Abstract:
A multi-layer wireless sensor construct is provided. The construct includes a first dielectric layer adapted to be attached to a portion of a first surface of an electrically-conductive material. A layer of mu metal is provided on the first dielectric layer. A second dielectric layer is provided on the layer of mu metal. An electrical conductor is provided on the second dielectric layer wherein the second dielectric layer separates the electrical conductor from the layer of mu metal. The electrical conductor has first and second ends and is shaped to form an unconnected open-circuit that, in the presence of a time-varying magnetic field, resonates to generate a harmonic magnetic field response having a frequency, amplitude and bandwidth.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION(S) 
       [0001]    This patent application claims the benefit of and priority to United States Provisional Application Ser. No., 61/895,129, filed on Oct. 24, 2013, the contents of which are hereby incorporated by reference in their entirety. In addition, this application is related to co-pending patent applications titled “ANTENNA FOR FAR FIELD TRANSCEIVING” and “PLASMA GENERATOR USING SPIRAL CONDUCTORS” filed on the same day and owned by the same assignee as this patent application. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C, § 202) and may be manufactured and used by or for the Government for governmental purposes without. the payment of any royalties thereon or therefore, In accordance with 35  . U.S.C. § 202, the contractor elected not to retain title. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Modern aerospace vehicles (e.g., airplanes, rotorcraft, unmanned aerial vehicles, airships, rockets, and spacecraft) are designed to utilize a variety of lightweight and strong composite materials. One class of these composite materials is referred to generally as “carbon fiber reinforced plastics” (CFRPs). These materials are being incorporated into frame structures as well as various components. For example, liquid reservoirs such as fuel tanks are being constructed from CFRPs. 
         [0004]    Measuring the quantity of fuel in an aerospace vehicle&#39;s fuel tank is a continuous and critical function. Traditionally, fuel quantity measurement has been accomplished by one or more probes that must be immersed in the fuel. The probes are typically of the electronic-capacitive type which requires electrical penetrations through the fuel storage tank. Electrical penetrations through a fuel tank as well as routing of wire within a fuel tank are not optimum solutions. Though the intended interrogation signals on the wires to the fuel probes are low in power level, the potential for other higher powered unwanted interference signals both man-made and natural can couple onto fuel tank electrical penetrations creating unwanted or even disastrous results. For example, a lightning strike on or in the near vicinity of the air vehicle can induce strong electrical currents on the wiring that penetrates the fuel tank. 
         [0005]    Recently, a new class of wireless sensing systems have been developed that use open-circuit, electrically-conductive spiral trace sensors. Details of these sensors and sensing systems are described in U.S. Pat. No. 8,430,327. Briefly, the described wireless sensing system includes a sensor made from an electrical conductor shaped to for an open-circuit, electrically-conductive spiral trace having inductance and capacitance. In the presence of a time-varying magnetic field, the sensor resonates to generate a harmonic response having a frequency, amplitude and bandwidth. A magnetic field response recorder wirelessly transmits the time-varying magnetic field to the sensor and wirelessly detects the sensor&#39;s response. Unfortunately, the above-described wireless sensor and sensing system will not function when used on or near conductive materials (such as components made from CFRPs) since the conductive material shields and absorbs the electromagnetic energy generated by the sensor. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention is a multi-layer wireless sensor construct for attachment to a first surface of an electrically-conductive material. The construct can be used in the sensing of the presence of a liquid at a second surface of the electrically-conductive material. The wireless sensor construct includes a first dielectric layer adapted to he attached to a portion of the first surface of the electrically-conductive material. A layer of mu metal is provided on the first dielectric layer wherein the first dielectric layer separates the layer of mu metal from the first surface of the electrically-conductive material. A second dielectric layer is provided on the layer of mu metal, An electrical conductor is provided on the second dielectric layer wherein the second dielectric layer separates the electrical conductor from the layer of mu metal. The electrical conductor has first and second ends and is shaped to form a spiral between its first and second ends. The first and second ends remain electrically unconnected such that the electrical conductor so-shaped is maintained as an unconnected single-component open-circuit having inductance and capacitance. In the presence of a time-varying magnetic field, the electrical conductor so-shaped resonates to generate a harmonic magnetic field response having a frequency, amplitude and bandwidth. 
         [0007]    One embodiment of the invention is a system for indicating a quantity of liquid in a container that is electrically conductive. A plurality of the above-described wireless sensor constructs are adapted to be attached to an exterior wail region of the container, N magnetic field response recorder having an antenna wirelessly transmits the time-varying magnetic field to each construct&#39;s electrical conductor so-shaped and wirelessly detects the harmonic magnetic field response generated thereby. The magnetic field response recorder has an electrical impedance Z SOURCE  that is exclusive of electrical impedance of the antenna. A total electrical impedance Z TOTAL  is approximately matched to the electrical impedance Z SOURCE , The total electrical impedance Z TOTAL  is defined by a parallel combination of the electrical impedance of the antenna, electrical impedance of the electrical conductor so-shaped, electrical impedance of the layer of mu metal, and electrical impedance of a portion of the container bounded by an outer periphery of the layer of mu metal. 
         [0008]    Another embodiment of the invention is a system for storing a liquid and for indicating a quantity of the liquid so-stored. The system includes a container made from an electrically conductive material and a plurality of the above-described wireless sensor constructs attached to the container. A magnetic field response recorder has an antenna that wirelessly transmits the time-varying magnetic field to the electrical conductor so-shaped and wirelessly detects the harmonic magnetic field response generated thereby. As in the previous embodiment, the magnetic field response recorder has an electrical impedance Z SOURCE  exclusive of electrical impedance of the antenna. A total electrical impedance Z TOTAL  is approximately matched to the electrical impedance Z SOURCE . The total electrical impedance Z TOTAL  is defined by a parallel combination of the electrical impedance of the antenna, electrical impedance of the electrical conductor so-shaped, electrical impedance of the layer of mu metal, and electrical impedance of a portion of the container bounded by an outer periphery of the layer of mu metal. 
         [0009]    Yet another embodiment of the invention is a system for storing a liquid and for indicating a quantity of the liquid so-stored. The system includes a container and a plurality of wireless sensor constructs. Each wireless sensor construct is attached to an exterior surface region of the container, Each wireless sensor construct includes an electrical conductor so-shaped as in the previous embodiments and an electromagnetic window that is transparent to the harmonic magnetic field response of the electrical conductor so-shaped. The electromagnetic window is disposed between the electrical conductor so-shaped and an interior region of the container. A magnetic field response recorder having an antenna is also included. A total electrical impedance Z TOTAL  is approximately matched to the electrical impedance Z SOURCE  of the magnetic field response recorder. The total electrical impedance Z TOTAL  is defined by a parallel combination of the electrical impedance of the antenna, electrical impedance of the electrical conductor so-shaped, and electrical impedance of the electromagnetic window. 
         [0010]    These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic view of a multi-layer wireless sensor construct for attachment to an electrically-conductive material in accordance with an embodiment of the present invention; 
           [0012]      FIG. 2  is an isolated schematic view of an electrically-unconnected spiral sensor used in the wireless sensor construct in accordance with an embodiment of the present invention; 
           [0013]      FIG. 3A  is an isolated plan view of the wireless sensor construct&#39;s mu metal layer in accordance with an embodiment of the present invention; 
           [0014]      FIG. 3B  is an isolated plan view of the wireless sensor construct&#39;s mu metal layer in accordance with another embodiment of the present invention; 
           [0015]      FIG. 4  is a schematic view of a system for indicating a quantity of liquid in an electrically-conductive container in accordance with an embodiment of the present invention; 
           [0016]      FIG. 5  is a schematic view of an electrical impedance circuit equivalent of a wireless sensor system using the sensor construct in accordance with an embodiment of the present invention; and 
           [0017]      FIG. 6  is a schematic view of a portion of a container configured with electromagnetic energy windows forming a part of a wireless sensor construct in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0019]    Referring now to the drawings and more particularly to FIG,  1 , a multi-layer wireless sensor construct in accordance with an embodiment of the present invention is shown and is referenced generally by numeral  10 . Wireless sensor construct  10  is shown attached to a surface  102  of an electrically-conductive material  100 . In general and as will be explained further below, wireless sensor construct  10  can be part of a system used to sense the presence or absence of liquid at an opposing surface  104  of material  100  at a region thereof aligned with wireless sensor construct  10 . The liquid (not shown) can be water, fuel, sewage, or any other liquid retained by material  100 . Accordingly, in many applications, material  100  is part of a container. 
         [0020]    Wireless sensor construct  10  is a multi-layer device that provides for non-invasive sensing of liquid adjacent surface  104  from surface  102 , does not require any electrical connections thereto, does not require any penetrations of material  100 , and does not require any electrical wires to pass through material  100 , Wireless sensor construct  10  includes an electrically unconnected, open-circuit spiral conductor sensor  12 . Sensor  12  and its attributes are described in detail in U.S. Pat. No. 8,430,327, the entire contents of which are hereby incorporated by reference, Briefly, and with reference to  FIG. 2 , spiral trace sensor  12  is made from an electrically-conductive run or trace. More specifically, spiral trace sensor  12  is a spiral winding of conductive material with its ends  12 A and  12 B remaining open or unconnected, Accordingly, spiral trace sensor  12  is said to be an open-circuit, Techniques used to construct or deposit spiral trace sensor  12  on a substrate material can be any conventional metal-conductor deposition process to include thin-film fabrication techniques. In the illustrated embodiment, spiral trace sensor  12  is constructed to have a uniform trace width throughout (i.e., trace width W is constant) with uniform spacing (i.e., spacing d is constant) between adjacent portions of the spiral trace. However, it is to be understood sensor  12  is not limited to a uniform width conductor spirally wound with uniform spacing as illustrated in  FIG. 2 . 
         [0021]    In order to sense the presence/absence of a liquid at surface  104 , the harmonic response of sensor  12  must be able to penetrate material  100 . That is, an electromagnetic window must be defined adjacent sensor  12  such that the harmonic response of sensor  12  can pass through material  100  and such that the harmonic response changes (owing to the presence or absence of a liquid at surface  104  aligned with sensor  12 ) can be detected by sensor  12 . In the illustrated embodiment, the electromagnetic window is provided by a layer  14  of a high permeability material such as the class of materials known as mu metals (e.g., nickel-iron alloy). 
         [0022]    Layer  14  is disposed between sensor  12  and conductive material  100  to control the field coupling and enable sensor operation. The depth that induced currents can penetrate into a material is affected by the frequency of the excitation current, and the conductivity and magnetic permeability of the substrate material. While the depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability, the degree of penetration can in principle be increased by the creating a saturation magnetic field. To create a saturation magnetic field, the present invention employs materials with high magnetic permeability. 
         [0023]    The general configuration of wireless sensor construct  10  includes a stack defined by sensor  12  over a thin electrical isolating dielectric layer  16  over mu metal layer  14  that is electrically isolated from material  100  by a dielectric layer  18 . Dielectric layers  16  and  18  can incorporate adhesive properties to maintain the integrity of construct  10  and provide the means to attach construct  10  to surface  102 . The high permeability layer  14  is used to concentrate the magnetic field inside the high permeability material to enable sensor  12  to self-resonate. By covering or tiling the sensor area with high permeability material, the current density can be increased at surface  102  of material  100  thereby allowing the magnetic field to be more effectively coupled, 
         [0024]    As mentioned above, the high permeability materials used for layer  14  can be thin metallic films composed of a nickel-iron alloy and are commonly referred to as mu metals or high μ materials. Mu metal layer  14  increases the magnetic field produced by sensor  12  and correspondingly increases the penetration depth of the magnetic field into material  100 . Layer  14  also maintains a high density magnetic flux in the very narrow space of the thickness of layer  14  to sustain the oscillating magnetic fields (both the external driving magnetic field and the sensor-produced magnetic field) from being totally shielded and absorbed by material  100 . 
         [0025]    Mu metal layer  14  can he a solid sheet of material spanning to or beyond the outer dimensions of sensor  12 . However, mu metal layer  14  could be configured in other ways to adjust its above-described functions. For example, mu metal layer  14  could have one or more holes as in the examples illustrated in  FIGS. 3A and 3B . More specifically,  FIG. 3A  illustrates a “picture frame” mu metal layer  14  defined by a frame  14 A of mu metal material surrounding a single hole  14 B.  FIG. 3B  illustrates a multiple-hole mu metal layer  14  defined by a contiguous layer  14 C of mu metal material  14 C with a plurality of holes  14 B. While holes  14 D are all circular and are the same size, the holes could be different shapes/sizes and be arranged irregularly in the mu metal layer without departing from the scope of the present invention. 
         [0026]    A plurality of the wireless sensor constructs of the present invention can be used as part of a liquid quantity indication system. A simple embodiment of such a system is shown by way of example in  FIG. 4  where a top view of a liquid container  200  is illustrated. The particular shape and/or size of container  200  are not limitations of the present invention. For this embodiment, container  200  is made from an electrically-conductive material. For example, container  200  could be made from an electrically-conductive composite known as carbon fiber reinforced plastic (CFRP). The particular composition of the material used to make container  200  is not a limitation of the present invention. 
         [0027]    A number of the above-described, wireless sensor constructs  10  are attached to exterior surface regions of container  200  using, for example, adhesive properties of each construct&#39;s dielectric layer  18  ( FIG. 1 ) as described above, The particular number of constructs  10  and their positions on container  200  are not limitations of the present invention. In general, the number and positions of wireless sensor constructs  10  are determined by the particular application. For example, if container  200  is a fuel tank for an aircraft or space vehicle, constructs  10  are positioned to provide useful information at a variety of container attitudes roll, pitch and yaw angles) so that a fuel quantity (as opposed to a mere fuel level) can be determined. 
         [0028]    Each wireless sensor construct is resonated and monitored by a magnetic field response recorder  20 , the details of which are described in the above-referenced U.S. Pat. No. 8,430,327, as well as in. U.S. Pat. Nos, 7, 086,593 and 7,159,774, the entire contents of which are hereby incorporated by reference. While only one recorder  20  is illustrated, additional ones could be used without departing from the scope of the present invention. Briefly, recorder  20  includes an antenna  22  for transmission of a broadband time-varying magnetic field (causing the constructs&#39; sensors to resonate) and for reception of the harmonic resonance response of the constructs&#39; sensors. 
         [0029]    In order to maximize the coupling of each construct&#39;s sensor (i.e., sensor  12 ) transmission and response through the walls of container  200 , the present invention applies an electrical impedance matching approach that will be explained with the aid of  FIG. 5 . The complex form of impedance or Z (i.e., an equivalent complex impedance that includes all real and imaginary components of resistance, inductance and capacitance) will be used for simplicity, The signal applied by recorder  20  to its antenna  22  has an electrical impedance Z SOURCE  that does not include the electrical impedance of antenna  22  (referred to herein as Z A ). The electrical impedance of the construct&#39;s sensor (i.e., sensor  12  shown in  FIG. 1 ) is Z S . The electrical impedance of the construct&#39;s mu metal layer (i.e., mu metal layer  14  shown in  FIG. 1 ) is Z MU . The electrical impedance of the portion of container  200  bounded by the periphery of the construct&#39;s mu metal layer is Z C . In the present invention, maximum coupling of resonance energy is achieved when Z SOURCE  is equal to A TOTAL  where Z TOTAL  is the parallel combination of Z A , Z S , Z MU  and Z C . The above-described multi-layer construct simplifies the adjustment of Z TOTAL  for a particular application. In particular, a wireless sensor construct&#39;s mu metal. layer is readily adaptable to provide the needed adjustments to achieve the optimal Z TOTAL . 
         [0030]    As described above, the mu metal layer of each wireless sensor construct helps define an “electromagnetic window” through a (container) wall made from an electrically-conductive material. This provides the basis for a wireless liquid quantity indicating system. However, the present invention is not so limited. For example,  FIG. 6  illustrates a portion of a container  300  that incorporates electromagnetic windows  302  in the walls thereof where each window  302  is transparent with respect to harmonic magnetic. fields (i.e., those generated by a wireless sensor  12  and received by wireless sensor  12 ). A wireless spiral sensor  12  (as described above) can be coupled directly or indirectly to each window  302  such that a wireless sensor construct is defined by each sensor/window combination. In this embodiment, electrical impedance matching is achieved when Z SOURCE  is equal to Z TOTAL  where Z TOTAL  is the parallel combination of Z A , Z S  and Z W  where Z W  is the electrical impedance of window  302 . 
         [0031]    The advantages of the present invention are numerous. The wireless sensor construct provides the framework for achieving wireless sensing functions through electrically-conductive materials/walls. When used in groups, the wireless sensor constructs can be activated and interrogated in a wireless fashion to indicate attributes of materials (e.g., liquids) contained by electrically-conductive walls. The present invention can be used to indicate the quantity of liquid (e.g., fuel) in electrically-conductive containers subject to a variety of attitudes.