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How to choose the right PLC? These 8 steps are enough.

EEPWweixin @ 2019/10/15

PLC ( Programmable Logic Controller Control system, programmable logic controller, designed for industrial production, a digital operation of electronic devices.

It uses a kind of programmable memory for its internal storage program, execution logic operation, sequential control, timing, counting and arithmetic operation, and other user oriented instructions, and controls the various types of machinery or production process through digital or analog input / output, which is the core part of industrial control.

Characteristics of PLC control system

  1. From switch control to sequential control and arithmetic processing, it is from bottom to top.

  2. Logic control, timing control, count control, step (sequential) control, continuous PID control and data control DDPLC have the functions of data processing, communication and networking.

  3. A PC can be used as the main station, and multiple PLC as the slave station.

  4. A PLC can also be used as the main station, with multiple PLC as the slave stations, which constitute the PLC network. This is more convenient than using PC as the master station. When users are programming, they do not need to know the communication protocol.

  5. PLC network can be used as an independent DCS/TDCS or a subsystem of DCS/TDCS.

  6. It is mainly used for sequential control in industrial process. The new PLC also has closed-loop control function.


8 steps of design and selection of PLC

The level of system design will directly affect the performance of the control system and the reliability of the equipment. The selection of PLC is very important. How to select the appropriate PLC according to different control requirements and design a control system that runs smoothly, reliably, safely and practically, is easy to debug and easy to maintain?

In the design of PLC system, the system plan should first be determined. The next step is to design and select PLC. The choice of PLC is mainly to determine the specific type of PLC manufacturer and PLC. For distributed system and remote I/O system, we also need to consider the requirement of network communication.

So how to select PLC specifically? There should be the following aspects.

1. the choice of PLC manufacturers

To determine the manufacturer of PLC, we should consider the requirements of equipment users, designers' familiarity with different manufacturers' PLC, design habits, consistency of supporting products, and technical services. Considering the reliability of PLC itself, in principle, as long as it is a product of a large foreign company, there should not be a problem of poor reliability.

Generally speaking, for the occasion of controlling independent equipment or simpler control system, supporting Japanese PLC products has certain advantages in terms of cost performance. For the distributed control system and remote I/O system with large scale network communication function and high openness, the PLC produced in Europe and the United States has more advantages in network communication function.
In addition, for some special industries (such as metallurgy, tobacco, etc.), we should choose to have a PLC system which is reliable and reliable in operation.

2. estimation of input and output (I/0) points

The number of input / output points of PLC is one of the basic parameters of PLC. The determination of I/O points should be based on the sum of all input / output points required by the control equipment.

In general, the I/O point of PLC should have an appropriate margin. Usually, according to the statistics of input and output points, and then increase the 10% to 20% expansion margin, as input and output points to estimate data. In actual ordering, we need to adjust the number of input and output points according to the product characteristics of manufacturer PLC.

3. PLC memory capacity estimation

Memory capacity refers to the size of the hardware storage unit that the programmable controller itself can provide, and the memory capacity of various PLC can be found from the basic parameter table of the PLC. For example, SIEMENS's S7-314 PLC user program storage capacity is 64KB, S7-315-2DP PLC's user storage capacity is 128KB.

The program capacity is the size of the storage cell used by the user program in memory, so the memory capacity should be greater than the program capacity. In the design stage, because the user application program is not yet compiled, it is necessary to estimate the program capacity.


How to estimate program capacity? In many documents, different formulas are given. In general, they are 10~15 times the number of I/O points, plus 100 times the number of analog I/O points, which is the total number of words in the memory (16 bits for one word). In addition, the remaining amount is considered at 25% of this number.

4. the choice of PLC communication function

Now the communication function of PLC is becoming more and more powerful. Many PLC support various communication protocols (some need to have corresponding communication modules), and choose the appropriate communication mode according to actual needs when choosing.

The communication network of PLC system mainly has the following forms:
  • PC is the main station, and multiple PLC of the same type are composed of simple PLC network.

  • 1 PLC as the main station, the other PLC with the same type as the slave station, forming a master-slave PLC network.

  • The PLC network is connected to a large DCS through a specific network interface as a subnet of DCS;

  • A dedicated PLC network (the PLC communication network of each manufacturer).

In order to reduce the CPU communication task, according to the actual needs of the network, we should select communication processors with different communication functions, such as peer to peer, fieldbus, industrial Ethernet, etc.

5. selection of PLC models

Type of PLC: PLC is divided into two types: the whole body and the modular type.

  • The overall PLC has fewer I/0 points and is relatively fixed, so the user has little choice and is usually used for small control systems. Representatives of this category of PLC include: SIEMENS's S7-200 series, Mitsubishi Co's FX series, Omron Corporation's CPM1A series and so on.

  • The modular PLC provides a variety of I/O modules that can be inserted on the PLC substrate, so that users can reasonably select and configure I/O points of the control system according to needs. Therefore, the modular PLC configuration is more flexible, and is generally used in large and medium-sized control systems. For example, SIEMENS's S7-300 series and S7-400 series, Mitsubishi Co's Q series, Omron Corporation's CVM1 series and so on.

6. the choice of I/O module

The selection of digital input and output module:

The application of digital input and output module should be considered. For example, for input module, the application level of input signal level and transmission distance should be considered. The output module also has many kinds, such as relay contact output type, AC120V/23V bidirectional thyristor output type, DC24V transistor drive type, DC48V transistor drive type and so on.

Usually the relay output and output module has the advantages of low cost and wide voltage range, but the service life is short and the response time is long. The surge absorbing circuit needs to be added when used for inductive load. The bidirectional thyristor output module has fast response time, which is suitable for frequent open and close, inductive low power factor load occasions, but the price is more expensive, and the overload capacity is poor.
In addition, the input and output modules can be divided into 8 points, 16 points and 32 points according to the number of input and output points.

The selection of analog input and output module:

The analog input module can be divided into three types according to the analog input signal type. Current input type, voltage input type, thermocouple input type, etc. The current input type usually has a signal level of 4 to 20mA or 0 to 20mA; the voltage input module usually has a signal level of 0 to 10V and -5V to +5V. Some analog input modules can be compatible with voltage or current input signals.
The analog output module also divides the voltage output module and the current mode output module, and the current output signal usually has 0 ~ 20mA and 4 ~ 20mA. Voltage output signals usually have 0 ~ 10V, -10V~+10V and so on.
The analog input and output module can be divided into 2 channels, 4 channels and 8 channels according to the number of input and output channels.


7. function module

The function modules include communication module, positioning module, pulse output module, high-speed counting module, PID control module, temperature control module, etc.

When choosing PLC, it is necessary to test the possibility of matching the functional modules. The choice of function modules involves two aspects: hardware and software.


  • In terms of hardware, we should first consider that functional modules can be conveniently connected to PLC, and PLC should have relevant connections, installation locations and interfaces, cables and other accessories.

  • In software, PLC should have corresponding control functions, which can facilitate programming of functional modules. For example, MITSUBISHI's FX series PLC can easily control the corresponding functional modules through the "FROM" and "TO" instructions.

8. general principles

After the PLC type and specification are roughly determined, the basic specifications and parameters of each component of PLC can be determined one by one according to the control requirements, and the models of each component module can be selected. When choosing module models, the following principles should be followed.


Generally speaking, as PLC, there are many kinds of modules that can meet the control requirements. The choice should be based on the principle of simplifying line design, convenient use and minimizing external controllers.
For example, for the input module, priority should be given to the input form that can be directly connected to the external detection element, so as to avoid using the interface circuit. For the output module, the output module that can directly drive the load should be selected first, and the intermediate relay and other components should be minimized.


When selecting type, we should take into account the unification and general-purpose of each module of PLC and avoid excessive types of modules. This will not only help procurement, reduce spare parts, but also increase the interchangeability of all components of the system, providing convenience for design, debugging and maintenance.


When selecting components of PLC system, compatibility should be fully considered. In order to avoid the problem of poor compatibility, manufacturers of the main components of PLC system should not be excessive. If possible, try to choose the same manufacturer's products.

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