Fundamental Theory And Its Implementation Method For Material-Independent Eddy Current Sensor

Aimed at eliminating the influence of the electromagnetic properties of measured material on the output voltage of eddy current sensor and improving the sensor's performances, this thesis focuses on studying the relationship between the electromagnetic properties of measured material and the coil impedance; presents the approach that the coil impedance vectors are projected onto the projection plane and the amplitude/phase mixed modulation approach to eliminate the influence of the electromagnetic properties of measured material on the output voltage of eddy current sensor. In addition, this thesis also researches the influences of the probe structure and its geometric parameters on the coil impedance, which are helpful to optimize the probe and improve the sensor's performances.To sum up, the following works are carried out:1. Research on the theoretical computational method to the coil impedanceThe theoretical computational model of eddy current sensor with the non-magneticand magnetic measured material is constructed, the coil impedance integral expression is deduced, and the coil impedance value is obtained by the Bessel series expansion using software package Mathematic?.2. Research on the numerical computation method to the coil impedanceBased on the geometric characteristics and boundary conditions of eddy currentsensor, 2D and 3D finite element models of eddy current sensor are built, respectively. The coil current is obtained by the time-harmonic electromagnetic analysis, and the coil impedance value is got according to the coil current obtained before.3. Fundamental theory for material-independent eddy current sensorThe approach that the coil impedance vectors are projected onto the projection plane is presented to eliminate the influence of the electromagnetic properties of measured material on the output voltage of eddy current sensor by extracting two signals: coil resistance signal and inductance signal; the effectivity and feasibility of the approach above is validated by finite element method and experiment method. Meanwhile, the nonlinear errors compensation approaches are presented according to the validated conclusions.4. The functional circuit design and simulation of material-independent eddy current sensorAccording to the validation conclusions and the presented approach eliminating the influence of the electromagnetic properties of measured material on the output voltage of eddy current sensor, the amplitude/phase mixed modulation approach are adopted to design, the designed circuit is simulated in software package Multisim, and the functional circuit and the fitting function method is used to compensate the nonlinear errors.5. The experiment on the functional circuitBased on the characteristics of designed circuit and the properties of key components, the layout and wiring of PCB is performed in software package Protel DXP; The circuit is debugged after the components are welded; The experiment platform is constructed and the function of the circuit is experimented.6. Research on the influence of the probe structures and its geometric parameters on the sensor's performancesThe influences of the probe structure and its geometric parameters on the coil impedance are investigated in terms of coil shape and its geometric parameters, the ferrite core and its geometric parameters by the theoretical deduction and finite element method. The influences of them on the sensor's performances are determined further.The researches above can eliminate the influence of the electromagnetic properties of measured material on the output voltage of eddy current sensor, as well as result in that the study on coil impedance from the nonmagnetic measured material to the magnetic one, that the conclusions on the influence of the probe structure and its geometric parameters arrived from the coil impedance is more effective to instruct the design of probe than those from the magnetic flux distribution.The originalities in this thesis can be summarized as follows:1. The computational models of coil impedance under a magnetic and nonmagnetic measured material are builtThe measurand of eddy current testing is embodied by the change of coil impedance. Therefore, the coil impedance is a very important physical quantity in eddy current testing. However, the present studies mainly focus on the nonmagnetic measured material, and the researches on the coil impedance under the magnetic one are limited. The mathematical model of coil impedance and finite element model under a magnetic and nonmagnetic measured material are built, respectively. Based on the mathematical model, the integral expression of coil impedance under the magnetic and nonmagnetic measured material is deduced in detail. To obtain the coil impedance value conveniently, the integral expression of coil impedance is expanded as the Bessel series and the air domain is truncated, and the impedance value is obtained by Mathematic^TM. Based on the finite element model, the magnetic flux distribution of sensor, the eddy current distribution in measured and the coil impedance value are computed, respectively. The above research develops the research on the coil impedance computational methods from the nonmagnetic measured material to the magnetic one, which promotes the eddy current testing theory perfect.2. The approach to eliminate the influence of the electromagnetic properties of measured material on the output voltage of sensor is presented.According to the energy conservation principle and the conclusions arrived from the analysis to the change of coil impedance under different measured material, the phenomenon that the coil resistance and inductance under different materials and the same measurement distance is linear is discovered. Therefore, the approach that the coil impedance vectors are projected onto the projection plane is put forward to eliminate the influence of the electromagnetic properties of measured material on the output voltage of sensor, and finite element method and experiment method validate the above approach is feasible and effective. Finally, the amplitude/phase mixed modulation approach is used to design the functional circuit.3. The relationships between the probe structure, geometric parameters and the sensor's performances are investigated in terms of coil impedance.When the measurand is constant, the changes of the probe structure and its geometric parameters not only show the changes of coil impedance, but also affect the sensor's performances. The present studies mainly pay attention to the influence of the probe structure and its geometric parameters on the electromagnetic field distribution of eddy current sensor. However, there are many hypotheses in the theoretical deduction about the electromagnetic field distribution, it is difficult and inaccurate to measure the electromagnetic field distribution. In addition, the instrument measuring the electromagnetic field distribution is expansive. Compared with the electromagnetic field distribution, the computation and measurement to the coil impedance is more accurate and convenient. Hence, the conclusions arrived from the influences of the probe structure and its geometric parameters on the sensor's performances are more effective to instruct the design of probe and improve the sensor's performances.