Fluxgate Signal Research Articles

A new method of fluxgate signal extraction

Using dedicated high-frequency measuring system the distribution of the Barkhausen jumps intensity along a reversal magnetization cycle was investigated for low noise fluxgate sensors of various core shapes. It is shown that Barkhausen (reversal magnetization) noise intensity is strongly inhomogeneous during an excitation cycle. In the traditional second harmonic fluxgate magnetometers the signals are extracted in the frequency domain, as a result, some average value of reversal magnetization noises is contributed to the output signals. In order to fit better the noise shape and minimize its transfer to the magnetometer output the new approach for demodulating signals of these sensors is proposed. The new demodulating method is based on information extraction in the time domain taking into account the statistical properties of cyclic reversal magnetization noises. This approach yields considerable reduction of the fluxgate magnetometer noise in comparison with demodulation of the signal filtered at the second harmonic of the excitation frequency.

The origin of noise and magnetic hysteresis in crystalline permalloy ring-core fluxgate sensors

Abstract. Developed in the 1960s for use in high-performance ring-core fluxgate sensors, 6–81.3 Mo permalloy remains the state of the art for permalloy-cored fluxgate magnetometers. The magnetic properties of 6–81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction, are all optimum in the Fe–Ni–Mo system. In such polycrystalline permalloy fluxgate sensors, a single phenomenon may cause both fluxgate noise and magnetic hysteresis; explain Barkhausen jumps, remanence and coercivity; and avoid domain denucleation. This phenomenon, domain wall reconnection, is presented as part of a theoretical model. In the unmagnetized state a coarse-grain high-quality permalloy foil ideally forms stripe domains, which present at the free surface as parallel, uniformly spaced domain walls that cross the entire thickness of the foil. Leakage flux "in" and "out" of alternating domains is a requirement of the random orientation, grain by grain, of magnetic easy axes' angles with respect to the foil free surface. Its magnetostatic energy together with domain wall energy determines an energy budget to be minimized. Throughout the magnetization cycle the free-surface domain pattern remains essentially unchanged, due to the magnetostatic energy cost such a change would elicit. Thus domain walls are "pinned" to free surfaces. Driven to saturation, domain walls first bulge then reconnect via Barkhausen jumps to form a new domain configuration that I have called "channel domains", which are attached to free surfaces. The approach to saturation now continues as reversible channel domain compression. Driving the permalloy deeper into saturation compresses the channel domains to arbitrarily small thickness, but will not cause them to denucleate. Returning from saturation the channel domain structure will survive through zero H, thus explaining remanence. The Barkhausen jumps, being irreversible exothermic events, are sources of fluxgate noise powered by the energy available from domain wall reconnection. A simplified domain energy model can then provide a predictive relation between ring-core magnetic properties and fluxgate sensor noise power. Four properties are predicted to affect noise power, two of which are well known: saturation total magnetic flux density and magnetic anisotropy. The two additional properties are easy axes alignment and foil thickness. Flux density and magnetic anisotropy are primary magnetic properties determined by an alloy's chemistry and crystalline lattice properties. Easy axes alignment and foil thickness are secondary, geometrical properties related to an alloy's polycrystalline fabric and manufacture. Improvements to fluxgate noise performance can in principle be achieved by optimizing any of these four properties in such a way as to minimize magnetostatic energy. Fluxgate signal power is proportional to B − H loop curvature [d2B/dH2]. The degree to which Barkhausen jumps coincide with loop curvature is a measure of noise that accompanies the fluxgate signal. B − H loops with significant curvature beyond the open hysteresis loop may be used to advantage to acquire the fluxgate signal with reduced noise.

Continuous Measurement-While-Drilling Utilizing Strap-Down Multi-Model Surveying System

In the oil drilling process, measurement-while-drilling system is usually used to provide real-time monitoring of the position and orientation of the bottom hole. Usually the bottom drill string attitude is measured on the condition that the drill string is static. Particularly, in rotary steerable drilling application, it is a challenging problem to accurately measure the spatial attitude (inclination, azimuth, and tool face) of the bottom drilling tool with the drill string rotating. A set of “strap-down” measurement system was developed in this paper, the triaxial accelerometer and triaxial fluxgate were installed near the bit, and real-time well deviation and azimuth can been measured. Laboratory experiments show that the dynamic solver methods will meet the project requirements. However, according to the drilling field tests, playback analysis on storage data of the underground measurement indicates that drill string vibration is the main reason for measurement errors. So, by setting a time window, the real-time measurement of the accelerometer and the flux gate signal are processed and analyzed, the movement patterns of the rotary drilling tool at the bottom were judged within the time window. Stick-slip phenomenon is utilized to improve the measurement accuracy and to reduce the impact of lateral vibration of the drilling string on the measurement system. We could take advantage of stick-slip phenomenon because of it cannot be avoided completely, although it should be avoided in order to improve the drilling string operating life. Simulation results showed that recommended dynamic survey approach could effectively enhance measurement precision, and deduce effect of vibration on resolve results.

Design and Realization of a Digital Multichannel Fluxgate Signal Processing System

Addressing drawbacks of analog components and questions of multi-channel fluxgate signal operation, a FPGA (Field Programmable Gate Array) based signal processing system is designed. Three copies of sub modules compose the whole system, each of which exclusively processes one of three outputs of the fluxgate sensor. A “Phase-Sensitive-Rectification & Low-Pass-Filtering” circuit structure is employed in the processing module, through which the fluxgate signal harmonics are extracted and converted into direct quantities according to detected magnetic intensities. Firstly designed in HDL (Hardware Description Language), afterward configurated in a FPGA chip, finally tested by processing outputs of a fluxgate sensor probe in real-time, the functionality of the designed system is verified. With inherent advantages, this FPGA based design is much reliable over temperature; by processing signals not time-sharingly but synchronously, its working speed is excellently high.

DI-flux measurement of the geomagnetic field using a three-axial fluxgate sensor

In geomagnetic observatories DI-flux measurements are the commonly used instruments for absolutely determining the Earth's magnetic field components. Introduced in the late 1980s, these instruments traditionally consist of a non-magnetic theodolite equipped with a single-axis fluxgate sensor. Angles of magnetic field declination and inclination are taken in positions where the fluxgate signal is zero. Nowadays, three-component fluxgate sensors are available small enough to fit on the telescope of a theodolite and able to measure the full range of the Earth's magnetic field with adequate linearity. By replacing the single-axis fluxgate sensor by a three-axial system, additional magnetic information can be used to improve the absolute measurement significantly without changing the standard procedure. The rotation of the fluxgate sensor allows us to calibrate and align the fluxgate sensor completely. Therefore, each of the angle readings provides the complete field vector in the geographic reference system which leads us to a welcome high number of single absolute measurements. This can be used to simplify the measurement procedure, to eliminate systematic effects or to suppress the random measurement error.

Generation of high-order harmonics in insulator magnetic fluxgate sensor cores

High-order harmonics generated in an insulator core of a magnetic fluxgate show insignificant attenuation at high frequencies. This suggests that coherent detection of the fluxgate signal can be applied to an insulator core, decoupling noise participation thereby enhancing the detection sensitivity.

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.