Search

The accuracy in the measurement of magnetization curve (M-H) for soft materials is easily influenced by the external static field in the unshielded environment. To eliminate the influence from the environmental field, the measurement system must be placed in a zero-gauss chamber. However, such a device is not widely available and also costly. In this work, the rapid M-H measurement system with a giant magnetoresistance (GMR) sensor serving as the zero-field detector is proposed. The M-H curve is obtained by integrating the voltage in the pickup coil to determine the magnetization of the test sample under a sweeping magnetic field. The environment field component along the direction of the sweeping field is suppressed by using the output of the GMR sensor as the trigger signal to determine the offset field in the measured M-H curve. The experimental results show that the M-H curves for various kinds of soft magnetic samples, e.g. mu-metal wire, can be obtained under the interference of an applied static field. The proposed method helps improve the accuracy of the rapid M-H measurement system in the unshielded environment.

The alternating uniform magnetic field is useful for the biomagnetic study with magnetic nanoparticles. However, for the coil system driven by the magnetic field generator with a conventional low-voltage power source, it is challenging to achieve the desired field strength with a large coil due to the large impedance at higher frequencies. This article proposed a Helmholtz coil system that can generate the dc and ac magnetic field up to 1.3 kHz with low distortion. In order to overcome the inductive load, a computer-controlled parallel capacitor array was designed and built to tune the series coil–capacitor circuit to the resonant frequency. Experimental results show that the coil system has a bandwidth over 1 kHz with a 1% uniform region covering the $10\,\,\times10$ mm sample area. The proposed system is promising for the application in biomagnetic assay using the tunnel magnetoresistance sensors.

In this work, we propose a real-time noncontact thickness measurement system for conductive materials based on an inductor–capacitor ( LC ) resonant eddy-current sensor. For the aluminum, copper, stainless steel 304, and carbon steel samples of various thicknesses, the in-phase and quadrature components of the sensor output taken during liftoff scans are found to be a series of quadratic-like curves on the phase versus logarithmic amplitude diagram. These curves can be used to generate the standard phase–amplitude relations for thickness conversion. The phase–amplitude relations of the thickness between the values of standard samples are calculated by cubic spline interpolation. With the standard database of phase–amplitude relations, the noncontact thickness measurement was implemented in real time for the copper foil sample with an error of ±3% when the liftoff distance is less than 5 mm. The liftoff tolerance is as large as 100% of the probe diameter, indicating that the proposed method is useful for online quality inspection of conductor films on flexible substrates.

The high sensitivity for low-frequency fields makes the tiny giant magnetoresistance (GMR) sensors suitable for high-resolution eddy-current (EC) imaging. In this work, we proposed a novel design of the miniature differential EC probe with a spin-valve GMR sensor for surface and subsurface EC imaging. The differential EC probe consists of a full-bridge GMR chip mounted vertically on the edge of a tiny printed circuit board to reduce the liftoff distance. The excitation signal is perpendicular to the sensing axis of the GMR chip, which makes it sensitive to the unbalance EC signal on the surface of a conductive sample. The miniature EC probe operated at frequencies from 5 to 65 kHz is sensitive to the hidden defects on a flat conductor. The spatial resolution and stability of the device are verified by taking the 2-D scan images over the copper film samples with hidden defects, showing that the spatial resolution is better than 2 mm for the flaw 0.5 mm below the top surface of a multilayer structure. The proposed device is useful in the detection of small surface and subsurface defects, such as cracks, metal loss, and other mechanical damages.

Objective: The quality and work-life of ball bearings depend on the material properties of the steel ball, hence it is necessary to carefully classify their properties for bearings and related applications. This paper presents the classification of steel balls based on the subtle differences in their electromagnetic properties. Approach: The conductivity and magnetic susceptibility for steel balls of the same kind are measured to investigate their correlation with eddy-current (EC) signals. Main results: The developed EC sensor works at the resonant frequency of 117 kHz with an optimal readout resistance of 15 kΩ, which helps to boost the signal level without a high-gain preamplifier. To detect the EC signal, the steel ball under test moves through the pickup coil, and the recorded data are used to build a voltage probability map (VPM) for the classification of the steel ball properties. Experimental results show that the steel balls with and without the hardening process can be identified by the change in the amplitude and phase of the EC signal, which is consistent with the observed change in the electromagnetic properties of steel balls. Significance: The built system can be applied to the related industries to check the quality of steel balls before use.