Evangelos Hristoforou

Magnetostrictive delay lines: engineering theory and sensing applications

A review of the engineering theory and the sensing element applications of the magnetostrictive delay line (MDL) technique is presented. The state of the art of magnetic materials and effects used in sensor design is overviewed and the operation of MDLs and their basic engineering properties are discussed. The resulting position, stress and field sensors based on this technique as well as their most significant applications are demonstrated. Finally, the industrialization process and the integration of the sensors with electronic circuitry as well as their evaluation with respect to the state of the art are discussed.

Fe-based amorphous thin film as a magnetoelastic sensor material

Abstract The aim of this paper is to analyze some Fe-based amorphous thin films as sensing elements for magnetoelastic microsensors. Fe-based amorphous thin films with thickness ranging from 0.1 to 1 μm are prepared by r.f. sputtering method. Their magnetic properties (saturation magnetization M s , magnetic anisotropy constant k u and Curie temperature T C ), determining the performance of the material used as magnetoelastic sensing element, are investigated in correlation with the magnetoelastic properties (saturation magnetostriction λ s , magnetoelastic coupling coefficient b γ ,2 , magnetostrictive strain coefficient D and Δ E -effect). The magnetic and magnetoelastic characterizations of the samples are performed using a torque magnetometer and a capacitive cantilever technique, respectively. Based on the obtained results, we discuss the specific qualities of the Fe-based amorphous thin films in terms of their relevant material characteristics for magnetoelastic sensing application in comparison to the conventional magnetostrictive thin films presently in use.

Magneto-surface-acoustic-waves microdevice using thin film technology: design and fabrication process

Abstract In this paper we describe the design and fabrication process of a magneto-surface-acoustic-waves (MSAWs) microdevice using Fe 70 B 15 Si 15 amorphous thin films and Fe 70 B 15 Si 15 /SiO 2 -type multilayers as active media. This simple and small device, able to be used as a sensor or implemented as a part of the magnetomechanical sensing integrated systems, is fabricated using thin film technology combined with photolithographic techniques. In order to use magnetostrictive Fe-metalloid amorphous thin films and (Fe-based magnetostrictive alloy/insulator) N multilayers as active media for (MSAWs) microdevices, the magnetic (saturation specific magnetization σ s and magnetic anisotropy constant k u ) and magnetoelastic (ME) (saturation magnetostriction λ s , magnetoelastic coupling coefficient b γ ,2 , magnetostrictive strain coefficient D and Δ E / E ratio) properties which determine their performance are studied. The possible use of MSAWs microdevice fabricated by us in microsensors applications is illustrated.

A novel micro-Fluxgate sensor based on the AMR effect of ferromagnetic film-resistors

Abstract A novel magnetic field sensor is presented in this paper. It combines the classical Fluxgate principle with the anisotropic magneto-resistance (AMR) effect exhibited by barber-pole biased AMR film-resistors. It is shown that such film-resistors can be used as Fluxgate magnetic cores, because they exhibit one high-resistivity state and one low-resistivity state depending on the film magnetization polarity. Periodical alteration of the magnetization polarity, forced by an excitation field, makes the film-resistivity become a rectangular function of time, whose duty-cycle is proportional to the measured ambient field intensity. The proposed design helps with the suppression of repeatability and time-drift errors that are common in AMR sensors; the excitation field enhances the spin-alignment along the easy-axis of the film-resistor. The design also provides excellent temperature stability. Moreover, it is shown how a single AMR film-resistor can be employed for simultaneous sensing of two field components (i.e. on a x – y plane); the first component modulates the duty-cycle of the rectangular function (Fluxgate principle) and the latter its amplitude (conventional AMR effect). A complete mathematical modeling of the proposed sensor is presented in this work. The theoretical results have been verified by the use of a Honeywell HMC1021 single-chip sensor. The calibration data fairly agrees with the theory.

Magnetoelastic characterization of thin films dedicated to magnetomechanical microsensor applications

Abstract This paper discusses the results of measurements on the saturation magnetostriction constant λ s , magnetoelastic coupling coefficient b γ ,2 and magnetostriction strain coefficient d for Ni-evaporated and Fe 70 B 20 Si 6 C 4 -sputtered thin films on silicon and mica substrates. These results are presented in correlation with other magnetic properties (i.e., saturation magnetization, Curie temperature and anisotropy field) which characterize magnetoelastic sensing applications. We have demonstrated that the relevant properties of the magnetoelastic media used for magnetomechanical microsensors can be considerably improved by using amorphous Fe-metalloid thin films sputtered onto a silicon substrate in comparison with conventional magnetostrictive thin films evaporated onto silicon and mica substrates.

Sensors based on eddy currents in a moving disk

A new kind of displacement sensor is proposed, based on the creation of eddy currents in a moving conducting disk, set above an amorphous wire magnetostrictive delay line, which is parallel to a pair of current conductors. Transmission of pulsed current through these conductors causes a pulsed eddy current loop in the conducting disk, which in turn causes an acoustic pulse in the delay line. The amplitude of this acoustic pulse depends on the distance between a moving conducting disk and delay line. Experimental results are given, concerning tests of as-cast and stress-current annealed delay lines. Applications of this sensing idea are presented. >

Amorphous magnetostrictive wires used in delay lines for sensing applications

In this paper we give a review on the use of amorphous magnetostrictive wires in delay lines for sensing applications. Initially, we demonstrate the engineering model of the operation of magnetostrictive delay lines (MDL), illustrating the micro-strain generation, propagation and detection. Accordingly, we present the developed sensing elements based on this technique. The sensing elements are based on the parameters affecting the operation of the MDL, which are the ambient field, the interrogating electromagnetic field and the mechanical action on the magnetic element. Finally, we discuss on the development of a new magnetostrictive device, which incorporate the excitation and sensing means and can be used in sensing applications.

Boosting the performance of miniature fluxgates with novel signal extraction techniques

Analytical modeling of classical fluxgate sensor function is presented, accompanied by computer simulation results for numerous actual core hysteresis loops that enables theoretical verification of fluxgate limitations which obstruct efficient miniaturization. This mathematical analysis leads to the proposition and experimental verification of two alternative fluxgate signal extraction techniques that can be optimally utilized with conventional thin film and fine wire cores, and which may improve miniature sensor characteristics, thus enabling micro-system fabrication without performance degradation. Two fluxgate systems which meet the theoretically predicted performance, based on these techniques have been developed.

Torsion and stress in amorphous positive magnetostrictive wires

In this paper we report some results on the dependence of the pulsed voltage output on the applied torsion or tensile stress in Fe/sub 77.5/Si/sub 7.5/B/sub 15/ amorphous wires used as magnetostrictive delay lines. The obtained results show a non monotonic dependence of the response of the delay line on applied torsion. For applied tensile stress the response could be fitted by a linear curve for as-cast wires and by an exponential curve for annealed wires. The good ratio between minimum and maximum magnetostrictive delay line signals and the absence of hysteresis suggest the possibility to use this technique in load cells and torque sensors.

On the Universality of the Dependence of Magnetic Parameters on Residual Stresses in Steels

A method for the monitoring of residual stress distribution in steels has been developed based on non-destructive magnetic permeability measurements. The dependence of differential permeability on residual stresses induced through a controlled process of applied tensile and compressive stress in the elastic region, of all three zones of the welded metal, yields the magnetic stress calibration curve (MASC). MASC is obtained on flawless welded steel plates and can be measured for any grade of ferromagnetic steels. A surface MASC correlates the magnetic permeability with the spatial stress distribution, as determined by the X-Ray Diffraction Bragg-Brentano diffraction. A bulk MASC correlates the bulk magnetic permeability with residual stresses, as determined by the neutron diffraction. The resulting calibration curves, obtained for several grades of ferromagnetic steels, have a sigmoid shape but are unique for each grade of steels. Normalizing the magnetic permeability and the stress values against the differential permeability measured at the yield point and yield stress, respectively, the dependence of the local magnetic permeability on residual stresses for all different tested grades of steels results in a universal curve relating magnetic and elastic properties of steels at the macroscopic level.