The type-selection method of the PLC

In the design of the PLC system, the control scheme should be determined first, and 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, standard, according to the easy and industrial control system to form a whole, easy to expand the function of the selection of PLC should be in the relevant industrial performance, mature and reliable system, PLC system hardware, software configuration and function should be adapted to the device scale and control requirements. Familiar with the programmable controller, function chart and related programming language is beneficial to shorten the programming time, therefore, the engineering design selection and estimation, the characteristics of the process, control requirements, clear control tasks and scope to determine the required operation and action, and then estimate the input and output points, required memory capacity, determine the function of PLC, external equipment characteristics, and finally select the PLC with high performance price ratio and design the corresponding control system.

1.Estimation of the input / output (I / O) points

Appropriate margin should be considered when estimI / O points. Usually, according to the statistical input and output points, the extensible margin of 10%~20% can be added as the estimation data of input and output points. In the actual order, the input and output points should be rounded according to the product characteristics of the manufacturer PLC.

2.Estimates of the memory capacity

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 project in the memory, so the program capacity is less than the memory capacity. In the design stage, because the user application has not been compiled, the program capacity is unknown in the design stage, and it needs to be known after the program debugging. In order to have a certain estimate of the program capacity in the design and selection, the memory capacity estimation is usually used to replace it.

There is no fixed formula for the estimation of memory capacity. Different formulas are given in many literature data, which are generally 10~15 times of the digital I / O points, plus 100 times of the simulated I / O points, this number is the total number of memory (16 bits is one word), and the margin is considered according to 25% of this number.

3.Selection of the control functions

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

1.Computing function

The operation functions of simple PLC include logical operation, timing and counting; ordinary PLC operations also include data shift and comparison; more complex operations include algebra operation and data transmission; and analog PID operation and other advanced computing functions in large PLC. With the emergence of open systems, there have communication functions in PLC, some products have communication with the lower computer, some products have communication with the same computer or upper computer, some products also have the function of data communication with the factory or enterprise network. The design and selection should start from the requirements of practical application, and the reasonable selection of the required computing functions. In most applications, only logical operations and timing and counting functions are needed, while some applications require data transfer and comparison, and when used for analog quantity detection and control, only algebraic operations, numerical conversion and PID operations are used. Operations such as decoding and encoding are needed to display the data.

(2) Control function
The control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., which shall be determined according to the control requirements. PLC is mainly used for sequential logic control, therefore, in most occasions, single loop or multi-loop controller is often used to solve the control of analog quantity, sometimes special intelligent input and output unit to complete the required control function, improve the processing speed of PLC and save the memory capacity. For example, using the PID control unit, the high-speed counter, the analog unit with speed compensation, the ASC code conversion unit, etc.
(3) Communication function

The large and medium-sized PLC system shall support various fieldbus and standard communication protocols (e. g., TCP / IP) and shall be able to connect to the Plant management network (TCP / IP) when required. The communication protocol shall comply with ISO / IEEE communication standards and shall be an open communication network.

The communication interfaces of PLC system shall include serial and parallel communication interfaces (RS2232C / 422A / 423 / 485), RIO communication port, industrial Ethernet, common DCS interface, etc. Large and medium-sized PLC communication buses (including interface equipment and cables) shall be 1:1 redundant, the communication bus shall meet international standards, and the communication distance shall meet the actual requirements of the device.

In the communication network of the PLC system, the communication rate of the superior network shall be greater than 1Mbps, and the communication load shall not be greater than 60%. The communication network of PLC system mainly has the following forms:

1) PC is the main station, and multiple PLC of the same model are the slave station, forming a simple PLC network;

2) One PLC is the main station, and other PLC of the same type are slave stations, forming the main-slave PLC network;
3) The PLC network is connected to the subnetwork of the large DCS used as a DCS through a specific network interface;

4) Dedicated PLC network (dedicated PLC communication network of each manufacturer).
In order to reduce the CPU communication tasks, the communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) should be selected according to the actual needs of the network composition.

(4) Programming function
Offline programming mode: the PLC and the programmer generally use one CPU. When the programmer is in the programming mode, the CPU only provides services for the programmer and does not control the field equipment. After the programming, the programmer switches to the operation mode, and the CPU controls the field equipment and cannot be programmed. Offline programming method can reduce the system cost, but it is inconvenient to use and debug. Online programming method: the CPU and the programmer have their own CPU. The host CPU is responsible for field control and conducts data exchange with the programmer in a scanning cycle. The programmer sends the online program or data to the host. In the next scanning cycle, the host will run according to the newly received program. This method is expensive, but the system debugging and operation is convenient, which is often used in large and medium-sized PLC.

Five standardized programming languages: sequential function diagram (SFC), ladder diagram (LD), functional module diagram (FBD), and statement table (IL), and structural text (ST). The selected programming language shall comply with its standard (IEC6113123), and shall also support multiple language programming forms, such as C, Basic, etc., to meet the control requirements of special control occasions.

(5) Diagnostic function

The diagnostic functions of PLC include both hardware and software diagnosis. 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 functions of PLC are diagnosed by the software, and the component information exchange function of PLC CPU and external input and output is diagnosed by the software.

The strength of PLC diagnostic function directly affects the technical ability requirements of operation and maintenance personnel, and affects the average maintenance time.
(6) Processing speedPLC
Work with the scanning mode. From the perspective of real-time requirements, the processing speed should be the faster, the better. If the signal duration is less than the scanning time, the PLC will not scan the signal, resulting in the loss of signal data.

Processing speed is related to the length of the user program, the CPU processing speed, the software quality, etc. At present, the PLC contact response is fast and high speed, and the execution time of each binary instruction is about 0.2~0.4Ls, so it can adapt to the application needs of high control requirements and fast corresponding requirements. The scanning period (processor scanning cycle) shall meet: the scanning time of small PLC shall not be greater than 0.5ms / K; the scanning time of large and medium-sized PLC shall not be greater than 0.2ms / K.

4. Selection of the model model 
(I) Type of PLC

PLC is divided into integral and modular two types according to structure, field installation and control room installation; according to CPU length, 1 bit, 4 bit, 8 bit, 16 bit, 32 bit, 64 bit, etc. From the perspective of application, usually according to the control function or the number of input and output points.

The I / O points of integral PLC is fixed, so the user has less choice for small control system; modular PLC provides various I / O cards or plug cards, so the user can reasonably choose and configure the I / O points of the control system, the function expansion is convenient and flexible, generally used for large and medium-sized control system. 
(2) Selection of the input and output module

The selection of input and output modules should be consistent with the application requirements. For example, for the input module, the application requirements of signal level, signal transmission distance, signal isolation, and signal power supply mode should be considered. For the output module, the selected output module type should be considered. Usually, the relay output module has the characteristics of low price, wide voltage range, short life and long response time; the SCR output module is suitable for frequent switches and low power factor load situations, but the price is more expensive and 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, the intelligent input and output module can be reasonably selected to improve the control level and reduce the application cost.
Consider the need for extended racks or remote I / O racks, etc.
(3) Selection of power supply
The power supply of PLC, in addition to the introduction of PLC according to the requirements of the product specification, the general PLC should be designed to choose 220VAC power supply, which is consistent with the domestic power grid voltage. For important applications, uninterruptible power supply or stationary power supply should be used.

If the PLC itself has a usable power supply, check whether the provided current can meet the application requirements, otherwise, the external power supply should be designed. In order to prevent the introduction of PLC by the external high voltage power supply, the isolation of input and output signals is necessary, sometimes using simple diode or fuse tube isolation.

(4) Selection of memory
Due to the development of computer integrated chip technology, the price of memory has decreased. Therefore, in order to ensure the normal operation of application projects, the memory capacity of PLC is generally required to select 256 I / O points. When complex control functions are needed, the memory with larger capacity and higher grade should be selected.

(5) Selection of redundant functions
1. Redundancy of the control units
(1) Important process units: CPU (including memory) and power supply shall be 1B1 redundancy.
(2) The thermal standby redundancy system composed of PLC hardware and thermal redundancy software can also be selected when needed. 

2. Redundancy of the I / O interface units
(1) The multi-point I / O card of the control loop shall be configured redundantly configured.
(2) Multi-point I / O cards of important detection points can be redundantly configured.
(3) According to the need of the important I / O signals, the 2 heavy or 3 heavy I / O interface units can be selected.
(6) Economic considerations
When selecting PLC, consider the performance price ratio. When considering the economy, the application scalability, operability, input-output ratio and other factors should be considered at the same time, compared and taken into account, and finally select the more satisfactory products.

Input and output points have a direct impact on the price. Each additional piece of input and output card needs to increase a certain cost. When the number of points increases to a certain value, the corresponding storage capacity, rack, bus board, etc., should also be increased accordingly, therefore, the increase of points depends on the selection of CPU, memory capacity, control function range and other choices. In the estimation and selection should be fully considered, so that the whole control system has a more reasonable performance and price ratio.