|Title:||Magnetoelectric sensors based on magnetic field gradient technique|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Pages:||xxvi, 181 pages : color illustrations|
|Abstract:||Magnetoelectric (ME) sensors have received considerable research and application attention as new generation passive magnetic field strength (MFS)-type magnetic sensors over the past decade because of their unique ability to directly detect MFSs and induce interestingly large ME voltages (on orders of mV vs. μV in Hall sensors) without the support of external power supplies, signal conditioners, and/or other auxiliary means as required by traditional active Hall sensors. Magnetic field gradients (MFGs) are the variations of MFSs per unit length in three-dimensional (3-D) space. MFG-type magnetic sensors, which generally consist of two MFS-type magnetic sensors (e.g., Hall sensors) spatially separated by a baseline to detect MFGs by differencing their output signals over the baseline, feature a unique and strong ability to suppress ambient (i.e., common-mode) noises. This distinct noise-suppression ability has enabled MFG-type magnetic sensors to play an increasingly crucial role in outdoor environments and critical applications involving high ambient noises. However, there does not appear to be any solid work to combine the merits of passive ME sensing with MFG noise suppression to realize standalone MFG-type ME sensors for the passive and direct detection of MFGs into electrical voltages. This thesis reports the theoretical and experimental studies on the realization of this novel type of ME sensors based on the MFG noise-suppression technique. The studies are also extended to the development of novel MFG-type ME current sensors, linear displacement transducers, and eddy-current probes for applications in electrical condition monitoring, electromagnetic positioning/navigation, and non-destructive evaluation, respectively. The proposed MFG-type ME sensors, in contrast to the state of-the-art MFS-type ME sensors, are essentially passive, small scale, and high performance in terms of simultaneously high detection sensitivity, strong common-mode noise rejection rate, small input-output nonlinearity, and low MFG noise over a broad frequency range under a small baseline. Original contributions of the works include: (1) A multi-objective and multi-constrain (MOMC) method to optimize configuration parameters (e.g., baseline, assembly configuration) as well as the figure-of-merits (e.g., detection sensitivity, common-mode noise rejection rate, input-output nonlinearity, and MFG noise) for general MFG-type magnetic sensors is proposed and developed by combining finite element analysis (FEA) and nonlinear programming theory. The thesis contributes a 3rd-order polynomial shape function to FEA for enabling a visualization of the 3-D distribution of the MFG matrix as well as a numerical verification of the symmetric and invariant zero-trace properties of the MFG matrix with excellent convergence of < 0.01%. Then, a signal-noise model is established to validate the MOMC optimization method. The optimized design for general MFG-type magnetic sensors is found to have baseline to signal-noise-distance ratio of 0.03-0.05 and position to signal-noise-distance ratio of <0.1. (2) A general fully-coupled multiphysics FEA model for the design, verification and optimization of gradient-type ME sensors is established by simultaneously solving the ME constitutive relations with magneto-mechano-electric governing equations. The FEA model is capable of performing dynamic analyses on spatial variables of magnetization, mechanical stresses, electric charges as well as lumped variables of resonant frequency, mode shape, ME voltage coefficients, detection sensitivity of the MFG-type ME sensors with different combinations of structures, material properties, and magneto-mechano-electric boundary conditions in different experiment configuration. Two detection modes, namely transverse mode and axial mode MFG-type ME sensors are empirically studied based on Terfenol-D/PZT 8/Terfenol-D sandwich-type ME composites. The ME voltage coefficients of the composites are calculated to be 231-974 V/T in the frequency range of 1 Hz-170 kHz with the peak of 974 V/T at the resonance frequency of 120.3 kHz. A transverse MFG detection mode sensitivity of 4.93-36.44 V/(T/m) is achieved in the frequency range of 1 Hz-170 kHz under a transversely separated baseline of 35 mm. The other axial MFG detection mode ME sensor has achieved detection sensitivity of 0.21-2.02 V/(T/m) in the frequency range of 1 Hz-340 kHz under an axially separated baseline of 15 mm. Relative error of <3% is achieved between the FEA and experimental results.|
(3) A novel MFG-type ME current sensor operating in transverse MFG detection and conversion mode is developed based on a pair of ME composites that have a back-to- back capacitor configuration under a baseline and a magnetic biasing in an electrically-shielded and mechanically-enclosed housing. The physics behind the current sensing process is the product effect of the current-induced MFG effect associated with vortex magnetic fields of current-carrying cables (i.e., MFG detection) and the MFG-induced ME effect in the ME composite pair (i.e., MFG conversion). The output voltage of the current sensor is directly obtained from the transverse MFG-induced difference in ME voltage between the two ME composites and is calibrated against transverse MFGs to give a high transverse MFG detection sensitivity of 0.4-30.6 V/(T/m), a strong common-mode magnetic field noise rejection rate of <-14.5 dB, a small input-output nonlinearity of <10 ppm, and a low gradient noise of 0.16-620 nT/m/√Hz in a broad frequency range of 1 Hz-170 kHz under a small baseline of 35 mm. An analysis of experimental gradient noise spectra obtained in a magnetically-unshielded laboratory environment reveals the domination of the pink (1/f ) noise, dielectric loss noise, and power-frequency noise below 3 kHz, in addition to the circuit noise above 3 kHz, in the transducer. The current sensing performance of the sensor is evaluated, both theoretically and experimentally, under multisource noises of electric fields, magnetic fields, vibrations, and thermals. The sensor combines the merits of small nonlinearity in the current-induced MFG effect with those of high sensitivity and high common-mode noise rejection rate in the MFG-induced ME effect to achieve a high current sensitivity of 0.65-12.55 mV/A in the frequency range of 1 Hz-170 kHz, a small input-output nonlinearity of <500 ppm, a small thermal drift of <0.2%/°C in the current range of 0-20 A, and a high common-mode noise rejection rate of 17-28 dB from multisource noises. (4) A novel MFG-type ME linear displacement transducer operating in axial MFG detection mode is developed based on the large ME effect and the MFG technique in a pair of magnetically-biased, electrically-shielded, and mechanically-enclosed ME composites having an axial orientation and separation. The output voltage of the transducer is directly obtained from the axial MFG-induced difference in ME voltage between the two ME composites and is calibrated against axial MFGs to give a high detection sensitivity of 1.4-71.6V/(T/m) in a broad frequency range of 1 Hz-170 kHz under a baseline of 180 mm. The transducer is prototyped and deployed in a 1/10-scaled inspection vehicle-overhead cable model to experimentally evaluate its sensitivity, nonlinearity, and errors between the theoretical, numerical, and experimental data at different transverse displacements and assembled heights. An optimal height-to-baseline ratio of 1/√8 for satisfying the linear response condition with the constraint of displacement to ±50% of base length is analytically derived and experimentally verified with relative error less than 2%. It is expected that the proposed LDT can overcome the discontinuous response and nonlinearity problems intrinsic in traditional magnetic field-based LDTs, thereby providing a continuous displacement (movement) feedback with an accuracy of ~ 1 mm. (5) A novel MFG-type ME eddy-current probe is developed based on the axial MFG detection mode to sense eddy current-induced MFGs. The probe consists of a pair of ME composites, an excitation coil, and a copper-screened plastic tube. The working principle is described by energizing the excitation coil with an AC current of constant amplitude to create a sinusoidal magnetic field and hence an eddy current in the conductive object to be evaluated; by generating an eddy current-induced magnetic field in the opposite direction; and by detecting the opposite MFG using the ME composite pair to give an output voltage. On the basis of MFG technique, the common-mode magnetic field generated by the excitation coil is suppressed, and the presence of a surface or subsurface flaw will disturb the induced eddy currents which, in turn, can be directly monitored by the change of the output voltage of the probe. The probe is calibrated to have a high detection sensitivity of 0.22-1.98 V/(T/m) in the frequency range of 1 Hz-340 kHz. The input resistance (excitation coil resistance) and output resistance of the probe are measured to be 0.85 Ω and 20.4 MΩ in DC condition, and 30.8 Ω and 2 kΩ at the resonance frequency of 238 kHz, respectively. Both 1-D and 2-D non-destructive evaluation experiments are conducted by energizing the excitation coil with an AC current of 65 mA peak at resonance and with a lift-off distance of 1 mm. The probe shows a strong capability of detecting groove, scratched, and erosion flaws with a resolution of ~0.1 mm and a positioning accuracy of 1.5 mm.
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