Pascal Druyts

Modeling the Response of Electromagnetic Induction Sensors to Inhomogeneous Magnetic Soils With Arbitrary Relief

A general model to compute the response of an electromagnetic induction sensor to a magnetic soil, in both time and frequency domains, is developed. The model requires modest computational resources and can be applied to arbitrary soil inhomogeneities and relief, and to arbitrary sensor coil shapes, orientations, and positions. Central to the model is the concept of a head sensitivity map, which can be used to characterize the sensor head as a function of the shape, size, and position of the sensor coils. Two further concepts related to the head sensitivity are presented, which are the zero equisensitivity surface and the volume of influence. We demonstrate that these concepts aid the understanding of the detector behavior. The general model is based on the Born approximation, which is valid if the soil magnetic susceptibility is sufficiently small. A simpler model, which is only valid for homogeneous half-space soils but does not require the Born approximation, is also developed. The responses predicted by both models are shown to be in good agreement with each other and also with available analytic solutions. Comparing the two models also enabled an expression for the error incurred when using the Born approximation to be established. We shown that, for most soils of relevance to mine clearance, the corresponding error is negligible.

Accurate and efficient modeling of monostatic GPR signal of dielectric targets buried in stratified media

This paper presents an accurate model of a monostatic stepped-frequency continuous-wave (SFCW) ground-penetrating radar (GPR). The model takes into account the multiple reflections occurring between the soil, target and antenna, which is a transverse electromagnetic (TEM) ultra-wide band (UWB) horn. The antenna radiation pattern is accounted for by a Huyghens cosinusoidal distribution of electric and magnetic current located on the aperture. The model is validated by experiments, involving dielectric targets embedded in a sandbox. These experiments validate altogether the radar modeling, as well as the MoM and the dyadic Greens functions (DGFs) used in the numerical algorithms.

Sensorless drive of surface mounted permanent-magnet brushless DC machines with diametric windings based on inductance measurements

Last decades, important progress have been made in sensorless control methods based on the tracking of magnetic anisotropies linked to the rotor. These methods allow to estimate the rotor position at low speed down to standstill. The magnetic anisotropy is generally approached by a sinusoidal shaped function. However, the theory must be adapted in case of additional harmonic content in the anisotropy function. This paper specifically addresses the problem for the surface-mounted permanent-magnet brush-less DC machines with diametric windings. They present a peculiar magnetic anisotropy that suggests a simple method to detect the inversion of the magnetic field as a replacement to the dedicated sensors which are often used.

Multilevel data fusion approach for gradually upgrading the performances of identity verification systems

The aim of this paper is to propose a strategy that uses data fusion at three different levels to gradually improve the performance of an identity verification system. In a first step temporal data fusion can be used to combine multiple instances of a single (mono-modal) expert to reduce its measurement variance. If system performance after this first step is not good enough to satisfy the end-users needs, one can improve it by fusing in a second step result of multiple experts working on the same modality. For this approach to work, it is supposed that the respective classification errors of the different experts are de-correlated. Finally, if the verification systems performance after this second step is still not good enough, one will be forced to move onto the third step in which performance can be improved by using multiple experts working on different (biometric) modalities. To be useful however, these experts have to be chosen in such a way that adding the extra modalities increases the separation in the multi-dimensional modality-space between the distributions of the different populations that have to be classified by the system. This kind of level-based strategy allow to gradually tune the performance of an identity verification system to the end-users requirements while controlling the increase of investment costs. In this paper results of several fusion modules will be shown at each level. All experiments have been performed on the same multi-modal database to be able to compare the gain in performance each time one goes up a level.

Compensating the influence of the stator resistor and inverter nonlinearities in signal-injection based sensorless strategies

Among the sensorless position estimation methods in electrical machines drives, the injection of voltage test pulses is a promising strategy. Several papers are studying and applying this strategy, in particular for permanent magnet synchronous machine (PMSM) at low speed or standstill. Recently, an adaptive test pulses sequence reducing the current distortions has been proposed. However, the test pulses can be influenced by the voltage drops such as across stator resistor and semiconductor switches. By neglecting these voltage drops, the distortion in the current samples can not be fully reduced. In this paper, we improve the adaptive test pulses strategy by estimating and compensating the resistive drops. We also discuss the impact of the inverter nonlinearities. Finally, we present experimental results on a PMSM.

Strategy to detect and prevent the current zero-crossing for inverter powered drives

The output voltage of an inverter is often assumed proportional to the drive modulation signal. However, in practice the inverter presents nonlinear characteristics that may significantly affect the output voltage, and this may be critical in the context of sensorless drives and parameter identification. Nowadays, several solutions exist to compensate for the nonlinear behaviour of the inverter. Nevertheless, for the nonlinear effects at zero-crossing currents, no satisfying solution has yet been found. This paper proposes a calculation method that detects the occurrence of one or multiple zero-crossings of the current during a switching period. Then, an offset, selected in order to minimize the disturbance on the torque, is injected in the current control which prevents the current to cross zero. In that way, the disturbing effects of the zero-crossing current on the output voltages can be prevented. The method is evaluated for a permanent-magnet synchronous machine.

Effect of the soil on the metal detector signature of a buried mine

This paper analyzes the effect of the soil on the response of a metal detector (MD). The total response is first decomposed in a direct coupling between the transmitter and the receiver, the mine contribution and the soil contribution. The mine contribution is further related to its free space signature by introducting a number of transfer functions (TFs). Those TFs characterize the effect of the soil on the field propagation, from the transmit coil to the mine and back to the receiver, and on the mine signature. The expressions derived are quite general. However the TFs and other quantities of interest can only be computed if the scattering problem has been solved. For this it is usually necessary to resort to numerical techniques. Such techniques are computationally expensive, especially to analyze the various effects of the soil as they require to compute the solution for a large set of parameters. Therefore, we propose to model a buried mine by a multilayered sphere. From outside to inside, the layers represent the air, the soil, the mine explosive and the mine metallic content. Further, the analytic solution for such a multilayered sphere is used to compute the mine and soil responses, the mine free space signature and the various TFs as a function of the parameters of interest such as the soil electromagnetic (EM) properties or the mine depth. Finally, the validity domain of a number of practical approximations is discussed.

Underwater magnetic target localization and characterization using a three-axis gradiometer

Magnetometers and magnetic gradiometers are commonly used to detect ferromagnetic targets. When the distance to the target is large compared to its size, the target can be modeled as a dipolar source and can then be characterized by a single vector: its magnetic moment. The estimation of the magnetic moment of the target can be used to reduce the false alarm rate. We consider a total-field three-axis gradiometer which measures the gradient of the magnitude of the magnetic field along three orthogonal axes. We show that for an ideal gradiometer, the inversion problem separates into two linear problems and can therefore be solved without any initial estimation of the parameters. Moreover, the method can be applied directly on the data gathered by the gradiometer, without any grid interpolation and in real time. For the considered gradiometer, a better estimation of the parameters can be obtained if the detailed geometry of the gradiometer (location of the magnetometers) is taken into account. We show that the method can be extended to magnetometers, vertical total-field gradiometers, horizontal total-field gradiometers and tensor gradiometers. Finally, we analyze the effect of various survey parameters (gradiometer altitude, inter-track distance and target magnetic moment) on the accuracy of the estimated parameters in the presence of noise (magnetic noise and error on the position of the gradiometer).

Volume of Influence for Magnetic Soils and Electromagnetic Induction Sensors

The concept of volume of influence (VoI) for electromagnetic induction (EMI) sensors is introduced and accurately defined. It enables one to better understand the response of a magnetic soil to an EMI sensor, as well as the effect of soil inhomogeneity on soil compensation. The VoI is first defined as the volume producing a fraction of the total response of a homogeneous half-space. As this basic definition is not appropriate for sensor heads with intrinsic soil compensation, a generalized definition is then proposed. These definitions still do not yield a unique VoI, and a constraint must be introduced to reach uniqueness. Two constraints are investigated: one yielding the smallest VoI and the other one the layer of influence. Those two specific volumes of influence have a number of practical applications which are discussed. The smallest VoI is illustrated for typical head geometries, and we prove that, apart from differential heads such as the quad head, the shape of the smallest VoI is independent of the head geometry and can be computed from the far-field approximation. In addition, quantitative head characteristics are provided and show-among others-that double-D heads allow for a good soil compensation, assuming, however, approximate homogeneity over a larger volume of soil. The effect of soil inhomogeneity is further discussed, and a worst case VoI is defined for inhomogeneous soils.

Buried target signature extraction from ground-penetrating radar signal: measurements and simulations

Ground-penetrating radar (GPR) proves to be a very valuable tool in the field of humanitarian demining, especially for the detection of plastic land-mines. Recently, a monostatic stepped-frequency continuous-wave (SFCW) GPR, together with a conceptual model of the radar-antenna-soil system, has been developed for the characterization of the electromagnetic parameters of soil, i.e. dielectric permittivity (epsilon), magnetic permeability (mu) and electric conductivity (sigma). This approach is extended here to the extraction of the GPR signal and to modelling the signatures of buried targets. The equivalence principle is used to decompose the GPR signal into its soil and target-in-soil components, as well as to model the radar-soil-target system. It permits the soil contribution to be subtracted from the total GPR signal to provide the signature of the buried target. This signature is compared to simulations. For a proof of the concept, the GPR return signal from a buried metal sphere has been simulated using the Method of Moments and it shows good agreement with its measured counterpart. We also have extracted clean frequency- and time-domain signatures of a PMN-2 plastic mine embedded in a multilayered medium, subject to various water contents. The method is also applied to a B-scan above a buried conducting cylinder. Finally, a study of the main sources of errors in the extraction of the signature of a buried target shows that mistakes in antenna height measurement lead to errors more important than those due to misestimating the relative dielectric permittivity of the soil.