introduction
Temperature is one of the most common and fundamental process parameters in industrial production. For example, various heating furnaces, heat treatment furnaces, and reaction furnaces widely used in industries such as machinery, electronics, petroleum, and chemicals require strict temperature control for workpieces. In the past, temperature control systems typically relied on instruments such as temperature controllers for direct control. However, with the development of computer communication technology and the diversification and functionality of PLC modules, distributed monitoring systems using PLCs as the underlying unit are increasingly adopted in industrial control. Connecting three PLCs to a computer via an RS-485 bus forms a distributed temperature control system. Data communication enables data sharing between the computer and PLCs, achieving centralized management and decentralized control of temperature. In this system, communication between the computer and PLCs is crucial; this article will detail the construction of the distributed network.
1. System Hardware Configuration
The system hardware structure is shown in Figure 1. The temperature control system consists of three parts: (1) information acquisition and signal input, which includes acquiring the temperature of multiple points; (2) information conversion and processing, whose main function is to convert the acquired information into standard quantity information that can be recognized by the computer for processing and output decision instructions; (3) output and control, which controls the heating valve, cooling valve, alarm and other systems to achieve satisfactory temperature control. The boiler contains heating wire, temperature sensor, etc. The temperature control principle of the PLC temperature control system is to use the PLC as the main controller of the system. The temperature feedback signal in the measurement circuit is converted by A/D and then sent to the PLC for processing. The PID output control signal controls the on and off of the thyristor and controls the power obtained by the heating furnace, thereby realizing the control of temperature.
This system employs a distributed data acquisition method, consisting of three FX2N PLCs, three FX2N-485BD communication modules, an ND-6520 signal converter, a signal acquisition module, a control module, and a computer. The communication lines are 22AWG. Each PLC is responsible for acquiring field signals from several nearby loops. The PLCs are connected to the host computer via communication. The overall system structure is shown in Figure 2. The host computer is the main computer, and the slave computers use Mitsubishi FX2N series PLCs. Each PLC forms an N:N network via the FX2N-485-BD communication modules. The host computer is connected to the main unit via the ND-6520 converter. In this monitoring system, the main task of the host computer is to acquire various information from the heating furnace, display parameters such as furnace temperature in real time, promptly alarm in case of faults, and take emergency measures to control the operation of the heating furnace. The main task of the slave computers is to automatically control the heating furnace and execute commands from the host computer in case of faults.
2. Communication module wiring
2.1 Communication module and wiring
The wiring diagram is shown below:
Figure 3 shows the communication programs for each PLC in the N:N network wiring:
First unit:
LD M8002
MOV H6080 D8120
MOV H0000 D8121
MOV K0 D8129
END
Second unit:
LD M8002
MOV H6080 D8120
MOV H0001 D8121
MOV K0 D8129
END
Third station:
LD M8002
MOV H6080 D8120
MOV H0002 D8121
MOV K0 D8129
END
2.2 Format of PLC Communication Data
In computer communication mode, the FX series PLC has two communication modes. This system uses mode 1, and its communication data format is shown in Figure 4:
A. Communication mode when reading PLC components and communication data
Set subroutine operations:
1) Input points X000 to X003 (M1000 to M1003) of the main station are output to output points Y010 to Y013 of station numbers 1 and 2;
2) The input points X000 to X003 (M1064 to M1067) of station 1 are output to the main station and the output points Y014 to Y017 of station 2;
3) The input points X000 to X003 (M1128 to M1131) of station 2 are output to the main station number and the output points Y020 to Y023 of station 1;
4) The data register D1 in the master station is set to the value of counter C1 in station 1. The contact status (M1070) of counter C1 is reflected on the output point Y005 of the master station;
5) Data register D2 in the master station is set to the value of counter C2 in station 2. The contact (M1140) status of counter C2 is reflected on output point Y006 of the master station;
6) The values of data register D10 in station 1 and data register D20 in station 2 are added to the main station and stored in data register D3;
7) The values of data register D10 in the main station and data register D20 in station 2 are added to station 1 and stored in data register D11;
8) The values of data register D10 in the main station and data register D1 in station 1 are added to station 2 and stored in data register D21.
Some subroutines are configured with the following requirements:
Refresh range: 32-bit soft elements and 4-word soft elements (Mode 1)
Number of retries: 3
Communication timeout: 5 (50ms)
To configure the master subroutine, the slave device only needs to configure the first line of the program:
Figure 5 PLC communication ladder diagram
3. Conclusion
Actual operation of the system has shown that the distributed furnace temperature control system built with PLC has simple hardware connections, quick programming, and can easily realize network communication between computers and multiple PLCs, thereby achieving centralized processing of distributed data in complex systems. Moreover, it operates stably and reliably, and can well meet the requirements of the control system.
