Implementation of a filament power supply solution for electron beam welding machine based on ARM

The traditional electron beam welding power supply system adopts industrial frequency or medium frequency technology, which has the disadvantages of large size, low efficiency, poor beam stability, etc. By analyzing the current shortcomings of the electron beam welding power supply and combining modern power electronics technology, this paper proposes a filament power supply solution based on Arm full digital control, and introduces the software and hardware implementation methods in detail. Experiments show that the filament power supply can accurately stabilize the cathode filament current, and the filament emission electron density is stable, meeting the good performance requirements.

introduction

The working principle of the electron beam welder is shown in Figure 1. When the cathode filament in the high-voltage electron gun is heated to a certain temperature, electrons will escape. The scattered electrons are accelerated to the speed of light or close to the speed of light in the high-voltage electric field. After the electrons are focused by the electromagnetic lens, an electron beam with ultra-high energy density is formed. When the electron beam bombards the surface of the weld, the powerful kinetic energy of the electrons is instantly converted into heat energy, causing the metal to melt and solidify naturally after cooling, thus achieving the purpose of welding.

The cathode filament power supply for electron beam welding is mainly used to heat the cathode filament so that it emits electrons after being heated. Controlling the output voltage or current of the filament heating power supply can achieve the purpose of controlling the overflow electrons, thereby indirectly controlling the size of the electron beam current. In the actual welding process, the cathode filament needs to be able to stably emit electrons and maintain the electron density in the electron gun almost unchanged, so the requirements for the filament heating power supply are very high.

System composition and main circuit design

Figure 2 is a circuit diagram of the cathode filament heating power supply of the digitally controlled electron beam welding machine . The filament power supply is mainly composed of a filter rectifier circuit, a Buck voltage regulator circuit, an inverter circuit, a signal processing circuit, an ARM control board, a filament transformer, and a high-frequency rectifier circuit. The single-phase 200V mains power is filtered by a full-bridge uncontrolled rectifier to obtain a smooth DC voltage of about 310V. The Buck circuit composed of IGBT completes the adjustment of the DC voltage amplitude, and the inverter circuit completes the DC/AC conversion. The signal acquisition circuit processes the primary current and voltage of the filament transformer fed back and sends them to the A/D input of the controller STM32. After the controller converts and completes the digital PI adjustment, the corresponding PWM wave is output, and then amplified by the drive circuit to drive the IGBT to complete the closed-loop control of the entire filament power supply. Changing the duty cycle of the PWM wave can change the amplitude and current of the output voltage.

1. CPU controller

The CPU is the core of the entire filament power supply, and is mainly responsible for the collection of feedback signals, digital PI closed-loop calculation, PWM wave output, parameter setting and external communication. The CPU uses the latest STM32F107 series ARM chip launched by ST. This series of chips uses ARM's 32-bit Cortex M3 as the core, with a maximum main frequency of 72MHz. The Cortex core has a single-cycle hardware multiplication and division unit, so it is suitable for high-speed data processing. The chip has three independent conversion cycles, a high-speed analog-to-digital converter with a minimum of 1μs, and three independent digital-to-analog converters with their own independent sampling and holding circuits, so it is particularly suitable for three-phase motor control, digital power supply and network applications. The chip also has a wealth of communication units, including 1 Ethernet interface, 5 asynchronous serial interfaces, 1 USB slave device, 1 CAN device, I2C and SPI modules.

2. Display circuit and other circuits

For the cathode filament heating power supply of the electron beam welding machine used independently, it is necessary to be able to set various parameters of the power supply, including setting the output current, PID parameters, etc., and display the current current and voltage values ​​in real time. When a fault occurs, it is also necessary to display the fault type. The display unit of the filament power supply uses a four-digit digital tube for dynamic display, which has the advantages of intuitive display and long life.

The filament power supply also includes driving circuit, signal conditioning circuit, protection circuit, communication circuit, etc.

Control method and software implementation

1. Digital PI closed-loop control

When the output power of the Buck circuit gradually increases, the Buck circuit working mode will enter the CCM state from the DCM state, so the controlled system is a typical nonlinear control system.

Since the single-phase mains supply voltage usually fluctuates by ±10%, the rectified voltage Us also fluctuates by at least ±10%. In addition, the cold and hot states of the cathode filament are very different. The task of the control system design is to suppress the influence of Us fluctuations and changes in filament resistance on Io. The main interference of the system comes from the large fluctuations of Us. In order to quickly suppress the system fluctuations, a negative feedback and Us feedforward control structure is adopted. The control system structure is shown in Figure 4.

The controller adopts digital PI control, namely:

When the electron beam welder is just turned on, the cathode filament is in a cold state. If the filament power is suddenly turned on at this time, if it is controlled by voltage, it will inevitably generate a large impact current, which will affect the life of the cathode filament. In order to avoid this situation, the primary current control method of the filament transformer is adopted. If current control is adopted, when the filament is open, a very high voltage will be generated, which is easy to damage the secondary side components of the transformer. In order to solve this problem, it is necessary to limit the output voltage, and promptly remind the user of the filament breakage fault, and then automatically reduce the output voltage to zero.

2. System software design

The main functions of the CPU are to complete the closed-loop PI control algorithm, send PWM pulses, fault protection, data display and remote communication. The system software is mainly used to program the STM32 chip, and the programming language is C language.

The program consists of a main program and several subprograms: communication program, sampling subprogram, PWM interrupt program, display program, etc. After entering the PWM interrupt, the feedback signals of each channel are first collected and processed, as shown in the flow chart in Figure 5, and then the PWM pulse output is generated after the digital PI regulator is operated, and the IGBT is driven after isolation and amplification by the drive circuit to realize the closed-loop control of the entire filament power supply system.

This power supply adopts a fully digital operation interface. All parameters can be set through the panel buttons, realizing the fully digital operation of the filament power supply. The digital tube can display the output current, output voltage, operating status, fault information, etc. of the filament power supply system in real time. When a fault occurs, the CPU will block all PWM pulses and then display fault information such as overvoltage, overcurrent and filament breakage information.

Experimental Results

The cathode filament of the electron beam welder is generally made of very thin sheet tungsten wire, and the resistance value is usually very small. Usually, an adjustable voltage of 0~6V is added to both ends of the filament, and the maximum current flowing through the filament can reach about 30A. We used the filament power supply of the electron beam welder produced by a well-known manufacturer. Figure 6 shows the waveform of the filament power supply when the vacuum system is working normally and the high-voltage power supply is not turned on.

Figure 6 shows the waveform when the output filament current is set to 21A. Oscilloscope CH1 and CH3 show the waveforms of the primary voltage and current of the filament transformer, and CH2 shows the waveform of the filament current. From the figure, we can see that the filament power supply can achieve the soft start function very well, with almost no overshoot, and the filament current ripple is very small, controlled within 5%, achieving a good control effect.

Summarize

This paper proposes a fully digital controlled filament power solution based on ARM , and analyzes the characteristics of inverter power supply, such as high precision, small size, and full digital. All power supply parameters are directly set and stored through the human-machine interface, and it has the function of remote communication with the host computer. In the actual welding experiment, the filament can realize the slow rise and slow fall function according to the set rise and fall time. When the filament breaks, it can also quickly identify and shut down the power output in time, and remind the user in time to replace the new cathode filament, realizing the intelligence of the filament power supply. It has been verified that the filament power supply in this solution can accurately stabilize the cathode filament current, and the filament emission electron density is stable, meeting the good performance requirements.