Abstract: This paper presents a design scheme for a new type of dedicated wire feeder, utilizing Siemens PLC, touch screen, frequency converter, and fieldbus for remote monitoring, to meet the actual requirements of the refining F furnace at Taiyuan No. 2 Steel Plant. The main components and characteristics of this system are described. This scheme effectively improves the automation level of ladle wire feeding and the quality of smelted steel.
Keywords: wire feeder; PLC; frequency converter; PROFIBUS-DP fieldbus
In the steelmaking process, various deoxidizers, desulfurizers, and other additives need to be added to the ladle. However, due to the high viscosity of molten steel, the depth of the ladle, and the presence of slag on the surface of the molten steel, uniformly melting the cored wire into the molten steel has always been a challenge. In recent years, wire feeding technology has been used to largely solve this problem. Wire feeding technology is a series of techniques that involve winding various additives into steel strips to form cored wire, and then feeding the cored wire into the ladle using a wire feeding machine. Currently, the wire feeding machine solutions for refining furnaces in various steel plants in China all use simple instrument control combined with manual operation. Since the wire feeding length and speed are closely related to the quality of steelmaking, and the wire feeding speed and length are difficult to control manually, often resulting in unstable product quality, the automation of the wire feeding process is an inevitable trend.
1. Determination of Control Scheme
Based on the actual conditions of the F refining furnace at Taiyuan No. 2 Steel Plant, the new wire feeder control system consists of two parts: a pneumatic circuit and an electrical circuit. The feeding speed and length are controlled by adjusting the speed and number of revolutions of the AC motor. The electrical control system uses a Siemens S7200 programmable controller for automatic control of the wire feeder, and a Siemens TP170A touchscreen for human-machine interaction, enabling precise setting of the feeding speed and length. Two Siemens MicroMaster420 frequency converters are used for variable frequency speed control of the wire feeder motor. A fieldbus control system is constructed using PROFIBUS-DP fieldbus, and network and hardware configurations are performed using STEP7-MICRO WIN 32. Dynamic linking between the upper PC monitoring configuration software Win CC and MICRO WIN 32 is achieved, enabling on-site operation, monitoring, and data acquisition from the control room. A schematic diagram of the system structure is shown in Figure 1.
2. Implementation of electrical control
2.1 Control System Composition
The system control core uses the German Siemens S7-200 programmable controller. The CPU provides high-speed I/O for controlling high-speed events, which facilitates the use of high-speed counters in the wire feeder, and also provides convenient, simple and open communication functions.
The control system utilizes two RS485 communication ports on the PLC: Port0 communicates with two frequency converters using the USS protocol to control the speed of the wire feeding motor; Port1 communicates with the TP170A touchscreen via the MPI protocol to enable human-machine interaction. Through the TP170A touchscreen, precise setting and real-time display of wire feeding parameters, jogging and debugging of the entire machine, automatic operation, and calculation of process parameters are possible. Two photoelectric high-speed counters are connected to the PLC's I/O interface to collect the angular velocity of the moving wire feeding wheel in real time. Simultaneously, the control system employs a Siemens EM-277 module, connecting the PLC and a control room computer with a CP-5611 network card via a PROFIBUS-DP fieldbus for remote system monitoring.
The selection of frequency converter is mainly based on the actual control accuracy and motor capacity [1] . Since the power of the feed motor is 11kW, the scheme selects the cost-effective non-vector control MicroMaster420 frequency converter. This frequency converter is used in conjunction with the PLC through the USS protocol instructions, and has the characteristics of wide speed range, high speed accuracy, high temperature resistance and reliable operation. The USS protocol instructions are the protocol instructions for communication between PLC and frequency converter in the STEP 7-MICRO/WIN32 software toolkit. The communication between PLC and frequency converter is realized by using pre-configured subroutines and interrupt programs designed specifically for USS protocol communication.
By configuring the inverter's remote control mode, parameters such as the inverter slave address, baud rate, operating frequency, start-up time, and braking method are set, enabling the PLC to control the inverter in real time and provide signal feedback. After receiving the command, the PLC calculates the motor speed and frequency based on the set wire feeding speed and transmits these values to the inverter via USS protocol. Upon receiving the command, the inverter adjusts the motor speed according to its internal settings. The speed adjustment method is V/F type, meaning that while changing the inverter's output frequency, the output voltage is adjusted accordingly to maintain a constant magnetic flux in the asynchronous motor. This speed adjustment method is simple and reliable, effectively meeting the speed control requirements of the wire feeder.
2.2 PC Control Program
The PLC control program is written using SIEMENS' STEP7 software with the Ladder Logic Language (LAD). STEP7-MICRO WIN32 is specialized software for the S7-200 series programmable controllers. This software runs on the WIN2000 platform, and the standard package includes three programming languages: Statement List Language (STL), Ladder Logic Language (LAD), and Function Block Diagram Language (FBD). The program consists of a main program, seven subroutines, and three interrupt routines. The main program performs logic function control and calls the subroutines and interrupt routines. A schematic diagram of the program operation is shown in Figure 3.
The program includes: SBR0 initialization subroutine, which initializes two high-speed counters and the USS protocol; SBR1-SBR5 subroutines required for the USS communication protocol; INT1-INT3 interrupt routines required for USS protocol communication instructions; SBR6 speed calculation subroutine, which calculates real-time values such as the current wire feeding speed and current wire feeding length, and displays them on the TP170A; SBR7 calculator subroutine, which calculates the theoretical wire feeding motor frequency based on the input wire feeding speed and length; and SBR8 panel switch control subroutine, which controls the start, stop, wire feeding selection, and working/maintenance status selection of the wire feeder, among other control requirements.
3. Implementation of Remote Monitoring
3.1 Fieldbus Selection
PROFIBUS is an international, open fieldbus standard primarily designed for industrial process control applications, used for high-speed data transmission in distributed control systems. PROFIBUS comes in three forms: PROFIBUS-FMS, PROFIBUS-PA, and PROFIBUS-DP. This design utilizes PROFIBUS-DP, based on the ISO 7498 international standard and using the Open Systems Interconnection (OIS) network as a reference model. It employs Layers 1 and 2 and a user interface. The physical layer of this bus uses RS485 transmission (with RS-485 twisted-pair cable or fiber optic cable as the transmission medium), achieving a transmission speed of 9.6 kbps to 12 Mbps, enabling communication between distributed I/O stations and field devices in the automation system.
3.2 Configuration Software Design
This paper uses the configuration software WinCC to implement a human-machine interface (HMI) and monitoring and management system. The system design consists of the following modules: a WinCC communication driver, which connects the PLC and the WinCC data manager; a graphical operating system, providing the HMI for setting and displaying wire feeding length and speed in real time; a variable recording operating system, which records historical data for variables such as wire feeding length and speed; and an alarm recording operating system, which provides real-time monitoring of wire feeding length values to ensure system safety. System data is stored in the SQLAnywhere system database within WinCC. During operation, the WinCC communication driver uses channel units to form the interface between the WinCC and PLC programs. It uses a communication processor to send request messages to the PLC, and after the PLC returns a response message, the communication processor returns the process values to the WinCC data manager. The WinCC data manager centrally manages various data, providing various data for functions such as wire feeding length and speed setting, real-time display, alarm functions, and historical record storage.
4. Conclusion
The wire feeder control system uses a TP170A touchscreen as the human-machine interface, increasing system reliability and precisely controlling the wire feeding speed and length, thus ensuring reliable steelmaking quality. It employs the S7-200 USS protocol to directly control the frequency converter, achieving advantages such as flexible control, high precision, and simple program development. The system utilizes WinCC, a PLC control system configuration software for industrial applications, for on-site monitoring, enabling on-site operation, monitoring, and data acquisition. Since its implementation, the system has remained highly stable, reliably guaranteeing the refining quality of the F furnace in Taiyuan Iron & Steel Group's No. 2 steelmaking plant, resulting in significant economic and social benefits.
References:
[1]Siemens AG.WinCC v5 The Configuration Manual and the Communication Manual[P].Germany:Siemens,2000.
[2] Zhang Yong, Li Yuejin, et al. Automatic control system for BNBM waterborne coating production line based on Profibus fieldbus technology [J]. Manufacturing Automation, 2000(22):45-46.
For details, please click: Research and Development of a New Automatic Control System for Wire Feeding Machines
