Magnetic field response measurement acquisition system Number:7,086,593 from the United States Patent and Trademark Office (PTO) owispatent

Title: Magnetic field response measurement acquisition system

Abstract: Magnetic field response sensors designed as passive inductor-capacitor circuits produce magnetic field responses whose harmonic frequencies correspond to states of physical properties for which the sensors measure. Power to the sensing element is acquired using Faraday induction. A radio frequency antenna produces the time varying magnetic field used for powering the sensor, as well as receiving the magnetic field response of the sensor. An interrogation architecture for discerning changes in sensor's response frequency, resistance and amplitude is integral to the method thus enabling a variety of measurements. Multiple sensors can be interrogated using this method, thus eliminating the need to have a data acquisition channel dedicated to each sensor. The method does not require the sensors to be in proximity to any form of acquisition hardware. A vast array of sensors can be used as interchangeable parts in an overall sensing system.

Patent Number: 7,086,593 Issued on 08/08/2006 to Woodard,   et al.

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Inventors:	Woodard; Stanley E. (Hampton, VA), Taylor; Bryant D. (Smithfield, VA), Shams; Qamar A. (Yorktown, VA), Fox, legal representative; Christopher L. (Yorktown, VA), Fox, legal representative; Melanie L. (Hayes, VA), Bryant; Robert G. (Lightfoot, VA), Fox, deceased; Robert L. (Hayes, VA)
Assignee:	The United States of America as represented by the Administrator of the National Aeronautics and Space Administration (Washington, DC) N/A (
Appl. No.:	10/839,445
Filed:	April 30, 2004

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Current U.S. Class:	235/449 ; 235/435
Current International Class:	G06K 7/08 (20060101)
Field of Search:	235/384,435,439,449

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References Cited [Referenced By]

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Primary Examiner: Stcyr; Daniel
Attorney, Agent or Firm: Edwards; Robin W.

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Government Interests

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in part by employees of the United States Government and may be manufactured and used by and for the Government of the United States for governmental purposes without the payment of any royalties thereon or therefore.

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Parent Case Text

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CLAIM OF BENEFIT OF PROVISIONAL APPLICATION

Pursuant to 35 U.S.C. .sctn. 119, the benefit of priority from provisional applications having U.S. Ser. Nos. 60/467,844, filed on Apr. 30, 2004; 60/467,840, filed on May 1, 2003; 60/467,841, filed on May 1, 2003; 60/467,113, filed on May 1, 2003; 60/467,839, filed on May 1, 2003; and 60/467,842 filed on May 1, 2003; 60/467,112, filed on May 1, 2003; and 60/467,194, filed May 1, 2003 is claimed for this nonprovisional application.

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Claims

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What is claimed is:

1. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor.

2. The acquisition system of claim 1, wherein said antenna means is a single switching antenna.

3. The acquisition system of claim 1, wherein said antenna means is separate transmission and receiving antennae.

4. The acquisition system of claim 1, wherein said interrogation means is portable.

5. The acquisition system of claim 1, wherein said interrogation means is handheld.

6. The acquisition system of claim 1, wherein said one or more sensors are selected from the group consisting of inductor-capacitor circuits powered by Faraday induction and inductor-capacitor-resistor circuits powered by Faraday induction.

7. The acquisition system of claim 6, wherein the response attributes of said sensors change corresponding to one or more unrelated physical states that said sensors measure.

8. The acquisition system of claim 6, wherein one or more sensors has a capacitor embedded in a conducting material and an inductor placed away from the surface of the conductive material.

9. The acquisition system of claim 1, wherein said system uses one or more of measured frequency, amplitude or bandwidth to determine one or more changes in sensor capacitor geometric, capacitor dielectric, inductor geometric, inductor permeability, inductor-antenna separation, inductor proximity to a conductive surface, inductor overlap of a conductive surface, identification of conductive properties and resistance.

10. The acquisition system of claim 1, wherein said antenna means are one or more broadband antennas.

11. The acquisition system of claim 1 having more than one sensor, wherein the range of measurement frequencies of said sensors are within the range of said antenna means but do not overlap.

12. The acquisition system of claim 1, wherein individual ranges of resonant frequencies correspond to the physical property values to be measured.

13. The acquisition system of claim 1, wherein said one or more sensors are embedded in material that is transmissive to radio frequency energy.

14. The acquisition system of claim 1, wherein said antenna means is a metallic foil.

15. The acquisition system of claim 1, wherein said antenna means is a thin film deposited on a dielectric membrane.

16. The acquisition system of claim 1, wherein said one or more sensors are fabricated using metal deposition.

17. The acquisition system of claim 1, wherein one or more said sensors is metamorphic.

18. The acquisition system of claim 1, wherein at least one said sensor simultaneously measures one or more unrelated physical states.

19. The acquisition system of claim 1, wherein at least one sensor measures material phase transition.

20. The acquisition system of claim 1, wherein at least one sensor measures one or more attributes selected from the group consisting of strain, fluid-level, proximity, displacement, shear, torsion, pressure, angular orientation, dielectric level, fluid level, solid particle level, material phase transition, moisture exposure, chemical exposure, stoichemetric changes, wear, bond separation, identification of conductive materials, relative place orientation, and displacement rate.

21. The acquisition system of claim 1, wherein at least one said sensor comprises one or more interdigital electrodes positioned such that said electrodes are parallel to a surface of wear.

22. The acquisition system of claim 1, wherein at least one said sensor comprises one or more interdigital electroplates.

23. The acquisition system of claim 22, wherein said sensor further comprises an element positioned between said electroplates, said elements selected from the group consisting of temperature sensitive dielectric, thermomagnetic, and phase transition dielectric.

24. The acquisition system of claim 1, wherein at least one said sensor comprises two parallel electroplates for displacement measurements.

25. The acquisition system of claim 1, wherein at least one said sensor comprises a dielectric affixed to a stationary electroplate for displacement measurements.

26. The acquisition system of claim 1, wherein at least one said sensor comprises n pairs of parallel electroplates separated by a dielectric medium, for fluid level measurement.

27. The acquisition system of claim 1, wherein at least one said sensor comprises two parallel electroplates separated by a dielectric medium, for fluid level measurement.

28. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor; wherein said antenna means is a single switching antenna, and further wherein said interrogation means comprises the following steps: (a) at the lower limit of a predetermined range, transmitting a radio frequency harmonic for a predetermined length of time from said antenna; (b) switching said transmission mode of said antenna off; (c) turning the receiving mode of said antenna on; (d) rectifying the received response from said sensor to determine its amplitude; (e) storing the amplitude, A.sub.i(t), of said rectified response and the frequency, .omega..sub.i(t), of said transmitted radio frequency harmonic; (f) switching the receiving mode off and the transmission mode on; (g) shifting the transmitted radio frequency harmonic by a predetermined amount; (h) transmitting the harmonic for a predetermined length of time; (i) switching the transmission mode off; (j) switching the receiving mode on; (k) rectifying the received response from said sensor to determine its amplitude; (1) storing said current amplitude, A.sub.i, and said frequency, .omega..sub.i; (m) comparing said amplitude, A.sub.i, to the two previously recorded amplitudes, A.sub.i-1 and A.sub.i-2; (n) if said previous amplitude, A.sub.i-1, is greater than said amplitude, A.sub.i, and the previous amplitude, A.sub.i-1, is greater than the amplitude prior to it, A.sub.i-2, storing said amplitude, A.sub.i-1, as the amplitude inflection and the corresponding frequency, .omega..sub.i-1, for the current frequency sweep; (o) comparing said amplitudes obtained in step (n) with the amplitudes of the next subsequent sweep; (p) repeating steps (f) through (l) if an amplitude inflection has not been reached; and (q) once amplitude inflection has been reached, continuing the sweep to said next sensor.

29. The acquisition system of claim 28, wherein the sweep rate for each said sensor is dependent on the rate of change of the physical state being measured.

30. The acquisition system of claim 28, wherein said sensors have one or more different resolutions.

31. The acquisition system of claim 28, wherein the resolution of one or more sensors is not fixed.

32. The acquisition system of claim 28, wherein dynamic measurements are obtained by comparing variation in one ore more responses selected from the group consisting of frequencies, of a current sweep with those of prior sweeps.

33. The acquisition system of claim 28, wherein a change in position of a sensor is obtained from comparison of amplitude variations of successive sweeps.

34. The acquisition system of claim 28, wherein said interrogation means determines amplitude, frequency and bandwidth variation with time.

35. The acquisition system of claim 28, wherein said first sweep determines all resonant frequencies and corresponding amplitudes.

36. The acquisition system of claim 28, wherein data is stored for the entire range, followed by peak amplitudes being determined for each said sensor, further wherein said peak amplitudes and corresponding frequencies are stored for comparisons to subsequent sweeps.

37. The acquisition system of claim 28, further comprising a data file corresponding to each said sensor, comprising said sensor type, response variation, frequency partition and measurement band for each said partition sweep after said resonant is identified on said initial sweep and further comprising a table that correlates response variation to a physical state, said data files for each said sensor concatenated to form an aggregate file.

38. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor; wherein said antenna means is separate transmission and receiving antennae; and further wberein said interrogation means comprises the following steps: (a) at the lower limit of a predetermined range, transmitting a radio frequency harmonic for a predetermined length of time from said antenna; (b) turning said transmission antenna; (c) turning said receiving antenna on; (d) rectifying the received response from said sensor to determine its amplitude; (e) storing the amplitude, A.sub.i(t), of said rectified response and the frequency, .omega..sub.i(t), of said transmitted radio frequency harmonic; (f) turning said receiving antenna off and transmission antenna on; (g) shifting the transmitted radio frequency harmonic by a predetermined amount; (h) transmitting the harmonic for a predetermined length of time; (i) turning said transmission antenna off; (j) turning said receiving antenna on; (k) rectifying the received response from said sensor to determine its amplitude; (l) storing said current amplitude, A.sub.i, and said frequency .omega..sub.i; (m) comparing said amplitude, A.sub.i, to the two previously recorded amplitudes, A.sub.i-1 and A.sub.i-2; (n) if said previous amplitude, A.sub.i-1, is greater than said amplitude, A.sub.i, and the previous amplitude, A.sub.i-1, is greater than the amplitude prior to it, A.sub.i-2, storing said amplitude, A.sub.i-1, as the amplitude inflection and the corresponding frequency, .omega..sub.i-1, for the current frequency sweep; (o) comparing said amplitudes obtained in step (n) with the amplitudes of the next subsequent sweep; (p) repeating steps (f) through (l) if an amplitude inflection has not been reached; and (q) once amplitude inflection has been reached, continuing the sweep to said next sensor.

39. The acquisition system of claim 38, wherein the sweep rate for each said sensor is dependent on the rate of change of the physical state being measured.

40. The acquisition system of claim 38, wherein said sensors have one or more different resolutions.

41. The acquisition system of claim 38, wherein the resolution of one or more sensors is not fixed.

42. The acquisition system of claim 38, wherein dynamic measurements are obtained by comparing variation in one ore more responses selected from the group consisting of frequencies, of a current sweep with those of prior sweeps.

43. The acquisition system of claim 38, wherein a change in position of a sensor is obtained from comparison of amplitude variations of successive sweeps.

44. The acquisition system of claim 38, wherein said interrogation means determines amplitude, frequency and bandwidth variation with time.

45. The acquisition system of claim 38, wherein said first sweep determines all resonant frequencies and corresponding amplitudes.

46. The acquisition system of claim 38, wherein data is stored for the entire range, followed by peak amplitudes being determined for each said sensor, further wherein said peak amplitudes and corresponding frequencies are stored for comparisons to subsequent sweeps.

47. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor, wherein said interrogation means comprises: an antenna for transmitting and receiving a varying magnetic field; a microcontroller that places said antenna into transmission mode and submits a binary code to a frequency synthesizer, said frequency synthesizer concerting said code into a square wave with the frequency of the wave dependent on said binary code; a high-speed amplifier that amplifies said square wave; a low pass filter that attenuates all frequencies that are higher than a prescribed frequency for application to said antenna for a prescribed number of cycles; applying said low pass filter signal to said antenna for a prescribed number of cycles; a radio frequency receiving/transmission switch for switching said antenna to receiving mode; a high speed amplifier that amplifies the signal from said sensor after it is received from said antenna; a diode peak detector that rectifies said amplified signal and creates a DC value proportional to signal amplitude; an op amp that amplifies said DC voltage from said peak detector; an analog to digital converter that converts said signal from said op amp to a digital signal; and said microcontroller storing the amplitude of digital signal and the transmission frequency.

48. The acquisition system of claim 47, comprising separate transmission and receiving antennae.

49. The acquisition system of claim 38, further comprising a data file corresponding to each said sensor, comprising said sensor type, response variation, frequency partition and measurement band for each said partition sweep after said resonant is identified on said initial sweep and further comprising a table that correlates response variation to a physical state, said data files for each said sensor concatenated to form an aggregate file.

50. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor; wherein at least one said sensor is mounted to a conductive surface, further wherein said sensor has an inductor that has a fixed separation from said conductive surface.

51. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consisting of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor; wherein at least one said sensor measures a physical state within a conductive cavity, further wherein the inductor of said sensor is mounted external to said cavity at a fixed distance and fixed orientation from said cavity wall, the capacitor of said sensor is mounted internal to said cavity, and said antenna is mounted external to said cavity.

52. A magnetic field response measurement acquisition system, comprising: one or more inductively powered magnetic field response sensors, wherein one or more attributes of the one or more sensor responses correspond to one or more measured unrelated physical states and further wherein said one or more attributes are selected from the group consistng of amplitude, frequency and bandwidth; antenna means for transmitting magnetic fields to power said one or more sensors and for receiving magnetic field responses from said one or more sensors; an interrogation means for regulating said magnetic field transmission from and reception to said antenna means, and for analyzing said one or more sensor response attributes received from said one or more sensors, wherein said interrogation means can interrogate multiple sensors concurrently using a single acquisition channel, does not require that said signals from said one or more sensors be transmitted as modulated signals on a radio frequency carrier, and can concurrently acquire measurements of more than one unrelated physical state from each said sensor; wherein multiple sensors measure multiple physical states within a conductive cavity, further wherein said inductors of said sensors are mounted external to said cavity at distances and fixed orientations from said cavity wall, said capacitors of said sensors are mounted internal to said cavity, and said antenna is mounted internal to said cavity.

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Description

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending, commonly owned patent application Ser. No. 10/839,448, filed Apr. 30, 2004, entitled "Magnetic Field Response Sensor for Conductive Media."

The invention described herein was made in part by employees of the United States Government and may be manufactured and used by and for the Government of the United States for governmental purposes without the payment of any royalities thereon or therefore.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a remote monitoring system. It relates in particular to a monitoring system comprising one or more sensors, which utilize L-C (inductance-capacitance) or L-C-R (inductance-capacitance-resistance) resonant circuits, in combination with an interrogation means, to monitor a variety of properties, including strain, temperature, pressure, identification, performance, chemical phase transition (such as melting and state-of-cure), fluid level, wear, rotation rate, location and proximity. The system eliminates the need for physical connection to a power source (i.e., no lead wires) or to data acquisition equipment, and allows for multiple measurements using a single acquisition channel. Additionally, it does not require that the sensors be in proximity to any form of acquisition hardware and it facilitates use of a portable handheld interrogation unit.

2. Description of the Related Art

A magnetic field response sensor is a passive inductor-capacitive circuit designed to change correspondingly with a change in the physical state that the sensor measures. Use of inductors and capacitors to form resonant circuits is established in the literature. See, for example, D. Halliday and R. Resnick, Fundamental of Physics, 2nd Edition, Wiley, New York, pp. 624 634 or similar basic physics or electronics texts. Wireless measurement acquisition systems that use existing sensors physically connected to a power source, microprocessor and transmitters are described in Woodard, S. E., Coffey, N. C., Gonzalez, G. A., Taylor, B. D., Brett, R. R., Woodman, K. L., Weathered, B. W. and Rollins, C. H., "Development and Flight Testing of an Adaptable Vehicle Health-Monitoring Architecture," Journal of Aircraft, Vol 1, No. 3, May June 2004, pp 531 539. A method of acquiring measurements without the need for physical connection to a power source is the use of radio frequency identification (RFID) tags. This method relies on the use of radio-frequency integrated circuits functionally coupled to sensors. Representative of patents covering RFID tags is U.S. Pat. No. 5,420,757. An example of a system for interrogating fluid level is that presented by Kochin et al. in U.S. Pat. No. 6,335,690, which teaches a preferred separation distance between the sensor and the interrogator of less than 3.5 cm. U.S. Pat. No. 6,111,520 (Allen) and Fonseca, M. A., English, J. M., Arx, M. V. Allen, M. G., "High Temperature Characterization of Ceramic Pressure Sensors,"Proceeding of 1999 IEEE MEMS Workshop, pp 146 149 discuss several methods of magnetic field response sensor interrogation having the sensors within the perimeter of the antenna used for interrogation. Planar or laminar designs of L-C circuits include rectangular inductors (e.g., U.S. Pat. No. 6,025,735), spiral inductors (e.g., U.S. Pat. No. 6,111,520), parallel place capacitors (e.g. U.S. Pat. No. 6,335,690) and interdigitated capacitors (e.g., see K. G. Ong and C. A. Gaines, Smart Materials Structure, (9) 2000; 421 428).

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a magnetic field response measurement acquisition system having increased interrogation antenna and sensor separation distance.

Another object is the interrogation of multiple sensors concurrently using a single acquisition channel.

Another object is to provide a magnetic field response measurement acquisition system having a portable interrogator.

An additional object is to provide a magnetic field response measurement acquisition system enabling the easy incorporation of additional sensors.

Another object is to provide a magnetic field response measurement acquisition system capable of acquiring more than one measurement from each sensing element.

A further object is to facilitate multiple measurements whose dynamic characteristics affect different attributes of the sensor's magnetic field response.

Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.

In accordance with the present invention, a magnetic field response wireless measurement acquisition system comprises an interrogator which may be portable and handheld, at least one inductively powered L-C sensor, and software to determine sensor properties (e.g., resonant frequency, bandwidth, amplitude, etc.). The interrogator and software can be used with L-C sensors that measure a variety of parameters, including temperature, pressure, strain, location, rotation rate, and other parameters. The sensors convey basic waveform information (e.g., frequency, bandwidth, etc.) that is dependent solely on the properties being measured, and do not require wide bandwidths to transmit modulated information. The sensors emit a single radio frequency (RF) transmission, thus there is no requirement that information be transmitted as a modulated signal on the RF carrier. As a result, the sensors can be designed to have a higher Q (i.e., narrower bandwidth) than existing wireless sensing systems. This higher Q sensor can be interrogated at a greater distance and at lower power than lower Q sensors. There is also potentially less interference from neighboring sensors and higher sensor densities. Additionally, simplified system architecture enables the interrogator to be built into a handheld unit. An algorithm quickly determines the characteristic sensor parameters in an efficient manner, not requiring storage of readings across a spectral range and subsequent analysis of the ordered pairs. A vast array of sensors can be used as interchangeable parts in an overall L-C sensing system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of an L-C measurement acquisition system in accordance with the present invention.

FIG. 2 is a schematic of magnetic field response sensor measurement bands.

FIG. 3 is a flowchart illustrating interrogation logic.

FIG. 4 is a graph of sensor response amplitude as excitation frequency approaches sensor resonant frequency.

FIG. 5 illustrates resistive response curves.

FIG. 6 is a schematic of the interrogation system.

FIG. 7 illustrates a sensor circuit.

FIG. 8 is a representative antenna.

FIGS. 9a and 9b are graphs of resistance measurements.

FIGS. 10a and 10b are graphs of inductance measurements.

FIGS. 11a and 11b are graphs of quality factor, Q.

FIG. 12 illustrates a square spiral inductor.

FIG. 13 is a graph of resistance versus inductor trace width.

FIG. 14 is a graph of quality factor, Q, versus inductor trace width.

FIG. 15 is an illustration of a sensor mounted on a conductive surface via a spacer.

FIG. 16 is an illustration of a sensor mounted on a conductive surface with the inductor projected away from the conductive surface.

FIG. 17 is a schematic of a conductive closed cavity sensor configuration.

FIG. 18 is a schematic of a sensor for a conductive closed cavity.

FIG. 19 is a schematic of a conductive cavity with antenna and multiple sensors located internal to the cavity.

FIG. 20a is a schematic of a sensor embodiment for phase transition and strain measurement.

FIG. 20b illustrates a sensor embodiment that can be used to distinguish parts during curing

FIG. 21 is a graph of time history of sensor response during resin curing.

FIG. 22 is a schematic of a sensor embodiment for wear or thermal measuring utilizing interdigital electrodes.

FIG. 23 illustrates an embodiment of an interdigital device with one of the electrodes having a temperature sensitive dielectric or a dielectric which has a phase transition when exposed to excessive temperature

FIG. 24 illustrates a sensor embodiment for wear or thermal measurement having the inductor embedded within the capacitor.

FIG. 25 illustrates a sensor embodiment for wear or thermal measurement having the inductor mounted externally.

FIG. 26 a sensor embodiment for wear or thermal measurement having a sensor embedded in a cube.

FIG. 27 illustrates interdigital electroplates.

FIG. 28 illustrates an embodiment of interdigital electroplates with temperature sensitive dielectric, thermomagnetic or a phase transition dielectric between the electroplates.

FIG. 29 illustrates a capacitor with a negative electroplate that translates perpendicular to its surface and a stationary plate.

FIG. 30 illustrates an embodiment of a sensor for displacement measurements.

FIG. 31 is a first graph of capacitor variation with displacement.

FIG. 32 is a second graph of capacitor variation with displacement.

FIG. 33 illustrates a second embodiment of a sensor for displacement measurements.

FIG. 34 illustrates a third embodiment of a sensor for displacement measurements.

FIG. 35 is a graph showing capacitance variation with displacement.

FIG. 36 illustrates a fourth embodiment of a sensor for displacement measurements.

FIG. 37 illustrates electroplates and dielectric medium for a first embodiment of a sensor for fluid level measurements.

FIG. 38 illustrates a first embodiment of a sensor for fluid level measurements.

FIG. 39 illustrates electroplates having residual fluid film.

FIG. 40 illustrates n pair of parallel electroplates and dielectric medium for a second embodiment of a sensor for fluid level measurements.

FIG. 41 illustrates a second embodiment of a sensor for fluid level measurements.

FIG. 42 illustrates a third embodiment of a sensor for fluid level measurements.

FIG. 43 illustrates a cross section of interdigital capacitor with electric field.

FIG. 44 illustrates a dielectric medium in contact with electrode pairs.

FIG. 45 illustrates electrodes with a thin residual film.

FIG. 46 is a graph of frequency measurement for two fluid level sensors.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, an embodiment of a magnetic field response measurement acquisition system in accordance with the present invention is shown and referenced generally by numeral 10. Acquisition system 10 will first be described in terms of a general overview with the aid of FIG. 1.

Radio Frequency (RF) broadband antenna 12 transmits and receives RF energy. Processor 14 regulates the RF transmission and reception. Processor 14 includes algorithms embodied in software for controlling the antenna 12 and for analyzing the RF signals received from the one or more magnetic field response sensors 16. Sensors 16 are passive inductor-capacitor L-C circuits or inductor-capacitor-resistor L-C-R circuits. Each inductor L is placed in parallel with a capacitor C, forming an L-C(p) circuit. Processor 14 modulates the input signal to the antenna 12 to produce either a broadband time-varying magnetic field or a single harmonic magnetic field. The variable magnetic field creates an electrical current in the sensors 16 as a result of Faraday induction. Each sensor 16 will electrically oscillate at resonant electrical frequencies that are dependent upon the capacitance and inductance of each sensor 16. The oscillation occurs as the energy is harmonically transferred between the inductor (as magnetic energy) and capacitor (as electrical energy). When the energy is in the inductors, the magnetic fields produced are single harmonic radio frequencies whose frequencies are the respective sensor 16 resonant frequencies, and are dependent on how the physical measured property changes the capacitance of the circuit. The antenna 12 is also used to receive the harmonic magnetic responses produced by the inductors. The receiving antenna can be the same antenna used to produce the initial broadcast of energy received by the L-C circuit or another antenna can be used. When the same antenna is used, it must be switched from a transmitting antenna to a receiving antenna. A simple microprocessor can be used to identify the frequencies of the signals received by the antenna 12. The measured frequencies are then correlated to measurement of physical states.

As illustrated in FIG. 2, the sensors 16 are designed such that their range of measurement frequencies do not overlap, but are within a frequency range of the antenna 12. The individual ranges of resonant frequencies correspond to physical property values that can be measured. The capacitors are designed such that, when electrically coupled to the inductors, their range of values will be a predetermined partition of the RF frequency band. This method allows for any number of sensors 16 within the range of the antenna 12 to be interrogated concurrently.

The use of magnetic field sensors 16 and the measurement architecture of the present invention greatly reduces measurement acquisition complexity. The magnetic field response sensor 16 is a passive inductor-capacitive circuit designed to change correspondingly with a change in the physical state that the sensor 16 measures, and acquires power via Faraday induction. Sensing is provided by measuring resonant frequency shifts due to changes in inductance or capacitance, requiring no batteries. The harmonic magnetic field response of the inductor serves as a means of transmitting the resonant. Key attributes of the magnetic field response are amplitude, frequency and bandwidth. The sensors 16 can be designed such that one of the attributes varies correspondingly with the measured physical state. A RF antenna can produce the time varying magnetic field used for the Faraday induction, as well as receive the magnetic fields of the the sensor 16. The use of magnetic fields for powering the sensors 16 and for acquiring the measurements from the sensors 16 eliminates the need for physical connection from the sensor 16 to a power source and data acquisition equipment. The architecture also eliminates the need to have a data acquisition channel dedicated to each sensor 16. Multiple concurrent measurements can be accomplished with a single acquisition channel and multiple sensors, each with a different resonant frequency, can be probed by the broadband antenna 12.

Capacitor geometric, capacitor dielectric, inductor geometric or inductor permeability changes of a sensor will result in magnetic field response frequency change. Any resistive change will result in a response bandwidth change. Dielectric variations (e.g., due to the presence of chemical species or due to a material phase transition) to the capacitor can be designed for specific measurements. Further, a resistive element whose resistance changes with a physical parameter can also be placed in circuits of fixed capacitance and inductance. Hence, the system has the potential for acquiring many different types of measurements. Because the sensors' 16 functionality is based upon magnetic fields, they have potential use at cryogenic temperatures, extremely hot temperatures, harsh chemical environments and radiative environments.

When a sensor's 16 inductor comes in proximity to a conductive material, energy is lost in the sensor due to eddy currents being produced in the conductive material. As the sensor is brought closer to the material, the response amplitude decreases while the response frequency increases. Hence, this effect can be used to determine proximity to conductive surfaces. Otherwise, it is necessary to maintain a fixed separation. If capacitance and inductance are fixed, changes to a sensor's 16 orientation or position with respect to interrogating antenna 12 changes response amplitude. The interrogation system of the present invention allows for the acquisition of measurements from any magnetic field response sensor 16 developed to exploit the aforementioned phenomena. The system also allows for autonomous sensor interrogation, analysis of collected response to value of physical state and comparison of current measurements with prior measurements to produce dynamic measurements.

The measurement acquisition method can be used to acquire measurements even when the sensor 16 is embedded in material that is transmissive to the RF energy that interrogates the sensor 16. An advantage of this method is that the components for the method can be non-obtrusively added to the vehicle/system for which it is being used. An antenna 12 can be produced as a metallic foil or as metal deposited on a thin dielectric film. Either aforementioned version of the antenna 12 can be mounted to an existing bulkhead or other structural component. For some applications, sensors 16 can be fabricated using metal deposition methods. Metal deposition can be used to add sensors to a vehicle/structure during manufacturing. Other advantages of the method include (1) no line of sight being required between the antenna 12 and sensor 16, (2) the ability of the entire sensor 16 to be embedded in a nonconductive material, (3) the ability to embed the capacitive element in a conducting material with the inductive element being placed away from the surface of the conductive material, (4) no specific orientation of the sensor 16 with respect to the antenna 12 is required except that they cannot be 90 degrees to one other, and (5) no wiring is required to add new measurements, only a partition of a RF bandwidth used in the measurement spectrum and a frequency/measurement correlation table.

Interrogation

Interrogation utilizes a scan-listen-compare technique, which allows for high signal-to-noise ratio. FIG. 3 illustrates the interrogation logic. Separate transmission and receiving antennae can be used or a single switching antenna can be used. Using two antennae provides a larger volumetric swath at which measurements can be taken, which is approximately double that of a single antenna. The interrogation procedure generally comprises the following steps: (a) At the lower limit of a predetermined range, a radio frequency harmonic is transmitted for a predetermined length of time and then the transmission mode is swtiched off (i.e., the transmission antenna is turned off if two antennae are used or, if a single antenna is used, it ceases transmission). (b) The receiving mode is then turned on (i.e., the receiving antenna is turned on if two antennae are used or, if a single antenna is used, it begins receiving). The received response from the sensor 16 is rectified to determine its amplitude. The amplitude, A.sub.i(t), and frequency, .omega..sub.i(t), are stored in memory. (c) The receiving mode is turned off and the transmission mode is turned on. The transmitted radio frequency harmonic is then shifted by a predetermined amount. The harmonic is transmitted for a predetermined length of time and then the transmission mode is turned off. (d) The receiving mode is turned on. The received response from the sensor 16 is rectified to determine its amplitude. The amplitude, A.sub.i, and frequency, .omega..sub.i, are stored in memory. (e) The current amplitude, A.sub.i, is compared to the two previously attained (recorded) amplitudes, A.sub.i-1 and A.sub.i-2. If the previous amplitude, A.sub.i-1, is greater than the current amplitude, A.sub.i, and the previous amplitude A.sub.i-1 is greater than amplitude prior to it, A.sub.i-2, the previous amplitude, A.sub.i-1, is the amplitude inflection. The amplitude inflection occurs when the excitation harmonic is equal to the resonant frequency of the sensor 16. The amplitude, A.sub.i-1, and the corresponding frequency, .omega..sub.i-1, are stored for the sensor 16 for the current frequency sweep. These values can be compared to the values aquired during the next sweep. If an amplitude inflection has not been identified, then steps (c) and (d) are repeated. (f) If amplitude inflection has been identified, the harmonic sweep continues to the next sensor 16.

FIG. 2 illustrates three antenna sweeps for n sensors 16. The initial frequency sweep can be used to identify and catalog (store) all key response attributes (resonant amplitudes and frequencies) associated with all n sensors 16 within the antenna's 12 range of interrogation. If a particular sensor 16 is resistive, its bandwidth will also be stored. The cataloged resonant amplitudes and frequencies for all sensors 16 can be used to reduce the sweep time for successive sweeps. For example, the next sweep to update each resonant frequency can start and end at a predetermined proximity to the cataloged resonant and then skip to the next resonant. Every sensor 16 does not need to be interrogated during each successive sweep. The interrogation rate for each sensor 16 should be dependent upon the rate that the physical state that sensor 16 measures changes. FIG. 2 illustrates interrogation of sensor 21 and sensor n during the second sweep. Sensors 21, 22 and 23 have frequency, bandwidth or amplitude changes corresponding to variations in their measured physical states. Sensor n only has amplitude variations corresponding to either a displacement or rotation measurement.

Measurement resolution is also depicted in FIG. 2. Each sensor 21, 22, 23 . . . n need not have the same resolution nor fixed resolution (e.g., sensor 23). The interrogation range of sensor 21 is reduced to be within a few frequency increments of the measurement acquired during the previous sweep. Dynamic measurements can also be produced by comparing variation in frequencies and amplitudes current sweeps with those of the prior sweeps. For example, if capacitance and inductance are fixed and if the circuit follows a known trajectory (e.g., displacement of a lever), the change in position of the sensor 16 is known by comparing the amplitude variations of successive sweeps. The method requires calibration to ascertain inductor magnetic response amplitude dependency to position from antenna 12 (i.e., (A(d))). The calibration correlates response amplitude with distance from the antenna 12. The time between measurements is .DELTA.T. Hence, displacement rate is derived as displacement rate=[d(A(sweep 1))-d(A(sweep 2))]/.DELTA.T.

Similarly, dynamic strain measurements can be determined by comparing the frequencies of successive amplitudes. The measurement system can also be used to identify an amplitude threshold at a set frequency. This is indicitive of a certain antenna-inductor separation. If motion is rotary, the rate that the threshold is exceeded (number of times during a fixed duration) is indicative of rotation rate.

The sweep of individual frequencies is used because it concentrates all energy used to excite the sensor 16 at a single frequency. FIG. 4 depicts a sensor's 16 response amplitude as the excitation frequency approaches the sensor's 16 resonant frequency. During each frequency sweep for each sensor 16 range, the current, A.sub.i, and previous two amplitudes (A.sub.i-1 and A.sub.i-2) and frequencies are stored. The amplitudes are compared to identify the amplitude inflection. The frequency at which the amplitude inflection occurs is the resonant frequency. The purpose of the initial sweep is to ascertain all resonant frequencies and their corresponding amplitudes. Frequencies and amplitude values of successive sweeps can be compared to previous sweeps to ascertain if there is any change to a measured property or if the sensor 16 has moved with respect to the antenna 12. If the physical state has changed, the resonant frequency will be different from the prior sweep. If a sensor 16 has moved with respect to the antenna 12, the amplitudes will be different (frequency will remain constant). The magnitude and sign of the difference can be used to determine how fast the sensor 16 is moving and whether the sensor 16 is moving toward the antenna 12 or away from the antenna 12.

The interrogation logic can be extended to allow for resistive measurements. Once the resonant frequency and its respective amplitude for a sensor 16 have been identified, the amplitude at a fixed frequency shift prior to the resonant is then acquired. The resistance is inversely proportional to the difference of the amplitudes. Resistive variations can be discerned using only two points of the magnetic field response curve. The bandwidth of the response is proportional to the circuit resistance. However, to measure bandwidth, it is necessary to identify the response peak and then measure the response curve on either side of