Micro-fluxgate Sensor Research Articles

Design of a Low-Cost Small-Size Fluxgate Sensor.

Traditional fluxgate sensors used in geomagnetic field observations are large, costly, power-consuming and often limited in their use. Although the size of the micro-fluxgate sensors has been significantly reduced, their performance, including indicators such as accuracy and signal-to-noise, does not meet observational requirements. To address these problems, a new race-track type probe is designed based on a magnetic core made of a Co-based amorphous ribbon. The size of this single-component probe is only Φ10 mm × 30 mm. The signal processing circuit is also optimized. The whole size of the sensor integrated with probes and data acquisition module is Φ70 mm × 100 mm. Compared with traditional fluxgate and micro-fluxgate sensors, the designed sensor is compact and provides excellent performance equal to traditional fluxgate sensors with good linearity and RMS noise of less than 0.1 nT. From operational tests, the results are in good agreement with those from a standard fluxgate magnetometer. Being more suitable for modern dense deployment of geomagnetic observations, this small-size fluxgate sensor offers promising research applications at lower costs.

Rectangular Array Electric Current Transducer with Integrated Fluxgate Sensors.

Novel rectangular yokeless current transducer with the range 400 A using 16 microfluxgate sensors around the busbar conductor is presented in this paper. Compared to yokeless transducers utilizing the differential pair of magnetic sensors, our solution has much better suppression of the external currents (lower crosstalk). Compared to industrial transducers with yoke, the new transducer has 15-times lower noise, 7-times better temperature stability, and same crosstalk. Sensor design and design of current monitoring system is presented together with the results of long-term field tests. Crosstalk error is examined in dependence on the number of the operating sensors and external current position.

A 3-phase current transducer based on microfluxgate sensors

Novel 3-phase DC/AC current transducer for 3-phase current lines is based on 8 integrated fluxgate sensors. Using full information from each sensor rather than gradiometric pairs, we suppress crosstalk between individual phases and external magnetic fields up to the 4th order gradients. The suppression of external currents at a distance of 1 m is 90 dB. The main advantage of the used microfluxgates is excellent temperature stability allowing operation outside the laboratory: the achieved temperature coefficient of the sensitivity is 50 ppm/K, and the offset stability is 1 mA/K. These values are 20-times better than temperature stability achievable with current transducers sensors based on Hall sensors and magnetoresistors.

Three-Phase Busbar Current Transducer

A true three-phase 1000 A busbar current transducer is based on six micro-fluxgate sensors. Instead of using three independent single-phase current transducers, we use the full information from each sensor. Two TI DRV425 micro-fluxgate sensors are inserted into a hole drilled in each busbar. Our method of data processing is optimized to compensate the crosstalk between the three phases and external fields and gradients up to second order. The crosstalk error between the phases was 0.23% or smaller. The suppression of field from an external current at a distance of 10 cm is improved by a factor of 25 to 150. Our transducer has compact size, high temperature offset stability of 8.5 mA/°C, high current range up to 1000 A, low power consumption, and linearity of 0.1%. The results from a three-dimensional finite-element model and analytical computations confirm the measurements.

Soft ferromagnetic amorphous microwires for GMI sensing cores

Abstract The recent work is focused on the development of suitable amorphous ferromagnetic microwires (CoFe-based) for use in giant magneto-impedance (GMI) magnetic sensing cores. Rapid solidification route is used for the preparation of microwires via in-rotating water quenching technique. The potential of the CoFeSiBCr soft magnetic microwires for GMI based sensing core is investigated by using the driving frequency of MHz range. The GMI characteristics of the developed materials are carried out via impedance-mode. The performance of the magnetic sensor is studied based on its maximum GMI ratio, magnetic field sensitivity and noise spectrum. The experimental results reveal that the maximum GMI ratio, GMImax of about 650% and field sensitivity of around 802 V/T is achieved at ac driving frequency of 5 MHz with 3 mA driving current when annealed at 750 K for 5 min. The noise level of the optimized microwire can be obtained under 10 nT/√Hz@1 Hz while driven at 5 MHz. By means of its high operating frequency regime, high sensitivity, low noise level and simple methodology for sensing, the GMI microwires working in impedance-mode can be a potential candidate for commercial sensing applications such as microfluxgate sensors.

Busbar Current Transducer With Suppression of External Fields and Gradients

Busbar Current Transducer With Suppression of External Fields and Gradients

A sensitive and innovative detection method for rapid C-reactive proteins analysis based on a micro-fluxgate sensor system

A sensitive and innovative assay system based on a micro-MEMS-fluxgate sensor and immunomagnetic beads-labels was developed for the rapid analysis of C-reactive proteins (CRP). The fluxgate sensor presented in this study was fabricated through standard micro-electro-mechanical system technology. A multi-loop magnetic core made of Fe-based amorphous ribbon was employed as the sensing element, and 3-D solenoid copper coils were used to control the sensing core. Antibody-conjugated immunomagnetic microbeads were strategically utilized as signal tags to label the CRP via the specific conjugation of CRP to polyclonal CRP antibodies. Separate Au film substrates were applied as immunoplatforms to immobilize CRP-beads labels through classical sandwich assays. Detection and quantification of the CRP at different concentrations were implemented by detecting the stray field of CRP labeled magnetic beads using the newly-developed micro-fluxgate sensor. The resulting system exhibited the required sensitivity, stability, reproducibility, and selectivity. A detection limit as low as 0.002 μg/mL CRP with a linearity range from 0.002 μg/mL to 10 μg/mL was achieved, and this suggested that the proposed biosystem possesses high sensitivity. In addition to the extremely low detection limit, the proposed method can be easily manipulated and possesses a quick response time. The response time of our sensor was less than 5 s, and the entire detection period for CRP analysis can be completed in less than 30 min using the current method. Given the detection performance and other advantages such as miniaturization, excellent stability and specificity, the proposed biosensor can be considered as a potential candidate for the rapid analysis of CRP, especially for point-of-care platforms.

Wide Linearity Range and Highly Sensitive MEMS-Based Micro-Fluxgate Sensor with Double-Layer Magnetic Core Made of Fe–Co–B Amorphous Alloy

This paper reports a novel micro-fluxgate sensor based on a double-layer magnetic core of a Fe–Co–B-based amorphous ribbon. The melt-spinning technique was carried out to obtain a Fe–Co–B-based amorphous ribbon composite of Fe58.1Co24.9B16Si1, and the obtained amorphous ribbon was then annealed at 595 K for 1 h to benefit soft magnetic properties. The prepared ribbon showed excellent soft magnetic behavior with a high saturated magnetic intensity (Bs) of 1.74 T and a coercivity (Hc) of less than 0.2 Oe. Afterward, a micro-fluxgate sensor based on the prepared amorphous ribbon was fabricated via microelectromechanical systems (MEMS) technology combined with chemical wet etching. The resulting sensor exhibited a sensitivity of 1985 V/T, a wide linearity range of ±1.05 mT, and a perming error below 0.4 μT under optimal operating conditions with an excitation current amplitude of 70 mA at 500 kHz frequency. The minimum magnetic field noise was about 36 pT/Hz1/2 at 1 Hz under the same excitation conditions; a superior resolution of 5 nT was also achieved in the fabricated sensor. To the best of our knowledge, a compact micro-fluxgate sensor with such a high-resolution capability has not been reported elsewhere. The microsensor presented here with such improved characteristics may considerably enhance the development of micro-fluxgate sensors.

Influence of External Current on Yokeless Electric Current Transducers

Yokeless electric current transducers have compact size, but they are sensitive to external magnetic fields, including those caused by electric currents in their vicinity. It is often believed that this unwanted sensitivity can be effectively suppressed by using a differential sensor. In this paper, we investigate the effect of external current with arbitrary position on busbar differential current sensor. We show the main disadvantage of the differential current sensor: increased sensitivity to currents in the transversal direction, which are not sensed by a single sensor. We analyze by finite element method simulation also the influence of real conductor size and uneven density of ac currents. The results were verified on 1000 A current transducer using a pair of microfluxgate sensors. The realistic suppression of close currents depends on the conductor angular position and in 10 cm distance it can be as low as 50, but it can be corrected if the geometry is known.

An integrated microfluidic system using a micro-fluxgate and micro spiral coil for magnetic microbeads trapping and detecting

We report an innovative integrated microfluidic platform based on micro-fluxgate and micro-coils for trapping and detecting magnetic beads. A micro-spiral coil fabricated by microfabrication technology is used to trap the magnetic beads, and the micro-fluxgate is employed to detect the weak magnetic field induced by the trapped magnetic beads. The fabrication process of the magnetic bead trapping system using a micro-coil is highly compatible with that of the micro-fluxgate sensor, making fabrication of this integrated microfluidic system convenient and efficient. It is observed that the magnetic bead trapping ratio increases as the number of magnetic beads is increased with a flow rate of 5 to 16.5 μL·min−1. Samples spiked with different concentrations of magnetic beads can be distinguished clearly using the micro-fluxgate sensor in this microfluidic system. In this study, the results demonstrate that the microfluidic system traps and detects magnetic beads efficiently and is a promising candidate for biomarker capture and detection.

Sensitive detection of cardiac troponin T based on superparamagnetic bead-labels using a flexible micro-fluxgate sensor

In this study, we describe an innovative micro-fluxgate immunosensor based on superparamagnetic Dynabeads for the detection of cardiac troponin T (cTnT), an important biomarker for cardiovascular diseases.

A MEMS-fluxgate-based sensing system for the detection of Dynabeads

In this paper, a microfluxgate sensor was introduced into the application of magnetic bead detection. Micro-electro-mechanical systems (MEMS) technology was used to realize the microfluxgate sensor. Making use of the superparamagnetic property that micron-sized magnetic beads can be magnetized in a magnetic field, a magnetic bead detection system based on this microfluxgate sensor was designed and established. Dynabeads obtained by purchase were diluted to different concentrations to characterize the detection performance of the system. Experimental results show that by applying a dc magnetic field in the range of 0.54 to 1.05 mT, Dynabeads with a concentration as low as 9000 beads ml–1 can be detected by the system. Furthermore, from the results, each sample can be clearly distinguished from the others, which prove that the detection system has the ability to make rough estimates to the concentrations of Dynabeads. The MEMS-fluxgate-based detection system possesses many advantages, such as a low minimum detectable limit, small size, light weight and compatibility with microelectronic technology. Moreover, a MEMS microfluxgate sensor can be made into an array for the synchronous detection of multi-target biomolecules and is very suitable for integration with interface circuits.

Responsivity and noise of a wire-bonded CMOS micro-fluxgate sensor

This paper presents a dual-core (Vacquier-type) micro-fluxgate magnetic sensor fabricated on a silicon chip based on standard CMOS technology. The silicon chip is 2.5mm×2.5mm in dimension, and the micro-fluxgate sensor occupies the area of 2.5mm×1.8mm. The sensor consists of magnetic cores, planar pick-up coils, bottom excitation coils (CMOS Al interconnections) and upper excitation coils (wire-bonding Al wires). The micro-solenoid excitation coils consisting of aluminum bonding wires and CMOS metallic layers can generate out-of-phase excitation magnetic fields strong enough to saturate the two magnetic cores. It was found that the sensor's sensitivity can be optimized and the field noise spectral density can be minimized by adjusting the excitation current as well as the excitation frequency. The maximum responsivity of 8.1V/T occurs at the second harmonic when the excitation frequency is 110kHz; however, the minimum magnetic field noise is found to be 2.6nT/√Hz at 1Hz under 20-kHz excitation. In comparison with other miniature planar fluxgates with similar dimensions reported to date, our device has a relatively low field noise spectral density.

Design and Simulation of Interface ASIC for Micro-fluxgate Sensor

Application Specific Integrated Circuits (ASIC) for micro-fluxgate sensor, based on the Complementary Metal Oxide Semiconductor (CMOS) fabrication, was designed by a 0.5μ m n-well 2p-2m process, which has the advantage of small dimension and low-power consumption. In consideration of the structure and specification of the micro-fluxgate sensor, the excitation and sensing circuits were designed based on second-harmonic operation principle. The functions and parameters of circuits were verified by simulation, and the area of chip was 2.1mm×2.8mm.

Fabrication of MEMS-based Micro-fluxgate Sensor with Runway-shaped Co-based Amorphous Alloy Core

High-precision magnetic micro-sensor is an interdisciplinary subject of magnetic field measurement techniques and micro-electromechanical systems (MEMS) technology. A micro-fluxgate magnetic sensor based MEMS technology was designed and fabricated in this paper. This device is a micro-magnetic sensor with a symmetric construction, closed magnetic circuits and differential form. A 25μm thick Fluxgate core of runway model, made by Co-based amorphous alloy, was etched by laser and pasted on the substrate accurately. Excitation coil and sensing coil of 3D solenoid structure were prepared by RF magnetron sputtering and UV-lithography. The minimum line width of the coil is 50 μm. The experimental result shows that micro-fluxgate devices with the size of 5.7mm×7.1mm×60μm had a stable structure.

A closed core microfluxgate sensor with cascaded planar FeNi rings

In this paper, we present a new microfabricated fluxgate sensor structure using cascaded planar rings as the ferromagnetic core. The planar ring structures provide closed magnetic excitation loops enabling uniform core saturation with relatively small excitation magnetic field. The magnetic excitation is provided with a rod passing through the ring cores. Planar coils placed under the edges of the core are used as sensing elements. By using this structure, fluxgate sensors having a closed core configuration are realized with a developed microfabrication process in a very small area (<0.1 mm 2) with reduced number of via connections. The ferromagnetic rings are realized with a standard FeNi (iron–nickel) electroplating process. The main advantage of this structure is the possibility to arrange the linear operation range of the sensor by only changing the number of ring cores, without affecting the excitation mechanism. This is demonstrated by simulations and microfabricated prototypes having 18 and 12 ring cores with ±300 μT and ±550 μT linear operation ranges, respectively. A maximum linear operation range of ±2 mT is achieved with a larger size, 4-ring core. This is the widest linear operation range achieved with the microfabricated fluxgate type sensors, without using a feedback loop, to the best of our knowledge.

Harmonic frequency characterisations of a CMOS micro fluxgate sensor for low magnetic field detection

A CMOS micro fluxgate sensor, including device design, manufacturing process and harmonic frequency characterizations, is described in this paper. The single-axis device is designed in a planar coil structure with a ferromagnetic core featuring low saturation magnetic induction of 10μT. The chip is measured 2.08 mm × 1.29 mm and fabricated by TSMC CMOS 0.35μm 2P4M process. The miniature magnetic core is formed by wet etching process and precisely allocated to the designated position of the sensor chip by using an aligner. To characterize the micro-fluxgate sensor, the output voltage of the induction coil and external magnetic field is especially measured at the excitation, second, third and forth harmonic frequencies respectively. Experimental results reveal that the B-V diagram at excitation and the 3rd harmonic frequencies are well described by evenfunction behavior while the characteristics at the 2nd and 4th harmonic frequencies are expressed by odd-function operation. Besides, it is also observed that the maximum output voltage amplitudes at odd-order harmonic frequencies tend to occur when the maximum field-to-voltage transfer coefficients (i.e., dV/dB) become prominent at even-order harmonic frequencies, and vice versa. To measure the low magnetic field, it is found that the micro fluxgate demonstrates a nearly linear B-V relationship around the zero-magnetism region while the 4th order harmonic frequency is employed. The maximum responsivity can be further enhanced up to 174.62 mV/T. The frequency response of the device is measured as 90 Hz, i.e., a rapid response time of 0.01 s.

Microfluxgate sensors for high frequency and low power applications

Low cost and low power consumption are key factors to the spreading of microsystems into the everyday life tools. Fluxgate magnetometer systems still request such an improvement. This paper presents compact thin film microfluxgate sensors working in the range of tenths of kHz to tenths of MHz which exhibit improved factor of merit (FOM) in the high frequency range when the sensor size is reduced. The architecture is based on interlaced excitation and detection solenoid coils wounded around two rectangular cores made of thin film soft magnetic sputtered NiFe permalloy material. Measurement axis and magnetic hard axis are along core length. These thin film sensors also proved to be working in a wide range of excitation currents since deep saturation is not requested thanks to a highly nonlinear hysteresis loop. Excitation currents as low as 6mA lead to still linear output response. Devices with monolayer cores were measured up to 20MHz. Laminated cores were measured up to 73MHz. Frequency response higher than 100MHz is achieved when magnetostatic coupling occurs between laminated layers. The power consumption can be monitored at system level to adjust the sensor sensitivity within a range of 0.5 to more than 220V/T.

Microfluxgate sensor with amorphous cobalt (Co–Nb–Zr) soft magnetic core for electronic compass

A silicon based microfluxgate sensor with a cobalt based amorphous soft magnetic core for electronic compass is presented in this paper. A sputtered Co85Nb12Zr3 magnetic core having a rectangular ring shape is combined with microcopper solenoid coils for excitation and pickup, which were wound alternately around the core to increase the number of coil turns. The Co85Nb12Zr3 as a core material is adopted for improving properties of the magnetic core and easy integration with micromachining processes to achieve a small size of the sensor. The sputtered Co85Nb12Zr3 showed dc effective permeability of ∼10000 and an extremely low coercivity of ∼0.03Oe with the thickness of 1μm. The Co85Nb12Zr3 as a thin film core with high permeability and low coercivity was easily saturated by a low excitation magnetic field, enhancing the sensitivity and linearity of the microfluxgate sensor. Finally, the sensor showed excellent linearity response over the range of −300 to 300μT with sensitivity of 60V∕T at the excitation condition of 3.0Vp-p and 5.0 MHz square wave form. The sensor size excluding pad region is about 0.55×1.4mm2.

Microfluxgate performance improvement in microtechnology

Microfluxgate sensors have been developed to measure very low magnetic fields, of about some nanotesla. This paper describes a recent integrated solution coupled with the basic microfluxgate component. The objective is to decrease the noise level of the sensor down to a few nanotesla per square root of hertz. The technological solution consists of adding coils or single layers, thanks to which the required ac transverse magnetic field can be applied. A large enhancement of the sensor sensitivity is put in evidence on output signal figures with and without stabilization signal. The specifications of the sensor connected with its electronics are also presented.