AUTOMATIC BRAKING DISC ANALYSIS SYSTEM - MDH · M alardalen University Master Thesis 2.1.2 Ultrasonic Testing Ultrasonic Testing (UT) is another NDT-method that is widely used to

Marlardalen UniversitySchool of Innovation Design and Engineering

Vasteras, Sweden

Thesis for the Degree of Master of Science in Engineering - Robotics30.0 credits

AUTOMATIC BRAKING DISCANALYSIS SYSTEM

Joakim [email protected]

Examiner: Giacomo SpampinatoMalardalen University, Vasteras, Sweden

Supervisors: Mikael EkstromMalardalen University, Vasteras, Sweden

Company supervisor: Ingemar Reyier,Robotdalen, Vasteras

May 19, 2015

Malardalen University Master Thesis

Abstract

Volvo Group Truck Technology has the ambition to automate parts of their routine service. There-fore a project was launched to investigate which parts of the routine that could be automated. Theidea of this project is to lower the time spent on the service and also improve the working envi-ronment for the personnel.

The purpose of this thesis is to develop and build a conceptional prototype for a low-cost crackdetecting sensor. This thesis is a part of a larger proof of concept project which Volvo GTT runsin cooperation with Robotdalen and Robot Application Center (RAC).

The work done in this thesis has been based on literature studies, interviews and companyvisits. The gathered knowledge and observations was translated into what would be required tofit the needs. This thesis covers different techniques that could be used to detect flaws in brakingdiscs. However, this thesis is mostly focused on one non-destructive method technique based oninduced eddy currents.

Several non-destructive techniques and conceptual designs has been tested and evaluated withvarying results during this project.

The result of this thesis was a design that reacts to discontinuities in conductive materials, suchas the grey cast iron material used in the Volvo trucks braking discs. The results are represented asa voltage drop change and can be visualized by an oscilloscope. This study shows that the methodof choice has the potential to be used as a crack detecting system and that the system can be builtreliable with rather cheap components.

Further development should aim towards making the design even cheaper and the componentsshould be assembled on a PCB instead of a breadboard in order to make the system less sensitiveto noise and easier to assemble alongside the trucks braking discs.

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Abbrevations

NDT Non destructive testing

ECT Eddy current testing

LDC Inductance to digital converter

GUI Graphical user interface

AC Alternating current

PCB Printed circuit board

IACS Electrical conductivity unit for metals relative to a standard copper conductor

ADC Analog to digital converter

BBB Beaglebone black

MCU Microcontroller unit

GPIO General-purpose input/output

SPI Serial peripheral interface

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Table of Contents

1 Introduction 41.1 Problem Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Background 52.1 State-of-the-art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Eddy-Current Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 Ultrasonic Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Method 73.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Data gathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.1 Literature studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.2 Company visits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3 Interviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4 Eddy current probe designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.4.1 Absolute probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4.2 Differential probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4.3 Reflection probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.5 Tested coil types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.6 Depth of penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.7 Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.8 Test system design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.8.1 Analog-to-digital converter . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.8.2 Multiplexer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.8.3 Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.8.4 Probe design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.8.5 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Results 214.1 Test system design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2 Coils with potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5 Discussion 24

6 Conclusions & Future work 256.1 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Time Plan 26

References 27

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1 Introduction

This master thesis is a part of a bigger project at Robotdalen in cooperation with Volvo GroupTruck Technology (Volvo GTT). Volvo GTT wants to investigate how they could automate someparts of their aftermarket strategy.

1.1 Problem Formulation

The goal of this thesis is to investigate and evaluate how an automatic analysis of a trucks brakingdiscs could be done with the use of suitable sensors mounted on a robotic arm. The sensorsystem should be developed and used to detect both surface and sub-surface cracks in the metallicstructure of a braking disc. The goal of this system is to automate some parts of the today manuallywork done by a trained technician during service routine. The following points below should beconsidered to solve the problem.

• A sensor should be developed and evaluated that is able to detect both surface and sub-surface discontinuities in a braking disc.

• Possibility to mount the sensor to a robotic arm which can move the sensor in a desired threedimensional trajectory, in order to scan a desired area of a braking disc.

• Possibility to place the sensor alongside the braking disc at all time.

The state-of-the-art study in this thesis should focus on non destructive testing for cracks inmetallic structures. Research about signal processing should also be done to get a good overallunderstanding of how the different parts should cooperate with each other in order to solve thisproblem.

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2 Background

A braking disc has to be robust and be able to withstand the force and heat changes created bybraking in a harsh environment. Occasionally the structure of the braking disc will break and apotential hazard is a fact. Although trucks goes through a service routine a couple of times a yeara broken braking disc can be hard to detect.

In a truck service routine today a specialized technician does a number of services on eachtruck. Upon suspicion of a bad braking disc a number of analysis methods is used to locate theproblem. One of the error-indicating systems is based on a pressure sensor that is connected to thebraking pads. This system will warn the driver if the braking pads are not forcing the braking discenough. Another more manual method of finding problems with a braking disc is by sensing thefeedback in the steering wheel generated while braking. One of the most common problem foundwith this method is wobbling braking discs, although cracks will most likely go undetected.

As of today there is no system implemented to analyse the braking discs for surface or sub-surface cracks. By developing a tool that can be used in an automated fashion that does bothsurface and sub-surface crack detection analyses one could possibly make the process of detectingbad braking discs both faster and more secure.

2.1 State-of-the-art

This section presents the state-of-the-art research that has been done to get an understanding ofwhat has been done before and which methods that could be used in this thesis.

2.1.1 Eddy-Current Testing

One of the most commonly used non-destructive testing (NDT) methods is Eddy-Current Testing(ECT), which can be used to detect both surface and sub-surface cracks in conductive materials.For example, this method is widely used in the aeroplane industry to detect cracks in wings [1].

In ECT a magnetic field is generated by inducing an alternating current into a coil. When themagnetic field is generated in an ECT-probe and then placed close to a conductive material, eddycurrents are produced inside the conductor. Pure conductive materials allows the eddy currentsto flow without much disturbances to the generated magnetic field. However, if the conductivematerial contains cracks the flow of eddy currents is disturbed and the magnetic field is thereforedisturbed. The disturbances in the magnetic field can be measured by measuring the voltage-variations of either a separate detection coil or by the excitation coil, depending of which coil typethat is being used [2].

By varying the frequency of the alternating current and matching this to the resonance fre-quency of the circuit one can get different penetration depths of the magnetic field. This allowsfor detection of discontinuities at a desired depth [3]. The circuitry choice is explained further inthe section 3.5.

Advantages

• Probes can be built portable since the electronic components that is needed can be smalland still be sensible to small discontinuities.

• The results are immediate so the sensor could be used while in motion.

• Can detect both surface and sub-surface discontinuities.

• Probes does not need to have physical contact with the target of interest.

• Inexpensive components.

Disadvantages

• Rough surfaces might interfere with the testing.

• Can only detect flaws in conductive materials.

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2.1.2 Ultrasonic Testing

Ultrasonic Testing (UT) is another NDT-method that is widely used to detect flaws in differenttypes of material. The technique can detect surface and sub-surface flaws that produce reflectiveinterfaces such as: cracks, shrinkage cavities, bursts and porosity [7]. In UT, the probe needs tohave physical contact with the surface of the target to get proper readings.

In UT, an ultrasonic probe sends a sound wave pulse into the target material, and listens forthe echo which indicates the traveling time of the pulsed signal. By determining the time of flightfor the signal to echo back, an estimation of the depth can be done. If there is a crack on thesurface or sub-surface, there will be more than one echo that makes it possible to estimate thedepth of the crack.

Advantages

• Probes can be built very portable.

• The results are immediate so the sensor could be used while in motion.

• Can detect both surface and sub-surface discontinuities.

• Can be used in a wide range of materials and thicknesses, not limited to conductive materials.

Disadvantages

• Needs physical contact with the target.

• Rough surfaces might interfere with the testing.

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3 Method

In this 20 weeks master thesis work there is a need to gather wide knowledge regarding the serviceroutine and the design of the trucks braking system so that the tool can be developed in a suitableway. This knowledge will be gathered by doing literature studies, company visits, interviews andprototype manufacturing.

Both UT and ECT was researched and both methods could potentially be used in this masterthesis. Compared to UT the ECT-method does not need physical contact with the target materialand was therefore chosen as the method to use in this thesis. An ECT-sensor can be built veryportable and also be shielded from external interferences such as dust and magnetic fields. Theability of sensing both surface and sub-surface discontinuities is also something that is highlydesired for this thesis.

Interviews and state-of-the-art research regarding Eddy Current techniques will lay the foun-dation for the development of the crack detection sensor. Different types of Eddy Current probeswill be developed and tested in different configurations. One of the most important thing thatwill be evaluated is what frequencies to use when generating the magnetic field in order to getdesired penetration depth. This will be done by tuning the oscillation part of the Eddy Currentprobe until a desired penetration depth is achieved. Different windings of the excitation coils willbe tested and evaluated to get a desired magnetic field.

The ECT-sensor will be developed in such way that it could easily be mounted to a robotic armso that a braking disc analysis can be done during the service routine. It should also be possibleto mount the sensor beside the braking disc so that analyses can be done while the truck is inmotion.

3.1 Methodology

The research methodology will follow the principles of exploratory engineering science. The objec-tives will be achieved through the following steps:

• Identification of technical challenges or technical problems that could occur.

• Identifying existing research, technology, and components that can be used.

• Develop new or adapt existing components.

• Build proof-of-concept implementation using the existing, adapted, and/or new components.

• Evaluate, qualitatively, these implementations with respect to the challenges or problems.

3.2 Data gathering

For this thesis, there was a need to gather information about both the process within the serviceroutine and about non-destructive testing methods. Both qualitative and quantitative data werevery important for the evaluation of different approaches to the problem.

3.2.1 Literature studies

Different forms of literature has been studied for this thesis, for example technical reports, booksand articles. This literature study has worked as a theoretical framework for the practical parts ofthe project. A good understanding of electromagnetic fields, resonant circuitry and programminghas been critical to be able to develop a conceptional sensor.

3.2.2 Company visits

By interviewing people and inspecting the routines at companies that are performing the serviceroutine, a better overall understanding of the service routine and the problem was gathered. Oneimportant company visit was done at Volvo Truck Center in Enkoping which included interviewingworkers at site and inspection of the Volvo trucks at site. An important observation was done duringthis visit, the braking discs was shielded and could not be inspected without removing the wheels.

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It could thereby be hard to inspect the braking discs with a robotic arm during the service routine.However, the project was reformulated to develop a low-cost sensor that could be placed besidethe braking disc at all time which also could be used to scan the braking disc with the help of arobotic arm.

3.3 Interviews

Interviews with workers, teachers and researchers has been important to the project. By gatheringknowledge from a broad range of experienced people, important decisions have been possible totake.

The people that has been interviewed are:

Per SchlundPer Schlund is a lecturer in the Division of Intelligent Future Technologies at Malardalen Universitywho has great experiences in non-destructive testing methods in the aerospace industry. Theconclusion of the meeting with Per is that Per thought that Eddy Current held the highest potentialfor this application.

Nikola PetrovicNikola Petrovic, Post Doc at Malardalen University with great experience in signal processingand visualizations. Nikola described his work about detecting breast cancer with the use of microwaves and also his studies about detecting cracks in bolts and nuts. Nikola recommended thatfor metallic structures Eddy Currents has good potential since it e.g. is used by the aeroplaneindustry for similar crack-detection applications. [8]

Brian CoxBrian Cox has experience in different NDT-methods for both metallic and biological structures.Brian explained another technique he had used to measure salinity in water. The system dependedon resonating circuits. The principle of resonating circuits was explained by Brian and he envisionedthat it could be used in this thesis work. Brian recommended an LDC1000 Evaluation Modulewith an on-board LC tank which comes with a GUI to plot the variances in the impedance of thecoil.

3.4 Eddy current probe designs

Eddy current probes comes in different designs based on which type of application they will beused in.

3.4.1 Absolute probes

Absolute probes are based on the principle of using a single coil for both generating the magneticfield and to sense the changes of the magnetic field. The magnetic field is generated by passing analternating current (AC) through the coil. When the probe and its magnetic field is placed neara conductive material, eddy currents are generated inside the material which interfere with themagnetic field of the probe. By sensing the variations of the magnetic field one could detect flawsin the material, conductivity changes and thickness estimations. [9]

3.4.2 Differential probes

Differential probes uses at least two active coils where the difference in the magnetic field ismeasured to find discontinuities. For example, if one of the two coils are placed over a flaw andthe other is not, a difference in the magnetic field will occur between the two coils which indicatesa discontinuity. [9]

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3.4.3 Reflection probes

Reflection probes uses multiple (2 or more) coils where the principle is based on using sendingcoils and receiving coils. Reflecting probes are for example used in metal detectors where a largercoil creates a magnetic field that a smaller coil can pick up, variations in this magnetic fieldrepresents interference with a conductive material. The receiving and sensing coils can be scaledand optimized for their indented purpose.[9]

3.5 Tested coil types

Different types of coils were tested and evaluated during the project. Air coils both with andwithout a ferromagnetic core to focus the magnetic field were tested in absolute, differential andreflective mode.

One of the first designs that showed potential was a combination between reflection mode anddifferential mode. Where one sender coil is placed in between two receiving coils, see figure 1. Theidea of this configuration is that the sending coil generates an electromagnetic field which whenplaced over a conductive material generates eddy currents in the conductive material. Ideally, whenthe receiving coils gets induced by the generated electromagnetic field, the impedance between thetwo receiving coils should be close to equal. When a discontinuity disturbs the eddy current insidethe conductive material, an impedance change in one of the receiving coils will occur. By measuringthis change one can estimate the size of the discontinuity (e.g. crack). However, this design shouldnot be practical and probably to expensive since three coils is needed for each point that is to bemeasured.

Figure 1: A printed circuit board with three coils with ferromagnetic cores

Another similar design to the one shown in figure 1 was tested, see figure 2. The design wasmeant to be simple in the sense that the coils are winded by hand with the possibility to add aferromagnetic core inside the coils for testing purpose. This coil configuration uses the principle ofreflection and differential configuration with a sender coil in the middle and two receiving beside.An insulated copper wire with a diameter of 0.10mm was winded in 40 turns to build up eachcoil. This design did work in some sense but it was very sensitive to noise, making the changes ofinterest hard to filter out. A good shielding of the magnetic field was thereby very desirable.

Influenced and inspired by a study about differential planar eddy current probes made bya couple of students in Portugal, the idea of a planar coil design printed on a PCB was born.Their design consists of two D-shaped sensing coils with a driving trace in between them. [10] Theresults of that probe is quite remarkable. It generates an easy to decode output since the differentialmethod outputs a voltage equilibrium of 0 when the two sensing coils are in a symmetric condition.

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Figure 2: 3D printed rake shaped probe with three air coils

When a defect or other discontinuities interferes with the magnetic field, the voltage equilibriumis changed and the output is no longer 0.

Based on the gathered information about a flat PCB coil design, a coil prototype was designedin Ultiboard, see figure 3. This type of coil generates a wide magnetic field when a current is runthrough it. The inductance is proportional to the amount of loops that builds up the coil and canbe calculated by the equation 1, where L is the inductance in µH, r is the mean radius of the coilin centimeter, N is the number of turns and d is the depth of coil in centimeters.

L =r2N2

20r + 28d(1)

This coil was later printed on a copper PCB, see figure 4. The inductance of the coil wasmeasured to be about 8 µH. The testing and evaluation of the coil was done by connecting itto an AC powered LC-circuit where a conductor of 220 nF and the PCB coil was connected inparallel followed by a resistor. An oscilloscope was connected to measure the voltage drop acrossthe coil and the conductor. The maximum voltage drop indicates that the impedance of the coiland conductor configuration is the highest at that specific frequency. The frequency of the ACpower source was therefore increased until a peak of voltage drop was reached over the coil andconductor. This peak-voltage drop frequency indicates the resonance frequency of the LC circuitwhere it is desirable to be for a couple of reasons, read more about the circuitry choice in section3.7. However, this coil seemed to pick up quite a lot of noise, which most likely is due to itsedgy design. The noise was high and could potentially be hard to filter out. Another design wasconsidered instead with a less edgy design.

The flat coil that showed the most desirable results with low noise sensitivity and calculableinductance can be seen in figure 5. This coil consists of 9 turns that is printed on a PCB with aninductance of about 8 µH. This coil design can easily be calculated upon and thereby adjustableto fit the need of inductance to achieve right resonance frequency. This coil design was tested inabsolute mode where it was evaluated before it was printed aligned with 3 more identical coils.The 4 coil design can be seen in figure 10.

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Figure 3: A single flat coil designed in Ultiboard

3.6 Depth of penetration

Based on a paper from Volvo Trucks, cracks deeper than 1.5 mm into the braking discs couldpotentially disrupt the disk further and the disc should be replaced if a crack deeper than 1.5 mmis observed. Based on this fact, the sensors’ depth of penetration had to be taken into accountwhen designing the circuit. The depth of penetration is basically how deep the eddy currents areinduced into the target material.

To calibrate the depth of penetration one need to understand that the depth of penetration isbased on different properties of the target material. More specifically, the depth of penetration isdetermined by the electrical conductivity, magnetic permeability of the target material and alsoby the frequency of the generated magnetic field by the probe. [11]

The depth of penetration can be calculated by formula 2. Where δ is the depth of penetrationin millimeters, f is the frequency in Hz, µ is the magnetic permeability in H/mm and σ is theelectrical conductivity in IACS.

δ ≈1

√πfµσ

(2)

Lower frequencies penetrates deeper into the material but lower frequency also makes thechanges in the magnetic field due to discontinuities less noticeable. So the relationship betweendepth of penetration and inductive sensitivity are two very important aspects to take into accountwhen designing an NDT system based on induced eddy currents.

3.7 Circuitry

Generating a magnetic field and sensing it can be done in multiple ways. Where in this masterthesis the choice of design was based on researched designs with the restrictions that was set forthe project. Since the system could potentially be used in all of Volvo trucks at all time, therestrictions for the system to be low-cost is crucial. However, the system should still be verydurable and sensitive.

The system designs tested in this project has been based on different coil and capacitor config-urations where the first system that was tested was based on a single coil and resistor connectedin series, see figure 6.

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Figure 4: A single flat coil printed on a PCB

By sweeping the frequency of the AC power source it is possible to determine the self-resonancefrequency of a coil. By configuring the AC power source to the self-resonance frequency of the coilthe voltage drop over the coil is varying a lot depending on the changes in the magnetic field. Thevoltage drop is changing because of the high impedance change near the specific frequency. Figure7 shows the impedance change relative to the frequency. This system has potential in the sensethat small variances in the magnetic field will give a large voltage difference which occurs whena discontinuity disturbs the eddy currents. However, at self-resonance the coil starts to get moreunpredictable and can be hard to control. Therefore, a more stable configuration is desirable.

Another more stable configuration to sense variations in the magnetic field in a circuit insteadof a single component is to connect a conductor and a coil in parallel to each other, see figure 8.This configuration allows to adjust the resonant frequency just by changing the inductance andconductance components [12]. By choosing the values correctly, the resonance frequency can bechosen to match a certain penetration depth of a given material.

The circuitry choice of this project landed on the principle of absolute probes extended witha switching circuit that enables to switch between coils that are aligned on a board. For theconceptional design a PCB was made that contains 4 flat coils with an inductance of about 8 µHeach. The reason for this design was based on the knowledge that the braking disc is spinning andthat the sensor could be moved by a robotic arm in a circular trajectory while the measurementare supposed to take place.

3.8 Test system design

The entire system is based on the principle of resonance, where an LC circuit is tuned to theresonance frequency of that circuit. This resonance frequency can be adjusted by changing eitherthe coil or the capacitor to achieve a certain depth of the eddy currents that are induced in theconductive material. The reason for the AC power source to be set to the resonance frequency ofthe LC circuit is that the system gets more sensitive to small variances that interferes with the coilsmagnetic field. This occurs because the impedance of the LC circuit is the highest at resonancefrequency, this can be seen in the figure 7.

To be able to detect the variations in the voltage drop, an ADC was used to sample the voltagedrop across the coil and the conductor that is connected in parallel in the LC circuit. The phase-shift of the changes can also be taken into consideration to give a better estimation of the flaws sizeand depth, however, in this thesis the phase-shift was not taken into account in the estimations of

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Figure 5: A single flat coil with 9 turns

the flaws.The conceptional system design that was connected onto a breadboard can be seen in figure 9,

where circle 1 shows the three control signals from the Beaglebone Black, circle 2 shows a transistorcoupling with four transistors, circle 3 shows the multiplexer, circle 4 shows the 220 nF conductorand circle 5 shows the four flat PCB coils boards backside. When a conductive material is movedunder the PCB coils, eddy currents are induced into the material which interferes with the coilsmagnetic fields which can be seen on the change of voltage drop across the coil.

3.8.1 Analog-to-digital converter

An LTC2380CMS 16-bit analog-to-digital converter (ADC) that has a sampling rate of 2 Ms/s wasbought and could be used in this project to sample the change of voltage drop over the coils. TheLinear Demo Circuit 1783A-A board that has an LTC2380CMS mounted could be used in testpurpose to be able to sample the data and send it over USB to a computer. This could be done inorder to evaluate if the signal analysis methods that was going to be used worked as intended beforescaling the system to a smaller PCB with a smaller MCU. At 16-bit resolution and a sampling rateof 2 Ms/s the ADC samples 4 MB data per second. The need for storage is a crucial parameter totake into account since most MCUs only has a small flash storage integrated.

3.8.2 Multiplexer

To be able to switch between the flat coils on the PCB-probe a CD4051BC multiplexer was used.This multiplexer opens the gates of the transistors in a transistor coupling. This coupling deter-mines which out of 8 coil channels the current should be lead through. A Beaglebone Black wasused to send the control signals to the multiplexer so that the right coil was activated at the righttime. There is three control signals, A, B and C that can be used in a binary fashion to togglebetween 8 channels, see table 1. The multiplexer has a crosswalk between any two channels of 3MHz which is way more than what is needed in this project. The switching frequency is consideredto be around 8 kHz to be able to do one complete reading of the eight coils every millisecond, thathas been considered to be needed when the wheel is spinning at high speeds. If the measurementsare taking place at a speed of 90 km/h, each measurement will scan approximately 3 mm of thebraking disc, which has been considered the upper scan length limit. At lower speeds a lowersampling rate could therefore be used to still get a good estimation.

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Figure 6: Coil and resistor in series connected to an AC power source

C B A Output Channel0 0 0 10 0 1 20 1 0 30 1 1 41 0 0 51 0 1 61 1 0 71 1 1 8

Table 1: Multiplexer control signals and output channel

3.8.3 Transistors

In the transistor coupling that is controlled by the gate controlling multiplexer there is a need fortransistors that tolerates signals with a peak-to-peak voltage of 5 volt at tops 500kHz that alsohas low internal resistance and low current leakage. The bipolar NPN transistor 2N3053A werethe first transistor that was used, tolerates signals up to 7 GHz. However, this type of transistordrains the gate-current and since the multiplexer only can deliver a smaller than needed amount ofcurrent this transistor could not be used. Another way of doing this since the multiplexer only candeliver a small amount of current was to use MOSFET transistors instead. MOSFET transistorscan be controlled by a small amount of current and are mostly dependent on the voltage becauseof its insulated design. The MOSFET transistor type used for this system was a BSP171.

3.8.4 Probe design

A flat PCB coil design with 4 aligned coils was chosen to prove the concept of switching coils. Thecoils has 9 turns and an inductance of about 8µH each. Figure 10 shows the coils printed on aPCB. The idea of this type of probe is to sample on each coil to detect changes in the voltage dropover each coil while the braking disc is spinning underneath the coils. For each measurement, onepeak-to-peak average voltage is calculated and stored in memory. This process is repeated until thewhole braking disc has been scanned. When the data has been calculated and stored, an analysisto find variances in the sampled signal will take place and indicate if there is any disturbances. Aconceptional PCB designed in Ultiboard with a mounted multiplexer can be seen in figure 11.

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Figure 7: Illustration of how the impedance of a coil and an LC circuit is changing with thefrequency. The self-resonance of a coil is in general higher than the resonance frequency of an LCcircuit

3.8.5 Software

A Beaglebone Black Rev C (BBB) was used to generate the control signals to switch between thecoils of the probe, a BBB can be seen in figure 12. The BBB is running an AM335x 1 GHz ARMCortex-A8 processor, 4 GB on-board flash storage, 512 MB RAM and 64 controllable GPIO pins.The BBB comes with an installed Linux Debian operating system.

To control the switching between the coils a separate code needed to be implemented. Thesystem uses three GPIO pins of the BBB to generate a 3-bit value to control the multiplexer.Since the BBB is running an operating system, access to the GPIO pins are exposed by the kernelin the /sys/class/gpio directory. To enable the desired GPIO pins, echo the GPIO port numbersto the /sys/class/gpio/export file. When the ports are enabled one need to set them to either actas an input or an output, this is done by echoing in/out to /sys/class/gpio/gpioX/direction,where X is the GPIO pin number. When the GPIO is set to the desired mode the GPIO pin caneither be set or read by reading or writing to the GPIO pins. To read from the GPIO pin, simplyuse the catenate (cat) command, like cat /sys/class/gpio/gpioX/value. The GPIO pin is setto a desired low (0 V) or high (5 V) value by echoing 0 or 1 to the /sys/class/gpio/gpioX/valuefile. A coil switching program written in C controls the switching between the GPIO pins in theright sequence to activate the desired coil. This sequence switching should preferably be done 8times a millisecond to get one complete reading every millisecond. Figure 13 shows a state-machineillustration of the code running on the BBB.

Another important part of the switching program is to indicate which coil is activated. Thisis important because another program will be running in parallel in a separate thread that needsto know which coil it is sampling on. A state-machine representation of the sampling system canbe seen in figure 14. In the initial state of the sampling system an 8 by N integer array/matrix iscreated, where 8 is the number of coils that will be used and N refers to the number of completescans (1 on each coil) that will be done. Another initialisation done in the beginning of the code isa temporary sampling buffer which holds all the samples that will be done on each coil. When the

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Figure 8: Coil and conductor in parallel with a resistor in series connected to an AC power source

initialisation is done, the ADC starts to sample and store the samples into the temporary buffercreated in the initial state. The sampling continues until the coil is switched. When the coil hasbeen switched, the absolute mean value is calculated by adding together all the absolute valuesof the samples and dividing them with the number of samples. The absolute mean value is thenstored in the 8 by N array. If the total scan of the entire disc is not completed, the system willstart over again and do a new scan of all the coils, this is repeated until the entire disc has beenscanned. When the disc has been scanned entirely further analysis of the sampled data will takeplace. Each row of the 8 by N array will be analysed separately, since each row represents onecoils’ readings throughout the scan. Any deviations in the signal indicates discontinuities in thetarget material. The size of the deviations indicates the size of the flaw in the braking disc.

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Figure 9: Conceptional breadboard connected circuit. 1, Control signals to control the multiplexer.2, Transistor coupling. 3, Multiplexer. 4, 220 nF conductor. 5, PCB coil board.

Figure 10: 4 flat coils printed on a PCB

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Figure 11: Concept design of a 4 coiled PCB card with a mounted multiplexer on the backside

Figure 12: Beaglebone Black, picture taken from [13]

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Figure 13: State-machine representing the system that is controlling the multiplexer

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Figure 14: State-machine representing the sampling system

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4 Results

This section presents the results of this thesis work. The results are presented with pictures andtexts to explain what has been accomplished.

4.1 Test system design

The design of the test system consists of a BBB, multiplexer, conductor, coils, resistor, oscilloscope,function generator and a lot of wires. The multiplexer, conductor, coils and resistor are connectedon a breadboard, which can be seen in figure 9.

The function generator generates an AC current to drive the circuit while an oscilloscope isconnected to measure the voltage drop over the active coil. A program is running on the BBBthat controls 3 GPIO ports on the BBB. These 3 GPIO pins acts as an input to the multiplexerto choose between 8 different outputs, in this thesis only 4 ports were used to prove the conceptof switching coils. The outputs of the multiplexer controls a MOSFET transistor coupling whichdid work, but with some current leakage which interfered with the signal.

A voltage drop indicating that a conductive material was interfering with the magnetic fieldgenerated by the active coil was seen. This variation was seen when an adjustable spanner madefrom cast iron was moved underneath the coil. The results from this test are explained further insection 4.2.

4.2 Coils with potential

The flat PCB coils, seen in figure 10, proved that the concept of switching between coils workedas intended with some varying features of the coils due to some limitations of the milling machine.However, these coils were not perfect in the sense that they had a very low inductance, which gavea very high resonance frequency when combined with a 220 nF conductor. A higher conductanceshould give a lower resonance frequency of the circuit, which would lead to an increased penetrationdepth. The conductor was changed to a conductor with higher conductance, without being ableto achieve the desired resonance frequency.

The coil that showed the most impressive results were the type of coil seen in figure 1. This coilis relatively expensive when compared to the printed flat PCB coils but are less sensitive to noise.The combination of this coil and a 220 nF capacitor results in a resonance frequency of 22.8kHz,which in the case of inspecting cast iron materials gives a desirable penetration depth. The figure15 shows a setup where a single coil is held up by a soldering stand. The back end of an adjustablespanner was used to prove the concept of detecting discontinuities in a cast iron specimen.

Figure 15: Single coil placed about 2 millimeters above an adjustable spanner made of cast iron

The results from this configuration showed that a noticeable change in the voltage drop occurred

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when the adjustable spanner was placed about 2 millimeters underneath the coil. The peak-to-peakvoltage drop in this case dropped from 328.68 mV down to 300.96 mV. These measured values canbe seen in figure 16 and 17 respectively.

Figure 16: No conductive material underneath the coil - Peak-to-peak voltage of 328.68 mV

Figure 17: Adjustable spanner directly underneath the coil - Peak-to-peak voltage of 300.96 mV

The results from this test indicates that there is a gradual sweet spot between 300 mV and 330mV which indicates variations of depth in the target material. This sweet spot was not calibratedfor the depth during the project since the ADC was not implemented. The variations were onlyobserved on an oscilloscope.

The same coil configuration was set up to be compared to a scanCONTROL 2600 laser scannerat Robotdalen in Vasteras. The setup can be seen in figure 18. The idea of this test was to provethat this sensor could indicate variations in a braking disc and also see how well a laser scannerwould perform. The setup consisted of a rig for spinning a braking disc based on the same principleused for spinning food in a micro wave oven, this rig was built by an employee at Robotdalen. This

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rig allowed to spin the braking disc while at the same time doing measurements with any sensor.The spinning simulated the fact that the sensor will do its analyses while the wheel is spinning. Noswitching circuitry was used during this test since the task for this test was simply to prove thatthe concept worked. The AC power source was set to 5 Volts at 22.8 kHz which is the resonancefrequency of the circuit. When letting the sensor hover above and pass by a visual crack, a clearvariation in the voltage drop was observed on an oscilloscope. This proves that the sensor reactsto discontinuities in braking discs. The scanCONTROL 2600 laser scanner was configured to scana diagonal line of the spinning braking disc so that the whole braking disc could be scanned. Thislaser scanner gave a pleasing crack depth estimation with a resolution down to 2 µm.

Figure 18: A scanCONTROL 2600 laser scanner to the left and the developed conceptional eddycurrent sensor to the right with one coil operating

These variations in the voltage drop across the coil shows that the system is sensitive to changesin conductive materials, which has been the task all along. Due to a limited amount of time thevariations has not yet been calibrated to give an estimation of the size of the discontinuities.

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5 Discussion

This thesis work has given a better technical understanding of generated magnetic fields, conductivematerials, electronics in general, controlling an industrial robot, C programming as well as projectmanagement. To get the responsibility of developing a system from scratch with great access tocomponents and materiel has been very pleasing. An important lesson that has been learnt fromthe project is that to always count with delays and redefinitions of the project. During the thesiswork, some parts had been promised to be delivered at a certain time but had either been delayedor not been delivered at all. These are things that should be considered from the beginning ofprojects and alternative ways of doing things should always be kept in mind so that the work cancontinue.

During the first 3 weeks some redefining of the project definition had to be done after anobservation at Volvo Truck Center in Enkoping. This redefinition meant that the sensor to bedeveloped had to be a low-cost system, since it would potentially be used on every Volvo truck atall time. This observation showed the importance of communicating with the people on site andlistening to their feedback. This is an important lesson that has been learnt during the project.

In general, ordered components have been delivered in time. Although, during the middleparts and the end of the project some late deliveries of components and materiel has delayed thedevelopment of some parts of the system. Thankfully, these late delivered components have notbeen crucial to prove the concept of the system.

Throughout the thesis work, different approaches to generate and sense changes in magneticfields has been tested. Based on these tests and doing research in NDT-methods, a couple ofconcepts has been tested and evaluated with varying results. Because of time related reasons,some ideas has not yet been fully tested and could be considered to be tested if someone continueson with the project. These ideas are believed to be working and the components that is neededwas delivered.

The sensor has not been fully developed which was the goal from the beginning. However, thesystem constructed to prove the concept of detecting cracks in metallic structures has been provensuccessful. How reliable and accurate the system is has not yet been tested, but changes in themetallic structures such as cracks has been observed on an oscilloscope. Since this sensor still isin a conceptional state, there is still things to work on before a working sensor with the right sizeand price can be developed.

Overall, this thesis work has been very interesting and given deeper knowledge in differentresearch fields. The communication with teachers, professors, workers and supervisors have beengreatly appreciated.

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6 Conclusions & Future work

The idea of a sensor based on induced eddy currents that can sense discontinuities in brakingdiscs has been proven successful. Since the sensor has shown great potential with relatively cheapcomponents, it is confirmed that induced eddy currents could be the main technique to be used ina system for detecting cracks in braking discs.

The author of this report sees good potential in this solution, because it have displayed pleasingresults at a low cost. Some further development and optimizations and the system could potentiallybe implemented on the Volvo Trucks.

Based on the shown results, the conceptional sensor developed during this thesis work couldpotentially be built into a reliable system. This system could warn the driver that the brakingdiscs are disrupted, avoiding potential hazards.

The purpose of this thesis was to investigate how a crack detecting system could be designed,in order to detect cracks in braking discs. The author of this thesis is pleased with the resultsand thinks that with the knowledge gathered during the project, which has been captured in thisreport, a sensor could be built that fulfills the goals of the thesis.

6.1 Future work

Due to some time related reasons, everything that had been planned to test has not yet been tested.These theoretical ideas has been discussed with the supervisor of the project Mikael Ekstrom andalso with Brian Cox.

First of all, since the conceptional system is connected on a breadboard using wires, a PCBlayout for the entire system should be considered to reduce the amount of noise in the system. Aproper shielding of the entire system should be considered as well.

The sensor is today depending on a function generator and an oscilloscope. The functiongenerator should be switched to a compactly designed sinus generating circuit that can generate asinusoidal signal at the resonance frequency of the circuit.

Instead of using the BBB, another cheaper PCB-mounted MCU could be considered that canstore the sampled data from an ADC. Another option to the ADC is to consider using an LDC1051that calculates and converts inductance into a digital signal which can be sent over SPI to the MCU.

An overview of the desired conceptional complete system can be seen in the figure 19. AnMCU should be scaled to fit the purpose of the system. The MCU controls the multiplexer whichenables and controls the switching of coils. To sample the signal variations for further analysis,implementation of the inductance to digital converter LDC1051 or an ADC could be considered.

Figure 19: State-machine representation of the conceptional complete system

Since cracks in conductive materials also shifts the phase of an induced current, one improve-ment could be to also take the phase shift into consideration when doing the crack depth estima-tions. The phase shift is often taken into consideration in other EDT studies and applications [14][2].

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7 Time Plan

Tasks Duration Start FinishState-of-the-art research and interviews 3 weeks Mon 19/1-15 Tue 10/2-15

Status and planning seminar 1 day Wed 11/2-15 Wed 11/2-15Concept development and writing report 13 weeks 5 days Thu 12/2-15 Tue 19/5-15

Report submission date 1 day Wed 20/5-15 Wed 20/5-15Preparing the presentation 2 weeks Thu 21/5-15 Thu 4/6-15

Demonstration of the system

The state-of-the-art research and interviews were completed in the first 3 weeks as planned.A status and planning report was written and handed in to the examination committee, followedby a status and planning seminar. The concept development was ongoing in parallel with writingdaily notes of the progress. These notes reflected my thoughts, progress and what people hadsaid during the interviews. The main focus until the end of April was to develop a reliable andcost effective conceptional sensor system before starting to put the report together. The last threeweeks before the report submission date were mostly focused on the report writing. The time planwas in general held quite well with some minor delays.

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References

[1] “Non-destructive testing in the aerospace industry,” 2015. [Online]. Available: https://www.nde-ed.org/AboutNDT/SelectedApplications/AircraftInspection/Aircraft%20Inspection.htm

[2] P. J. H. Geirinhas Ramos, A. Lopes Ribeirol and J. Neskudla, Eddy Current Testing of Con-ductive Materials, 2008.

[3] P. Schlund, personal communication, 2015.

[4] Y.-M. Q. Xiao-Me Pei, Hong-Sheng Liang, A frequency spectrum analysis method for eddycurrent non-destructive testing, 2002.

[5] B. Petkovic, Assessment of Linear Inverse Problems in Magnetocardiography and LorentzForce Eddy Current Testing, 2013.

[6] “Pulsed eddy current testing,” 2015. [Online]. Avail-able: http://www.tuv.com/en/corporate/business customers/materials testing andinspection/advanced ndt/pulsed eddy current/pulsed eddy current.html

[7] F. A. Administration, Acceptable Methods, Techniques and Practices - Aircraft InspectionRepair and Alterations, 1998.

[8] N. Petrovic, personal communication, 2015.

[9] “Probes - mode of operation,” 2015. [Online]. Avail-able: https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/ProbesModeOp.htm

[10] P. M. R. P. V. M. P. Luis S. Rosadoa, Telmo G. Santos, A differential planar eddy currentsprobe: Fundamentals, modeling and experimental evaluation, 2012.

[11] “Probes - depth of penetration & current density,” 2015. [Online]. Avail-able: https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm

[12] B. Cox, personal communication, 2015.

[13] [Online]. Available: http://regmedia.co.uk

[14] P. M. R. M. P. Diogo E. Aguiam, Luis S. Rosado, Portable Instrument for Eddy CurrentsNon-Destructive Testing based on Heterodyning Techniques, 2014.

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https://www.nde-ed.org/AboutNDT/SelectedApplications/AircraftInspection/Aircraft%20Inspection.htm

https://www.nde-ed.org/AboutNDT/SelectedApplications/AircraftInspection/Aircraft%20Inspection.htm

http://www.tuv.com/en/corporate/business_customers/materials_testing_and_inspection/advanced_ndt/pulsed_eddy_current/pulsed_eddy_current.html

http://www.tuv.com/en/corporate/business_customers/materials_testing_and_inspection/advanced_ndt/pulsed_eddy_current/pulsed_eddy_current.html

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/ProbesModeOp.htm

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/ProbesModeOp.htm

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm

http://regmedia.co.uk

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