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Application of PLC in Control System of Vacuum Heat Treatment Furnace

Application of PLC in Control System of Vacuum Heat Treatment Furnace

Vacuum heat treatment is a necessary intermediate process used in manufacturing industry. It is a branch of heat treatment. The main purpose of vacuum heat treatment is to prevent oxidation and decarbonization of workpiece during processing. Because the programmable controller applies microelectronic technology and computer technology, various control functions are realized by software. As long as the program is changed, it can adapt to the change of production process, so it has strong adaptability. It can not only complete the functions of logic operation, timing and counting, but also perform arithmetic operation. Therefore, it can control not only the switching quantity, but also the analog quantity. It can also connect with the computer to realize the hierarchical control. Moreover, the PLC has the advantages of small size, light weight, compact structure, short development cycle, and small installation and maintenance workload. Therefore, in the process of transforming traditional industries with microelectronics technology, the traditional relay control system is mostly replaced by PLC. In this paper, through elaborating the combination of PLC and vacuum heat treatment furnace control system, the overall idea is put forward, and the system hardware and software design is carried out.

1. The determination of control system scheme:
Vacuum Heat Treatment Furnace


1.1 The working principle of the control system is as follows: firstly, the scheduled PLC and the controlled vacuum heat treatment are powered on, and the ready signal is sent to the industrial control computer. Thereafter, the industrial control computer is used to start the control operation of the whole system. During the working process, PLC realizes data communication with industrial control computer on the one hand, and controls the normal operation of vacuum heat treatment furnace on the other. When the PLC receives the data extraction command from the industrial control computer, it sends the data collected from the vacuum heat treatment furnace, such as the vacuum degree, temperature, cooling water status and switching status of each component to the industrial control computer. When the data is sent, the PLC continues to control the vacuum heat treatment furnace. PLC controls the equipment mainly through the peripheral circuit composed of contactor, solid-state relay and other electrical components to control the operation, air supply operation, limit and fault alarm of air pressure valve, vacuum pump, resistance wire and other components. On the one hand, the industrial control computer processes and displays the received data, on the other hand, it waits for the arrival of the second clock interrupt signal, ready to issue the next command to extract data and receive and receive data. The control system is designed to connect two PLCs with an industrial control computer. The data of each PLC will be extracted regularly and the two PLCs will be controlled alternately.

1.2 The overall layout of the control system takes PLC as the control core and industrial control computer as the supervision and display. The temperature signal is returned by thermocouple, and the controlled pulse is output to the solid state relay to control the temperature by comparing with the set value. The vacuum sensor is used to return the vacuum signal, then the next step is carried out according to the vacuum value, and the solenoid valve and vacuum pump are driven to achieve the required vacuum in the furnace. The circulating water sensor is monitored and handled in time when the alarm state occurs. By controlling the opening of the circulating water valve and the circulating water pump, the water consumption of all the vacuum furnaces in the factory building is coordinated. Linear switch and rotary switch of furnace door are controlled by PLC program.

2. Hardware design of the system:

The hardware of PLC control system is composed of PLC, input/output circuit and peripheral equipment. The scale of the system depends on the vacuum heat treatment process. Because vacuum heat treatment furnace has closed-loop control, PID regulation, communication network and so on, the control circuit is more complex, so the selection of large and medium-sized PLC can meet the requirements. Because the control object of this control system is the heat treatment vacuum furnace with a volume of 10 cubic meters, the model of PLC is S7-300. The connection of the external components of the vacuum system is shown in Figure 4. NL1-NL3 is a mechanical pump, ND1, ND2 is a diffusion pump, NZ1 is a Rhodes pump, VP1-VP13 is a pneumatic valve, BW1-BW5 is a pump cooler, PT1-PT4, PA1, PA2. PA3 is a vacuum sensor and PD1-PD3 is a pressure gauge. The primary vacuum is pumped by mechanical pumps NL1, NL2 and NL3, the intermediate vacuum is pumped by NZ1, and the advanced vacuum is pumped by ND1 and ND2.

3. Software design steps of PLC control system:

(1) Working out the operation plan of vacuum furnace. According to the requirement of heat treatment process, the logical relationship between input parameters such as thermocouple, vacuum degree and output signals of control contactor and solid state relay and various operations is analyzed, and the method of measurement and control is determined. The operation contents and sequence of each equipment in the vacuum furnace system are designed. Flow charts can be drawn accordingly.

(2) Draw flow charts of vacuum pumping and temperature control.

(3) Develop anti-interference measures of PLC control system, such as power isolation, photoelectric coupling of peripheral circuit.

(4) Programming, according to the input of controlled object such as thermocouple and output signal such as controlling the pulse of solid state relay and selected PLC model, allocating hardware resources, numbering various relays or contacts of ladder diagram, and then programming with ladder diagram according to technical instructions.

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