Search

We have developed a steel tube axis alignment method using magnetic sensors associated with a commercial nondestructive evaluation (NDE) probe. The probe consists of an eddy-current encircling coil and a magnetization yoke. The accurate steel tube axis alignment method uses eight commercial Hall sensors, placed in optimal gradiometric configuration. We present a theoretical analysis of the system by means of 2-D finite-element simulations and the expected analytical expressions. Our results predict a high theoretical accurate tube axis alignment of around 10-50 [micro]m. Experimental results are in good agreement with simulations and show that, for a steel tube, the axis alignment measurement accuracy can reach to 100 [micro]m in a true experimental laboratory magnetic environment, including the intrinsic magnetic noise of the magnetization yoke. Index Terms--Hall effect devices, magnetometers, nondestructive testing.

A study of highly sensitive magnetometers is presented with emphasis on their performance. Their characteristics such as sensitivity and bandwidth and noise are examined and their potential is explored.

A temperature control scheme with intrinsic noise limitations was analyzed. The scheme uses YBCO high temperature sensors. A current pre-biasing method was applied to the microbridge to permit very large offset compensations and for circuit redesigning. A temperature control with fluctuations less than 1.6 microKpp over a two hour recording time was achieved through a transfer coefficient value of 441 V/K.

The light-induced bolometric effects in YBCO microbridges engraved in thin films deposited on MgO by laser ablation were studied using the device's critical current versus temperature dependence curves. Optical measurements were performed under varying optical power and voltage values at a temperature level of 77 degrees Kelvin. Results indicate that current versus temperature dependence curves are very useful for estimating bolometric effects in thin films.

This paper describes a theoretical study of the sensitivity and intrinsic noise response in a longitudinal giant magnetoimpedance sensor of a magnetically soft microwire. We show that the ratio of the intrinsic magnetic noise voltage spectral density to the sensitivity is proportional to [epsilon]/sin 0, where [epsilon] depends upon the internal field and frequency and 0 is the angle between the equilibrium magnetization and the axial applied static field. The model allows us to calculate the influence of the easy axis angle and the applied fields on sensor response and to obtain a working point. We show the conditions necessary to reduce the intrinsic noise level to the fT/[check]Hz level. Index Terms--Giant magnetoimpedance (GMI), magnetic field sensors, magnetic noise, soft magnetic wires.

A nonlinearity in the magnetoelectric coefficient, αMENonlin, of Metglas/Pb(Zr,Ti)O3 (PZT) and Metglas/Pb(Mg1/3,Nb2/3)O3-PbTiO3 (PMN-PT) laminate sensors has been observed. This nonlinearity was found to be dependent on the dc magnetic bias (Hdc) and frequency of the ac drive field (Hac). The maximum value of αMENonlin for both types of composites was found near the electromechanical resonance. For Metglas/PZT laminates, the maximum occurred under a finite bias of Hdc ≈5 Oe; whereas, for Metglas/PMN-PT, the maximum was found near zero dc bias. One application for αMENonlin is a cross-modulation scheme that can shift low frequency signals to higher frequency to achieve lower noise floor. For Metglas/PMN-PT, αMENonlin has another application: removal of the necessity of a dc bias, which helps to design high-sensitivity sensor arrays and gradiometers. [ABSTRACT FROM AUTHOR]

The equivalent magnetic noise spectral density level for long type magnetostrictive-piezoelectric laminated composites has been investigated by using a 1D equivalent circuit model, and exemplified for a longitudinal-transverse mode. The theoretical developments explain well our experimental results. The findings show that similar ultimate magnetic noise spectral noise density can be expected whether using either charge or voltage amplifier detection methods. The findings show that a volume effect and the dielectric loss factor of the piezoelectric layer are the dominant sources of the noise floor. Our findings show that the noise scales as 1/[Square_Root]f. The lowest equivalent noise floor value that has been observed is 10/[Square_Root]f pT/[Square_Root]Hz for f < 10 kHz, with a white noise level of 100 fT/[Square_Root]Hz above 10 kHz. [ABSTRACT FROM AUTHOR]

The performance of magnetic field sensors based upon the giant magnetoimpedance (GMI) effect in soft magnetic wires is investigated in the MHz frequency range. The performance of the sensor is based on its sensitivity, voltage noise level and (voltage) noise-to-sensitivity ratio, or intrinsic magnetic noise level. Optimization of the sensitivity and noise response of the sensor through variation of the sample anisotropy direction and external applied axial field and dc bias current suggest an intrinsic noise level in the [formula] level. Qualitative agreement is obtained between theory and experiment on a CoFeSiB microwire, for the maximum sensitivity and the corresponding noise, as a function of the external field and dc bias current. [ABSTRACT FROM AUTHOR]

The equivalent magnetic noise spectral density of giant magnetoimpedance (GMI)-based sensors exhibits a low-frequency excess noise inversely proportional to the frequency (1/f noise). In order to enhance the performance and reach the ultimate and intrinsic limit of the magnetometer, it is necessary to investigate the origin of this excess noise. Cross correlation measurements have permitted the demonstration that the excess low-frequency noise arises from the sensor, under defined excitation conditions, particularly the amplitude of the excitation current and the dc bias current. Given that the GMI effect is based on an impedance variation governed by the magnetization direction under an applied external magnetic field, the low-frequency magnetization fluctuations within the GMI wire have been considered to be a possible noise source. The fluctuation-dissipation theorem permits evaluation of the spectral power density of these fluctuations, which is proportional to the imaginary part of the susceptibility, \chi ^\prime \prime . The latter may be evaluated by two different methods, based on the measurement of the real part of the wire impedance and on the evaluation of the circumferential hysteresis loop area. The results obtained by both methods are compared and discussed. The values of \chi ^\prime \prime thus obtained are then used in order to predict the equivalent magnetic noise level at 1 Hz, and are compared with that measured at 1 Hz. The comparison is conducted for different amplitudes of the dc bias current, and the results obtained indicate that the proposed methods have the potential of predicting the excess noise in GMI sensors. [ABSTRACT FROM PUBLISHER]

In the paper the magneto-impedance effect in the Co68.15Fe4.35Si12.55B15 microwire with diameter of about 100 μm is carefully studied. The measurements in close contact and contactless geometry were carried out in the static field ranging from 0 to 6 kA/m and frequencies of the alternating field from 20 Hz to 2 MHz. It is shown that the examined microwire shows high anisotropy of soft magnetic properties i.e. the circumferential permeability is at least 7 times higher than the longitudinal one. Moreover, the microwire can be used as highly sensitive magneto-impedance sensor working in contactless geometry especially at fields below 200 A/m. [ABSTRACT FROM AUTHOR]