GB2496136A

GB2496136A - Passive capacitive moisture detector

Info

Publication number: GB2496136A
Application number: GB201118839A
Authority: GB
Inventors: Harvey John Burd; David Edwards
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2011-11-01
Filing date: 2011-11-01
Publication date: 2013-05-08
Also published as: WO2013064829A2; WO2013064829A3; GB201118839D0

Abstract

A sensor comprises a sensor resonator member 110 configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency, and a capacitive element 140 coupled to the sensor resonator member, the capacitive element comprising first and second capacitor plates 144 configured to be deposited in a granular material such that a portion of the granular material interposed between the plates forms a dielectric layer 142 of the capacitive element, wherein the sensor is configured such that the resonant frequency is dependent on a capacitance of the capacitive element. The member 110 comprises inductive element 120 and capacitor 130. The sensor may be located near buried pipes to locate leaks such that changes in moisture content of the material 142 affect the resonant frequency.

Description

PASSIVE FLUID DETECTOR

The present invention relates to inductively excited resonators and to a method of passively detecting the presence of fluids using inductively excited resonators. In particular, but not exclusively, the present invention relates to the detection of fluid around buried assets using inductively excited resonators.

BACKGROUND

Determining the location and identity of a buried asset can be a challenging task.

Traditionally, determination of the location may be performed by systematically digging holes until the asset is found. More recently, ground penetrating radar (GPR) has been used in order to locate a buried asset based on a signal reflected by the asset. (Reference to GPR includes radiation having a frequency in the range of from around 200MHz to around 1GHz.

Other frequencies are also useful).

However, assets made of certain materials may not provide a strong enough reflected signal to allow the location of the asset to be clearly identified. Furthermore, radiation can be reflected by a number of features of a volume of ground, including variations in moisture content, solids composition, the presence of wildlife, and voids formed for example by tunnelling wildlife. Thus it can be difficult to reliably identify a location of a buried asset using GFR.

In WO 2009/101450, which is incorporated herein in its entirety by reference, a technique has been described that allows such assets to be tagged using a resonant radar reflector assembly. The described resonant radar reflector assembly includes one or more resonant radar reflector members arranged to reflect radiation in the GPR frequency range, so as to provide a clear reflected signal that can be used to identify the location of the buried asset.

Furthermore, by combining resonant reflector members having different associated resonant frequencies, each asset may be identified by the combination of frequencies reflected.

Thus, using the tagging technique described in WO 20091101450 the presence of a specific buried asset can be detected, and its location more easily determined.

Common types of buried asset which may need to be located include fluid carrying pipes such as water pipes. A common scenario in which it is necessary to locate such an asset is in the event of the asset requiring maintenance such as to fix a leak.

While the above described techniques allow the location of a buried asset to be determined, pinpointing an area associated with a leak may sometimes require a return to the traditional technique of systematically digging holes along the length of the asset to determine the site of the leak, which is inefficient and may lead to significant disruption.

Prior art techniques for detecting and locating a leak site along a water pipe are available and include: placing a conductor along the interior of the pipe and monitoring changes in earth loop electrical resistance; acoustic/vibration detectors that measure vibrations caused by fluid leaking from the pipe; and the use of a distributed network of pressure sensors along the pipe. However, these techniques require active monitoring of the pipe and may provide a relatively inaccurate estimate of the location of the leak.

It is an aim of at least some embodiments of the present invention to at least partly mitigate one or more problems associated with the prior art.

It is an aim of certain embodiments of the present invention to enable the location of a buried asset, and also the location of a site of a leak from the buried asset, to be more easily determined.

BRIEF SUMMARY OF THE DISCLOSURE

According to a first aspect of the invention, there is provided a sensor comprising a sensor resonator member configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency, and a capacitive element coupled to the sensor resonator member, the capacitive element comprising first and second capacitor plates configured to be deposited in a granular material such that a portion of the granular material interposed between the first and second capacitor plates forms a dielectric layer of the capacitive element, wherein the sensor is configured such that the resonant frequency is dependent on a capacitance of the capacitive element.

According to some embodiments, the capacitance of the capacitive element may be arranged to be dependent on a water content of the granular material. The first and second capacitor plates may be configured to be deposited in a regolith. and wherein a portion of the regolith interposed between the first and second capacitor plates forms the dielectric layer of the capacitive element.

The sensor may further comprise an inductive element and a further capacitive element, and the capacitive element may be connected in parallel with the further capacitive element. The inductive element and the further capacitive element may be embedded in an electrically insulating medium, and the first and second capacitor plates may be disposed external to the electrically insulating medium.

According to some embodiments, the sensor may further comprise a reference resonant member in addition to the sensor resonant member.

According to a further aspect of the invention, there is provided a method of detecting a change in water content level in a granular material, the method comprising providing a sensor comprising a sensor resonator member configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency and a capacitive element coupled to the sensor member, the capacitive element comprising first and second capacitor plates, depositing the first and second capacitor plates in the granular material such that a portion of the granular material interposed between the first and second capacitor plates forms a dielectric layer of the capacitive element, exposing the sensor to a radio frequency electromagnetic field emission including radiation of a frequency corresponding to the resonant frequency of the sensor material, and detecting an

electromagnetic field caused by the sensor.

Further advantages of the present invention will be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter by way of example only with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a water sensitive inductive element according to some embodiments of the invention; Figure 2 shows a pipe having a sensor resonant assembly according to an embodiment of the invention; and Figure 3 shows a pipe having a plurality of sensor assemblies provided thereon.

DETAILED DESCRIPTION

Embodiments of the present invention provide a method and apparatus for directly detecting a location of both a buried asset and the site of a leak associated with the buried asset.

Figure 1 schematically illustrates an inductively excited resonator member 110 in accordance with embodiments of the invention. The resonator member 110 comprises an inductive element 120 coupled to a first capacitive element 130, the inductive element 120 and the tirst capacitive element 130 being isolated from the environment as signified by enclosure 150.

The resonator member 110 further comprises a second capacitive element 140 connected in parallel with the first capacitive element 130 and located outside of the enclosure 150. The second capacitive element 140 comprises two insulated plates or members 144A, 144B configured such that soil or other subsurface material can occupy the space between the plates or members of the second capacitive element 140 when buried. In some embodiments, the insulated plates or members 144A, 144B may comprise insulated rods or bars.

For a resonator member 110 affixed to a buried asset, such as a water pipe, the soil or other subsurface material occupying the space between the insulated plates 144A, 144B forms a dielectric layer 142 for the second capacitive element 140. In the event of a water leak near the resonator member 110, the soil in the vicinity of the resonator member 110, including the soil forming the dielectric layer 142 of the second capacitive element 140, will experience an increase in water content. As the water content of the soil changes the dielectric constant of the soil also changes causing the capacitance of the second capacitive element 140 to change.

As the inductive element 120 and the first capacitive element 130 are isolated from the environment within enclosure 150, the change in water content in the soil surrounding the resonator member 110 has no effect on the reactance of these elements.

Thus, an increase in the water content of the soil between the insulated plates 144A, 144B can be expected to lead to an increase in the dielectric constant for the dielectric layer 142, and therefore an increase in the capacitance of the second capacitive element 140.

As mentioned above, the first and second capacitive elements are connected in parallel, and therefore the combined capacitance of the first and second capacitive members 130, 140 is determined by adding their capacitance values. This means that a change in the capacitance of the second capacitive element 140 results in a change in the combined capacitance of the first and second capacitive elements.

The resonant member 110 will resonate at a resonant frequency when excited by a suitable signal dependent on the values of inductance of the inductive member 120 and the combined capacitance of the first and second capacitive elements. Thus, the combined effect of the inductive member 120 and the two capacitive members causes the resonant frequency of the device to be dependent on the water content of the soil. A change in resonant frequency of the resonant member 110, or tag, can therefore be used to identify the presence of elevated moisture levels in the vicinity of the resonant member 110.

Furthermore, in some embodiments, a magnitude of the change in frequency may be used to estimate, or determine, a magnitude of a change in moisture level which may be used to localise a water leak.

By specifying the capacitance values of the fixed first capacitive element 130 and the second capacitive element 140, it is possible to control the magnitude of the shift in resonant frequency for the resonant member 110 when moisture levels around the resonant member change. For example, by choosing suitable values for the capacitance of the first capacitive element and the second capacitive element, the range of frequencies at which the resonant member 110 will resonate can be limited to within a certain desired frequency window.

Once a leak has been fixed, the soil or other subsurface material used as the dielectric 142 of the second capacitive element 140 will dry out naturally, returning the moisture levels in the soil to a more normal level, and thereby reversing the effect on the resonant frequency of the resonant member 110. Thus, the tag can be re-used when the leak is fixed.

Figure 2 shows a scenario in which an underground asset (a water pipe 160) has been provided with a resonator assembly 100 according to an embodiment of the invention.

Interrogation apparatus 180 having a transmitter element 182 and a receiver element 184 has been arranged to induce an RE signal in the vacinity of the pipe 160 and to detect a signal reflected by the resonator assembly 100.

The resonator assembly 100 is provided with a resonant member 110. The interrogation apparatus 180 is arranged to iiradiate the resonator assembly with an RF field of a frequency corresponding to the resonant frequency of the resonator member 110.

By way of example, Figure 3 shows a pipe 160 having four resonator assemblies 301, 302, 303, 304 as described above provided thereon. In the embodiment of Figure 3, the resonator assemblies are substantially equally spaced apart along the pipe. In alternative embodiments, the resonator assemblies are not substantially equally spaced apart along the pipe. The water pipe of Figure 3 further comprises a leak site 120 located between first and second resonator assemblies 301 302.

Over time, some of the contents of the pipe 160 will escape from the leak site 120 into the surrounding soil or subsurface material raising the water content in the soil in the vicinity of the leak site 120. As the contents of the pipe continue to escape from the leak site 120, soil comprising a dielectric for the resonator assemblies nearest the leak site will become increasingly damp, and the resonant frequency of those resonator assemblies will change. In the example shown in Figure 3, the first and second resonator assemblies 301, 302 will be the first to be effected by the leak. As the leak of the contents of the pipe 160 continues, further resonator assemblies may be affected.

Furthermore, those resonator assemblies nearest to the leak site 120 will experience the greatest change in water content in the surrounding soil, and can therefore be expected to show the largest change in resonant frequency in response to the leak.

Thus, by measuring the resonant response of the resonator assemblies along the length of the pipe, the presence of a leak from the pipe 160 can be determined by detecting a change in resonant frequency for one or more resonator assemblies. Furthermore, the location of the leak site 120 can be localised to a region along the length of the pipe 160 by determining a region containing one or more resonator assemblies 301, 302 having the greatest change in resonant frequency.

In some embodiments, rather than being substantially equally spaced apart along the pipe 160, resonator assemblies may be concentrated around areas of interest, such as joints or valves, at which a leak may be expected to form.

According to some embodiments, each resonator assembly may comprise at least two resonant members. A sensor resonant member including a second capacitive member as described above, and sensitive to the water content in the surrounding soil, and a reference resonant member isolated from the environment and having a fixed, reference, resonant frequency.

In some embodiments, the reference resonant frequency is arranged to be substantially the same as the resonant frequency of the sensor resonant member when no elevated water content levels are present. Thus, when interrogated by the interrogation apparatus 180 the reference resonant member and the sensor resonant member will each return substantially the same frequency.

However, upon elevation of water content levels in the soil around the sensor member, for example due to water leaking from the pipe 160, the resonant frequency of the sensor member will change relative to that of the reference member sufficiently to allow the interrogation apparatus 180 to detect a difference between the resonant frequencies of the sensor member and the reference member.

In some embodiments, the reference member may be configured to have a known reference resonant frequency different from the resonant frequency of the sensor member in the absence of elevated moisture levels. The interrogation apparatus 180 may then be configured to determine a frequency offset between the reference resonant frequency and the resonant frequency of the sensor.

In each case, the presence of a reference member allows detection of elevated water content levels in the soil by detecting relative changes in resonant frequency of a sensor member with respect to the reference member, rather than requiring an absolute change in resonant frequency of one or more sensor members to be detected. In some embodiments, this has the advantage of enabling smaller changes in resonant frequency to be detected, thereby increasing a sensitivity ot a detection system to the presence of elevated moisture levels.

A further advantage of the presence of a reference member is that in some embodiments changes in an environment that result in a change in the resonant frequency of a sensor member but which are not due to changes in moisture levels in the surrounding soil, for example changes in temperature, are automatically compensated for.

According to some embodiments, the reference member and the sensor member may be configured to have axes of polarisation that are normal to each other to thereby allow the reference member and sensor member to be interrogated independently while operating at similar resonant frequencies.

According to some embodiments, multiple inductive elements may be provided in each sensor assembly, with each inductive element configured to resonate at a distinct frequency.

Such an arrangement would allow different sensor assemblies to have different frequency signatures' based on the resonant frequencies associated with each sensor assembly, thereby allowing individual sensor assemblies to be identified remotely by the interrogation apparatus 180.

The above embodiments have been described in the context of a buried resonator assembly in which soil or other subsurface material is present between the insulated plates 144A, 144B of the second capacitive member 140 to form the dielectric 142. However, it will be recognized that embodiments of the invention may equally be applied to any material that is able to act as a dielectric 142 between the insulated plates 144A, 144B. For example, sensors according to some embodiments may be used to detect elevated moisture levels in stored granular materials, such as cereals or other foodstuffs.

Furthermore, sensors according to embodiments of the invention could be disposed in the ground beneath building foundations to allow changes in water content in the underlying soil to be monitored. For example, it is well known that certain soils, such as clay soils, expand and contract due to changes in water content, and therefore monitoring the water content of the soil may assist in the diagnosis of seasonal structural movements.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Claims

<claim-text>CLAIMS: 1. A sensor comprising: a sensor resonator member configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency; and a capacitive element coupled to the sensor resonator member, the capacitive element comprising first and second capacitor plates configured to be deposited in a granular material such that a portion of the granular material interposed between the first and second capacitor plates forms a dielectric layer of the capacitive element; wherein the sensor is configured such that the resonant frequency is dependent on a capacitance of the capacitive element.</claim-text> <claim-text>2. The sensor of claim 1, wherein the capacitance of the capacitive element is dependent on a water content of the granular material.</claim-text> <claim-text>3. The sensor of claim 1 or claim 2, wherein the first and second capacitor plates are configured to be deposited in a regolith. and wherein a portion of the regolith interposed between the first and second capacitor plates forms the dielectric layer of the capacitive element.</claim-text> <claim-text>4. The sensor of any preceding claim, further comprising an inductive element and a further capacitive element.</claim-text> <claim-text>5. The sensor of claim 4, wherein the capacitive element is connected in parallel with the further capacitive element.</claim-text> <claim-text>6. The sensor of claim 4 or claim 5, wherein the inductive element and the further capacitive element are embedded in an electrically insulating medium, and wherein the first and second capacitor plates are external to the electrically insulating medium.</claim-text> <claim-text>7. The sensor of any preceding claim, further comprising a reference resonant member in addition to the sensor resonant member.</claim-text> <claim-text>8. A method of detecting a change in water content level in a granular material, the method comprising: providing a sensor comprising a sensor resonator member configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency and a capacitive element coupled to the sensor member, the capacitive element comprising first and second capacitor plates; depositing the first and second capacitor plates in the granular material such that a portion of the granular material interposed between the first and second capacitor plates forms a dielectric layer of the capacitive element; exposing the sensor to a radio frequency electromagnetic field emission including radiation of a frequency corresponding to the resonant frequency of the sensor material; and detecting an electromagnetic field caused by the sensor.</claim-text> <claim-text>9. A sensor substantially as hereinbefore described with reference to the accompanying drawings.</claim-text> <claim-text>10. A method substantially as hereinbefore described with reference to the accompanying drawings.</claim-text>