Abstract: The anti-interference technique for arc-submerging welder controlled by microcomputer is developed. The possible origins of abnormal welding operation are analyzed. Based on the analysis, some measures are taken in the circuit design. The hardware includes shielding, decoupling, filtering, isolation, and grounding. Software includes watchdog timer, redundant instruction, and digital filtering. Testing results show that these measures can solve the problem of interference affecting the control system. Keywords : arc-submerging welder; control system; shielding technology ; digital filtering Introduction to Filtering Submerged arc welding (SAW) is widely used in industrial production, offering advantages such as high production efficiency, high weld quality, and relatively good working conditions. However, traditional SAW power supplies are not only energy-intensive and resource-intensive but also have complex wire feeding circuits. This is inconsistent with the current development level of microelectronics, computer control, and power electronics technologies. Inverter power supplies are characterized by small size, light weight, low power consumption, and high efficiency. With the development of power electronics technology, the emergence of new high-voltage, high-capacity, integrated, fully controllable, high-frequency, multi-functional, and intelligent power module devices has brought new opportunities for the development of inverter power supplies. Using a microcontroller-controlled arc welding inverter power supply, leveraging its excellent dynamic characteristics and welding process performance, will be more conducive to controlling the entire arc welding process and improving welding quality. The welding machine's control circuit uses an Intel 16-bit microcontroller (8OC196KC) for control. The control circuit must complete functions such as data acquisition, program control, and power supply external characteristic control. Therefore, the reliable operation of the microcontroller is undoubtedly one of the key factors for the reliable operation of the entire welding machine. To minimize the adverse effects of various interferences, in addition to carefully selecting highly integrated, anti-interference, and low-power components and precisely adjusting parts, it is also necessary to take specific anti-interference measures from both hardware and software perspectives to ensure stable and reliable system operation. 1. Sources of Interference Interference sources are the components, devices, or signals that generate interference. The main sources of interference in microcontroller systems include the thermal noise of electronic components themselves, electromagnetic interference generated by electrical and electronic equipment, the impact of high-power equipment on the power grid, electromagnetic waves emitted by high-power broadcasting, television, and communication equipment, the transient processes of the system's own circuits, and unreasonable printed circuit board design layouts, etc. Interference can enter the microcontroller system along various lines. Interference in the industrial environment generally enters the system in the form of pulses, mainly through three channels: first, spatial interference (magnetic field interference), where electromagnetic signals enter the system through spatial radiation; second, process channel interference, where interference enters through forward channels, backward channels, and mutual channels with other systems connected to the system; and third, power supply system interference, where electromagnetic signals enter the system through power supply lines. 2 Hardware Anti-interference Technology Hardware anti-interference design is the preferred anti-interference design scheme when designing a system. It can effectively suppress interference sources and block interference propagation paths. Reasonable arrangement and selection of relevant parameters can suppress most interference in the system. 2.1 Anti-interference Configuration of the Host Unit The parts of the microcontroller system most susceptible to interference include the reset circuit and the clock circuit. A typical reset circuit uses an RC reset circuit, as shown in Figure 1a. This method provides a reset time of less than 50ms. Increasing the reset capacitor further will make it difficult for the reset signal generated by the watchdog timer (WDT) to function. This system uses the MAX705 monitoring chip, which can output a low-level reset pulse with a width of up to 200ms, sufficient to ensure reliable reset of the 80C196KC. In addition, it also has voltage monitoring and watchdog functions. To prevent stray charges from forcing the RESET to an erroneous state, a pull-down resistor is connected to the RESET pin to absorb stray charges and keep the reset signal low. Furthermore, to avoid frequent resets caused by power supply voltage glitches due to interference, a 0.1 μF ceramic capacitor is connected between the power supply pins of the microcontroller, digital IC, and monitor and ground. The reset circuit using the MAX705 is shown in Figure 1b. [align=center] Figure 1 Microcontroller Reset Circuit[/align] The clock circuit generates the CPU's operating timing pulses and is a key component for the normal operation of the CPU. Many interferences ultimately disrupt the normal operation of the clock, thus causing the CPU to... The operation is out of control. Figure 2 shows that after the noise interference is superimposed on the clock signal, it will change the clock division signal, causing the CPU working timing to be disordered. The system uses a 12MHz crystal oscillator. The most influential high-frequency noise generated by the microcontroller is about 3 times its clock frequency. [align=center] Figure 2 Noise interference to clock signal[/align] In order to avoid interference caused by the clock circuit, the following anti-interference measures were taken in the PCB wiring of this system: (1) The clock pulse circuit is configured close to the microcontroller 80C196KC and uses thick and short leads. (2) The oscillation circuit is surrounded by ground wire and the crystal shell is grounded. (3) The capacitors of the crystal oscillator circuit are used with stable performance and accurate capacitance value, and are far away from heat-generating components. (4) The high current signal lines and power transformers on the printed circuit board are far away from the crystal oscillator signal connection. 2.2 Power supply anti-interference In the microcontroller control system, the most serious interference comes from power supply noise. The internal resistance of the power supply is the main cause of power supply interference noise. Without this internal resistance, any interference noise would be short-circuited by the power supply, causing no interference voltage in the circuit. However, in reality, power supply internal resistance is impossible to eliminate. This is because power supply noise mainly originates from the use of high-power equipment and unreasonable power supply configuration. Power supply interference in microcontroller data acquisition is primarily caused by overvoltage, undervoltage, or voltage spikes. The hazards of overvoltage and undervoltage are obvious; they cause the system's DC supply voltage to deviate from the allowable range, resulting in abnormal operation. As for voltage spikes, although their duration is very short, they generally do not damage the system, but they can severely disrupt the normal operation of the microcontroller, causing logic malfunctions and even damaging the source code. The solution to these problems is to use AC power regulators and voltage stabilizers with noise suppression capabilities. 2.3 Electromagnetic Interference The high-frequency transformer in the welding machine operates at 20kHz, and its leakage flux is a significant source of interference. Furthermore, the high-frequency current generates electric and magnetic lines of force, which undergo high-frequency changes, thus forming electromagnetic waves propagating in space. This system employs both shielding and twisted-pair transmission. First, a metal shield isolates the entire welding machine into several different areas. The microcontroller control system is placed in one area, while the transformer and power switching transistors, which generate strong electric, magnetic, and electromagnetic interference, are placed in another area, utilizing the surrounding shield to block the coupling of spatial fields. Additionally, an iron plate is wrapped around the side of the transformer to provide a circuit for the leakage flux; alternatively, non-magnetic materials such as aluminum plates can be used to block leakage flux. Twisted-pair cables have a strong ability to suppress electromagnetic induction interference; the system uses twisted-pair cables in conjunction with optocouplers to complete the transmission between the main control board and the wire feeding circuit board. 2.4 Isolation and Grounding The welding machine includes both low-voltage and high-voltage control components. Maintaining control signal connection between the two while isolating electrical connections, i.e., implementing weak current and strong current isolation, is an important measure to ensure system stability and the safety of equipment operators. The system uses two isolation methods: optocouplers and relays. All analog and digital input and output channels in the system are generally isolated using optocouplers; the power interface between weak current and strong current is isolated using relays; some parts also use both optocouplers and relays for isolation. This avoids direct contact between them and reduces their harmfulness. In addition to isolation, grounding technology is also an important means of suppressing noise. Good grounding can greatly suppress internal noise coupling, prevent the intrusion of external interference, and improve the anti-interference capability of the system. The following measures were taken: (1) On the printed circuit board, digital ground and analog ground are routed separately and finally connected to the ground wire at the power supply end. (2) Analog signal lines should avoid corner routing. In addition, the ground wire should be made as thick and wide as possible and form a loop. The power supply leads of the printed circuit board are grounded at a single point. (3) The digital ground and analog ground of the control system are floating, while the casing of the welding machine and control power supply is shielded and grounded. This can effectively prevent electrostatic and electromagnetic interference. In addition, the designed hardware anti-interference system also includes other anti-interference measures, such as electromagnetic anti-interference, decoupling capacitor anti-interference, inductive load anti-interference, and mechanical switch contact anti-interference. 3 Software Anti-interference Measures In order to improve the reliability of the control system, hardware anti-interference measures alone are far from enough. Appropriate software anti-interference measures are also needed. Software anti-interference technology is an auxiliary method to eliminate false signals and restore the system to normal operation after the input signal is interfered with. 3.1 Watchdog Timer (WDT) The WDT can be implemented in hardware or software. The MAX705 reset chip used in this system has its own hardware watchdog function. At the same time, this system uses the watchdog function built into the 80Cl96KC, which is essentially a 16-bit counter. When it starts, its count increases by 1 in each state cycle. If it is not cleared by instruction within 64 K state cycles (this system uses a 12 MHz crystal oscillator, which is about 11 ms), the counter will overflow and pull the RESET pin low for at least one state cycle to reset the system and reinitialize it. 3.2 Data Acquisition Anti-interference Measures For real-time data acquisition systems, digital filtering methods are used to resist interference. The main digital filtering methods are: program judgment filtering, median filtering, arithmetic mean filtering, weighted average filtering, first-order lag filtering, composite filtering, etc. Digital filtering has the following advantages: (1) Digital filtering is implemented by program, without the need to add hardware equipment, and has high reliability and good stability. (2) Digital filtering can filter signals with very low frequencies, while analog filters cannot be too low due to the influence of capacitor capacitance. (3) Digital filtering can use different filtering methods or filtering parameters according to different signals, and has the characteristics of flexibility, convenience and strong function. 3.3 Redundant instructions The value of the internal program counter PC of the microcontroller is most susceptible to interference. After being subjected to strong interference, the value of PC is changed. The changed value is random and uncertain. It often causes the microcontroller to jump from the correct position to an uncertain area to continue execution, or to treat the operand as an opcode, causing program chaos. Therefore, when writing programs, minimize the use of multi-byte instructions and consciously insert some no-operation instructions (NOP) at critical points; these are redundant instructions. Inserting redundant instructions reduces the number of program crashes. When a crashed program encounters these NOP instructions, the PC content is adjusted, quickly bringing the program back on track. Additionally, fill all unused memory locations with 0FFH values, i.e., the destination code value of the reset instruction RST. This way, when the program crashes into these areas, it will be reset, preventing system crashes. 3.4 Software Traps When a crashed program enters the non-program area, redundant instructions become ineffective. At this time, software traps can be used to intercept the crashed program, redirecting it to a specified location for error handling. Software traps are used to forcibly redirect captured crashed programs to the entry address of a dedicated error-handling program. Assuming the entry address of this error-handling program is ERROR, the following three instructions constitute a software trap: NOP, NOP, LJMP. ERROR is typically filled into the non-program area of ​​the EPROM with such software traps. Since software traps are all placed in places where normal programs cannot be executed, they will not affect the program's execution efficiency. 4 Conclusions (1) The system analyzed various interferences that may affect the normal operation of the welding machine control system and their causes, and took corresponding anti-interference measures from both hardware and software aspects. (2) The fundamental anti-interference of the microcontroller practical application system lies in the fact that hardware and software anti-interference only play a supplementary role. Therefore, when designing the system, only by taking both into account, complementing each other, and combining them can a good anti-interference effect be achieved. (3) The welding machine test showed that the above-mentioned hardware and software measures were effective and achieved good results, solving the interference problem of the inverter submerged arc welding machine control system. References: (1) Li Heqi, Zhang Zhijian. Anti-interference design of microcomputer-controlled pulse MIG welding machine control system. Journal of Gansu University of Technology, 2002(4): 10-11. (2) Li Chunxu, Du Limin, Tang Xuefeng, Li Heqi. Research on anti-interference technology design of digital control inverter welding machine. Electric Welding Machine, 2006(11). (3) Yu Qiuhong, Ren Liye. Review of anti-interference technology of single-chip microcomputer application system. Journal of Changchun University, 2007(10).