Davorin Ambrus

Pulsed eddy current nondestructive testing of ferromagnetic tubes

In sinusoidal eddy current nondestructive testing (NDT) of thick ferromagnetic tubes, wall thickness is measured with the coils displayed for 2-3 tube diameters (remote field eddy current technique). The tube inner diameter is measured at higher frequency with another pair of coils displaced for around 1 tube diameter (electrical caliper). In this paper, we give a thorough analysis of the excitation frequency and the distance between the coils for measurement of the tube inner diameter and wall thickness as a background for the application of pulsed eddy current (PEC). We propose application of one pair of coils displaced for 1-2 tube diameters for measurement of those tube parameters employing the features of response to the pulsed excitation. Results of our experimental work confirm that PEC technique provides a significant improvement of present eddy current systems for NDT of the ferromagnetic tubes.

Validation of a coil impedance model for simultaneous measurement of electromagnetic properties and inner diameter of a conductive tube

In eddy-current nondestructive testing of conductive tubes such as oil-well casings, the measured tube wall thickness and inner diameter must be compensated for variations of electromagnetic properties of a tube material -magnetic permeability and electrical conductivity. Commercial systems for eddy-current nondestructive testing of oil-well casings incorporate multiple coils and multifrequency excitation for measurement of the inner diameter and the electromagnetic properties. This paper investigates the feasibility of the electromagnetic properties and inner diameter measurement with only one coil excited at one frequency. The authors have derived a coil impedance model and developed an inversion procedure for the determination of tube properties. Measurements have been performed on several tubes made of different materials. In order to validate the model and the optimization algorithm, measurement results were compared with the predictions of the model and finite-element analysis. Applicability of the method has been confirmed for the investigated frequency range (1-50 kHz).

Robust Estimation of Metal Target Shape Using Time-Domain Electromagnetic Induction Data

Discriminating metal parts of buried hazardous targets from ordinary metallic clutter is a very difficult and time-consuming task. For that purpose, electromagnetic induction (EMI) sensors composed of multiple transmitter/receiver coils in multiaxis arrangements are commonly used. In this way, data of high fidelity and spatial diversity are obtained so that the target can be characterized in terms of its geometric and electromagnetic properties. However, this often increases sensor size and reduces its portability, which is a problem for humanitarian demining applications, where compact, robust, and lightweight sensors are needed. If such sensors are to be used for target characterization, robust estimation algorithms are required, capable of coping with limited spatial information content and uncertainties related to sensor positioning and its coil geometry model. In this paper, we present a robust concept for estimating the general shape of magnetic metal targets using time-domain EMI sensors with single-axis coil geometries. We introduce the signature matrix, a parameter derived from time-dependent eigenvalues of the targets magnetic polarizability tensor, and use it for shape estimation. The proposed method was evaluated both through simulations and experiments, using two different sensor platforms (laboratory-based experimental sensor platform and a commercial metal detector mounted on a mobile robot). The obtained results clearly indicate that the target shape can be estimated from sensor data of limited spatial diversity and under the uncertainties of sensor positioning and coil geometry.

Active induction balance method for metal detector sensing head utilizing transmitter-bucking and dual current source

A central problem in a design of frequency domain electromagnetic induction sensors used in landmine detection is an effective suppression of a direct inductive coupling between the transmitter and the receiver coil (induction balance, IB). In sensing heads based on the transmitter-bucking configuration, IB is achieved by using two concentric transmitter coils with opposing exciter fields in order to create a central magnetic cavity for the receiver coil. This design has numerous advantages over other IB methods in terms of detection sensitivity, spatial resolution, sensor dimensions and suitability for model-based measurements. However, very careful design and precise sensing head geometry are required if a single excitation source is used for driving both transmitter coils. In this paper we analyze the IB sensitivity to small perturbations of geometrical properties of coils. We propose a sensor design with dual current source and active induction balance scheme which overcomes the limitations of geometry-based balancing and potentially provides more efficient compensation of soil effects.

A digital tachometer for high-temperature telemetry utilizing thermally uprated commercial electronic components

The high-temperature environment of an oil well requires reliable downhole instrumentation, typically based on simple and robust circuits, low power consumption, and smart measurement algorithms. In this paper, we present an electronic design and measurement methodology for a simple microcontroller-based digital tachometer, optimized for downhole spinner-flowmeter applications and multichannel telemetry. We have developed and experimentally evaluated an effective algorithm for direction-sensitive rotational speed measurement, which combines quadrature pulse decoding with the method of dependent count. The algorithm employs a temperature compensation technique, making it suitable for the high-temperature application of commercial-grade electronic parts. Our results confirm the functionality and reliable operation of the tachometer, targeted at high-temperature short-term oil-well logging applications.

Model-based target classification using spatial and temporal features of metal detector response

The paper presents a novel model-based algorithm for classifying buried metallic targets using spatial and temporal response properties of a pulse induction metal detector mounted on a mobile robot for autonomous landmine detection. In the proposed approach, we firstly derive a simplified analytical model for spatial distribution of the primary magnetic field that corresponds to transmitter/receiver coil geometry of a given metal detector. The sensing head model is then coupled to a metallic target analytical dipole model whose parameters are the magnetic polarizability tensor and the target location. Finally, the forward sensor/target model is fitted to sensor data obtained by spatially mapping the suspected target area using a mobile robot. Inverted magnetic polarizability tensors corresponding to sensor data acquired at different time instances (gates) are used for target characterization and classification. The algorithm is experimentally evaluated on a dataset collected from a test site containing surrogate mines (metallic spheres) and clutter targets.

Analytical modelling of soil effects on electromagnetic induction sensor for humanitarian demining

Accurate compensation of the soil effect is essential for a new generation of sensitive classification-based electromagnetic induction landmine detectors. We present an analytical model for evaluation of the soil effect suitable for straightforward numerical implementation. The modelled soil consists of arbitrary number of conductive and magnetic layers. The solution region is truncated leading to the solution in form of a series rather than infinite integrals. Frequency-dependent permeability is inherent to the model, and time domain analysis can be made using DFT. In order to illustrate the model usage, we evaluate performances of three metal detector designs.

Compensation of Coil Radial Offset in Single-Coil Measurement of Metal Tube Properties

Measurements of inner radius and electromagnetic properties (magnetic permeability and electrical conductivity) of metal, ferromagnetic tubes, such as oil-well casings, are essential in the assessment of their condition or as inputs for other measurement applications, e.g. reservoir electromagnetic evaluation through casing. In our previous work, we have shown that the inner radius and the permeability-to-conductivity ratio of the tube can be obtained by measurement of the impedance of a single coil axially centered within the tube. In this paper, we propose and experimentally verify the coil impedance model that is applicable for measurement of the tube properties when the tube and the coil axes are misaligned. Correction of the measured impedance for coil radial offset improves measurement accuracy and reduces the effect of the coil wobble.

UWB localization for discrimination-enabled metal detectors in humanitarian demining

Handheld metal detectors are ubiquitous devices in humanitarian demining. They are sensitive to extremely small metallic objects as in low-metallic content landmines. However, this leads to enormous false alarm rates (1 to 1000). Recent research in electromagnetic induction techniques for discrimination of hazardous metallic targets and clutter gives rise to a hope of faster, smarter and safer mine detection. These techniques require that position of the metal detectors sensor head be known to sub-centimeter accuracy. To this aim, we present and evaluate an ultra-wide bandwidth positioning embedded system. We evaluate its precision and accuracy, and show that it can be used for tracking of the sensor head with at least 40 Hz positon refresh rate. The achieved measurement uncertainty is better than 10 cm. We discuss the further improvements that can be made in such a localization system.

Automatic compensation of primary field coupling for a frequency-domain electromagnetic induction sensor

Residual mutual coupling between the transmitter and receiver coil(s) is a well-known problem in frequency-domain (FD) electromagnetic induction (EMI) sensors. In cases where a ratio of the measured signal to primary field coupling signal is sufficiently high, careful geometrical arrangements of coils, such as those used in EMI gradiometers, are normally sufficient for suppressing the primary field coupling. However, in applications requiring very high sensitivity, additional active means of sensor compensation are needed, since the measured object response may become commensurable with the primary field response and its thermally-induced fluctuations. In this paper, we investigate a technique for closed-loop automatic compensation of residual primary field coupling and its thermal drift, based on the use of an active sensor balancing. The technique is experimentally validated in a laboratory environment using a prototype of FD EMI sensor with magnetic cavity type design. Preliminary results indicate that direct coupling voltage can be effectively controlled, providing larger dynamic range and increased accuracy in measurements of EMI responses of small metallic objects.