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| {{Unreferenced|date=December 2009}}
| | Wilber Berryhill is the title his parents gave him and he completely digs that name. Some time in the past she chose to live in Alaska and her parents live nearby. To play lacross is the factor I love most of all. Distributing manufacturing is how he tends to make a residing.<br><br>Feel free to visit my site; psychic readers ([http://modenpeople.co.kr/modn/qna/292291 http://modenpeople.co.kr/modn/qna/292291]) |
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| The '''search coil''' (also known as [[induction loop|Inductive Sensor]]) is a [[sensor]] which measures the variation of the [[Magnetic Flux|magnetic flux]]. The search-coil can measure magnetic field from mHz up to hundreds of MHz. It is associated to a conditioning electronic to make a search coil magnetometer (also known as induction magnetometer).
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| == Principle ==
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| The induction sensor is based on [[Faraday's law of induction]]. The temporal variation of the [[Magnetic Flux]] <math> \Phi </math> through a ''N'' turns circuit will induce a voltage ''<math>e</math>'' which follows:
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| :<math> e = - N\frac{\mathrm d \Phi}{\mathrm dt}</math>
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| which can be express in a simpler way:
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| :<math> e = - N\times S\frac{\mathrm dB}{\mathrm dt}</math>
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| by assuming that induction magnetic field '''B''' is homogeneous over a section ''S'' (the [[Magnetic flux]] will be expressed <math> \Phi = B\times S </math>).
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| [[File:Fluxmètre à noyau ferromagnétique.jpg|thumb|Figure 1: Induction sensor constituting with a winding (orange) surrounding a ferromagnetic core (grey)]]
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| To increase the induced voltage (''<math>e</math>'') we can:
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| * increase the surface (''S''),
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| * increase the turn number (''N''),
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| * use a ferromagnetic core.
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| == Search coil using a ferromagnetic core ==
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| When a coil is wounded around a [[Ferromagnetism|ferromagnetic]] core, that increases the sensitivity of the sensor thanks to the apparent permeability of the ferromagnetic core.
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| === Apparent permeability ===
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| The magnetic amplification, known as apparent permeability ''<math>\mu_{app}</math>'', is the result of the [[magnetization]] of the ferromagnetic core response to an external magnetic field. The magnetization is reduced by the demagnetizing field.
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| :<math> \mu_{app} = \frac{\mathrm \mu_r}{\mathrm 1+N_z\times(\mu_r-1)}</math>
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| where ''<math>\mu_r</math>'' is the '''relative permeability''', ''<math> N_z </math>'' is the demagnetizing coefficient in the ''z'' direction.
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| The induced voltage will be written
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| :<math> e = - NS\mu_{app}\frac{\mathrm dB}{\mathrm dt}</math>
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| The demagnetizing coefficient can easily be computed in the case of simple shapes (spheres and ellipsoids).
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| == Applications ==
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| * [[Eye tracker]]
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| A '''search coil''' is used to measure [[Eye movement (sensory)|eye movement]] using [[coil]]s that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the [[human eye|eye]]. In animal studies, the search coil may be surgically implanted into the [[sclera]] of the eye. Alternating [[magnetic field]]s are generated by magnets positioned around the eye. Through [[electromagnetic induction]], [[Current (electricity)|electric currents]] are generated in the search coils. The polarity and amplitude of the current generated varies with the direction and [[angular displacement]] of the eye. By measuring these values, the position of the eye can be determined. Search coils can be applied to one or both eyes.
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| In order to detect eye orientation in more than one dimension (e.g. up/down vs. left/right), multiple magnets oriented [[orthogonal]]ly to each other may be used. Each magnet generates a field using a different frequency, allowing the readings from the search coil to be analyzed by computer to determine displacement in multiple dimensions. Additionally, a second search coil can be added to measure [[Torsion tensor|torsional]] rotation.
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| * Education
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| A more crude search coil also called exploring coil is also used in laboratory experiments in schools to measure the [[magnetic field]] in a certain region of space. In this case the search coil consists of a simple wire coil or solenoid connected to a sensitive [[ammeter]] or [[galvanometer]]. The coil is placed in the magnetic field to be measured and quickly withdrawn to a region of space with a negligible magnetic field. As the search coil moves the [[magnetic flux]] linked with the coil changes. This induces a current in the coil which can be registered on the [[galvanometer]]. Since induced current is directly proportional to rate of change of flux linkage and assuming the coil is removed from the magnetic field very quickly, the maximum current measured by the ammeter is proportional to the magnetic field <math>\ \mathbf{B}</math>. The search coil can be calibrated by repeating this in a known magnetic field.
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| A reference table shows open coil voltage versus magnetic field A/m (or teslas) or short current versus magnetic field.
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| * [[Non Destructive Testing]]
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| * [[Magnetotellurics]]
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| * [[Space research]]
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| Spacecraft on boarding [[Spacecraft magnetometer|search coil magnetometers]] has been built since many decades to investigate waves in space plasma.{{Clarify|date=April 2013}}
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| * Natural electromagnetic waves observations on Earth.
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| Many electromagnetic waves are observable on Earth. Let's quote Auroral research project (cf. Reference HAARP link).
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| ==References==
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| {{reflist}}
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| * Pavel Ripka, ''Magnetic Sensors and Magnetometers'', Artech House Publishers
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| * [http://www.intechopen.com/books/magnetic-sensors-principles-and-applications/induction-magnetometers-principle-modeling-and-ways-of-improvement- C. Coillot & P. Leroy, ''Induction Magnetometers: Principle, Modelling and ways of improvement'', Intech Open Access Publisher]
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| * [http://www.haarp.alaska.edu/haarp/magind.html]
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| {{DEFAULTSORT:Search Coil}}
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| [[Category:Vision]]
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Wilber Berryhill is the title his parents gave him and he completely digs that name. Some time in the past she chose to live in Alaska and her parents live nearby. To play lacross is the factor I love most of all. Distributing manufacturing is how he tends to make a residing.
Feel free to visit my site; psychic readers (http://modenpeople.co.kr/modn/qna/292291)