Abstract: The process control system adopts SIEMENS PCS7 system and the PLC adopts SIEMENS S7-400 series. The system integrates data acquisition, process monitoring and process control. The vacuum treatment of billet refining has been put into production, providing a good process link for the production of high-quality steel, especially heavy rail steel.
Keywords: Top gun position control; Heating and combustion control; PID
Abstract: The process control system uses SIEMENS the PCS7 system, PLC uses SIEMENS the S7-400 series, the overall system collection data acquisition, the process surveillance, the process control are a body system, the side semifinished product fining vacuum treated goes into production, for the fine steel production, specially the rail-steel production, has provided the good craft link.
Key words: Goes against the gun position control; Heats up the combustion control; PID
1 Introduction
Vacuum treatment of molten steel is used for dehydrogenation, denitrification, deoxidation (light treatment), and desulfurization, and also has the functions of alloy fine-tuning and temperature adjustment. Panzhihua Iron and Steel Group (Pangang) produces special steels, such as rail steel, low-alloy structural steel, gear steel, and oxygen cylinder steel, all of which require vacuum treatment, making it a crucial step in billet continuous casting production. The top lance is used in the non-vacuum treatment state to maintain the temperature of the vacuum chamber, reduce temperature drop, and remove nodules from the inner wall surface of the vacuum chamber. According to the steel plant's production requirements for the vacuum system, the control system must have high reliability, ease of operation, convenient maintenance, and comprehensive control functions. The programmable controller of the vacuum control system [1-2] adopts the SEIMENS SIMATIC S7-400 PLC. The SIMATIC S7-400 PLC is a mid-to-high-end PLC with modularity and ease of user mastery. It also features high-speed instruction processing, a human-machine interface, intelligent CPU diagnostics, network communication, and a rich selection of software programming languages.
2. Automated control system
Based on the vacuum process equipment, the vacuum control system is designed using SIEMENS' SIMATIC PCS7 process control system. The system mainly consists of an S7 400 PLC , two monitoring stations, and a secondary computer system.
The RH control system consists of a two-level control system, a first-level basic automation system and a second-level computer control system. The first-level basic automation system completes its logic control and control loop regulation control. The second-level computer system takes process production data from the basic automation system, performs model calculations based on the production plan and steel arrival situation, optimizes the production parameters, and sends them to the basic automation system for execution. The RH basic automation system uses Siemens' S7 400 series PLC controller to form the process control system. The network system uses Siemens' industrial Ethernet to connect the LF system, alloy feeding system, central control room operator station, report printer, and RH's second-level computer and other equipment. The process control system uses Siemens' S7 400 series PLC control system [4]. The operator station uses ET200M and uses PROFIBUS-DP network to access the PLC main controller to reduce cable wiring. The electromechanical equipment uses RS232 protocol to communicate with the PLC main controller. The operator station and the PLC system communicate using industrial Ethernet protocol. The first-level operator station and the second-level computer use Siemens OPC protocol [5] to complete data exchange. To ensure precise positioning of the control equipment, its transmission system employs VVVF technology for speed control.
3. Descriptive Study of the Heating Process of the Top Gun
During non-processing periods, the top gun begins to descend from its parking position. When it descends to the preset heating height, the heating gas and oxygen valves open, the gas is ignited and burned, and the gas and oxygen flow rates are adjusted to a given value to heat the vacuum chamber or remove tumors.
Figure 1 Heating process of the top gun
The core device for position, heating, and auxiliary control is the PLC, and the software is the same as that used by RH PLCs. The control program is written in the STEP 7 programming language, using ladder diagrams, function block diagrams, and statement lists. The program is written using a modular approach.
3.1 Implementation of Operation Mode by PLC
(1) Emergency Operation Mode (Pneumatic Motor): In an emergency, turn the pneumatic motor coupling handle to the pneumatic motor position and use N2 to raise and lower the top gun. Electric operation and pneumatic motor operation are switched through a coupler, and a limit switch detects whether the top gun is in pneumatic or electric mode.
(2) Local Operation Mode (Field Control Box): Select "Local" on the field control box, press the "Up" button (self-reset button), the gun will rise at the given speed, and will automatically stop when the gun reaches the emergency upper limit; press the "Down" button (self-reset button), the gun will descend at the given speed, and will automatically stop when the gun reaches the emergency lower limit. After operation, set the selector switch to "Remote Control" (Field Control Panel) mode. When the field control box is set to "Local Operation", the field control panel, PLC and LEVEL 2 cannot operate the top gun.
(3) Local manual operation mode (on-site control panel): On the on-site control panel, set the key switch to "Local". Ascending process: First, loosen the expansion seal ring. When the gun is far from the predetermined position, press the "High-speed Ascend" button (self-reset button). The gun will ascend at high speed. When the gun is close to the predetermined position, press the "Low-speed Ascend" button. The gun will ascend at low speed. Release the button when the gun reaches the predetermined position, and the gun will stop. Tighten the expansion seal ring. During the ascending process, the gun will automatically stop when it reaches the upper limit. Descending process: First, loosen the expansion seal ring. When the gun is far from the predetermined position, press the "High-speed Descend" button (self-reset button). The gun will descend at high speed. When the gun is close to the predetermined position, press the "Low-speed Descend" button. The gun will descend at low speed. Release the button when the gun reaches the predetermined position, and the gun will stop. Tighten the expansion seal ring. During the descent process, the gun will automatically stop when it reaches the lower limit. The on-site control panel is equipped with a BCD digital display showing the gun position (distance from the bottom of the vacuum chamber) of the top gun. It includes indicator lights for "Local/Remote," "High-Speed Ascent," "Low-Speed Ascent," "High-Speed Descent," "Low-Speed Descent," and position limits. After operation, the key switch on the on-site control panel should be set to "Remote" mode. The control panel has a remote I/O station for the PLC, connected to the master station via a PROFIBUS-DP network with a transmission rate of 12M/S. The PROFIBUS-DP protocol is used.
(4) Remote Operation Mode (OSM Operator Station Operation): Remote operation is only possible when the actual gun position is <= the parking position. The operator directly inputs the gun position setting value on the operator station, and the PLC compares the actual gun position with the set gun position to control the raising and lowering of the top gun. When the gun position difference is >100mm, the top gun runs at high speed (raising/lowering); when the gun position difference is <100mm, the top gun runs at low speed (raising/lowering); when the gun position difference is <10mm, the top gun stops running, ensuring position control accuracy within +/-10mm. During remote operation, the operator station is connected to the PLC via industrial Ethernet with a transmission rate of 100M/s. The TCP/IP protocol is used.
3.2 Top Gun Position Control
The top gun position control is a closed-loop control based on position deviation. In automatic operation, the operator presets the desired heating position of the top gun on the operator station based on the vacuum chamber temperature. This data is transmitted via Ethernet to the CP module, which then transfers it to the CPU's storage area. The application program written in STEP7 software executes the corresponding control program, controlling the top gun position deviation based on encoder feedback. The top gun position is measured by an absolute encoder. The pulse signal generated by the encoder is read by an FM451 counting module. The CPU reads the signal from the FM451 counting module into the program via a function block, converting the count value into a corresponding height value. This height value is subtracted from the preset height value to obtain the deviation value. The top gun's speed and stop are controlled based on this deviation value. Numerical and comparative calculations in the program generate control signals, which are output from the signal module to the frequency converter to control the motor, thus achieving the lifting and lowering control of the top gun. The top gun performs initial position correction at its lower limit. The top gun's stroke ranges from 0 to 6 meters; the gun position displayed on the OMS and the on-site control panel is the distance between the top gun head and the bottom of the vacuum chamber. A stroke greater than 100mm is considered fast, between 10 and 100mm is slow, and below 10mm, the PLC issues a stop command. The software program control design block diagram is as follows.
Figure 2. Software program control design block diagram
3.3 Combustion Heating Control
Combustion heating control involves heating the vacuum chamber or removing slag under atmospheric heating during non-processing periods using the top lance. This includes maintaining the vacuum chamber temperature and removing residue. Upon heating startup, when the top lance reaches the predetermined heating height, the PLC-controlled combustion control unit opens the shut-off valve, initiating combustion. The PLC-controlled regulating valve adjusts the flow rate to achieve the given flow. A flame detector monitors the combustion status of the top lance flame; if the combustion intensity is insufficient, the lance is automatically lifted. The given flow rate is set at the operator station.
Figure 3. Simplified diagram of signal transmission structure
The gases passing through the top gun are N2, O2, and CO. Combustion heating control involves regulating the ratio and flow rate of CO and O2. Flow regulation is a simple closed-loop control system, employing a PID algorithm. The mathematical model is as follows:
e=PV-SV
(1)
(2)
PV: Actual flow rate; SP: Flow rate setpoint; e: Flow rate difference; OUT: Controller output; δ: Proportional coefficient; Ti: Integral time; Td: Derivative time.
In the program, data calculation can be effectively achieved by directly calling the PID function block. Here, the FB41 continuous control PID function block is used, which allows for convenient parameter configuration. After appropriately adjusting the values of the three parameters δ, Ti, and Td, good control quality can be obtained. During heating, the flow meter detects the flow rate, and the controller outputs the opening value of the regulating valve to control the valve opening, thereby controlling the gas flow to reach the predetermined value. The flow rate can also be adjusted manually by controlling the valve opening; this is an open-loop control. At the beginning and end of the process, the top gun needs to be purged with N2 to clean the gas in the pipeline. Gas flow is controlled by the PLC through a solenoid valve. After processing, pressure tests are performed on the CO and O2 pipelines using an electric contact pressure gauge and a shut-off valve, respectively, to ensure there are no leaks in the gas pipelines.
Figure 4 Software Programming Block Diagram
3.4 Top Gun Cooling Control
The top gun is cooled by an MCW (Mechanical Controlled Water Flow Meter), and the inlet and outlet flow rates of the cooling water are measured using electromagnetic flow meters. The inlet valve can be opened remotely and manually, and it will automatically close when the inlet flow rate is >98t/h or the difference between the inlet and outlet flow rates is >900L/h.
3.5 Expansion Seal Ring Control
When the top gun is raised or lowered, the expansion sealing ring should be loosened. When the top gun stops, the expansion sealing ring should be inflated to prevent it from burning out due to the high temperature inside the vacuum chamber. While the expansion sealing ring is loosened, the gun hole should be purged with N2 to prevent the sealing ring from burning.
4. Conclusion
In the past, some accidents occurred in the actual use of the factory, which led to production interruption. The innovation of this paper is that the system has strong data processing capabilities, realizes automatic generation of data reports, access to databases and other functions. The programmable controller of the top gun system is based on microprocessor, integrates computer technology, automatic control technology and communication technology, and has the characteristics of reliability, ease of operation, flexibility, mechatronics and strong adaptability to harsh environments. In the current actual use, the power supply system of the PLC system is very reliable, the human-machine interface is friendly, easy to operate and convenient to use and maintain. The PLC meets the needs of various control functions of the top gun [3], reflects its powerful and reliable control function, and has been widely used in Panzhihua Iron and Steel Group, saving the company nearly one million yuan in costs every year.
References
[1] Chen Zaiping, Zhao Xiangbin. Programmable Controller Technology and Application System Design [M]. Beijing: China Machine Press, 2002. 98-103.
[2] He Yanqing, Dai Zixiang, Yu Jinshou. Programmable Logic Controller Principles and Application Techniques [M]. Beijing: Chemical Industry Press, 1998.
[3] Du Xiaorong. Chemical Instrumentation and Automation [M]. Beijing: Chemical Industry Press, 1994. 76-108.
[4] Xu Zengqi. Refining Outside the Furnace [M]. Beijing: Metallurgical Industry Press, 2002, pp. 10-66.
[5] Wan Shudong, Ge Yunping. Application of PLC in fuel preparation system of thermal power plant [J]. Beijing: Microcomputer Information, 2007, 6: 67-69.
