Three-axis Magnetometer Research Articles

FingerAuth: 3D magnetic finger motion pattern based implicit authentication for mobile devices

Smart devices, as the most widely used platforms for the mobile cyber–physical system (CPS) applications, such as smart home and health care systems, are becoming the prime targets of various attackers for users’ considerable private and confidential data in them. To fight against side channel attacks aiming to obtain credentials, e.g., passwords, during the process of user authentication, touch pattern based implicit authentication has been proposed. However, such a defensive technique fails to obtain an entire pattern of user operation by deriving user operation data via a touch-enabled screen. Considering that user operations, including on-screen and in-air finger movements, are performed in three-dimensional (3D) space, we propose a novel 3D magnetic finger motion pattern based implicit authentication technique, referred to as FingerAuth. To use FingerAuth, a user operates on her mobile device, e.g., texting a message and browsing websites, with a magnetic ring on the finger she uses. With the help of a built-in three-axis magnetometer on the mobile device, we can derive the 3D magnetic finger motion pattern as a human behavioral feature for implicitly authenticating the user. By using machine learning techniques, a robust 3D magnetic finger motion pattern detection model can be constructed. Two rounds of usability tests are conducted for the evaluation of FingerAuth. In the initial usability test targeting a given group of smart device users, we test the uniqueness of the proposed trait in typing scenario, achieving high average accuracy of 96.38%, low average false acceptance rate (FAR) of 4.06%, and false rejection rate (FRR) of 3.18%. In the second user usability test, we further evaluate the permanence of 3D finger motion pattern in multiple user–device interaction scenarios. There is an interim of two-week period between the training data collection phase and the testing data collection phase. The results of the high accuracy of over 80%, as well as the FAR and FRR of below 15%, indicate the applicability of FingerAuth.

Performance study of aluminum shielded room for ultra-low-field magnetic resonance imaging based on SQUID: Simulations and experiments**Project supported in part by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB04020200) and in part by the National Natural Science Foundation of China (Grant No. 11204339).

The aluminum shielded room has been an important part of ultra-low-field magnetic resonance imaging (ULF MRI) based on the superconducting quantum interference device (SQUID). The shielded room is effective to attenuate the external radio-frequency field and keep the extremely sensitive detector, SQUID, working properly. A high-performance shielded room can increase the signal-to-noise ratio (SNR) and improve image quality. In this study, a circular coil with a diameter of 50 cm and a square coil with a side length of 2.0 m was used to simulate the magnetic fields from the nearby electric apparatuses and the distant environmental noise sources. The shielding effectivenesses (SE) of the shielded room with different thicknesses of aluminum sheets were calculated and simulated. A room using 6-mm-thick aluminum plates with a dimension of 1.5 m × 1.5 m × 2.0 m was then constructed. The SE was experimentally measured by using three-axis SQUID magnetometers, with tranisent magnetic field induced in the aluminum plates by the strong pre-polarization pulses. The results of the measured SE agreed with that from the simulation. In addition, the introduction of a 0.5-mm gap caused the obvious reduction of SE indicating the importance of door design. The nuclear magnetic resonance (NMR) signals of water at 5.9 kHz were measured in free space and in a shielded room, and the SNR was improved from 3 to 15. The simulation and experimental results will help us design an aluminum shielded room which satisfies the requirements for future ULF human brain imaging. Finally, the cancellation technique of the transient eddy current was tried, the simulation of the cancellation technique will lead us to finding an appropriate way to suppress the eddy current fields.

Entropy-based adaptive attitude estimation

Gaussian approximation filters have increasingly been developed to enhance the accuracy of attitude estimation in space missions. The effective employment of these algorithms demands accurate knowledge of system dynamics and measurement models, as well as their noise characteristics, which are usually unavailable or unreliable. An innovation-based adaptive filtering approach has been adopted as a solution to this problem; however, it exhibits two major challenges, namely appropriate window size selection and guaranteed assurance of positive definiteness for the estimated noise covariance matrices. The current work presents two novel techniques based on relative entropy and confidence level concepts in order to address the abovementioned drawbacks. The proposed adaptation techniques are applied to two nonlinear state estimation algorithms of the extended Kalman filter and cubature Kalman filter for attitude estimation of a low earth orbit satellite equipped with three-axis magnetometers and Sun sensors. The effectiveness of the proposed adaptation scheme is demonstrated by means of comprehensive sensitivity analysis on the system and environmental parameters by using extensive independent Monte Carlo simulations.

방위각 추정용 3축 마그네토미터를 위한 타원체 피팅 방식 보정 기법 비교

Three-axis magnetometers are widely used in various fields requiring azimuth information. However, accuracy of azimuth estimation based on magnetometer signals may be degraded because of errors such as offset, scale factor, nonorthogonality, hard-iron distortion, and soft-iron distortion. Recently, several ellipsoid-fitting calibration techniques have been proposed and have received much attention. However, comparative analysis of calibration accuracies between these techniques has not been conducted. This study compared and analyzed performance of four ellipsoid-fitting magnetometer calibration techniques such as the linear least square method, the two-step algorithm, and two different nonlinear least square methods. Our analysis and experimental results reveal superiority of the linear least square method compared to other methods in terms of calibration accuracy as well as ease of use in practice.

Design and Numerical Validation of an Algorithm for the Detumbling and Angular Rate Determination of a CubeSat Using Only Three-Axis Magnetometer Data

A detumbling algorithm is developed to yield three-axis magnetic stabilization of a CubeSat deployed with unknown RAAN, orbit phase angle, inclination, attitude, and angular rate. Data from a three-axis magnetometer are the only input to determine both the control torque and the angular rate of the spacecraft. The algorithm is designed to produce a magnetic dipole moment which is constantly orthogonal to the geomagnetic field vector, independently of both the attitude and the angular rate of the rigid spacecraft. The angular rates are calculated in real time from magnetometer data, and the use of a second-order low-pass filter allows to rapidly reduce the measurement error within ±0.2 deg/sec. Numerical validation of the algorithm is performed, and a variety of feasible scenarios is simulated assuming the CubeSat to operate in low Earth orbit. The robustness of the algorithm, with respect to unknown deployment conditions, different sampling rates, and uncertainties on the moments of inertia of the CubeSat, is verified.

Vector Calibration Method for Orthogonality Correction of Three-axis Magnetometers

提出一种基于误差分离的矢量校准法用于三轴感应式磁传感器的正交性校正.建立了正交性校正的矢量校准数学模型，针对校正过程中的安装误差进行误差项分离校正，结合卡尔曼滤波与最小二乘法对模型参数进行求解得到正交性校正矩阵.仿真和实验结果表明，基于矢量校准数学模型的校准方法可行且有效，在输入磁场2.5nT情况下，测量误差由传统方法的0.0962nT减小到0.0019nT.此方法已应用于某星载感应式磁力仪的正交性校正.

Complete Calibration of Three-Axis Strapdown Magnetometer in Mounting Frame

Three-axis magnetometers (TAMs) are widely used in connection with vehicle navigation systems and many other engineering applications. Technological limitations in sensor manufacturing and unwanted magnetic fields will corrupt the measurements of TAMs. In this paper, a new method is proposed to calibrate strapdown magnetometer in sensor frame. A complete error model, including instrumentation errors (body frame misalignment, scale factors, non-orthogonality, and offsets) and magnetic deviations (soft and hard iron), is elaborated. The calibration process is divided into two steps. The first step is providing a consistent solution to the ellipsoid fitting problem using a non-linear least squares estimator and genetic algorithm. The experimental system mainly consists of tri-axis HMC5843 AMR sensor and proton magnetometer. The true scalar value of magnetic field was obtained with proton magnetometer. Simulated results show that the error average of nonlinear least square is much lower than the genetic algorithm. The results of the calibration algorithm in the first step provide magnetic readings in the calibration frame. In the next step, the three calibrated axes of the given magnetometer, which are nominally orthogonal, are aligned into a true orthogonal sensor mounting frame by offering a rigorous vector analysis of rotational kinematics named vector product operations. In order to determine the magnetometer misalignment in the mounting frame, a surface plate is used as the main reference plate. Therefore, the calibration and alignment methodology is formulated in the sensor frame, without resorting to external information or models about the magnetic field.

Three-Axis Atomic Magnetometer Employing Longitudinal Field Modulation

A three-axis atomic magnetometer employing longitudinal field modulation is demonstrated theoretically and experimentally. The operational condition of this magnetometer is that the longitudinal z-component of the external magnetic field is dominant, which is achieved by applying a bias magnetic field. The longitudinal z-component of the external magnetic field is extracted from the modulation frequency, which tracks the resonance frequency, and the transverse x - and y - components are, respectively, obtained from the quadrature and the in-phase signals of a lock-in amplifier. Through feedback control, the magnitude and direction of an external magnetic field can be measured in real time, and an excellent orthogonality of the three-axis magnetometer response is obtained. By optimizing the parameters of this three-axis magnetometer, a longitudinal field sensitivity of 0.3 ${\rm{pT}/ \rm{Hz}}^{1/ 2}$ and a transverse field sensitivity of 2 ${\rm{pT}/ \rm{Hz}}^{1/ 2}$ are achieved.

Robust unscented Kalman filter for nanosat attitude estimation

A robust unscented Kalman filter based on a multiplicative quaternion-error approach is proposed for nanosat estimation in the presence of measurement faults. The global attitude parameterization is given by a quaternion, while the local attitude error is defined using a generalized three-dimensional attitude representation. The proposed algorithm uses a statistical function including measurement residuals to detect measurement faults and then uses an adaptation scheme based on multiple measurement scale factor for filter robustness against faulty measurements. The proposed algorithm is demonstrated for the attitude estimation of a nanosat with an on-board three-axis magnetometer and rate-integrating gyros in the presence of measurement faults as well as satellite orbit errors. To compare the estimation performance of the proposed algorithm, the robust unscented Kalman filter with single measurement noise scale factor, the standard extended Kalman filter and the unscented Kalman filter are also implemented under the same simulation conditions.

3D human gesture capturing and recognition by the IMMU-based data glove

Gestures recognition provides an intelligent, natural, and convenient way for human–robot interaction (HRI). This paper presents a novel data glove for gestures capturing and recognition based on inertial and magnetic measurement units (IMMUs), which are made up of three-axis gyroscopes, three-axis accelerometers and three-axis magnetometers. The proposed data glove has eighteen low-cost IMMUs, which are compact and small enough to wear. The gestures included the three-dimensional motions of arm, palm and fingers are completely captured by the data glove. Meanwhile, we attempt to use extreme learning machine (ELM) for gesture recognition which has not found yet in the relevant application. The ELM-based recognition methods for both static gestures and dynamic gestures are respectively presented. The experimental results of gestures capturing and recognition verify the effectiveness of the proposed methods.

A three-axis atomic magnetometer for temperature-dependence measurements of fields in a magnetically shielded environment

High performance spin-exchange-relaxation-free (SERF) gyroscopes require stable and homogeneous magnetic fields. These fields are usually sensitive to temperature. In this paper, a three-axis atomic magnetometer to measure the temperature dependence of fields inside the magnetic shields for a SERF gyroscope prototype is constructed and tested. Based on a three-beam configuration, three-axis vector capability is obtained by a cross-modulation scheme of the magnetic field components along orthogonal axes and subsequent demodulation of the relevant probe signals. The relative temperature dependence of magnetic fields inside the prototype shields is measured to be –. The results are useful for estimating the necessary precision of temperature control in both a SERF gyroscope and its fundamental physics applications.

Novel Bio-Logging Tool for Studying Fine-Scale Behaviors of Marine Turtles in Response to Sound

Increases in the spatial scale and intensity of activities that produce marine anthropogenic sound highlight the importance of understanding the impacts and effects of sound on threatened species such as marine turtles. Marine turtles detect and behaviorally respond to low-frequency sounds, however few studies have directly examined their behavioral responses to specific types or intensities of anthropogenic or natural sounds. Recent advances in the development of bio-logging tools, which combine acoustic and fine-scale movement measurements, have allowed for evaluations of animal responses to sound. Here, we describe these tools and present a case study demonstrating the potential application of a newly developed technology (ROTAG, Loggerhead Instruments, Inc.) to examine behavioral responses of freely swimming marine turtles to sound. The ROTAG incorporates a three-axis accelerometer, gyroscope, and magnetometer to record the turtle’s pitch, roll, and heading; a pressure sensor to record turtle depth; a hydrophone to record the turtle’s received underwater acoustic sound field; a temperature gauge; and two VHF radio telemetry transmitters and antennas for tag and turtle tracking. Tags can be programmed to automatically release via a timed corrodible link several hours or days after deployment. We describe an example of the data collected with these tags and present a case study of a successful ROTAG deployment on a juvenile green turtle (Chelonia mydas) in the Paranagua Estuary Complex, Brazil. The tag was deployed for 221 minutes, during which several vessels passed closely (<2km) by the turtle. The concurrent movement and acoustic data collected by the ROTAG were examined during these times to determine if the turtle responded to these anthropogenic sound sources. While fine-scale behavioral responses were not apparent (second-by-second), the turtle did appear to perform dives during which it remained still on or near the sea floor during several of the vessel passes. This case study provides proof of concept that ROTAGs can successfully be applied to free-ranging marine turtles to examine their behavioral response to sound. Finally, we discuss the broad applications that these tools have to study the fine-scale behaviors of marine turtles and highlight their use to aid in marine turtle conservation and management.

Characterization and demonstration results of a SQUID magnetometer system developed for geomagnetic field measurements

We characterized a low temperature superconducting quantum interference device (SQUID) magnetometer system developed for high-sensitivity geomagnetic field measurement, and demonstrated the detection of weak geomagnetic signals. The SQUID magnetometer system is comprised of three-axis SQUID magnetometers housed in a glass fiber reinforced plastic cryostat, readout electronics with flux locked loop (FLL), a 24-bit data logger with a global positioning system and batteries. The system noise was approximately 0.2 pT √Hz−1/2 in the 1–50 Hz frequency range. This performance was determined by including the thermal noise and the shielding effect of the copper shield, which covered the SQUID magnetometers to eliminate high-frequency interference. The temperature drift of the system was ∼0.8 pT °C−1 in an FLL operation. The system operated for a month using 33 l liquid helium. Using this system, we performed the measurements of geomagnetic field in the open-air, far away from the city. The system could detect weak geomagnetic signals such as the Schumann resonance with sixth harmonics, and the ionospheric Alfvén resonance appearing at night, for the north–south and east–west components of the geomagnetic field. We confirm that the system was capable of high-sensitivity measurement of the weak geomagnetic activities.

Low-noise tunneling-magnetoresistance vector magnetometers with flux chopping technique

A concept for a low-noise three-axis magnetometer consisting of tunneling magnetoresistance (TMR) sensors and a flux chopper was designed, implemented, and characterized in this work. The TMR sensors used in this study were the model of TMR2102D from Multidimension Technology Inc. Three TMR sensors were aligned orthogonally on a printed circuit board (PCB) and mounted inside a cylindrical flux chopper. The cylindrical flux chopper including a soft magnetic shielding tube and enameled copper wires was 16mm in length and 8mm in diameter. The shielding tube was made of a cobalt-based soft magnetic ribbon, Metglas-2714A, from Metglas Inc. The flux chopper modulated the external magnetic flux density using the fluxgate effect, which made the sensors respond to the quasi-static field at the chopping frequency. The demodulated output showed a reduction in low-frequency noise to the level of 0.17nT/√Hz@1Hz. To demonstrate the technical feasibility for electronic compass application, the demodulated output of the vector magnetometer prototype was recorded by rotating the device in Earth’s magnetic fields about a fixed axis. The Cartesian components Bx, By, and Bz of the Earth’s fields at various azimuth angles were retrieved by performing a calibration algorithm to correct the non-orthogonality caused by the misalignment of TMR sensors and the chopper. The calibrated outputs were linear and orthogonal to each other with an angle error less than 1° and nonlinearity of 0.7%, indicating that the chopping technique is useful to realize an extremely low-noise three-dimensional magnetometer for the geomagnetic application.

A Modified Tolles–Lawson Model Robust to the Errors of the Three-Axis Strapdown Magnetometer

The estimating of the Tolles–Lawson model’s coefficients plays an important role in aeromagnetic compensation. The directional cosines, which are indispensable for estimating the coefficients, are usually measured by a three-axis strapdown magnetometer in a general aeromagnetic survey system. However, in some cases, the scalar magnetometer may have a much higher accuracy than the three-axis strapdown magnetometer. This imbalance of the measurement accuracy is then introduced into the Tolles–Lawson model and affects the estimation of the coefficients. In this letter, a modified Tolles–Lawson model is introduced to reduce the imbalance through substituting the error model of the three-axis strapdown magnetometer into the calculation of the directional cosines. The characteristics of the modified model are analyzed and the corresponding coefficient-estimating system is developed. Simulation results illustrate that the modified model is more robust to the larger errors of the three-axis strapdown magnetometer than the classical model.

Performance Improvement of an INS by using a Magnetometer with Pedestrian Dynamic Constraints

This paper proposes to improve the performance of a strap down inertial navigation system using a foot-mounted low-cost inertial measurement unit/magnetometer by configuring an attitude and heading reference system. To track position accurately and for attitude estimations, considering different dynamic constraints, magnetic measurement and a zero velocity update technique is used. A conventional strap down method based on integrating angular rate to determine attitude will inevitably induce long-term drift, while magnetometers are subject to shortterm orientation errors. To eliminate this accumulative error, and thus, use the navigation system for a long-duration mission, a hybrid configuration by integrating a miniature micro electromechanical system (MEMS)-based attitude and heading detector with the conventional navigation system is proposed in this paper. The attitude and heading detector is composed of three-axis MEMS accelerometers and three-axis MEMS magnetometers. With an absolute algorithm based on gravity and Earth’s magnetic field, rather than an integral algorithm, the attitude detector can obtain an absolute attitude and heading estimation without drift errors, so it can be used to adjust the attitude and orientation of the strap down system. Finally, we verify (by both formula analysis and from test results) that the accumulative errors are effectively eliminated via this hybrid scheme.

Aerodynamic Coefficient Identification of a Space Vehicle from Multiple Free-Flight Tests

The identification of aerodynamic coefficients, based on free-flight measurements, remains complex and challenging for vehicles such as space probes, unmanned aerial vehicles, or ammunition. In this paper, a detailed procedure for the identification of the drag, pitching moment slope, and pitch damping coefficients of a reentry space vehicle is presented. New models of these aerodynamic coefficients, based on polynomial functions of Mach numbers and incidence angles, are proposed, and an estimation strategy using multiple data series is applied. The free-flight data correspond to three-axis magnetometer and radar measurements for different experimental conditions. The resulting model is tested with new data sets to ascertain the model validity. The results show that the proposed description of the three aerodynamic coefficients and the estimation strategy are relevant.

Magneto-Gyro Wearable Sensor Algorithm for Trunk Sway Estimation During Walking and Running Gait

Trunk sway is a critical gait parameter associated with walking stability and joint loading. This paper introduces a novel algorithm to estimate trunk sway based on magnetometer and gyroscope sensor fusion. An initial experiment was performed to determine the optimal placement of the wearable sensor device on the back and a second experiment was performed to characterize the accuracy of the algorithm. Ten human subjects walked, fast walked, and ran on a treadmill with normal, slightly increased or significantly increased trunk sway. Subjects wore a single magneto-inertial measurement unit (IMU) and a standard set of reflective motion-capture markers on their back. Magneto-gyro algorithm trunk sway estimations were compared with estimations from other common wearable sensor algorithms and errors were determined via comparison with trunk sway measured from motion capture. Overall, the magneto-gyro algorithm was the most accurate (RMSE = 1.7 ± 0.7°) followed by algorithms based on a single three-axis magnetometer (RMSE = 2.5 ± 1.8°), IMU gradient descent (RMSE = 2.9 ± 3.4°), a single three-axis gyroscope (RMSE = 3.2 ± 2.4°), magneto-IMU Kalman filter (RMSE = 8.5 ± 5.5°), and a single accelerometer (RMSE = 16.7 ± 11.2°). Optimal placement of a wearable sensor for estimating trunk sway was along the spine between the $T7$ – $T12$ vertebrae. The presented algorithm based on magnetometer and gyroscope sensor fusion could enable more precise trunk sensing for clinical gait applications outside the laboratory.

Hybrid Calibration Method for Three-Axis Gradiometer

The performance of a three-axis gradiometer (TAG) is limited by measurement and misalignment errors of two three-axis magnetometers (TAMs). We describe a simplified calibration method for TAGs. One TAM is calibrated with a scalar method. Self-errors of the other TAM and misalignment errors between the two TAMs are then calibrated as integrated errors based on a vector calibration. The experimental results show that the scalar output errors of the two TAMs and the vector output errors of the TAG are greatly reduced and the performance of the TAG is improved dramatically.

Distortion Magnetic Field Compensation of Geomagnetic Vector Measurement System Using a 3-D Helmholtz Coil

The magnetic interferential fields, such as soft-iron and hard-iron interferences, will seriously affect the accuracy of geomagnetic vector measurement system, and thus should be compensated. In this letter, a new compensation method using a 3-D Helmholtz coil is proposed. As a first step, the geomagnetic vector measurement system is exposed to different directions and the magnitudes of magnetic field generated by a 3-D Helmholtz coil to construct the equations of error model, and soft-iron parameters can be estimated by solving linear equations. Then, hard-iron parameters are estimated by changing the fixation direction of the three-axis magnetometer. Finally, all the estimated parameters are used for compensating distortion magnetic fields. In order to verify the effectiveness of the proposed method, the experiment is conducted, and the results demonstrate that the proposed method contributes to the accuracy improvement of geomagnetic vector measurement system.