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The coils in a three-axis fluxgate magnetometer have been investigated to enhance the noise and power performance for the use in geomagnetic stations. The turn number of the excitation and pickup coils in the fluxgate sensors is carefully optimized by measuring the equivalent magnetic noise spectral density of the magnetometers. Correspondingly, the values of the tuning and demodulation capacitors are delicately designed to match the optimized coils. The noise of the fluxgate magnetometer is 6 pT/ $\surd $ Hz at 1 Hz with a power consumption lower than 360 mW. The size of the packaged magnetometer is $30 \times 30 \times 113$ mm3, and its weight is less than 80 g (without cable). The features of the fluxgate magnetometer make it a suitable instrument for geomagnetic stations and other applications.

The magnetic gradient tensor system (MGTS) based on the fluxgate sensors array replaces partial differentials with the sensors reading difference to approximate the magnetic field gradient, but it introduces structural errors at the mean time. A good structure design with reasonable parameters can reduce structural errors and expand the theoretical detection range of the MGTS. From the analysis of the complete 18-uniaxial MGTS, five simplified structures of the MGTS were summarized, which include the plane cross, equilateral triangle, square, right-angle tetrahedron, and regular tetrahedron. The differential magnetic gradient tensor measurement matrixes of different structures were also provided, where we found the plane-cross structure produces the smallest structural error. Moreover, the theoretical detection limits expression of the MGTS to the magnetic dipole was derived. The detection limit boundary of the MGTS is related to the sensor’s accuracy, the array baseline distance, and the magnetization direction and magnetic moment magnitude of the source. In actual heading-line surveys, the magnetic moments of four typical small magnets were successfully estimated by the magnetic dipole positioning method, and the theoretical detection limits of the built plane-cross MGTS aiming at the four magnets were estimated. The results show that the estimation accuracy of the theoretical detection limits of MGTs is ±0.5 m.

The industrial investment in unmanned aerial vehicles (UAVs) is soaring due to their multiple autonomous applications, such as aerial photography, rescue operations, surveillance, and scientific data collection. Current sensing is critical for determining battery capacity in charging and discharging process and alerting for system fault during the flight. Shunt resistors and Hall-effect sensors are traditionally used in UAVs. Recently, magnetoresistive (MR) sensors are gaining enormous attention from researchers. MR sensors tend to consume less power, and they are smaller in size than the Hall-effect sensors. In this paper, a number of off-the-shelf MR sensors were investigated to evaluate the possibility of applying them for UAVs. Another type of magnetic sensor (fluxgate) and shunt resistor was also studied and compared as a reference. The relative scoring method is adopted to evaluate the sensor performance under different metrics, and the results disclose that the MR sensors are highly suitable for current sensing of UAVs due to their higher accuracy, lower energy consumption, wider temperature endurance, smaller size, and less weight than other magnetic current sensors. They are also very competitive with traditional shunt resistors through the overall comparison. The remanence, thermal stability, and cross-field sensitivity of MR sensors are further discussed. This finding provides insight into the selection strategy of current sensors for UAVs and can enhance the industrial development of UAV potentially.

Beam diagnostic and especially beam current measurements represent a key aspect in the production of a high-quality beam. Due to the known interplay between beam current and beam and plasma properties, many solutions have been addressed to solve this need. Although precise beam current measuring systems exist, they cannot provide noninvasive solutions for the measurement of very low-frequency currents in electron cyclotron resonance ion source (ECRIS) beamline, with a range from tens to hundreds of microamperes. To address the above-mentioned needs, this paper proposes an indirect measurement methodology based on the estimation of the magnetic field produced by target current exploiting a residence time difference fluxgate and a flux concentrator (a mechanical structure able to constrain produced magnetic field in a given area). The main advantage of this approach is related to the possibility to perform a noninvasive current measurement. An optimal geometry for the flux concentrator has been investigated by using a finite-element method analysis that has led to the definition of a minimum value of the magnetic field generated by the minimum value of the target current, around $5.0~\mu \text{A}$ , equal to 3.07 nT. To assess the solution, an experimental setup, miming real magnetic flux intensity inside ECRIS beamlines, has been realized. The measuring strategy has a magnetic resolution of 1 nT and a current resolution of $1.6~\mu \text{A}$ , that is, in line with requirements. Results obtained demonstrate the suitability of the measurement system for the specific application addressed in this paper, both in terms of operating range and resolution.

Coupled-core fluxgate magnetometer (CCFM), which is based on the coupling-induced oscillations in bistable nonlinear dynamics, can achieve self-excitation and obtain superior performances compared with the single core fluxgate magnetometer under the premise of utilizing the same residence times difference readout method. One of the pivotal factors determining the performances of CCFM is the critical coupling factor. To get control of it and further enhance the sensor performances, the negative feedback is exploited and implemented by feeding the magnetization from induction coil to excitation coil of the same fluxgate element (self-feedback). Theoretical analyses, Simulink simulations with a quantitative model, and experimental results obtained in the magnetic shielded environment reveal the critical coupling factor adjustment effect of negative feedback on CCFM and confirm the substantial enhancement of sensor performances. Finally, the sensitivity of CCFM for the static magnetic field detection is improved from 0.0253 to $0.7895~\mu \text{s}$ /nT, and the magnetic resolution of CCFM is optimized from 0.395 to 0.049 nT. Therefore, the negative feedback is an effective and reliable method for performances enhancement of CCFM, and it has potential utilization on the oscillation systems with similar dynamic peculiarities for factor adjustment.

A quasi-digital dc current sensor with rated current of ±600 A while overload current of about ±750 A is proposed in this paper. The new sensor is based on the open-loop self-oscillating fluxgate technology, but its originality is using a microcontroller unit to detect the duty cycle of the exciting voltage of the fluxgate. Compared with the published similar method, the whole signal chain of the new sensor is quasi-digital and without low-pass filter and analog-to-digital converter required when connected to digital systems. A precisely theoretical equation with respect to the linear dependence between the duty cycle and the current to be measured is established. Based on the equation, factors affecting the sensor sensitivity, accuracy, and resolution are determined, which constitutes the theoretical basis on the optimization design of the new sensor. The sensor linearity is improved using the least-squares polynomial fitting method. Some key specifications including the linearity, repeatability, and power supply effect of the sensor are characterized. The measurement results show that the linearity of the new sensor with the theoretical equation is better than 1.7% in the full scale of ±600 A, and can be improved to 0.3% when the fifth-order polynomial fitting method is used.

A new airborne platform named the unmanned aerial vehicle (UAV) is used to detect the geomagnetic anomaly with low magnetic interference. Nevertheless, there are steering errors of three-axis fluxgate magnetometers (TFMs) with the changes of UAV flight course. The main reasons are due to the effects of nonorthogonality, scale factors, and zero shifts. Therefore, it is quite necessary to establish calibration methods to get magnetic information of high precision. The methods of least squares (LS) and backpropagation artificial neural network (BPANN) are proposed to correct the system errors of measured data in this paper. The results show that the errors are suppressed using LS and BPANN methods. The measured errors of geomagnetic field decrease obviously after calibration. Furthermore, the BPANN method is more effective to calibrate the data error of TFMs than LS method when UAV changes its flight direction. Moreover, the spatial distributions of magnetic fields for TFMs after calibration using LS and BPANN methods are quite consistent with the distributions for optical pump magnetometers. This paper can provide a better way to improve the performance of TFMs and be widely used in the data error calibration of multiaxis sensors.

A setup has been developed for determining the axial position of the active magnetic sensor element inside its case with respect to a reference face at the outside of the sensor case. For an axial Hall-sensor providing a sensitivity of 1 mV/muT, an uncertainty in the order of 6 mum is achieved, i.e., the uncertainty of the distance between reference point and Hall element, as specified by the producer, can be reduced by a factor of about 50. For a fluxgate sensor having two rods of 5 mm in length, it is possible to localize its axial magnetic center within nearly the same uncertainty

In this paper, analytical and experimental results on fluxgate magnetometers, intended to sense dc magnetic field, that make use of a readout technique based on the estimation of the residence times in the stable attractors are presented. The approach, exploiting the inherent nonlinear character of the bistable core dynamics, is based on the time-domain characterization of the transitions between the two saturation states of the hysteresis loop that is inherent in the ferromagnetic core dynamics. This readout technique affords the possibility of low amplitude and frequency reference (or bias) driving signals (sinusoidal or triangular), compared to conventional fluxgates, thereby reducing the device power requirements. The efficacy of this strategy is shown through an analytical approach, and via experimental results that suggest guidelines for the optimal design of the device. Considerations of the effects of a suitable choice of the magnetic core (in terms of its hysteresis behavior) have also been included. Experiments, carried out on miniaturized laboratory fluxgate prototypes with different ferromagnetic materials, reveal good performances of the proposed methodology, including suitable sensitivity and resolution, as well as low cost.

This paper presents a weak magnetic field measurement system using micro-fluxgate (FG) sensors and a sensor signal processing technique using the delta-sigma modulation in the negative feedback loop. The feedback of the lowpass filtered bitstream output of a delta-sigma modulator to the magnetic field improves system linearity, hysteresis, and stability. In spite of the fact that the second-order delta-sigma modulator is used, the third-order noise shaping can be obtained in the digital output bit-stream by the use of an integrator in the loop. This improves the SNR of the digital output. The measured noise of the implemented system meets the measured noise of the FG sensing element itself. The weak magnetic field in the range of the Earth's magnetic field is successfully measured. The nonlinearity error is less than 0.4% in the range of /spl plusmn/100 /spl mu/T.

Based on analyzing the magnetic leakage feature of wire rope defects, the longitudinal resolved vector B/sub ot/ of the defect leakage field is taken for the object to be tested. In order to detect the weak magnetic leakage field, a fluxgate sensor of single core and single winding is developed. Using the fluxgate, a new kind of transducer for testing defects of wire rope has been designed. The structure and interface circuit of the transducer are studied. The experimental results of detecting the LF and LMA defects of the wire rope are presented.

A new fluxgate magnetometer has been developed for measuring magnetic fields with frequencies up to 2 kHz, based on a novel sensor design and readout electronics which ensure the stabilization of the premagnetizing field. The sensor is made from an amorphous core material, 25-/spl mu/m thick, and provides a closed path for the premagnetizing field. The magnetometer has been designed using a pulse-width modulator and has a resolution of 10 nT. Errors due to nonlinearity and frequency response are smaller than 0.1 mT/T over the specified frequency range.