Wireless open-circuit in-plane strain and displacement sensor requiring no electrical connections

A wireless in-plane strain and displacement sensor includes an electrical conductor fixedly coupled to a substrate subject to strain conditions. The electrical conductor is shaped between its ends for storage of an electric field and a magnetic field, and remains electrically unconnected to define an unconnected open-circuit having inductance and capacitance. In the presence of a time-varying magnetic field, the electrical conductor so-shaped resonates to generate harmonic electric and magnetic field responses. The sensor also includes at least one electrically unconnected electrode having an end and a free portion extending from the end thereof. The end of each electrode is fixedly coupled to the substrate and the free portion thereof remains unencumbered and spaced apart from a portion of the electrical conductor so-shaped. More specifically, at least some of the free portion is disposed at a location lying within the magnetic field response generated by the electrical conductor. A motion guidance structure is slidingly engaged with each electrode's free portion in order to maintain each free portion parallel to the electrical conductor so-shaped.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to wireless electrical devices. More specifically, the invention is a wireless in-plane strain and displacement sensor requiring no electrical connections.

2. Description of the Related Art

Electrical devices typically utilize a plurality of circuit elements wired together to form a circuit. As is well understood in the art, such electrical devices function for a designed purpose when electric current flows through the circuit. If an unwanted break occurs in the circuit, electric current ceases to flow and the circuit must be repaired or replaced to restore device function. Circuit repair or replacement causes downtime, requires manpower, and can be expensive.

In addition, electrical circuits typically use solder to connect circuit elements to one another. The use of solder poses a number of problems. Solder increases the cost of electrical devices and requires the use of venting and air filtration systems during fabrication due to the toxic nature of solder. Further, the high heat required to melt solder can stress or damage circuit boards, and the presence of toxic solder also poses waste issues when old electrical circuits must be disposed of or recycled. For all of these reasons, the typical electrical device has a number of inherent flaws.

One type of electrical device used in monitoring the “health” of structures (e.g., dynamic structures such as aircraft and other vehicles, static structures such as buildings and bridges, etc.) is known as an electrical strain sensor. An electrical strain sensor directly or indirectly relates any mechanical strain to a change in an electrical response. One of the earliest strain gauge designs used a foil of electrically conductive material. When stretched within a material's elastic limits, the foil's resistance increases as the material's longer and narrower shape increases its electrical resistance. When the material is compressed, it becomes shorter and wider thus decreasing the electrical resistance. Strain is directly proportional to the ratio of change in resistance as compared to the resistance of the sensor when it is not deformed. This property is used to make a strain gauge that requires the strain sensor (i.e., the foil) to be directly electrically connected to a resistance measuring circuit such as a Wheatstone bridge.

Other types of electrical strain sensors include capacitive strain sensors, fiber optic strain sensors, and piezoelectric strain sensors. Capacitive strain sensors the displacement between capacitive plates or between neighboring interdigital electrodes. Similar to resistive strain sensors, strain is directly proportional to the ratio of change in capacitance relative to the non-deformed-sensor capacitance. Fiber optics sensors use Bragg gratings that alter the wavelength at which light is reflected and/or transmitted through the fiber. During strain, the grating separation distance changes thus changing the Bragg wavelength (reflected wavelength). The change in wavelength is correlated to strain. The direct optical change can be related to an electrical signal using optoelectronics. A piezoelectric strain sensor uses the changing resistivity of a semiconductor caused by applied strain. All of the above sensors require being part of closed electrical circuits for power and interrogation. Further, because solder and printed circuit boards are typically used to make closed circuits for the sensors discussed above, any reliability, hazardous material, and waste issues associated with solder directly affect them.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a strain sensor requiring no electrical connections.

In accordance with the present invention, a wireless in-plane strain and displacement sensor includes an electrical conductor fixedly coupled to a substrate subject to strain conditions. The electrical conductor has first and second ends and is shaped between the first and second ends for storage of an electric field and a magnetic field. The first and second ends remain electrically unconnected such that the electrical conductor so-shaped defines an unconnected open-circuit having inductance and capacitance. In the presence of a time-varying magnetic field, the electrical conductor so-shaped resonates to generate harmonic electric and magnetic field responses, each of which has a frequency, amplitude and bandwidth associated therewith. The sensor also includes at least one electrically unconnected electrode having an end and a free portion extending from the end thereof. The end of each electrode is fixedly coupled to the substrate and the free portion thereof remains unencumbered. The free portion is also parallel to and spaced apart from a portion of the electrical conductor so-shaped. More specifically, at least some of the free portion is disposed at a location lying within the magnetic field response generated by the electrical conductor. A motion guidance structure is slidingly engaged with each electrode's free portion in order to maintain each free portion parallel to the electrical conductor so-shaped.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and more particularly toFIGS. 1A and 1B, a wireless in-plane strain and displacement sensor in accordance with an embodiment of the present invention is shown and is referenced generally by numeral100. The illustrated sensor100is presented as an exemplary embodiment as there will be many possible embodiments that can be constructed based on the basic principles of the present invention without departing from the scope thereof. In the illustrated embodiment, sensor100includes a pattern10of electrically conductive material and an electrically unconnected electrode20spaced apart from pattern10.

Electrical conductor pattern10is any electrical conductor (e.g., wire, run, thin-film trace, etc.) that can be shaped to form an open-circuit pattern that can store an electric field and a magnetic field. Pattern10is a single-component open-circuit element with no electrical connections being made thereto. The term “open-circuit pattern” as used herein means that the conductor has two ends that remain electrically unconnected so that the resulting conductor pattern is an electrical open-circuit having inductance and capacitance attributes.

Pattern10can be made from an electrically-conductive run or thin-film trace that can be deposited directly onto or embedded within a substrate material30such that pattern10is fixed to substrate30. Dashed lines are used for substrate30to indicate substrate30does not form part of the present invention. Generally, substrate30is an electrically insulating and non-conductive material. If not, some electrically non-conductive material (e.g., an adhesive, a mounting black, etc.) will be disposed between pattern10and substrate30. In either case, substrate30is a material structure (e.g., dynamic structure, static structure, etc.) that is expected to undergo strain to be sensed by sensor100. Thus, the particular substrate/embedding material structure can vary without departing from the scope of the present invention. Although not a requirement of the present invention, the portion of substrate30on which pattern10is deposited is typically planar. Techniques used to deposit pattern10directly onto substrate30can be any conventional, metal-conductor deposition process to include thin-film fabrication techniques. As will be explained further below, pattern10can be constructed to have a uniform or non-uniform width, and/or uniform or non-uniform spacing between adjacent portions of the pattern's runs/traces.

The basic features of pattern10and the principles of operation for sensor100will be explained for a spiral-shaped conductor pattern. However, it is to be understood that the present invention could be practiced using other geometrically-patterned conductors provided the pattern has the attributes described herein. The basic features of a spiral-shaped conductor that can function as pattern10are described in detail in U.S. Patent Publication No. 2007/0181683, the contents of which are hereby incorporated by reference in their entirety. For purpose of a complete description of the present invention, the relevant portions of this publication will be repeated herein.

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 pattern. However, in the present invention, pattern10exploits parasitic capacitance. The capacitance of pattern10is operatively coupled with the pattern's inductance such that magnetic and electrical energy can be stored and exchanged by the pattern thereby creating a damped simple harmonic resonator. Since other geometric patterns of a conductor could also provide such a magnetic/electrical energy storage and exchange, it is to be understood that the present invention could be realized using any such geometrically-patterned conductor and is not limited to a spiral-shaped pattern.

The amount of inductance along any portion of a conductive run of pattern10is directly related to the length thereof and inversely related to the width thereof. The amount of capacitance between portions of adjacent conductive runs of pattern10is directly related to the length by which the runs overlap each other and is inversely related to the spacing between the adjacent conductive runs. The amount of resistance along any portion of a conductive run of pattern10is directly related to the length and inversely related to the width of the portion. Total capacitance, total inductance, and total resistance for a spiral pattern are determined simply by adding the effective contributions due to individual portions of the pattern. For example, the effective inductance contribution of a trace portion is the resultant change in the total inductance of pattern10due to the changes in the pattern's distributed self-inductance and distributed mutual inductance due to the addition of the trace. The effective capacitance contribution of a trace portion is the resulting change in the capacitance of pattern10due to the addition of the trace portion as a result of the charge in the portion creating electric fields with the charges in other parts of pattern increasing the total distributed capacitance. The geometries of the various portions of the conductive runs of the pattern can be used to define the pattern's resonant frequency.

Pattern10with its distributed inductance operatively coupled to its distributed capacitance defines a magnetic field response sensor. In the presence of a time-varying magnetic field, pattern10electrically oscillates at a resonant frequency that is dependent upon the capacitance, inductance and resistance of pattern10. This oscillation occurs as the energy in the magnetic field along the length of pattern10is harmonically transferred to the electric field between parallel portions of pattern10. That is, when excited by a time-varying magnetic field, pattern10resonates a harmonic electric field and a harmonic magnetic field with each field being defined by a frequency, amplitude, and bandwidth.

The application of an oscillating magnetic field to pattern10as well as the reading of the induced harmonic response at a resonant frequency can be accomplished by a magnetic field response recorder. The operating principles and construction details of such a recorder are provided in U.S. Pat. Nos. 7,086,593 and 7,159,774, the contents of which are hereby incorporated by reference in their entirety. Briefly, as shown inFIG. 2, a magnetic field response recorder50includes a processor52and a broadband radio frequency (RE) antenna54capable of transmitting and receiving RE energy. Processor includes algorithms embodied in software for controlling a antenna and for analyzing the RF signals received from the magnetic field response sensor defined by pattern10. On the transmission side, processor52modulates an input signal that is then supplied to antenna54so that antenna54produces either a broadband time-varying magnetic field or a single harmonic field. On the reception side, antenna54receives harmonic magnetic responses produced by pattern10. Antenna54can be realized by two separate antennas or a single antenna that is switched between transmission and reception.

Referring again toFIGS. 1A and 1B, electrode20is representative of one or more electrical conductors having no electrical connections made thereto (i.e., it is electrically unconnected) and capable of supporting movement of electrical charges therein. In terms of the in-plane strain and displacement sensor of the present invention, electrode20has one end20A electrically insulated from and fixedly coupled (e.g., using a mounting block or adhesive as indicated by reference numeral22) to substrate30with the remaining part20B of electrode20being unencumbered to its opposing end200. At least some of unencumbered part20B overlaps and is spaced-apart from pattern10at a location that lies within the magnetic field response (not shown) generated by pattern10when pattern10is wirelessly excited by, for example, recorder50as explained above. That is, some of unencumbered part20B of electrode20overlaps a portion of pattern10at some non-zero angle (e.g., 90° in the illustrated embodiment). To keep unencumbered part20B properly coupled to the magnetic field response of pattern10, unencumbered part20B (and generally all of electrode20) is maintained parallel to pattern10at a selected distance therefrom. With just end20A fixed to substrate30, elongation or compression strain (indicated by two-headed arrow40) experienced by substrate30will cause electrode20to move relative to pattern10. To insure that unencumbered part20B remains spaced apart and parallel to pattern10during strain-induced movement of electrode20, an electrically non-conductive support housing24cooperates with unencumbered part20B. For example, housing24can be fixedly coupled to substrate30and function as a sleeve with an opening24A (FIG. 1B) formed all the way through housing24. Opening24A would be sized for the sliding engagement of unencumbered part203. Note that inFIG. 1A, the top portion of housing24is not shown to more clearly illustrate unencumbered part203. It is to be understood that housing24could be replaced by any number of support devices/mechanisms that allowed unencumbered part203to move as described above.

For purpose of the present invention, electrode20must support the hi-directional movement of electric charges therealong. For a linear (or substantially linear) electrode such as electrode20, the charges should move along the length of electrode20. The use of such charge movement in a wireless electrical device is disclosed in U.S. Patent Publication No. 2010/0109818, the contents of which are hereby incorporated by reference in their entirety. In accordance with the teachings of this patent publication, electrode20should have a length-to-width aspect ratio (i.e., length divided by width) that is large enough such that the effects of linear movement of electric charges along the length of electrode20outweigh the effects of eddy currents in electrode20when electrode20is positioned in the magnetic field response of pattern10. The length-to-width aspect ratio of electrode20will typically be designed to satisfy a particular sensor's performance criteria. Accordingly, it is to be understood that the particular length-to-width aspect ratio of electrode20is not a limitation of the present invention.

In operation, when Pattern10is exposed to a time-varying magnetic field (e.g., as generated by recorder50, a moving magnet, or any other source/method that generates an oscillating magnetic field), pattern10resonates harmonic electric and magnetic fields. The generated magnetic field is generally spatially larger than the generated electric field. At least some of unencumbered part20B of electrode20is positioned relative to pattern10such that it will lie with at least the generated magnetic field.

In the presence of a time-varying magnetic field, pattern10resonates to generate harmonic electric and magnetic field responses. With electrode20configured and positioned as described above, the magnetic field response of pattern10generates an electromotive force in electrode20such that electric charges flow linearly in both directions along the length of electrode20as indicated by two-headed arrow26. Note that the current flow in electrode20by linear charge flow26is achieved without any electrical contact with (i) pattern10, (ii) electrode20, or (iii) between pattern10, electrode20and antenna54.

In general, for fixed excitation conditions, the magnetic; field response frequency, amplitude, and bandwidth of pattern10are dependent upon the electric conductivity of any material placed within its magnetic field and electric field. As mentioned above, the conductive material area of electrode20defines a relatively large length-to-width aspect ratio. In this way, electrode20is electrically powered via oscillating harmonics from pattern10. In addition, electrode20has a magnetic field formed along its length due to the current created in the electrode20that is coupled to that of pattern10. The charge on the electrode20will result in an electric field between the charge on pattern10and electrode20. Therefore, electrode20and the overlapped portions of pattern10will behave somewhat like capacitor plates in a closed electrical circuit except electrode20also has a current that creates a magnetic field that is also coupled to the magnetic field of pattern10. The magnetic field on electrode20increases as the spacing between electrode20and pattern10decreases because electrode20is exposed to a higher magnetic strength.

If the magnetic field of electrode20is oriented 90° with respect to the overlapped portion of pattern10, any destructive interference between electrode20and pattern10should vanish. Accordingly, if the relative positions and orientations of pattern10with respect to electrode20remain fixed (i.e., there is no strain being experienced by substrate30), then the magnetic field response of sensor100remains unchanged for fixed excitation conditions. These fixed conditions and resulting magnetic field response of sensor100define a baseline frequency, amplitude, and bandwidth response for sensor100that is recorded prior to using sensor100.

Changes in the baseline response of sensor100will occur wherever linear charge flow26changes. This will happen if the amount of overlap between pattern10and electrode20changes due to elongation or compression strain40. The change in charge flow26causes a change in at least one of the frequency, amplitude and bandwidth response of sensor100with respect to the baseline response of sensor100. The frequency response ω of pattern10changes with the amount that electrode20overlaps pattern10in accordance with the relationship ω=1/2π(sqrt(LC)) where the inductance L and capacitance C of pattern10are functions of the position of electrode20. Accordingly, if electrode20shifts by an amount Δx and the overall length of sensor100is x, the frequency response ω of pattern10will deviate from its baseline frequency. Since strain is defined by Δx/x and x is known as a baseline attribute of sensor100, the frequency response of pattern10is indicative of both the amount of displacement Δx as well as the in-plane strain experienced by substrate30. Once the baseline response of sensor100is known and sensor100is placed in use, interrogation or monitoring of electrical device100(for changes in response relative to the baseline response) can be carried out continuously, periodically, on-demand, etc., without departing from the scope of the present invention.

As mentioned above, both the width of the pattern's conductive runs/traces and the spacing between adjacent portions of the conductive runs/traces can be uniform as in the illustrated embodiment. However, the present invention is not so limited. For example, a spiral pattern's conductive trace width could be non-uniform while the spacing between adjacent portions of the conductive trace could be uniform. Another possibility is that the spiral pattern's conductive trace width could be uniform, but the spacing between adjacent portions of the conductive trace could be non-uniform. Still, further, the spiral pattern's conductive trace width could be non-uniform and the spacing between adjacent portions of the conductive trace could be non-uniform.

A variety of electrode configurations can also be used without departing from the scope of the present invention. For example, although a single electrode has been shown in the above-described embodiment, the present invention is not so limited. Accordingly, the embodiment inFIG. 3illustrates the use of a number of electrodes20with at least some of their corresponding unencumbered parts205overlapping pattern10. Once again, the top portion of support housing24is not shown to more clearly illustrate unencumbered parts205. The greater number of electrodes20produces a greater response sensitivity so that smaller amounts of in-plane strain and displacement can be discerned.

The advantages of the present invention are numerous. The sensor is a passive open-circuit device that significantly reduces manufacturing cost. No solder connections are needed to form the sensor. Therefore, the sensor can be completely recyclable. The sensor uses only two components and no physical or electrical connections between the components are required thereby making the sensor inherently more reliable then any device that depends upon connections between components. For example, the sensor could be powered and interrogated after most damage events. The sensor can be placed on a system during any phase of fabrication or use. If placed on a component or in a mold of a non-conductive component, the sensor could also be used to track the component during manufacturing.

The sensor could also be used as a human implanted sensor, e.g., incorporated into hip or joint replacements. This has many benefits over what is currently being done in that all the advantages above apply and the sensor could be wirelessly powered and interrogated external, to the body, i.e., no electrical connections or leads are placed inside the body. Further, no surgery would be necessary to discern if there is any damage to the sensor.

What is claimed as new and desired to be secured by Letters Patent of the United States is: