Remote field eddy current (RFEC) testing is an eddy current non-destructive test method that has become widely used for the examination of carbon steel tubes, such as those found in heat exchangers and boilers. RFEC testing relies on the use of very low intensity electromagnetic fields on the exciting probe. The defect detection is carried out considering the phase difference between the exciting and pickup signals. However, various testing effects were produced because the affection of different electromagnetic characteristics of tubes and types of defects The purpose of this study is to analyze the influential factors of RFEC testing accuracy by means of the finite element method (FEM). In order to investigate the numerical simulation, the researcher used the finite element software package FEMLAB to create an electromagnetic model of RFEC testing. Then he applied the mathematics software package MATLAB to plot the voltage plane diagram and defect-phase difference diagram. After the comparison of the simulation with experiments, the researcher confirmed the reliability of electromagnetic simulation. As a result, this study provides the variations in RFEC testing result diagrams for the influential factors. Over all, this study has successfully established an FEM RFEC electromagnetic model. After the FEM simulation, this method can provide many comparable data for experimenter. This study may remedy the different types of defects and reduce assessing error. Therefore, the accuracy and reliability of testing of RFEC can be accessed.

Eddy current testing is a non-destructive testing method, which is usually used for an examination of thin metal material. It can acquire the coil impedance variety between the eddy current magnetic field and the coil magnetic field by electromagnetic induction. By prognosticating its phase angle, testing can exam the influence of the examination through the impedance plane diagram. However, the eddy current testing is usually influenced by different types of the defects and factors of examination. The purpose of this study is to analyze the examination factors of eddy current testing accuracy by means of the finite element method (FEM). This study is to create two dimensional axial symmetry model of simplification in eddy current testing by finite element software package Femlab. After that, the researcher applied the mathematics software package Matlab to plot the impedance plane diagram and evaluation curve. Furthermore, the researcher analyzed the impedance plane diagram and evaluation curve in different examination factors. After the FEM simulation in different types of the defects and the examination factors, this study can provide many comparable data for experimentalists, to judge the condition of the defects more correctly and to reduce testing errors more effectively in the eddy current testing.

This thesis describes the development of an eddy current instrument and its application in detecting early-stage pitting corrosion. Eddy current testing has previously been used in Non-Destructive Testing (NDT) applications detecting large defects, like cracks. However, the challenge of detecting corrosion pits of less than 1mm³ remains unaddressed. This research involved the design of a Field Programmable Gate Array (FPGA)-based eddy current instrument, and the design and modelling of a novel differential electromagnetic sensor. The FPGA provided accurate synchronisation among the major electronic components. The firmware developed as part of this research allowed for exact interfacing to A/D and D/A converters, performed a real-time demodulation and signal generation, the instrument also supported a multi-frequency eddy current application. The firmware showed promising end-results in terms of sensitivity and stability in relation to pitting corrosion detection. In summary, this instrument offered significant improvement in sensitivity; the size of corrosion detected is improved more than 10 per cent compared to the previously reported, which enabled the detection of pits smaller than 1 mm³. For the sensor probe, a novel differential sensor was proposed to minimise the background signal for plate scanning and improve the sensitivity. The designed probe has an advantageous feature: the sensor response can be analysed using a closed form analytical solution.

While evaluating the depth of corrosive defect of the pipe through Remote Field Eddy Current (RFEC) Testing technology, the critical principle of the process is to use the interaction of the magnetic field. Generally, exciter coils in the low-frequency alternating current and detector coils can generate the magnetic field. The signal curve can be transformed by receiving and plotting the induction voltage of detector coils. In addition, the signal curve can be used to identify the existence and the depth of corrosive defect from the shape and angle of the curve. Thus, the structure of the detector has a great influence on the exactitude of the testing. However, in the real experiment, RFEC probe is covered by shell and hard to disassemble. Thus, few people doing the research to analyze the structure of the RFEC probes. This research is based on two-dimensional axial-symmetry models and using Finite Element Method to simulate different structures or designs, such as the distance between exciter coils and detector coils, the amplitude and frequency of current in exciter coils, and even the material and size of shield. The simulation results show the influences of changing these important characteristics. Therefore, with these scenarios, the RFEC testing technology can be understood more completely and be improved the accuracy and reliability of the experiment by optimizing the sensibility of the RFEC probe.

This thesis first explains the eddy current and the phenomenon of skin effect, where the resultant flux flows near the surface of the metal. A new flux direction perspective is created for steel laminations, from which derivations of the eddy current resistance and power losses in different directions are developed assuming uniform flux conditions. The developed method compares with a proposed theory through experimental data. The results from the comparison support the validity of the developed derivations. Two uniform flux generators and their billets construction are introduced. The power loss between two cubic billets with different orientations is compared. A Finite Element Analysis (FEA) program is used to show the difference between lamination alignments. To prove the validity of the developed theory, two experiments were performed using two different electroheating apparatus. The results give scale factors from which the theoretical values can be matched to the experimental ones. Due to the poorer construction of the first apparatus, the scale factor of measured to computed losses is 1.15. The scale factor for the second apparatus can be taken as unity, revealing a good match between theory and measurements. After verification of the developed equations for uniform flux experiments, the focus of the eddy current loss calculation turned to partial core transformers. The flux background of a cubical core is reviewed. Three key factors ( L', Kec and βa) are introduced into the eddy current power loss model. L' is a length which indicates the region of the flux spreading at the ends of the core. Kec as a ratio indicates how much of the main flux spreads at the ends of the core. βa is the ratio of the winding axial length and winding thickness. Using simulations from the Finite Element Analysis (FEA) program MagNet, a partial core side view with the flux distribution and flux density from two orthogonal angles is created. A flux linkage comparison between the experimental results and the returned values from MagNet verifies the high accuracy of the flux plot in MagNet. The eddy current power loss model is then built up with equations. The relationships amongst the three key factors are studied and confirmed using the experimental results. Normally, a partial core transformer uses a cylindrical partial core rather than a cubical partial core, to reduce the amount of winding material. Therefore, a further goal was to prove the developed model for cylindrical partial core transformers. The construction differences between the cubical and cylindrical core is discussed. The orthogonal flux assumptions for the cylindrical core in two directions are reviewed. The flux penetration between two adjacent blocks is considered and explained. The mathematical core loss model is created for a cylindrical core composing by ten blocks. Three tests were performed using the developed core loss model. The results visualize the power loss from the core by its temperature distribution, and consequently prove the validity of the developed core loss model. An eddy current loss comparison and the discussion are made between the previous method and the developed method. Overall, the results confirm a significant improvement using the developed core loss model, and a generic form of the partial core can be used for designing future models of partial core transformers which have a stacking factor greater than 0.96.

Eddy current (EC) based testing and measurement methods are well known in non-destructive testing (NDT) world. EC sensors are extensively studied and used for material health monitoring and its property measurement. Target displacement measurement is one of the well-known applications of EC method. The main advantage of EC sensor is its working capability in harsh environment like humidity, contamination etc. It is non-contact, rugged and requires less maintenance. The range and sensitivity of target displacement is mainly determined by the probe geometry and its construction method. Also displacement measurement depends upon geometry and electromagnetic (EM) properties of the target plate. Any variation of ambient temperature alters the EM properties of the probe as well as EM properties of the target. Thus, many parameters like geometry, EM properties and temperature involved in target displacement measurement. Hence, while using EC sensor for displacement measurement, it demands careful design and measurement procedure to achieve high sensitivity and high precision with low temperature drift. To achieve these, we present the following. 1) A temperature compensation technique 2) Optimization of probe geometry and its construction method to increase the range and sensitivity 3) Selection of suitable probe measurement parameter (Z, R, X) based on target material properties 4) Making the displacement measurement less sensitive to tolerance in probe construction parameter. A temperature compensation technique for target displacement measurement, using a self-running LC oscillator has been presented. A sensing coil is energized by a Hartley oscillator. The oscillator voltage is maintained at a constant level by a closed loop feedback circuit and the average feedback current to the oscillator is measured for target displacement detection. The temperature drift of the feedback current is compensated by applying temperature compensation function (TCF) and this is verified experimentally. Cold rolled mild steel (carbon steel) is taken as a target material and the sensor is tested over a temperature range of 20 °C – 80 °C. It shows that the temperature drift is less than ±30 ppm/°C over 3 mm target displacement. To match all the sensor modules in mass production, components selection procedure is presented. To avoid mismatch across sensors in manufacturing process, the transistor based oscillator is modified with operational trans-conductance amplifier (OTA). The same temperature compensation formula (TCF) is applied to compensate the temperature drift of feedback current and achieved intended accuracy. Geometry and construction parameters of the eddy current sensing probe is optimized for target displacement measurement using Ansoft Maxwell, electromagnetic design software. EC probe with different geometry are analyzed in search of suitable geometry for target displacement measurement. Four shapes of commercially available core have been chosen for probe construction. For each shape of sensing probe, the radius and height of the probe is increased by 0 mm to 9 mm to find the effect of them on sensitivity and range of target displacement measurement. It has been observed that the probe with less height and maximum diameter has shown better performance. In addition to that, the probe geometry is optimized to achieve more sensitivity and range within the space available for probe mounting. It helps to utilize the available space effectively for probe design. Coil winding and mount-ing it inside the core window also important parameter in probe design. It has been observed that de-pressing the sensing coil inside the core window from sensing face by 3 mm decreases the sensitivity by 40 %. Hence, it is recommended to place the coil on the extreme end of the sensing face of the core. To know the effect of core permeability, it is varied from 1000 to 15000. It has been observed that it has no effect on sensitivity and measurement range. Only optimizing the probe geometry and its construction method is not adequate for target displacement measurement. We know that the EC based displacement measurement is also target material dependent. Generally probe impedance is measured and then the temperature drift of the sensing coil resistance is compensated to know the target displacement. Most of the temperature compensation techniques use this compensation technique and it is shown that those are suitable for high conductivity targets like copper. Choosing Z for displacement measurement may not be only best choice for all target materials. The displacement can be measured also through either R or X of the probe. Choosing the proper probe parameter for a given target material will provide a less temperature drift for target displacement measurement. To know about this, a simulation has been made for target displacement measurement with target metal of μr = 1, relative permittivity εr =1, and temperature coefficient of resistivity ∝ = 0.004 K-1. The conductivity (σ) of the target is varied from 1×106 S/m to 62×106 S/m in the temperature range of 20 ℃ – 80 ℃. Now the simulation has been repeated by fixing  as a constant and varying target μr. The metal plate with  = 1×106 S/m, εr=1 and ∝ = 0.004 K – 1 is taken as a target and μr is varied from 100 to 10000. For both conductivity and permeability sweep analysis, the target displacement is measured as a function of Z, R and X independently. The temperature drift in displacement measurement is also analysed for the above temperature range. An experiment has been conducted with copper, stainless steel and mild steel as target metal in the temperature range of 20 ℃ – 80 ℃. The temperature drift is calculated when the displacement is measured as function of Z, R and X. Based on the results, we have identified that the target material relative permeability determines the selection of probe measurement parameter for target displacement measurement. Hence, knowing tar-get r alone suffice to select the probe measurement parameter (Z or R or X) for displacement measurement. Optimizing the probe geometry, selecting the proper probe measurement parameter and temperature compensation technique suffice to provide a good sensitivity, range and low temperature drift for a single probe. But in general, one of the mass produced probes is selected as a reference probe and it is calibrated against the ambient temperature and target displacement. And the calibration curves are loaded to all the probes. Matching the probe construction parameters to each other across the production patches is not possible in mass production. This makes the temperature compensation function and displacement calibration are different for every individual probes for displacement measurement. This degrades the measurement accuracy. A simulation has been performed with pot core with commercial tolerance. Using this, we have obtained 24 probes due to variations in 1) Individual and few combinational variations in core and coil dimensions 2) Core permeability variation and 3) relative position of the coil with respect to core. Finally, we have quantified the displacement error for each probe. We have identified the important probe dimensional parameters that have to be controlled precisely in mass production to improve the measurement accuracy. It shows error of 0.86 % in the displacement measurement when the relative reactance and relative displacement is used for measurement. In practice, error in displacement measurement due to both the ambient temperature drift and the tolerance in probe construction parameter exist simultaneously. Hence, the combined error is computed for the target displacement range of 0 mm – 3 mm for the temperature range of 0 °C – 100 °C. The total error of less than 1 % is achieved for commercial standard probe tolerance. Finally, we have provided general factory production procedure and user calibration procedure of probe design to achieve cost effective displacement measurement with sensitivity and range with low temperature drift.

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The in-mold electromagnetic stirrer is a kind of device which is utilized to stir the molten steel in the steel factory. This thesis provides a detailed three-dimensional electromagnetic analysis of an in-mold electromagnetic stirrer driven by the moving magnetic field produced from stator winding currents. A commercial finite element analysis software will be utilized to calculate the flux density, eddy current, and electromagnetic force from static and dynamic analyses, and the above three physical phenomena are also discussed to obtain the 3-D electromagnetic characteristics. In order to improve the operational properties of the in-mold electromagnetic stirrer, the various position of the stator is modified to observe the distribution of the electromagnetic force. Besides, the magnitude and frequency of the input currents are also adjusted to predict the probable performances during on-site operation.

For the evaluation of non-ferrous heat exchanger tube, there are many non-destructive testing methods; however, the eddy current testing (ECT) method is the most popular one. By using of ECT, you may find out the defects existing inside or outside the tube wall, diagnose the heat exchanger system and find out the latent problems. The problem is that an improper signal analysis will result in error in the range of 15〜25% of the tube wall thickness, or even over 40% error. This is a great discouragement to the ECT inspectors, and will reduce the confidence of the proprietors of power plants or petro-chemical industries to the use of ECT. Therefore, in this thesis, the study is mainly focus on the problems of the aluminum brass tubes in condenser using ECT method. This thesis will analyze the causes of error of aluminum brass tubes when using ECT, prepare calibration and reference tubes, and test them using eddy current instruments. The relationship among the raw data with volts, phase angle and depth has been found. Two data evaluation methods are developed, one is the defect depth modification equation and the other is the auxiliary evaluation curve. The new methods are proved to be more accurate and practical in the evaluation of heat exchanger tube after more than one year of verification by field testing in the power plant. The results obtained in this thesis are very helpful to reduce the probability of tube failure.