Abstract:
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 die 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. Preferably, the conductors are mutually coupled and exhibit a mutual inductance between each other so that detectable changes in the current distribution between the conductors caused by an external magnetic field may be detected while the net current flowing in the conductors remains sufficient to maintain a magnetic field of sufficient magnitude in the material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims benefits from U.S. Provisional Patent Application No. 60/348,668 filed Jan. 15, 2002, the contents of which are hereby incorporated herein by reference 

   FIELD OF THE INVENTION 
   This invention relates generally to sensing magnetic fields, and more particularly to methods and devices for sensing very small magnetic fields such as those emitted by a biological body. 
   BACKGROUND OF THE INVENTION 
   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&#39;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 clinical 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. 
   SUMMARY OF THE INVENTION 
   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. 
   Preferably, the conductors are mutually coupled and exhibit a mutual inductance between each other so that detectable changes in the current distribution between the conductors caused by an external magnetic field may be detected while the net current flowing in the conductors remains sufficient to maintain a magnetic field of sufficient magnitude in the material. The sensitivity of the sensor may be increased by increasing the number of conductors. 
   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. Furthermore, such sensors do not require cooling and lend themselves to portability and thin film formation. 
   In accordance with an aspect of the invention, there is provided a magnetic field sensor, including: a shell made of a magnetic material defining an interior; a first conductor within the interior; a second conductor within the interior, nested within the first conductor; the first and second conductors exhibiting a mutual inductance between each other; a source of electrical energy, interconnected with the first and second conductors, to establish an alternating electric current through at least one of the first and second conductors, and thereby a magnetic field in the shell; a monitor, interconnected with one of the first and second conductors, to monitor changes in current therethrough as the shell is brought into proximity with an external magnetic field, to sense a change in permeability of the magnetic material. 
   In accordance with another aspect of the invention, there is provided a magnetic field sensor, including a magnetic material; two mutually-coupled conductors driven by a source of alternating electricity to generate a magnetic field in the magnetic material, and mutually coupled so that a change in permeability of the magnetic material results in re-distribution of current through the conductors; a meter for measuring current through at least one of the first and second conductors to sense a change in permeability of the magnetic material caused by an external magnetic field, and thereby the external magnetic field. 
   In accordance with yet another aspect of the invention, there is provided a method of sensing an external magnetic field, including inducing a magnetic field in a magnetic element by providing alternating current through first and second mutually-coupled electric conductors; measuring a change in distribution of current in the two conductors, resulting from the external magnetic field varying permeability of the magnetic element. 
   In accordance with a further aspect of the invention, there is provided a magnetic field sensor, including a conductive core, surrounded by a magnetic layer; a coil wound about the magnetic layer; a source of electrical energy, providing electrical energy to the conductive core to establish a magnetic field in the magnetic layer, the magnetic field having a magnitude at which there is a linear relationship between the magnetic field and permeability of the magnetic layer; a sensor for detecting changes in voltage across the coil, resulting from changes in the permeability of the magnetic layer, attributable to an external magnetic field. 
   In accordance with another aspect of the invention, there is provided a method of sensing a low frequency external magnetic field including: exciting a magnetic element with a time varying magnetic field having a frequency and amplitude to maximize the sensitivity of the permeability of the magnetic element to the magnetic field in the magnetic element; sensing the external magnetic field by measuring variation of the permeability of the magnetic element, resulting from the external magnetic field. 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In figures, which illustrate by way of example only, embodiments of the present invention, 
       FIG. 1  is a μ-H curve graphically illustrating the variation of permeability with magnetic field in a magnetic material used in example embodiments of the present invention; 
       FIG. 2  is a simplified schematic diagram of a magnetic field sensor; 
       FIG. 3  is a simplified schematic diagram of another magnetic field sensor, exemplary of an embodiment of the present invention; 
       FIG. 4  is a simplified schematic diagram of a further magnetic field sensor; 
       FIGS. 5 and 6  are simplified schematic diagrams of a magnetic field sensor, exemplary of other embodiments of the present invention; 
       FIGS. 7 and 8  illustrate thin film magnetic field sensors, exemplary of embodiments of the present invention; 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a μ-H curve  10  illustrating the relationship between the permeability (μ) of a particular ferromagnetic material and a magnetic field in the material having a magnitude |H core |. As illustrated, the permeability is highly dependent on the strength of magnetic field H core  until the magnetic material is saturated. At saturation, the permeability remains relatively constant despite an increase in the magnetic field strength. In region  12 , the variation of permeability μ is generally, and steeply, linear with H core : a slight increase in H results in a significant increase in 
         μ   ⁡     (       i   .   e   .           ⁢       Δ   ⁢           ⁢   μ       Δ   ⁢           ⁢   H         &gt;&gt;   1     )       .         
Curve  10  illustrates the relationship between μ and H core  for a particular range of frequencies of H core  within the material. Empirically, it has been observed that the value of μ max  increases as the frequency of H core  is increased. Thus, the sensitivity of μ in relation to H core  may increase further in the presence of a time varying H core . An optimal frequency of frequency for H core , providing maximum sensitivity of μ in relation to H core  may be determined for any particular material by trial and error.
 
   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 H core  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. 
     FIG. 2  illustrates a magnetic field sensor  20 , in a form of ferromagnetic core inductor, including of a ferromagnetic core  22  surrounded by a coil  24 . Core  22  is made of ferromagnetic material having a μ-H curve generally as depicted in FIG.  1 . Core  22  may, for example, be formed of NiFe, NiFeCo, Co-based amorphous material, Fe-based nano-crystalline material, a ferrite, or similar material. Coil  24  is formed of a conducting material, such as copper and is connected as part of an alternating current (AC) circuit having an AC voltage source  26  and a controller  28 . For sensing the external magnetic field, AC source  26  provides a generally constant alternating current i loop  flowing in the coil  24 . A generally constant current source (such as AC source  26  and controller  28 ) may be formed in any number of ways. Controller  28  may for example take the form of a simple current limiting resistor. 
   The alternating current i loop  induces a magnetic field H source  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 H source  having an optimal frequency, maximizing the sensitivity of μ in relation to H core . H source  brings H core  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 H core  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 H core  including the magnetic field H source  generated by the current in the coil  24  and any external magnetic field. 
   In operation then, an external magnetic field (H ext ) to be sensed acts on core  22  in the direction of the axis of core  22 . This external field, H ext , is coupled to core  22  and thus increases H core  in core  22  from H source  to H source +H ext . This increase in H core  causes a change in the permeability of core  22  from μ 0  to μ 0 +Δμ (as for example illustrated in FIG.  1 ). This increases the self-inductance of coil  24  and hence its impedance. The change in impedance causes a change in voltage across coil  24 . As i loop  is kept constant, the variation of the external magnetic field intensity H ext  may be determined by measuring the variation of the voltage across coil  24  at voltmeter  30 . For low frequency external magnetic fields (about 100 Hz) voltage induced by the external field may be ignored. Nevertheless, accurately, detecting small values of H ext  and very slight variations in H ext  becomes difficult because of the multiple mechanisms affecting voltage across coil  24 : V out  reflects changes in permeability, and H ext  and H source . 
     FIG. 3  therefore illustrates an alternate magnetic field sensor  40 . Sensor  40  may be formed by a sensing element  42  of ferromagnetic composite wire or thin film having a conductive core  44  coated with a high permeability magnetic layer  46 , and an induction coil  48  coiled about ferromagnetic layer  46 . Optionally, an insulating layer (not shown) may be provided between conductive core  44  and ferromagnetic layer  46 . Sensing element  42  is connected to a circuit of an alternating current source  50 , generating an alternating loop current i loop . Induction coil  48  is connected in parallel with a capacitor  54 . The voltage across coil  48  and capacitor  54  may be measured by a monitoring device such as voltmeter  56 . 
   In operation, sensing element  42  is driven by the AC current provided by source  50  to provide a magnetic field H source  in the ferromagnetic layer  46  bringing its magnetic permeability to a linear region on the μ-H curve. Preferably, source  50  is driven at a frequency providing a high measurable μ max  and high sensitivity of μ in relation to H core . Unlike in sensor  20  ( FIG. 2 ) the magnetic field H source  attributable to source  50  in sensing element  42  is generated circumferentially around the central axis of core  44 . As such, the variation of H source  does not directly induce a current in coil  48 . However, in the absence of an external field, the permeability of layer  46  varies with the frequency and magnitude of the AC driving current from source  50  in all directions and thus the magnetic flux in magnetic material  46  varies along the lengthwise extending axis of sensing element  42 . This change results in a change in inductance as seen by coil  48  and induces a current in coil  48  or an electrical potential difference V out =V 0  across coil  48 . 
   In the presence of an external magnetic field, H ext , the permeability of sensing element  42  further changes, causing additional variation of magnetic flux in sensing element  42 , thus inducing additional current in coil  48  or additional electrical potential difference across coil  48 , giving a value of V out  different from V 0 . The difference between the two values (V out −V 0 ) reflects H ext . 
   In order to assist in sensing changes in V out  the value of capacitor  54  is chosen so that the resonant frequency of the circuit including capacitor  54  and coil  48  equals the 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. 
   As such,  FIG. 4  illustrates a further magnetic field sensor  60 , in a form of self-inductance composite wire inductor, including an insulated conductive wire  62  surrounded by a ferromagnetic shell  64 . Again shell  64  may be formed of NiFe, NiFeCo, Co-based amorphous material, Fe-based nano-crystalline material, a ferrite, or similar material. Conductive wire  62  is connected to a circuit having an AC source  66  and a controller  68 . Conductive wire  62  is of high conductivity. For sensing an external magnetic field, an alternating current flows in the conductive wire  62 , generating a circumferential magnetic field H core =H source  which magnetizes ferromagnetic shell  64 . The magnetization H core  depends on the magnetic permeability of ferromagnetic shell  64 . The alternating current flowing in the conductive wire  62  is arranged such that the permeability of the ferromagnetic shell  64  is excited to a dynamic state in variation with external magnetic field. Again, AC source  66  is operated to provide an H source  having an optimal frequency, maximizing the sensitivity of μ in relation to H core . The magnetic flux in ferromagnetic shell  64  resists a change in current through conductive wire  62 , thereby producing a self-inductance on the conductive wire  62 . The voltage across wire  62  may be measured by a monitoring device such as voltmeter  70 . The self-inductance of wire  62  varies as the magnetic permeability of the ferromagnetic shell  64  varies with the magnetizing field, including any external magnetic field. 
   In operation, an external magnetic field parallel to the axis of the conductive wire  62 , H ext , 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 V out  at terminals across the conductive wire  62 . Sensing voltage V out  provides an indicator of the magnitude of the external magnetic field H ext . Conveniently, as H ext  is parallel to wire  62  it does not directly induce a current in wire  62 . 
   However, again, as should be appreciated in the presence H ext , 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 i loop . As a consequence the magnetic field in core  64  attributable to source  66  H source  decreases. The combination of an increase in H ext  and a decrease in H source  may be difficult to meaningfully measure. Moreover, possibly, the change in H core  brought on by the decrease in H source  may move H core  out of the linear region on the μ-H curve. 
     FIG. 5  therefore illustrates further embodiment of a magnetic sensor  80  exemplary of an embodiment of the present invention. Sensor  80  includes first and second lengthwise extending nested conductors  82  and  84 . As illustrated, example outer conductor  82  takes the form of an insulated cylinder. Inner conductor  84 , nested within the exterior conductor  82 , takes the form of lengthwise extending wire, having a generally uniform cross-section. Preferably, the cross-sectional area of outer conductor  82  is significantly larger than that of inner conductor  84 . Inner conductor  84  is also preferably surrounded by an insulation layer (oxide, for example) electrically insulating conductor  82  from conductor  84 . 
   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 i loop . 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, i loop  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 H source  having an optimal frequency, maximizing the sensitivity of μ in relation to H core . 
   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 i loop  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 i loop  generally constant, the external magnetic field H ext  can be sensed by measuring the variation of the current flowing through the outer conductor  82  i out . 
   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. 
     FIG. 6  shows a further example sensor  80 ′, substantially similar to sensor  80  (FIG.  5 ). Like elements are thus labeled with like numerals but include a prime (′) symbol. As illustrated, instead of a single interior conductor  84 , sensor  80 ′, includes a plurality (n) of parallel inner conductors  94 . Again, the conducting surface area of outer conductor  82 ′ is larger than the area of each self-inductance of conductors  94 . Both the outer conductor  82 ′ and self-inductance wires  94  are made of high conductivity materials and are connected in parallel to a circuit having an AC source  88 ′ and a controller  90 ′. For sensing the external magnetic field as an alternating current flows in the circuit, a circumferential magnetic field is generated by the current in the outer conductor  82 ′ and in addition by the currents flowing in the inner conductors  94 . 
   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, H ext  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 i loop  is generally constant, current passing through each of inner conductors  94  decreases and current through outer conductor  32 ′ increases. As there are plurality of inner conductors  94 , changes in current in outer conductor  82 ′ win be more significant than in the presence of a single inner conductor. Again, the external magnetic field intensity H ext  can be determined by measuring the variation of the current flowing through the outer conductor  82 ′ i out  by a monitoring device such as ammeter  96 . 
   Advantageously sensor  80 ′ as described with reference to  FIG. 6 , unlike sensor  80  (FIG.  5 ), includes more than one embedded inner conductor nested within the outer conductor. As a result, the sensitivity to the external field can be substantially increased by increasing the number of the interior conductors as well as the length of the sensing element can be shortened. 
   Conveniently, sensors  100  and  110  functionally similar to sensors  80  and  80 ′ ( FIGS. 5 and 6 ) may be formed as thin film sensors as illustrated in  FIGS. 7 and 8 . As illustrated, inner conductors  102  ( 112 ) may be formed as insulated metal strips embedded nested within outer conductors  104  ( 114 ) within a ferromagnetic shell  106  (or  116 ). Again, the cross-sectional urea of inner conductors  102  ( 112 ) is significantly smaller than the cross-section of outer conductor  104 ( 114 ). Conductors  102  ( 112 ) and  104  ( 114 ) are driven by source  108  ( 118 ) controlled by controller  109  ( 119 ). An external magnetic field H ext  may be detected as described with reference to  FIGS. 5 and 6 , by measuring current through one of the conductors using a monitoring device such as ammeter  105  ( 115 ). 
   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.