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| {{lead missing|date=April 2011}}
| | 37 yr old Crop Farmers Jerrod from Mont-Tremblant, loves to spend time magic, Clash Of Lords 2 Hack and aircraft spotting. Has just finished a trip to Fossil Hominid Sites of Sterkfontein.<br><br>Feel free to visit my weblog; [http://tinyurl.com/q38lk2a clash of lords 2 cheats] |
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| ==Introduction==
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| [[Magnetic field sensors|Magnetic field sensing]] can be categorized into four general types<ref name="Lenz">Lenz, J., Edelstein, A.S., "Magnetic sensors and their applications." IEEE Sensors J. 2006, 6, 631-649.</ref> depending on the magnitude of the measured field. If the targeted [[B-field]] is larger than the earth [[magnetic field]] (maximum value around 60 <math>\mu</math>T), the requirement on the sensitivity of the sensor is not aggressive. To measure the earth field larger than the geomagnetic noise(around 0.1 nT), better sensors are required. For the application of [[Magnetic anomaly detector|magnetic anomaly detection]], sensors at different spots have to be used to cancel the spatial-correlated noise in order to achieve a better [[spatial resolution]]. To measure the field below the geomagnetic noise, much more sensitive magnetic field sensors have to be employed. These sensors are mainly used in [[medical]] and [[biomedical]] applications, such as [[MRI]], molecule tagging and etc.
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| There are many approaches for magnetic sensing, including [[Hall effect sensor]], magneto-diode, magneto-transistor, [[Magnetoresistance|AMR]] [[magnetometer]], [[Giant magnetoresistance|GMR]] magnetometer, [[magnetic tunnel junction]] magnetometer, magneto-optical sensor, [[Lorentz force]] based [[Microelectromechanical systems|MEMS]] sensor, [[Electron tunneling|Electron Tunneling]] based MEMS sensor, MEMS [[compass]], Nuclear precession magnetic field sensor, optically pumped magnetic field sensor, [[fluxgate magnetometer]], [[search coil]] magnetic field sensor and [[SQUID magnetometers|SQUID magnetometer]]. MEMS-based magnetic field sensor can offer small-size solution for magnetic field sensing. Smaller device can be placed closer to the measurement spots and thereby achieving higher spatial resolution. Additionally, MEMS magnetic field sensor does not involve the [[microfabrication]] of magnetic material. Therefore, the cost of the sensor can be largely reduced. Integration of MEMS sensor and [[microelectronics]] can further reduce the size of the entire magnetic field sensing system.
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| ==Lorentz-force-based MEMS sensor==
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| This type sensor relies on the mechanical motion of the MEMS structure due to the Lorentz force acting on the current-carrying conductor in the magnetic field. The mechanical motion of the micro-structure is sensed either electronically or optically. The mechanical structure is often driven to its [[resonance]] in order to obtain the maximum output signal. Piezoresistive and electrostatic transduction method can be used in the electronic detection. Displacement measurement with laser source or LED source can also be used in the [[Optical detector|optical detection]]. Several sensors will be discussed in the following subsections in terms of different output for the sensor.
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| ===Voltage sensing===
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| Beroulle et al.<ref name="Beroulle">Beroulle, V.; Bertrand, Y.; Latorre, L.; Nouet, P. Monolithic Piezoresistive CMOS magnetic field sensors. Sens. Actuators A 2003, 103, 23-32</ref> have fabricated a U-shape [[cantilever beam]] on silicon substrate. Two piezo-resistors are laid on the support ends. There are 80-turns Al coil passing current along the U-shape beam. Wheatstone bridge is formed by connecting two "active" resistor with another two "passive" resistor free of strain. When there is an external magnetic field applying to the current carry conductor, motion of the U-shape beam will induce strain in two "active" piezo-resistors and thereby generating an output voltage across the [[Wheatstone bridge]] which is proportional to the magnetic field flux density. The reported sensitivity for this sensor is 530 m Vrms/T with a resolution 2 µT. Note that the frequency of the exciting current is set to be equal to the resonant frequency of the U-shape beam in order to maximize the sensitivity.
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| Herrera-May et al.<ref name="autogenerated2">Herrera-May, A.L.; García-Ramírez, P.J.; Aguilera-Cortés, L.A.; Martínez-Castillo, J.; Sauceda-Carvajal, A.; García-González, L.; Figueras-Costa, E. A resonant magnetic field microsensor with high quality factor at atmospheric pressure. J. Micromech. Microeng. 2009, 19, 015016.</ref> fabricate a sensor with similar piezoresitive read-out approach but with different mechanical motion. Their sensor relies on the torsional motion of a micro-plate fabricated from silicon substrate. The exciting [[current loop]] contains 8 turns of aluminum coil. The location of the current loop enables a more uniform Lorentz force distribution compared with the aforementioned U-shape cantilever beam. The reported sensitivity is 403 mVrms/T with a resolution 143 nT.
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| Kádár et al.<ref name="Kádár">Kádár, Z.; Bossche, A.; Sarro, P.M.; Mollinger, J.R. Magnetic-field measurements using an integrated resonant magnetic-field sensor. Sens. Actuators A 1998, 70, 225-232.</ref> also chose the micro-torsional beam as the mechanical structure. Their read-out approach is different. Instead of using piezoresitive transduction, their sensor relies on electrostatic transduction. They patterned several [[electrode]]s on the surface of the micro-plate and another external glass wafer. The glass wafer is then boned with the silicon substrate to form a [[variable capacitor]] array. Lorentz force generated by the external magnetic field will results in the change of capacitor array. The reported sensitivity is 500 Vrms/T with a resolution of a few mT. The resolution can reach 1 nT with vacuum operation.
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| Emmerich et al.<ref name="Emmerich">Emmerich, H.; Schöfthaler, M. Magnetic field measurements with a novel surface micromachined magnetic-field sensor. IEEE Tans. Electron Dev. 2000, 47, 972-977.</ref> fabricated the variable capacitor array on a single silicon substrate with comb-figure structure. The reported sensitivity is 820 Vrms/T with a resolution 200 nT at the pressure level of 1mbar.
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| ===Frequency shift sensing===
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| Another type of Lorentz force based MEMS magnetic field sensor utilize the shift of [[mechanical resonance]] due to the Lorentz force applying to certain mechanical structures.
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| Sunier et al.<ref name="Sunier">Sunier, R.; Vancura, T.; Li, Y.; Kay-Uwe, K.; Baltes, H.; Brand, O. Resonant magnetic field sensor with frequency output. J. Microelectromech. Syst. 2006, 15, 1098-1107.</ref> change the structure of aforementioned U-shape cantilever beam by adding a curved-in support. The piezoresistive sensing bridge is laid between two heating actuation resistors. 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 current. The mechanical structure is actually driven by the heating resistor at its resonance. Lorentz force applying at the U-shape beam will change the resonant frequency of the beam and thereby change the frequency response of the output voltage. The reported sensitivity is 60 kHz/T with a resolution of 1 µT.
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| Bahreyni et al.<ref name="Bahreyni">Bahreyni, B.; Shafai, C. A resonant micromachined magnetic field sensor. IEEE Sensor J. 2007, 7, 1326-1334.</ref> fabricated a comb figure structure on top of the silicon substrate. The center shuttle are connected to two clamped-clamped conductors used to change the internal stress of the moving structure when external magnetic field is applied. This will induce the change of the resonant frequency of the comb finger structure. This sensor use electrostatic transduction to measure the output signal. The reported sensitivity is improved to 69.6 Hz/T thanks to the high mechanical quality factor (Q = 15000 @ 2 Pa) structure in the vacuum environment. The reported resolution is 217 nT.
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| ===Optical sensing===
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| The optical sensing is to directly measure the mechanical displacement of the MEMS structure to find the external magnetic field.
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| Zanetti et al.<ref name="Zanetti">Zanetti, L.J.; Potemra, T.A.; Oursler, D.A.; Lohr, D.A.; Anderson, B.J.; Givens, R.B.; Wickenden, D.K.; Osiander, R.; Kistenmacher, T.J.; Jenkins, R.E. Miniature magnetic field sensors based on xylophone resonators. In Science Closure and Enabling Technologies for Constellation Class Missions; Angelopoulos, V., Panetta, P.V., Eds.; University of California: Berkeley, CA, USA, 1998; pp. 149-151.</ref> fabricated a Xylophone beam. Current is flowing through the center conductor and the Xylophone beam will be deflected as the Lorentz force is induced. Direct mechanical displacement is measured by an external laser source and a detector. The resolution of 1 nT can be reached. Wickenden<ref name="Wickenden">Wickenden, D.K.; Champion, J.L.; Osiander, R.; Givens, R.B.; Lamb, J.L.; Miragliotta, J.A.; Oursler, D.A.; Kistenmacher, T.J. Micromachined polysilicon resonating xylophone bar magnetometer. Acta Astronautica 2003, 52, 421-425.</ref> had tried to shrink the footprint of this type of device by 100 times. But a much lower resolution of 150 µT was reported.
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| Keplinger et al.<ref name="Keplinger">Keplinger, F.; Kvasnica, S.; Hauser, H.; Grössinger, R. Optical readouts of cantilever bending designed for high magnetic field application. IEEE Trans. Magn. 2003, 39, 3304-3306.</ref><ref name="autogenerated1">Keplinger, F.; Kvasnica, S.; Jachimowicz, A.; Kohl, F.; Steurer, J.; Hauser, H. Lorentz force based magnetic field sensor with optical readout. Sens. Actuators A 2004, 110, 12-118.</ref> were trying to use an LED source for optical sensing instead of using an external laser source. Optical fibers were aligned on the silicon substrate with different arrangements for the displacement sensing. A resolution 10 mT is reported.
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| ==
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| When the temperature increases, the Young’s modulus of the material used to fabricate the moving structure decreases. This will leads to the softening of the moving structure. Meanwhile, thermal expansion and thermal conductivity will increase with the temperature inducing an internal stress in the moving structure. These effects can result in the shift of the resonant frequency of the moving structure which is equivalent noise for resonant frequency shift sensing and the voltage sensing as well. In addition, temperature rise will generate larger Johnson noise (affect the piezoresitive transduction) and also large mechanical fluctuation noise (affect the optical sensing). Therefore, advanced electronics for temperature effect compensation have to be used to improve the sensitivity.
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| ==References==
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| <references/>
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| [[Category:Magnetic devices]]
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37 yr old Crop Farmers Jerrod from Mont-Tremblant, loves to spend time magic, Clash Of Lords 2 Hack and aircraft spotting. Has just finished a trip to Fossil Hominid Sites of Sterkfontein.
Feel free to visit my weblog; clash of lords 2 cheats