application requirements. Scan cycle (processor scan cycle) should meet: small PLC scan time is not greater than 0.5ms / K; large and medium-sized PLC scan time is not greater than 0.2ms / K.

Five, PLC models

PLC type: PLC according to the structure is divided into two types of integrated and modular.

Integral PLC I / 0 points less and relatively fixed, so users have less choice, usually for small control systems. The representatives of this type of PLC are: Siemens S7-200 series, Mitsubishi FX series, Omron CPM1A series.

Modular PLC provides a variety of I / O modules can be plugged in the PLC substrate, user-friendly selection and configuration of reasonable control system I / O points. Therefore, the modular PLC configuration more flexible, generally used for medium and large control systems. For example, Siemens S7-300 series and S7-400 series, Mitsubishi Q series, Omron CVM1 series.

Six, a variety of modules to choose from

(A) digital I / O modules

Digital input and output module selection 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 drive, DC48V transistor drive and so on.

Relay output output module is usually low cost, the use of a wide range of voltage and other advantages, but the shorter life expectancy, longer response time, for the inductive load when the need to increase the surge absorber circuit; bidirectional thyristor output module faster response time For switching frequent, inductive low power factor load situation, but the price is more expensive, overload is poor.

In addition, the input and output modules in accordance with the input and output points can be divided into: 8:00, 16:00, 32 points and other specifications, the choice should be based on the actual needs of a reasonable allocation.

(B) analog I / O modules

Analog input module, according to the analog input signal type can be divided into: current input type, voltage input type, thermocouple input type. The current signal level of the input type is 4 ~ 20mA or 0 ~ 20mA; the signal level of the voltage type input module is usually 0 ~ 10V, -5V ~ + 5V and so on. Some analog input modules are compatible with voltage or current input signals.

Analog output module The same sub-voltage output module and current output module, the current output signal is usually 0 ~ 20mA, 4 ~ 20mA. The voltage type output signal usually has 0 ~ 10V, -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) functional modules

Functional modules include communication module, positioning module, pulse output module, high-speed counting module, PID control module, temperature control module. PLC should be selected when testing rate to the possibility of supporting functional modules, select functional modules involving both hardware and software.

In terms of hardware, we should first consider the function modules can be easily connected with the PLC, PLC should have related connections, installation location and interface, connecting cables and other accessories. In the software, PLC should have the corresponding control function, can carry on programming to the function module conveniently. For example, FX series PLC of Mitsubishi can control the corresponding function module conveniently by "FROM" and "TO" commands.

Seven, redundant functions

(A) control unit redundancy

1, an important process unit: CPU (including memory) and power should be 1B1 redundancy.

2, when needed, can also be used by the PLC hardware and hot standby software hot standby system, 2 or 3 heavy red fault-tolerant systems.

(B) I / O interface unit redundancy

1, multi-point control loop I / O card should be redundant configuration.

2, an important test point of multi-point I / O card can be redundant configuration. 3) As important I / O signals, 2-fold or 3-fold I / O interface units are available.

General principle

After the PLC models 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 the respective modules are selected. When selecting a module model, the following guidelines should be observed.

(A) economy

Choose PLC, should consider the cost performance. When considering the economy, we should consider the application of scalability, operability, input-output ratio and other factors, to compare and take into account the final selection of more satisfactory products.

Input and output points have a direct impact on the price. Each additional piece of input and output cards need to increase the cost. When the number of points increases to a certain value, the corresponding memory capacity, rack, motherboard, etc. should also be increased accordingly, therefore, the increase of points on CPU selection, memory capacity, control function range and other options have an impact. In the estimation and selection should give full consideration to the entire control system has a more reasonable cost performance.

(B) convenience

In general, as a PLC, to meet the control requirements of the module there are often many choices should be to simplify the circuit design, easy to use, as far as possible to reduce the external control device is the principle.

For example: For the input module, the input form that can be directly connected with the external detection element should be selected, avoiding the use of the interface circuit. For the output module, you should give priority to the output module that can directly drive the load, to minimize the components such as the intermediate relay.

(C) versatility

When making the selection, we must take into account the unity of PLC modules and common, to avoid too many types of modules. This is not only conducive to procurement, reduce spare parts, but also can increase the interchangeability of the various components of the system, to facilitate the design, commissioning and maintenance.

(D) compatibility

Select the PLC system components, should take full account of compatibility. In order to avoid the problem of poor compatibility, the main components of the PLC system should not be too many manufacturers. If possible, try to choose the same manufacturer's products.