Feilu Luo

Reduction of Lift-Off Effects in Pulsed Eddy Current for Defect Classification

Pulsed eddy-current (PEC) testing is an electromagnetic nondestructive testing & evaluation (NDT&E) technique and defect classification is one of the most important steps in PEC defect characterization. With pulse excitation, the PEC response signals contain more features in time domain and rich information in frequency domain. This paper investigates feature extraction techniques for PEC defect classification including rising time, differential time to peak and differential time to zero, spectrum amplitude, and differential spectrum amplitude. Experimental study has been undertaken on Al-Mn 3003 alloy samples with artificial surface defects, sub-surface defects, and defects in two-layer structures under different lift-off. Experimental results show that methods are effective to classify the defects both in single-layer structures and two-layer structures. Comparing the results of different methods, it is found that differential process can eliminate the lift-off in defect classification in both time domain and frequency domain. The study can be extended to defect classification in complex structures, where lift-off effects are significant.

Defect characterisation based on heat diffusion using induction thermography testing.

Pulsed eddy current (PEC) thermography (a.k.a. induction thermography) has been successfully applied to detect defects (corrosion, cracks, impact, and delamination) in metal alloy and carbon fiber reinforced plastic. During these applications, the defect detection mechanism is mainly investigated based on the eddy current interaction with defect. In this paper, defect characterisation for wall thinning defect and inner defect in steel is investigated based on heat diffusion. The paper presents the PEC thermography testing, which integrates the reflection mode and transmission mode by means of configuring two cameras on both sides of sample. The defect characterisation methods under transmission mode and reflection mode are investigated and compared through 1D analytical analysis, 3D numerical studies, and experimental studies. The suitable detection mode for wall thinning and inner defects quantification is concluded.

PEC Frequency Band Selection for Locating Defects in Two-Layer Aircraft Structures With Air Gap Variations

Multilayer structures are widely used in aircraft fuselage. Because of the interlayer air gap caused by deformation or disbonding, conventional single-frequency eddy current cannot discriminate between second-layer defect signals and gap signals. In this paper, several defects at varied locations (i.e., first-layer surface, first-layer subsurface, second-layer surface, and second-layer subsurface) are manufactured into two-layer Al-Mn 3003 alloy specimen with various air gaps. Pulsed eddy current (PEC) is investigated in combination with principal component analysis (PCA) to classify and locate defects in the specimen. The new feature named differential frequency to zero is proposed, and the frequency responses of selected frequency band are processed through PCA. The principal components are used for locating defects. The experimental results show that first-layer surface defects, first-layer subsurface defects, second-layer surface defects, and second-layer subsurface defects can be classified when air gap is varied from 0 to 1.4 mm through the proposed methods. In conclusion, PEC testing with the help of PCA can eliminate the interlayer air gap and liftoff effect, which has potential for defect characterization in multilayer aircraft structures.

1/f noise suppression of giant magnetoresistive sensors with vertical motion flux modulation

The 1/f resistance noise is one of the main noise sources of giant magnetoresistive sensors, which will cause intrinsic detection limit at low frequency. To suppress this noise, a vertical motion flux modulation (VMFM) scheme with high efficiency and simple structures is proposed. And the electrical coupling effect is investigated with an equivalent circuit model. We found that the electrical coupling disturbance can be suppressed by improving the symmetry of VMFM sensors. The modulation efficiency of VMFM sensors has reached 18.8%, which is higher than most prototype sensors with other flux modulation schemes.

Integrating magnetoresistive sensors with microelectromechanical systems for noise reduction

1/f noise is the dominant detection limit of magnetoresistive (MR) sensors at low frequency. The vertical motion flux modulation (VMFM) integrating with microelectromechanical systems (MEMS) can reduce 1/f noise by tens or hundreds of times, although thermal-mechanical noise possibly has strong impact on the detection ability of VMFM sensors like common MEMS sensors. Surprisingly, the voltage noise originated from thermal-mechanical noise is actually far less than the noise base of MR sensors, which indicates a great perspective for the integration of MEMS and MR sensors.

Integrated Compensation of Magnetometer Array Magnetic Distortion Field and Improvement of Magnetic Object Localization

A fluxgate magnetometer array for magnetic object localization is designed, where hard-iron and soft-iron magnetic distortion fields are the major factors influencing measurement accuracy. A vector compensation method is proposed to suppress error, in which magnetometer error, misalignment error, and magnetic distortion fields are considered. The experimental system mainly consists of a plane cross magnetometer array, a magnet (to be hard-iron), a steel block (to be soft-iron), and a deployment platform (to change the attitude of the magnetometer array). Experimental results show that integrated compensation parameters can be obtained accurately, and array difference errors are reduced about two orders, thus proving the effectiveness of the vector compensation method. The compensated array is used for static and dynamic localization in 3-D. In static situation, localization errors are reduced from 0.17 m, 0.28 m, and 0.27 m to 0.03 m, 0.05 m, and 0.14 m, respectively. On the object deployment trace, error intensity is reduced from 0.17 to 0.04 m. In particular, the dynamic localization results are unreliable without compensation, and the error intensity is reduced from 2.47 to 0.05 m using the proposed method, thus improving the localization accuracy.

Nonlinear temperature compensation of fluxgate magnetometers with a least-squares support vector machine

Fluxgate magnetometers are widely used for magnetic field measurement. However, their accuracy is influenced by temperature. In this paper, a new method was proposed to compensate the temperature drift of fluxgate magnetometers, in which a least-squares support vector machine (LSSVM) is utilized. The compensation performance was analyzed by simulation, which shows that the LSSVM has better performance and less training time than backpropagation and radical basis function neural networks. The temperature characteristics of a DM fluxgate magnetometer were measured with a temperature experiment box. Forty-five measured data under different magnetic fields and temperatures were obtained and divided into 36 training data and nine test data. The training data were used to obtain the parameters of the LSSVM model, and the compensation performance of the LSSVM model was verified by the test data. Experimental results show that the temperature drift of magnetometer is reduced from 109.3 to 3.3 nT after compensation, which suggests that this compensation method is effective for the accuracy improvement of fluxgate magnetometers.

A New Calibration Method of Three Axis Magnetometer With Nonlinearity Suppression

Nonlinearity is a prominent limitation to the calibration performance of vector magnetometers. A new calibration model is proposed to suppress the nonlinearity of magnetometers and improve calibration performance, in which the nonlinearity coefficients of scale factors are considered. The experimental system mainly consists of a three-axis fluxgate magnetometer (MAG3300), a 2D nonmagnetic rotation equipment, and a proton magnetometer (CZM-3), in which the nonmagnetic rotation equipment is used to change the position of three-axis fluxgate magnetometer, and the scalar value of magnetic field is obtained with the proton magnetometer and considered to be the true value. The principle of this new calibration method is analyzed, and the calibration procedures are introduced. Experimental results show that after nonlinearity suppression, the root-mean-square error of calibration can be reduced by three times. It suggests an effective way to improve the calibration performance of three-axis magnetometers.

Magnetic interferential field compensation in geomagnetic measurement

The accurate measurement of a geomagnetic field is a key technology in geomagnetic navigation. Many magnetic interferential fields on navigation vehicles, such as ferromagnetic parts and electric equipments, will seriously disturb the magnetometer, and thus should be compensated. This paper develops a new magnetic interferential field compensation method using three-axial magnetometer measurements. The method relies on a formulation derived from the difference of the measured geomagnetic magnitude and its true value. The goal is to evaluate the magnetic interferential field parameters first and then compensate the measurements with them. The trust-region method is adopted to evaluate the parameters in the formulation. A simulation is conducted to test the validity of the method. An experiment is designed and implemented on an underwater vehicle. Both simulation and experimental results indicate that vehicle magnetic interferential field can be well compensated with this method.

Designs of Slope Magnetic Flux Guides for 3-Axis Magnetic Sensor

Generally, giant magnetoresistive (GMR) sensors are only sensitive to the magnetic field in the plane of the substrate due to fabrication restraints. This paper designs and models slope magnetic flux guides that are deposited on the slope surface of a silicon substrate. Finite element method (FEM) simulations are used to optimize the flux guide designs. The flux guides can be deposited with good symmetry and are able to convert the out-of-plane magnetic flux to the GMR sensor plane. Meanwhile, a Wheatstone bridge is configured to deduce a differential voltage only relative to the z-component of the magnetic field. Additionally, the magnetic field in the active region of the GMR sensor would be intensified. With these flux guides, the magnetic field perpendicular to the chip surface can be detected with the GMR sensors in-plane. Also, the sensitivity of the sensor can be improved due to the amplification ability of the flux guides. An integrated 3-axis magnetic sensor with better angular position can be realized with the slope flux guides.