0 Introduction Refractory materials play a crucial role in various industries such as national defense, aerospace, electronics, energy, chemicals, metallurgy, and nuclear power. Electron beam melting furnaces are specialized equipment used for melting refractory metals, requiring a vacuum environment to ensure proper operation. The mechanical drive system, cooling system, and electron gun system work together according to the process requirements to achieve efficient and safe melting.
1 Vacuum System Configuration and Single Crystal Process Flow 1.1 Vacuum System Configuration The vacuum system of a 20kW electron beam furnace consists of a mechanical pump and a diffusion pump connected in series to achieve the required vacuum level for the melting process.
1.2 Single Crystal Pulling Process 1. First, turn on the cooling water and compressed air. 2. Power on the furnace and start the mechanical pump. 3. Immediately activate the diffusion pump; after 5 minutes, open the main pump. 4. Open the deflation valve and clamp the rod. 5. Charge the system. Once completed, close the furnace, shut off the main pumping valve, and open the pre-pumping valve. 6. When the vacuum reaches a certain level (in Pa), close the pre-pumping valve, open the main pumping valve, and simultaneously open the large valve. 7. Continue pumping until the vacuum level stabilizes. 8. Turn on the cathode, apply the high-voltage power supply, and start the electron gun to melt the metal. Adjust the filament current to 10A and activate the excitation. 9. Melt the rod and break the high pressure. 10. After cooling down, close the large valve and open the air valve. If stopping the furnace, first stop the diffusion pump, wait about an hour, then close the large valve, main pump, mechanical pump, cooling water, compressed air, and finally cut off the power.
2 Control System Design Scheme During the electron beam melting process, key parameters include vacuum level, melting power, and melting speed. A high vacuum helps remove impurities both thermodynamically and kinetically. It also affects the electron beam emission. If the vacuum is too low, glow discharge may cause overcurrent and prevent melting. High voltage and current from the electron acceleration circuit determine the melting power. Increasing the power at a constant melting speed raises the molten pool temperature, aiding impurity removal but increasing metal evaporation and energy consumption. Therefore, power must be carefully controlled. Melting speed is mainly determined by the linear and rotational speeds of the feeding rod and the tow bar. Thus, seven critical parameters—vacuum level, high voltage, current, feeding rod speed, and tow bar speed—must be precisely controlled to ensure automation and safety.
Additionally, fluctuations in the power supply can affect the motor speed of the mechanical pump, leading to unstable vacuum and potential equipment failure. Overcurrent or phase loss can damage the motor and cause production losses. Therefore, vacuum control is essential.
2.1 Vacuum Gauge Output The vacuum gauge outputs a 4–20 mA current signal. After PLC processing, it controls the AC inverter, maintaining stable motor speed for the mechanical pump. The AC drive includes overcurrent and phase loss protection.
2.2 Hardware Configuration The vacuum gauge uses a microcomputer-type ionization vacuum gauge (ZJ-10 series). The control system is based on an MCU, while the inverter uses the ABB ACS600 series, known for its high performance, direct torque control, and advanced features. The PLC is a Siemens S7-200 series, chosen for its compact size, cost-effectiveness, and ease of installation.
2.3 Vacuum Degree PID Algorithm Vacuum control in the chamber is nonlinear, time-varying, and subject to many disturbances. Traditional control methods struggle to achieve good dynamic and static performance. An incremental PID algorithm is used to improve accuracy and reliability. The formula involves proportional, integral, and derivative coefficients, with the output calculated based on the deviation between the setpoint and actual value.
The control object is the electron beam furnace, with the motor speed of the mechanical pump as the control variable. The setpoint is given in Pa, and the vacuum gauge converts this into a 4–20 mA signal. The PLC processes the signal using a PID instruction (PIDTABLE) to adjust the inverter frequency, ensuring the system reaches a stable state.
2.4 Monitoring System Design The system uses a two-level control mode: the upper computer manages production operations, while the lower machine handles data acquisition and process control. The upper computer uses an Advantech IPC with Win98, and the lower machine uses a Siemens S7-200 PLC along with intelligent instruments like ionization gauges, current and voltage transmitters. An RS232/485 communication card ensures real-time monitoring.
The lower machine controls manual and automatic operations through input signals and outputs to contactors, relays, and solenoids. Due to the complexity of the electron beam furnace, modular programming and state design methods are used to simplify the logic. Interlocks and fault handling are implemented to improve system reliability.
2.4.2 PC Software Design Kingview 6.0 is used as the configuration software, supporting all hardware devices. The system includes modules for data acquisition and processing. The main screen displays the header, single crystal workshop, monitoring center, and main menu. Real-time trend curves automatically scroll, and alarm windows display events with time, variable name, and limits. Sound and light alarms are triggered via animation and sound functions. Parameters can be color-coded for easy monitoring, and historical data is stored and queried.
Reports, both real-time and historical, are generated using Kingview’s tools. Historical reports include daily, monthly, and annual summaries, with Excel integration for easier management. The system significantly improves operational efficiency and meets all process requirements.
3 Conclusions By April 2003, the equipment had been in use for nearly three months. The new control system proved reliable, accurate, and efficient, reducing labor and improving vacuum stability. By implementing a two-level control system based on PLC and configuration software, the outdated equipment was upgraded to an intelligent, well-managed system. The new setup is user-friendly, stable, and delivers excellent results, fully meeting the process needs.
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