CN111413654B - A system and method for reducing noise of tunneling magnetoresistance sensor - Google Patents
A system and method for reducing noise of tunneling magnetoresistance sensor Download PDFInfo
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Abstract
本发明公开了一种降低隧道磁电阻传感器噪声的系统及方法，包括：无线圈屏蔽圆筒和驱动电路；其中，无线圈屏蔽圆筒的材料为高磁导率软磁材料；待处理的TMR传感器放置在无线圈屏蔽圆筒中心；TMR传感器的输出端与驱动电路的输入端相连；无线圈屏蔽圆筒基于高磁导率软磁材料的过饱和特性，在通入交流电流时，其屏蔽能力发生高频变化，从而将将TMR传感器的待测直流磁场转变为高频交流磁场，降低TMR传感器的1/f噪声；驱动电路基于参考信号，对TMR传感器的输出信号进行相敏检波和低通滤波，滤除和频信号，保留差频信号，进一步滤除TMR传感器的1/f噪声。本发明中的无线圈屏蔽圆筒无需缠绕多匝线圈，不会引入新的噪声，降噪效果较好。
The invention discloses a system and method for reducing noise of a tunnel magnetoresistance sensor, comprising: a non-coil shielding cylinder and a driving circuit; wherein, the material of the non-coil shielding cylinder is a high magnetic permeability soft magnetic material; the TMR to be processed The sensor is placed in the center of the non-coil shielding cylinder; the output end of the TMR sensor is connected to the input end of the drive circuit; the non-coil shielding cylinder is based on the supersaturation characteristics of the high permeability soft magnetic material, when the alternating current is passed through, its shielding The ability of high-frequency changes, so that the DC magnetic field to be measured of the TMR sensor is converted into a high-frequency AC magnetic field, and the 1/f noise of the TMR sensor is reduced; the drive circuit is based on the reference signal. Through filtering, the sum frequency signal is filtered out, the difference frequency signal is retained, and the 1/f noise of the TMR sensor is further filtered out. The non-coil shielding cylinder of the present invention does not need to be wound with multiple turns of coils, does not introduce new noise, and has a better noise reduction effect.
Description
Technical Field
The invention belongs to the field of magnetic sensor noise reduction, and particularly relates to a system and a method for reducing noise of a tunnel magneto-resistance sensor.
Background
A Tunneling magnetoresistive sensor (TMR) is a magnetic field measurement sensor with high sensitivity, high response frequency and low power consumption, and has wide applications in various fields such as industrial control, financial instruments, biomedical, consumer electronics, and automobiles. The TMR sensor has many advantages, but the noise is large, so that the research on a method for reducing the noise of the tunnel magneto-resistance sensor is of great significance.
The existing methods for reducing the noise of the tunnel magneto-resistance sensor comprise a sigma-delta circuit modulation method, a magnetic tunnel junction array method, an MEMS type magnetic flux modulation method and a magnetic shielding chopping modulation method. The sigma-delta circuit modulation reduces noise through oversampling and noise shaping technologies, but the multi-stage sigma-delta circuit is complex in structure, consumes more elements, and is easy to introduce new noise. The magnetic tunnel junction array method improves the signal-to-noise ratio by arranging N magnetic tunnel junctions in series1/2However, as the signal-to-noise ratio requirement is increased, the number of the magnetic tunnel junctions connected in series is increased, and high requirements are put on the process. The MEMS type magnetic flux modulation method reduces 1/f noise by an MEMS circuit and a magnetic flux concentrator, but the magnetic flux concentrator needs to be provided with a complicated mechanical structure. The methods are all based on complicated circuit structures or complex processes to realize noise reduction, and are not suitable for commercial chips or commercial sensors. The magnetic shielding chopping modulation method can realize noise reduction of the TMR sensor by a simple structure, the chopping principle of the magnetic shielding chopping modulation method is similar to that of an electrical chopping technology, and conversion from direct current magnetic field measurement to alternating current magnetic field measurement is realized by modulating the magnitude or the existence of magnetic flux, so that the aim of reducing 1/f noise is fulfilled. However, this method needs to wind a multi-turn coil, and has a complex structure, introduces new noise, and has a poor noise reduction effect.
Disclosure of Invention
In view of the above defects or improvement requirements of the prior art, the present invention provides a system and a method for reducing noise of a tunneling magneto-resistance sensor, so as to solve the technical problem of poor noise reduction effect caused by winding a multi-turn coil in the prior art.
In order to achieve the above object, in a first aspect, the present invention provides a system for reducing noise of a tunneling magnetoresistive sensor, including: a coil-less shield cylinder and a drive circuit;
wherein, the material of the coil-free shielding cylinder is a soft magnetic material with high magnetic permeability; the TMR sensor to be processed is placed in the center of the wireless coil shielding cylinder; the output end of the TMR sensor is connected with the input end of the drive circuit;
the coil-free shielding cylinder is based on the supersaturation characteristic of a soft magnetic material with high magnetic conductivity, and the shielding capability of the coil-free shielding cylinder is subjected to high-frequency change when alternating current is introduced, so that a direct current magnetic field to be detected of the TMR sensor is converted into a high-frequency alternating current magnetic field, and the 1/f noise of the TMR sensor is reduced;
the driving circuit carries out phase-sensitive detection and low-pass filtering on the output signal of the TMR sensor based on the reference signal, so as to filter a sum frequency signal, retain a difference frequency signal and further filter 1/f noise of the TMR sensor.
Further preferably, the coil-less shielding cylinder is wrapped with an insulating material to insulate it from the outside.
Further preferably, the drive circuit includes: the pre-amplifier circuit, the band-pass filter circuit and the phase-locked amplifier circuit;
the output end of the pre-amplification circuit is connected with the input ends of the band-pass filter circuit and the phase-locked amplifier circuit, and the output end of the band-pass filter circuit is connected with the input end of the phase-locked amplifier circuit;
the pre-amplification circuit is used for amplifying the reference signal and the output signal of the TMR sensor, outputting the amplified reference signal to the phase-locked amplifier circuit, and outputting the amplified output signal of the TMR sensor to the band-pass filter circuit;
the band-pass filter circuit is used for preliminarily filtering noise in the amplified TMR sensor output signal and outputting the preliminarily noise-reduced signal to the phase-locked amplifier circuit;
the phase-locked amplifier circuit is used for realizing phase-sensitive detection and low-pass filtering on the signals input by the band-pass filter circuit based on the amplified reference signals, realizing the demodulation process of alternating current to direct current and filtering 1/f noise of the TMR sensor.
Further preferably, the lock-in amplifier circuit includes a phase-sensitive detection circuit and a low-pass filter circuit connected in series.
In a second aspect, the present invention provides a method for reducing noise of a tunneling magnetoresistive sensor, including the following steps:
s1, placing the TMR sensor to be processed in the center of the wireless coil shielding cylinder;
s2, alternating current is introduced into the wireless coil shielding cylinder, and the shielding capability of the wireless coil shielding cylinder is changed at high frequency based on the supersaturation characteristic of the high-permeability soft magnetic material of the wireless coil shielding cylinder, so that a direct current magnetic field to be measured of the TMR sensor is converted into a high-frequency alternating current magnetic field, and the 1/f noise of the TMR sensor is reduced;
s3, performing phase-sensitive detection and low-pass filtering on the alternating current output of the TMR sensor, filtering out sum frequency signals, reserving difference frequency signals, and further filtering out 1/f noise of the TMR sensor.
Further preferably, when the high magnetic permeability soft magnetic material of the coil-less shielding cylinder is supersaturated, the magnitude of the shielding capability of the coil-less shielding cylinder is controlled by controlling the amplitude of the alternating current, and the period of change of the shielding capability of the coil-less shielding cylinder is controlled by controlling the frequency of the alternating current.
Further preferably, the method for reducing noise of the tunneling magneto-resistance sensor provided by the second aspect of the invention is applied to the field of noise reduction of magnetic sensors.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
1. the invention provides a system for reducing noise of a tunnel magneto-resistance sensor, which comprises a coil-free shielding cylinder and a driving circuit, wherein the coil-free shielding cylinder is made of a high-permeability soft magnetic material, and the shielding capability of the coil-free shielding cylinder changes at high frequency when alternating current is introduced based on the supersaturation characteristic of the high-permeability soft magnetic material, so that a direct-current magnetic field to be detected of a TMR sensor is converted into a high-frequency alternating-current magnetic field, and the 1/f noise of the TMR sensor to be processed is reduced; the magnetic shielding chopping is utilized to reduce noise, the frequency limitation of mechanical chopping is avoided, extremely low frequency noise can be inhibited, and the hysteresis effect can be reduced; in addition, the coil-free shielding cylinder does not need to be wound with a plurality of turns of coils, so that new noise is not introduced, and the noise reduction effect is good.
2. The system for reducing the noise of the tunnel magneto-resistance sensor provided by the invention adopts the coil-free shielding cylinder, has simple structure, lower power consumption and convenient carrying, is suitable for a commercial TMR chip or sensor, and can improve the signal-to-noise ratio of the commercial TMR chip or sensor.
3. The invention provides a method for reducing noise of a tunnel magneto-resistance sensor, which is characterized in that alternating current is introduced into a coil-free shielding cylinder, high-frequency change of shielding performance of the shielding cylinder is realized based on the supersaturation characteristic of a soft magnetic material with high magnetic conductivity, a direct-current magnetic field to be measured outside the shielding cylinder is converted into a high-frequency alternating-current magnetic field, and a TMR sensor generates alternating-current output, so that low-frequency noise of the TMR sensor is effectively suppressed.
4. The method for reducing the noise of the tunnel magneto-resistance sensor is based on magnetic flux chopping shielding modulation, is implemented by using a wireless coil magnetic shielding cylinder without using a coil, reduces the loss caused by the heating of the coil, and simultaneously further improves the temperature stability and the portability.
Drawings
FIG. 1 is a diagram of a system for reducing noise of a tunneling magnetoresistive sensor according to embodiment 1 of the present invention;
FIG. 2 is a schematic diagram of the overall structure and a schematic diagram of a partial structure of a system for reducing noise of a tunneling magnetoresistive sensor according to embodiment 1 of the present invention; wherein, figure (a) is the overall structure schematic diagram of the system for reducing the noise of the tunnel magnetoresistive sensor; FIG. (b) is a schematic diagram of the relative position of a coil-less shield cylinder to a TMR sensor in a system for reducing the noise of a tunneling magnetoresistive sensor;
fig. 3 is a schematic circuit diagram of a preamplifier circuit according to embodiment 1 of the present invention;
fig. 4 is a schematic circuit diagram of a bandpass filter circuit according to embodiment 1 of the present invention;
FIG. 5 is a diagram of a process for reducing noise of a tunneling magnetoresistive sensor according to embodiment 2 of the present invention;
FIG. 6 is a graph showing the magnetic field curves obtained during the process of reducing the noise of the tunneling magnetoresistive sensor according to embodiment 2 of the present inventionAn intent; wherein, the diagram (a) is the external DC magnetic field B to be measuredoutSchematic diagram of the magnetic field curve of (a); graph (B) shows the induced magnetic field BeSchematic diagram of the magnetic field curve of (a); FIG. (c) is a graph schematically showing the magnetic shielding ability SE of the magnetic shielding cartridge; FIG. d shows an AC magnetic field B inside a coil-less shield cylinderinA schematic diagram of a curve of (a);
fig. 7 is an axial schematic view of a magnetic shield cylinder without coil provided in embodiment 2 of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Examples 1,
A system for reducing noise in a tunneling magnetoresistive sensor, as shown in fig. 1, comprising: a coil-less shield cylinder 1 and a drive circuit 2; the material of the coil-free shielding cylinder 1 is a soft magnetic material with high magnetic permeability, namely a soft magnetic material with relative magnetic permeability greater than 5000. In the embodiment, the high-permeability soft magnetic material is 1J85 type permalloy, and the permeability of the high-permeability soft magnetic material is increased or even saturated under the influence of a weaker magnetic field; under the condition that a small current is applied to generate a small induced magnetic field, the magnetic permeability of the magnetic shielding material enters a saturation region and is reduced along with the increase of the induced magnetic field, and the corresponding magnetic shielding capacity is also reduced. There is the circuit connection point on no coil shielding drum 1, can insert the alternating current of high-frequency, realizes shielding performance's high frequency variation to wrap up with insulating material, with external insulation. Specifically, the overall structure of the noise reduction system for a tunnel magnetoresistive sensor proposed in the present embodiment is as shown in fig. 2 (a), and a coil-free shielding cylinder 1, a TMR sensor 3 to be processed, and a driving circuit are packaged in a plastic structure, wherein, a schematic diagram of the relative positions of the coil-free shielding cylinder 1 and the TMR sensor 3 in the system is as shown in fig. 2 (b), the TMR sensor 3 to be processed is placed at the center of the coil-free shielding cylinder 1, and the output terminal of the TMR sensor is connected to the input terminal of the driving circuit. The coil-free shielding cylinder 1 is based on the supersaturation characteristic of the soft magnetic material with high magnetic conductivity, and when the alternating current is conducted, the shielding capability of the coil-free shielding cylinder is changed at high frequency, so that the direct current magnetic field to be measured of the TMR sensor is converted into a high-frequency alternating current magnetic field, and the 1/f noise of the TMR sensor is reduced. The drive circuit 2 performs phase-sensitive detection and low-pass filtering on the output signal of the TMR sensor based on the reference signal, filters out a sum frequency signal, retains a difference frequency signal, and further filters out 1/f noise of the TMR sensor. Specifically, the drive circuit 2 includes: a pre-amplifier circuit 21, a band-pass filter circuit 22 and a lock-in amplifier circuit 23; the output end of the pre-amplifier circuit 21 is connected to the input ends of the band-pass filter circuit 22 and the lock-in amplifier circuit 23, and the output end of the band-pass filter circuit 22 is connected to the input end of the lock-in amplifier circuit 23.
The preamp circuit 21 is used for amplifying the reference signal and the output signal of the TMR sensor, and outputting the amplified reference signal to the lock-in amplifier circuit 23, and outputting the amplified output signal of the TMR sensor to the band-pass filter circuit 22; specifically, the schematic circuit diagram of the preamplifier circuit 21 is shown in fig. 3, and the amplification factor of the output and the differential input is obtained by the ratio of the degeneration resistor R6 and the inversion input resistor R3; the method specifically comprises the following steps:
the band-pass filter circuit 22 is used for preliminarily filtering noise in the amplified TMR sensor output signal, and outputting the preliminarily noise-reduced signal to the lock-in amplifier circuit 23; specifically, the circuit schematic of the band-pass filter circuit 22 is shown in fig. 4, and the band-pass center frequency f thereof is0By a sliding resistor Rm, a parallel input resistor R11And R12And a first order capacitance C8And a second order capacitance C9The determination specifically comprises:
the center frequency, the quality factor and the working bandwidth can be adjusted by adjusting the sliding resistor Rm.
The lock-in amplifier circuit 23 is used for realizing phase-sensitive detection and low-pass filtering on the signal input by the band-pass filter circuit 22 based on the amplified reference signal, realizing the demodulation process of alternating current to direct current, and filtering 1/f noise of the TMR sensor. The lock-in amplifier circuit 23 includes a phase-sensitive detection circuit 231 and a low-pass filter circuit 232 connected in series, and the phase-sensitive detection function is realized by a chip having a multiplier function through a signal after band-pass filtering and an amplified reference signal, and the sum frequency signal is filtered by a low-pass filter, so that the lock-in amplification function is realized.
Specifically, in the embodiment, the signal input by the bandpass filter circuit 22 is a sine wave with an amplitude of asAngular frequency of ω0And the phase is represented as theta + pi/2: vs(t)＝Assin(ω0t+θ+π/2)＝Ascos(ω0t + θ). Taking the amplitude as ArefAngular frequency of ω0And the sine wave signal with the phase pi/2 is a reference signal and is recorded as: vref(t)＝Arefsin(ω0t+π/2)＝Arefcos(ω0t). After the reference signal and the signal input by the band-pass filter circuit pass through a multiplier of phase-sensitive detection, the output signal is as follows:
wherein, the former term is a difference frequency signal, the latter term is a sum frequency signal, and θ is a phase difference between the input signal and the reference signal. After low-pass filtering and frequency sum term filtering, the output result is a constant containing the phase difference between the input signal and the reference signal
Therefore, the phase-locked amplifying module realizes the conversion from alternating current to direct current, completes the demodulation function of the system, and realizes the conversion from alternating current to human beingAnd D, outputting the direct current to obtain a direct current value reflecting the magnetic field intensity to be measured, and filtering out 1/f noise of the TMR sensor.
Examples 2,
A method of reducing noise in a tunneling magnetoresistive sensor, comprising the steps of:
s1, placing the TMR sensor to be processed in the center of the wireless coil shielding cylinder;
s2, alternating current is introduced into the wireless coil shielding cylinder, and the shielding capability of the wireless coil shielding cylinder is changed at high frequency based on the supersaturation characteristic of the high-permeability soft magnetic material of the wireless coil shielding cylinder, so that a direct current magnetic field to be detected outside the shielding cylinder is converted into a high-frequency alternating current magnetic field, and the 1/f noise of the TMR sensor is reduced;
s3, performing phase-sensitive detection and low-pass filtering on the alternating current output of the TMR sensor, filtering out sum frequency signals, reserving difference frequency signals, and further filtering out 1/f noise of the TMR sensor.
Further, the TMR sensor is placed in the center of the coil-less shield cylinder, and an alternating current I is passed to the coil-less shield cylinder, as shown in fig. 5. When TMR sensor is used for magnetic field measurement, the external DC magnetic field to be measured is recorded as BoutThe magnetic field curve is shown in fig. 6 (a). At this time, an axial AC current is applied along the wall of the shielding cylinder without coil to generate an induced magnetic field BeSince the high-permeability soft magnetic material of the coil-less shield cylinder has a super-saturation characteristic, it follows the induced magnetic field BeAnd when the magnetic shielding cylinder is used, the magnetic field of the magnetic shielding cylinder is increased, the magnetic permeability of the magnetic shielding cylinder is reduced, and the shielding capacity of the magnetic shielding cylinder is weakened. Therefore, the shielding capability SE of the magnetic shielding cylinder changes with the change of the induced magnetic field. Wherein a magnetic field B is inducedeThe magnetic field curve of (b) is shown in fig. 6, and the curve of the magnetic shielding capability SE of the magnetic shielding cylinder is shown in (c) of fig. 6. Under the action of magnetic shielding capability SE, the external DC magnetic field to be detected is recorded as BoutThe magnetic field of the person passing through the shielding cylinder without coil is changed into an alternating current magnetic field BinThe magnetic field curve is shown in fig. 6 (d).
Specifically, fig. 7 is an axial schematic view of a magnetic shielding cylinder without a coil. Alternating current I is introduced along the axial direction of the shielding cylinder to generate a tangential induced magnetic field Be. Induced magnetic field BeThe frequency is consistent with the alternating current frequency I, and the magnitude is in direct proportion to the alternating current amplitude. When the amplitude of the alternating current is large enough to make the soft magnetic material with high magnetic conductivity enter the saturation region, the relative magnetic conductivity mu of the materialrAnd BeIs inversely related. At this time, the magnetic shielding capability of the magnetic shielding cylinder can be obtained by Schwanofu's formula
Wherein d is the thickness of the magnetic shielding cylinder material, murIs the relative magnetic permeability of the magnetic shielding cylinder material, qrTo shield the cartridge material from relative electrical conductivity. In summary, when the high-permeability soft magnetic material of the magnetic shielding cylinder enters the saturation region, the magnetic shielding capability SE of the magnetic shielding cylinder is in positive correlation with the permeability of the high-permeability soft magnetic material and is in positive correlation with the induced magnetic field BeIs negatively correlated with the alternating current I and the shielding ability variation frequency is correlated with the alternating current frequency.
Further, based on the above process, the output voltage of the TMR sensor is:
wherein S isENIs the sensor sensitivity. It can be seen that the output of the TMR sensor is related to the ac current I passing through the coil-less shielded cylinder. The 1/f noise of the TMR sensor is inversely proportional to the output signal frequency of the TMR sensor, and the higher the frequency of the alternating current I introduced into the wireless coil shielding cylinder is, the higher the alternating magnetic field BinThe higher the frequency of TMR sensor, the higher the signal frequency of TMR sensor, and the smaller the 1/f noise of TMR sensor.
According to the invention, the alternating current is introduced into the coil-free shielding cylinder, so that the high-frequency change of the shielding performance of the shielding cylinder is realized, the direct current magnetic field to be detected outside the shielding cylinder is converted into the high-frequency alternating current magnetic field, and the TMR sensor generates alternating current output, thereby effectively inhibiting the low-frequency noise of the TMR sensor, and simultaneously inhibiting the noise, having lower power consumption and being more convenient to carry.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (5)
1. A system for reducing noise in a tunneling magnetoresistive sensor, comprising: a coil-less shield cylinder and a drive circuit;
the material of the coil-free shielding cylinder is a soft magnetic material with high magnetic permeability; a TMR sensor to be processed is placed in the center of the coil-free shielding cylinder; the output end of the TMR sensor is connected with the input end of the driving circuit; the coil-free shielding cylinder is wrapped by an insulating material to be insulated from the outside;
the coil-free shielding cylinder is based on the supersaturation characteristic of a soft magnetic material with high magnetic conductivity, and the shielding capability of the coil-free shielding cylinder is subjected to high-frequency change when alternating current is introduced, so that an external direct current magnetic field to be detected is converted into a high-frequency alternating current magnetic field, and the 1/f noise of the TMR sensor is reduced;
the driving circuit carries out phase-sensitive detection and low-pass filtering on the output signal of the TMR sensor based on the reference signal, so as to filter a sum frequency signal, retain a difference frequency signal and further filter 1/f noise of the TMR sensor.
2. The system of claim 1, wherein the driving circuit comprises: the pre-amplifier circuit, the band-pass filter circuit and the phase-locked amplifier circuit;
the output end of the pre-amplification circuit is connected with the input ends of the band-pass filter circuit and the phase-locked amplifier circuit, and the output end of the band-pass filter circuit is connected with the input end of the phase-locked amplifier circuit;
the pre-amplification circuit is used for amplifying a reference signal and an output signal of the TMR sensor, outputting the amplified reference signal to the phase-locked amplifier circuit, and outputting the amplified output signal of the TMR sensor to the band-pass filter circuit;
the band-pass filter circuit is used for preliminarily filtering noise in the amplified TMR sensor output signal and outputting the preliminarily noise-reduced signal to the phase-locked amplifier circuit;
the phase-locked amplifier circuit is used for realizing phase-sensitive detection and low-pass filtering on the signals input by the band-pass filter circuit based on the amplified reference signals, realizing the demodulation process of alternating current to direct current and filtering 1/f noise of the TMR sensor.
3. The system for reducing noise of a tunneling magneto-resistive sensor according to claim 2, wherein the lock-in amplifier circuit comprises a phase-sensitive detector circuit and a low-pass filter circuit connected in series.
4. A method of reducing noise in a tunneling magnetoresistive sensor, comprising the steps of:
s1, placing the TMR sensor to be processed in the center of the wireless coil shielding cylinder;
s2, alternating current is introduced into the wireless coil shielding cylinder, and the shielding capability of the wireless coil shielding cylinder is changed at high frequency based on the supersaturation characteristic of the high-permeability soft magnetic material of the wireless coil shielding cylinder, so that an external direct-current magnetic field to be measured is converted into a high-frequency alternating-current magnetic field, and the 1/f noise of the TMR sensor is reduced; the coil-free shielding cylinder is wrapped by an insulating material to be insulated from the outside;
s3, performing phase-sensitive detection and low-pass filtering on the alternating current output of the TMR sensor, filtering out sum frequency signals, reserving difference frequency signals, and further filtering out 1/f noise of the TMR sensor.
5. The method of claim 4, wherein the magnitude of the shielding capability of the coil-less shielding cylinder is controlled by controlling the amplitude of the alternating current and the period of change of the shielding capability of the coil-less shielding cylinder is controlled by controlling the frequency of the alternating current when the high permeability soft magnetic material of the coil-less shielding cylinder is supersaturated.
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| CN112698253B (en) * | 2020-12-11 | 2023-09-29 | 哈尔滨工程大学 | Digital triaxial TMR magnetic sensing system |
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