Tunnel Magnetoresistance Sensors Research Articles

Electron irradiation total ionizing dose (TID) effect of tunneling magnetoresistance sensors

Tunnel magnetoresistance (TMR) sensors are rapidly gaining attention for magnetic field sensing applications owing to their high sensitivity, low power consumption, small size, and low cost. Their metallic, non-semiconductor construction makes them excellent candidates for use in the harsh environments present in nuclear and space applications. In this work, the impacts of electron irradiation on the magnetoelectric transport properties of the two types of TMR sensors were investigated. The electron fluence ranged from 0 Mrad(Si) to 400 Mrad(Si). No significant evidence of degradation of the sensor characteristics, such as sensitivity, resistance, TMR ratio and detectivity, was observed. Our results verify the robustness and adaptability of the TMR sensors.

TMR Sensor-Based Detection of EVs in Semi-Dynamic Traffic for Optimal Charging

One of the challenges faced in the design of inductive power transfer (IPT) highway is the detection of electric vehicle (EV), to enable charging, as it travels along the highway. When a primary coil buried under the highway has sufficient coupling with the secondary coil mounted on the vehicle, it is energized to initiate IPT. In this paper, a tunneling magnetoresistance (TMR) sensor based EV detection method on an IPT highway is proposed. In the proposed scheme, a detection coil is wound around the outer boundary of the secondary coil and it is excited at 400 kHz. The TMR sensors are arranged close to the primary coil such that it senses the magnetic field generated by the detection coil. The TMR sensors are placed over the leading and trailing edges (in the direction of EV travel) of the primary coil. From the leading edge TMR sensor outputs, the energization point of that primary coil to begin IPT is decided. Similarly, from the trailing edge TMR sensor outputs, the de-energization point to stop IPT from that primary coil is decided. Also, the effective area of overlap between the coils due to lateral misalignment is estimated. This enables the driver in correcting the lateral misalignment so that efficient charging is possible from the subsequent primary pads. A laboratory scale prototype was built and was tested. The results obtained prove the usefulness of the scheme.

Detecting Magnetic Ink Barcodes With Handheld Magnetoresistive Sensors

Information encoding in barcodes using magnetic-based technology is a unique strategy to read data buried underneath non-transparent surfaces since a direct line-of-sight between the code and the reader is not required. This technology is of particular interest in secure labelling and recyclable packaging applications. However, current magnetic reading heads, such as those employed for magnetic ink character recognition, need to be placed in contact with the magnetic structures, limiting the depths at which the information can be read. This paper describes a strategy to overcome that limitation by replacing the traditional inductive heads with tunnel magnetoresistive (TMR) sensors. Soft-magnetic codes can be printed using conventional LaserJet toners and, by having their magnetisation set with a permanent magnet included in the device, the resulting magnetic field can be read using a TMR sensor. We demonstrate that such a device can read barcodes at depths of at least 1 mm. It can also resolve individual structures as thin as 200 {\mu}m when used in contact.

Multi-frequency imaging with non-linear calibration of magnetoresistance sensors for surface and buried defects inspection

Timely inspecting defects has both safety and economic significance in industry. Eddy current testing (ECT) probes with array sensors are widely used for metal structures inspection. However, the sensitivities of array sensors are typically not consistent, resulting in different responses when measuring the same defect signal. Therefore, the array sensors of an array probe are desired to be calibrated. This paper proposes an ECT probe with an array of tunnel magnetoresistance (TMR) sensors and two excitation coils for surface and buried defects inspection. Alternating currents with different frequencies and amplitudes are driven through the two coils simultaneously. The TMR sensors have non-linear resistance vs. magnetic field curves. A non-linear calibration method based on the non-linear characteristic is proposed to calibrate the high frequency signals of the sensors. This calibration method does not need any extra equipment and takes account of the effect of the non-uniformity of the excitation magnetic field to some extent. Experimental result shows that the signal becomes much flatter after the calibration. The relative flat index is reduced from 38.6% to 11% by the calibration process. To validate the detectability of the proposed probe for different kinds of defects, surface and buried defects in aluminum samples are tested by the probe. It is seen that a small surface defect with sizes 1 mm (length) x 0.5 mm (width) x 0.5 mm (height) can be clearly identified from the high-frequency image. Two defects buried 3 mm below the top surface are also inspected. The peak signal values of the two defects are 0.69 V and 0.97 V, respectively, demonstrating that the probe can distinguish the 0.1 mm height difference of the two defects. In addition, a defect buried 10 mm under the surface is imaged by the probe. Thanks to the special probe design and excellent sensitivity of the TMR sensors, the probe has good sensitivity for both surface and buried defects.

Vector Magnetic Field Imaging With High-Resolution TMR Sensor Arrays for Metal Structure Inspection

This paper proposes an eddy current testing (ECT) probe with high-resolution tunneling magnetoresistance (TMR) array sensors for vector magnetic field measurement. The probe consists of two printed circuit boards (PCBs), which are placed perpendicular to each other. Three arrays of TMR sensors are wire bonded on the two PCBs to measure the three components of the magnetic field. Each array contains 32 sensors and the distance between the centers of two adjacent sensors in an array is 0.5 mm. The 32 output signals of each array are multiplexed, amplified and filtered by using a circuit on the PCB. An experimental system is set up and aluminum samples with machined defects are imaged by the probe. It is seen that the patterns of the images along the x-, y- and z-axis are different, meaning the information in the three images can compensate each other to a certain extent. The images change their orientations if the defects are oriented along different angles. An image processing algorithm including image segmentation, normalization, edge detection and K-means classification is developed to class the defects. It is seen that the defects can be classified accurately with the algorithm. Then, the orientations of the defects are calculated from the images with a linear regression of the angles of the three axis images. It is found that the mean error of the orientation quantification is only 4.14°, which is much smaller than the mean error calculated from each axis image separately.

Measurement of Small-Magnitude Direct Current Mixed With Alternating Current by Tunneling Magnetoresistive Sensor

Measurement of small-magnitude direct current (dc) mixed with alternating current (ac) is crucial in the power systems in which large-scale renewable energy and power electronics are used. This research proposes a method to design a nonintrusive surface-mount current sensor that can measure milliampere dc mixed with ac of different amplitudes based on the tunneling magnetoresistive (TMR) sensors. A sensor module configuration that uses a high-sensitivity TMR sensor to detect small-magnitude dc and ac was proposed. Moreover, because the high-sensitivity TMR sensor was susceptible to saturation, a medium-sensitivity TMR sensor was used to detect the large-magnitude ac. A signal processing framework was built, in which the mean and standard deviation of the detected waveforms were calculated to obtain the dc and ac components of the current under measurement, and the measurement error due to the TMR sensor saturation was also reduced by a compensation algorithm. The results showed that the sensor could accurately measure dc lower than 40 mA mixed with the root-mean-square ac ranging from 100 mA to 2 A.

Developments and applications of tunneling magnetoresistance sensors

Magnetic sensors based on tunneling magnetoresistance (TMR) effect exhibit high sensitivity, small size, and low power consumption. They have gained a lot of attention and have potential applications in various domains. This study first introduces the development history and basic principles of TMR sensors. Then, a comprehensive description of TMR sensors linearization and Wheatstone bridge configuration is presented. Two key performance parameters, the field sensitivity and noise mechanisms, are considered. Finally, the emerging applications of TMR sensors are discussed.

Tunnel-magnetoresistive-based Pulsed Eddy Current Probe for Inspection of Corrosion under Insulation

The problem of corrosion in the industrial oil and gas pipes has been one of the major contributors in catastrophic structural failures. Among the various types of corrosion, corrosion under insulation (CUI) has been known to cause serious problems. Pulsed eddy current (PEC) non-destructive testing has shown its effectiveness in detecting hidden CUI. Most PEC systems have been developed by using an inductive coil as their sensing device, while some use a magnetic sensor which potentially offers better resolution. A new probe design based on a solid-state Tunnel magnetoresistance (TMR) is presented. TMR sensors offer higher sensitivities compared to other commercially available sensors. The performance of the probe is evaluated by using ferromagnetic mild steel plates that have thicknesses in the range of 8mm to 12mm at different stand-offs with a thin aluminium sheet under the probe. The different thicknesses represent different corrosion levels, while the stand-offs and thin aluminium sheet are to mimic the insulation of different thicknesses and the cladding in the real pipeline structure. The results show an overall mean absolute error (MAE) of 0.19 mm, which is better than our existing Hall-device-based PEC probe.

Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

Non-invasive human brain functional imaging with millisecond resolution can be achieved only with magnetoencephalography (MEG) and electroencephalography (EEG). MEG has better spatial resolution than EEG because signal distortion due to inhomogeneous head conductivity is negligible in MEG but serious in EEG. However, this advantage has been practically limited by the necessary setback distances between the sensors and scalp, because the Dewar vessel containing liquid helium for superconducting quantum interference devices (SQUIDs) requires a thick vacuum wall. Latest developments of high critical temperature (high-Tc) SQUIDs or optically pumped magnetometers have allowed closer placement of MEG sensors to the scalp. Here we introduce the use of tunnel magneto-resistive (TMR) sensors for scalp-attached MEG. Improvement of TMR sensitivity with magnetic flux concentrators enabled scalp-tangential MEG at 2.6 mm above the scalp, to target the largest signal component produced by the neural current below. In a healthy subject, our single-channel TMR-MEG system clearly demonstrated the N20m, the initial cortical component of the somatosensory evoked response after median nerve stimulation. Multisite measurement confirmed a spatially and temporally steep peak of N20m, immediately above the source at a latency around 20 ms, indicating a new approach to non-invasive functional brain imaging with millimeter and millisecond resolutions.

Short-Circuit and Over-Current Fault Detection for SiC MOSFET Modules Based on Tunnel Magnetoresistance With Predictive Capabilities

This letter proposes a short-circuit and over-current fault detection solution for silicon-carbide (SiC) <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> modules based on tunnel magnetoresistance (TMR). The TMR sensor is integrated into an SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> module to noninvasively measure its current. The measured current is compared with a threshold to detect short-circuit and over-current faults. The TMR sensor is installed in a chosen location, where the TMR sensor gain is automatically doubled in the event of short-circuit fault. As a result, a short-circuit fault can be predicted, i.e., the fault is detected before the actual short-circuit current reaches the threshold. Besides, only one TMR sensor is required to detect both short-circuit and over-current faults for a half-bridge power module. According to the experimental results, the short-circuit fault is detected when the fault current reaches half of the fault threshold current. The experimental results indicate general superiority over desaturation technology and better performance in detection time, reaction time, and total protection time compared with Rogowski switch-current sensor-based technology.

Inspection of Defect Under Thick Insulation Based on Magnetic Imaging With TMR Array Sensors

A lot of industrial structures are covered by insulation. Defects under insulation (DUI) frequently distress the structure maintenance and may lead to serious safety concerns. In this article, a probe with tunneling magnetoresistance (TMR) array sensors is proposed to inspect defects under thick insulation. The probe contains 64 TMR sensors with the size of each sensor length <inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> width &#x003D; 0.45 mm <inline-formula> <tex-math notation="LaTeX">$\times0.45$ </tex-math></inline-formula> mm, which can generate a magnetic field image with high spatial resolution. Excitation current with extremely low frequency (in the order of a few hundred Hertz) is utilized for inspection of defects with large liftoff distances. In this low-frequency range, the TMR sensors have much better sensitivity than conventional induction coils. A data processing method based on discrete wavelet transform (DWT) is utilized to reduce noise and highlight defect indication. Experimental results demonstrate that the probe is capable of detecting a machined rectangular defect with dimensions of 6 mm (length), 3 mm (width), and 1 mm (depth) under 10 mm thick insulation. The minimum dimensions of the machined defect under 20 mm insulation that can be detected by the probe are 9 mm (length), 3 mm (width), and 1 mm (depth). As a comparison, for a commercial pulsed eddy current probe (PECA-HR-SM), the length of minimum detectable detect is 19 mm under 18 mm thick insulation. If the thickness of the insulation is 30 mm, a machined rectangular defect with dimensions 21 mm (length), 3 mm (width), and 7 mm (depth) can be detected by the proposed probe. The experimental results demonstrate that the proposed probe is a promising alternation with excellent sensitivity for inspection of defects with large liftoff distances.

Research on crack monitoring technology of flexible eddy current array sensor based on TMR sensors

The TMR (Tunnel MagnetoResistance) sensors are used to replace the original induction coils, and the TMR sensors are arrayed along the x-axis direction, which quantitatively monitor crack propagation through the response sequence of TMR sensors. Firstly, a current dipole model is proposed to characterize the influence of disturbed magnetic induction intensity at the test point, and the method of quantitative monitoring of subsurface crack is proposed. Then, the finite element model of the sensor is established to analyze the disturbed effect of crack propagation on eddy current with different excitation frequencies. Finally, the experiments on the surface and subsurface crack monitoring with different depths are carried out. The experimental results show that the crack propagation can be monitored quantitatively through the response sequence of the TMR sensors. Also, the effect of subsurface crack monitoring can be improved by increasing the longitudinal distance between the test point and the crack (LDTC).

Dynamic noise characteristics of tunneling magnetoresistance sensor under AC excitation

The tunnel magnetoresistance (TMR) sensor has the advantages of high sensitivity, easy miniaturization, and low power consumption, and it has the great potential to become a high-performance magnetic field measurement sensor. As the TMR magnetic sensor has complex noise characteristics, it is of great significance to explore the noise characteristics of TMR magnetic sensor for guiding its application. In this paper, the noise characteristics of the TMR magnetic sensor under different excitations are studied. It is found that the noise of TMR magnetic sensor under AC excitation is separated: some of the noise is dependent on the excitation and is moved to the excitation frequency synchronously with the output signal. The remaining part of the noise is independent of the excitation and remains in the low frequency band. Through the quantitative analysis of noise data, we find that the noise related to excitation accounts for about 70% of the total noise, while the noise unrelated to excitation accounts for about 30%. The results show that under AC excitation, 70% of the noise is synchronously modulated and 30% of the noise is separated from the signal, which provides an important guidance for the application of TMR magnetic sensor.

A Study of Hysteresis Reduction of Small AC Magnetic Field Modulated Tunneling Magnetoresistive Sensor

In this study, a hysteresis reduction technique for tunneling magnetoresistive (TMR) sensor for low magnetic field sensing is proposed, where a small ac modulation magnetic field is applied. The magnetic tunnel junctions (MTJs) were connected into an asymmetric Wheatstone bridge to form a sensor, and the output voltage of the TMR sensor was measured using a small ac modulation field and test field along the sensor’s sensing direction. The results showed that hysteresis in the TMR sensor can be effectively reduced to exhibit a linear response. Further, the frequency response of the TMR sensor was studied. Finally, the feasibility of the TMR sensor for low-field detection with small ac modulation was demonstrated.

Tunnel Magnetoresistance Sensor with AC Modulation and Impedance Compensation for Ultra-Weak Magnetic Field Measurement.

Tunnel magnetoresistance (TMR) is a kind of magnetic sensor with the advantages of low cost and high sensitivity. For ultra-weak and low-frequency magnetic field measurement, the TMR sensor is affected by the 1/f noise. This paper proposes an AC modulation method with impedance compensation to improve the performance. The DC and AC characteristics of the sensors were measured and are presented here. It was found that both the equivalent resistance and capacitor of the sensors are affected by the external magnetic field. The TMR sensors are connected as a push–pull bridge circuit to measure the magnetic field. To reduce the common-mode noise, two similar bridge circuits form a magnetic gradiometer. Experimental results show that the sensor’s sensitivity in the low-frequency range is obviously improved by the modulation and impedance compensation. The signal-to-noise ratio of the sensor at 1 Hz was increased about 25.3 dB by the AC modulation, impedance compensation, and gradiometer measurement setup. In addition, the sensitivity of the sensor was improved from 165.2 to 222.1 mV/V/mT. Ultra-weak magnetic signals, namely magnetocardiography signals of two human bodies, were measured by the sensor in an unshielded environment. It was seen that the R peak of MCG can be clearly visualized from the recorded signal.

A technology of full perimeter inspection for steel pipeline without removing cladding

This study proposes a rotating excitation magnetic field detection method to identify in-service cladding steel pipeline damage. A ring of uniformly arranged excitation coils generates excitation magnetic fields under the action of a three-phase current, which synthesize in the pipe as a rotating magnetic excitation field based on the principle of vector synthesis. Tunneling magneto-resistive (TMR) sensors work as detection probes for measuring magnetic induction strength. The induced current flows circularly in the pipe's body under the influence of a rotating excitation field. Therefore, this excitation method has a high sensitivity to defects in different length directions and can detect damage information in the whole circumference of the pipe body. The TMR sensors are uniformly arranged along the pipe's rim and measure the magnetic field in the radial direction with high sensitivity and low background noise (10 pT/Hz@1 kHz). The finite element method investigated the operating principle and performance of two excitation coils probe structures. The specimen probe and signal acquisition setup was developed and examined, with which artificial damages in the pipeline with cladding were inspected. The test result indicates that the proposed probe can penetrate the surface cladding of the pipe, effectively identifying defects in the pipeline.

Flexible Probe With Array Tunneling Magnetoresistance Sensors for Curved Structure Inspection

The 3-D curved structures are widely used in industries due to their advantages of flowing geometry, high structural strength, and excellent aerodynamic performance. Damages may occur in curved structures during manufacture or use, which may endanger the equipment’s operation safety. Efficient tools for inspection of buried defects in curved structures are still lacking. This article proposes a flexible nondestructive testing probe with tunneling magnetoresistance (TMR) sensors for inspecting buried defects in curved structures for the first time. The probe contains 16 TMR sensors and the distance between each 2 adjacent sensors is 2 mm. The TMR sensors are fetched from a customized wafer and screened to guarantee the consistency. In this probe, the TMR sensors are wire bonded on a flexible printed circuit board and then protected by polyurethane pouring sealant. The probe can be bent to fit an irregular surface with minimum bending radius 5.5 mm. The selection of the excitation frequency and the proposed probe performance for buried defects’ inspection are studied using a 3-D finite-element numerical model. Experimental results demonstrate that defects buried 3 mm below the top surface of cylindrical samples can be accurately detected. A chromatic confocal distance sensor is employed to sketch the curved surface of an irregular shaped sample. The electromagnetic image and optical image of this sample with curved surface are plotted in a fused image, which presents comprehensive information of the sample about both the structural pattern and the buried defects’ indications.

Comparison of performance among five types of tunneling magnetoresistance linear sensing units based on MgO magnetic tunnel junction

Magnetic tunnel junction (MTJ) has been successfully used in spintronic devices, such as magnetoresistive random access memory, tunneling magnetoresistance (TMR) sensor, magnetic logic. In the TMR sensor a special magnetic structure is used between the free layer and the pinned layer of an MTJ to realize a linear output. So far, five types of TMR linear sensing units (TMR-LSNs) have been developed based on MgO MTJs, which are artificial-indirect-double-exchange-coupling-, magnetic-field-biased-, in-plane-, perpendicular-, and superparamagnetic-TMR-LSN, respectively. In this paper, the five types of TMR-LSNs are combed and their magnetic sensing performances are systematically compared with each other. First, the five types of TMR-LSNs each have a linear resistance response to the external magnetic field with a changeable sensitivity, a linear field range and a low frequency noise level. Second, in the five types of TMR-LSNs different magnetic structures are used to realize the same aim that is to obtain the optimized performance parameters, which is of significance for putting TMR sensors into practical applications. Third, the five types of TMR-LSNs are suitable for different application scenarios due to their respective performance parameters. Therefore, we believe that our summarized discussion in this paper will help people to explore and find the relevant applications of TMR sensors based on the five types of TMR-LSNs.

A Magnetic Field Imaging System Based on TMR Sensors for Banknote Recognition

The high sensitivity and MEMS-compatible process of tunneling magnetoresistance (TMR) sensors provide great opportunities to develop high-resolution magnetic field detection and imaging systems. In this work, we designed a magnetic field imaging (MFI) system based on TMR sensors to detect the uniaxial magnetic field components of banknotes including US dollars (USD) and Chinese Yuan (CNY), which clearly reveals the distribution of magnetic anti-counterfeiting markers inside the banknotes. Measurement results of the USD show that the magnetic ink signal is around μT level, and the person portraits on banknotes can be revealed by the sensor signals. The testing results of the CNY banknotes indicate magnetic color-shifting ink images, as well as a clear magnetic anti-counterfeiting thread. Different from the traditional optical detection, magnetic detection provides unique signals and identification methods. With a satisfactory recognition rate, the proposed MFI system based on TMR sensors demonstrates its promising potential forgery detection and denomination identification of banknotes.

Signal Processing and Defect Characterization of Pulsed Eddy Current Testing Based on Cross-Correlation

In order to solve the problems of the low signal-to-noise ratio and inaccurate feature extraction of time domain signal in pulsed eddy current detection, a method of signal processing and feature extraction for pulsed eddy current detection based on cross correlation is proposed. the pulsed eddy current testing of cracks in different sizes and positions of multi-layer aluminum alloy specimens is carried out by using magnetic differential probe based on Tunnel Magnetoresistance sensor (TMR). Experimental results show that compared with the traditional method, the proposed method can characterize the defects at different position depths more clearly and accurately, which improves the detection depth and accuracy of pulsed eddy current detection. According to the variation rule between the size and depth of the defect and the cross-correlation eigenvalue, the exponential change curve of peak-to-peak time of cross-correlation can be determined, which can effectively identify the defects exceeding the geometric size of threshold at different depths.