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Journal of Materials Processing Technology 189 (2007) 384–391

Development of digital gas metal arc welding system

Manh Dung Ngo, Vo Hoang Duy, Nguyen Thanh Phuong,Hak Kyeong Kim, Sang Bong Kim ∗

School of Mechanical Engineering, Pukyong National University, San 100, Yongdang-dong,Nam-gu, Busan 608-739, South Korea

Received 27 July 2006; received in revised form 29 January 2007; accepted 3 February 2007

bstract

This paper presents a relatively new method for achieving better welding results of gas metal arc welding (GMAW) system by using a decentralizedontrol method. The GMAW system is an arc welding process which incorporates the power source of GMAW (PS-GMAW) with the wire feednit (WFU). In this paper, the GMAW system is considered as two separated subsystems, those are PS-GMAW and the WFU. The mathematicalodeling of PS-GMAW and WFU are presented. Based on two subsystems, a sliding mode controller is designed and applied to control theFU meanwhile a proportional controller is designed and applied to control the PS-GMAW. Furthermore, two decentralized controllers have to

e designed to control the welding arc of GMAW, be stable and control the output welding current and welding voltage for tracking the constantetting values accurately during the welding process, respectively. The simulation and experimental results are presented to show the effectivenessf the proposed controllers. 2007 Elsevier B.V. All rights reserved.

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eywords: Decentralized control method; Sliding mode controller; Proportiona

. Introduction

The GMAW process have been developed and made commer-ially available in 1948. Although the basic concept was actuallyntroduced in the 1920s. In its early commercial applications, therocess was used to weld aluminum with an inert shielding gas,iving rise to the term “MIG” (metal inert gas) which is stillommonly used when referring to the process. Variations haveeen added to the process. One of the variations was the use ofctive shielding gases, particularly CO2, for welding certain fer-ous metals. This method eventually led to be formally acceptederm as gas metal arc welding (GMAW) by American weld-ng system. The GMAW process uses either semi-automatic orutomatic equipment and it is principally applied in high pro-uction welding. Most of metals can be welded with this processn all positions with the lower energy variations of the process.

MAW is an economical process that requires little or no clean-

ng of the weld deposit. Warpage is reduced and metal finishings minimal compared to stick-electrode welding [1].

∗ Corresponding author. Tel.: +82 51 620 1606; fax: +82 51 621 1611.E-mail address: kimsb@pknu.ac.kr (S.B. Kim).

oststdi

924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.02.010

troller; PS-GMAW; WFU

Nowadays, the GMAW is almost substituted for the stick-lectrode arc welding process in industrial factories (Lincolnlectric [2], etc.) because most industrial automatic welding

obots include a GMAW system. Usually, the conventionalMAW system has an analogical circuit and open loop con-

rol method. So the conventional GMAW requires a well-trainedechnician to operate [1]. Furthermore, the quality of outputelding current and voltage of a conventional GMAW sys-

em has much influence on welding environment. So its outputelding current and voltage must be checked and readjusted

egularly. In the welding process, the output welding currentnd voltage are the decisive factors on the quality of weldingroduct. In order to get the best quality of welding results, theutput welding current and voltage must be constantly controlleduring the welding process [7].

To solve these problems, a new GMAW system must be devel-ped. The developed digital GMAW system is considered as twoeparate subsystems such as PS-GMAW and WFU. This decen-ralized control method is proposed and applied to the developed

ystem. A sliding mode controller [4] and a proportional con-roller are designed for WFU and PS-GMAW, respectively. Twoecentralized controllers have to be designed to control the weld-ng arc of GMAW, be stable and control the output welding

rocessing Technology 189 (2007) 384–391 385

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M.D. Ngo et al. / Journal of Materials P

urrent and welding voltage for tracking the constant settingalues accurately during the welding process, respectively. Theeveloped digital GMAW system makes output welding cur-ent and voltage track any setting values easily and quickly. Theeveloped GMAW has setting values ranges from 16 VDC to6 VDC for welding voltage and from 50 to 220 A for weldingurrent. The error vector is defined as the difference between theetting values and the output values. Based on the error vector,he above two controllers of the GMAW system are designedo ensure the error vector to converge to zero. Furthermore,he simulation and experimental results are shown to prove theffectiveness of the proposed controllers.

. Dynamic system modeling

The flowchart of the developed GMAW system is depicted inig. 1 and the basic of GMAW equipment is shown in Fig. 2. Theire feed speed of WFU has range from about 1.9 to 30 m/min. It

onsists of DC motor, gearbox, guide tube clamp and feed rolls.he welding electrode wire is fed by WFU to compensate anlectrode melting-rate in order to continuously keep a weldingrc during the welding process. The changed electrode feed-

rbas

Fig. 1. Flow chart for con

ig. 2. Basic GMAW equipment. (1) Shielding gas; (2) DC motor and gearox; (3) welding electrode roll; (4) WFU; (5) PS-GMAW; (6) control box; (7)elding gun; (8) workpiece; (9) control cable.

ate makes the output welding current change. The WFU muste controlled in order to achieve the setting welding currentnd to get the results in clean positive arc starting with eachtrike. Furthermore, it must minimize stubbing, skipping and

trolling the GMAW.

386 M.D. Ngo et al. / Journal of Materials Proc

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w

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ig. 3. Relationship between the electrode feed-rate and welding current.

patter, and also maintain steady wire feeding during the weldingrocess.

To clearly illustrate this concept, Fig. 3 shows a relationshipetween the welding current and electrode feed-rate for alu-inum wire positive electrode with argon shielding gas of the

incoln Electric Company (MIG/MAG welding Giude).

Fig. 4 depicts the PS-GMAW system and its control box. TheGBT’s technique is applied for the inverter of PS-GMAW. Theulse width modulator (PWM) of IGBT gate of the inverter is

Fig. 4. Scheme of PS-GMAW and control box.

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f

essing Technology 189 (2007) 384–391

perated at 20 kHz. The average output voltage of PS-GMAWepends on the time of duty ON of PWM. In the GMAW system,he average output voltage is the main factor which effects theelding voltage. So to control the welding voltage for tracking

he setting value, the duty of PWM is controlled.

.1. Dynamic model of WFU

In this system, a DC servomotor is utilized to control the DCotor of WFU which controls the electrode feed-rate to keep

he welding arc. The dynamic relationship between the electrodeeed-rate and the voltage of a DC servomotor can be expressednto a second-order dynamic equation as the following:

m(s) = Wf(s)

Vm(s)= b0

s2 + a1s + a0(1)

here Wf(s) (m/min) and Vm(s) (V) denote the electrode feed-ate and the DC voltage of a DC servomotor, respectively. Theurpose of controlling WFU is to change the electrode feed-ate in order to achieve the welding current Iw and to track theetting value Is. In the GMAW process, the electrode feed-rateust be equal to the electrode melting-rate to maintain a stable

rc length.

f = Wm (2)

here Wm is the electrode melting-rate.The electrode melting-rate Wm can be stated as a function of

elding current Iw and welding voltage Uw as the following:

m = KiIw − KuUw (3)

here Ki and Ku are the coefficient ratios of the melting-rate tohe welding current and welding voltage, respectively.

From Eqs. (2) and (3), the electrode feed-rate can bexpressed as follows

f(s) = KiIw − KuUw (4)

Therefore, the welding current can be expressed as follows

w = Wf(s)

Ki

+ �G

Ki

(5)

here �G = Ku × Uw.In practice, the welding current influences the electrode feed-

ate much more than the welding voltage. If the welding voltages considered as a disturbance in Eq. (4), the block diagram for

n open loop transfer function of the WFU can be described inig. 5.

If �G is considered as disturbance of the system, the transferunction G(s) of the WFU between the Vm(s) and Iw can be

Fig. 5. Block diagram of open loop of WFU.

M.D. Ngo et al. / Journal of Materials Processing Technology 189 (2007) 384–391 387

circuit of PS-GMAW.

e

G

G

w

2

Iaaistasmowsbtdi

Facow

U

ws

Ψ

3

FpcS → 0 and the predictive starting average voltage output for PS-GMAW (Ua) can be calculated using Eq. (8) with assumption

Fig. 6. Schematic

xpressed as follows:

(s) = Iw(s)

Vm(s)= 1

Ki

[Gm(s) + �Gd] (6)

(s) = Iw(s)

Vm(s)= 1

Ki

[b0

(s2 + a1s + a0)+ �Gd

](7)

here �Gd = �G/Vm.

.2. Dynamic model of PS-GMAW

The schematic of PS-GMAW that consists of a rectifier, anGBT module, a ferrite transformer, a high frequency rectifiernd an inductor is presented in Fig. 6 [6]. The PWM is oper-ted at 20 kHz for controlling the IGBT’s gates. The PS-GMAWs designed with two kinds of feedback which have differentampling time. Firstly, RCT feedback is set to have a samplingime 50 ns. This means that it is operated at frequency as sames IGBT’s gate frequency. The RCT feedback has no relation-hip with the controllers. It is only used to protect the IGBTodule from being damaged by over-current when short-circuit

ccurred. Secondly, the sampling time of the feedback outputelding current and welding voltage is the same as controllers’

ampling time 10 ms. Based on the setting values and the feed-ack values, the error of voltage is determined. Using this error,he proportional controller is designed and applied to control theuty of PWM of an IGBT’s gate such that the welding voltages able to track the setting value during the welding process.

The circuit layout of the GMAW system can be expressed asig. 7. Ua, Ra, Rp, Rn and L represent the average output volt-ge of PS-GMAW, resistance of PS, parasitic resistance in theircuit, resistance between contact tip and wire, and inductance

f the PS-GMAW, respectively. The Kirchhoff’s voltage law forelding circuit can be expressed as follows.

a = LdIw

dt+ (Ra + Rp + Rn)Iw + (Uw + Usheath)Ψ (8)

Ltdm

Fig. 7. Circuit layout of the GMAW system.

here Usheath is given as a constant value of 14.5 V [3], Ψ is awitch parameter and defined as follows:

={

1 if arc is on

0 if short circuit(9)

. Design of decentralized controllers

As presented in Section 2, the welding process is shown inig. 1. In practice, to achieve the welding arc ON for the weldingrocess, the predictive starting voltage for DC servomotor (Vm)an be calculated using Eq. (7) with assumption of �Gd = 0,

(dIw/dt) = 0 and Iw = Is. When welding arc is on, an error vec-or is defined. The main objective of this part is how to designecentralized controllers which stabilize the welding arc andake the error vector converge to zero as t → ∞.

388 M.D. Ngo et al. / Journal of Materials Processing Technology 189 (2007) 384–391

3

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w

aiT

S

w

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u

wu

3

Wi

U

tf

U

w

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u

w

P

u

U

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a

Fig. 8. Block diagram of a closed loop WFU.

.1. Controller design for WFU

The block diagram of a closed loop WFU is shown in Fig. 8.ecalling Eqs. (1) and (7), the state-equation with zero initialalues of system can be expressed as follows:

x1 = Wf

x2 = Wf

x1 = x2

x2 = −a1x2 − a0x1 + b0ui

y = Iw = c[x1 + �G]

(10)

here x ∈ R2 = [ Wf Wf ]T is the state vector, y = Iw is thecalar output, and ui = Vm is the scalar controller for WFU, and= 1/Ki.

The current error ei is defined as follows:

i = Is − Iw

c(11)

here Is is the constant setting current value.Derivative of the error in Eq. (11) can be expressed as:

˙i = −x1 − �G (12)

In order to obtain the controller of sliding mode, the slidingurface is defined as follows [5]:

= ei(t) + λei(t) (13)

here λ is a positive constant value.It is easy to see that the error ei in the (13) converges to zero

long the trajectory of S equals to zero. From Eq. (13), if S = 0, ei

s negative when ei is positive, and vice versa. That is, eiei ≤ 0.hus, the equilibrium point of ei converges to zero as t → ∞.

Derivative of sliding surface can be calculated as follows:

˙ = [(λ − a1)ei + a0

c(Is − cei) − b0ui] + d (14)

here d = −a0�G − a1�G − �G is bounded value.Now, our objective is to design a controller ui which stabi-

izes the system and makes the sliding surface converge to zeroased on Lyapunov’s method with the Lyapunov’s function as= 1/2S2 [8]. The sliding mode controller can be drawn from

he Lyapunov’s condition:

˙ = SS ≤ 0 (15)

The proposed controller is given as follows:

i = ei

b0(λ−a1) + a0

cb0(Is − cei) + QS + β × sign (S(t)) (16)

aa

u

Fig. 9. Proportional controller for PS-GMAW.

here β and Q are positive constant values, and β is named aspper bound of the disturbance.

Eq. (16) satisfies the condition in Eq. (15) if |d| ≤ β.

.2. Controller design for PS-GMAW

Fig. 9 illustrates the proportional controller for PS-GMAW.hen the arc is ON, the average voltage output of PS-GMAW

n Eq. (8) can be rewriten as follows:

a = (Rs + Rp + Rn)Iw + (Uw + Usheath) (17)

The PS-GMAW in Fig. 6 is based on the DC converter. Sohe average output voltage of PS-GMAW can be expressed asollows:

a = 2 × D × 86 (18)

here D is duty of PMW that ranges from 0 to 48%.Using Eqs. (17) and (18), the duty D can be expressed as

= 1

172[(Rs + Rp + Rn)Iw + (Uw + Usheath)] (19)

In this system, the duty of PWM is coded by 8 bit digitalumber in the microprocessor. So the relationship between theontroller uu and D can be written as:

u = 16 × 100

3D = Ua

H(20)

here H = 129/400.From Eqs. (17), (18) and (20), the average voltage output of

S-GMAW can be rewritten as follows:

u = 400

129[(Rs + Rp + Rn)Iw + (Uw + Usheath)] (21)

w = Ua − Usheath − �D = H × uu − Usheath − �D (22)

here �D = R × Iw is considered as disturbance, and= Rs + Rp + Rn.As explained in Eq. (5) and Fig. 3, the welding current Iw

lmost depends on the electrode feed-rate which is changed byFU.So according to Eq. (22) the controller uu is directly propor-

ional to the Uw.The error voltage eu is defined as follows:

u(t) = (Us − Uw) Kw (23)

The proportional controller uu is applied to adjust the aver-ge output voltage of PS-GMAW such that the welding voltage

chieves the setting value. The proposed controller can be stateds:

u(i) = uu(i−1) + Kp × eu(i−1) (24)

M.D. Ngo et al. / Journal of Materials Proces

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uu equals 158 at steady state.Fig. 15 shows that the simulation result for the tracking error

Fig. 10. Developed GMAW system.

here Kp, Kw are positive constant values. uu(i−1), eu(i−1) arehe values of controller and the error at period of (i − 1)th,espectively.

. Simulation and experimental results

.1. Hardware design

The developed full automatic GMAW system is presented inig. 10.

It includes GMAW and a travel beam. The travel beam moveshe welding torch along the welding line with constant velocity.he PS-GMAW has LCD display and analog display for showing

he welding current and welding voltage to easily compare theigital values with analog values.

.2. Identification of parameters for the transfer function

The transfer function in Eq. (1) can be rewritten as follows:

m = ωmotor × b′

Vm= Wf

Vm= b′/ke

(sτm + 1)(sτe + 1)(25)

For the specific WFU, the ratio of gear box Kgear = 1/34, theiameter of feed roll Droll = 40 × 10−3 and parameters of DCotor of WFU are shown in Table 1, ke the back-electromotive

orce constant, kT the motor torque constant, Ram the armature

able 1umerical values of DC motor of WFU

arameters Values Units

e 57.3 × 10−3 V s/rad

T 4.8 × 10−2 N × m/A

am 1.1 �

am 0.9 × 10−3 H

m 0.157 × 10−4 kg m2

m = RamJm/kekT 6.28 × 10−3 s

e = Lam/Ram 0.82 × 10−3 s

ea

TN

P

SSDRRRUSII

sing Technology 189 (2007) 384–391 389

esistance, Lam the armature inductance of DC motor, Jm isoment of inertia. τm is mechanical time constant and τe is

lectrical time constant.The parameters of transfer function Gm(s) can be calculated

s follows:

0 = b × b′ = 119, 631 (26)

here b = 1/(ke × τm × τe) = 3,389,002, b′ = Wf/ωmotor =Droll/2) × Kgear × 60 = 20 × 10−3 × 1/34 × 60 = 0.0353m/min)/(rad/s)

1 = τm + τe

τmτe= 1378.7 (27)

0 = 1

τmτe= 194189.8 (28)

The Ki depends on a diameter of welding electrode. Basedn the experimental results, the gain Ki can be approximatedanging from 0.041 to 0.046 for 1.2 mm aluminum electrode. Inhis paper, Ki = 0.043 is chosen.

The nominal mathematical model of the transfer functionm(s) is as follows:

m(s) = 119, 631

s2 + 1387.5s + 194189.8(29)

.3. Simulation and experimental results

To verify the effectiveness of the proposed controllers, simu-ation and experiment are done for the developed GMAW systemn Fig. 10.

Numerical values and initial values used in simulation andxperiment are shown in Tables 1–3.

Fig. 11 shows that the simulation result of control signal inputf ui for WFU. The control signal becomes stable after 3.5 s andas the value about 7.5 V.

The sliding surface and tracking error ei are shown inigs. 12 and 13. The control signal ui make the sliding surfaceconverge to zero and then the tracking error ei go to zero.The control signal input uu for PS-GMAW is shown in Fig. 14.

u. The simulation result shows that the error converges to zerofter 3.5 s.

able 2umerical values and initial values of system

arameters Values Units

etting current 110 Aetting voltage 22 Viameter of electrode 1.2 mm

a 0.019 �

p 0.096 �

n 0.017 �

sheath 14.5 Vampling time 0.01 snitial ui for WFU 7.67 Vnitial uu for PS-GMAW 157 digit num.

390 M.D. Ngo et al. / Journal of Materials Processing Technology 189 (2007) 384–391

Table 3The gains of GMAW system

Gains Values

Ki 0.043Q 30β 200,000dMin 30,000λ 350Kw 0.14Kp 0.51dMax 185,000

it

ssws

Fig. 13. Tracking error ei (A).

Fig. 11. Control signal input of ui (V).

Fig. 16 shows that the simulation results of output weld-ng current and welding voltage are 110 A and 22 V, respec-ively.

The experimental results of welding current and voltage arehown in Fig. 17. These results show that the output values are

table after 3.5 s and track the setting values very well during theelding process. However, there is a little difference between the

imulation and experimental results at the beginning of welding

Fig. 12. Sliding surface s (A/s).

pw

c

Fig. 14. Control signal uu (digit num.).

rocess because the welding arc is established at the beginning

elding process.The initial duty for experiment of PWM corresponding to the

ontrol signal input for uu = 158 (digit num.) is shown in Fig. 18.

Fig. 15. Tracking error of welding voltage eu (V).

M.D. Ngo et al. / Journal of Materials Processing Technology 189 (2007) 384–391 391

Fig. 16. Tracking current and voltage.

Fig. 17. Experimental results of developed GMAW.

Fig. 18. Initial PWM for one gate of IGBT.

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5

trGaespvimtG

R

[

[

[

[

[

[

[

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Fig. 19. Welding results of GMAW.

Fig. 19 shows that welding seam around of 110 A–22 V isery nice and neat.

. Conclusion

In this paper, a developed digital automatic GMAW sys-em is presented. A new method for achieving better weldingesults using a decentralized control method is proposed. TheMAW system is considered as two separate subsystems such

s WFU and PS-GMAW. Furthermore, the mathematical mod-ls of two subsystems are presented. Based on these models,liding mode control and proportional control methods are pro-osed and applied to control the output welding current andoltage for tracking the setting value. The simulation and exper-ment results show that the developed digital automatic GMAW

akes the output welding values track the setting values. Fur-hermore, the quality of welding line was assured by developedMAW system.

eferences

1] W.H. Chu, P.C. Tung, Development of an automatic arc welding sys-tem using a sliding mode control, J. Mach. Tools Manuf. (2005) 933–939.

2] MIG/MAG Welding Giude for Gas Metal Arc Welding (GMAW), third ed.,Lincoln Electric Company, 1997.

3] H. Terasaki, S.W. Simpson, Circuit simulation for gas arc welding system, in:The 47th IEEE International Midwest Symposium on Circuits and Systems,(2004), III.387–III.390.

4] J.X. Xu, T.H. Lee, Y.J. Pan, On the sliding mode control for dc servo mech-anisms in the presence of unmodeled dynamics, Mechatronics 13 (2003)755–770.

5] M.D. Ngo, Y.G. Kim, H.K. Kim, S.B. Kim, Adaptive sliding mode controlof nonholonomic two-wheeled mobile robot for tracking smooth curvedpath, Korean Society for Power System Engineering, Pukyong NationalUniversity, Busan, 2003, pp. 167–172.

6] M.D. Ngo, B.D. Park, D.J. Yeong, S.B. Kim, Development of digitaldc-arc welding machine, in: The International Symposium on Electrical–Electronics Engineering, Hochiminh, Vietnam, 2005, pp. 33–38.

7] P. Praveen, P.K.D.V. Yarlagadda, M.J. Kang, Advancement in pulse gasmetal arc welding, J. Mater. Proces. Technol. 164–165 (2005) 1113–1119.

8] C. Edwards, S.K. Spurgeon, Sliding Mode Control Theory and Applications,Taylor & Francis, UK, 1998.