A. C. Bruno

A Magnetostrictive Composite-Fiber Bragg Grating Sensor

This paper presents a light and compact optical fiber Bragg Grating sensor for DC and AC magnetic field measurements. The fiber is coated by a thick layer of a magnetostrictive composite consisting of particles of Terfenol-D dispersed in a polymeric matrix. Among the different compositions for the coating that were tested, the best magnetostrictive response was obtained using an epoxy resin as binder and a 30% volume fraction of Terfenol-D particles with sizes ranging from 212 to 300 μm. The effect of a compressive preload in the sensor was also investigated. The achieved resolution was 0.4 mT without a preload or 0.3 mT with a compressive pre-stress of 8.6 MPa. The sensor was tested at magnetic fields of up to 750 mT under static conditions. Dynamic measurements were conducted with a magnetic unbalanced four-pole rotor.

Detection of reentry currents in atrial flutter by magnetocardiography

The magnetic signal produced by atrial flutter induced in isolated rabbit hearts has been detected. A simple model describing a circus reentry path is discussed and fitted to the experimental data. Agreement between the simulated magnetic field obtained using the model and animal experimental results suggests that, at least in the preparation used, the presence of a reentry current can be checked and described in a zero-order approximation by the circular motion of a constant-intensity current dipole. It appears that magnetocardiography can be used as a noninvasive technique to locate and to provide information about the radius of the circuit of such reentry currents.<<ETX>>

Discrete spatial filtering with SQUID gradiometers in biomagnetism

First‐, second‐, and third‐order gradiometers used in detecting biomagnetic signals are analyzed as spatial filters. Their transfer functions independent of the source to be measured are presented and both the magnitude and phase characteristics of the transfer functions are analyzed. The distortion introduced by the gradiometer can be estimated from these characteristics. In order to treat the signal in that approach, the spatial Fourier transform of a magnetic signal produced by a current dipole at a given distance is discussed.

A Noncontact Force Sensor Based on a Fiber Bragg Grating and Its Application for Corrosion Measurement

A simple noncontact force sensor based on an optical fiber Bragg grating attached to a small magnet has been proposed and built. The sensor measures the force between the magnet and any ferromagnetic material placed within a few millimeters of the sensor. Maintaining the sensor at a constant standoff distance, material loss due to corrosion increases the distance between the magnet and the corroded surface, which decreases the magnetic force. This will decrease the strain in the optical fiber shifting the reflected Bragg wavelength. The measured shift for the optical fiber used was 1.36 nm per Newton. Models were developed to optimize the magnet geometry for a specific sensor standoff distance and for particular corrosion pit depths. The sensor was able to detect corrosion pits on a fuel storage tank bottom with depths in the sub-millimeter range.

In-fiber Fabry-Perot interferometer for strain and magnetic field sensing.

In this paper we discuss the results obtained with an in-fiber Fabry-Perot interferometer (FPI) used in strain and magnetic field (or force) sensing. The intrinsic FPI was constructed by splicing a small section of a capillary optical fiber between two pieces of standard telecommunication fiber. The sensor was built by attaching the FPI to a magnetostrictive alloy in one configuration and also by attaching the FPI to a small magnet in another. Our sensors were found to be over 4 times more sensitive to magnetic fields and around 10 times less sensitive to temperature when compared to sensors constructed with Fiber Bragg Grating (FBG).

Spatial Fourier transform method for evaluating SQUID gradiometers

A simple method of measuring the spatial transfer function of a gradiometer, consisting of a flux transformer coupled to a SQUID, is presented and it is compared with theoretical predictions. Based, on this approach, a new method of reporting a gradiometer’s performance is proposed; the rejection factor is expressed in decibels obtained directly from the transfer function plot.

New magnetic techniques for inspection and metal-loss assessment of oil pipelines

Superconducting quantum interference device (SQUID) magnetometer has been used to detect metal-loss on the outside surface of small diameter pipes covered with thermal insulation at lift-offs up to 40 mm. In addition, an inversion method based on a finite element model was developed to recover of the metal-loss three-dimensional shape from fields obtained with high sensitivity Hall sensors due to metal-loss in both wall surfaces of a large diameter pipe.

Two-dimensional spatial frequency response of SQUID planar gradiometers

Planar gradiometers can be modelled as two-dimensional spatial filters, taking into account area, baseline and shape of the coils. We associate a spatial frequency response with each configuration studied and show that planar gradiometers behave as band-pass spatial filters. Also, in order to determine a spatial frequency range for typical magnetic field sources, we calculate the two-dimensional Fourier transform of the field due to a current dipole for several liftoffs. Important issues such as gradiometer spatial cutoff frequencies, bandwidth and symmetry are discussed.

Digital filter design approach for SQUID gradiometers

A review of the traditional method for designing gradiometers is made. A nonrecursive digital filter model for the gradiometer is presented, giving a new set of parameters for the gradiometer identification. Some designs are analyzed using the proposed set. As an example, a true differentiator is designed to be used as the SQUID input coil. It is shown that the differentiator has the same noise rejection as the conventional gradiometer but provides more signal sensitivity.

Improving the detection of flaws in steel pipes using SQUID planar gradiometers

The detection by magnetic techniques of corrosion pits on pipes covered with thermal insulation is critical. Without removing the insulating layer, SQUID technology is the only one available, because the sensor must be placed at a distance from the pipe surface. In this paper we compare the performance of different SQUID gradiometer designs, varying coil area and gradiometer baseline, to determine the best configuration for this task. The gradiometer response was simulated and analyzed by using a spatial frequency model that incorporates the design parameters mentioned before. A three-dimensional finite element model was developed to simulate a typical flaw geometry in small diameter steel pipes due to corrosion pitting. It is shown that planar gradiometers enhance dramatically flaw signal visualization when compared with a typical axial gradiometer design.