A MEMS magnetic field sensor is a small-scale microelectromechanical systems (MEMS) system for detecting and measuring magnetic fields (Magnetometers). Many of these operate by detecting the effects of Lorentz forces: changes in voltage or resonant frequency can be measured electronically, or mechanical displacement can be measured optically. Compensation for temperature effects is required. Its use as a miniature compass may be one example of a simple application.
Video MEMS magnetic field sensor
Magnetic field sensing
Magnetometers can be categorized into four general types depending on the size of the measured field. If the targeted B field is larger than the Earth's magnetic field (a maximum value of about 60 ÂμT), the sensor need not be very sensitive. To measure the earth's larger area of ​​geomagnetic noise (about 0.1 nT), better sensors are needed. For magnetic anomaly detection applications, sensors in different locations should be used to cancel spatial-related noise to achieve better spatial resolution. To measure the field under geomagnetic noise, a more sensitive magnetic field sensor should be used. These sensors are mainly used in medical and biomedical applications, such as MRI and molecular marking.
There are many approaches for magnetic sensing, including Hall effect sensor, magneto-diode, magneto-transistor, AMR magnetometer, GMR magnetometer, magnetic tunnel magnetic intersection, magneto-optical sensor, Lorentz power-based MEMS sensor, MEMS sensor based Electron Tunneling, MEMS compass , Nuclear presession magnetic field sensor, optically pumped magnetic field sensor, fluxgate magnetometer, magnetic field coil sensor search and SQUID magnetometer.
Prominent magnetic sensor character MEMS
The MEMS magnetic sensor has several parameters: Quality Factor (Q), Resonance Frequency, Mode Shape, Responsive, and Resolution.
The quality factor is a measure of how much energy can be sustained during resonator vibration. There may be several factors that can dampen the resonator, such as the mechanical damping of the resonator itself or the attenuation of external pressure and temperature.
The resonant frequency is the frequency at which the device vibrates with the highest amplitude (or longest, like a bell or tune fork). Resonance frequency is governed by device geometry. We can calculate the resonant frequency when we know the device dimensions, Young's modulus of equivalent devices, and equivalent device densities.
The shape of the mode is the resonator vibration pattern.
The responsiveness (which contributes to the resolution) describes the size of the oscillations we can get from devices with the same external conditions. If we apply the B field and the same current to some resonators, devices that exhibit larger amplitude of vibration are said to have higher responsiveness. All other things are considered equal, higher responsiveness devices are more sensitive. The magnetometer range based on the piezoelectric resonator is mV/T (millivolt/Tesla), so higher responsivity is generally better.
Resolution refers to the smallest magnetic field that the device can measure. The smaller the number, the more sensitive the device. The magnetometer range based on a piezoelectric resonator is some nT (nanoTesla).
Advantages of MEMS-based sensors
A small magnetic field-based MEMS sensor, so it can be placed close to the measurement location and thus achieves a higher spatial resolution than any other magnetic field sensor. In addition, building a MEMS magnetic field sensor does not require microfabrication of magnetic materials. Therefore, the cost of the sensor can be greatly reduced. Integration of MEMS and microelectronic sensors can reduce the size of the entire magnetic field sensor system.
Maps MEMS magnetic field sensor
Lorentz power-based MEMS sensor
This type of sensor relies on the mechanical movement of the MEMS structure because of the Lorentz force acting on the current carrying conductor in the magnetic field. The mechanical movement of the microstructure is felt both electronically and optically. The mechanical structure is often driven to resonance to obtain the maximum output signal. Piezoresistive and electrostatic transduction methods can be used in electronic detection. Measurement of displacement by laser source or LED source can also be used in optical detection. Some sensors will be discussed in the following subsections in terms of different outputs for the sensor.
Sensing voltages
Beroulle et al. has made a U-shape cantilever beam on a silicon substrate. Two piezo-resistors are placed on the supporting ends. There is an 80-turn Alcoil that passes the current along the U-shape beam. Wheatstone bridge is formed by connecting two "active" resistors with two other "passive" resistors, which are free from tension. When an external magnetic field is applied to the current carrying conductor, the U-shaped beam motion induces a strain in the two active "piezo-resistors" and thereby produces an output voltage across the Wheatstone bridge that is proportional to the magnetic field. flux density. The reported sensitivity for this sensor is 530 mM Vrms/T with a resolution of 2 Ã,ÂμT. Note that the interesting current frequency is adjusted to equal the resonant frequency of U-shaped rays to maximize sensitivity.
Herrera-May et al. create sensors with the same piezoresistive readout approach but with different mechanical motions. Their sensors depend on the torsional motion of the micro plate made from the silicon substrate. The interesting current circle contains 8 rounds of aluminum coil. The current loop location allows a more uniform distribution of Lorentz styles compared to the previously mentioned U-shape cantilever blocks. The reported sensitivity was 403 mVrms/T with a resolution of 143 nT.
KÃÆ'¡dÃÆ'¡r et al. also chose a micro-torsional ray as a mechanical structure. Their reading approaches are different. Instead of using piezoresistive transduction, their sensors depend on electrostatic transduction. They modeled several electrodes on the surface of micro-plates and other external glass wafers. The glass wafer is then fastened with a silicon substrate to form a variable capacitor array. Lorentz power generated by an external magnetic field produces a change in the array of capacitors. The reported sensitivity was 500 Vrms/T with a resolution of several mT. Resolution can reach 1 nT with vacuum operation.
Emmerich et al. fabricated variable capacitor arrays on a single silicon substrate with a comb-figure structure. The reported sensitivity is 820 Vrms/T with a resolution of 200 nT at a pressure level of 1mbar.
Sensing frequency shift
Another type of Lorentz power-based MEMS magnetic field sensor utilizes mechanical resonance shifts due to the Lorentz force applied to specific mechanical structures.
Sunier et al. alter the structure of the previously mentioned U-shape cantilever beam by adding curved support. The piezoresistive sensing bridge is placed between two actuation heating resistors. The frequency response of the output voltage of the sensing bridge is measured to determine the resonant frequency of the structure. Note that in this sensor, the current flowing through the aluminum coil is DC. The mechanical structure is actually driven by the heater resistor at its resonance. The Lorentz style applying to U-shaped rays changes the resonance frequency of the beam and thereby alters the frequency response of the output voltage. The reported sensitivity is 60 kHz/T with a resolution of 1 Ã,ÂμT.
Bahreyni et al. fabricated the image structure of the comb on the silicon substrate. The central shuttle is connected to two clamped conductors used to change the internal stress of the moving structure when an external magnetic field is applied. This will induce a change in the resonance frequency of the finger joint structure. This sensor uses electrostatic transduction to measure the output signal. The reported sensitivity was increased to 69.6 Hz/T thanks to a high mechanical quality factor structure (Q = 15000 @ 2 Pa) in a vacuum environment. The reported resolution was 217 nT.
Optical sensing
Optical sensing is to directly measure the mechanical displacement of the MEMS structure to find an external magnetic field.
Zanetti et al. make Xylophone beam. The current flowing through the center of the conductor and the mine gambang beam will be deflected as Lorentz force induced. Direct mechanical displacement is measured by external laser sources and detectors. Resolution 1 nT can be reached. Wickenden has tried to minimize this type of device trace 100 times. But a much lower resolution of 150 Ã,ÂμT is reported.
Keplinger et al. try to use LED source for optical sensing instead of using external laser source. Optical fibers are parallel to the silicon substrate with different settings for displacement sensing. 10 mT resolution reported.
Temperature effect
As the temperature increases, Young's modulus of the material used to make the structure move down, or more simply, the structure moves soft. Meanwhile, thermal expansion and increase of thermal conductivity, with temperature induces internal stress within the moving structure. This effect may result in a resonance frequency shift from a moving structure equivalent to noise for sensing resonance frequency shift or voltage sensing. In addition, the temperature rise will result in greater Johnson noise (affecting piezoresiative transduction) and increase mechanical fluctuation noise (which affects optical sensing). Therefore, sophisticated electronics for temperature effect compensation should be used to maintain sensitivity as temperature changes.
Apps
Detects defects in electrically conductive materials
Magnetometers based on piezoelectric resonators can be applied to find flaws in highly secure metal structures, such as aircraft propellers, engines, airframe and wing structures, or oil pipes or high-pressure gas pipelines. When a magnet (generally an electromagnet creates a varying frequency field) creates an eddy current in the material, the eddy current generates another magnetic field in the material that the magnetometer can perceive. If there is no defect or crack in the pipe, the magnetic field of the eddy current shows a constant pattern as it travels along the material under test. But the gap or hole in the material disrupts the eddy current, so the magnetic field changes, allowing a sensitive magnetometer to sense and localize the defect.
Monitor the health of chest cavity organs
When we breathe, the nerves and muscles of our thoracic cavity create a weak magnetic field. Magnetometers based on piezoelectric resonators have a high resolution (in the nT range), enabling the sensing of the solid state of our respiratory system.
References
Source of the article : Wikipedia
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