Real-time Electrode Misalignment Detection Device for RSW Basing

on Magnetic Fields

D. Ibáñez

, E. García

, J. Martos

and J. Soret

Dept. of Electrical and Electronic Engineering, University of Valencia, Burjassot, Valencia, Spain

Ford Valencia, 46440, Valencia, Spain

Keywords: Resistance Spot Welding, Detection, Hall Effect Sensors, Electrodes, Misalignment, Magnetic Field,

Simulation.

Abstract: Electrodes misalignments are considered one of the most important mechanical factors involved in RSW

(Resistance Spot Welding). Misalignment causes quality problems as undersized weld, expulsions or

nonrounded-weld. Man-power needed in the automotive production lines is increased so as to repair the lack

of quality, which means an increase in the cost of production. Consequently, an implantable solution for the

automotive industry should be developed in order to detect misalignment when this happens. This research

gives an answer by measuring the electrode misalignment by means of the generated magnetic field for the

electrodes. The proposed method is validated by Multiphysics simulation measurement. Finally, this method

is put into practice by creating a device tested in an automotive production line at the assembly and body plant

in Ford Valencia. Together with the device, a communication system is implemented to carry out predictive

management. This research initiates a novel line of research for the early and online detection of

misalignment problems in welding guns.

https://orcid.org/0000-0002-3917-9875

https://orcid.org/0000-0002-4210-9835

https://orcid.org/0000-0002-8455-6369

https://orcid.org/0000-0001-8695-6334

INTRODUCTION

The resistance spot welding process bases its

operation on a current flow from the tip of the upper

electrode between the metals to be welded to the tip

of the lower electrode. When the current circulates

through the metals, and due to Joule's law, the heat

generated melts the metals forming a weld joint

between the metal sheets through the fusion and the

result is a strong weld between sheets without

additional substances. Therefore, the growth of the

weld (welding nugget) depends on the density of

welding current, the welding time, the force exerted

by the electrodes on the sheets and the area of the

electrode tip (Aravinthan et al., 2011)

This dependence is different depending on the

parameter, since the time and the current make the

welding, while the pressure and the area of the

electrode tip have a direct relationship with the final

quality of the welding.

Misalignment of the electrodes causes a

variation in the contact area between the electrode

tips. The alignment of electrodes can be classified

into three types depending on their orientation. The

electrodes can be perfectly aligned, axially

misaligned or angularly misaligned, as shown in

Figure 1 (Zhang et al., 2005).

Figure 1: Types of electrode misalignment RSW.

142

Ibáñez, D., García, E., Martos, J. and Soret, J.

Real-time Electrode Misalignment Detection Device for RSW Basing on Magnetic Fields.

DOI: 10.5220/0009820801420149

In Proceedings of the 17th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2020), pages 142-149

ISBN: 978-989-758-442-8

Different studies have shown the alignment of the

electrode plays an important role in the geometry of

the welding point, Figure 2, in addition the

misalignment of the electrode causes expulsion,

which leads to poor welding zones Tang et al.,2003),

(Charde,2012).

Figure 2: Poor-quality spot causes by misalignment.

Some authors have proposed different methods

for detecting misalignment using image processing

methods, by which they are able to determine the

direction and angle of misalignment with good results

(Li et al.2019). The main problem presented by this

method is the cost of implementation for a high

production line with many welding guns. Therefore,

there is not a viable method for detecting

misalignment in this type of production lines.

Due to the reasons mentioned above, there is a

need to investigate a simple and economical method

for the detection of misalignment, since what is

sought is that it can be used in the high-production

automotive industry.

The theoretical basis of this method is laid on the

variation of the current density at the tip of the

electrode. Specifically, what is intended with this

method is to identify the change in current density as

the misalignment appears.

Figure 3: Geometric model for calculating contact area

due to axial misalignment.

When the electrodes are misaligned, as shown in

Figure 3, the contact area (Ca) of the electrode tips

varies according to the equation 1.

𝐶



2𝑟



sin







1

𝛿

2𝑟





𝛿



𝑟





𝛿





where Ca is the contact area of the electrodes, r

is the radius of the electrode and δ is the amount of

misalignment.

It can be seen from the equation that the

reduction of the contact area is strongly correlated

with the axial misalignment.

From the Ampère's Integral Law can be obtained

the relationship between the magnetic field (B) and

the current density. This law relates the magnetic field

intensity to its source, the current density.

𝐵



⃗

∙𝑑𝑙

⃗

µ



𝐽

⃗

∙𝑑𝑆

⃗





µ



𝜀



𝐸



⃗

∙𝑑𝑆

⃗



Where the magnetic field is described by the

variable B, the current density by J and the electric

field by E.

Following the equation 2, it can be affirmed that

if the current flows through the electrodes, and

therefore there is a current density, a rotational

magnetic field around the electrodes appear and the

rotor of the magnetic field points in the same

direction that the current density, this behaviour is

represented in Figure 4.

Figure 4: Generated Magnetic Field.

As a result, basing in this equation, it can be

affirmed that when electrodes are misaligned, the

contact area of the electrodes decreases, which

produces a raising on the current density and

consequently a magnetic field higher than which is

generated with a correct electrode alignment.

MISALIGNMENT DETECTION

METHOD

As shown in the previous section, a possible

relationship between the magnetic field generated by

the short-circuited electrodes and their misalignment

can be deduced mathematically. The proposed

method for the detection of the misalignment is

Real-time Electrode Misalignment Detection Device for RSW Basing on Magnetic Fields

143

based on the measurement of the magnetic field in

the contact plane of the electrodes.

The measurements should be carried out in such a

way that a value is obtained for determining the

direction of the misalignment of the electrodes.

For this, it is postulate that if the contact plane is

divided into the Cartesian axes and measurements are

made at different distances from the centre of the

ideal contact surface of the electrodes, could be

detected the misalignment of the electrodes and its

direction.

This means that if the magnetic field can be

measured in both Cartesian axes, both in positive and

in negative, it will be possible to determine the

difference of the magnetic field generated by the

misaligned electrodes in comparison with the one

generated by the perfectly aligned electrodes.

As it is a novel hypothesis, due to the fact that

other researchers haven´t published anything related

to the relationship between the magnetic field and the

misalignment, it is fundamental to demonstrate it.

Firstly, performing a validation by means of

simulation of the physical phenomenon, to verify that,

in fact, the mathematical assumption is fulfilled.

For the validation of this method, software of

simulation of the physical phenomenon based on

magnetic field theory is used.

Once the hypothesis has been validated for the

proposed method, a device would be developed for

taking measurements in an industrial environment,

capable of determining the differences between

simulation and real experimentation to finally design

an automatic system for detecting problems of

alignment of welding electrodes in real time.

MATERIALS AND METHODS

For the analysis of the behaviour of the magnetic

field depending on the state of the misalignment, a

physical phenomenon simulation software is drawn

on. The simulations are carried out for the symmetry

of an F- type electrode (ISO 5821, 2007) with the

following data:

•

Current flowing through the electrode: 8 kA.

•

Diameter of the electrode tip: 6mm.

•

Electrode body diameter: 20 mm.

•

Cone height: 5mm.

These simulations are performed simulating a

current flowing between the electrodes shorted. To

obtain the relationship between misalignment and

generated magnetic field.

During this validation, three tests will be

performed. In the first one, the magnetic field

generated for an electrode in which δ = 0 mm, i.e., a

perfectly aligned electrodes, is simulated. In the

second, the value of δ is increased up to 1mm and the

magnetic field is simulated, comparing the values of

both cases. Finally, the value δ is increased again up

to 2mm and the simulation is carried out, comparing

all the obtained values.

For each of the cases, two simulations are carried

out. In the first simulation, the values of the magnetic

field are collected depending on the distance on the x-

axis to the centre of the electrode. These values are

simulated for both the contact plane of the electrodes,

z = 0 mm, as for planes situated z = 10mm and z = -

10mm.

In the second simulation, the data is acquired in

this case as a function of the displacement in the z-

axis. In this simulation, two data curves are obtained:

the variation of the magnetic field on the z-axis when

x = 20mm and the variation of the magnetic field on

the Z-axis when x = -20mm.

MAGNETIC FIELD

SIMULATION

This section shows the results of the different simulat-

ions carried out as described in the previous section.

4.1

δ=0mm

As mentioned, simulations are carried out for three

different scenarios. In this first case, two electrodes

perfectly aligned are simulated, δ=0 mm and S=

50.26 mm

. This first case points what is the ideal

value of the magnetic field generated by the

electrodes. The following cases should therefore be

compared with this to determine if there is certainly a

relationship between the misalignment and the

generated magnetic field.

Figure 5: Magnetic Field Generated for aligned electrodes.

X-axis displacement.

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From this simulation, the data represented in

Figure 5 and Figure 6 are obtained. As it can be seen,

both in the data of Figure 5, which shows the

evolution of the magnetic field on the x axis, and the

one of Figure 6, which shows the evolution of the

magnetic field in the y-axis, there is a symmetry in

the generated magnetic field.

Hence, this means that when measuring at the

same distances from the centre of the generated

magnetic field, the same value is obtained. This point

is very important because what is sought in this study

is to be able to determine the misalignment but also

the direction of it.

In addition, this data also shows how the Ampere

law is fulfilled. By analysing the three curves of

Figure 7, it can be observed how those taken on the

planes z = -10mm and z = 10mm, present a lower

magnetic field value. This is because at this height,

the surface through which the intensity flows are S

= 314.15mm

. This makes the current density lower

and therefore the magnetic field is lower too

Figure 6: Magnetic Field Generated for aligned electrodes.

z-Axis displacement.

4.2

δ=1mm

In this second case, electrodes with a displacement

of the upper electrode of 1mm are simulated, that is,

δ = 1mm. Using equation 1 it can be calculated that

the contact surface for this case will be S = SꞏCr =

50.26ꞏ0.7623= 38.31 mm

Therefore, according to the hypothesis, the

magnetic field generated by the electrodes must be

higher since the current density increases.

Figure 7 shows how, as expected, the simulated

magnetic field is greater than the magnetic field of

Figure 5. The maximum value of the generated

magnetic field is 340 mT, which represents an

increase in the magnetic field by approximately 111%

compared to the field generated by the perfectly

aligned electrodes.

Figure 7: Magnetic Field Generated for electrodes with

δ=1mm. X-axis displacement.

To make the comparison between the different

cases, the data is analysed at 20mm from the centre of

the magnetic field for the perfectly aligned electrodes.

This centre is shown in the graphs as the zero of the

coordinate axes. This analysis is done in a more

graphical way observing the Figure 6 and 8 that

represents the displacement in the Z-axis, since the

data represented in it are those corresponding to the

distances 20mm and -20mm.

If the data of x = 20mm and x = -20mm are

taken at the time when z = 0mm, it can be observed

that for the aligned electrodes this takes a similar

value of 72 mT approximately.

Figure 8: Magnetic Field Generated for electrodes with

δ=1mm. z-Axis displacement.

On the other hand, with the data collected for this

second case, it is observed that for x = 20mm the

value of the magnetic field is equal to 74.5 mT, while

Real-time Electrode Misalignment Detection Device for RSW Basing on Magnetic Fields

145

for x = -20mm the magnetic field is equal to 69,5mT.

This means that while in the first case the difference

between x = 20mm and x = -20mm is 0 mT, in the

second case this difference increases significantly up

to 5 mT.

It is also important to point out that because of the

fact that one of the electrodes has moved but the other

has been fixed, the centre of the magnetic field has

been shifted 0.5 mm. This also rise the difference

between the values in the x positive and negative x.

4.3

δ=3mm

In this last case the misalignment of the electrodes

increases δ up to 3 mm. This supposes a misalignment

of 50% of the maxim misalignment. This last

simulation helps to determine in a more reliable way

if it is possible to differentiate different states of

misalignment.

Figure 9: Magnetic Field Generated for electrodes with

δ=2mm. X-axis displacement.

In this case, using equation 1 again, the actual

contact surface of the electrodes can be calculated.

Since δ has increased to 2mm, the ratio between the

ideal surface and this new surface decreases to

0.5309. Therefore, the current surface is 26.68 mm

As in the previous case, it can be seen in Figure 9

that the maximum magnetic field generated is higher

than the other two cases, taking a value of 376 mT.

This represents an increase of 123% and 111%

respectively.

Following in this case the previous analysis about

the measurement differences between x = 20mm and

x = -20 can be seen, as in the previous result, that there

is no symmetry. For x = 20mm a value of 81 mT is

recorded, while for x = -20mm a value of 65mT is

recorded.

Figure 10: Magnetic Field Generated for electrodes with

δ=2mm. X-axis displacement.

Therefore, in this third case it can be seen how the

difference between both measures increases again

while the misalignment increases too, going from a

difference of 0 mT for the misaligned electrodes to a

difference of 16 mT

Figure 11: Summary of simulation result.

Finally, Figure 11 summarizes the most relevant

values of this validation by simulation. Then, the

results show that there is a strong relationship

between the generated magnetic field and the

alignment of the electrodes. So, the hypothesis has

been validated.

SIMPLE DEVELOPMENT OF A

DEVICE FOR THE DETECTION

Once the hypothesis has been validated, it is

necessary to study how this new method can be

applied to the high production industry of the

automobile.

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As it has been explained, it is necessary to make

measurements of the magnetic field in the contact

plane of the electrodes. Therefore, it is necessary to

perform four measurements of the magnetic field, two

for each of the axes x-y. For this, it is necessary to

located sensors capable of performing these

measurements at x = 20mm, x = -20mm, y = 20mm

and y = -20mm.

In this case, two PCB are manufactured, one in

which four low-cost hall effect sensors are located,

which will be where the measurement is made. In this

first PCB it is designed with a circular hole in the

middle, so that the four hall effect sensors are

distributed to perform measurements on the two

cartesian axes.

In the second PCB the microcontroller used to

control the signals collected by the sensors is located.

To isolate the two PCBs, a 3D printed PCA

encapsulation is performed. This design is made to

optimize costs and increase the robustness of the

design.

This device is designed so that the electrodes can

be closed and positioned in the middle of the four

sensors. Once located in that position, a current is

passed between the short-circuited electrodes and the

sensors measure the generated magnetic field.

Figure 12 shows the location on the real welding

line of the PCB on which the Hall Effect sensors are

mounted.

Figure 12: Actual PCB location for magnetic field

measurement.

This measured magnetic field data is sent to the

second PCB where the microprocessor that manages

these signals is located. There the voltage values

measured by the sensors are converted to magnetic

field units.

Figure 13: Actual PCB location for collection of the data.

Figure 13 shows the situation of the PCB, in

which the microprocessor is located, in the actual

welding line.

The signal management is carried out following

the flow chart of Figure 15. Once the microcontroller

is initialized, a first measurement of the magnetic

field is made to determine the offset of the sensors,

thus eliminating the possible differences between the

measurements.

Figure 14: Operating flow chart.

Once this action is carried out, the four sensors

begin to record the magnetic field value, if the

measured magnetic field is zero, the microcontroller

does not perform any calculation. Once the electrodes

are short-circuited and generate a magnetic field, the

Real-time Electrode Misalignment Detection Device for RSW Basing on Magnetic Fields

147

Figure 15: Measurement process flow.

microcontroller begins to register the values by

calculating at all times the average value of the

magnetic field measured in that period of time, so that

the possible peaks that could appear are reduced by

Use the average.

When the electrodes stop conducting current, the

sensors re-measure the absence of magnetic field

and at that time, the microcontroller sends the values

to the PLC.

After communication with the PLC, the variables

are reset to 0 and the measurement is restarted

waiting again for the magnetic field generated by the

electrodes in short circuit.

REAL-TIME CONTROL

APPLICATION

Once a device capable of detecting the magnetic

field has been developed, and therefore, the problems

of alignment of the electrodes in the welding clamps,

it is necessary to develop a final system capable of

carrying out preventive maintenance on the actual

welding line.

In such a way that patterns of misalignment

behaviour and work limits are established based on

history and experimentation. To do this, once the

process described in Figure 14 is finished, when the

data is already in the PLC, this data is stored in a

database and sent to a web server. This whole process

can be summarized in Figure 15.

In this case, in order to reduce the number of

variables on which to perform the analysis, the four

variables of each of the sensors are reduced by only

two.

These two variables are calculated by subtraction

between the sensors placed antiparallel, therefore, the

two final variables will represent the displacement of

the centre of the magnetic field on the X and Y axes,

eliminating the absolute value of each of the four

sensors.

Starting from the established values of the

simulation first and after the values stored in the

history, the alarm and pre-alarm levels for each of the

Cartesian axes can be established. As shown in figure

16.

Figure 16: Example of alarms based on simulation.

Currently, this whole real-time medicine process

is installed in a welding line. This application allows

to obtain the data of each check in the attempt in

which it is performed.

This last part of the investigation is still in an

exact process of validation, since it is convenient to

test all the cases and the real behaviour of a welding

clamp, beyond the simulation results.

CONCLUSION

This article tries to give a solution to this important

problem of misalignment in the electrodes of the

welding guns. This problem directly affects the costs

of production of automobile manufacturing, so it is

mandatory to find a solution that can be implantable,

i.e., a solution that does not involve a high cost of

implementation. This article responds positively,

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148

presenting a method for detecting the misalignment

by magnetic fields.

For the method validation, mathematical

calculations and simulations are used, in which it is

observed how, unequivocally, there is a strong

relationship between both factors.

Finally, a device for measuring the magnetic field

is proposed, this device is composed of four hall

effect sensors managed by a microcontroller. For the

validation of the sensor, tests in a line of production

of the automobile in Valencia Body and Assembly

plant are carried out.

Although the data collection is not yet extensive

enough, a historical data collection is begun to

monitor the behaviour of the magnetic field generated

by real misaligned electrodes.

In future investigations, the data obtained in the

actual welding line should be analysed. Based on

these results, a possible update of the method and the

designed device should be performed.

In addition, an exhaustive study of the influence

of the existing noises on the production lines and

their influence on the measures taken must be made.

ACKNOWLEDGEMENT

The authors wish to thank Ford España S.A, in

particular, the Almussafes factory for their support in

the present research.

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