Since the advent of the arc welding inverter in the 1980s, it has become the mainstream product of welding power supply after continuous development and improvement. The inverter frequency of arc welding inverter is generally 20-100kHz. Since the current inverter power supply mostly adopts analog circuit control, the improvement of inverter power supply performance is limited. The digitization of welding machine is the trend of the development of welding equipment today, which makes it possible to apply digital control to arc welding inverter power supply. In China, the research on digital welding machine is still in its infancy, and there is still a big gap compared with the world's advanced level. This makes it very necessary for us to study arc welding inverter power supply. This paper will systematically introduce the design of MIG arc welding power supply control system based on DSC+MCU, and propose the implementation of fuzzy control on DSC.
1. Power supply main circuit design
1.1 System overview
Pulse MIG welding is a melting inert gas shielded welding with periodic changes in welding current, which has high requirements for welding equipment. The switching frequency of the main circuit should be high, the response speed should be fast, the dynamic response performance should be good, the output current ripple should be small, and it should be able to adapt to the welding needs of various welding materials and various welding wire diameters under different conditions. The pulse MIG welding machine is mainly composed of 6 parts.
1.1.1 Main circuit
The main circuit is the power conversion circuit of the system. Its function is to rectify the 380V three-phase AC power on the power grid and perform medium-frequency bridge inversion. After rectification again, the output current can reach 450-500A.
1.1.2 Drive circuit
The drive circuit has a protection function to prevent overcurrent, overheating and other conditions. The insulated gate high-power transistor (IGBT) in the drive inverter circuit has strict requirements on the trigger pulse, and its switching loss is also closely related to the slope and amplitude of the trigger pulse.
1.1.3 Main control board circuit
The main control board circuit is the core part of the welding machine control. Variable parameter current closed-loop PI control and voltage closed-loop PI control are used in the control. The system is sent to the analog input port of the drive board through D/A conversion and output through the drive IGBT. The feedback of the control is the sampling value of the final output voltage and current.
1.1.4 Wire feeder circuit
The wire feeder adjusts the speed of the wire feeder motor to achieve uniform or variable speed wire feeding. The wire feeder circuit controls the solenoid valve to switch the shielding gas on and off.
1.1.5 Welding machine panel
The welding machine panel is the interface for the welding machine and the user to interact. The user can observe and set the working mode of the welding machine, the status of the welding machine, the type of welding wire and shielding gas, etc. through the panel.
1.1.6 Remote control box
When the user is far away from the welding machine, the remote control box can be used to replace the function of the control panel to achieve remote control.
1.2 Main circuit design
It works in soft switching mode and adopts an improved shift resonant circuit.
The inverter part N1 and N3 are leading bridge arms, and N2 and N4 are lagging bridge arms. C1 and C3 are parallel capacitors of the leading arm, C2 and C4 are parallel capacitors of the lagging arm, and C1=C3%26gt;%26gt;C2=C4. L2 is a saturated inductor, and Cs is a circulating current suppression series capacitor. T1 is the main transformer. "+" and "-" are welding gun outputs. S1 is an air switch. The grid voltage is rectified and stabilized by Cp and L1 to provide DC input for the inverter.
2. Design of power supply control system
2.1 Overall scheme of pulse MIG arc welding power supply control system
The overall design of pulse MIG arc welding control system is the key to system design, which involves fields such as automatic control, computer and welding.
The pulse MIG arc welding power supply control system has three parts: current waveform control system, arc length control system, and expert system. Among them, the current waveform control system and arc length control system are closed-loop control systems, and the output of the arc length control system is the input of the current waveform control system. Therefore, the whole system is a double closed-loop control system, with current waveform control as the inner loop and arc length control as the outer loop. The expert system is the adjustment part of the whole double closed-loop system.
2.2 Design of each part of the control system
2.2.1 Current waveform control system
In order to achieve the accuracy of current waveform control, this system should adopt PID control. Since ideal differential control reacts quickly to strong disturbances, the thermal inertia of the arc makes the system unable to respond to differential control in time. In addition, ideal differential control will amplify the noise interference in the deviation signal e(t), produce a large noise output, and affect the system performance. Therefore, this system adopts a variable parameter PI controller. For pulse MIG welding, the transition form of one pulse and one droplet has the best welding quality among all transition forms. At this time, the size of the molten droplet is equivalent to the diameter of the welding wire. Therefore, the better control idea of the molten droplet transition is to control the shape of the waveform to ensure the consistency of the size of the molten droplet. Waveform control determines the behavior characteristics of a single molten droplet. In a cycle of molten droplet transition, the transition of the molten droplet can be divided into 6 stages. In these stages, different proportional coefficients and integral times are used respectively to obtain better system dynamic response speed and steady-state accuracy of base current time.
2.2.2 Arc length control system
The stability of the arc length of the welding arc mainly depends on two aspects: one is the self-regulation of the arc; the other is the welding parameters and the selected process method. The traditional arc control method can only guarantee the best arc length control effect in a certain interval because the parameter selection of its arc voltage controller must take into account the entire wire feeding speed range. In addition, it is hoped that the arc length adjustment process can be carried out quickly and stably. Considering the shortcomings of traditional PID control and the fact that fuzzy control is relatively mature, this system uses a fuzzy controller to design the arc voltage feedback part.
First, each pulse is sampled with a peak value as the starting point and a sampling period of 100μs to calculate the average voltage, and the average voltage is compared with the given voltage value to obtain the voltage deviation and the rate of change of the voltage deviation. Then, according to the welder's experience, a fuzzy controller is designed, and the input is the voltage deviation and the rate of change of the voltage deviation. After fuzzy reasoning and defuzzification, the base time of the next pulse is obtained, and the calculated base time is sent to the current waveform control part to modify the current control waveform, thereby adjusting the melting speed so that the average voltage of the pulse is equal to the given voltage.
2.2.3 Expert control system
Expert system is a computer program system that simulates human experts to solve problems in specific fields. The welding expert system contains a large amount of expert-level knowledge and experience in the welding field, and can solve parameter adjustment and coordination problems in the welding field according to the knowledge of welding experts. The welding expert system is an analytical expert system that solves classification problems. The operation usually performed on classification problems is an interpretation operation. The interpretation operation is mainly an identification operation. In the welding expert system, the working conditions must be identified and the corresponding solution must be obtained from the expert system database based on the input. This solution may not be unique and may be wrong. The solution is screened by the inference engine to obtain the optimal solution. The following is the specific working process:
First, on the welding machine display panel, the welder selects the welding conditions, including welding materials, base material type, base material grade, base material thickness, welding wire diameter, shielding gas, wire feeding speed, etc. At the same time, appropriate measures are taken to ensure that there are no contradictions between the related conditions, and the legality of the input conditions is checked. Then, based on the selected welding conditions, the expert system determines the parameters of the current control waveform through reasoning. Finally, the given voltage of the arc control part is determined. At the same time, in order to save the welding specifications, a knowledge acquisition machine is designed.
3. System software design
The pulse MIG inverter welding machine is a complex multi-processor system. Its software design includes the programming of the MC56F8523 DSC on the main control board, the programming of the welding expert system of the MC9S12DP256 microcontroller on the main control board, the software design of the MC9S08GT60 microcontroller on the wire feeder, and the software design of the AT89C51 microcontroller on the panel and the remote control box.
The software of each subsystem realizes part of the functions of the welding machine. The subsystems communicate with each other through the RS-485 bus and are scheduled by the MC9S12DP256. The DSC on the main control board mainly completes the control of the welding process, including the control of the current closed loop and the voltage closed loop. The system software adopts a modular design, which makes the program structure clear and facilitates the expansion of the system structure.
Keywords：DSC MCU
Reference address：Design of MIG arc welding power supply control system based on DSC+MCU
1. Power supply main circuit design
1.1 System overview
Pulse MIG welding is a melting inert gas shielded welding with periodic changes in welding current, which has high requirements for welding equipment. The switching frequency of the main circuit should be high, the response speed should be fast, the dynamic response performance should be good, the output current ripple should be small, and it should be able to adapt to the welding needs of various welding materials and various welding wire diameters under different conditions. The pulse MIG welding machine is mainly composed of 6 parts.
1.1.1 Main circuit
The main circuit is the power conversion circuit of the system. Its function is to rectify the 380V three-phase AC power on the power grid and perform medium-frequency bridge inversion. After rectification again, the output current can reach 450-500A.
1.1.2 Drive circuit
The drive circuit has a protection function to prevent overcurrent, overheating and other conditions. The insulated gate high-power transistor (IGBT) in the drive inverter circuit has strict requirements on the trigger pulse, and its switching loss is also closely related to the slope and amplitude of the trigger pulse.
1.1.3 Main control board circuit
The main control board circuit is the core part of the welding machine control. Variable parameter current closed-loop PI control and voltage closed-loop PI control are used in the control. The system is sent to the analog input port of the drive board through D/A conversion and output through the drive IGBT. The feedback of the control is the sampling value of the final output voltage and current.
1.1.4 Wire feeder circuit
The wire feeder adjusts the speed of the wire feeder motor to achieve uniform or variable speed wire feeding. The wire feeder circuit controls the solenoid valve to switch the shielding gas on and off.
1.1.5 Welding machine panel
The welding machine panel is the interface for the welding machine and the user to interact. The user can observe and set the working mode of the welding machine, the status of the welding machine, the type of welding wire and shielding gas, etc. through the panel.
1.1.6 Remote control box
When the user is far away from the welding machine, the remote control box can be used to replace the function of the control panel to achieve remote control.
1.2 Main circuit design
It works in soft switching mode and adopts an improved shift resonant circuit.
The inverter part N1 and N3 are leading bridge arms, and N2 and N4 are lagging bridge arms. C1 and C3 are parallel capacitors of the leading arm, C2 and C4 are parallel capacitors of the lagging arm, and C1=C3%26gt;%26gt;C2=C4. L2 is a saturated inductor, and Cs is a circulating current suppression series capacitor. T1 is the main transformer. "+" and "-" are welding gun outputs. S1 is an air switch. The grid voltage is rectified and stabilized by Cp and L1 to provide DC input for the inverter.
2. Design of power supply control system
2.1 Overall scheme of pulse MIG arc welding power supply control system
The overall design of pulse MIG arc welding control system is the key to system design, which involves fields such as automatic control, computer and welding.
The pulse MIG arc welding power supply control system has three parts: current waveform control system, arc length control system, and expert system. Among them, the current waveform control system and arc length control system are closed-loop control systems, and the output of the arc length control system is the input of the current waveform control system. Therefore, the whole system is a double closed-loop control system, with current waveform control as the inner loop and arc length control as the outer loop. The expert system is the adjustment part of the whole double closed-loop system.
2.2 Design of each part of the control system
2.2.1 Current waveform control system
In order to achieve the accuracy of current waveform control, this system should adopt PID control. Since ideal differential control reacts quickly to strong disturbances, the thermal inertia of the arc makes the system unable to respond to differential control in time. In addition, ideal differential control will amplify the noise interference in the deviation signal e(t), produce a large noise output, and affect the system performance. Therefore, this system adopts a variable parameter PI controller. For pulse MIG welding, the transition form of one pulse and one droplet has the best welding quality among all transition forms. At this time, the size of the molten droplet is equivalent to the diameter of the welding wire. Therefore, the better control idea of the molten droplet transition is to control the shape of the waveform to ensure the consistency of the size of the molten droplet. Waveform control determines the behavior characteristics of a single molten droplet. In a cycle of molten droplet transition, the transition of the molten droplet can be divided into 6 stages. In these stages, different proportional coefficients and integral times are used respectively to obtain better system dynamic response speed and steady-state accuracy of base current time.
2.2.2 Arc length control system
The stability of the arc length of the welding arc mainly depends on two aspects: one is the self-regulation of the arc; the other is the welding parameters and the selected process method. The traditional arc control method can only guarantee the best arc length control effect in a certain interval because the parameter selection of its arc voltage controller must take into account the entire wire feeding speed range. In addition, it is hoped that the arc length adjustment process can be carried out quickly and stably. Considering the shortcomings of traditional PID control and the fact that fuzzy control is relatively mature, this system uses a fuzzy controller to design the arc voltage feedback part.
First, each pulse is sampled with a peak value as the starting point and a sampling period of 100μs to calculate the average voltage, and the average voltage is compared with the given voltage value to obtain the voltage deviation and the rate of change of the voltage deviation. Then, according to the welder's experience, a fuzzy controller is designed, and the input is the voltage deviation and the rate of change of the voltage deviation. After fuzzy reasoning and defuzzification, the base time of the next pulse is obtained, and the calculated base time is sent to the current waveform control part to modify the current control waveform, thereby adjusting the melting speed so that the average voltage of the pulse is equal to the given voltage.
2.2.3 Expert control system
Expert system is a computer program system that simulates human experts to solve problems in specific fields. The welding expert system contains a large amount of expert-level knowledge and experience in the welding field, and can solve parameter adjustment and coordination problems in the welding field according to the knowledge of welding experts. The welding expert system is an analytical expert system that solves classification problems. The operation usually performed on classification problems is an interpretation operation. The interpretation operation is mainly an identification operation. In the welding expert system, the working conditions must be identified and the corresponding solution must be obtained from the expert system database based on the input. This solution may not be unique and may be wrong. The solution is screened by the inference engine to obtain the optimal solution. The following is the specific working process:
First, on the welding machine display panel, the welder selects the welding conditions, including welding materials, base material type, base material grade, base material thickness, welding wire diameter, shielding gas, wire feeding speed, etc. At the same time, appropriate measures are taken to ensure that there are no contradictions between the related conditions, and the legality of the input conditions is checked. Then, based on the selected welding conditions, the expert system determines the parameters of the current control waveform through reasoning. Finally, the given voltage of the arc control part is determined. At the same time, in order to save the welding specifications, a knowledge acquisition machine is designed.
3. System software design
The pulse MIG inverter welding machine is a complex multi-processor system. Its software design includes the programming of the MC56F8523 DSC on the main control board, the programming of the welding expert system of the MC9S12DP256 microcontroller on the main control board, the software design of the MC9S08GT60 microcontroller on the wire feeder, and the software design of the AT89C51 microcontroller on the panel and the remote control box.
The software of each subsystem realizes part of the functions of the welding machine. The subsystems communicate with each other through the RS-485 bus and are scheduled by the MC9S12DP256. The DSC on the main control board mainly completes the control of the welding process, including the control of the current closed loop and the voltage closed loop. The system software adopts a modular design, which makes the program structure clear and facilitates the expansion of the system structure.
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