0 Introduction Refractory materials play an essential role in various critical 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 the purity and quality of the final product. These systems consist of mechanical drive units, cooling systems, and electron gun components that operate in coordination with the process requirements.
1 Vacuum System Configuration and Single Crystal Pulling Process 1.1 Vacuum System Configuration The vacuum system of a 20kW electron beam furnace is composed of a mechanical pump and a diffusion pump connected in series. This combination ensures efficient pumping and maintenance of the required vacuum level throughout the melting process.
1.2 Single Crystal Pulling Process The process begins by circulating cooling water and activating compressed air. Then, the power is turned on, and the mechanical pump starts. After 5 minutes, the diffusion pump is activated, followed by the main pump. The furnace is then vented, and the clamping rod is engaged. Once the system is ready, the furnace is sealed, and the pre-pumping valve is opened while the main pumping valve is closed. When the vacuum reaches a certain level, the pre-pumping valve is closed, and the main pumping and large valves are opened. The vacuum is maintained until the desired level is achieved. The cathode is then activated, and the high-voltage power supply is applied, allowing the electron gun to begin melting the metal. The filament current is adjusted to 10A, and the excitation is turned on. The rod is melted, and high pressure is released. After cooling, the large valve is closed, and the air valve is opened. If the furnace needs to be stopped, the diffusion pump is turned off first, allowed to cool for about an hour, and then the large valve, main pump, mechanical pump, cooling water, compressed air, and power are sequentially shut down.
2 Control System Design In the electron beam melting process, key parameters include vacuum level, melting power, and melting speed. A high vacuum level is crucial for removing impurities both thermodynamically and kinetically. It also affects the electron beam emission. If the vacuum is too low, glow discharge may occur, causing overcurrent and preventing proper melting. The high voltage and acceleration circuit determine the melting power. Increasing the power at a constant melting speed raises the molten pool temperature, aiding in impurity removal but also increasing metal vaporization and energy consumption. Therefore, power must be carefully controlled. Melting speed depends on the linear and rotational speeds of the feed rod and the tow bar. Seven critical parameters—vacuum level, high voltage, current, feed rod speed, and tow bar speed—must be monitored and controlled to ensure automation and safety.
Additionally, fluctuations in power supply voltage can affect the motor speed of the mechanical pump, leading to unstable vacuum levels or even equipment failure. Overcurrent or phase loss can damage motors and cause production losses. Hence, precise control of the vacuum system is vital.
2.1 Vacuum Gauge and PLC Integration The vacuum gauge outputs a 4–20 mA signal, which is processed by the PLC to control the AC inverter. This ensures the mechanical pump motor runs at a stable speed. The inverter includes overcurrent and phase loss protection for added reliability.
2.2 Hardware Configuration The vacuum gauge uses a microcomputer-type ionization gauge (ZJ-10 series), while the inverter is an ABB ACS600 model, known for its advanced torque control and protection features. The PLC is a Siemens S7-200, chosen for its compact size, cost-effectiveness, and ease of installation.
2.3 PID Control Algorithm Due to the nonlinear and time-varying nature of the vacuum system, traditional control methods struggle to achieve satisfactory performance. An incremental PID control method is used instead. The algorithm calculates the output based on the error between the setpoint and actual value, ensuring accurate and stable control. The PLC executes the PID command, adjusts the inverter frequency, and maintains the desired vacuum level.
2.4 Monitoring System Design The system employs a two-level control structure: the upper level handles management tasks, while the lower level manages field data acquisition and process control. The upper computer uses an Advantech IPC with Windows 98, while the lower level uses a Siemens S7-200 PLC and other intelligent instruments. Communication is facilitated via an RS232/485 card for real-time monitoring.
2.4.1 PLC Programming The PLC controls manual and automatic operations, managing input signals from buttons and limit switches. Output terminals control contactors, relays, solenoids, and alarms. The system uses a modular programming approach, implementing state design to manage complex sequences. Interlocks and fault detection are integrated to enhance system reliability.
2.4.2 PC Software Design Using Kingview 6.0, the system configures hardware devices, displays real-time data, and generates reports. The software supports trend charts, alarm windows, and historical data retrieval. Parameters can be dynamically adjusted, and alerts are visually and audibly signaled. Reports, including daily, monthly, and annual summaries, are generated using Excel integration, simplifying data analysis and reporting.
3 Conclusions By April 2003, the system had been in operation for nearly three months. The new control system proved reliable, precise, and user-friendly, significantly improving vacuum stability and reducing labor intensity. By adopting a two-level control architecture, the outdated equipment was upgraded into an intelligent, optimized production system. The improved furnace operates smoothly, offers excellent melting performance, and fully meets all process requirements.
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