Content area
In the anticorrosive coating line of a welded tube plant, the current status and existing problems of the medium-frequency induction heating equipment were discussed. Partial renovations of the power control cabinet have been conducted. Parameters such as the DC current, DC voltage, intermediate frequency power, heating temperature and the positioning signal at the pipe end were collected. A data acquisition and processing system, which can process data according to user needs and provide convenient data processing functions, has been developed using LabVIEW software. This system has been successfully applied in the coating line for the automatic control of high-power induction heating equipment, production management, and digital steel tube and/or digital delivery.
Abstract: In the anticorrosive coating line of a welded tube plant, the current status and existing problems of the medium-frequency induction heating equipment were discussed. Partial renovations of the power control cabinet have been conducted. Parameters such as the DC current, DC voltage, intermediate frequency power, heating temperature and the positioning signal at the pipe end were collected. A data acquisition and processing system, which can process data according to user needs and provide convenient data processing functions, has been developed using LabVIEW software. This system has been successfully applied in the coating line for the automatic control of high-power induction heating equipment, production management, and digital steel tube and/or digital delivery.
Key words: induction heating; data acquisition; data processing; coating line; welded steel tube
1 Introduction
Induction heating based on the principle of electromagnetic induction, generates eddy currents in metallic materials when exposed to an alternating magnetic field, thereby heating the metals to high temperatures. This method is widely used in the melting,rolling, welding, and heat treatment of various metals and their alloys. Induction heating with a frequency ranging from 50 Hz to less than 10 kHz is commonly referred to as medium-frequency induction heating. By the end of the 20" century, domestic technology for medium-frequency induction heating equipment had become well-developed and began to see widespread industrial applications '?. In the steel pipe manufacturing industry, high-power medium-frequency induction heating equipment is widely used for seamless steel pipe heat treatment, high-frequency welded pipe heat treatment, and the production of anticorrosion coatings on the outer surface of large-diameter pipeline pipes. However, in the early stages, the automation control level of the medium-frequency heating equipment was relatively low, the manufacturing technology in small and medium-sized enterprises was outdated, and many users had limited or basic requirements for the control functions of the equipment. Problems such as incomplete protection, lack of process data recording, and absence of temperature closed-loop control still exist in medium-frequency induction heating equipment to some extent.
At Baosteel, the welded pipe anti-corrosive coating production line was put into operation in 2009, featuring 3 200 kW-1 000 Hz medium-frequency induction heating equipment designed and manufactured domestically. The DC current, DC voltage , medium-frequency voltage, output power, and other parameters of the induction heating power supply are displayed using pointer-type instruments. The heating temperature of the steel pipe is measured by an infrared thermometer and displayed on a digital display meter, allowing the operator to view or manually record the data. With the promotion of Baosteel 's intelligent manufacturing strategy and the implementation of the production process control plan, the process control parameters of key equipment have become a focus. In addition, many customers have introduced clear and traceable requirements for the process control parameters of induction heating, including output power, current parameters, and heating temperature.
The medium-frequency induction heating equipment was locally modified to meet these requirements. The main control parameters of the equipment were collected and a data acquisition system was developed to gather , process , and store the data. This system provides data support to meet the production management needs and the varying requirements of different customers. Additionally, this study contributes to the ongoing efforts at Baosteel to address the issues of "digital steel pipe" and "digital delivery".
2 Power cabinet modification
The control cabinet of the intermediate frequency power supply was partially reformed to collect the current, voltage , power , temperature , and other parameters during induction heating. The acquisitions of DC voltage and DC current were added to the output side of the rectifier circuit. The DC signal (0-75 mV) was obtained through the shunt and then converted into a 0-10 V voltage signal using the isolation module GLQ2. The DC voltage signal obtained by the attenuating card SJB1 was converted to a 0-10 V voltage signal through the isolation module GLQI , which is shown in Fig. 1 (a). The medium-frequency voltage signal was collected through the intermediate frequency transformer on the output side of the inverter circuit and then converted into a 0-10 V DC signal using the rectifier board ZLBI. This converted into a 0-10 V DC voltage signal through the isolation module GLQ3, as shown in Fig. 1(b). The heating temperature of the steel pipe was measured by a Williamson high-precision infrared thermometer installed at the outlet of the intermediate frequency induction furnace, and the temperature signal was converted into a 4-20 mA current signal using the isolation module GLQ7. The output current and voltage signals of the isolation module were collected to the industrial computer through the analog input channel of the data acquisition card PCI-6221, and the data were processed therein. The output power was calculated based on the DC voltage and the DC current.
3 Hardware of data acquisition system
The data acquisition hardware system comprises an industrial computer, data acquisition card, analog quantity isolation board, digital quantity isolation board, signal conversion unit, and other components, as shown in Fig. 2. The system uses an Advantech industrial computer with a 610H chassis, which is known for its strong resistance to anti-electromagnetic interference and efficient airflow. The data acquisition card is a PCI-6221 data acquisition card from NI Corporation, a general-purpose acquisition control board based on the PCI bus, which can be directly inserted into a computer with a PCI slot. This component forms the hardware system for acquiring analog and digital voltage signals, monitoring inputs, outputting analog or digital voltage signals, The data acquisition card of PCI-6221 provides users with the counting, timing, and other functions. following features; 16 analog input channels, 2 analog output channels with 16-bit resolution, bidirectional 24-digit I/O channel, and two 24-bit counters. This card has a 250 000 samples per second sampling rate, a 16-bit resolution, a maximum voltage range of - 10 to +10 V,and a minimum voltage range of -200 to +200 mV'··.
The data acquisition card requires corresponding signal processing and transmission accessories to capture external data. The DC voltage, DC current , and intermediate frequency voltage signals are inputted into the signal attenuation cards and analog signal isolation boards. The infrared thermometer, through a matching transmitter, sends data to the analog signal isolation board. PLC digital signals are routed to the digital isolation board. After all signals pass through their respective isolation boards, input and output signals are transmitted to the data acquisition card (PCI-6221) via the NI standard 68-core (PIN) ГО connector and 68-core shielded cable.
4 Software development based on LabVIEW
4.1 LabVIEW software introduction
LabVIEW is a virtual instrument software developed by NI Corporation in the United States of America. This software uses graphical language programming to perform tasks in laboratory research and industrial automation, such as data acquisition, instrument control, process monitoring, and automatic testing. Using LabVIEW to design virtual instruments, users can not only achieve the functions of general measuring instruments but also customize the software to meet specific requirements and even add other functions that traditional measuring instruments do not offer. The LabVIEW program mainly includes the following three parts: front panel, block diagram program, and icon/ connection port. The front panel is the interactive graphical user interface in LabVIEW , allowing users to input data and display program outputs, simulating the front panel of a real instrument. The block diagram program uses graphic language to control the quantity and indicator quantity on the front panel. The icon/ connection port is used to define LabVIEW programs as subroutines that can be called by other programs, enabling hierarchical and modular programming in LabVIEW".
4.2 Program development and implementation
The data acquisition system is configured with the Winl0 operating system on the Advantech 610H industrial computer. LabVIEW 2018 is used as the development platform. The system utilizes the NI PCI-6221 data acquisition card, which supports the DAQ MX driver. NI provides dedicated management software, Measurement & Automation Explorer, to detect and conduct performance tests on all NI data acquisition cards, ensuring the accuracy of hardware installation and normal working conditions. After testing the data acquisition card, the software can be used to configure the analog input/ output and digital I/O channels, including selecting physical channels, setting the sampling mode, frequency and number of samples per channel, сопfiguring the input mode, and setting the maximum and minimum sampling values "·'.
The graphical programming language of LabVIEW is used for program development. For a new project,the first step is to design the user interface, or front panel, based on the functional requirements of the software. The front panel not only accepts instructions from the program block diagram but also serves as the interface for the user to interact With the source program. On the front panel, various controls can be set, including switches, buttons, data input & output charts,text boxes,and other controls.
The developed user-oriented main interface is shown in Fig. 3. The main functions of this interface include displaying key parameters such as DC voltage, DC current , medium-frequency voltage, medium-frequency power frequency and temperature ¡real-time display of temperature curve; historical record queries; and the recording/ generation of heating temperature or output power data.
In the graphical programming language of LabVIEW, Virtual Instrument (abbreviated as VI) is the basic programming unit, comprising three parts; front panel, block diagram, and icon/connector. The front panel of each VI program corresponds to its block diagram. The block diagram program is written in the graphic programming language of LabVIEW and is composed of four elements: nodes, terminals, frames, and wires. Among them, the terminals are used to transfer data between the controls and indicators on the front panel of the program. The nodes are used to implement functions and function calls. The frames are used to structure program control commands, while the wires represent the data flow during program execution,defining the direction of data flow in the block diagram. When compiling the block diagram program, the required node icons or frames are selected from the function template and then connected to the terminals in the block diagram". The main interface of the system focuses primarily on displaying key power parameters and temperature curves.
4.3 Data processing and file storage
LabVIEW supports a variety of formats for file and data input/ output, including text files, binary files, data recording files, waveform files, measurement files, and TDM/TDMS files. Among these, the spreadsheet Excel file is a special type of text file. In this study, the Write To Spreadsheet File function is used to save the collected data in text format. Users can set the storage path themselves, and the file can be accessed directly in the background.
The system collects all data based on the set time interval and generates a text format file, which occupies minimal memory and effectively reduces storage space usage. For process control data that must be delivered to the user, choosing to save only the steel pipe temperature, or temperature + power, is possible according to the needs of users. The system will generate the corresponding data file based on the selection and store each steel pipe"s data as an Excel file. The file can be named based on the production time or the pipe number. Feeding information is confirmed through the coating position, with communication to the PLC ма L2, sending the pipe information data packet in real time to obtain the pipe number.
By default, the system stores all collected process data in ". txt" format and user record data in ". xls format, respectively, in the "record" folder and "recordexcel" folder on disk D.
5 Tube end identification
In the data acquisition and processing system, the data provided to the user must be segmented and stored in accordance with each steel pipe. Therefore, for steel pipes that are continuously heated in the preheating section of the roller table, with temperature data being continuously collected, accurately identifying the position of the pipe end to determine the time point for data segmentation is crucial.
Three methods were used for comparative analysis for precise identification of steel pipe end: temperature fluctuation data analysis, contact probe detection, and laser sensor detection.
(1) Pipe end temperature fluctuation data analysis: The temperature measured by the thermometer at this location may fluctuate and substantially deviate from the normal temperature value due to the attached insulation paper at the end of the steel pipe connection. Therefore , based on the temperature fluctuations at this point, the steel pipe is at the pipe end boundary , which can serve as the cutting signal for pipe end identification. However, extensive onsite experiments have shown that signals collected by infrared thermometers can notably deviate due to changes in environmental and on-site conditions. The temperature fluctuations at the pipe end are occasionally not evident. Therefore ,if the pipe end temperature signal is used as the data segmentation signal, then the accuracy of the pipe end identification cannot be reliably guaranteed.
(2) Contact probe detection method; Two probes are connected to the steel pipe, and a 24 V DC power supply is applied between them. When the probes come into contact with the pipe body, the circuit becomes conductive. However, when the steel pipe passes through the end, the insulation paper on the pipe end can cause a brief disconnection in the circuit, which may serve as the signal for identifying the pipe end. In practice, the mechanical contact probe using this method is susceptible to damage due to friction caused by the spiral movement and jumping of the steel pipe. Furthermore, the long-term use of a 24 V DC power supply can lead to damage.
(3) Laser sensor detection method; The Keyence 3D laser sensor LJ-V7300, paired with an LJ-V7001 controller,1s used to identify the shape of steel pipe ends.
Therefore ,the laser sensor detection method is configured with an Advantech industrial computer to develop the graphics processing system and corresponding image functions. The industrial computer communicates with the PLC via PROFINET. When the graphics processing system detects the position of the tube end, a signal is sent to the PLC, which then transmits the pipe end signal to the data processing system. The structure of this graphic recognition system is shown in Fig. 4. By further developing the laser sensor control system, pattern recognition can be performed based on identifying the position of the steel pipe end connector, thereby enabling highly accurate determination of the steel pipe end position.
6 Conclusions
By partially modifying the control cabinet of the intermediate frequency induction heating equip ment, parameters such as DC current, DC voltage, intermediate frequency power, heating temperature, and pipe end signal were automatically collected. A data acquisition and processing system has been developed using LabVIEW software, which not only provides technical support for the needs of different users but also offers data support for equipment operation. This system has been applied to the auto matic control of the intermediate frequency induction heating power supply, production management, and dig ital tracking of steel pipes or deliveries in the anticor rosive coating line of a welded tube plant, achieving positive results.
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