International Conference on Smart Manufacturing ApplicationApril. 9-11, 2008 in KINTEX, Gyeonggi-do, Korea

Design of LC Resonant Inverter for Ultrasonic Metal Welding system

Yu Tao, Lee Hwachun, Lee Dongheon, Song Sunggun, Kim Dongok, Park SungjunDepartment of Electrical Engineering, Chonnam National University, Gwangju, Korea

(Tel: +82-62-530-0743; E-mail: sjparklgchonnam.ac.kr)

Abstract: The ultrasonic welding is with features of high electric conductivity and hot conductivity when it is usedin various metallic bonds, excellent quality when it is used for advanced junction, easiness when it is used in variousmetallic bonds, needless for the exhaustive material and benefit for the environment. Currently the use of ultrasonicwelding is increasing in the industrial fields such as the automobile battle, the refrigerator, the air conditioner, thebattery and the solar cell junction. In this paper, the output LC resonance filter and 35kHz square-wave-generating FullBridge circuit was designed and tested. The output examination of the ultrasonic oscillator and the ultrasonic weldingexamination were done. The method for getting more efficient result in the ultrasonic welding system was researched.

Keywords: Ultrasonic welding, DSP, LC resonant

1. INTRODUCTION

In this paper, utilizing DSP technology, a LCresonant inverter for ultrasonic metal welding systemthat can weld the metal material is designed. In Europeit is paid attention to the rule of environmentalprotection more now. The use of some material such asPb-free, Cd-free and Halogen is being reduced gradually.In view of implementation of the environmentalprotection rule and endeavor of improving theproductivity, it is very important to develop theultrasonic welding system that can protect theenvironment. Especially the characteristics of weldingparts are different according to the difference of thecoating material and welded material used. For raisingthe efficiency and improving the trust of the weldingparts, it should be further investigated on the ultrasoniccharacteristic. In addition, the ultrasonic weldingtechnology has been already widely applied to suchfields as the automobile, the refrigerator, conditioner,the battery, the solar panel, etc. Its use has a very wideprospect.

The simulation has been done on the output LCresonance inverter part of the ultrasonic welding systemby P-spice, the feasibility has been proved, the realoutput LC resonance part has been designed accordingto the result of simulation. The 35kHzsquare-wave-generating full-bridge circuit and DSPcontrol part have been made. By utilizing ultrasonicwelding machine the welding without the solderingpharmaceutical can be realized. The remarkableenvironmental benefit will be brought by applying thistechnology in the industrial field.

2. OSCILLATOR OF ULTRASONIC METALWELDING SYSTEM

2.1 Characteristic of oscillatorOscillator of the ultrasonic metal welding system is

similar as of the ultrasonic plastic welding system. Butin metal welding system the followings should be pay

attention to.First, to minimize the loss and enhance the energy

density the frequencies of oscillator and of mechanicalresonant should be same. The characteristics of theconverter frequency impedance are shown in figure 1.As shown in figure the minimum frequency ofimpedance is the series resonant frequency (fs) and themaximum is the parallel resonant frequency (fp), whichare frequencies of the piezoelectric element. Butactually there are the booster and the horn in the systemexcept the piezoelectric element, so other resonantpoints (fxl,fx2 ) exist except the series and parallelresonant points (fs,fp ). The resonant frequency shouldbe restricted near the series resonant frequency pointand kept away from the mechanical resonant points andthe parallel resonant point as shown in figure 1.

Phase

FrequencyPLL Lock R'ange

Fig. 1 Frequency characteristics ofultrasonic welding system

Second, in the case of the mechanical and electricalcharacteristics are variational each parameter ofoscillator should be adjusted automatically for theoptimum welding result.

Third, the amplitude of oscillator output should notbe influenced even if the mechanical load is increased toboost the diffuseness of the atoms in the metal weldingsystem. So the ultrasonic output amplitude should bekept by the appropriate control in the electricaloscillator part. The appropriate mechanical amplitudemust be kept to ensure the effective welding result.As said before the mechanical amplitude need be

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oo.

adjusted according to various load conditions. Themechanical amplitude can be adjusted by changing thegeometric construction of welding part, but in this waythe change of the amplitude is much great. Theamplitude need be adjusted by inches according to theoutput of the oscillator part. Namely for gettingappropriate amplitude according to the load in the metalwelding system the electrical output adjusting method ismore effective than the mechanical constructionchanging method.

2.2 Construction of oscillator

The construction of inverter that shown as figure 2 isnecessary to achieve the appropriate characteristic ofoscillator described above. The rectifier part fills therole of changing the common AC voltage to the DCvoltage. The Full-Bridged inverter fills the role ofgenerating the various high-frequency outputs. Theoutput and input of inverter is isolated by using thehigh-frequency isolated transformer. The LC seriesresonant filter is used in the output part to generate thesinusoidal high-frequency output from the rectangularoutput in the inverter.

Independent of the load to minimize the energy lossthe mechanical resonant frequency of output in theoscillator should be controlled. So as shown in thefigure the phase difference of signals in the voltagesensors and current sensors are detected through thecomparator, and these signals are controlled by the PLLmethod through the PI controller. It is described indetail about the output resonant filter and the PLLcontrol method in the next chapter.

Vllf

Oac.xu Detection of Current Sensor

Phase Shift Voltage Sensor

V-1 Voltage 4Amplitude

Fig. 2 Inverter's structure in ultrasonic welding system

2.2.1 L-C resonant filter

As described before there are many resonantfrequencies in the converter. If the output of inverter isthe rectangular wave, for there being many harmonics,the mechanical resonant happens and the energy densitymay be less. For avoiding it the pure sinusoidal waveshould be input to the converter. So the LC resonantfilter is used for getting resonant near in the outputfrequency of inverter oscillator. As shown in figure 3LC filter plays the role of not only low frequency passfilter but also amplifying the voltage near the resonantof the filter. But as shown in figure the amplitude ofvoltage near the resonant point is very large. Theresonant frequency of LC filter should be designed

outside the PLL Lock Range of the frequency change ofthe oscillator output. And when welding in practice toget the sufficient mechanical amplitude the output filtershould be designed properly for generating high voltageabout several kVs.

PLL Lock Range

Frequency

Fig. 3 Frequency characteristics of output LC filter

2.2.2 Detecting of phase signals

Sigl D S1g D0 &Sig2 D

Sig2 Sg

Sig1

Sig2

Sig0_D

Sig2_D

Sigl & !Sig2_D

-. -. j {iiiL.f171 t-V7'i.iimi

Fig. 4 Structure and wave ofphase detection circuit

To uniform the frequencies of the oscillator outputand the resonant frequency of the converter the PLLcontrol method should be applied in the oscillator.When the converter is in resonant, the phases of outputvoltage and current should be detected first to make thevoltage and current in the same phase. The phasedetecting circuit is shown as figure 4 and the operationprinciple is described below.Two analog signals SigI and Sig2 turn to SigID and

Sig2_D through the comparator. As shown blow thesignal Sigl_D&!Sig2_D is got through the Not Gateand the And Gate. The pulse width of this signal isdetected through the Capture port of DSP. The detectedpulse width and the frequency of signal are detected toachieve the phase difference. And the earliness anddelay ofphases are decided by using the D-Q Flip Flop.

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2.3 Defining parameters of oscillator

The most important points deciding the effect ofwelding is the welding time, the force between materialsneed to be welded and the mechanical amplitude of theconverter when metal is welded in the system. Theparameters should be adjusted in the oscillator variouslyaccording to the kind of material, the welding area, thewelding thickness and the surface state etc. The properprograms have been proposed that let the parametersadjusting easier, shown as below.

Welding press means that the press between thehammer and the horn is set to the value that can ensurethe sufficient press always and welding quality. Thispress is defined by the air press. In practice the force isabout 400 - 1500N and the pressure can be controlbetween 0 - 6Bar in the developed oscillator.

The relation between output voltages and weldingtimes is shown as figure 5 and it is known from thefigure that variables Trigger Time, Weld Time, AfterBurst Time and After Duration Time can be adjustedeasily by the user.

Trigger ~~ ~ ~ ~ ~ ~ ~ Sot After;Trigger Soft Weld Time After Burst Start DurationTime Start

Fig. 5 Characteristics of output frequency in inverter

The amplitude is adjusted by the booster roughly inmechanical and by the oscillator minutely. In practicethe amplitude is about 15 - 45mm and should becontrolled each 1% from 25% to 100%.

3. SIMULATION AND RESULTS

3.1 Digital simulation

the simulation. In the simulation the reactor of outputfilter, capacitor and load resistor was set to variousvalues and the frequency response is simulated in theP-Spice.When the reactor is changed from 0.95mH to 1.3OmH

each 0.05mH the frequency characteristic of outputvoltage is shown as figure 7(a). It can be known fromthe figure that the resonant frequency and resonantvoltage amplitude decrease together with the increase ofthe value of reactor. When the capacitor is changedfrom 14nF to 18nF each 0.5nF the frequencycharacteristic of output voltage is shown as figure 7(b).It can be known that the resonant frequency decreasesand the resonant voltage amplitude increases with theincrease of the value of capacitor. When the reactor isset to 1.lmH, the capacitor is set to 15.7nF and theoutput resistor is changed from 1 kQ to 10 kQ each0.5nF the frequency characteristic of output voltage isshown as figure 7(c). It can be known that the resonantfrequency of filter is unaltered. The output voltage offilter is greater than the input voltage in the mechanicalresonant frequency 35kHz. As the load increases tocertain extent, the output voltage will not change. Thatis to say, though the load is changed, the mechanicaldisplacement of ultrasonic welding system will notchange.

25000 30000 35000 40000 45000 50000

Frequency

(a)Variable reactor value

The circuit of the output LC resonant filter inoscillator is shown as figure 6. The resistor R is the loadwhen the converter is resonant. The output simulation ofthe oscillator has been done according to changes of thereactor, capacitor and load in the LC filter in 35kHzultrasonic welding system. The resonant frequency ofseries L-C is show as formula below

04I

2;c XLC (1)

ACIL

Fig. 6 Simulation circuit ofLC filter

The frequency of output resonant filter should be setlarger than PLL frequency of oscillator. It is set to38.5kHz with reactance 1.lmH and capacitor 15.7nF in

25000 30000 35000 40000 45000 50000

Frequency

(b) Variable capacitor value

I25000 30000 35000 40000 45000 50000

Frequency

(c) Variable resistance valueFig. 7 Frequency characteristics ofLC filter

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.-

3000-

2500-

2000-

1500-

1000-

500-

ol

3000-

2500-

2000-

1500-

1000-

500-

4

(1-a

(1-a

Q~

12000-

10000-

8000-

6000-

4000-

2000-

0-I

For showing the characteristic of the output L-C filterand the amplitude of voltage in the time domain thesimulation has been done as the circuit shown as figure8. The output voltage of inverter and filter are shown asfigure 9. In figure 9(a) the maximum output voltage isabout 1.4kV when the pulse amplitude of inverter is45°0 and in figure 9(b) the maximum output voltage isabout 2.2kV when the pulse amplitude of inverter is90°0. It is sufficient for the request of piezoelectricelement.

TX1OFFTIME 8 S m l ic2 E2 R29 TN33 20c11t2P90 R2LtONTIME1

OPPVAL= -0(GN GN°0 |l1C >

OFFTIME= 2~142 M3 E4 _ONTIME-14

OPPVAL-= 0 V/7NL,%I 7 R6V

Fig. 8 Simulation circuit in time domain

2500-

2000-

500-

000-

~500-a)

0-

> -500-

-1 000-

-1 500-

-2000-

The free admittance loop of 35kHz metal weldingconverter is shown in figure 11. The input is set tosinusoidal voltage with 1 OOVp-p and variable frequencyfor detecting the admittance loops according toconditions. The admittance loops with pressure OBar (noload), 3Bar and 5Bar when welding the same metal areshown in the figure. It can be known that the resonantpoint is 35.146kHz and the half resonant point is35.21kHz when there is no load; there is no resonantpoint when it is on welding course. And the outputfrequency of oscillator needs to be controlled forminimizing the reactive power and enhancing theenergy density.

No Loaded2.5-1

V:

V:

(a) Pulse amplitude is 45°0

-1-\------ Filter,--Output

1--------

nverterOutput

_-

70x10O 75x.10 80x.10 85x.10 9.0x00 95x10, 1.Ox10Time(s)

(b) Pulse amplitude is 90%0Fig. 9 P-Spice simulation results in time domain

3.2 Converter free admittance loop

The STAPLA 35kHz metal ultrasonic welding partmade in Germany is shown in figure 10. It consists ofhorn, height adjustment, converter, anvil, air cylinder,flow control valve and air solenoid valve.

Height Adjustment

;. 146kHz

CONDUCTANCE (mS)

Fig. 11 Free admittance loop of transducer

3.3 Oscillator

Block diagram of developed inverter in the metalultrasonic welding system is shown as figure 12. Thegraphic LCD, Key and LED are applied for userinterface and serial RS-323 is applied forcommunication in the oscillator. There are isolated 24VDI(Digital Input)/DO(Digital Output) with port couplerfor providing the input/output signals and thevoltage-controlled air regulator that can provide the airpressure of 0 - 6Bar. The voltage and current sensorsare used in the phase and output voltage control. Andhigh-speed DSP TMS320F2812 (150MHz) is alsoapplied in the system as user interface, output detectingand inverter control.

Horn

Fig. 10 Photograph ofmetal ultrasonic welding systemFig. 12 Block diagram of developed metal ultrasonic

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-2500

Converter

welding system's inverterThe 35 kHz metal welding ultrasonic oscillator are

made up of rectifier part, inverter part, control part andinterface shown as figure 13. The non-isolated resistorsproportion circuit is used for detecting the outputvoltage signals and the high frequency current sensor isused for current detecting. The DSP chipTMS320F2812 is used in system for one-chip interfaceand control. This DSP has the large memory ( Flash:128k x 16bit, Ram : 18k x 16 bit), 16 12bit inside highspeed A/Ds, 12 PWM outputs, 2 UARTs, 56 P/O portsand 150MHz work frequency.

Transformer Full Bnrdge RegulatorRectfier& SMPS

LC Filte

DSPControll

Gra4hic Key & LEDLCD

Fig. 13 Photograph of the produced inverter

3.4 Output wave and welding result of oscillator

Shown as figure 14 when Welding Time is set to 1second, After Burst is set to 0.2 second and AfterDuration is set to 0.5 second after the Trigger Time theoutput voltage presents in Soft Start state. When thereis no load the inverter output and voltage/current outputof welding converter are shown as figure 15(a). Here theresonant point is 35.146 kHz and the voltage is in samephase as the current. In resonant state the equivalentimpedance of piezoelectric element is the minimum andthe output voltage is the maximum IOOV. The outputvoltage of inverter and output voltage/current ofwelding converter are shown as figure 15(b). It can beknown that when in load there is no resonant point andthe voltage is not in same phase as the current. Here themaximum value of output voltage is about 1.6kV.Though the output of inverter is rectangular wave, thefinal output is the pure sinusoidal by using the L-Cfilter.Tek-l1

Fig. 14 Output wave of inverterIn figure 16 the real welding results of the made

oscillator and STAPLA metal ultrasonic welding systemare show as (a) 0.5t aluminum, (b) 0.9t aluminum, (c)2SQ electrical wire and aluminum, (d) the terminalport and 2SQ electrical wire. It can be known from thefigure that there is no distortion outside the welding partand the welding result is excellent. It is also knownthat the ultrasonic welding can be used not only to weldthe same kind material (aluminum, copper) but also toweld the different kinds (aluminum-copper).

a150 20 %

(b)With loadFig. 15 Output wave ofinverter when No-load and With-load

(c) 2SQ electrical wire and 0.5t aluminum(d) Terminal port and 2SQ electrical wire

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1'1-' rs ns)4-.

Fig. 16 Photograph ofmetal weldingFrom the voltage and current waves shown in figure

17(a) it can be known that the voltage control workswell when 2 0.5t-aluminums are welded and theparameters are pressure 3Bar, amplitude 500 u, weldingtime 1 second and After Duration time 0.5 second. Itis shown as figure 17(b) that the maximum outputvoltage of oscillator is about 1200V and the current isabout 700mA. The phase difference between voltageand current is about 50° and the efficiency power isabout 270W.

Tek ^ 1-, 1,L -

.s

~~~~~~Outp9ut

~~~~Output

L.

Chll 1.OOkV 1 910mlAl Ch3 7 Iso.

HE

(a)

Fig.

the efficiency power is about 640W. It is obvious thatthe ultrasonic metal welding system has thecharacteristic of shot time and little power.

4. CONCLUSION

It is obvious that the ultrasonic metal welding systemhas the characteristic of shot time and little power.When ultrasonic welding technology is used in metalwelding there are excellent effect because of highelectricity conductivity and calorific conductivity insolidity solder. It is also an environmental protectionwelding method for needless of solders, no exhaust gasand smell. It is definite that the ultrasonic metal weldingtechnology will get the extensive application in theindustrial field for the possible of welding the thinmaterials and different materials, the advantage such asthe shot welding time etc.

ACKNOWLEDGEMENTS

This research was financially supported by theMinistry of Commerce, Industry and Energy (MOCIE)and Korea Industrial Technology Foundation (KOTEF)through the Human Resource Training Project forRegional Innovation.

REFERENCES

[1] Lorenzo Parrini, "Design of AdvancedUltrasonic Transducers for Welding Devices",IEEE Trans. Sonics Ultrason. Vol 48, No 6,1632 1639, Nov 2001.

[2] J.Tsujino, T.Ueok, Y Suzaki, K Uchida,"Ultrasonic Butt Welding of Thick Metal Plate

Specimens Using a Capacity Static InductionThyristor Power Amplifier" IEEE UltrasonicSymposium, 371 374, 1990.

[3] Ned Mohan, Tore M. Undeland, William P.Robbins, "Power Electronics", WILEY

[4] Timothy L. Skvarenina, "The Power ElectronicsHandbook", CRC PRESS

Fig. 18 Voltage and current wave when 0.9t aluminum welding

The wave of voltage and current are shown as figure18(a) when 2 0.9t-aluminums are welded and theparameters are pressure 5Bar, amplitude 7000, weldingtime 1 second and After Duration time 0.5 second.Shown as figure 18(b) the maximum output voltage isabout 1500V, the current is about 1200mA, the phasedifference between voltage and current is about 45°, and

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