Transmission line strain sensor

Abstract

A sample for use in determining strain, the sample comprising: a substrate, wherein the external surface of the substrate is partially, or fully, coated in a conductive to form a transmission line, the coated substrate being configured to be connected to electronic equipment such that the strain on the sample is determinable by applying and measuring a signal to the coated substrate using the electronic equipment.

Description

[0001] Transmission Line Strain Sensor

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a sample for use in a strain sensor and methods of measuring strain on a transmission line. Particularly, but not exclusively, it relates to a transmission line strain sensor for use on geosynthetics in aggressive environments such as in sub-soil applications.

[0004] BACKGROUND

[0005] It is known to use a vibrating wire strain gauge for civil engineering applications. Such sensors have issues related to their comparatively large volume, low durability in harsh environments and difficulty bonding to certain materials, for example, geosynthetics.

[0006] The use of geosynthetics is increasingly widespread due to their utility in enhancing the safety of designs and reducing material consumption. Despite their increased prevalence, the true mechanical response of such geosynthetics once installed is largely unknown due to their use being largely limited to sub-soil applications where direct observation is challenging. Use of typical sensor packages such as foil strain gauges have been limited in utility as they significantly alter the geosynthetic-soil interaction characteristics and face issues in bonding to the geosynthetic. Such sensors also have limited utility in live engineering projects due to their limited resilience to environmental conditions such as moisture, pressure, and abrasion. Additionally, laboratory based testing on geosynthetic responses are limited in usefulness as it is difficult to accurately represent the conditions and demands placed on the geosynthetic on live full-scale construction projects.

[0007] Current technologies for measuring strain in geosynthetics include strain gauges, optical fibres and extensometers where these devices are attached / bonded to a geosynthetic layer. Such devices have issues in terms of their reliability when the device is placed in soil and due to the bonding nature of the devices with the geosynthetic can change the characteristic properties of the geosynthetics. i Existing strain sensors for geosynthetics use an additional strain sensing cable which can be attached to the geosynthetic using glue. The glue can cause a deviation in the results of the strain sensor thus making such technologies unreliable.

[0008] Other strain sensing solutions include the application of a conductive filler to provide the geosynthetic with conductivity properties which can be measured. Doping may be used to provide such fillers. However, this changes the inherent properties of the geosynthetic so that measurements of strain may not be suitable for geosynthetics with such doping.

[0009] Accordingly, the ability to measure strain in harsh environments is desirable to improve design methodologies and to provide safer and more economic structures.

[0010] An object of the present invention is to mitigate some of the deficiencies of the prior art mentioned above.

[0011] SUMMARY OF THE INVENTION

[0012] Aspects and embodiments of the invention provide a sample capable of working as a strain sensor when used with electronic equipment. In particular, the described sample is able to operate in harsh environments such as in sub-soil applications. This is particularly advantageous for use in geosynthetics to monitor the strain of geogrids installed in soil. The sample comprises a substrate (such as a geogrid) coated in a conductive coating. This conductive coating does not alter the samplesoil interaction characteristics thus providing more accurate results. The claimed invention allows for improved design methodologies, and safer and more economic structures. Furthermore, the use of live monitoring systems allows for early identification of faults in samples installed in harsh environments which may be difficult to monitor which will subsequently facilitate the pre-emptive maintenance of the structure, leading to cost savings and enhanced safety.

[0013] According to an aspect of the invention there is provided a sample for use in determining strain, the sample comprising: a substrate, wherein the external surface of the substrate is partially, or fully, coated in a conductive coating to form a transmission line, the coated substrate being configured to be connected to electronic equipment such that the strain on the sample is determinable by applying and measuring a signal to the coated substrate using the electronic equipment.

[0014] For example, the transmission line may be a parallel plate-like or waveguide-like transmission line. The term transmission line refers to a medium which the transmission and distribution of electric power occurs. The transmission lines discussed in the present case are intended for installation in earth materials such as soils and rocks. They are not for use in overhead power lines.

[0015] The sample is able to work as a self-contained component of a strain sensor without altering the properties of the substrate and whilst only requiring electronic equipment to supply a signal.

[0016] Optionally, wherein the substrate comprises geosynthetic and / or the sample comprises a geogrid rib. This is particularly useful in geosynthetic applications.

[0017] Optionally, wherein the conductive coating is a conductive flex paint. This is advantageous as it adheres well to the substrate, is easy to apply and functions well as a carrier for the signal. Furthermore, the use of flex paint prevents cracks in the coating. Alternatively, wherein the conductive coating is a conductive epoxy resin, conductive epoxy paint, or conductive silicon.

[0018] Optionally, wherein the coating is applied with the sample in tension.

[0019] Optionally, wherein the coated substrate is configured to be connected to electronic equipment by comprising connectors, such as radio frequency connectors, conductively or wirelessly coupled to the coated substrate. This enables connection of the sample to electronic equipment, such as a signal generator or network analyser.

[0020] There is also provided a strain sensor comprising the sample, wherein the coated substrate is connected to the electronic equipment. Optionally, wherein the electronic equipment comprises a signal generator and oscilloscope. Optionally, wherein the electronic equipment comprises a network analyser. Optionally, wherein the electronic equipment is connected via the connectors. There is also provided a sample for use in determining strain, the sample comprising: a substrate; and a conductive resiliently deformable material applied to the substrate, wherein the conductive resiliently deformable material is configured to be connected to electronic equipment such that the strain on the sample is determinable by applying and measuring a signal to the conductive resiliently deformable material using the electronic equipment.

[0021] Advantageously, the sample is able to operate in harsh environments and can be used as part of a strain sensor to determine the amount of strain imparted on a sample.

[0022] Optionally, wherein the conductive resiliently deformable material comprises a conductive mesh. This allows the sample to be manufactured in a more robust and repeatable manner. The mesh is also highly compatible with fibre-based geosynthetics as the mesh can be embedded into the fibre-based structure of the geosynthetic. The mesh provides a high conductivity, signal integrity and redundancy.

[0023] Optionally, wherein the conductive mesh comprises a plurality of fibres and the plurality of fibres are placed at an angle relative to the longitudinal axis of the sample. Preferably, wherein the angle is 45°. Having the fibres at an angle allows for much greater flexibility and extension.

[0024] Optionally, wherein the resiliently deformable material is applied to the substrate using an adhesive.

[0025] Optionally, wherein the resiliently deformable material is applied to the substrate using one or more connectors.

[0026] Optionally, wherein the resiliently deformable material is applied to the substrate using heat and / or pressure to embed the resiliently deformable material within the substrate.

[0027] Optionally, wherein the conductive resiliently deformable material is applied with the sample in tension. This ensures that the conductive resiliently deformable material is in contact with the substrate at a neutral sample state which subsequently ensures high conductivity and signal integrity. Optionally, wherein the conductive resiliently deformable material is configured to be connected to electronic equipment by comprising connectors conductively or wirelessly coupled to the coated substrate.

[0028] Optionally, wherein the conductive resiliently deformable material is connected to the electronic equipment.

[0029] Optionally, wherein the electronic equipment is connected via the connectors.

[0030] Optionally, wherein the electronic equipment comprises a signal generator and oscilloscope.

[0031] Optionally, wherein the electronic equipment comprises a network analyser.

[0032] Optionally, wherein the substrate comprises geosynthetic and / or the sample comprises a geogrid rib.

[0033] There is also provided a method of measuring the strain through a sample comprising a transmission line, using electronic equipment connected to the transmission line, the method comprising: supplying, using the electronic equipment, a signal to the transmission line; applying, using the electronic equipment, a sweep of frequencies to the transmission line; measuring, using the electronic equipment, at least one propagation characteristic of the signal in the transmission line at each frequency of the sweep of frequencies; and determining, using the electronic equipment, the strain on the transmission line based on the measured at least one propagation characteristic.

[0034] The frequency signals may have the same or different amplitudes and phases.

[0035] There is also provided a method of measuring the strain through a sample comprising a transmission line, using electronic equipment connected to the transmission line, the method comprising: supplying, using the electronic equipment, a signal to the transmission line; applying, using the electronic equipment, a sweep of frequencies to the transmission line; measuring, using the electronic equipment, the phase of the signal in the transmission line at each frequency of the sweep of frequencies; and determining, using the electronic equipment, the strain on the transmission line based at least partly on the measured phase.

[0036] In particular, the phase characteristic method is easy to detect in real lines and is less prone to random effects.

[0037] The sweep of frequencies may be a broad spectrum sweep of frequencies.

[0038] The sample can comprise a coating to form a waveguide for RF signal propagation which allows measurement of propagation characteristics (phase, attenuation etc). As described above, a conductive mesh can be used in lieu of the conductive coating to form a waveguide.

[0039] Optionally, wherein the electronic equipment is used to measure one or more propagation characteristics in addition to the phase of the signal in the transmission line and the strain is additionally determined based on the measured propagation characteristics.

[0040] Optionally, wherein the one or more propagation characteristics includes the amplitude of the signal.

[0041] Optionally, wherein the supplied signal sent for at least some of the swept frequencies is an electrical sinusoidal signal, the method further comprising: short or open circuiting the transmission line; forming standing waves in the short or open circuited transmission line; measuring, using the electronic equipment, a change in the at least one propagation characteristic of the signal in the transmission line by measuring changes to the standing waves; and determining, using the electronic equipment, the strain on the transmission line based on the measured change in the at least one propagation characteristic.

[0042] Optionally, wherein the supplied signal sent for at least some of the swept frequencies is an electrical sinusoidal signal, the method further comprising: short or open circuiting the transmission line; forming standing waves in the short or open circuited transmission line; measuring, using the electronic equipment, a change in the phase of the signal in the transmission line by measuring changes to the standing waves; and determining, using the electronic equipment, the strain on the transmission line based on the measured change in the phase. The above methods enable the strain to be determined whilst the strain sensor is installed in harsh environments and is sensitive to both low and high strains.

[0043] Optionally, wherein the electronic equipment is used to measure a change in one or more propagation characteristics in addition to a change in the phase of the signal in the transmission line and the strain is additionally determined based on the measured change in the one or more propagation characteristics.

[0044] Optionally, wherein the one or more propagation characteristics includes the amplitude of the signal.

[0045] Optionally, wherein the supplied signal is an electrical sinusoidal signal for all of the swept frequencies.

[0046] There is also provided a method of measuring the strain through a sample comprising a transmission line using electronic equipment connected to the transmission line, the method comprising the steps of: applying, using the electronic equipment, electrical pulses to the transmission line; measuring, using the electronic equipment, at least one propagation characteristic of the transmission line, wherein the at least one propagation characteristic is the transmission time and / or shape of the electrical pulses through the transmission line; and determining, using the electronic equipment and signal processing, the strain on the transmission line based on the measured at least one propagation characteristic.

[0047] This method enables the strain to be determined whilst the strain sensor is installed in harsh environments and is sensitive to both low and high strains.

[0048] Optionally, further comprising calibrating the transmission line by measuring at least one calibration propagation characteristic of the transmission line before strain is applied to the transmission line.

[0049] Optionally, wherein the at least one propagation characteristic and the at least one calibration propagation characteristic are the same kind of propagation characteristic. Optionally, wherein the strain on the transmission line is determined based on a comparison of the at least one propagation characteristic and the at least one calibration propagation characteristic.

[0050] Optionally, wherein the sample is or has one or more of the features of the sample or strain sensor discussed above.

[0051] Other aspects of the invention will be apparent from the appended claim set.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0054] Figure la is an image of a sample according to an aspect of the invention;

[0055] Figure lb is a schematic of the sample of Figure la;

[0056] Figure 2 is a schematic representation of a strain sensor arrangement according to an aspect of the invention;

[0057] Figure 3 is a flowchart of the process of determining strain according to an aspect of the invention;

[0058] Figure 4 is a plot of fundamental coefficient against line voltage for use in determining strain according to an aspect of the invention;

[0059] Figure 5 is a plot of signal frequency against signal strength for use in determining strain according to an aspect of the invention;

[0060] Figure 6 is a plot of signal frequency against phase for use in determining strain according to an aspect of the invention; and

[0061] Figure 7 is a flowchart of the process of determining strain according to an aspect of the invention.

[0062] Figure 8a is a side view of a schematic representation of a sample according to an aspect of the invention.

[0063] Figure 8b is a plan view of a schematic representation of the sample shown in

[0064] Figure 8a.

[0065] Figure 9a shows a mesh aligned orthogonally to the longitudinal axis of the sample according to an aspect of the invention.

[0066] Figure 9b shows a mesh aligned at an angle to the longitudinal axis of the sample according to an aspect of the invention. DETAILED DESCRIPTION

[0067] The present invention provides a sample 100 such as that shown in Figures la and lb which is configured to operate in harsh environments. Such a sample can be used as part of a strain sensor to determine the amount of strain imparted on the sample. This will be discussed further with reference to Figure 2.

[0068] The sample 100 has a substrate 101 coated on its external surface in a conductive substance to act as a transmission line 104. The coating is applied partially or completely to the external surface.

[0069] In the illustrated example, the sample 100 is a geogrid rib used for geosynthetic applications. The conductive substance may be a coating of conductive paint. Advantageously, the conductive paint has good adhering characteristics, is easy to apply and functions well as a carrier. The conductive paint can be a conductive flex paint to prevent cracks in the paint coating and improve the survivability of the sample 100. The paint can be applied with a form of masking so that it can be applied in a straight line or other shape appropriate for acting as a transmission line, and for example prevent the variations or discontinuities in the transmission line 104 that might be caused if the paint went beyond one surface around the edges of substrate 101. Alternatively, the conductive coating may be formed from conductive silicon. The conductive coating may alternatively be a conductive epoxy resin or paint. The conductive coating is applied to both faces of the geogrid. The coating may be acrylic.

[0070] Masking is used to avoid premature short circuiting between the layers of paint arising from paint overspill over the edge of the sample 100. Alternatively, or additionally, a sharp scalpel blade (or similar) can be used to remove any paint overspill.

[0071] Optionally, the conductive coating could be applied to parallel ribs of the geogrid for additional sensing capabilities.

[0072] The coating can be applied to the sample whilst it is in light tension, for example conductive flex paint can be allowed to cure with the sample (geogrid rib) in tension. The paint is applied under tension for ease of manufacturing and improves the accuracy of applying the conductive substance. The tension also mitigates detrimental effects such as flexural cracking or delamination. When the sample 100 is a geogrid, applying under tension is advantageous as geogrids are produced commercially under slight tension.

[0073] Before applying the conductive substance, a spray adhesive coating may be applied to the substrate 101 to provide abrasion resistance. Such an adhesive does not fundamentally change the structure of the substrate 101 itself. As an example, the spray adhesive may be a polyolefin adhesion promotor. This additionally improves adhesion strength. Abrasion resistance may also or alternatively be provided using a spray coating, such as a polyurethane conformal coating, applied after the conductive substance.

[0074] The substrate 101 may additionally or alternatively be treated with plasma under vacuum or cold atmospheric conditions to increase the surface free energy of the substrate and to produce more durable coatings of the conductive substance. Plasma treating also cleans the surfaces of the substrate 101 and improves adhesion of coatings.

[0075] The adhesion of the conductive substance, such as conductive paint, to the substrate can be improved under vacuum and atmospheric pressure.

[0076] As shown in Figures la and lb, the illustrated sample 100 further comprises connectors 102 to facilitate connection of the sample to electronic equipment. Connectors 102 are electrically connected to the sample 100. Any connection method could be used so long as they are electromagnetically coupled (e.g., connectors 102 could be soldered to the sample 100). The connectors 102 are radio frequency, RF, connectors such as SMA (SubMiniature version A) connectors.

[0077] Optionally, wireless coupling could be used.

[0078] The sample 100 in accordance with Figures la and lb is configured such that electronic equipment can be connected to the sample 100 and the strain on the sample can be determined.

[0079] The sample 100 is resilient against environmental moisture both in terms of electrical survivability and transmission efficacy. Thus, the system is capable of sensing strain in extreme moisture conditions. In particular, the sample is operable at high confining pressures (e.g. 200kPa) and high soil water contents and capable of continued sensing despite damage which may occur under such conditions.

[0080] Figure 2 is a schematic representation of a strain sensor 200 including the sample 100 of Figures la and lb.

[0081] The sample 100 of Figure 2 shows the conductive coating 204 over the surface of the substrate 101 to form the sample 100. The connectors 102 are connected to electronic equipment 208 via connections 206.

[0082] Connections 206 are cables, preferably coaxial cables to facilitate a connection between the connectors 102 of the sample 100 and the electronic equipment 208. Two connections 206 are shown for illustration purposes only. The skilled person understands that one connection, or more than two connections may be used to achieve the same purpose and can be placed appropriately. As an example, connections 206 can be directly mounted to the sample 100 with one at each end of the conductive substance.

[0083] The electronic equipment 208 is used to supply a signal to the sample 100 via connections 206. The electronic equipment 208 can be any commercially available equipment known to the skilled person to provide signals. The electronic equipment 208 includes a signal generator and oscilloscope, and alternatively may be a vector network analyser.

[0084] Advantageously, the strain sensor 200 is capable of surviving and sensing strain under confining pressure.

[0085] The strain sensor 200 can be used to determine the strain acting on the sample 100. This is described further with reference to Figures 3 to 7.

[0086] Figure 3 is a flowchart of the process of determining strain in the transmission line by applying a sweep of frequencies to the transmission line.

[0087] The process of Figure 3 can be used in combination with the sample 100 of Figures la and lb and the strain sensor 200 of Figure 2. A transmission line is connected to electrical equipment as discussed in relation to Figure 2 to form a strain sensor 200. The strain sensor 200 may comprise the sample 100 as part of the transmission line.

[0088] In a first step 302, the transmission line is terminated with a load. The termination can include conditions where the transmission line is short or open circuited. The transmission line could also be matched or sandwiched. Where the transmission line is matched, the impedance of the transmission line is matched to the network analyser. This provides no reflection of the signal. Where the transmission line is sandwiched, a transmission line of different impedance is placed between lengths of the same impedance.

[0089] A sweep of frequencies is then applied to the transmission line (either short circuited / open circuited / matched / sandwiched) at step 304 to form standing waves in the transmission line. Frequencies ranging from 0Hz to 40GHz can be applied.

[0090] At each frequency of the broad spectrum sweep of frequencies, a propagation characteristic of the transmission line is measured at step 306.

[0091] The propagation characteristic is the phase or voltage amplitude of the signal in the transmission line. In some cases, both the voltage amplitude and the phase of the signal in the transmission line may be measured to provide more accurate strain sensing by employing both measurement techniques.

[0092] Measuring the phase of the transmission line is particularly advantageous as it is less prone to random effects. Further, there is no change in the signal magnitude as, for the ideal case of a lossless transmission line, there is no attenuation or dispersion. Consequently, the delay of the pulse / sinewave (phase shift) caused by the change in length of the line is advantageous for measurements as it is affected by fewer variables introducing random noise.

[0093] Once the propagation characteristic (voltage and / or phase) of the signal in the transmission line is measured for each frequency of the sweep of frequencies, the strain on the transmission line is determined at step 308. The strain is determined based on the measured voltage and / or phase values. Considering the case where the propagation characteristic is the voltage amplitude of the transmission line, a plot of frequency against transmission line voltage amplitude / signal strength can be generated to determine the strain on the transmission line.

[0094] For a transmission line of a fixed length and electrical properties, standing waves will form when the length of the line is a1 / 4 of the signal wavelength (referred to as the "fundamental frequency"). For an open circuited line, the voltage at the source is minimum while at the output end of the transmission line is at maximum.

[0095] When the signal frequency is increased to 2 times the fundamental frequency, both the source voltage and load voltage are now of the same value (continuity) such that there is no signal loss.

[0096] Furthermore, continuity is achieved at harmonic frequencies with even coefficients (2, 4 shown on the fundamental coefficient axis) and no voltage is measured at harmonic frequencies with odd coefficients (1, 3, 5 shown on the fundamental coefficient axis) as shown in Figure 4.

[0097] Consider a line of initial length l0. For a signal generated at the fundamental frequency f0of the line:

[0098] If the transmission line were to be subjected to an arbitrary strain, the length of the line will increase by a length n, where

[0099] AQ

[0100] ZO+ n —

[0101] 4-

[0102] This means that there will only be weak resonance within the strained transmission line when a signal of y is passed through. Thus, the transmission line voltage will be reduced from the maximum value. The line of length Zo+ n will instead generate wave minimum at a different fundamental frequency. A plot of frequency against signal strength provides a new location of the peak voltages as a result of strain being applied to the transmission line. Figure 5 shows the locations 502 of the signal strength peaks (as determined by the voltage measurements) when no strain is applied (represented by the grey line). When strain is applied, for example 3% strain, the signal strength peaks shift as represented by locations 504 (represented by the black line).

[0103] The shifts in the signal strength peaks, represented by arrow 506, can then be calibrated against strain in order to use the transmission line as a strain sensor.

[0104] Considering the case where the propagation characteristic is the phase of the transmission line, a plot of frequency against phase can be generated to determine the strain on the transmission line as shown by Figure 6.

[0105] Similar to the voltage characteristics discussed directly above in relation to Figures 4 and 5, the phase shift is related to the resonance shift as the phase is affected by the formation of harmonics. The phase, 0, of a system could be described by the following equation which relates relative permittivity, er, transmission line length, L, and signal frequency, f:

[0106] As such, for a set input frequency and environmental conditions, the phase of the system will change as the transmission line length changes. This is shown in Figure 6 which shows phase location 602 (represented by the dashed line) when no strain is applied and phase location 604 (represented by the solid line) when 3% strain is applied. The shifts in the phase, represented by arrow 606, is then used to determine the strain on the transmission line.

[0107] The phase shift method is advantageous as it is easier to detect phase in real lines compared to the voltage characteristic method.

[0108] Figure 7 is a flowchart of the process of determining strain according to an aspect of the invention wherein an electrical pulse is applied to the transmission line. This is preferably done in addition to one or both the standing wave method described above.

[0109] In the first step 702, electrical pulses or sinusoidal frequencies are applied to the transmission line using the electronic equipment connected to the transmission line. Before applying the electrical signal, the transmission line may be terminated as either shorted or kept as an open circuit to attain strong reflection of the signal at the end of the line. Alternatively, the transmission line could be terminated with any other load.

[0110] At step 704, one or more propagation characteristics of the transmission line are measured. The propagation characteristics may be the transmission time of the electrical pulse or the phase shift of the sinusoidal signal. In some cases, both the pulse transmission time and shape (impedance) and the sinusoidal signal phase and amplitude for a range of frequencies may be measured to provide more accurate strain measurements.

[0111] Once the propagation characteristic(s) of the signals in the transmission line is measured, the strain on the transmission line is determined at step 706. The strain is determined based on the measured transmission time / shape and / or phase shift / amplitude.

[0112] Considering the transmission time as the propagation characteristic, as the transmission line lengthens under strain, the time taken for the signal to both propagate and reflect increases. This change in signal time can be measured and calibrated against mechanical strain as a means of strain sensing.

[0113] Considering the impedance (shape) as the propagation characteristic, the input impedance of a transmission line is a function of the length, I, of the line. This can be used to determine the characteristic impedance, Zo, and phase constant, p, of a transmission line using the following equations, where Zsc,in is the short circuited input impedance and Zoc,in is the open circuited input impedance :

[0114] The propagation of a pulsed signal through a transmission line will cause a change in the 'shape' of the pulse. In lossy lines, the pulse will be attenuated. The amount of attenuation depends on the length of the line. Hence, the attenuation of the signal can be numerically measured and this can be compared against calibrated figures in order to determine the extension of the line. Additionally, signal processing can be used to determine the extension of the line when a more complex pulse propagation is taking place.

[0115] The methods described in relation to Figures 3 to 7 include a calibration step before carrying out the described methods. The calibration step is used to determine the length of the transmission line and the initial propagation characteristics before strain is applied to the system. Knowing the initial length enables the subsequent strain on the system to be determined by comparison of the initial length with the determined lengths. Measurements can be taken for all cases: open and short circuited lines as well sandwiched and matched lines.

[0116] There may be a general calibration step for each sample that the transmission line sensor is used on because there will be slightly different strain and strain transferral properties from product to product. This will involve subjecting the sample, along the anticipated length that will be sensed in industry, to known strain values and recording the propagation characteristics at each strain value. This can be repeated to comprehensively understand the properties for the required product.

[0117] Additionally or alternatively, there may be a calibration step applied once the sensor is installed to account for environmental effects such as moisture and temperature. This calibration step may not be necessary when, for example, an independent moisture / temperature probe is used.

[0118] The above embodiments can be combined. In particular, there is no significant attenuation of the signal across the entire bandwidth when using the voltage characteristic method such that the system is also suitable for testing using electrical pulses. Thus, the voltage characteristic method can be used alongside the transmission time and / or impedance methods described above. This provides more accurate strain sensing. Further, the phase method can be used with the transmission time method as a shift in phase will lead to a shift in signal propagation time.

[0119] Figures 8a and 8b show an alternative embodiment of the present invention in which the sample comprises a conductive resiliently deformable material. The conductive resiliently deformable material can act as a trace element for parallel plate transmission lines used for geosynthetic strain sensing. The sample of Figures 8a and 8b is similar to the sample discussed with respect to Figures la and lb in that it is configured to operate in harsh environments and can be used as part of a strain sensor to determine the amount of strain imparted on the sample.

[0120] The sample shown in Figures 8a and 8b uses the same methods to determine the amount of strain imparted on the sample as discussed in relation to Figures 3 and 7 but instead of sample 100, sample 800 is used. In particular, the sample 800 is compatible with detecting length changes when strain is imparted. As such, the above discussion in relation to Figures 3 to 7 also applies to the following embodiment which uses sample 800. The phase-shift method and the transmission time method are preferably used with sample 800.

[0121] Figure 8a shows a side view of the sample 800 and Figure 8b shows a plan view of the sample 800.

[0122] The sample 800 has a substrate 802. The substrate is a dielectric substrate and is preferably a geosynthetic substrate. A conductive resiliently deformable material 804 is applied to the substrate 802 to act as a transmission line. The conductive resiliently deformable material is, for example, an array of conductive traces which can be orthogonal or otherwise. The conductive resiliently deformable material 804 preferably comprises a conductive mesh such as mesh 900a or 900b discussed with respect to Figures 9a and 9b.

[0123] Advantageously, the conductive mesh enables the sample to be manufactured in a more robust and repeatable manner. Further, where the sample is for use with fibre-based geosynthetics, the mesh is highly compatible as the mesh can be embedded into the fibre structure of the geosynthetic. The mesh also helps to provide a high conductivity, signal integrity and redundancy.

[0124] The mesh can comprise a fine wire mesh, a randomly distributed mesh (e.g., the sizes of the gaps in the mesh are not consistent), a coarse mesh (e.g., the size of the gaps in the mesh are larger than the gaps used in fine wire mesh), a loop mesh (e.g., a chain-mail construction) and / or a woven mesh. Preferably, the mesh is formed from steel or copper although any suitable conductive material can be used.

[0125] The resiliently deformable material 804 (e.g., the mesh) can be created using any suitable technique known to the skilled person. For example, the resiliently deformable material 804 may be created using inkjet printing, CNC application (computer numerical control machining), or any other relevant printing method.

[0126] The conductive resiliently deformable material 804 (e.g., the mesh) is applied to a first and second surface of the substrate 802, preferably the top and bottom surface of the substrate 802 when in use.

[0127] In one example, the substrate is formed from nitrile rubber, FR4 (a glass-reinforced epoxy resin laminate) and / or PET (polyethylene terephthalate).

[0128] The conductive resiliently deformable material 804 can be applied to the substrate 802 using any appropriate method known to the skilled person. In one example, the conductive resiliently deformable material 804 is applied using mechanical means. For example, the conductive resiliently deformable material 804 can be applied using heat and / or pressure to embed the material 804 in the substrate 802. In another example, a connector can be used to apply the material 804 to the substrate 802. An example of such a connector is an insulating pin. One or more connectors may be used. Alternatively, the conductive resiliently deformable material 804 can be applied to the substrate 802 via chemical means. For example, through the use of an adhesive with sufficient strain transferal properties. One or more adhesives such as epoxy resin, conductive tape and / or insulating doublesided tape can be used to adhere the conductive resiliently deformable material 804 to the substrate 802.

[0129] Regardless of the method of fixing the conductive resiliently deformable material 804 to the substrate 802, the material 804 is preferably applied to the substrate 802 whilst the sample 800 is held under tension. This ensures that the conductive resiliently deformable material 804 is in contact with the substrate 802 at a neutral sample state. For example, it is preferably to ensure that the strands in a woven mesh are in contact at a neutral sample state. In order to ensure a contact between the fibres, the mesh needs to be slightly stretched sufficiently to ensure a conduct (typically, the stretch is not very much as it will be stretched further by the ground movement). This starting (stretched) position is the neutral sample state (i.e., reference position from which extensions are measured). This subsequently ensures that there is high conductivity and signal integrity along the entire length of the sample even when no strain is applied. As shown in Figures 8a and 8b, the sample 800 can further comprise connectors 806a and 806b to facilitate connection of the sample to electronic equipment as discussed with respect to connectors 102 of Figures la and lb. Connectors 806a and 806b are electrically connected to the conductive resiliently deformable material 804 via any suitable connection method which provides electromagnetic coupling (e.g., soldering). The connectors 806a, 806b can be 50Q coaxial connectors and are preferably RF connectors such as SMA connectors.

[0130] Alternatively, wireless coupling could be used.

[0131] The sample 800 is configured such that electronic equipment can be connected to the sample 800 and the strain on the sample can be determined. Such a connection can be established using cables as discussed with respect to sample 100 of Figure 2. Preferably, coaxial cables are used. Similar to sample 100, strain sensor 200 can use one connection, or more than two connections may be used to achieve the same purpose and can be placed appropriately. As an example, two connections can be directly mounted to the sample 800 with one at each end of the conductive substance as shown in Figures 8a and 8b.

[0132] Electronic equipment, such as electronic equipment 208, is used to supply a signal to the sample 800 via the connectors 806a and 806b. The electronic equipment can be any commercially available equipment known to the skilled person to provide signals. The electronic equipment includes a signal generator and oscilloscope, and alternatively may be a vector network analyser.

[0133] The sample 800 when connected to electronic equipment can be used to determine the strain acting on the sample 800. This is described further with reference to Figures 3 to 7 where reference to sample 100 also applies to sample 800.

[0134] Figures 9a and 9b each show different orientations of the mesh that can be applied to the substrate 802.

[0135] Whilst the mesh is shown in Figures 9a and 9b to be comprised of orthogonal strands, the mesh could be formed from parallel, connected, 'zig-zag' or serpentine traces (e.g., wires) that allow for extension and development of a parallel-plate transmission line.

[0136] Figure 9a shows mesh 900a which has been orientated so that the fibres 904a are aligned orthogonal to the longitudinal axis of the sample 800 in which strain 902a is applied. As the mesh is subjected to any arbitrary stress, this is immediately taken up by fibres 904b which align with the direction of strain 902a which reduces the extensibility of the sample 800 to largely match the sample 800 stress-strain response of the constituent mesh material.

[0137] In comparison, the use of a non-aligned mesh 900b provides more favourable results as it allows for much greater flexibility and extension. As shown in Figure 9b, the non-aligned mesh 900b has been orientated such that the fibres 906a and 906b are placed at an angle relative to the longitudinal axis of the sample 800 where strain 902b is applied. The angle is preferably 45°. Positioning the fibres 906a and 906b at an angle allows for much greater flexibility and extension. This is because the fibres 906a and 906b are able to redistribute in response to stress / strain application which causes a geometric change in the mesh 900b structure. The non-aligned mesh 900b allows for the transverse fibres to contribute to the stress response of the trace as they are partially aligned to the stress axis of strain 902b which should increase the overall strength of the mesh 900b.

[0138] Use of copper mesh or steel mesh exhibit high conductivity, low attenuation, and high extensibility in the non-aligned mesh 900b.

[0139] Samples 100 and 800 discussed above can be used as a geosynthetic and can also be used in wearable electronic devices to detect strain / movement of a user.

Claims

CLAIMS1. A sample for use in determining strain, the sample comprising: a substrate; and a conductive resiliently deformable material applied to the substrate, wherein the conductive resiliently deformable material is configured to be connected to electronic equipment such that the strain on the sample is determinable by applying and measuring a signal to the conductive resiliently deformable material using the electronic equipment.

2. The sample of claim 1, wherein the conductive resiliently deformable material comprises a conductive mesh.

3. The sample of claim 2, wherein the conductive mesh comprises a plurality of fibres and the plurality of fibres are placed at an angle relative to the longitudinal axis of the sample.

4. The sample of claim 3, wherein the angle is 45°.

5. The sample of any of claims 1-4, wherein the resiliently deformable material is applied to the substrate using an adhesive.

6. The sample of any of claims 1-4, wherein the resiliently deformable material is applied to the substrate using one or more connectors.

7. The sample of any of claims 1-4, wherein the resiliently deformable material is applied to the substrate using heat and / or pressure to embed the resiliently deformable material within the substrate.

8. A sample for use in determining strain, the sample comprising: a geogrid substrate, wherein the external surface of the geogrid substrate is partially, or fully, coated in a conductive to form a transmission line, the coated geogrid substrate being configured to be connected to electronic equipment such that the strain on the sample is determinable by applying and measuring a signal to the coated geogrid substrate using the electronic equipment.

9. The sample of claim 8, wherein the conductive coating is a conductive flex paint or a conductive epoxy resin or a conductive epoxy paint.

10. The sample of any of claims 8 or 9, wherein the conductive coating is conductive silicon.

11. The sample of claims 8-10 wherein the coating is applied with the sample in tension or the sample of claims 1-7 wherein the conductive resiliently deformable material is applied with the sample in tension.

12. The sample of any of claims 8-10 wherein the coated geogrid substrate is configured to be connected to electronic equipment by comprising connectors conductively or wirelessly coupled to the coated geogrid substrate or the sample of any of claims 1-7 wherein the conductive resiliently deformable material is configured to be connected to electronic equipment by comprising connectors conductively or wirelessly coupled to the coated substrate.

13. A strain sensor comprising the sample of claim 12 wherein the coated geogrid substrate or the conductive resiliently deformable material is connected to the electronic equipment.

14. The strain sensor of claim 13 wherein the electronic equipment is connected via the connectors.

15. The strain sensor of claim 13 or 14 wherein the electronic equipment comprises a signal generator and oscilloscope.

16. The strain sensor of claim 15 wherein the electronic equipment comprises a network analyser.

17. The sample of any of claims 8-10, wherein the substrate comprises geosynthetic and / or the sample comprises a geogrid rib.

18. A method of measuring the strain through a sample comprising a transmission line, using electronic equipment connected to the transmission line, the method comprising:supplying, using the electronic equipment, a signal to the transmission line; applying, using the electronic equipment, a sweep of frequencies to the transmission line; measuring, using the electronic equipment, the phase of the signal in the transmission line at each frequency of the sweep of frequencies; and determining, using the electronic equipment, the strain on the transmission line based at least partly on the measured phase.

19. The method of claim 18, wherein the electronic equipment is used to measure one or more propagation characteristics in addition to the phase of the signal in the transmission line and the strain is additionally determined based on the measured propagation characteristics.

20. The method of claim 19 wherein the one or more propagation characteristics includes the amplitude of the signal.

21. The method of claim 18, wherein the supplied signal sent for at least some of the swept frequencies is an electrical sinusoidal signal, the method further comprising: short or open circuiting the transmission line; forming standing waves in the short or open circuited transmission line; measuring, using the electronic equipment, a change in the phase of the signal in the transmission line by measuring changes to the standing waves; and determining, using the electronic equipment, the strain on the transmission line based on the measured change in the phase.

22. The method of claim 18, wherein the electronic equipment is used to measure a change in one or more propagation characteristics in addition to a change in the phase of the signal in the transmission line and the strain is additionally determined based on the measured change in the one or more propagation characteristics.

23. The method of claim 22 wherein the one or more propagation characteristics includes the amplitude of the signal.

24. The method of any of claims 18-23, wherein the supplied signal is an electrical sinusoidal signal for all of the swept frequencies.

25. The method of measuring the strain through a sample comprising a transmission line, using electronic equipment connected to the transmission line, of claims 18 to 24 or independent thereof, the method comprising the steps of: applying, using the electronic equipment, electrical pulses to the transmission line; measuring, using the electronic equipment, at least one propagation characteristic of the transmission line, wherein the at least one propagation characteristic is the transmission time and / or shape of the electrical pulses through the transmission line; and determining, using the electronic equipment and signal processing, the strain on the transmission line based on the measured at least one propagation characteristic.

26. The method of any of claims 18 to 25, further comprising calibrating the transmission line by measuring at least one calibration propagation characteristic of the transmission line before strain is applied to the transmission line.

27. The method of claim 26, wherein the at least one propagation characteristic and the at least one calibration propagation characteristic are the same kind of propagation characteristic.

28. The method of claim 27 , wherein the strain on the transmission line is determined based on a comparison of the at least one propagation characteristic and the at least one calibration propagation characteristic.

29. The method of any of claims 18 to 28 wherein the sample is or has one or more of the features of the sample or strain sensor of any claims 1 to 17.

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