Electrical Time Domain Reflectometry Research Articles

Numerical and Experimental Investigation of Time-Domain-Reflectometry-Based Sensors for Foreign Object Detection in Wireless Power Transfer Systems

Foreign object detection (FOD) is considered a key method for detecting objects in the air gap of a wireless charging system that could pose a risk due to strong inductive heating. This paper describes a novel method for the detection of metallic objects utilizing the principle of electric time domain reflectometry. Through an analytical, numerical and experimental investigation, two key parameters for the design of transmission lines are identified and investigated with respect to the specific constraints of inductive power transfer. For this purpose, a transient electromagnetic simulation model is established to obtain and compare the sensor impedance and reflection coefficients with experimental data. The measurement setup is based on parametrically designed sensors in laboratory scale, using an EUR 2 coin as an exemplary test object. Consequently, the proposed simulation model has been successfully validated in this study, providing a comprehensive quantitative and qualitative analysis of the major transmission line design parameters for such applications.

Changes in electric resistance of cracked copper-coated, pitch-based carbon fibres for structural health monitoring in a glider wing

Structural health monitoring based on detecting strain enables further exploitation of the lightweight potential of aircraft structures. Since carbon fibres (CF) can be easily integrated into aircraft structures made of fibre-reinforced plastics (FRP), their use in sensor applications has become subject of research. One of many novel approaches is the use of cracked CF for strain detection. During loading and unloading, the fibre cracks are opening and closing respectively, resulting in substantial changes in the electric resistance of the CF. The high dependence of the electric resistance on the mechanical strain enables spatially resolved strain sensing along the CF based on the electric time domain reflectometry (ETDR) principle. However, due to high inhomogeneity of the used pitch-based CF and the resulting electric properties, the ETDR-signal is degraded largely by impedance mismatch along the transmission line, which limits the measuring length of a spatially resolved sensor. In this contribution, a concept for a spatially resolved CF-based strain sensor is described, outlining the challenges that need to be addressed. One approach for the extension of the measuring length is lowering the base resistivity of the used CF, which is discussed in detail. In order to achieve a low base resistivity while maintaining the characteristic of opening and closing fibre cracks, copper-coated CF are investigated. The copper-coated pitch-based CF are integrated into test specimens consisting of a composite material and are subjected to tensile loading. Simultaneously to the deflection, the electrical resistance is measured. Consequently, the sensitivity to strain of the CF is determined and discussed.

Laboratory Evaluation of Railroad Crosslevel Tilt Sensing Using Electrical Time Domain Reflectometry.

Crosslevel is defined as the difference in elevation between the top surface of two railroad tracks. Severe changes in crosslevel, for example, due to earthquakes, ground settlement, or crushed ballasts, affect track geometry and can cause train derailment. Therefore, the objective of this study was to monitoring railroad crosslevel by using electrical time domain reflectometry (ETDR) to simultaneously interrogate multiple capacitive tilt sensor prototypes connected in a transmission line. ETDR works by propagating an electrical pulse signal from one end of the transmission line and then monitoring the characteristics of each reflected pulse, which is affected by the capacitance (or tilt) of the sensors. This study begins with a discussion of the capacitive tilt sensor’s design. These 3D-printed sensors were tested to characterize their tilt sensing performance. Then, multiple tilt sensors were connected in a transmission line and interrogated by ETDR. The ability to use ETDR to multiplex and interrogate sensors subjected to different angles of tilt was validated.

Distributed Strain Sensing Using Electrical Time Domain Reflectometry With Nanocomposites

The objective of this study is to validate distributed strain sensing using electrical time domain reflectometry (ETDR) with multi-walled carbon nanotube (MWCNT)-based thin film sensing elements. The proposed ETDR sensor composed of two types of transmission lines: parallel-wire-type transmission line and parallel-plate-type transmission line, ( i.e. , with MWCNT-based sensing elements). The hypothesis was that the greater strain-induced impedance changes of the nanocomposite would enhance ETDR sensing performance. In this paper, four different types of ETDR sensing elements were subjected to one-cycle uniaxial tensile strains to validate strain sensing. Three sensing elements were then integrated in an ETDR setup, and strain patterns were applied for validating their distributed strain sensing behavior.

A sensor detecting kissing bonds in adhesively bonded joints using electric time domain reflectometry

The joining technology with the highest potential in lightweight construction is the adhesive bonding. In an adhesively bonded joint different defects can occur. Alongside gross defects like areas of uncured adhesive and voids, adhesion defects like kissing bonds pose a serious problem to the confidence in these kind of joints in the aircraft industry and other industries. Since kissing bonds are small interfacial defects of as few as nanometers in thickness, the detection by conventional nondestructive testing methods is not possible in most applications. In this paper a novel adhesive sensor based on the electric time domain reflectometry is proposed detecting differences in the deformation of the adherents in an adhesively bonded joint to infer that a kissing bond is taking affect. Numerical calculations are used to show that the proposed sensor principle poses a productive approach. For the numerical calculations a finite difference time domain model is used. The sensor is then validated experimentally by shear tension testing of single lap shear specimens with the adhesive sensor integrated into the joint.

Inverse calculation of strain profiles from ETDR measurements using artificial neural networks

Abstract. A novel carbon fibre sensor is developed for the spatially resolved strain measurement. A unique feature of the sensor is the fibre-break resistive measurement principle and the two-core transmission line design. The electrical time domain reflectometry (ETDR) is used in order to realize a spatially resolved measurement of the electrical parameters of the sensor. In this contribution, the process of mapping between the ETDR signals to the existing strain profile is described. Artificial neural networks (ANNs) are used to solve the inverse electromagnetic problem. The investigations were carried out with a sensor patch in a cantilever arm configuration. Overall, 136 experiments with varying strain distribution over the sensor length were performed to generate the necessary training data to learn the ANN model. The validation of the ANN highlights the feasibility as well as the current limits concerning the quantitative accuracy of mapping ETDR signals to strain profiles.

Phenomenological investigation of a carbon fibre based strain sensor with spatial resolution by means of time domain reflectometry

Fibre reinforced plastics feature versatile function‐integrative capabilities, e.g. the possibility to realise embedded Structural Health Monitoring (SHM) systems. Material‐compatible sensors are a prerequisite for a robust and reliable function of such systems. Among others, sensors based on carbon fibres are in the focal point of research due to their high material compatibility. The contribution proposes a novel continuous strain sensor based on embedded carbon fibres. In opposition to typical carbon fibre sensors, the presented measurement principle is based on the reversible opening and closing of aligned carbon fibre fragments. The phenomenological effects are investigated by a combined electrical, mechanical and optical analysis. The sensor features a strain sensitivity that is up to four orders of magnitude higher than the one of current carbon fibre sensors. For the first time, the application of the electrical time domain reflectometry for a spatially resolved strain measurement with carbon fibre sensors is presented here. In addition a damage localisation capability with an observed spatial resolution in the lower mm‐range is possible.

Rapid and innovative instrumental approaches for quality and authenticity of olive oils

The quality of virgin olive oils is assessed through the determination of several analytical parameters, whose values must be within the ranges established by the different institutions involved. In addition to official methods, there is a strong need for simple, rapid, and environmentally friendly techniques for the quality control of virgin olive oils and for addressing the challenging task of determining geographical origin and detecting adulterants. Toward this purpose, some of the most interesting applications based on optical spectroscopic techniques, on the measurement of electrical characteristics and on the use of instruments equipped with electronic chemical sensors, including also other promising techniques are herein discussed. These techniques, adequately coupled with an appropriate statistical approach, appear to be promising for assessment of several quality‐related parameters. The prediction of sensory attributes and of the oxidative status of virgin olive oils have also been reviewed by adopting these selected techniques, which are also considered to be potentially appropriate solutions for identification of the geographical origin of virgin olive oils and to assess their adulteration with cheaper oils. Overall, the techniques discussed are promising and cutting‐edge approaches for the establishment of useful portable instruments for in situ monitoring of the quality of virgin olive oils.Practical applications: The simple, rapid, and environmentally friendly analytical approaches discussed herein represent promising analytical tools for assuring the authenticity and monitoring the quality of virgin olive oils. Such innovative techniques and tools need to be ring‐tested and validated. Some innovative reviewed approaches will permit to develop useful portable instruments able to perform in situ appropriate controls also by small laboratories or olive oil mills with limited technical facilities. These equipments will be potentially usable also by trained “non‐professional analytical skilled” people. Some other approaches, rapid but more expensive, will be applicable mainly by quality control labs and will increase the number of samples analyzed per day, thus fostering laboratory proficiency and an effective fighting against olive oil fraud.Quality analysis of virgin olive oil can be achieved by a set of chemical and sensory determinations. Reference chemical analyses are usually carried out in a laboratory by trained personnel and requires high costs and lengthy times to obtain results. Innovative techniques characterized by high accuracy, short response time, and possibility of in situ analysis to evaluate olive oil quality are reviewed herein. Such techniques can be divided in different groups depending on the principle of the methodology: 1) optical techniques, such as UV‐Vis, NIR, MIR, Raman, and fluorescence spectroscopy; 2) electrical techniques, such as electrical impedance spectroscopy, amperometry, and time domain reflectometry; and 3) electronic noses, electronic tongues and other promising approaches, where an array of electrodes or other technical solutions (PTR‐MS, FGC E‐nose, NMR) are used in conjunction with multivariate data analysis to obtain a chemical fingerprint of the product.

Investigation of Electrical Time Domain Reflectometry for infusion and cure monitoring in combination with electrically conductive fibers and tooling materials

Abstract The presented work investigates Differential Electrical Time Domain Reflectometry (D-TDR) as a method to monitor the flow front position and curing degree of resin during manufacturing of composite structures by vacuum infusion technology. The sensor concept is based on the physical principle that a voltage pulse introduced into a transmission line, which serves as a sensor, is reflected at impedance changes, which correlate to changes of the dielectric properties along the line. The method's accuracy under different boundary conditions is investigated by infiltrating glass and carbon fiber preforms on metallic and wooden toolings with epoxy resins. A shielded sensor concept is presented, which eliminates the influence of conductive materials on the signal. For all tested fiber/tooling combinations, the monitoring method shows a good agreement between the flow front position and optically determined flow fronts. Degree of cure measurements at ambient and elevated temperatures show a correlation with Differential Scanning Calorimetry (DSC) and rheology analyses.

Slime thickness evaluation of bored piles by electrical resistivity probe

The bottoms of bored piles are generally stacked with soil particles, both while boreholes are being drilled, and afterward. The stacked soils are called slime, and when loads are applied on the pile, increase the pile settlement. Thus to guarantee the end bearing capacity of bored piles, the slime thickness should be precisely detected. The objective of this study is to suggest a new method for evaluating the slime thickness, using temperature compensated electrical resistivity. Laboratory studies are performed in advance, to estimate and compare the resolution of the electrical resistivity probe (ERP) and time domain reflectometry (TDR). The electrical properties of the ERP and TDR are measured using coaxial type electrodes and parallel type two-wire electrodes, respectively. Penetration tests , conducted in the fully saturated sand–clay mixtures, demonstrate that the ERP produces a better resolution of layer detection than TDR. Thus, field application tests using the ERP with a diameter of 35.7 mm are conducted for the investigation of slime thickness in large diameter bored piles. Field tests show that the slime layers are clearly identified by the ERP: the electrical resistivity dramatically increases at the interface between the slurry and slime layer. The electrical resistivity in the slurry layer inversely correlates with the amount of circulated water. This study suggests that the new electrical resistivity method may be a useful method for the investigation of the slime thickness in bored piles. • The slime thickness in bored piles is evaluated by electrical resistivity. • The ERP produces a better resolution of layer detection than TDR. • Electrical resistivity increases at the interface of slurry and slime layers.

Electrical resistivity tomography to understand clay behavior during seasonal water content variations

Problems with foundations highlight a lack of understanding surrounding factors which influence ground movements during wet–dry cycles (Vincent, 2009). To address this issue, geotechnical characterizations of a clayey formation can be used to identify significant variability in lithological facies, both vertically and horizontally over very short distances. Soil heterogeneities explain in this case a wide range of geotechnical parameters and weak correlations between them, assessed on soil behavior observed on site. This paper focuses on soil water deficits and benefits over time, related to soil composition in a heterogeneous clay formation through the use of electrical resistivity tomography (ERT). Electrical resistivity tomography and time domain reflectometry (TDR) were used simultaneously to measure resistivity and soil moisture at an experimental field site with the unique objective of qualifying soil moisture in this first stage of the study. The resistivity variations obtained from ERT were compared to local effective rainfall and soil moisture measurements from time domain reflectometry (TDR) down to a depth of 3m. Results show the potential of qualifying soil water content variations over the seasons, and especially of detecting a rapid increase in humidity thanks to spatial soil heterogeneity at a decimetric scale. ERT proves to be a useful method of delineating soil facies based on their drying and humidification behavior.

Smart tooling with integrated time domain reflectometry sensing line for non-invasive flow and cure monitoring during composites manufacturing

An electrical transmission line integrated into composite tooling has been developed to facilitate non-contact multipoint flow and cure monitoring during vacuum assisted resin transfer molding processing. The sensor is made of conductive aramid (Kevlar®) fibers and is an integral part of the glass fiber composite tooling. Electrostatic simulations in COMSOL® have been performed to optimize the wire spacing maximizing the sensitivity of the electric time domain reflectometry measurements. Tooling with integrated wires placed with optimal spacing has been fabricated. TDR flow monitoring experiments with the integrated sensor have been conducted and compared visually using digital image processing. TDR data obtained from the same sensor also enables measurement of the resin cure state. Fast and slow curing resin systems have been studied using the transmission line integrated tooling and differential scanning calorimetry experiments have been performed to validate the cure measurements obtained from TDR. Major benefits of the sensor implementation include elimination of ingress/egress issues associated with standard sensor integration into composite tooling as well as reduced maintenance of the non-contact solution.

Localized functionally modified glass fibers with carbon nanotube networks for crack sensing in composites using time domain reflectometry

An electric time domain reflectometry (TDR) based sensing approach with an external parallel plate transmission line has been developed to evaluate high-frequency electromagnetic changes in composites due to applied load and internal damage. A model system of cross-ply glass fiber/vinyl ester composites with and without the selective integration of localized carbon nanotube (CNT) networks was studied where microcracking and delamination are introduced during tensile loading. A sizing technique has been used for localized functional modification using CNTs. The TDR sensing approach has been correlated with strain and acoustic emission (AE) measurements as well as micrographs of edge replicas capturing the damage state. Both the nanotube modified and baseline composites have similar mechanical properties and damage progression which is reflected in similar stress–strain plots, AE measurements and edge replica studies. However, the CNT introduced composites have enhanced strain and damage dependent TDR response. Hence, through localized functional modification of the composite electromagnetic properties using CNTs and the electromagnetic–mechanical property coupling of CNTs, it is possible to (1) increase TDR sensitivity to strain and (2) sense development of micro-scale cracks. This approach offers potential for use in existing composite structures or permanently integrated during the manufacturing process and is in situ and non-invasive.

A Prototype System for Time-Lapse Electrical Resistivity Tomographies

A prototype system for time-lapse acquisition of 2D electrical resistivity tomography (ERT) and time domain reflectometry (TDR) measurements was installed in a test site affected by a landslide in Basilicata region (southern Italy). The aim of the system is to monitor in real-time the rainwater infiltration into the soil and obtain information about the variation of the water content in the first layers of the subsoil and the possible influence of this variation on landslide activity. A rain gauge placed in the test site gives information on the rainfall intensity and frequency and suggests the acquisition time interval. The installed system and the preliminary results are presented in this paper.

Comparison Study of Three Common Technologies for Freezing-Thawing Measurement

This paper describes a comparison study on three different technologies (i.e., thermocouple, electrical resistivity probe and Time Domain Reflectometry (TDR)) that are commonly used for frost measurement. Specially, the paper developed an analyses procedure to estimate the freezing-thawing status based on the dielectric properties of freezing soil. Experiments were conducted where the data of temperature, electrical resistivity, and dielectric constant were simultaneously monitored during the freezing/thawing process. The comparison uncovered the advantages and limitations of these technologies for frost measurement. The experimental results indicated that TDR measured soil dielectric constant clearly indicates the different stages of the freezing/thawing process. Analyses method was developed to determine not only the onset of freezing or thawing, but also the extent of their development. This is a major advantage of TDR over other technologies.

Quantifying field-scale soil moisture using electrical resistivity imaging

Summary Electrical resistivity imaging (ERI) and time domain reflectometry (TDR) were used to simultaneously measure resistivity and soil moisture at an experimental field site with the objective of quantifying soil moisture in 2D unsaturated zone profiles (>40 m long and 9 m deep). With a time-series data set and a modified form of Archie’s Law, we used bulk conductivity derived from ERI models of soil resistivity, pore water conductivity derived using an exchangeable cation proxy (Ca + Mg), and percent clay content of soils, to convert 2D ERI profiles into 2D soil moisture profiles. We optimized the model using 1D access-tube time domain reflectometry (TDR) derived soil moisture measurements. Results show that this approach can be used to quantify soil moisture in heterogeneous soils, and thus provides a useful method for measuring 2D soil moisture at a variety of field sites and scales.

Development of an electrical time domain reflectometry (ETDR) distributed moisture measurement technique for porous media

In this paper, a novel electrical time domain reflectometry (ETDR) measurement technique capable of measuring moisture distribution profile in a porous medium is presented. A signal calibration algorithm to interpret the ETDR waveform based on an empirical relation between moisture content and differential impedance of the medium was developed. The transient distribution of the moisture content in a concrete block covered with fibre reinforced polymer laminate on one end surface was experimentally measured. The experimental results were then compared with the solution of a finite element analysis. Good agreement was shown for the trend of the transient moisture distribution profile. It was demonstrated that the ETDR distributed moisture sensor developed in the current study was capable of measuring the variation of the moisture content distribution in a porous medium.

Electric time-domain reflectometry distributed flow sensor

Liquid composite molding (LCM) has become an important processing technique to manufacture high-performance composite parts. The sensing of the process parameters, such as resin fill of the porous material, are key to improve repeatability, maximize quality and minimize cost. This paper describes a distributed flow sensor, which considerably decreases tooling integration costs and improves spatial resolution by allowing sensing of hundreds of sensing elements with a single input/output port. The transmission line sensor is virtually divided into a large number of small discrete transmission lines treated as a long array of sensing elements. Piecewise sensing is achieved by electric time-domain reflectometry and inversion of a non-uniform transmission line model. The paper describes the distributed sensing approach, experimentally validates distributed sensing in a LCM setup, and analyzes critical sensor parameters.

Structural Deflection Monitoring Using an Embedded ETDR Distributed Strain Sensor

Feasible application of the electrical time domain reflectometry (ETDR) technique for distributed strain measurement is investigated in this study. A prototype coaxial ETDR distributed strain sensor is embedded into a laboratory-scale reinforced concrete beam specimen, and three-point bending tests are conducted. The ETDR signal waveforms of the embedded sensor are used to determine the deflection profiles of the beam under various bending loads. The resulting deflection profiles are validated by their good agreements with the deflection measurements by LVDT displacement sensors at four discrete locations. It is shown that the ETDR distributed sensing technique has the capability to capture the variation of the load-induced axial strain along the sensing line. In addition, the ETDR distributed sensor could also clearly capture the location of localized high strain points.

Structural Damage Detection Using an Embedded ETDR Distributed Strain Sensor

Feasible application of the Electrical Time Domain Reflectometry (ETDR) distributed strain sensing technique for structural damage detection was investigated in this study. A small-scale concrete beam specimen with an embedded ETDR distributed strain sensor was tested in three-point bending. The resulting ETDR waveforms of the sensor were studied in conjunction with the load–displacement curve of the concrete beam and the video images of the specimen recorded during the test. It was shown that the embedded ETDR sensor was capable of detecting weak points in the structure resulted from structural defects. The embedded sensor also had the capability to detect a load-induced damage at its early stage when no surface crack line was visible. On the occurrence of an apparent crack damage that passed through the sensing line, the embedded sensor could locate its position with a precision within 0.25′′. Moreover, the ETDR distributed sensor had the capability to detect multiple cracks simultaneously along a single sensing line.