University of Twente
EEMCS / Electrical Engineering
Control Engineering
Magnetic Flux Leakage Floorscanner
Maarten Koster
Individual Design Assignment
Supervisors:
prof.dr.ir. P.P.L. Regtien
ir. W. van Hoorn, ApplusRTD
ing. H. Rundberg, ApplusRTD
September 2008
Report nr. 029CE2008
Control Engineering
EE-Math-CS
University of Twente
P.O.Box 217
7500 AE Enschede
The Netherlands
1 Summary This study is performed as a bachelor assignment at the University of Twente in cooperation with Applus RTD. It is a preliminary study because the total labor time is limited. The assignment is to improve an existing inspection device for large storage tanks called the Floorscanner. A leaking storage tank can threaten public health, pollute the environment and lead to millions of direct costs. The main reason for storage tank failure is corrosion; this can weaken or destroy components of the tank system, causing holes or possible structural failure. An important problem is measuring the remaining wall thickness and the detection of defects on the floor. For this application the Floorscanner has been developed by Applus RTD. The Floorscanner however superseded by new techniques and competitors. In this study new techniques are investigated for a future Floorscanner. To prevent a narrow approach nearly all commonly used nondestructive testing techniques have been included in the preliminary investigation. Based on the objectives of the Floorscanner and preferences of Applus RTD this is narrowed to Magnetic Flux Leakage. Next techniques as Magnetic Reluctance, Pulsed Magnetic Flux Leakage and Barkhausen Noise are presented for possible integration with magnetic flux leakages. A combination of these techniques is proposed to improve the detection and characterization of defects. For the detection of flux leakage, sensitive magnetic field sensors are presented and compared. Three different sensor techniques: AMR, GMR and GMI are used in the experiments. The rest of the research is more experimental and accompanied with FEMM simulations and experiments. A core with the two electromagnets is used as a starting point for simulations and experiments. First the permeability of the core material is determined. Next the plate permeability and the influence of an air gap is investigated. Finally the AMR/GMR sensors are calibrated. Further simulations and experiments focus on MFL; defects with varying depth and diameter are scanned. During the experiments variables as: magnetization current and lift off are introduced and the results are analyzed. The output of the GMR sensor and coil is moreover analyzed by a spectrum analyzer for the detection of possible Barkhausen Noise. Also experiments with Pulsed Magnetic Flux Leakage are performed. Magnetic Flux Leakage Floorscanner Page ‐ 1 ‐ 2 Table of content 1 SUMMARY ......................................................................................................................................... ‐ 1 ‐ 3 INTRODUCTION .................................................................................................................................. ‐ 4 ‐ 4 OBJECTIVES ........................................................................................................................................ ‐ 5 ‐ 5 NON DESTRUCTIVE TEST TECHNIQUES ................................................................................................ ‐ 6 ‐ 5.1 5.2 5.3 5.4 5.5 5.6 6 RADIOLOGY: X‐RAY AND GAMMA ................................................................................................................ ‐ 6 ‐ ULTRASONIC ............................................................................................................................................ ‐ 7 ‐ EDDY CURRENT ......................................................................................................................................... ‐ 8 ‐ MAGNETIC FLUX LEAKAGE (MFL) ................................................................................................................ ‐ 9 ‐ MAGNETIC PARTICLE ................................................................................................................................ ‐ 10 ‐ CONCLUSION .......................................................................................................................................... ‐ 10 ‐ EXTENDED “MFL” TECHNIQUES ........................................................................................................ ‐ 11 ‐ 6.1 PULSED MAGNETIC FLUX LEAKAGE ............................................................................................................. ‐ 11 ‐ 6.2 MAGNETIC RELUCTANCE .......................................................................................................................... ‐ 12 ‐ 6.3 MULTIDIRECTIONAL FLUX FIELD ................................................................................................................. ‐ 13 ‐ 6.4 BARKHAUSEN NOISE ................................................................................................................................ ‐ 13 ‐ 6.4.1 Magnetic Induced Potential Noise ............................................................................................ ‐ 14 ‐ 6.5 COMBINING TEST METHODS ...................................................................................................................... ‐ 14 ‐ 6.6 CONCLUSION .......................................................................................................................................... ‐ 14 ‐ 7 MFL MEASUREMENT SYSTEM ........................................................................................................... ‐ 15 ‐ 7.1 MAGNETS .............................................................................................................................................. ‐ 15 ‐ 7.1.1 Magnets and laminated iron core simulation. .......................................................................... ‐ 16 ‐ 7.2 MAGNETIC FIELD SENSORS TECHNIQUES ....................................................................................................... ‐ 17 ‐ 7.2.1 Coil ............................................................................................................................................. ‐ 17 ‐ 7.2.2 Hall ............................................................................................................................................ ‐ 17 ‐ 7.2.3 Anisotropic Magnetoresistive (AMR) ........................................................................................ ‐ 17 ‐ 7.2.4 Giant Magnetoresistive (GMR) .................................................................................................. ‐ 18 ‐ 7.2.5 Giant Magnetic Impedance (GMI) sensors ................................................................................ ‐ 18 ‐ 7.2.6 Fluxgate ..................................................................................................................................... ‐ 18 ‐ 7.2.7 Superconducting Quantum Interference Device (SQUID) and other low field sensors .............. ‐ 18 ‐ 7.2.8 Conclusion ................................................................................................................................. ‐ 19 ‐ 7.3 MAGNETIC FIELD SENSORS ........................................................................................................................ ‐ 19 ‐ 7.3.1 HMC1052 (AMR) ....................................................................................................................... ‐ 20 ‐ 7.3.2 AAH002‐02 (GMR) ..................................................................................................................... ‐ 20 ‐ 7.3.3 AGMI302 (GMI) ......................................................................................................................... ‐ 21 ‐ 8 TESTING ........................................................................................................................................... ‐ 22 ‐ 8.1 8.2 8.3 8.4 9 CORE PERMEABILITY ................................................................................................................................. ‐ 22 ‐ TEST PLATE PERMEABILITY ......................................................................................................................... ‐ 24 ‐ INFLUENCE LIFT OFF ON PERMEABILITY ......................................................................................................... ‐ 26 ‐ CALIBRATING THE GMR SENSOR ................................................................................................................ ‐ 28 ‐ SIMULATIONS .................................................................................................................................. ‐ 29 ‐ Magnetic Flux Leakage Floorscanner Page ‐ 2 ‐ 9.1 10 CONCLUSION .......................................................................................................................................... ‐ 32 ‐ MFL TESTING ................................................................................................................................ ‐ 33 ‐ 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 11 VARIABLE DC CURRENT STATIC SITUATION .................................................................................................... ‐ 33 ‐ DEFECT DEPTH ........................................................................................................................................ ‐ 33 ‐ VARIABLE DC CURRENT, DYNAMIC SITUATION ............................................................................................... ‐ 35 ‐ LIFT OFF (VARIABLE AIR GAP) ..................................................................................................................... ‐ 36 ‐ DEFECT DIAMETER ................................................................................................................................... ‐ 37 ‐ CRACK .................................................................................................................................................. ‐ 38 ‐ BARKHAUSEN NOISE ................................................................................................................................ ‐ 39 ‐ PULSED MAGNETIC FLUX LEAKAGE ............................................................................................................. ‐ 40 ‐ CONCLUSION .......................................................................................................................................... ‐ 40 ‐ CONCLUSION ................................................................................................................................ ‐ 41 ‐ 11.1 RECOMMENDATIONS ............................................................................................................................... ‐ 42 ‐ 12 REFERENCES ................................................................................................................................. ‐ 43 ‐ 13 APPENDIX .................................................................................................................................... ‐ 44 ‐ 13.1 CALIBRATION OF AGMI302 MAGNETIC FIELD SENSOR .................................................................................... ‐ 44 ‐ 13.2 GMR SENSOR CALIBRATION: LABVIEW INSTRUMENT ...................................................................................... ‐ 44 ‐ 13.3 HYSTERESIS MEASUREMENT: LABVIEW INSTRUMENT ...................................................................................... ‐ 45 ‐ Magnetic Flux Leakage Floorscanner Page ‐ 3 ‐ 3 Introduction A leaking storage tank can threaten public health, pollute the environment and lead to millions of direct costs. The main reason for storage tank failure is corrosion. Corrosion is caused by the deterioration of a material, usually from a reaction with the environment inside and outside the tank. Most tanks are made of steel, a material vulnerable to corrosion. Corrosion can weaken or destroy components of the tank system, causing holes or possible structural failure. The stored products will penetrate into the ground, and hence harm the environment. An important problem is measuring the remaining wall thickness in tanks. Indications obtained during inspection need to be interpreted and evaluated. Any indication that is found is a possible defect. It is essential to know whether the possible defect is likely to shorten the life or performance of the tank. The defect cannot be found by visual detection because there is only access to the top surface of the floor. For the inspection a technique is needed that can detect corrosion on the underside and can give a full coverage of the floor. For this application the Floorscanner has been developed by Applus RTD. The Floorscanner uses Eddy currents to detect defects, but is at the moment superseded by new techniques and competitors. At present Applus RTD requested help of the University Twente. The assignment is a theoretical/practical research for a new Floorscanner. The new Floorscanner should be based on Magnetic Flux Leakage. Figure 1 Floorscanner developed for the first experiments Magnetic Flux Leakage Floorscanner Page ‐ 4 ‐ 4 Objectives The new Floorscanner has to meet the following requirements as described by Applus RTD. ‐
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Capable of testing plates up to 16mm thick Detecting defects on surface and on the underside of the plates. Visualize the depth of the defect. Detect different sorts of defects and characterize these. Resolution better than 1mm for small defects in both the horizontal and vertical direction of the plate. Portable and suited to inspect a large amount of space inside the tank Entrance of a tank 0.5 m in diameter, compact dimensions that fit through the hole Fast: inspection of tank 100 m diameters should take less than 1.5 day Inspection must be automated Self propelled Average one inspection in 5 years Apart from the demands, there are several obstructions inside the tank. The Floorscanner has to deal with these obstructions to cover up the floor. The obstructions are: ‐ Heating tubes ‐ Oil drains ‐ Pillars below the plates to support the floor ‐ Plate crossings More complications: ‐ Walls not perpendicular to the floor. ‐ Several layers of coating on the floor (up to 6mm) ‐ Composition of the material not similar ‐ Rough surface Magnetic Flux Leakage Floorscanner Page ‐ 5 ‐ 5 Non destructive test techniques The old Floorscanner uses a non destructive test technique called Eddy current to inspect the tank floor for defects. Besides Eddy current there are several other non destructive techniques for possible testing. Non destructive testing includes all the test techniques in which the material is tested without harming its future usefulness. Not all Non destructive Test (NDT) techniques can detect all sorts of defects; each one has its own limitations. Care is required for selecting the best technique for an inspection task. It’s also possible to use more than one technique; this provides additional information and enhanced inspection. In this chapter commonly used non destructive test techniques are described. Furthermore a consideration is made whether the demands of the Floorscanner could be realized. This chapter intends to give a general overview of all possible techniques to prevent a narrow approach that only includes magnetic flux leakage. 5.1 Radiology: X­ray and gamma X‐rays are generated electrically and Gamma rays are emitted from radio‐active isotopes. Both are penetrating radiation which are absorbed by the material through which it passes. The greater the thickness and material density, the greater the absorption. To produce an X or Gamma radiograph a film is placed close to the surface of the object (Figure 2). The source of radiation is positioned on the other side of the object some distance away, so that the radiation passes through the object and on to the film. After the exposure the film can be viewed by transmitted light on a viewer. i
Figure 2 principle radiography Advantages: ‐ Suitable for almost any material ‐ 2D images of an area Disadvantages: ‐ Not suitable for thick materials: max 5mm steel (relation between power of radiation source and radiation penetrating depth) ‐ No information about the depth of defects ‐ Two‐sided access is needed ‐ Possible health hazard ‐ Not suitable for automation. Conclusion: Radiology has some major disadvantage in automation, penetration and depth information. The demands for the Floorscanner cannot be realized with radiology. Magnetic Flux Leakage Floorscanner Page ‐ 6 ‐ 5.2 Ultrasonic The ultrasonic test method uses high frequency (1‐ 10 MHz) ultrasound to detect defects and or material properties (Figure 3). The transducer is controlled by the pulser and transmits a pulse of sound energy into the material. The sound energy propagates through the materials in the form of sound waves. When there is a defect, part of the energy will be reflected back. The reflected wave is transformed into an electrical signal by the transducer and is displayed on a screen. Reflections from the back surface of the material are detected by the same transducer. The travel time is related to the distance that the signal has traveled. The travel time and amplitude give information about the defect depth, size and orientation. It is necessary to make the wavelength smaller or equal to the expected defect size. For the detection of small defects it is necessary to use high frequency. The higher frequency, the smaller is the penetration depth. Figure 3 principle ultrasonic testing ii Advantages ‐ Ultrasonic attenuation depends on defect size, depth and stress ‐ By measuring ultrasonic velocity, information about stress can be obtained. ‐ Can be used in the plane normal to the surface or under an angle. ‐ Sensitive to both surface and subsurface defects ‐ Only single side access is needed ‐ Instantaneous results ‐ Can be used for thickness measurements ‐ Non conductive materials can be inspected Disadvantages ‐ Surface must be accessible ‐ Skill and training needed to interpreted measurements. ‐ Not suitable for full automation ‐ Requires a coupling medium ‐ Difficult to inspect materials that are: rough, grained, irregular of shape or not homogeneous. Therefore service preparation needed for better/easy testing ‐ Defect oriented parallel to the sound beam may go undetected ‐ Reference standards required for calibration and characterize the defects. ‐ Not sensitive for surface defects: erratic effects of the probe’s near field and ring down
‐ Thin surface can be difficult to inspect
‐ Interferences from sound pulse, reflections from tank wall
Conclusion: Ultrasonic has the disadvantage that it requires a coupling medium and surface preparation. However the demands for the Floorscanner could mostly be realized with ultrasound. Magnetic Flux Leakage Floorscanner Page ‐ 7 ‐ 5.3 Eddy Current Eddy Current testing uses the principle of electromagnetism. The Eddy Currents are created by electromagnetic induction. When a coil is excited with sinusoidal current (f= 5 Hz‐ 10 MHz) a magnetic field develops in and around the coil. If another electrical conductor is brought into this changing magnetic field, current will be induced in this second conductor. Eddy Currents are induced electrical currents that flow in a circular path. These currents create a secondary electromagnetic field that opposes the effect of the applied magnetic field (Lenz’s law). At regions of defects, material property variations and surface characteristics distort the Eddy Currents. The secondary field changes and so does the coil impedance, these impedance changes are measured and correlated with the defects (Figure 4). Figure 4 principle Eddy Current iii Advantages: ‐ Pulsed Eddy Current can give information about the depth of the defect. Detecting sub‐surface defects. ‐ Can inspect complex shapes ‐ Instantaneous results ‐ Can be used for thickness measurements ‐ Test probe does not need to contact the inspected surface ‐ Only single sided access is needed ‐ High inspection speeds possible (5m/s) ‐ Can detect very small defects ‐ No contact needed with inspected surface ‐ Suitable for full automation ‐ Simple ‐ Low cost Disadvantages ‐ Only conductive (and/or ferromagnetic) materials can be inspected ‐ Surface must be accessible ‐ Skill and training needed to interpreted measurements ‐ Reference standards required for calibration and characterize the defects. ‐ Defect oriented parallel to the probe coil may go undetected ‐ Maximum inspectable thickness 15mm ‐ Difficult to isolate different parameters which affect eddy current response ‐ Best suitable for surface or near surface defects Conclusion: Eddy Current has the disadvantage that the signals are difficult to interpret, and it can be used with difficulty for thick materials. However the demands for the Floorscanner could mostly be realized with Eddy Current. Magnetic Flux Leakage Floorscanner Page ‐ 8 ‐ 5.4 Magnetic Flux Leakage (MFL) The material under test has to be magnetized. This is done by a permanent magnet near the surface. The magnetization generates magnetic flux in the material. A defect inside the material will cause an abrupt change of magnetic permeability. The permeability of the defect is lower than outside the defect. This high resistance forces the flux to take a different route. In cases where the other routes are saturated, flux leaves the material to the surrounding space causing flux leakage (Figure 5). This leakage can be detected by a magnetic sensor located between the two poles of the magnet near the tested material surface (Figure 6). . Figure 5 principle Magnetic Flux leakage iv Figure 6 MFL testing Advantages ‐ Leakage flux depends on the size, orientation, level of magnetization and inspection speed of the defect ‐ Automatic testing and quantitative evaluation without human inspectors ‐ MFL unit can be portable/ compact and battery powered ‐ Only single side access is needed ‐ No contact needed with inspected surface ‐ Suitable for full automation ‐ Simple ‐ Low cost Disadvantages ‐ Only (ferro)magnetic materials can be inspected (narrowing conducting materials) ‐ Surface must be accessible ‐ Skill and training needed to interpret measurements ‐ Reference standards required for calibration and characterization of the defects ‐ Defects oriented in parallel to the probe coil may go undetected ‐ Maximum inspectable thickness 15mm ‐ Difficult to isolate different parameters which affect magnetic flux leakage response ‐ Best suitable for surface or near surface detects Conclusion: MFL has the disadvantage that the signals are difficult to interpret, and it can be used with difficulty for thick materials. However the demands for the Floorscanner could mostly be realized with MFL. Magnetic Flux Leakage Floorscanner Page ‐ 9 ‐ 5.5 Magnetic particle Magnetic particle testing is a combination of magnetic flux leakage and visual testing. First the material that is to be tested needs to be magnetized. This is done by a permanent magnet near the surface. If any defects are present the defects will create a magnetic flux leakage field. The flux will stray out into the air and the “defect” becomes magnetic (Figure 7). To visualize the leakage flux, iron particles in a dry or wet form are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, forming a visible indication. Figure 7 magnetic particle testing v Advantages ‐ Simple ‐ Low cost ‐ Direct view of results ‐ Only single side access is needed Disadvantages ‐ Can detect only surface or near surface defects ‐ No depth indication and orientation defects ‐ Only ferromagnetic materials can be inspected ‐ Consumables needed ‐ Difficult to automate, needs eye inspection Conclusion: Magnetic particle has the disadvantage that iron particles in a dry or wet form have to be applied to the surface therefore demands for the Floorscanner cannot be realized with Magnetic particle. 5.6 Conclusion Five commonly used non destructive testing techniques are introduced. Every technique has it own applications and limitations. Based on the objectives earlier specified three techniques have potential to be used in the Floorscanner. These techniques are: magnetic flux leakage, Eddy current and to a lesser extent ultrasonic. At this point Applus RTD has a preference for Magnetic Flux leakage. In the next chapter the focus will be on magnetic flux leakage. Magnetic Flux Leakage Floorscanner Page ‐ 10 ‐ 6 Extended “MFL” techniques Magnetic flux leakage testing has the advantage that it is compact, low cost, uses no consumables and only single sided excess is needed (Figure 8). However the technique is not very sensitive to small defects because signals are caused by both defects as well as permeability variations in the material. Because of these permeability variations there is need for calibration. Even after the calibration, characterization of defects is difficult. The characterization of defects is normally done by a skilled operator to interpret the signals correctly. By automation of the characterization new development are needed for extraction of defects. Figure 8 MFL test setup vi This chapter will suggest some techniques that can be integrated with magnetic flux leakages to improve the defect detection and characterization. A combination of these techniques is proposed in order to improve the detection and characterization of defects. Because of conclusions from the previous chapter and the preference of the client this chapter will focus on magnetic flux leakage, especially enhancement with new techniques and recent developments. 6.1 Pulsed Magnetic Flux Leakage Pulse Magnetic Flux Leakage (PMFL) is an extension of normal MFL and can provide additional depth information of the defect. With PMFL the magnetic field is driven by AC current or a square waveform current which includes many frequency components (in this study a sinus is used). This is different compared to MFL which uses permanent magnets or electromagnets that are driven by a DC current. The use of frequency components provides information from different depths in the tested material due to the skin effect. The skin effect provides the skin depth which is a measure for the penetration depth of a magnetic field in a conductor. The skin depth is defined as the distance below the surface of the material at which the current density decays with a factor 1/e of the current density at the surface. f = frequency, = permeability test material, = conductivity test material The skin depth of a magnetic field decreases with the applied frequency. High frequency excitation will create a magnetic field that is concentrated at the surface of the tested material. Low frequency excitation will have a greater penetration depth and provides information about the depth of the defect. It is easy to produce a square wave; this can be done with simple digital electronics. Pulsed excitation creates magnetic fields with several frequency components at the same time. The frequency components of a pulsed signal vary with time, high frequency field components at the start of the waveform, reducing in frequency as time continues. Variations in the induced signal at the beginning of the excitation pulse will correspond to surface details; signals induced towards the end of the excitation pulse will correspond to details deeper in the material. Magnetic Flux Leakage Floorscanner Page ‐ 11 ‐ For the steel test plate (chapter 8.1): = 2122 , = 107 The skin depth of the magnetic field is calculated for frequencies used in the experiments X(f = 0Hz) = ∞ (magnetic fields will penetrate through the entire plate) X(f= 10Hz)= 1.2 µm X(f=20Hz) = 0.9 µm The calculations show that a static magnetic field penetrates through the entire test plate. This way the detection of surface and bottom‐side defects can be done simultaneously, every defect is detected. However the discrimination between surface and bottom‐side defects is more difficult, the depth at which a defect is positioned cannot be determined. In a situation with two defects on top of each other, no distinction can be made between the two because only one MFL signal is received. For low frequencies (10Hz, 20Hz) the skin depth is in the micrometer range. A possible application of PMFL can be corrosion detection. Corrosion will mostly start at the surface and can be detected in the first stages, before any harm can be done. For testing of holes and scratches a skin depth of several millimeters is required. For testing of bottom‐side defects an even larger skin depth is required. This cannot be realized by pulsed magnetic fields, only static fields can penetrate this deep. 6.2 Magnetic Reluctance A technique called Magnetic Reluctance can offer complementary defect information; it measures the magnetic reluctance of the excitation magnet circuit, this can be done using a pickup coil wrapped around the magnetic circuit (Figure 9). Magnetic reluctance is analogous to resistance in an electric circuit. It is a measurement of the opposition of a material to an applied magnetic field. The magnetic reluctance of a lossless, uniform magnetic circuit is given by: = magnetic permeability, l = length flux path, S = cross section area flux path The magnetic reluctance is sensitive to both geometrical and permeability changes in the material. This gives complementary defect information. The setup with pickup coil (Figure 9) is first used for permeability testing (chapter 8). Figure 9 Magnetic Reluctance setup This technique is relatively simple to integrate with MFL; the only demand is a pickup coil for flux measurements. The signal conditioning and processing can be done in parallel with the MFL measurements. Magnetic Flux Leakage Floorscanner Page ‐ 12 ‐ 6.3 Multidirectional Flux Field The principle of the Multidirectional Flux Field depends on the simultaneous imposition of a longitudinal flux field produced by the electromagnets and a circulating field produced by two electrodes (Figure 10). With changes in amplitude relative to each of the imposed longitudinal and circulating flux fields the vector angle will change to reveal more details about defects. Figure 10 multi directional flux field, yellow arrow: tangential field produced by electrodes, green arrow: flux produced by two electromagnets In Figure 10 a DC current source is used, resulting in a static magnetic field (yellow arrows). By using and AC current an alternating magnetic field can be produced. This technique can only be used when the tank‐floor is electrically accessible, so floors without isolating coating on top. Another disadvantage is the extreme large current that is necessary to produce a measurable circulating flux field. These extreme high currents are necessary because of the low resistivity of the plate material. Extreme high currents do also put high demands on the two contact points on the material. The applicability of this technique is very limited in tank inspection because a tank floor has often a rough surface and a non conducting material on top. Moreover it is difficult to produce high currents in an experimental situation; therefore, this technique will not further investigated. 6.4 Barkhausen Noise Ferromagnetic materials are characterized by spontaneous magnetization. The magnetic structures consist of unaligned magnetic domains to minimize the free energy of the material. External magnetic fields rearrange the magnetic domains; this magnetization process is very sensitive to the material structure. Therefore, the magnetization process is very sensitive to the structure of the material and can give more information of defects. Magnetization changes within a material are not continuous processes. There are discontinuous in the magnetization. These discontinuities occur randomly and are known as Barkhausen Noise. Barkhausen Noise can be related to discontinuities in the magnetization process described by the magnetic hysteresis loop (Figure 11). Figure 11 Barkhausen noise visible in hysteresis loop vii Magnetic Flux Leakage Floorscanner Page ‐ 13 ‐ Electromagnetic noise signals show a widespread frequency range and a characteristic dependence on the external magnetic field applied to the ferromagnetic material. By extracting this information more characteristics of defects are revealed. Barkhausen Noise can be measured at the MFL magnetic sensor. The only restriction is that the magnetic field sensor is sensitive to the high frequency Barkhausen Noise. With the use of a special sensor, matching signal conditioning and fast AD converters, Barkhausen Noise measurements can be integrated. 6.4.1 Magnetic Induced Potential Noise In addition to Barkhausen Noise several other magnetic effects have been detected in ferromagnetic materials. One of these effects is Magnetic Induced Potential Noise and can be used in the Floorscanner. Using an alternating AC current source, the potential noise can be detected at any two points within the electrical circuit containing the ferromagnetic material (Figure 12). The current is much lower compared to the Multi directional flux field. Figure 12 Magnetic Induced Potential Noise setup viii This technique can only be used when the tank floor is electrically accessible, so without isolating coating on top. There is a need for electrodes placed directly on the surface, this technique can only be used in liGMIted cases and will therefore not further be investigated 6.5 Combining test methods Most of the techniques can be combined: Pulsed Magnetic Flux Leakage is the alternating current version of MFL and can be combined with for example Magnetic Reluctance because the coil for Reluctance measurements can only operate with alternating magnetic fields. Also Barkhausen noise is only visible with alternating fields. Multidirectional Flux Field and Magnetic Induced Potential Noise both use two electrodes on the electric conducting test surface. If contact can be made by the electrodes then both the techniques can be used simultaneously. 6.6 Conclusion Five different techniques are proposed as extensions of the standard MFL. Techniques such as Magnetic Reluctance, Pulsed Magnetic Flux Leakage and Barkhausen Noise can be integrated without too much difficulty. These last two techniques will be tested in chapter 10. Magnetic reluctance is used for permeability measurements and described in chapter 8. Multidirectional Flux Field and Magnetic Induced Potential Noise need direct contact with the test surface. This contact is not always possible and will result in extra noise. These two techniques have a minor chance of succeeding and will not be investigated. Further experiments focus on Magnetic Reluctance, Pulsed Magnetic Flux Leakage, Barkhausen noise and combinations of these techniques. Magnetic Flux Leakage Floorscanner Page ‐ 14 ‐ 7 MFL measurement system The key parts in a Magnetic Flux Leakage measurement system are the strong magnets that magnetize the material and the sensitive magnetic field sensor for detecting the flux leakage. This chapter presents a U‐shaped lamented iron core with two electromagnets as magnetizer. Furthermore commonly used magnetic field sensor techniques are presented and compared. Finally three different sensors are presented for the experiments. 7.1 Magnets To perform magnetic flux leakage the material under test must be magnetized. This is done by two electromagnets. If a material has not been magnetized to a suitable extent, the defects cannot leak enough magnetic flux. Creating a suited magnetization is an important factor in an MFL. Until saturation of the material the following holds: for the same defect, the stronger the applied magnetic field, the larger the generated magnetic flux leakage. However, to provide a high magnetic strength, the size of the magnets must be increased. The magnetic force between the magnet and the floor will increase so does the weight of the system. As a result, the driving power of the electromagnets needs to be increased. Hence, it is very important to design an optimal magnetic circuit and select an appropriate magnetic field strength. The magnetic circuit consists of a U‐shaped laminated iron core. The optimal dimensions have to be derived by simulation and experiment. A core with the two electromagnets is used as a starting point for simulations and experiments This core was chosen because it was available for experimenting. The dimensions of this core are shown in Figure 13. 15 cm
4 cm
Core
13 cm
Coil 1
4 cm
9 cm
Coil 2
7 cm
depth 4 cm
Figure 13 dimensions of core
The specifications of the core: ‐ dimensions: ( Figure 13) ‐ lamination thickness = 0.5mm ‐ lamination fill factor = 0.97 (total thickness – airspaces between the lamination) x 100% ‐ permeability = μ = 2122* *permeability is experimentally determined (chapter 8.1) Magnetic Flux Leakage Floorscanner Page ‐ 15 ‐ It is assumed that the BH curve of the core material is linear. This is a valid assumption for low magnetic fields because the core material is still in the linear range by the maximum external magnetic field. The specifications of the coils: measured by HP4275A ‐ Inductance = L = 15.4mH (coil 1), L = 15.2mH(coil 2) ‐ Resistivity = R= 9.9 Ω measured by Agilent 34401A ‐ Winding turns = n = 1000 marked on the coils ‐ diameter wire = 0.7mm measured by Mitutoyo sliding ‐ length of coil = 5cm measured by Mitutoyo sliding fitted to experimental results* ‐ conductivity wire σ = 20 MS/m *the conductivity is fitted in such a way that the resistivity and inductance in the simulation is comparable to the measured values. Simulating the magnetic circuit via solution of the Maxwell equations is difficult due to the nonlinear behavior. The magnetic circuit can be simulated by a finite element method. The simulation of the magnetic circuit is done in FEMM (Finite Element Method Magnetics) which is a free program for simulating electromagnetic problems. FEMM can solve some limited cases of Maxwell’s equations. This solving can be done in two dimensions and can be used for axisymetric problems. It is also possible to address nonlinear magnetic problems, which is particular suitable for modeling MFL. Several simulations of the magnetic circuit are shown in the next paragraph. For these simulations the above specifications are used. 7.1.1 Magnets and laminated iron core simulation. The first simulation visualizes the magnetic field lines of the electromagnets and core material through air. For the core the dimensions as shown in Figure 13 are used. In FEMM it is possible to draw the U‐shape and enter the specifications of the material as specified above. The shape is drawn in 2D, the third dimension is added by entering a depth of 4 cm in the drawing. For each electromagnet two squares are drawn, one with the current inside and one with the current outside the screen. This creates a 2D version of a round coil/electromagnet. The air around the core and magnets has a relative permeability of μair = 1. The grid is chosen in such a way that important regions between the legs of the core are calculated more precisely. In Figure 14 the FEMM drawing is made visible. The yellow grid is dense in the regions of interest. Air
Core iron
0.7mm
[Coil 1:1000]
0.7mm
[Coil 1:-1000]
Figure 14 FEMM drawing 0.7mm
[coil 2:-1000]
0.7mm
[coil 2:1000]
The current of each coil is chosen 1.25A AC; this is the same current as in chapter 8.1 and the maximum driving current for the coils. The FEMM simulation visualizes the maximum magnetic field that can be created with the magnets (Figure 15) Magnetic Flux Leakage Floorscanner Page ‐ 16 ‐ Figure 15 FEMM simulation of the magnets and the U‐core Figure 15 FEMM simulation of the magnets and the U‐core shows that the magnetic field is for the most part in the iron core. At the position of the magnetic field sensor (between the two legs) is a field of 10mT visible. It is assumed that this will reduce if the material for testing is positioned below the two legs of the core. 7.2 Magnetic field sensors techniques Magnetic field detection has a variety of magnetic sensors to detect magnetic fields. In the next paragraphs commonly used magnetic field sensors are presented and compared. 7.2.1 Coil The coil sensor is based on induction: the voltage induced in a coil is proportional to the changing magnetic field in the coil. This induced voltage creates a current that is proportional to the rate of change of the field. Coils cannot detect static or slowly changing fields. Signal strength decreases with decreasing speed and below a certain flux change rate the signal disappears into the noise. Coils are inexpensive and easily manufactured. In this research a coil is used for permeability measurements (chapter 8). One coil is placed on the magnetic core and one coil is placed on the test plate to measure the coupled flux. 7.2.2 Hall The Hall sensor is based on the Lorentz force in semiconductor materials. When a current is applied to the material, charge carriers begin to flow. If at the same time a perpendicular magnetic field is applied, the current carriers are deflected to the side by the Lorentz force. The voltage across the semiconductor is a measure for the magnetic field strength. The Hall sensor is frequently used in magnetic sensing; the prices there are lower then GMR, AMR and GMI. The sensitivity of the hall sensor is however small compared to these other techniques and 7.2.3 Anisotropic Magnetoresistive (AMR) Anisotropic Magnetoresistivity is a magnetoresistive effect in ferromagnetic metals. These devices are made of a thin film. The film's resistance changes in the presence of a magnetic field. If a voltage is applied along the length of a thin film of semiconductor material, a current will flow and the electrical resistance can be measured. When a magnetic field is applied perpendicular to the slab, the Lorentz force will deflect the charge carriers. If the width of the slab is greater than the length, the charge carriers will cross the slab without a significant number of them collecting along the sides. The effect of the magnetic field is to increase the length of their path and, thus, the resistance. The bandwidth is usually in the 1‐5 MHz range. The reaction of the magnetoresistive effect is very fast and not limited by coils or oscillating frequencies. They have a high sensitivity and are small sized. In this research an AMR sensor is used for MFL measurements (chapter 10). Magnetic Flux Leakage Floorscanner Page ‐ 17 ‐ 7.2.4 Giant Magnetoresistive (GMR) The giant magnetoresistive sensor is just as AMR based on resistance changes of semiconductor material. GMR has compared to AMR the advantage of a higher increase in resistance: an increase in resistance of several hundred percent is possible in large fields. GMR sensors do often use a Wheatstone bridge configuration. In this configuration, four of these GMR resistors create a higher output signal. Just as AMR has GMR low power consumption, bandwidth 1‐5MHz and small size. In this research a GMR sensor is used for MFL measurements (chapter 10). 7.2.5 Giant Magnetic Impedance (GMI) sensors The GMI sensor is based on the change of impedance of certain amorphous materials with an applied magnetic field. This effect is a function of the material’s permeability, conductivity and the current frequency. One, two and three axis GMI sensors are produced. The GMI sensor consists of an amorphous metal wire with pickup coil wrapped around the wire. The amorphous wire is driven by an AC current. The frequency range of this sensor is however low compared to GMR and AMR. This operating frequency is lower because the amorphous wire in the GMI sensor needs to be driven by an AC current. A low pass filter is needed for filtering the high frequency AC current. 7.2.6 Fluxgate The fluxgate sensor is based on a magnetic saturation circuit. There are a variety of configurations; the most common is the ring core fluxgate which incorporates an excitation‐ and a sense coil, wrapped around a ferromagnetic core (Figure 16). The core's magnetic induction changes in the presence of an external magnetic field. The excitation coil causes the core to oscillate between saturation points, the sense coil outputs a signal that is caused by changes of an external magnetic field. Fluxgates are really sensitive but expensive for the use in the MFL system. The size of a fluxgate is several centimeters and the costs at least 250 euro. No fluxgate is therefore used in this research. Figure 16 fluxgate ix 7.2.7 Superconducting Quantum Interference Device (SQUID) and other low field sensors The SQUID is base on a superconducting Josephson junction and can be used to measure extremely small magnetic fields. The performances are superior compared to all other magnetic sensors. However the SQUID is much too expensive and not portable for the use in a MFL system. Besides the SQUID there are several other low field sensors, these sensors are in an experimental stage or too expensive. Magnetic Flux Leakage Floorscanner Page ‐ 18 ‐ 7.2.8 Conclusion Figure 16 shows the range for various magnetic field sensors for comparison. Figure 17 gives an indication of the costs and power for some sensors. Finally, Table 1 lists typical specifications of the sensors described in this section. Coil Hall AMR
GMR
GMI
Size mm mm m
m
m
Fluxgate SQUID
cm m 10‐6‐ 102 10‐10‐ 105
100‐ 105 10‐8‐ 1010 (frequency dependent) Temperature high low stability 10‐6‐ 102
10‐1‐ 108
medium
high
high
high high Power ‐ consumption (mW) 10 10
10
100
100 106 (cooling) Cost (euro) 1 ‐ 50 5 ‐ 50
1 ‐ 50
10 ‐ 100
250 ‐ 1000 10.000 –
1.000.000 Sensitivity (gauss) 0.1 ‐ 10 Figure 17 comparing techniques x For DC and AC inspection, Hall, GMR, AMR, Fluxgate and SQUID can be used to measure the leakage field. SQUID is too expensive, leaving Hall, Coil, GMR, AMR and Fluxgate. Coils can only be used for AC inspection. 7.3 Magnetic field sensors For the permeability experiments the flux through the core has to be known. The only way is using a coil wrapped around the core. For this purpose a self manufactured coil is used (chapter 8). In the MFL experiments, sensitive magnetic field sensors are needed. A higher sensitivity of the sensor means that smaller defects can be detect. Even defects at the bottom side can be detected by high sensitive sensors. The magnetic flux leakage for top‐side defects can be 1 gauss, matching sensors with such a small range are needed. Additionally, the price of the sensor should be minimal. Based on the demands above and the availability in small quantities the following sensors are chosen (Figure 18). Sensor name HMC1052 AAH002‐02
AGMI302 Technique Number of axis AMR 2 GMR
1
GMI 3
Sensitivity (mV/Gauss) Linear magnetic field range (gauss) Saturation (gauss) Lowest detectable field (gauss) Frequency range (kHz) Price (euro) Type of output signal 5 6 75
2
240
2 (minimal) 20 0.12 6
5000 20 (evaluation board)
voltage differential
>1000
5(sample)
voltage differential
1
20(calibrated sample)
voltage Figure 18 comparing the magnetic field sensors Magnetic Flux Leakage Floorscanner Page ‐ 19 ‐ 7.3.1 HMC1052 (AMR) The first magnetic field sensor is the anisotropic magnetoresistive (AMR) sensor: HMC1052, specifications are shown in Figure 18. The sensor uses two Wheatstone bridges to measure magnetic fields in two orthogonal directions (Figure 19). With a voltage applied to a bridge, the sensor converts every external magnetic field into a differential voltage output. The magnetoresistive sensors inside the bridge are made of a nickel‐iron film. In addition to the bridge circuits, the sensor has a set/reset coil. The coil can be pulsed with high currents to enable the sensor to perform high sensitivity measurements and improve linearity. Figure 19 HMC1052 The HMC1052 is placed on a simple evaluation board which includes a mosfet for driving the set/reset coil, and an amplifier. This board can be connected directly to the DAQ‐card for Labview experiments. For three‐axis sensing a single‐axis HMC1051 sensor can be used in combination with the HMC1052, another option is the three‐axis AMR sensor: HMC1053. Unfortunately no evaluation board is available for the HMC1053 3‐axis sensor, without this board it is impossible to experiment with the sensor because the package (BGA) cannot be connected due to the small size. The HMC1052 evaluation board was ordered but never arrived. So sadly enough no AMR sensors could be used in the MFL experiments. 7.3.2 AAH002­02 (GMR) The second sensor is a giant magnetoresistive (GMR) sensor. For this study the AG001‐01 analog sensor evaluation kit of the NVE‐corporation was ordered. This evaluation kit consists of an assortment of GMR sensors with different sensitivity and applicable range.. For the MFL experiments the AAH002‐02 single axis GMR sensor is used, this is the most sensitive sensor manufactured by NVE; the specifications are shown in Figure 18. Just as the HMC1052 it uses a Wheatstone bridge to measure magnetic fields. The output of the bridge is a differential voltage. For the signal conditioning an instrumentation amplifier is used. The amplification is chosen so that during a maximum applied magnetic field, the output swing is lower than the voltage supply. Figure 20 GMR sensor signal conditioning Magnetic Flux Leakage Floorscanner Page ‐ 20 ‐ Because the GMR sensor is sensitive in only one magnetic axis, additional sensor bridges (at orthogonal directions) are placed to permit sensing of arbitrary field directions. For the experiments only the x‐axis of the magnetic field (parallel to the magnetic field of the electromagnets) is used. This is done so because otherwise three sensors would be needed for 3‐axis sensing, while only one sensor of this type was provided in the evaluation kit. 7.3.3 AGMI302 (GMI) The third sensor is a three axis magnetic inductance (GMI) sensor: AGMI302. Three sensors are incorporated in a single package to sense the X‐axis, Y‐axis and Z‐axis of the magnetic field. The specifications are shown in Figure 18. The AGMI302 is a pre‐calibrated sample; the calibration curve is shown in chapter 13.1. This GMI sensor consists of an amorphous metal wire with a pickup coil wrapped around the wire (Figure 21). The amorphous wire is driven by an AC current of 200 kHz. The output sensitivity of this sensor is higher than the AMR and GMR sensors. The frequency range of this sensor is however drastically lower compared to GMR and AMR: DC‐1kHz. This is because of the low pass filter of the AC driving current. The output signals of the three sensors are buffered in a sampling‐hold circuit, amplified and multiplexed. The drawback of this multiplexer is the inevitable demultiplexing of the signal before further processing can be done. Figure 21 AGMI302 In addition of the AGMI302, sensors are available that integrates a 3‐axis GMI sensor with a 3‐axis MEMS accelerometer and a controller IC in a single small package. This accelerometer could be a solution for the position detection of the Floorscanner. Because the accelerometers and signal processing are integrated in the same package as the GMI sensors no additional hardware is required. In this research however, is chosen for the AGMI302 (without accelerometers) because it’s the only 3‐axis GMI sensor with an analog output. The more advanced sensors that include an accelerometer communicate with I2C. This is difficult to integrate in a Labview environment. Figure 22 the sensors are placed in a case between the two coils, direct above the test plate Magnetic Flux Leakage Floorscanner Page ‐ 21 ‐ 8 Testing In this chapter several experiments are performed on the magnets and magnetic circuit. First the permeability of the core material is determined. Next the plate permeability and the influence of an air gap is investigated. Finally the GMR sensor is calibrated. 8.1 Core permeability The estimation of the core permeability is essential because this parameter is unknown and no simulations can be performed unless the permeability is known. For the estimation of the permeability the setup in Figure 23 is used. Figure 23 core permeability, setup In this setup the magnetic circuit consists of a core and a yoke. This yoke is made from the same material as the core and creates a closed magnetic circuit. Now the total magnetic circuit consists of the same core material. This makes it easy to determine the core material permeability. The magnetic circuit is shown in Figure 24. Figure 24 magnetic circuit including plate reluctance In the magnetic circuit, R is the reluctance. This parameter determines the flux that flows through the core. The reluctance depends on the total length of the core material l, the permeability μ and the cross sectional area A. lf
f
f
f
f
f
f
f
f
R = 8:1 μA
The magnetic flux: Φ is analog to the current in an electrical circuit and can be measured using the pickup coil. Magnetic Flux Leakage Floorscanner Page ‐ 22 ‐ Φ=
Hf
f
f
f
f
f
R
8:2 This pickup coil is wrapped around the core. The flux density through the coil is defined by: ` a
` a
1f
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f
f
f
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f
f
Z V coil t dt B coil t =@
8:3 NA
The permeability µ gives the relation between the magnetic field strength H and the magnetic induction B. This relation is nonlinear but can be assumed linear if the exposed magnetic field H is low: no saturation in the material. The permeability of the core material is high, it is assumed that during the experiment the core material will not saturate. The core material permeability is approximated linear. B = μH =μ 0 μ material H 8:4 μ 0 = 4π10@ 7 8:5 The magnetic field H is applied by the two cylindrical coils acting as electromagnets. The field strength of the magnet is defined by: H = NI 8:6 N = number of turns, I = current In this experiment: I core = 1.25A
N core = two coils x 1000 turns = 2000
`
a
l core = 2x 0.11 + 2 x 0.13 = 0.48m
Acore =0.04 2 m2
H = NI = 2500
μ core = to be determined
For the permeability measurements a Labview instrument is developed (chapter 13.3) for visualizing the HB / hysteresis plot. Additionally some other parameters are measured and calculated: coil current, coil voltage, coil loss, frequency and coil inductance. The Labview instrument front plate (Figure 25) shows the Flux density of the pickup coil as function of the magnetization current (HB plot). Figure 25 H‐B plot yoke on magnetic core Using Figure 25 and: Magnetic Flux Leakage Floorscanner Page ‐ 23 ‐ dB
f
f
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= μ dH
8:7 The permeability of the core can be determined: μ core = 2122 . The derivative is taken at the point where the flux density is zero. This is a rather high permeability but this is typical for weak iron. 8.2 Test plate permeability The permeability of the core is determined. The next step is replacing the yoke with test plates (Figure 26). These test plates are originating from used tank floors and are comparable with the real situation. Figure 26 determining test plate permeability, setup Two test plates are supplied by Applus RTD, respectively 4mm and 8mm thick. The plates contain several holes and other sorts of defects for later MFL testing. At the moment, a surface, free of defects, is used for the permeability measurements. The yoke in the magnetic circuit is replaced by the test plate. In the experiment it is important not to create an air gap between the core and the test plate; this influences the total permeability (chapter 8.3). The resulting magnetic circuit is shown in Figure 27. Figure 27 magnetic circuit including core reluctance R tot = R core + R plate =
lf
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core
tot
=
+
= NI μ tot μ 0 Atot μ core μ 0 Acore μ plate μ 0 Aplate
8:8 Rcore is known Rtot is measured in this experiment; Rplate can be determined and so can the plate permeability: Magnetic Flux Leakage Floorscanner Page ‐ 24 ‐ μ plate =
lf
`
plate
f
f
f
f
f
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Aplate μ 0
R core @ R tot a
8:9 The test was performed with an AC current of 20Hz (Figure 28) and 10 Hz. Figure 28 1 H‐B plot of 8mm plate (left), 4mm plate (right) In the hysteresis plot of the 8mm plate two H‐B curves are visible. The green plot is the flux through the magnetic core sensed by the coil in the core. The red plot is the flux through the 8mm thick test plate sensed by the second coil in the 8mm plate. From the H‐B plot of the 8mm plate the following can be concluded: - The total permeability: μtot = 298 (20Hz), 424 (10Hz) - The flux coupling between of the plate is about 50% -
No saturation is visible From the H‐B plot of the 4mm plate can be concluded: - The total permeability: μtot = 293(20Hz), 398 (10Hz) - No saturation is visible l plate = 0.11m
Aplate = 0.04 x 0.008 = 0.32x10@ 3 m2 From equation 8:9 it can be concluded that: μ plate 8mm, 20Hz = 408 `
a
The plate permeability is lower compared to the core; this is expected since the plate material is not weak iron but a sort of industrial steel with a much lower permeability. Magnetic Flux Leakage Floorscanner Page ‐ 25 ‐ 8.3 Influence lift off on permeability The permeability of the core and plates is determined. The following experiment shows the influence of an air gap between the core and test plate, lift off. This test is performed because during the MFL measurement the magnet is not hold at a fixed distance but at 0.5 ‐ 1 cm above the plate. The magnetic circuit is in this case extended with two regions of air (Figure 29 shows only one air reluctance). It is presumed that the total permeability will drop dramatically as a result of the air permeability (μair = 1). Figure 29 magnetic circuit including air reluctance The experiment is performed with a lift off of 0mm, 3.5mm, 7.5mm and infinity between core and test plate. In Figure 30 the HB / hysteresis plot is shown. Figure 30 H‐B plot 8mm plate with 0mm (left), 3.5mm (middle), 7.5mm (right) air spacing between core and plate From the H‐B plot of the 8mm plate with lift off, the following conclusions can be made: -
The total permeability: μtot = 298 (0 mm), 88 (3.5 mm), 55 (7.5 mm) With the plate on distance infinity (Figure 31) the total permeability: μtot = 32 is dropped drastically. This is caused by the dominant air permeability. Figure 31 H‐B plot, no plate Magnetic Flux Leakage Floorscanner Page ‐ 26 ‐ Furthermore there are some tests performed with lift off on the 4mm plate: - The total permeability of the 4mm plate: μtot = 89 (0 mm), 88 (3.5 mm), 57 (7.5 mm) Similar results are obtained at a frequency of 10 Hz by both the 8mm and 4mm plate Lift off 0mm 3.5mm
7.5mm
∞
yoke 298 88 55
32
2122 151 36 19
‐
‐ 424 84 58
32
2340 µplate 196 42 25
‐
‐ 4mm plate 293 89 57
32
2122 398 85 60
32
2340 8mm plate 20Hz µcore µplate 8mm plate 10Hz µcore 20Hz µcore 4mm plate 10Hz µcore Figure 32 permeability test results Magnetic Flux Leakage Floorscanner Page ‐ 27 ‐ 8.4 Calibrating the GMR sensor For the calibration of the AMR / GMR sensors a Labview instrument is developed (chapter 13.2). The magnetic field sensor is place inside a coil acting as an electromagnet (Figure 33). Figure 33 calibrating the GMR sensor The electromagnet is controlled by the Labview instrument that applies a DC current to the coil. The current is varied in steps; the whole (linear) magnetic range of the sensor is covered. Figure 34 calibration of GMR sensor It can be seen (Figure 34) that the AAH002‐02 sensor has a linear range of 2 gauss, just as described by the manufacturer. The signal conditioning amplification is varied and fits to the known magnetic field of the electromagnet with the sensor. There is no need for calibration of the GMI sensor, this sensor is pre‐calibrated by the manufacture. It is however possible to calibrate the GMI sensor (and AMR sensor) with the use of this setup and Labview instrument. Magnetic Flux Leakage Floorscanner Page ‐ 28 ‐ 9 Simulations In the simulations the specifications of chapter 7.1 have been used. Additional specifications of the plate are: ‐ μplate = 408 ‐ thickness plate: 8mm Each figure shows two situations at which one variable is changed; the comments are at the end of this chapter. The scale of the magnetic field is in every figure the same. Figure 35 1ADC (left) 2.5ADC (right) 3.5mm lift off Figure 36 3.5mm lift off (left), 10mm lift off (right) 0.5ADC Figure 37 0Hz (left), 20Hz (right) 0.5A Figure 38 no defect (left), 10mm(l) x 2mm(d) defect (right) lift off 3.5mm, 0.5ADC Magnetic Flux Leakage Floorscanner Page ‐ 29 ‐ Figure 39 3.5mm lift off (left), 10mm lift off (right) 0.5ADC, 10mm(l) x 2mm(d) defect Figure 40 0Hz (left), 20Hz (right) 0.5A, 10mm(l) x 2mm(d) defect Figure 41 0Hz (left), 20Hz (right) 0.5A, 10mm(l) x 6mm(d) defect Figure 42 0Hz (left), 20Hz (right) 0.5A, 2mm(l) x 2mm(d) defect Magnetic Flux Leakage Floorscanner Page ‐ 30 ‐ Figure 43 0Hz (left), 20Hz (right) 0.5A, 2mm(l) x 6mm(d) defect Figure 44 0Hz (left), 20Hz (right) 0.5A, 10mm(l) x 6mm(d) bottom defect Figure 45 (left), 20Hz (right) 0.5A, 10mm(l) x 2mm(d) bottom defect Figure 46 0Hz (left), 20Hz (right) 0.5A, 2mm(l) x 2mm(d) bottom defect Magnetic Flux Leakage Floorscanner Page ‐ 31 ‐ In Figure 35 the current and therefore the applied magnetic field is changed. It shows that a larger current creates a larger flux density around the legs of the core. The MFL sensor (positioned between the two legs) senses a magnetic field of 50 gauss (1ADC) and 250 gauss (2.5ADC). Without magnetic shielding the MFL sensors are in overload because this magnetic field is intense, especially at 2.5ADC. In Figure 36 the lift off is changed. It shows that the flux density inside the core with a 10mm lift off is lower. Also the field between the legs is more intense because the plate is at a larger distance. In Figure 37 an alternating current (sinus) is exciting the coils, creating a PMFL situation. For the frequency is chosen 20Hz. By varying the frequency from 1mHz – 10kHz no difference in the efflux density plot is observed compared to the 20Hz situations. This is probably the result of the small skin depth. All PMFL simulations are performed at 20Hz; this is done also in the PMFL experiments (chapter 10.8). In the PMFL situation a dense field is observed around the test plate. This is because of the skin effect, the alternating field has a skin depth of µm’s and therefore the flux is concentrated around the surface. The magnetic field between the legs is 0.1T; this field is too strong for sensitive sensors. Some sort of shielding is necessary. In Figure 38 a defect with the dimensions: 10mm(l) x 2mm(d) is simulated at the top side of the plate. The magnetic field above the defect is lower (<50 gauss). This reduction of field density can be sensed at the MFL sensor. In Figure 40 the same defect while using PMFL instead. It is shown that the flux density around the defect is more intense, by positioning the sensor at a close distance of the effect, the flux leakage is strong and could provide information about dimensions of the defect. In Figure 41, Figure 42 and Figure 43 the size (and shape) of the defect is changed. The flux density above the defect is characterizing for the defect. By positioning several MFL sensors in the horizontal direction more information about the defect can be obtained. It could be possible to fit these experiments with the simulation. In Figure 44, Figure 45 and Figure 46 a bottom‐side effect is simulated with changing shapes. In the 0Hz situation a reduction of the flux density above the defect is visible. The field density however, does not show noticeable changes by changing the defect dimensions. The PMFL simulations show a flux field (above the defect) that strongly depends on the defect dimensions. 9.1 Conclusion The simulations show that a static magnetic field creates a flux density <50 gauss (0.5A) between the legs of the core. The MFL sensor therefore requires magnetic shielding for operation. By applying an alternating current to the electromagnets, PMFL can be simulated. It is shown that PMFL creates a dense flux density around the plat because of the skin effect. The PMFL simulations do not show that the flux density noticeable depends on the frequency of the alternating field. This is probably the result of the small skin depth. Simulations of defects with different dimensions show in case of 0Hz a reduction in the flux density above the defect. Not much information can be obtained about the position (top or bottom) of the defect. During PMFL the flux density above the defect strongly depends on the dimensions of the defect. The position of the defect could be obtained by positioning several MFL sensors horizontal above the defect. Magnetic Flux Leakage Floorscanner Page ‐ 32 ‐ 10 MFL testing 10.1 Variable DC current static situation In the first MFL test the Floorscanner is positioned at a fixed distance of 5mm of the test plate (8mm thickness without defect , corrosion or cracks). The current in the electromagnet is varied from 0.8 ADC to 0 ADC in steps of 0.1ADC. The magnetic field detected by the GMI and GMR sensor is shown in Figure 47. Figure 47 xyz varying DC current (3,5mm lift off) It shows that without an external magnetic the MFL sensors sense a nonzero field. This is caused by the earth magnetic field and electromagnetic sources like CR‐displays near the setup. The sensors are sensitive and can be used as a sort compass. During this test the sensors were placed in an iron case for attenuation of external electromagnetic fields. By applying an external field ‐produced by the electromagnets for the MFL experiments‐ the sensed fields are even more intense. This is not surprising, a part of the magnetic field flows through the air between the two coils, as shown in the simulations (chapter 9). With the external field applied and no saturation in the test plate, the flux leakage should be near zero. However the sensors detect magnetic fields. This is because the two sensors are very sensitive; the electromagnet produces a small electromagnetic field between the two coils, just as in the simulations. It can be seen that the field in x‐direction is more intense than in the y‐ and z‐direction. This is not surprising since the x‐axis of the sensor is parallel to the magnetic field. The linear range of the GMI and GMR sensors is limited however to 6 gauss, this range is reached at a 0.6ADC. This means that with a static MFL setup, the electromagnetic field is leaking in such an amount that the sensors are already out of linear range. The GMR sensor shows even some signs of saturation, and is sometimes at its limit. Better shielding is required or else the sensors are in the non linear range or could even overload. 10.2 Defect depth In the second MFL experiment round defects have been made in the 8mm steel plate. The defects heave the same diameter: 10mm, the only variables are the total depth in the plate and the position of the defect: topside or bottom‐side (Figure 48): - one defect: 8mm depth - two defects: 6mm depth (top‐side/bottom‐side) - two defects: 4mm depth (top‐side/bottom‐side) - two defects: 2mm depth (top‐side/bottom‐side) Magnetic Flux Leakage Floorscanner Page ‐ 33 ‐ Figure 48 defects with different depth, top‐side/bottom‐side Figure 49 shows the results of a scan. During this scan the magnetic sensor (placed at the Floorscanner) is moved forward from the 8mm depth to the 2 mm depth defect (8mm thick test plate). Afterwards the sensor is moved backwards over the defects. Figure 49 scanning defects forwards/backwards (+0.5ADC 3.5mm lift off) In the first part of the scan four peaks are visible. These peaks correspond to 8mm/6mm/4mm/2mm defects. All these defects are placed at the topside of the plate. This means that the Floorscanner does not detect bottom‐side defects. The second part of the scan is mirrored; this part is the result of moving the Floorscanner backwards over the defects. The x‐axis is the most sensitive for the flux leakage of the defects. The z‐axis is the least sensitive. The GMR sensor has an overshoot at 0.5ADC and cannot be visualized. Figure 50 scanning defects 8mm depth till 2 mm depth (0.5ADC, x‐axis, 3.5mm lift off) In Figure 50 another scan is performed. In this scan the x‐axis of the GMI sensor is shown. Again only the top side effects are visible. Noticeable is the magnetic field before the scan (3 gauss) and afterwards (0 gauss), somehow the magnetic field of the plate depends on the position. Figure 51 scanning defects forwards/backwards y axis (0.5ADC, 3.5mm lift off) In Figure 51 the y‐axis is shown. Four peaks with overshoot are visible.
Magnetic Flux Leakage Floorscanner Page ‐ 34 ‐ 10.3 Variable DC current, dynamic situation In the following experiments the current of the electromagnets is varied. The current is chosen: ‐1.75ADC, ‐0.5ADC, + 0.5ADC, 1.75ADC. It is assumed that no noticeable difference should be seen because the GMI sensor is linear for both positive and negative fields. During the test the Floorscanner is moved over the defects with variable depth. The results are shown in Figure 52. Most of the time, the GMR sensor was in saturation and therefore not shown in the results. ‐1.75ADC ‐0.5ADC +0.5ADC +1.75ADC Figure 52 varying current, scanning forwards/backwards (3,5mm lift off) The following can be concluded: There are noticeable differences visible by flipping the direction of the DC magnetic field. During the experiments with the strong magnetic field, saturation of the x‐sensor is visible. During saturation the information of the sensors is not reliable anymore. To prevent saturation, better shielding of the sensors will be needed. The y‐ and z‐sensors however, are more sensitive since more flux leakage is present at these high currents.
Magnetic Flux Leakage Floorscanner Page ‐ 35 ‐ 10.4 Lift off (variable air gap) In this experiment the air gap between the U‐core and the test plate is varied. The lift off is chosen: 3,5mm (standard lift off used in all the experiments) and 5mm. In this setup the lift off cannot be smaller than 3,5 mm because otherwise the magnet is stuck to the test plate. This is due to the rails of Floorscanner which are made of plastic to prevent the flux flowing through the rails and thereby influencing the measurements. The plastic deforms and the magnet is stuck to the plate. The lift off could be increased to more than 5mm but defect detection is difficult at this long distances. Figure 53 xyz 3.5mm lift off (‐0.5ADC) Figure 54 xyz 5mm lift off (‐0.5ADC) Comparing Figure 53 and Figure 54 the following can be concluded: The sensitivity at larger lift offs is reduced. At 5mm lift off the resulting signals are reduced to more than a half compared to 3.5mm lift off. Another disadvantage of a larger lift off is: the magnetic field of the electromagnet flows more easily through air and excites the MFL sensors resulting in erroneous measurements. This is visible at the x‐axis with 5mm lift off.
Magnetic Flux Leakage Floorscanner Page ‐ 36 ‐ 10.5 Defect diameter In this experiment the diameter of a round defect is varied from 8mm to 1.5mm, the depth is constant at 3mm; in Figure 55 the orientation of the defect on the test plate is shown. Figure 55 defects with different diameter In Figure 56 the results of a scan at 0.5ADC are shown. In this test the Floorscanner is moved over the defect from 8mm to 1.5 mm and backwards. In the figure two maxima are visible, there should be 5 peaks. This signal only shows the two biggest defects, the other defects are simply not detected at this magnetic field, because the flux leakage is too weak. Figure 56 scanning defects forwards/backwards 0.5ADC ( axis, 3.5mm lift off) Figure 57 scanning defects forwards/backwards 1ADC (axis, 3.5mm lift off) In Figure 57 the results of a scan at 1ADC are shown. Compared to the scan with 0.5ADC the results are more precise. More peaks are visible, showing a better sensitivity for defects. However the smallest defect is still not detected. Magnetic Flux Leakage Floorscanner Page ‐ 37 ‐ 10.6 Crack In this experiment the Floorscanner is moved over a crack with the following dimensions: 3mm width, 1mm depth, the length is 50cm. Figure 58 shows a scan in which the Floorscanner is moved slowly from one side to the other side of the crack and backwards. In the second part of the figure the Floorscanner is moved over the defect with more speed. Figure 58 scanning crack forwards/backwards ‐0.5AAC, first slow scanning then fast (x‐axis, 3.5mm lift off) From the figure can be concluded that the GMI is very sensitive for cracks. The flux leakage is changing with 6 gauss. At the fast scan an overshoot is visible. Figure 59 xyx scanning crack forwards/backwards +0.5AAC (3.5mm lift off) Figure 59 shows another scan. The x‐axis is the most sensitive to the crack. The GMR sensor also shows the crack (blue line). Both figures show that the magnetic field depends on the side of the crack: as if the plate is separated in two different magnetic regions. Magnetic Flux Leakage Floorscanner Page ‐ 38 ‐ 10.7 Barkhausen Noise Barkhausen Noise can be related to discontinuities in the magnetization process described by the magnetic hysteresis loop. It has a widespread frequency range and a characteristic dependence on the external magnetic field applied to the material. The coil wrapped around the core can be used for the detection of Barkhausen noise. Also the GMR sensor for detection of the flux leakage has a bandwidth until 1MHz and could be used for Barkhausen Noise detection. The frequency range of the GMI sensor (1 kHz) is insufficient for the noise experiments. Another requirement for the detection is a matching signal conditioning and fast AD converters or spectrum analyzer. The DAQ‐card in combination with Labview is not fast enough (maximum sampling rate 1 kHz) so no Barkhausen Noise is visible in the Hysteresis graph. But by using a scope (Agilent 54622A) with a build in FFT analyzer and fast ADC, the Barkhausen Noise experiment is performed: Both the output signals of the coil and the GMR are analyzed. The spectrum of the GMR sensor is shown in Figure 60. In the left figure the magnetization current is switched off, in the right figure the magnetization current is switched on. In the figure on the right, the 20Hz peak is visible; the noise amplitude is slightly higher. Part of this additional noise is Barkhausen Noise. The noise however is uniform spread across the spectrum and no distinction can be made between thermal (and other unwanted noise) and Barkhausen Noise. Similar spectra are obtained at the output of the coil. The two spectrums do not give information about Barkhausen Noise. A possible explanation is that the dynamic range of the scope is not sufficient for the distinction of Barkhausen Noise out of the thermal noise. Another possibility could be that the bandwidth of the GMR/coil sensor is limited due to attenuation in the cable and signal conditioning circuits. Therefore the high frequency range of Barkhausen Noise, which could be more prominent, is not reached. Figure 60 FFT of the GMR sensor output, sensing no field (left), sensing 20Hz field and flux leakage of defect (right) Magnetic Flux Leakage Floorscanner Page ‐ 39 ‐ 10.8 Pulsed Magnetic Flux Leakage In this experiment the electromagnets are excited by an alternating current. The frequency is 20Hz and this corresponds to a skin depth of 0.9µm. To generate a higher frequency a higher voltage is required because of the inductance of the coils. The maximum current at 20Hz was limited by the voltage range of the power supply (75VAC). Figure 61 scanning defects forwards/backwards 0.25AAC (z‐axis, 3.5mm lift off) In Figure 61 the Floorscanner is moved over the defect with different depth (chapter 10.2). Four defects can be distinguished. The detected defects are four top‐side defects. Figure 62 defects forwards/backwards 0.5AAC (z‐axis, 3.5mm lift off) In Figure 62 the experiment is repeated with 0.5AAC. Four defects are visible; the x‐axis sensor is however at maximum range. Therefore the AAC can not be increased any further. The result of PFML fall short compared to the simulations. The results during the MFL experiments give less information about the defects. The alternating field causes the MFL sensor to overload at large currents. 10.9 Conclusion The experiments with the Floorscanner show that the GMR and GMI sensors are sensitive for the earth magnetic field. Scans on a defect free surface show large magnetic field at the sensors. Magnetic shielding attenuates the magnetic fields partly, for larger fields the shielding is inadequate. Defects with different depths are analyzed. Only top side effects are detected, flux leakage of bottom side defects is for detection insufficient. Small defects of 2mm in diameter and 3mm depth can be detected. The Floorscanner is sensitive for cracks: large flux leakage changes are shown. Barkhausen can be detected in the MFL sensor spectrum. However it is difficult to distinguish the Barkhausen noise from other noise sources. PMFL experiments do no show much information about the defect, the sensitivity is low compared to the simulations. Magnetic Flux Leakage Floorscanner Page ‐ 40 ‐ 11 Conclusion The assignment of this study is to improve an existing inspection device for large storage tanks called the Floorscanner. All commonly used nondestructive testing techniques have been investigated. Because of the Floorscanner objectives the focus is on magnetic flux leakage (MFL). Techniques as Magnetic Reluctance, Pulsed Magnetic Flux Leakage (PMFL) and Barkhausen Noise are presented for integration with magnetic flux leakages. A combination of these techniques is proposed to improve the detection and characterization of defects. For the detection of flux leakage, sensitive magnetic field sensors are presented and compared. Two different sensor techniques: GMR and GMI are used in the experiments because of their high sensitivity. A core with the two electromagnets is used as a starting point for simulations and experiments. In the first experiment the permeability of the core and plate material is determined; this information is indispensible in the further MFL simulations and experiments. From the MFL simulations the following can be concluded: By applying an alternating current to the electromagnets, PMFL can be simulated. It is shown that PMFL creates a dense flux density around the plate because of the skin effect. The flux density does not noticeably depend on the frequency of the alternating field. Simulations of defects (0Hz magnetic field) show a reduction in the flux density above the defect. This flux reduction does not give much information about the position (top‐ or bottom‐side) of the defect. From PMFL simulations the flux density above the defect appears to strongly depend on the dimensions of the defect. The position of the defect can be obtained by positioning several MFL sensors horizontally above the defect. The MFL experiments show that the magnetic field sensors are sensitive for the earth magnetic field and other noise. Magnetic shielding is used for attenuating of the sensor but works partially: there is saturation of the sensor at the x‐axis (parallel to the excitation magnetic field). This saturation deteriorates the defect detection. Defects with different depths are analyzed. Only top side effects are detected: flux leakage of bottom side defects is insufficient for detection. Small defects of 2mm in diameter and 3mm depth can be detected. The Floorscanner is sensitive for cracks. Barkhausen can be detected in the MFL sensor spectrum. However it is difficult to distinguish the Barkhausen from other noise sources. PMFL experiments do no show much information about the defect. The sensitivity is low compared to the simulations; this is because of sensor overload. Magnetic Flux Leakage Floorscanner Page ‐ 41 ‐ 11.1 Recommendations In the MFL experiments only the GMR and GMI sensors are used because the AMR sensor did not arrive in time. A next study could include AMR also because three‐axis AMR sensors are already available. The magnetic shielding is insufficient for large magnetic fields; a better attenuation could be obtained with the use of mu‐metal. For the position detection of the Floorscanner accelerometers can be used. Sensors are available which integrate a 3‐axis GMI sensor and a 3‐axis MEMS accelerometer with a controller IC in a single small package. Because the accelerometers and signal processing are integrated in the same package as the GMI sensors no additional hardware is required. The autonomy of the Floorscanner can be increased. In the last years new developments such as on‐chip magnetic sensor arrays are presented. Arrays of GMR sensors are expected in the near future. These arrays can be used in the Floorscanner for faster scanning and better defect detection. Instead of one sensor, an array can be placed above the defect(s). During the experiments with large magnetic fields, the GMR sensor was in overload. A possible solution for this is a gradiometer; this GMR sensor detects only magnetic gradients and has low sensitivity to external fields. Since the x‐axis of the magnetic field sensor experiences the magnetic field of the electromagnet this axis can be implemented by a gradiometer. For the y‐ and z‐axis, higher sensitive (normal) GMR sensors can be used. Both sensors are manufactured by the same company. This combination of sensors could improve the sensitivity of a new Floorscanner resulting in better defect and corrosion detection. The magnetic field sensor in the Floorscanner is not at a fixed distance above the plate. This changing position influences the sensitivity. A magnetic field sensor attached to a piëzo (or other actuator) could provide fast control and fixed distance. In case the sensor is near saturation, the distance could be adjusted too. The Barkhausen Noise is still an option to be further investigated, by studying the spectrum of this noise and the noise from other sources. Magnetic Flux Leakage Floorscanner Page ‐ 42 ‐ 12 References -
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(Footnotes of pictures: see end of appendix) Magnetic Flux Leakage Floorscanner Page ‐ 43 ‐ 13 Appendix 13.1 Calibration of AGMI302 magnetic field sensor 13.2 GMR sensor calibration: Labview instrument Magnetic Flux Leakage Floorscanner Page ‐ 44 ‐ 13.3 Hysteresis measurement: Labview instrument i
http://www.insight‐ndt.com/papers/technical/t001.pdf ii
http://www.ndt‐ed.org/EducationResources/CommunityCollege/Ultrasonics/Introduction iii
http://www.engineersedge.com/inspection iv
http://www.ndt‐
ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/basicprinciples.htm v
http://www.ndt‐
ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/basicprinciples.htm vi
Non destructive crack detection by capturing local flux leakage field by Mustafa Goktepe
vii
An Introduction to Barkhausen Noise and its Applications by Mark Willcox & Todd Mysak viii
Magnetically Induced Potential Noise(GMIPN)—A New Method for the Characterization of Magnetic Materials and for New Sensor Applications ix
www.sensorland.com/HowPage071.html x
http://archives.sensorsmag.com/articles/1298/mag1298/index.htm Magnetic Flux Leakage Floorscanner Page ‐ 45 ‐