US 7196514 B2
A magnetic field sensor relies on variations in permeability of magnetic material to detect an external field. An exemplary magnetic field sensor includes a lengthwise extending sensing element, and a coil wound about the sensing element. The sensing element may be formed as a number of geometrically and magnetically similar or identical conductive cores that are ferromagnetic, driven by an AC magnetic field. The multiple conductive cores are connected in parallel. An external magnetic field changes the magnetic characteristics of the sensing element and induces a change in electric potential of the induction coil. The sensor output is proportional to the number of conductive cores.
1. A magnetic field sensor, comprising
a magnetic sensing element comprising a plurality of parallel, lengthwise extending conductive cores that are ferromagnetic, connected in parallel;
a coil wound about said plurality of conductive cores;
a source of electrical energy, providing alternating electrical energy to said conductive cores to establish an alternating magnetic field in said conductive cores, said magnetic field having a magnitude at which there is a linear relationship between said magnetic field and permeability of said magnetic sensing element;
a circuit for detecting changes in voltage across said coil, resulting from changes in said permeability of said magnetic sensing element, attributable to an external magnetic field.
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9. A method of sensing a low frequency external magnetic field comprising:
exciting a magnetic element with a time varying magnetic field parallel to the longitudinal extent of said magnetic element, said magnetic element comprising plurality of lengthwise extending conductive ferromagnetic cores, said magnetic field having a frequency and amplitude to maximize the sensitivity of the permeability of said magnetic element to said magnetic field in said magnetic element;
sensing said external magnetic field by measuring variation of said permeability of said magnetic element, resulting from said external magnetic field by measuring an induced voltage in a coil coiling said magnetic element.
10. The method of
11. The method of
12. A magnetic field sensor, comprising
a magnetic sensing element comprising a plurality of parallel, lengthwise extending ferromagnetic conductive cores;
a coil wound about said magnetic sensing element;
a source of electrical energy, providing electrical energy to said coil to establish a magnetic field in said magnetic sensing element, said magnetic field having a magnitude at which there is a linear relationship between said magnetic field and permeability of said magnetic sensing element;
a circuit for detecting changes in voltage across said coil, resulting from changes in said permeability of said magnetic sensing element, attributable to an external magnetic field.
13. The sensor of
14. The sensor of
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This application claims benefits from U.S. Provisional Patent Application No. 60/348,668 filed Jan. 15, 2002, and is a continuation-in-part of U.S. patent application Ser. No. 10/342,612 filed Jan. 15, 2003 now U.S. Pat. No. 6,835,186 the contents of both of which are hereby incorporated herein by reference.
This invention relates generally to sensing magnetic fields, and more particularly to methods and devices for sensing very small magnetic fields such those emitted by a biological body.
The body of a human being is governed by his/her brain. Understanding and monitoring brain activity could potentially improve the quality of life and work efficiency. Monitoring brain activity may, for example, aid in the cure of sleep disorders; in detecting sleep onset during attentive tasks such as driving; in detecting pilot blackout or disorientation in flight; in monitoring attention and consciousness; in and sensing brain activity of those not otherwise able to communicate.
All of these applications require portable devices that sense brain activities or status of consciousness. The brain works by communication between the neuron cells, which emit electrical pulses and thus produce an electrical field and an accompanied magnetic field. A current source in the neurons results in a current and thus causes an electrical field on the scalp. A corresponding potential difference may be detected (measured with EEG). Similarly, a magnetic field outside the head may be detected (measured with MEG). By measuring the electric or magnetic field, the activities of the brain can be detected.
As such, two known methods may be used to sense human brain activities—electroencephalography (EEG) and magnetoencephalography (MEG), which work by measuring the electric and magnetic fields corresponding to the brain activities, respectively. In EEG, electric signals measured through a set of electrodes (placed on the scalp of the subject) are amplified, digitized, and interpreted by using EEG software that creates real-time brain waves. In MEG, the data is a measurement of the accompanied magnetic field generated by the same electrical currents that produce the EEG data, and roughly resembles EEG recordings. Both EEG and MEG can be used to interpret brain activity. However, most existing EEG devices are suitable only for clinical or laboratory applications since the electrodes must be in contact with the subject's scalp. MEG measurement is non-contact and non-invasive and thus it could be suitable for both clinical and non-clinical applications.
However, the magnetic field intensity of the brain is very weak (typically at the level below 10−12 Tesla), therefore conventional methods of measuring magnetic fields, such as the field detection coils, the Hall element, the magneto-resistance (MR) element, the giant magneto-resistance (GMR) element, and the thin film fluxgate sensor (FGS) are not sensitive enough to detect the magnetic field of the brain.
The Superconducting Quantum Interference Device (SQUID) has remained to be the only commercially available medical apparatus that can detect the magnetic field of the brain. However, Since the SQUID uses field detection coils made of superconducting material operated at temperature—269° C. with circulated liquid helium cooling, it requires a huge cooling system and a magnetic shielding room, and thus its applications are limited to laboratory and dinical conditions.
It is expected that portable MEG devices equipped with micro MEG sensors have a potential for a spectrum of applications of non-contact sensing and monitoring of brain activities or status of consciousness.
Accordingly, there is a need for improved magnetic field sensors, and methods.
In accordance with the present invention, a magnetic field sensor relies on variations in permeability of magnetic material to detect an external field. An exemplary magnetic field sensor includes a magnetic material and two or more conductors, at least one of which is connected to an electrical energy source. Current flowing through at least one of the conductors establishes a magnetic field in the magnetic material at a magnitude at which there is a generally linear relationship between the magnetic field and the permeability of the material. An external field to be sensed influences the permeability of the material. Sensing variations in the permeability of the magnetic material allows the external magnetic field to be sensed.
In accordance with an aspect of the present invention, a magnetic field sensor includes at least one conductive core, surrounded by a magnetic layer. A coil is wound about the magnetic layer. A source of electrical energy provides electrical energy to the conductive core to establish a magnetic field in the magnetic layer. This magnetic field has a magnitude at which there is a generally linear relationship between the magnetic field and permeability of the magnetic layer. A sensor detects changes in voltage across the coil, resulting from changes in permeability of the magnetic layer, attributable to an external magnetic field.
In accordance with another aspect of the present invention, a method of sensing a low frequency external magnetic field includes exciting a magnetic element with a time varying magnetic field parallel to the longitudinal extent of the magnetic element. This magnetic field has a frequency and amplitude to maximize the sensitivity of the permeability of the magnetic element to magnetic field in the magnetic element. The method further includes sensing the external magnetic field by measuring variation of the permeability of the magnetic element, resulting from the external magnetic field by measuring an induced voltage in a coil coiling said magnetic element.
In accordance with another aspect of the present invention, a magnetic field sensor includes a sensing element including magnetic material; a coil wound about said sensing element; a source of electrical energy, providing electrical energy to said sensing element to establish a magnetic field in said sensing element, said magnetic field having a magnitude at which there is a linear relationship between said magnetic field and permeability of the magnetic material in said sensing element. A circuit detects changes in voltage across the coil, resulting from changes in the permeability of the sensing element, attributable to an external magnetic field.
Advantageously, exemplary magnetic sensors are able to detect relatively weak external magnetic fields, such as those emitted by the human brain and other biological bodies. Such magnetic sensors may be used in a variety of applications involving non-contact sensing and monitoring of brain activities or status of consciousness.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In figures, which illustrate by way of example only, embodiments of the present invention,
As will become apparent, magnetic field sensors exemplary of embodiments of the present invention, exploit the increased permeability and generally linear relationship between μ and Hcore in a ferromagnetic material to detect the presence of an external magnetic field to be sensed. Conveniently, such sensors may detect very small magnetic fields (e.g. in the order of about 10−12 Tesla) and thus lend themselves to the detection of small magnetic fields emanating from a biological body, such as an animal or human brain. Such sensors are thus well suited for uses as MEG sensors.
The alternating current iloop induces a magnetic field Hsource in core 22 along the axis of core 22, which, in turn, magnetizes the core 22. The degree of magnetization of the core depends on its permeability. Preferably, AC source 26 is operated to provide an Hsource having an optimal frequency, maximizing the sensitivity of μ in relation to Hcore. Hsource brings Hcore within the linear region of the μ-H curve for the material, at the operating frequency of source 26. That is, source 26 is controlled so that Hcore within ferromagnetic core 22 is in a range where the relationship between H and μ is generally steeply linear. Due to the magnetic flux generated in the ferromagnetic core 22, coil 24 experiences a self-inductance resisting the variation of the alternating current in coil 24. The voltage across coil 24 may be measured by a monitoring device such as voltmeter 30. The magnitude of this self-inductance depends on, among other things, the size, shape and number of turns of coil 24. These characteristics are static and depend on design choice.
The self-inductance also depends on the permeability of the ferromagnetic core 22. This permeability varies in relation to the magnetizing field Hcore including the magnetic field Hsource generated by the current in the coil 24 and any external magnetic field.
In operation then, an external magnetic field (Hext) to be sensed acts on core 22 in the direction of the axis of core 22. This external field, Hext, is coupled to core 22 and thus increases Hcore in core 22 from Hsource to Hsource+Hext. This increase in Hcore causes a change in the permeability of core 22 from μ0 to μ0+Δμ (as for example illustrated in
Alternatively, as illustrated in
In operation, sensing element 42/42′ is driven by the AC current provided by source 50/50′ to provide a magnetic field Hsource in the ferromagnetic layer 46 (or the magnetic material of components 43) bringing its/their magnetic permeability to a linear region on the μ-H curve. Preferably, source 50/50′ is driven at a frequency providing a high measurable μmax and high sensitivity of μ in relation to Hcore. Unlike in sensor 20 (
In the presence of an external magnetic field, Hext, the permeability of sensing element 42/42′ further changes, causing additional variation of magnetic flux in sensing element 42/42′, thus inducing additional current in coil 48/48′ or additional electrical potential difference across coil 48/48′, giving a value of vout different from v0. The difference between the two values (vout−v0) reflects Hext.
Notably, for sensing element 42′ of
In order to assist in sensing changes in vout the value of capacitor 54 may be chosen so that the resonant frequency of the circuit including capacitor 54 and coil 48 equals the frequency of AC current through sensing element 42.
Unfortunately, the presence of coils 24 and 48 make formation of sensors 20 and 40 using thin films difficult, if not impossible.
In operation, an external magnetic field parallel to the axis of the conductive wire 62, Hext, changes the permeability of shell 64, and thereby the self-inductance of wire 62 and its impedance. The change in permeability, in turn, induces an electrical potential difference vout at terminals across the conductive wire 62. Sensing voltage vout provides an indicator of the magnitude of the external magnetic field Hext. Conveniently, as Hext is parallel to wire 62 it does not directly induce a current in wire 62.
However, again, as should be appreciated in the presence Hext, the self-inductance and thus impedance of conductive wire 62 increases. This increase in self-inductance results in an increased impedance of conductive wire 62. This, in turn, results in a decrease in the current in iloop. As a consequence the magnetic field in core 64 attributable to source 66 Hsource decreases. The combination of an increase in Hext and a decrease in Hsource may be difficult to meaningfully measure. Moreover, possibly, the change in Hcore brought on by the decrease in Hsource may move Hcore out of the linear region on the μ-H curve.
Both the outer conductor 82 and inner conductor 84 are made of high conductivity materials and are connected to a circuit having an AC source 88 and a controller 90. AC source 88 and controller 90 provide a generally constant loop current iloop. As illustrated, inner and outer conductors 84 and 82 are electrically connected in parallel. As such, current through each conductor is proportional to the impedance of the other conductor. Current through one of the conductors (e.g. conductor 82) may be measured by a monitoring device such as ammeter 92.
A ferromagnetic shell 86 of generally uniform thickness covers the exterior of outer conductor 82. Again shell 86 may be formed of NiFe, NiFeCo, Co-based amorphous material, Fe-based nano-crystalline material, a ferrite, or similar material. As a result of the geometry of sensor 80, magnetic flux attributable to current in inner conductor 84 is coupled to shell 86. Similarly, flux attributable to current in outer conductor 82 is coupled to shell 86. As such, there is a mutual inductance between inner and outer conductors.
In operation, iloop flows in the loop including voltage source 88, inducing a circumferential magnetic field as a result of the current flowing in outer conductor 82 and the current flowing in inner conductor 84. Ferromagnetic shell 86 is magnetized by the circumferential magnetic field. Source 88 provides sufficient AC current in outer conductor 82 so that the permeability of the ferromagnetic shell 86 is excited to a linear point on the μ-H curve, providing sensitivity between μ and H. Again, source 88 is operated to provide an Hsource having an optimal frequency, maximizing the sensitivity of μ in relation to Hcore.
Shell 86 is brought into proximity with the source of the external magnetic field to be sensed so as to couple the external magnetic field to shell 86 in a direction parallel to the axis of the ferromagnetic shell 86. This results in a change in the self and mutual inductance of both inner and outer conductors 84 and 82, and a change in the flow of current in both these conductors. As will be appreciated, the self and mutual inductance of inner conductor 84 is significantly more sensitive than that of outer conductor 82. As such, the impedance of inner conductor 84 is affected more significantly than that of outer conductor 82.
Conveniently, increases in the permeability of core 86 results in increases of the inductance of both inner and outer conductors 84 and 82. However, as the current iloop remains generally constant, a decrease in current in inner conductor 84 resulting from an increased inductance of inner conductor 84 results in increased current flowing through outer conductor 82. As a result, current in outer conductor 82 continues to ensure that core 86 is magnetized to operate within the linear region of μ-H curve for core 86.
Keeping iloop generally constant, the external magnetic field Hext can be sensed by measuring the variation of the current flowing through the outer conductor 82 iout.
Advantageously, use of two conductors 82 and 84, allows a change in permeability to be sensed by sensing the relative change in inductance (self and mutual) of conductors 82 and 84. As such, very small absolute changes in inductance may be reflected in significant and detectable changes in current flow in these conductors.
In operation, the ferromagnetic shell 86′ is magnetized by this circumferential magnetic field generated by conductors 82′ and 94 and by the magnetic field to be sensed, Hext, in parallel to the axis of the ferromagnetic shell 86′. Source 88′ provides sufficient AC loop current flowing through the outer-conductor 82′ so that the permeability of the ferromagnetic shell 86′ is excited to a dynamic state in the linear region of the μ-H curve, in the absence of an external magnetic field. In the presence of an external magnetic field, conductors 82′ and 94 experience a change in self and mutual inductance. Again the inductance of each of conductors 94 will be more significant than the change in inductance on the outer conductor 82′. As iloop is generally constant, current passing through each of inner conductors 94 decreases and current through outer conductor 82′ increases. As there are plurality of inner conductors 94, changes in current in outer conductor 82′ will be more significant than in the presence of a single inner conductor. Again, the external magnetic field intensity Hext can be determined by measuring the variation of the current flowing through the outer conductor 82′ iout.
Advantageously sensor 80′ as described with reference to
Conveniently, sensors 100 and 110 functionally similar to sensors 80 and 80′ (
The thin film sensors 100 and 110 may be formed using soft ferromagnetic top and bottom layers of about 1 μm defining shell 106 (116) and a copper middle layer of about 5 μm in thickness, the width and length of which may be about 1.5 mm and 5 mm respectively defining conductors 102 (112) and 104 (114). Sensors 100 and 110 can be produced by physical vapour deposition using a thin film sputtering machine, in a conventional manner understood by those of ordinary skill.
Conveniently, sensors exemplary of embodiments of the present invention may be used as MEG sensors, and may monitor brain activity and may thus monitor the onset of sleep; blackouts; attention and consciousness; disorientation; and attempts at communication. Such a sensor may for example be used in conjunction with other medical sensors.
Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.