A Novel Welding Inverter System for High Speed Automatic Manufacturing of Containers 1

中国科技论文在线 http://www.paper.edu.cn A Novel Welding Inverter System for High Speed Automatic Manufacturing of Containers1 Wu Xiangmiao Wang Zhenmin Hua...
Author: Bethany Bates
1 downloads 0 Views 325KB Size
中国科技论文在线

http://www.paper.edu.cn

A Novel Welding Inverter System for High Speed Automatic Manufacturing of Containers1 Wu Xiangmiao Wang Zhenmin Huang Shisheng South China University of Technology, South China University of Technology, South China University of Technology, Guangzhou City, P.R.China 510640 Guangzhou City, P.R.China 510640 Guangzhou City, P.R.China 510640 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

Abstract The property of the welding inverter system takes a great role on the container manufacturing production efficiency and welding quality. In this paper, a novel digital controlled welding inverter system based on Digital Signal Processor (DSP) was proposed, and the hardware architecture and software control flow chart were designed. The full bridge soft switching topology was adopted and a special drive circuit was designed to improve the energy transfer efficiency and reliability of the welding inverter. A new current waveform control method with medium current waveform was proposed to control and adjust the growth and transition of each droplet, therefore to improve the welding quality when the welding speed increased. The experimental results indicate that the developed welding inverter system can reach perfect welding quality in the condition of high speed welding process. Keywords: Container manufacturing;high speed welding;welding inverter;DSP

0 INTRODUCTION In container manufacturing field, the working condition is very poor and the Electromagnetic Interference (EMI) caused by the strong arc is very serious[1-2]. To improve the production efficiency, the welding system should satisfy the requirements of high speed processing with good bead formation, less spatter and high welding quality which put forward more requirements for the welding inverter system[1-2]. Nowadays, with the increasing of the welding speed, the single wire high speed welding process always meet the defects such as undercut, poor bead formation and so on. Many researches and attempts on how to improve the welding quality and production efficiency have been done and a lot of solutions are obtained[3-5]. The Pulse Gas Shielded Metal Arc Welding (GMAW-P) process is a promising method [3-7]. To achieve one pulse one droplet transfer effect and to ensure the same size of each droplet, the droplet growth and transition should be controlled accurately. The current waveform control technology is regarded as one of the most promising solutions [5-7]. However, according to the randomicity and decentralization of the droplet transfer, the synchronization between the preset current waveform and the droplet transient transfer process is very difficult. What’s more, the self-regulation and reliability of the arc will be affected when the current waveform is under control. Finally, the waveform control during the upper period of the shrinkage process of each droplet is short of pertinence, thus it is very difficult to achieve a prospective effect [3-7]. A novel high speed Pulse Metal Inert Gas (P-MIG) welding inverter system based on DSP, soft switching technology, digital signal processing technology and current waveform control technology was developed in this paper, and this welding inverter is with multi-functions such as MMA, Direct Current MIG and P-MIG. By fully using the capability of the DSP control system, according to the transient energy control idea, a new current waveform control method with medium current waveform was proposed to improve the welding quality when the welding speed 1

Support in part by the National Natural Science Foundation of China under Grant E50805051, the Research Fund for the Doctoral Program of Higher Education of China under Grant 200805611086 and Guangdong Provincial Science and Technology Project of China under Grant 2008B010400041. -1-

中国科技论文在线

http://www.paper.edu.cn

increased.

1 System structure and hardware design

Fig.1 System structure of the welding inverter

Fig.1 shows the system structure of the novel welding inverter system. The whole system mainly consists of two parts: one is the soft switching main circuit and the other is control system based on DSP processor. In this inverter, the phase shifting soft switching technology is adopted. The DSP control circuit is the core of the welding power supply which is in charge of the calculation of the feedback signals of the current and voltage, the control of the welding procedure, the generation of the digital Pulse Width Modulation (PWM) signals, the disposal of failures and information display etc[5-7].

1.1 Main circuit of the welding inverter Fig.2 shows the diagram of the Soft switching main circuit, where VT1-4 are the Insulated Gate Bipolar Transistor (IGBT) power switching tubes; Ud is the voltage power source; VD1-4 are the anti-parallel diodes; C1-4 are the parallel capacitances; T1 is the high frequency transformer; VD5-6 are rectifier diodes; VD7 is the free-wheeling diode; LO is the output inductance; and RO is the output load. The full bridge phase-shifting soft switching topology makes full use of parasitic components and operates on high frequency zero-voltage soft-switching status with convenient phase-shifting control and less current and voltage stress [8-11]. However, the power switch IGBT of the lag leg works in a very small Zero voltage switching range, especially in the open circuit condition and low output power condition. In order to insure the soft switching range of the lag leg, the parameters of the main circuit should be calculated and designed carefully [8-11]. VT 1 VD 1

VT 3

C1

Ud

VD 3

C3

T 1 L lk

VT 2 VD 2 C 2

VT 4 VD 4

C4

Lo VD 7

Ro

VD 5 VD 6

Fig.2 Diagram of the Soft switching circuit

1.2 Control circuit design based on DSP processor The TMS320LF2407A processor is used in the DSP control circuit and the architecture of -2-

中国科技论文在线

http://www.paper.edu.cn

whole control system is shown as Fig.3. It is mainly composed of a smallest DSP system, IGBT drive circuit, feedback circuit, wire feeder circuit, protection circuit, parameter setting circuit, keyboard and display circuit etc[5]. The smallest system is mainly composed of the DSP, the crystal oscillator, the reset circuit, the decoding circuit, the power supply circuit, the Joint Test Action Group (JTAG) interface and external Random Access Memory (RAM) etc. The reset chip MAX811 is used to reset the system which can monitor the voltage of general power supply of 2.0V, 2.8V, 3.0V, 3.3V and 5.0V with high accuracy. A crystal oscillator of 15MHz is used as the external clock and the frequency of the DSP is 40MHz, which can be obtained by frequency multiplication through software programming. the nine A/D channels of the DSP are adopted to collect the feedback signals and the parameter setting signals, and allocated as follow: AD0 is used to collect welding current signal; AD1 is used to collect the arc voltage; AD2 is used to set the pulse frequency; AD3 is used to set the duty cycle; AD4 is used to set the peak current value; AD5 is used to set the base current value; AD6 is used to set the medium current value; AD7 is used to set the medium current time; and AD8 is used to set the wire feeding speed value[5-7].

Fig.3 The architecture of the control system

1.3 IGBT drive circuit design C2 R3

R1

VCC

ZD1

Q1

R2

R4 ZD2

T3 PWM

R5

C1

IGBT

G E

Fig.4 Scheme of the drive circuit of the IGBT

By using high frequency pulse transformer, a IGBT gate drive circuit is proposed as Fig.4. The transformer T3 is used to transmit the rising and falling edges of the PWM signal, and the bi-stable circuit (ZD1, ZD2) is adopted to regenerate the transition PWM signals and convert the transition pulses to the original signal. The driving energy can be transmitted through the transformer and the gate capacity C1 can be charged very quickly by supplying an important -3-

中国科技论文在线

http://www.paper.edu.cn

current during a short time to obtain good switching.

1.4 System protection circuit design To obtain safe and reliable operation of the power conversion circuit and the drive circuit, the TMS320LF2407A provides pins PDPINTA and PDPINTB for the input of power drive protection interrupt. Making use of them, it is very convenient to release such kinds of protection functions as overvoltage protection, undervoltage protection, overcurrent protection, overheat protection and so on[5-7]. The schematic diagram is shown in Fig.5. Once the value detected is higher than the given value, the protection circuit will generate two signals. One is sent directly to the DSP as an fault category signal and the other one will cause an interrupt. In the interrupt service program, the relative fault input signal is queried and the corresponding fault handling is carried out. At the same time, the fault signal will work as a blockade signal of PWM, which sets the PWM output pin to a high resistance state so as to block the drive signal, then shut down the power device and fulfill the protection function in time. The whole procedure can be done automatically without any intervention of the program, which is very important to realize real time and rapid disposal of all kinds of faults.

Fig.5 Schematic diagram of power drive protection

2 Medium waveform control principle Essentially, the current waveform control method with medium current waveform is based on the transient energy control theory. In the droplet growth and transition process, the transient energy supplied for the arc can be strictly controlled by changing the output current waveform, including the parameters such as the peak current value, the persistent time, the pulse frequency, the medium current value and so on[5]. The medium current waveform control technology is realized by modulation of the high frequency (around 20~30 kHz) pulse width during the inverting process and its modulation principle is shown as Fig.6. By using low frequency signal as the modulated signal, the output current waveform of the welding inverter can be modulated into a regular waveform with alternating periods of wide pulse and narrow pulse, where the wide pulse period (PULSEW) corresponds to the peak current Ip and the narrow pulse period (PULSEN) corresponds to the base current Ib. The effective value of output current and voltage can be calculated as follows: T

I f 2 = (1/ T ) ∫ i 2 (t )dt

(1)

0

T

U f 2 = (1/ T ) ∫ u 2 (t )dt

(2)

0

-4-

中国科技论文在线

http://www.paper.edu.cn

f = 1/ T

(3)

δ = tp / T

(4)

Therefore: T

I f 2 = (1/ T ) ∫ i 2 (t )d t 0

tp

T

0

tp

= (1/ T )∫ ip 2 (t )dt + (1/ T ) ∫ ib 2 (t )dt

= (1/ T ) ⎡⎣ I p 2tp + I b 2 (T − tp ) ⎤⎦

(5)

So:

I f 2 = I p 2δ + I b 2 (1 − δ )

(6)

I f = ( I p 2 − I b 2 )δ + I b 2

(7)

U f = (U p 2 − U b 2 )δ + U b 2

(8)

Where: I f -The effective value of output current

U f - The effective value of output voltage T - The pulse duty cycle

f - The pulse frequency i(t ) - The instantaneous current value u (t ) - The instantaneous voltage value

δ - The duty ratio of the pulse tp

-Peak current time From the equation (7) and equation (8), it is obvious that the effective value of output current and voltage are close related to the pulse peak current Ip, the pulse peak voltage Up, the base current Ib, the base voltage Ub and the pulse duty cycle δ.

Fig.6 Chart of the waveform modulation principle

3 Work flow chart design The software of the control system is mainly composed of the following parts, such as the -5-

中国科技论文在线

http://www.paper.edu.cn

main program, the timer interrupt service program, the power drive protection interrupt service program, the control algorithm subprogram, the A/D sample subprogram, the D/A conversion subprogram and so on. The flow chart of the main program is shown in Fig.7.

Fig.7 The main work flow chart of the welding system

4 Experimental result and analysis By using the TDS2012B, the Wave-star oscilloscope software and the high power resistance load, the IGBT drive circuit output waveform and the operation waveform of the IGBT are tested. The developed welding inverter system was on site used in the container automatic production line for a long time.

4.1 Static characteristic test results Fig.8 is the obtained static characteristics of the novel soft-switching welding inverter under 160A, 250A and 500A output current. From the test results, it is obvious that the novel welding inverter has perfect constant current characteristic.

-6-

中国科技论文在线

http://www.paper.edu.cn

80

160A 250A 500A

70 60

Voltage U /V

50 40 30 20 10 0 0

100

200

300

400

500

Current I /A

Fig.8 The static characteristic test results

4.2 Soft switching experimental result The convenient phase-shifting FB ZVS topology makes full use of parasitic components and operates on high frequency zero-voltage soft-switching status with convenient phase shifting control and less current and voltage stress. However, there are several drawbacks, such as a narrow ZVS range of lagging leg, duty cycle loss, and voltage surge of rectifying diode [8-10]. When the inverter works under open circuit condition, the IGBT of the lagging leg is very difficult to realize soft switching. To improve the soft switching range of lagging leg, the parameters of the main circuit should be designed carefully.

CH1-Vge, 10 v/div, CH2-Vce, 200v/div X-coordinate, time, 10us/div Fig.9 The soft switching test result of lagging leg

Fig.9 shows the soft switching test result of the lagging leg in the working condition of open circuit, where CH1 is the drive voltage waveform of the IGBT of the lagging leg (VGE), and CH2 is the voltage waveform between collection polar and emission polar (VCE) of IGBT of the lagging leg. From this figure, the time that VCE descends to zero is much earlier than the time that VGE reaches the gate threshold voltage of IGBT which means that the IGBT turns on with full soft switching. While the time that VGE descends to zero is almost as the time that VCE is beginning to go up which means that the IGBT turns off with very narrow soft switching. This experiment demonstrates that the lagging leg can realize soft switching with no load, and it also demonstrates that the design of magnetizing inductance, resonant capacitors and dead time is suitable.

4.3 Medium current waveform control experiment

-7-

中国科技论文在线

http://www.paper.edu.cn

(a) Output current and voltage waveform

(b) Weld bead formation under high speed welding condition Fig.10 Medium current waveform and the weld bead formation

Fig.10 (a) shows the output current and voltage waveform of the proposed welding inverter system with medium current control, where Fig.8(b) shows the weld bead formation of panel welding process in container manufacturing. The welding parameters are as follows: the wire is ER-506, and the diameter is 1.2mm; the effective current values If is 100A; the duty cycle δ is 30%; the pulse frequency f is 30Hz; the peak current value IP is 300A; the base current value Ib is 50A; the medium current value Im is 200A; the medium time is 3ms; the shielding air is Ar(80%) +CO2(20%); the gas-flow rate is 14L/min and the welding speed is 1.5m/min. the welding process is very stable, and there is less spatter. The weld appearance is smooth, and the weld bead formation and welding quality can satisfy the container manufacturing requirement.

5 Conclusions A digital controlled welding inverter system based on DSP is proposed, and the hardware architecture and software flow chart are analyzed and designed. A drive circuit using high frequency pulse transformer and MOSFET special for high power IGBT is designed. The principle of the current waveform control with medium current is analyzed, and the proposed welding inverter system has the characteristic of soft switching capability. By using this welding inverter system, the welding speed can reach 1.5m/min with perfect weld bead formation in the panel welding process of container manufacturing.

References [1] Huang Shisheng, Wang Zhenmin. A NBJ type gas shielded metal arc welding inverter for container manufacturing. ZL200620154526.6 [P]. 2006-12-08 [2] Chen Qiang. Development and current status of arc welding technology. Symposia of the High Productivity Welding Forum, 2002, n 11, pp.109~110 [3] Zhu Jinhong, Lu Kaitong, Shi Hongxin, et al. Full digital inverter power supply for CO2 arc welding based on ARM controller[J]. Transactions of the China Welding Institution, 2007,Vol.28,No.8:5-8 [4] Chen Wenjie, Chen Shanben, Lin Tao. Analysis of RTOS and its application in digital welding power source [J]. Electric Welding Machine, 2004,34(9): 29-34 [5] Wu Kaiyuan. Study on DSP based digital intelligent control system for GMAW-P inverter [D]. Guangzhou: South China University of Technology, 2005 [6] Liu Jia. Research on the digital control of welding inverter [D].Beijing: Beijing University of Technology, 2002 [7] Xue Jiaxiang,Yao Ping, Dong Fei, etc. Forward median waveform control process parameter in pulsed MIG welding. Transactions of the China Welding Institution,2008, v 29, n 1, pp 73-76+80 [8] R.A.Fisher et al. A 500kHz 250W DC/DC converter with multi-Outputs controlled by phase shifted PWM and -8-

中国科技论文在线

http://www.paper.edu.cn

magnetic amplifiers. Proceedings of High Frequency Power Conversion. 1988,100~110 [9] Wang Zhenmin, Huang Shisheng,Xue Jiaxiang. Inverter for soft-switching double-wire pulse metal active gas welding. Journal of South China University of Tech. (Science edition), 2006, 34(7):31-34,59 [10] J.A.Sabate et al. Design considerations for high-voltage full bridge zero-voltage-switched PWM converter. IEEE APEC. 1990,274~284 [11] Sable.D.M, Lee.F.C. The operation of a full-bridge, zero-voltage-switched PWM converter[C]. In: Proceedings of VPEC. Virginia, USA, 1989. 92-97

Acknowledgments Thanks to the supports of International Marine Containers (Group) Ltd. (CIMC) and Guangzhou Unison Welding Equipment and Technology Ltd. Author Brief Introduction Wang Zhenmin is with the South China University of Technology, Wushan Road, Guangzhou City, Guangdong Province, 510640 P.R.China (phone: +86-20-87114484; fax: +86-20-87114484; e-mail: [email protected]). Wu Xiangmiao is with the South China University of Technology, Wushan Road, Guangzhou City, Guangdong Province, 510640 P.R.China (phone: +86-20-87114484; fax: +86-20-87114484; e-mail: [email protected]). Huang Shisheng is with the South China University of Technology, Wushan Road, Guangzhou City, Guangdong Province, 510640 P.R.China (phone: +86-20-87114484; fax: +86-20-87114484; e-mail: [email protected]).

-9-

Suggest Documents