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Sensing of Weld Pool Surface Using Non-Transferred Plasma Charge Sensor

W. Lu and Y. M. Zhang

Welding Research Laboratory, Center for Manufacturing and Department of Electrical and Computer Engineering, University of Kentucky, Kentucky 40513, USA

John Emmerson

Magnatech Limited Partnership, East Granby, Connecticut 06026, USA Corresponding Author: Y.M. Zhang (ymzhang@engr.uky.edu)

Abstract: Gas Tungsten Arc Welding (GTAW) is the primary process for precision joining

of metals due to its capability for accurate control of heat input. As a close relative and

modification of GTAW, Plasma Arc Welding (PAW) has higher energy density and can

penetrate thicker workpieces while maintaining the desired capability for accurate control of

heat input. In order to produce quality welds consistently using PAW, in addition to accurate

control of heat input, the sensing and control of the weld pool surface is also critical. It has

been found that the non-transferred plasma arc, in the absence of the transferred arc or the

main plasma arc for welding, can establish a plasma charge potential which is indicative of

the arc length. In order to take good advantage of this intrinsic characteristic of the non-

transferred arc and eliminate the influence of the transferred arc in a normal PAW process, a

power module is used to cut off the main arc current periodically for a very short period of

time to acquire accurate information monitoring the weld pool surface. A non-transferred

plasma charge sensor is proposed based on this mechanism and experiments verified its

effectiveness.

1. Introduction

GTAW is the primary process for precision joining of metals due to its capability for accurate

control of heat input. Although PAW is a close relative and modification of GTAW and can

accurately control the heat input like GTAW, it can achieve much deeper penetration. In fact,

its highly concentrated arc can easily melt the metal and form a deep weld pool, even a small

funnel-shaped cavity referred to as keyhole [1], just as laser welding does. It is this keyhole

that significantly increases heat transfer efficiency and hence penetration capability. In order

to maintain an appropriate state of the keyhole, the welding current must be adjusted at a

suitable level so that the workpiece is fully penetrated as desired but the melted metal is not

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blown away by high arc pressure, a phenomenon known as burn-through. This suitable level

of welding current must be determined by monitoring a key parameter which can accurately

depict the state of the process such as the weld pool surface or weld pool depth. Hence, in

order to achieve quality welds, a reliable sensor is critical.

Detection of weld pool has been attracting the interest of researchers from the

beginning of arc welding applications. Methods proposed include weld pool oscillation [2] [3]

[4], ultrasound [5] [6], arc light spectral lines [7] and infrared sensing [8]. At the Welding

Research Laboratory at the University of Kentucky, a number of arc welding sensors have

been successfully proposed and implemented, including weld sag sensing [9], weld pool

vision sensing [10] [11] [12], arc light spectra analysis [13] [14], plasma cloud charge sensing

[15], efflux plasma charge sensing [16], light beam aided sensing [17] [18], etc. All these

sensing mechanisms are based on the intrinsic characteristics of the welding process and have

useful application in arc welding processes. However, they all lack wide application either

because of sensor complexity, high cost, or unavailability for specific applications. Thus, a

reliable, robust and easy-to-use arc welding sensor is still actively being sought.

In the present paper, a new sensing technique for PAW is proposed, based on the

space charge between the PAW torch nozzle and the workpiece when only the non-

transferred plasma arc is present. In PAW, two arcs are induced by two separate power

supplies. One is the pilot arc or non-transferred arc, induced by the power between tungsten

electrode and plasma torch nozzle. Another is the main arc or transferred arc, established by

the power from the workpiece to the electrode. It has been found that the plasma arc is not

electrically neutral [19] and an electrical potential exists between the two ends of the plasma

jet due to the difference in mass and thermal velocity between ionized electrons and positive

ions [20]. Previous study [21] shows that when only the non-transferred arc is present, there

will be a voltage drop between the torch nozzle and the work and this voltage is a good

indictor of the non-transferred arc length, thus the surface condition of the workpiece, in

terms of sensitivity and reliability. Because the non-transferred arc does not obey the

minimum voltage principle (which governs all transferred arcs and commands them to follow

the minimum path between the electrode and work), its flow is straight and its path is

independent of workpiece geometry. However, if a transferred arc is present so that the

minimum voltage principle applies, the arc path will be affected by the geometry of the

workpiece in that the arc will no longer be directly related to the shape of the weld pool

surface. Moreover, the electrical force from the main arc plays a much more important role

on the movement of charged particles than thermal energy, resulting in a different pattern of

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plasma charge. Therefore, the non-transferred arc sensing mechanism would become invalid.

In order to implement the mechanism in normal PAW process, in which the transferred arc is

always present, an Insulated Gate Bipolar Transistor (IGBT) power module is used in this

study to isolate the main power for a very short period of time so that during this period, the

transferred arc extinguishes and only the non-transferred arc is present. In this way, the non-

transferred arc sensing environment can be constructed artificially and the indicative arc

voltage can be measured effectively. Hence, the proposed sensing approach can be

applicable in a real time welding process.

In this study, the electrical potential between the torch nozzle and the work due to the

non-transferred plasma charge is sensed by a resistor and a capacitor in series. The sensor is

referred to as the non-transferred plasma charge sensor (NTPCS). Because this sensor

requires no additional attachment, the torch’s accessibility, an important factor in determining

the practicality of welding sensing and control systems, will not be affected.

2. Non-transferred plasma charge sensor (NTPCS) description

Figure 1 shows the operating configuration for the NTPCS. The main power supply is

connected to the tungsten electrode and the work through an IGBT power module. By

controlling the voltage between the Gate and Emitter of the IGBT, one can control the on/off

state of the IGBT, and, thus, the connection of the main power supply to the welding process.

Most of the time, the IGBT is on in order to provide welding power, as in a normal welding

process. For a very short period of time, normally on the order of ms, the IGBT is turned off

so that the main power is separated from the welding system. With the pilot arc power supply

on all the time, only non-transferred arc exists during this interval, which is exactly the same

as non-transferred micro-plasma sensing [21]. By monitoring the nozzle-to-work voltage, one

can tell the length of the non-transferred arc, which is straight typically. If the torch standoff

is kept constant during the whole process, the weld pool surface, especially its depth, can be

obtained by subtracting the standoff from the arc length. Hence, this period is referred to as

the sensing period.

In order to sense the electrical potential between the nozzle and the work, a low pass

filter (a resistor and a capacitor) is used. The voltage across the capacitor is sampled as the

output of the NTPCS sensor. In order to provide an appropriate load, the typical resistance

and capacitance at the load terminal are chosen as 53KΩ and 0.01µF, respectively, giving the

cutoff frequency of 2K Hz. Thus, the resistor and capacitor circuit serves both sensing and

low-pass filtering purposes.

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3. Theoretical background

To understand the cause of the signal and its relationship to the weld pool surface, it is

necessary to understand the physical phenomena involved in the welding process. Because

the sensing signal is the voltage between the torch nozzle and the workpiece due to electron

and positive ion distribution in this region, anything that influences the charge distribution

will affect the sensor output.

Consider the non-transferred arc case first. The inert gas is ionized and blown out of

the torch nozzle chamber by the plasma gas, forming a plasma jet referred to as non-

transferred plasma. The non-transferred plasma is subject to a considerably greater flux of

electrons than of positive ions due to the much higher thermal velocity of electrons [20] [21].

The mean electron velocity ev is determined by the mean electron temperature eT and

Boltzmann’s constant k according to Equation (1) [22]:

2/1

8⎟⎟⎠

⎞⎜⎜⎝

⎛=

e

ee m

kTvπ

(1)

where em is the mass of an electron. For the typical plasma electron temperature of 5300-

7000K, the mean electron velocity is in the range of 4.5×105 to 5.2×105 m/s.

On the other hand, the ion speed vs is determined by Equation (2) [23] given below:

2/1

)(⎟⎟⎠

⎞⎜⎜⎝

⎛ +=

i

ies m

TTkv (2)

where Ti and mi are the temperature (K) and mass (kg) of the positive ions. In the case

where the temperatures of electrons and positive ions are both equal to the plasma

temperature (i.e. T T Te i= = ) and where the lightest species of positive ion has a mass of

approximately 2000 me , the typical ion velocity is in the range of 9.0×103 to 10.0×103 m/s.

Comparing the two velocities it is obvious that the flux of electrons greatly exceeds that of

positive ions. Thus, more electrons approach the bottom side of the pilot arc, i.e., the

workpiece, while more positive ions accumulate at the top, i.e., the nozzle, resulting in an

electrical field being established at the two ends of the plasma jet until it retards the

approaching charged particles causing a charge balance. The established electrical field

points from the nozzle to the workpiece.

According to [20], the plasma charge potential 0V can be estimated as:

ekTV 5.2

0 ≈ (3)

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where T is the temperature of the plasma jet, k the Boltzmann’s constant, and e the electron

charge. For a given experimental condition, the plasma temperature can be considered

constant, resulting in a nearly fixed plasma charge voltage independent of the arc length.

The arc resistance varies with the arc length. To determine the resistance of the non-

transferred arc, the resistivity can be calculated as [24]:

ln1053.6 2/33

TΛ

×=η (4)

where Λln is a constant depending on the electron density and the temperature. Hence, the

arc resistance aR can be calculated as:

SlRa

×=η (5)

where l and S are the length and cross section area of the arc, respectively.

In a stable plasma jet, the electron density and temperature are nearly fixed, implying

a constant arc resistivity, as well as a constant cross section area. Hence, the non-transferred

plasma arc resistance is variable and depends mainly on the arc length.

Combining the voltage and resistance analysis, the non-transferred plasma arc can be

modeled as a constant voltage source 0V in series with a variable resistor aR which is mainly

determined by the length of the arc [21]. The Thevenin source model of the NTPCS sensor is

shown in Figure 2. Hence, the sensor signal, the voltage on the capacitor, can be expressed in

frequency domain as:

]1)[()( 0

++=

CsRRsV

sVa

a (6)

where aV is the voltage on the capacitor, 0V the equivalent voltage source due to the plasma

charge, aR the equivalent resistance of the arc, and R and C the sensing resistance and

capacitance, respectively.

aV can be expressed in time domain as:

⎥⎦

⎤⎢⎣

⎡+

−−×= ))(

exp(1)( 0 CRRtVtv

aa (7)

Since the sensing period and the time constant CRRa )( + are both on the order of ms, the

exponential component can be expanded with only the first two terms, yielding:

tCRR

VCRR

tVtvaa

a )()

)(1(1)( 0

0 +=⎥

⎦

⎤⎢⎣

⎡+

−−×= (8)

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Thus, should the sampling time T be chosen, the signal av is inversely proportional to the

arc resistance aR , and in turn, the arc length l . Equation (8) can be reorganized for the arc

length measurement in terms of the sensed voltage:

BnTVARS

CnTVnTSV

laa

−=−=)()(

0

ηη (9)

where ηCnTSV

A 0= and ηRSB = are two constants, and nT represents the thn sampling time.

Assuming the non-transferred arc approaches the weld pool surface and the torch

standoff L is constant, the weld pool depth d can be calculated by subtracting standoff from

arc length:

Lld −= (10)

If the standoff distance may vary, a low current can be applied periodically to resume a flat

weld pool surface for short periods of time. The corresponding measurements will give real-

time adjustments to the standoff distance L in Equation (10).

During the normal welding period, i.e., the IGBT is turned on and the main power is

supplied, the workpiece becomes the positive terminal, resulting in a negative NTPCS signal.

For a data acquisition system taking only positive signals, the negative signal is read as zero.

When the IGBT is turned off, positively charged particles do not vanish immediately. Instead,

it will take some time to neutralize them, as well as the negatively charged capacitor. Thus,

for the first few samples in every sensing period, the signal will remain at zero until it rises

gradually to steady state. Experimental results show that this time normally lasts about

20~30ms. However, one does not need to wait until the capacitor is fully charged. Should the

sensing period be fixed and greater than the charge recovery time, the maximum sample in

every sensing period, normally exactly when the IGBT is turned back on, is able to reveal the

weld pool depth during this period. Hence, 10ms is typically chosen as the sensing period and

the signal sampled just at the end of the sensing period is taken as the output of the NTPC

sensor. Figure 3 shows a typical waveform of the sampled signal.

Theoretically, Equation (10) gives the absolute value of the weld pool surface depth.

However, due to the harsh nature of the welding environment and a great number of

uncertainties involved in the arc and weld pool, especially when the main arc current is

applied, the pool surface depth may differ from the prediction given by Equations (1)~(10)

considerably. Thus, instead of calculating the absolute weld pool surface depth, a relative

height is estimated with respect to a reference, which will be discussed in greater detail later.

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4. Experimental procedure

In order to test the response of the proposed NTPCS at various welding conditions, a series of

experiments have been conducted in PAW. In these experiments, two inverters designed for

gas tungsten arc welding and plasma arc welding are used as the main power supply and pilot

arc power supply, respectively. The IGBT used is CM300H-12A, rated at maximum capacity

of 300A and 600V. The host computer adjusts the on/off state of the IGBT by controlling the

voltage level between the Gate and Emitter of the IGBT, as well as the main welding current

via the analog output interface to the main power supply. The torch, a regular commercial

straight-polarity plasma arc welding torch, is attached to a manipulator. The motion of the

manipulator is controllable and the moving direction is parallel to the workpiece to maintain a

constant torch standoff.

All of the experiments are bead-on-plate welds on stainless steel (type 304). For

simplicity, the pilot arc current is fixed as 15A. Table 1 lists additional common parameters

used in the experiments.

5. Experimental results

5.1 Validity test experiment

In order to verify that the non-transferred plasma arc signal is not affected by the main power

supply when the IGBT is turned off, the welding current is pulsed between 25A and 10A

repeatedly every 400ms. Each 100ms, the IGBT is turned off for 10ms in order to sense the

non-transferred plasma arc signal. The torch travels at a fast speed so that though welding

current is applied, no metal is melted. Figure 4 shows the sampled signal together with the

welding current. It can be seen that although the main current varies the signal remains nearly

constant, implying that the non-transferred plasma arc signal does not depend on the external

main power supply once it is isolated by the IGBT.

5.2 Pulsed current experiment

Pulsing current gives better process controllability and reduces heat input. In order to test the

validity of NTPCS under pulsed current, experiments are conducted on a 1.9mm thick

workpiece at the welding speed of 2mm/s. The welding current is pulsed between 25A and

10A every 400ms so that the workpiece is just penetrated and the back side bead remains

nearly constant.

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It is apparent that under pulsed current the weld pool grows deeper and larger during

peak current duration due to higher heat input and arc pressure, while it grows narrower and

smaller during base level, as shown in Figure 5. According to the inversely proportional

relationship between the non-transferred plasma signal and the weld pool surface depth, the

signal should be larger during base period and smaller during peak period. Moreover, the

weld pool grows and shrinks with the pulsed current’s pattern repeatedly, suggesting

periodical rises and falls of the weld pool and its surface depth.

The sampled signal is shown in Figure 6. It can be seen that the non-transferred

plasma arc signal increases gradually under base current and decreases under peak current,

responding to the transition of welding current levels. This behavior is exactly in accordance

with the assumption of the weld pool surface variation.

5.3 Speed variation experiment

In order to verify that the non-transferred plasma arc signal can be used to detect the variation

of the weld pool surface depth, the welding speed is varied in order to obtain different

penetration and weld pool surface conditions. The experiment is conducted on 2mm thick

plate with a welding current of 25A. At the beginning, the torch moves at 2mm/s, causing

insufficient heat input and a non-penetrated weld. After 30 seconds, the speed is decreased to

1mm/s and full penetration is established. Then after another 20 seconds, the speed is

changed back to 2mm/s. The original signal is shown in Figure 7 (a). Figure 7 (b) shows the

plot of the maximum signal in every sensing period, i.e., the signal sampled just before the

IGBT is turned back on. Figure 7 (c) and (d) show the front side and back side of the weld,

respectively.

As can be seen, during the first 30 seconds, because the workpiece is not penetrated,

the weld pool is relatively narrow, corresponding to a higher non-transferred plasma arc

signal. When the torch moves more slowly so that the heat input increases, the backside

beads appear and the weld pool becomes deeper. This is reflected in a smaller non-transferred

plasma arc signal. After the speed is increased again, the signal rises because the weld pool

gets shallower and narrower. The weld pictures verify the variation of the weld pool.

5.4 Welding current variation experiment

As shown above, the variation in welding current does not impact the signal if the weld pool

is not undergoing changes. When the weld pool does have considerable variation, however,

the NTPCS should be able to detect it no matter what welding current is being used. For

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verification, the welding current is varied every 10 seconds from 25A, 45A, 15A to 40A, with

1.7mm/s welding speed and 1.9mm workpiece thickness. Figure 8 (a) shows the original

sensor signals together with the welding current. To examine the data more closely, the

maximum during every sensing period is extracted from the original and redrawn in Figure 8

(b). The front side and back side of the weld are shown in Figure 8 (c) and (d), respectively.

Figure 8 (d) shows that during the first 10 seconds, the workpiece is not penetrated

due to the low welding current. The non-transferred plasma arc signal is relatively high,

implying that the arc is short. When the higher welding current (45A) is applied, the

workpiece is penetrated quickly. This is reflected in the signal by an immediate drop in the

non-transferred plasma arc signal. Moreover, because of the large heat input, the weld bead

becomes bigger and bigger, as can be seen from the back side of the work. As a result, the

signal dwindles gradually. With the welding current decreasing and the backside beads

disappearing again during the third 10 seconds, the sensor signal rises again. Then the sensor

signal descends as expected when the current becomes large enough for penetration in the

final 10 seconds. However, due to different thermal stress under different currents, the

workpiece has been deformed during the third 10 seconds, resulting a longer standoff

distance. This explains why the signal at the third current transit is not as distinct as those

during the first two.

5.5 Quasi-keyhole plasma arc welding experiment

Quasi-keyhole plasma arc welding, pulsing the welding current between peak and base levels

in order to open and close the keyhole repeatedly, has proven to be an efficient approach for

plasma arc welding process control [25] [26]. With the existing keyhole sensor, for example,

the Efflux Plasma Charge Sensor (EPCS) [16], one can detect the instant at which the

keyhole is established, as well as the state of the keyhole.

EPCS measures the plasma from the back side of the workpiece as shown in Figure 9.

It gives a non-zero output when and only when the keyhole is established such that the efflux

plasma charges a voltage eV between the electrically isolated workpiece and detection plate,

typically 2V in this experiment. Should the keyhole not be established, there is no efflux

plasma between the workpiece and the detection plate, resulting in zero output signal eV .

Moreover, a larger eV implies a larger keyhole diameter and a wider backside bead. Hence,

the NTPCS signal can be compared with the EPCS signal during the same welding process to

examine the response of the proposed NTPCS.

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Experiments are conducted on 4.7mm thick stainless steel workpiece with a welding

speed of 2 mm/s. The welding current is pulsed between peak level and base level

periodically.

Figure 10 (a) and (b) show the resultant signals for two quasi-keyhole plasma arc

welding cases, partial keyhole and full keyhole. In case (a), because the EPCS signal never

exceeds the threshold, the keyhole is not established due to the relatively low peak current. In

case (b), the keyhole is established during the peak level and closed during the base level. It

can be seen that if the keyhole has been established by the peak current, the signals sampled

in peak and base levels differ by as much as 0.5V or more. However, if the keyhole is not

established during the peak current period, the difference is much smaller. This phenomenon

can be reasonably explained by the distinction of keyhole profiles between these two cases,

as shown in Figure 10 (c) and (d). When the keyhole is open in the peak duration, the arc

length becomes much longer than that in the base duration, during which the keyhole is

supposed to be closed due to low heat input. As a result, the voltage signal differs

significantly. On the other hand, if the keyhole has never been established during either peak

or base level period, the difference in weld pool surface depth is relatively small, resulting in

a small NTPCS signal variation for the two durations. Hence, by computing the signal

difference between peak and base durations, one can detect the establishment of the keyhole.

Figure 11 shows the plots of EPCS signal eV , NTPCS signal aV and the pulsed

welding current I (shown as 50/I ) for a weld in which the keyhole has never been closed in

either peak or base duration. As can be seen that during the peak duration a big keyhole is

established, corresponding to high eV and low aV . When the current is switched to base level,

the keyhole is still open but decays due to less heat input. This is shown by the decreasing eV

and increasing aV . The weld pool dwindles till it approaches the minimum and then grows

again. With the growth of the weld pool, the EPCS signal eV keeps increasing and the

NTPCS signal aV keeps decreasing at the same time until the welding current is switched to

the peak level again. Thus, the NTPCS signal is in good accordance with the weld pool

pattern as well as the EPCS signal.

As mentioned earlier, the weld pool surface depth can be estimated by comparing the

signal with a reference level due to possible uncertainty in welding conditions such as the

standoff distance and distortion caused by workpiece deformation. For quasi-keyhole plasma

arc welding, the keyhole is open during peak durations and closed during base ones. The

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weld pool profiles during base level and peak level are similar to those shown in Figure 5 (a)

and Figure 10 (d), respectively. As can be seen, the weld pool in the base duration is much

flatter and more stable than it is in the peak. This is because the base current has much

weaker capability to melt the metal and thus forms a much narrower and smoother weld pool.

Hence, the signal sampled during base duration is suitable for use as the reference for the

calculation of the weld pool surface depth in the succeeding peak duration.

In order to test the applicability of this mechanism, preliminary experiments are

conducted on 4.3mm thick workpiece with pulsed welding current. The base current is fixed

at 30A, while the peak current is varied every 10 seconds from 120A, 90A, 110A to 80A in

order to obtain different keyhole diameter. The base duration and peak duration are 350ms

and 250ms, respectively. Figure 12 (a) shows the original NTPCS signal together with the

welding current. Extracting the maximum in each sensing period and redrawing the signal

sampled in base and peak durations separately are shown in Figure 12 (b) and (c). Figure 12

(d) shows the back side of the weld.

It can be seen that the workpiece is fully penetrated during the whole process although

the penetration level varies. Signal sampled during base duration centers at an average level

but varies upward and downward shortly during the whole process, implying that the weld

pool undergoes only a smooth variation. By contrast, the variation in the sensor signal is

considerably larger for different peak levels and the higher the current, the lower the signal.

This is because higher welding current causes bigger and deeper keyhole and weld pool as

can be seen from the back side of the workpiece.

7. Conclusions

• NTPCS technology has been developed for plasma arc welding. Experimental results

verified that the NTPCS is capable of monitoring the weld penetration and providing

reliable information about the weld pool surface, especially the weld pool surface depth.

The NTPCS signal, sampled when the main power supply is isolated from the process,

decreases as the arc length or the weld pool surface depth increases.

• The NTPCS signal may be affected by parameters such as pilot arc current, plasma gas

flow rate, etc. To assure the effectiveness of the NTPCS technology, experiments must be

conducted in order to obtain the optimal combination of these parameters and thus the

most indicative sensing signal.

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Acknowledgement

This work is funded by the National Science Foundation under Grant DMI-0114982, the

Magnatech Limited Partnership, and the University of Kentucky Center for Manufacturing.

The authors sincerely thank Dr. Arthur Radun and Mr. Yuchi Liu for fruitful technical

discussions.

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26. Y. M. Zhang and Y. C. Liu, 2003. “Modeling and control of quasi-keyhole arc welding process,” Control Engineering Practice, 11(12): 1401-1411.

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List of Tables and Figures Table 1. Common parameters of the experiments Figure 1. The schematic of NTPCS system Figure 2. Thevenin source model of NTPCS system (a) The real system (b) Equivalent model circuit Figure 3. A typical waveform of NTPCS signal Figure 4. Sampled signal with no molten metal Figure 5. Illustration of weld pool surface under base and peak current levels (a) Weld pool under base current (b) Weld pool under peak current Figure 6. Pulsed current experimental result Figure 7. Speed variation experimental result (a) Original signals (b) The maximum signals during every sensing period (c) Front side of the weld (d) Back side of the weld Figure 8. Current variation experimental result (a) Original signals (b) The maximum signals during every sensing period (c) Front side of the weld (d) Back side of the weld Figure 9. Illustration of EPCS configuration Figure 10. Partial keyhole and full keyhole experimental results (a) Signal for partial keyhole case (b) Signal for full keyhole case (c) Partial keyhole profile (d) Full keyhole profile Figure 11. Signals for a keyhole plasma arc welding under pulsed current Figure 12. Peak current variation experimental result under pulsed welding current (a) Original signals (b) Signals during base duration (c) Signals during peak duration (d) Back side of the weld

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Table1. Common parameters of the experiments Sensing period 10ms

Normal welding period 90ms Sampling frequency 1KHz

Nozzle orifice diameter 1.8 mm Standoff distance 5mm

Plasma gas Pure Argon Plasma gas flow rate 5 feet3/h

Shielding gas Pure Argon Shielding gas flow rate 25 feet3/h Back-side shielding gas Pure Argon

Back-side shielding gas flow rate 45 feet3/h Pilot arc current 15A

Figure 2. Thevenin source model of NTPCS system. (a) The real system. (b) Equivalent model circuit

Arc

Nozzle

Electrode

R

C

Workpiece

(a) (b)

Ra

V0

R

C aV

Figure 1. Schematic of NTPCS system.

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Figure 3. A typical waveform of NTPCS signal

Figure 4. Sampled signal with no molten metal

17

Figure 5. Illustration of weld pool surface under base and peak current levels (a) Weld pool under base current (b) Weld pool under peak current

Figure 6. Pulsed current experimental result

Non-transferred Plasma

Plasma Torch

Workpiece

Weld Pool

(a)

Non-transferred Plasma

Plasma Torch

Workpiece

Weld Pool

(b)

18

(a)

(b)

19

(c)

(d)

Figure 7. Speed variation experimental result (a) Original signals (b) The maximum signals during every sensing period (c) Front side of the weld (d) Back side of the weld

20

(a)

(b)

21

(c)

(d)

Figure 8. Current variation experimental result (a) Original signals (b) The maximum signals during every sensing period (c) Front side of the weld (d) Back side of the weld

22

Figure 9. Illustration of EPCS configuration

23

(a)

(b)

24

Figure 10. Partial keyhole and full keyhole experimental results (a) Signal for partial keyhole case (b) Signal for full keyhole case (c) Partial keyhole profile. (d) Full keyhole profile

Figure 11. Signals for a keyhole plasma arc welding under pulsed current

Non-transferred Plasma

Plasma Torch

Workpiece

Weld Pool Partial Keyhole

(c)

Non-transferred Plasma

Plasma Torch

Workpiece

Weld Pool Full Keyhole

(d)

25

(a)

(b)

26

(c)

(d)

Figure 12. Peak current variation experimental result under pulsed welding current (a) Original signals (b) Signals during base duration (c) Signals during peak duration (d) Back side of the weld