Because of its low construction cost, low operation and maintenance cost, high flexibility, high expansibility and high product quality, all hydrogen bell type furnace is often used in the annealing process of magnetic materials and steel materials, mainly to improve the physical and mechanical properties of materials.
However, in actual production, the process of bell type annealing furnace is very complex and the production conditions are harsh, so the requirements for the control system are much higher than other heating equipment. Taking the automation control system of 12 bell type annealing furnaces as an example, this paper introduces a fieldbus control system composed of PLC and PID intelligent temperature control instrument.
1、 Brief introduction of annealing process in bell type furnace
The main equipment of bell type furnace includes annealing hearth, heating hood, inner hood, cooling hood, valve station, etc. The hearth is the basic equipment of the whole bell type furnace. The motor and impeller are installed at the bottom of the furnace, which are beneficial to the thermal convection circulation. The burden is placed in the inner hood for annealing. The resistance wires in the heating hood are evenly distributed in two areas. When heating, the inner hood is outside the inner hood. After heating, the cooling hood is replaced. The valve station is composed of public and auxiliary pipelines, control valves and instruments, which is used to provide the input and output of hydrogen, nitrogen, water and other media, as well as the control of pressure and flow.
The process is as follows: after the furnace charge is installed, the inner hood is fastened and hydraulically locked, and the cooling circulation system in the connecting flange between the inner hood and the hearth starts to work. After the inner hood is locked, confirm that the hydrogen valve, the inner hood, the hearth, the circulating fan and the exhaust pipeline are well sealed, then vacuum, and the furnace pressure drops to - 90kpa; start the automatic nitrogen purging, replace the air in the inner hood, and reduce the oxygen content in the furnace At the same time, the heating hood is hoisted to the hearth, and heating and heat preservation are started according to the set annealing curve. The furnace charge is annealed under full hydrogen. Before the end of the heat preservation stage, the inner hood of the hearth is automatically detected for leakage. When the safety is detected, the heating hood can be lifted away, then the cooling hood can be lifted to the furnace platform, and the cooling fan can be started. When the temperature of the inner hood is below 400 ℃, the water will be cooled until the furnace charge reaches the furnace discharge temperature, and the nitrogen will be used for purging. After the replacement, the circulating fan will stop working, the cooling hood will be lifted away, the inner hood will be loosened, and the furnace charge will be discharged for the next week The production of the period. The annealing process is shown in the figure
2、 System configuration
The annealing process of bell type annealing furnace is complex, and there are many logic interlocks. Using S7-300 PLC as the controller can meet the control requirements. A set of PLC is used to control two hearth. PLC master station includes power module and communication module. Four digital input / output modules are equipped in the valve station of the hearth to deal with the actions of gas valves, switches and other switching values. The two analog input modules are mainly used to collect various analog signals such as furnace pressure and gas flow. The frequency converter controls the hearth circulating fan. The PLC master station establishes PROFIBUS field control network through the DP interface on the CPU, and connects the frequency converter of the boiler fan and the field slave station. The touch screen is TFT LCD of MCGS, which is connected to Ethernet through Ethernet, and can meet the use requirements.
A total of 12 stoves are controlled by six PLCs. Through the photoelectric switch, all PLC and upper computer constitute Industrial Ethernet to realize real-time communication and data exchange between multiple PLCs and between PLC and upper computer, so as to ensure real-time and efficient data transmission. The upper computer is composed of two industrial control computers, which are used as engineer station and operation station respectively, and equipped with ups and printer. As the management layer of network communication, the upper computer is installed with industrial configuration software to monitor the production process.
Each control system is equipped with an xma-5000 series temperature control instrument with intelligent PID function. The xma-5000 instrument has communication function, and the communication interface complies with RS485 standard. Multiple xma-5000 instruments are connected to the serial port of upper computer through RS485 communication converter. Xma-5000 instrument, as the lower computer of the system, measures and adjusts the furnace temperature on site according to the process parameters given by the upper computer. When the network fails, xma-5000 instrument can also be separated from the upper computer to complete the task alone.
Xma-5000 instrument regulates the heating power through periodic zero crossing trigger mode. The inner cover is equipped with double input K-type thermocouple, one is sent to temperature control meter for closed-loop temperature control, and the other is sent to upper computer for temperature curve recording and over temperature alarm. Xma-5000 instrument has PID parameter self-tuning function. After the temperature curve is set, start the instrument and start self-tuning. Besides PID, the self-tuning parameters also include output limiting parameters and anti overshoot simulation parameters. The output control quantity of the instrument is 4-20mA, and the output power of the thyristor regulator can be adjusted in real time.
The system adopts the control idea of centralized management and decentralized control. The sequence control, data acquisition and fault handling are completed by PLC. The temperature control is completed by intelligent instrument xma-5000. The upper computer is responsible for the monitoring and configuration management of the whole annealing process. The control task of the system is decentralized, but the information of each measurement and control unit is open, which can exchange data with each other and cooperate with each other to complete the control process of the whole system.
3、 PC control system and professional configuration software
The upper computer is mainly composed of two industrial control computers, printers and other peripherals. The professional configuration software of upper computer is used to realize the man-machine operation interface, including the overall situation of the scene, alarm record, data display and record, operation command execution and other functions.
In the actual production, due to the different product specifications and process requirements, the annealing curve often needs to change parameters frequently in the annealing process. The setting and modification of the parameters of the traditional manual operation instrument is very complicated. The application of the upper computer configuration software can realize the quick and direct setting of system parameters or the modification of the process curve through the computer keyboard. At the same time, the configuration software can realize the centralized monitoring of the furnace group and the separate monitoring of each heating furnace, and can display the working status and various real-time data of each heating equipment in real time. When the system fails, when the control cabinet sends out a beep alarm, the computer interface of the upper computer displays the alarm prompt at the same time, and provides the function of fault inquiry and prompt the corresponding processing and solving measures. In the aspect of management, the heat treatment automation software has the function of administrator permission setting. According to the needs of different levels of managers and operators, multiple users can set login permissions. High level users can have more operation and view permissions. Similarly, front-line operators have limited operation permissions to ensure the safety of production.
4、 Summary
The whole annealing furnace group control system consists of intelligent temperature control instrument with field communication ability and PLC to form a system network, which enables each heating equipment to exchange information with the host computer. It can realize signal acquisition, control, fault handling and other functions through the field temperature control instrument, and can also realize data viewing and processing, control action and other functions through the monitoring computer to realize centralized management Management control. The PLC and the temperature control instrument can run independently to complete the control, each system does not interfere with each other, and can achieve centralized monitoring and management through the upper computer. After the system is put into use, the temperature control accuracy is good, the operation is stable, and a higher level of production automation is realized. Practice has proved that the control system can also be upgraded to the workshop management level to control more heat treatment workshops with bell type furnaces and meet higher management requirements.
