Abstract:
A wireless sensing system includes a sensor made from an electrical conductor shaped to form 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 frequency, amplitude and bandwidth.

Description:
ORIGIN OF THE INVENTION 
     This invention was made in part by an employee of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. Pursuant to 35 U.S.C. §119, the benefit of priority from provisional application 60/774,803, with a filing date of Feb. 6, 2006, is claimed for this non-provisional application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to wireless sensing systems. On More specifically, the invention&#39;s a wireless sensing system that uses an open-circuit electrically-conductive spiral trace as the system&#39;s sensor. 
     2. Description of the Related Art 
     wireless sensors and wireless sensor measurement acquisition systems are known in the art. For example, U.S. Pat. Nos. 7,086,593 and 7,159,774 disclose magnetic field response sensors designed as passive inductor-capacitor circuits and passive inductor-capacitor-resistor circuits that produce magnetic field responses whose harmonic frequencies correspond to states of physical properties of interest. A sensor is made by electrically connecting a spiral trace inductor to a capacitor. A magnetic field response recorder wirelessly transmits a time-varying magnetic field that powers each sensor using Faraday induction. Each sensor then electrically oscillates at a resonant frequency that is dependent upon the capacitance, inductance and resistance of each sensor. The frequency, amplitude and bandwidth of this oscillation is wirelessly sensed by the magnetic field response recorder. The sensor&#39;s response is indicative of one or more parameters that are to be measured. 
     While the above-described magnetic field response measurement acquisition system greatly improves the state-of-the-art of wireless sensing, sensor reliability is greatly improved by eliminating their electrical connections. Furthermore, a sensor design that can be readily modified to provide a different response characteristic would be desirable. In this way, sensors could be mass produced in one configuration and then quickly customized by a user for their particular application. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a wireless sensing system. 
     Another object of the present invention is to provide a wireless sensing system utilizing a simple sensor that is easy to fabricate and more reliable than existing designs. 
     Still another object of the present invention is to provide a magnetic field response-based wireless sensing system whose sensor can be readily adjusted in terms of its frequency response. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a wireless sensing system includes a sensor made from an electrical conductor having first and second ends and shaped to form a spiral between the first and second ends. The shaped conductor is an open-circuit having inductance and capacitance. In the presence of a time-varying magnetic field, the shaped conductor 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 shaped conductor and wirelessly detects its response frequency, amplitude and bandwidth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an open-circuit spiral trace sensor in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic view of an embodiment of a magnetic field response recorder used in the present invention; 
         FIG. 3  is a schematic view of a spiral trace sensor whose traces are non-uniform in width; 
         FIG. 4  is a schematic view of a spiral trace sensor having non-uniform spacing between the traces thereof; 
         FIG. 5  is a schematic view of a spiral trace sensor having non-uniform trace width and non-uniform trace spacing; 
         FIG. 6A  is a cross-sectional view of a spiral trace sensor assembly with a dielectric material disposed between the sensor&#39;s traces; 
         FIG. 6B  is a cross-sectional view of a spiral trace sensor assembly with a dielectric material layer disposed on top of the spiral trace; 
         FIG. 6C  is a cross-sectional view of a spiral trace sensor assembly with a dielectric material disposed between the sensor&#39;s traces and on top of the spiral trace; 
         FIG. 7A  is a schematic view of a linear arrangement of open-circuit spiral trace sensors mutually inductively coupled and interrogated by a magnetic field response recorder; and 
         FIG. 7B  is a schematic view of a non-linear arrangement of open-circuit spiral trace sensors mutually inductively coupled and interrogated by a magnetic field response recorder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and more particularly to  FIG. 1 , a wireless sensing system using an open-circuit spiral trace sensor in accordance with an embodiment of the present invention is shown and is referenced generally by numeral  10 . In general, system  10  includes a sensor assembly  20  and a magnetic field response recorder  30 . Sensor assembly  20  will typically be attached to or incorporated in a structure, machine, environment, etc. (not shown), and designed to sense/measure some physical parameter of interest. It is to be understood that the particular mounting location and/or parameter(s) to be sensed are not limitations of the present invention. 
     Sensor assembly  20  typically includes a substrate material  22  that is electrically non-conductive and can be flexible to facilitate a variety of mounting scenarios. The particular choice of substrate material  22  will vary depending on the ultimate application incorporating sensor assembly  20 . Accordingly, the choice of substrate material  22  is not a limitation of the present invention. 
     Deposited on substrate material  22  is a spiral trace sensor  24  made from an electrically-conductive run or trace. More specifically, spiral trace sensor  24  is a spiral winding of conductive material with its ends  24 A and  24 B remaining open or unconnected. Accordingly, spiral trace sensor  24  is said to be an open-circuit. Techniques used to deposit spiral trace sensor  24  on substrate material  22  can be any conventional metal-conductor deposition process to include thin-film fabrication techniques. In the illustrated embodiment, spiral trace sensor  24  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, as will be explained further below, the present invention is not limited to a uniform width conductor spirally wound with uniform spacing as illustrated in  FIG. 1 . 
     As is well known and accepted in the art, a spiral inductor is ideally constructed/configured to minimize parasitic capacitance so as not to influence other electrical components that will be electrically coupled thereto. This is typically achieved by increasing the spacing between adjacent conductive portions or runs of the conductive spiral trace. However, in the present invention, spiral trace sensor  24  is constructed/configured to have a relatively large parasitic capacitance. The capacitance of spiral trace sensor  24  is operatively coupled with the sensor&#39;s inductance such that energy can be exchanged between the sensor&#39;s magnetic field and its electric field. The amount of inductance along any portion of a conductive run of sensor  24  is directly related to the length thereof and inversely related to the width thereof. The amount of capacitance between portions of adjacent conductive runs of sensor  24  is directly related to the length by which the runs overlap each other and&#39;s inversely related to the spacing between the adjacent conductive runs. The amount of resistance along any portion of a conductive run of sensor  24  is directly related to the length and inversely related to the width of the portion. Total capacitance, total inductance and total resistance for spiral trace sensor  24  is determined simply by adding these values from the individual portions of sensor  24 . The geometries of the various portions of the conductive runs of the sensor can be used to define the sensor&#39;s resonant frequency. 
     Spiral trace sensor  24  with its inductance operatively coupled to its capacitance defines a magnetic field response sensor. In the presence of a time-varying magnetic field, spiral trace sensor  24  electrically oscillates at a resonant frequency that is dependent upon the capacitance, inductance and resistance of spiral trace sensor  24 . This oscillation occurs as the energy is harmonically transferred between the inductive portion of spiral trace sensor  24  (as magnetic energy) and the capacitive portion of sensor  24  (as electrical energy). In order to be readily detectable, the capacitance and inductance of spiral trace sensor  24  should be such that the amplitude of the sensor&#39;s harmonic response is at least 10 dB greater than any ambient noise where such harmonic response is being measured. 
     The application of the magnetic field to spiral trace sensor  24  as well as the reading of the induced harmonic response at a resonant frequency is accomplished by magnetic field response recorder  30 . The operating principles and construction details of recorder  30  are provided in U.S. Pat. Nos. 7,086,593 and 7,159,774, the contents of which are hereby incorporated by reference. Briefly, as shown in  FIG. 2 , magnetic field response recorder  30  includes a processor  32  and a broadband radio frequency (RF) antenna  34  capable of transmitting and receiving RF energy. Processor  32  includes algorithms embodied in software for controlling antenna  34  and for analyzing the RF signals received from the magnetic field response sensor defined ay spiral trace sensor  24 . On the transmission side, processor  32  modulates an input signal that is then supplied to antenna  34  so that antenna  34  produces either a broadband time-varying magnetic field or a single harmonic field. On the reception side, antenna  34  receives harmonic magnetic responses produced by spiral trace sensor  24 . Antenna  34  can be realized by two separate antennas or a single antenna that is switched between transmission and reception. 
     As mentioned above, both the width of the spiral trace sensor&#39;s conductive trace and the spacing between adjacent portions of the conductive trace can be uniform as shown in  FIG. 1 . However, the present invention is not so limited. For example,  FIG. 3  illustrates a spiral trace sensor  44  in which the width of the conductive trace is non-uniform while the spacing between adjacent portions of the conductive trace is uniform.  FIG. 4  illustrates a spiral trace sensor  54  in which the width of the conductive trace is uniform, but the spacing between adjacent portions of the conductive trace is non-uniform. Finally,  FIG. 5  illustrates a spiral trace sensor  64  having both a non-uniform width conductive trace and non-uniform spacing between adjacent portions of the conductive trace. 
     As described above, the length/width of the conductive trace and the spacing between adjacent portions of the conductive trace determine the capacitance, inductance and resistance (and, therefore, the resonant frequency) of a spiral trace sensor in the present invention. In addition, the sensor&#39;s resonant frequency can be modified by providing a dielectric material (i) between adjacent portions of the spiral trace sensor&#39;s conductive trace, and/or (ii) on top of the spiral trace. This is illustrated in  FIGS. 6A-6C  where a cross-sectional view of a sensor assembly in accordance with the present invention (e.g., sensor assembly  20  in  FIG. 1 ) has been modified by adding a dielectric material thereto. For example, in  FIG. 6A , a dielectric material  70  is added between the conductive traces of spiral trace sensor  24  in  FIG. 6B , a dielectric material  72  is overlaid as a layer on top of spiral trace sensor  24 . Finally, in  FIG. 6C , a dielectric material  74  is added between the conductive traces of spiral trace sensor  24  as well as top thereof so that substrate material  22  and dielectric material  74  effectively encase spiral trace sensor  24 . 
     The advantages of the present invention are numerous. The low profile magnetic field response sensor has a smaller profile than previous designs as no separate capacitor is required. This eliminates the need for a separate capacitor as well as the electrical connection between an inductor and capacitor. This makes the sensor easy to fabricate as a simple electrically-conductive trace. The resonant frequency response is easily adjusted by modifying the length of the spiral conductor. For example, a 24×18 inch spiral trace sensor with uniform 0.080 inch width traces and uniform spacing of 0.005 inches yields a response of 0.9 MHz. The resonant frequency was boosted to 6.0 MHz simply by cutting the circuit to a 5×9 inch area. Thus, the present invention is well suited to be manufactured to a standard size with subsequent simple modification for a specific application. 
     Previously-cited U.S. Pat. Nos. 7,086,593 and 7,159,774 discuss methods by which multiple sensors can be interrogated provided that they are within the magnetic field of the magnetic field response recorder and their response is large enough to be received by the response recorder. By means of inductive coupling, an arrangement of sensors discussed herein in close enough proximity to be inductively coupled to each other allows the measurement of each sensor to be interrogated by a magnetic field response recorder without the recorder&#39;s magnetic field directly interrogating each sensor. That is, just one sensor is powered directly by the recorder, and the recorder directly receives the response (for the whole arrangement) from this sensor. The remaining sensors in the arrangement are communicated via inductive coupling as their response is superimposed upon that of the sensor being powered and interrogated directly. Hence, the sensor being directly powered/interrogated has a response containing the resonant responses of all sensors in the arrangement that are inductively coupled thereto. Each response can be correlated to the magnitude of one or more physical quantities. Two simple sensing arrangements illustrating this concept are shown in  FIGS. 7A and 7B . 
       FIG. 7A  illustrates an arrangement  80  of spiral trace sensors  84 A- 84 E all aligned in a row. Response recorder  30  is positioned to power and receive responses from sensor  84 A. Because all the sensors are inductively coupled, their response will be superimposed upon the response of sensor  84 A via inductive coupling. Each sensor is designed so that its frequency does not overlap that of any other sensor. If any sensor in the array should have its response change (as a result of the change in a physical quantity that it is measuring), the change will manifest itself in the frequency response of sensor  84 A.  FIG. 7B  illustrates an arrangement  90  of spiral trace sensors  94 A- 94 G not aligned in a row. That is, the previously described approach of powering/interrogating an arrangement sensors via inductive coupling does not require that the sensors be aligned in any particular arrangement. The only requirement for interrogating the sensors via inductive coupling is that the relative position of the sensors remain fixed. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.