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Purpose The purpose of this paper is to propose an approach, which is easy to implement, for estimating the thickness of the air layer that may separate metallic parts in some aeronautical assemblies, by using the eddy current method. Design/methodology/approach Based on an experimental study of the coupling of a magnetic cup core coil sensor with a metallic layered structure (consisting of first metal layer/air layer/second metal layer), which is confirmed by finite element modelling simulations, an inversion technique relying on a polynomial forward model of the coupling is proposed to estimate the air layer thickness. The least squares and the nonnegative least squares algorithms are applied and analysed to obtain the estimation results. Findings The choice of an appropriate inversion technique to optimize the estimation results is dependent on the signal-to-noise ratio of measured data. The obtained estimation error is smaller than a few percent, for both simulated and experimental data. The proposed approach can be used to estimate both the air layer thickness and the second metal layer thickness simultaneously/separately. Originality/value This model-based approach is easy to implement and available to all types of eddy current sensors.

This chapter is focused on biomaterial interaction with electromagnetic fields. It gives a wide and deep introduction of biological medium and their electromagnetic properties, which can be used to sense and analyze tissue or cells within lab-on-chip. It consists of three complementary parts. A recall of electrokinetics laws is done and is applied to biological medium (electrolyte with fluid motion) in the first section. The scaling factor towards nano-scale is highlighted considering the ionic concentration distribution. The second section deals with dielectrophoresis forces (DEP), which results from the cell polarization under non-uniform electric field. The dielectric modeling of cells and their DEP response are depicted. Taking benefits of cell dielectric behavior, the applications such as cell sorting, trapping and characterization with electrorotation are presented. The last section introduces the biomaterial behavior at radiofrequency range (from MHz up to several tens of GHz). Applications using thee relaxation times occurring at such high frequency are described to sense or characterize biological medium. Electric Impedance Spectroscopy and Radio-Frequency devices are thus introduced.

The aim of this paper is to experimentally assess the capacity of radio-frequency flexible sensors to localize a dielectric object inside a fluid, which could be used as a non contact sensor detected to the detection or the monitoring a local modification of a tissue, such as a tumor or a lesion. The used sensor is an MRI-like antenna, which consists in a flat, flexible and low cost multi-turn split resonator (MTLR), which features a geometrically pre-determined resonance frequency. The MTLR is used here as a transmit and receive sensor monitored by a distant loop coil. The complex impedance changes observed at the ends of the monitoring coil is known to be linked to the dielectric properties of the resonator environment. If a dielectric object is placed close to the resonator, the complex impedance is altered. In this work, an experimental set up is used to assess the relevance of such a measurement scheme to detect the presence of a dielectric inclusion embedded in another dielectric medium. The setup includes a spherical object of 1.5 cm diameter filled with various NaCl solutions moved vertically inside a tank filled with deionized water by means of a three axis robotic arm, to create an electrical conductivity contrast between the inclusion and the media, and three 2 cm diameter MTLR sensors featuring 30, 47 and 70 MHz resonance frequencies, respectively. The sensors are operated through the use of monitoring coils connected to a network analyzer, and measurements are carried out at each position of the spherical object. The resulting sensor responses are plotted and used for the assessment of sensor performances. In addition, a method to combine the multi-frequency data provided by the three different sensors is proposed. Two different metrics regarding the spatial resolution (SR) and the peak signal to noise ratio (PSNR) are computed to characterize the single sensor performances, as well as the enhancement provided by the proposed multi-frequency approach.

PDMS foams have been recently considered to design enhanced flexible capacitive pressure sensors. Indeed, PDMS foams offer a promising tradeoff between bulky and structured PDMS film, allowing enhanced and easily adjustable sensor performances to be obtained. Here we explore the use of 3D printed polyvinyl alcohol (PVA) as a sacrificial mold in a polymer casting process. The PVA mold network defines channels within the PDMS film and is dissolved after curing the polymer casted around it. Sensors fabricated with PDMS scaffolds and bulk PDMS are electromechanically characterized under normal stress and compared. PDMS foam based sensors are shown to exhibits enhanced and adjustable performances compared to bulk PDMS based sensor of same dimensions. This work opens the way to the accurate design of pressure sensors for wearable pressure sensing applications.

The evaluation of dielectric properties in the radio frequency (RF) bandwidth is highly relevant to characterize the physical or physiological state of organic materials. This paper reports on the implementation of an easy-to-use and contactless RF dielectric sensing technique used for the monitoring of the heat-induced gelation of a white egg sample. The sensing technique basically consists in the distant monitoring of a high-quality factor inductive RF resonator operating at 98 MHz, which is electromagnetically coupled to the egg white sample. A lumped element modeling of the contactless RF probe interacting with the egg white is proposed. It is used to estimate both the conductivity and the permittivity of the sample during its gelation, starting from the impedance changes measured at the ends of the RF probe, and after probe calibration carried out using a reference NaCl solution. The results obtained during a 10 h monitoring experience allow the dynamics of the egg white gelation to be analyzed. These results open the way to the development of easy-to-implement and non-invasive dielectric monitoring of organic materials, with many prospects in agrifood as well as health monitoring applications.

In this paper, a contactless radio-frequency (RF) inductive probe is used to spatially localize and characterize a complex dielectric organic inclusion in a fluid. The effect of dielectric properties (DP) of this organic material is investigated experimentally and by numerical computations.The used RF probe is a 135 MHz 3 cm diameter and 10 cm long, cylindrical bracelet resonator, placed close to a water tank filled with deionized water which includes a 1.5 cm diameter inclusion filled of air or NaCl solutions and placed in arbitrary positions. The water tank and the inclusion are used to model an organic material including a tumor. The RF probe is used as a transmit and receive sensor. It induces a magnetic field inside the water tank, which, by reciprocity, conveys information about the DP of the investigated material. The impedance changes at the end of the RF probe are directly related to the modifications of the magnetic field, and are measured by means of a network analyzer. A complex fit of the impedance frequency response around the resonance frequency gives access to two quantities proportional to the electrical conductivity and dielectric constant of the inclusion. The inclusion is moved into the water tank along the three axes by means of a robotic arm, so that two three dimensional maps of the equivalent dielectric quantities in function of the inclusion position are sensed by the probe. Then, the inclusion is filled with different conductive NaCl solutions from 0.1 to 1.1 S/m in order to test the ability of the probe to sense the modifications of the dielectric properties of the inclusion. Experimental as well as computation results obtained using the Distributed Point Source Method (DPSM) validate the ability of the proposed probe to localize the inclusion as deep as 1 cm into the water, and the ability of the probe to sense the dielectric property changes of the inclusion.

Centimetric capacitive sensors made out of PDMS foams of controlled porosity are fabricated and electromechanically characterized under either normal stress and shear stress. For normal stress characterizations, it can be concluded that the density of the foam strongly affects the sensor performances expressed in sensitivity and measurement range thanks to dedicated behavioral sensor models. Also, it was found that the actual size of the foam pores plays a minor role in the electromechanical behavior of the sensor. Similar conclusions can be derived shear stress sensor characterizations. This work opens the way to the design of flexible sensors with adjustable performances for normal and shear stress measurements.

This paper reports on the study of microporous polydimethylsiloxane (PDMS) foams as a highly deformable dielectric material used in the composition of flexible capacitive pressure sensors dedicated to wearable use. A fabrication process allowing the porosity of the foams to be adjusted was proposed and the fabricated foams were characterized. Then, elementary capacitive pressure sensors (15 &times, 15 mm2 square shaped electrodes) were elaborated with fabricated foams (5 mm or 10 mm thick) and were electromechanically characterized. Since the sensor responses under load are strongly non-linear, a behavioral non-linear model (first order exponential) was proposed, adjusted to the experimental data, and used to objectively estimate the sensor performances in terms of sensitivity and measurement range. The main conclusions of this study are that the porosity of the PDMS foams can be adjusted through the sugar:PDMS volume ratio and the size of sugar crystals used to fabricate the foams. Additionally, the porosity of the foams significantly modified the sensor performances. Indeed, compared to bulk PDMS sensors of the same size, the sensitivity of porous PDMS sensors could be multiplied by a factor up to 100 (the sensitivity is 0.14 %.kPa&minus, 1 for a bulk PDMS sensor and up to 13.7 %.kPa&minus, 1 for a porous PDMS sensor of the same dimensions), while the measurement range was reduced from a factor of 2 to 3 (from 594 kPa for a bulk PDMS sensor down to between 255 and 177 kPa for a PDMS foam sensor of the same dimensions, according to the porosity). This study opens the way to the design and fabrication of wearable flexible pressure sensors with adjustable performances through the control of the porosity of the fabricated PDMS foams.

This paper focuses on the use of microporous PDMS foams as a highly deformable film to improve the sensitivity of flexible capacitive pressure sensor dedicated to wearable use. A fabrication process allowing the mechanical properties of foams to be adjusted is proposed together with a non-linear behavioral model used to objectively estimate the sensor performances in terms of sensitivity and measurement range. Sensors fabricated and characterized in this study show that the sensitivity and the measurement range can be adjusted from 0.14%/kPa up to 13.07%/kPa, and from 594 kPa to 183 kPa, respectively, while the PDMS film porosity ranges from 0% up to 85%.

flat circular transmission-line based 300 MHz resonator is implemented for the noncontactassessment of burn depths in biological tissue. Used as a transmit-and-receive sensor, it isplaced here at a 2 mm distance from organic material test samples (pork fillet samples) which havebeen previously heated on one face in various heating conditions involving various temperatures,durations and procedures. Data extracted from the sensor by means of a distant monitoring coilwere found to clearly correlate with the depth of burn observed on the tissue samples (up to 40%sensor output changes for a 7 mm burn depth) and with the heating conditions (around 5% sensoroutput changes for 5.5 mm burn depth obtained at 75 °C or 150 °C). These results open the way tothe development of easy to implement burn assessment and monitoring techniques, which could beintegrated in wearable medical dressing-like monitoring devices.

Multi-axial accelerometers are invaluable for the analysis of anisotropic cardiac vibrations during the first-heart-sound (S1). However, most studies simply consider the multi-axial accelerometer components independently along the different anatomical axes and rarely properly account for this anisotropy. The objective of this study was to evaluate a simple method, intended for chronically implanted medical devices, to identify a physiological reference frame maximizing S1 vibrations in the transverse plane. We attached a tri-axial accelerometer on the tip of a cardiac pacing lead to the chest of 10 human volunteers and implanted it subcutaneously in one pig. The volunteers and pig underwent separate experimental protocols capturing a wide range of conditions. The acceleration components of the tri-axial accelerometer were projected along the identified reference frame and a frame perpendicular to this. These were denoted the cardiac systolic mechanical axis and perpendicular axis, respectively. Results showed that projection along the mechanical axis tended to maximize signal-to-noise ratio and minimize cycle-to-cycle morphological variability. Significant differences were also observed between the mechanical and perpendicular axes projections, demonstrating sensibility to orientation. Along the mechanical axis, signal-to-noise-ratio was up to 21.72 dB greater, and cycle-to-cycle variability was up to 7.47 dB smaller, than along the perpendicular axis. The transverse plane angle defining the reference frame was relatively similar throughout the volunteers (28.5–44.0° from the medial-lateral axis), demonstrating inter-subject homogeneity. Also, it varied little across the different conditions (15.6° mean standard deviation across the volunteers), demonstrating stability. In conclusion, the proposed algorithm optimized signal quality at low computational cost.

Measuring cardiac hemodynamics subcutaneously offers a novel and minimally-invasive means of monitoring mechanical heart performance. Four accelerometer-based hemodynamic sensors were implanted at various subcutaneous sites of the thorax in one dog. Three of the sensors were on the tips of cardiac pacing leads, one was embedded inside an inactive pacemaker device can. The first heart sound could be well identified and its amplitude quantified in all subcutaneous sensors on a beat-to-beat basis. Its magnitude was comparable to that of the external signal, and about half that of the intracardiac signal. It also correlated well to cardiac contractility during transient pharmacologically induced hemodynamic challenges. This is the first evidence that heart sounds can be measured in the subcutaneous space to monitor heart function.

The paper reports on a numerical study aiming at assessing the influence of the frequency on the sensitivity of an inductive sensing method dedicated to the non contact dielectric characterization of organic materials. A three dimensional electromagnetic mesh-free modeling of the interactions between a single loop radio frequency antenna and a dielectric material mimicking a tissue (skin) shows that frequencies allowing a loss factor $\mathbf{tan}\delta$ close to 1 in the material is particularly relevant to sense its complex permittivity. Moreover, the inverse problem which consists in estimating the complex permittivity of the material starting from the antenna impedance changes is carried out using an artificial neural network. Estimation results shows that a combination of three frequencies providing loss factors in the material such as $\mathbf{tan}\delta = 1,\mathbf{tan}\delta >> 1$ and $\mathbf{tan}\delta enable to significantly enhance the estimation of the dielectric properties, even if sensor positioning uncertainty is involved.

This paper reports on the optimization of flexible PDMS-based normal pressure capacitive microsensors dedicated to wearable applications. The operating principle and the fabrication process of such microsensors are presented. Then, the deformations under local pressure of PDMS thin films of thicknesses ranging from 100 μm to 10 mm are studied by means of numerical simulations in order to foresee the sensitivity of the considered microsensors. The study points out that, for a given PDMS type, the sensor form ratio plays a major role in its sensitivity. Indeed, for a given PDMS film, the expected capacitance change under a 10 N load applied on a 1.7 mm radius electrode varies from a few percent to almost 40% according to the initial PDMS film thickness. These observations are validated by experimental characterizations carried out on PDMS film samples of various thicknesses (10 μm to 10 mm) and on actual microsensors. Further computations enable generalized sensor design rules to be highlighted. Considering practical limitations in the fabrication and in the implementation of the actual microsensors, design rules based on computed form ratio optimization lead to the elaboration of flexible pressure microsensors exhibiting a sensitivity which reaches up to10%/N.

International audience

This paper deals with an innovative implementation of a semi-analytical modeling method, called the Distributed Points Source Method (DPSM), in the case of an eddy current problem. The DPSM has already shown great potentialities for the versatile and computationally efficient modeling of complex electrostatic, electromagnetic or ultrasonic problems. In this paper, we report a new implementation of the DPSM, called differential DPSM, which shows interesting prospects for the modeling of complex eddy current problems such as met in the non-destructive imaging of metallic parts. In this paper, the used eddy current imaging device is firstly presented. It is composed of an eddy current (EC) inducer and a magneto optical set-up used to translate the magnetic field distribution appearing at the surface of the imaged part, into a recordable optical image. In this study, the device is implemented for the time-harmonics (900 Hz) imaging of a two-layer aluminum based assembly, featuring surface-breaking and buried defects. Then, the basics of the time-harmonics DPSM modeling are recalled, and the differential approach is presented. It is implemented for the modeling of the interactions of the eddy current imaging device with the considered flawed assembly in the same operating conditions as the experimental implementation. The comparison between experimental and computed data obtained for millimetric surface and buried defects is presented in the form of complex magnetic cartographies and Lissajous plots. The obtained results show good agreement and open the way to the modeling of complex EC problems. Furthermore, the low computational complexity of the differential DPSM modelings makes it promising to consider for the solving of EC inverse problems.

International audience

The formation of acid milk gels is the crucial first step in the manufacturing of yogurts. The changes of the physical state of yogurts during gelation affect their dielectric properties. In this study, the authors aim at assessing the relevance of a contactless and easy-to-implement radiofrequency (RF) method dedicated to the monitoring of the dielectric properties of yogurts during gelation. The sensing technique lies in the distant monitoring of a high-quality factor inductive RF resonator, electromagnetically coupled to the yogurt in formation. A lumped element modeling of the contactless RF probe interacting with the yogurt is proposed. It is used to estimate both the conductivity and the permittivity of the yogurt, starting from the impedance changes monitored by the RF probe. The monitoring technique is implemented during yogurt formation induced under various operating conditions and preparation parameters including incubation temperatures and doses of acids (glucono-δ-lactone). The obtained preliminary results open the way to the development of easy-to-implement and non-invasive dielectric monitoring techniques of organic materials, with many prospects in agrifood as well as health monitoring applications.

International audience; This paper deals with the experimental characterization of an array probe dedicated to the eddy current imaging of sub-millimetric surface breaking defects appearing in bore holes of metallic parts. The probe is constituted of a large inducer generating an uniformly oriented EC flow within the inspected material, and a sensing array probe featuring bobbin coils to sense the radial component of the magnetic field resulting from the eddy currents/defects interactions within the wall of the bore hole. The probe was designed with accurate defect characterization in view, i.e. to provide multi-frequency and high spatial resolution images with a reduced acquisition time, so as to enhance the informative content of the acquired eddy current data. An experimental set-up was build in order to validate the imaging performances of such a probe. A prototype featuring a large inducer and a single sensing coil which can be accurately positioned in the sensing area, has been developed in order to evaluate the sensing performances as well as to study the influence of the sensing array configuration on the imaging performances. The experimental results demonstrate a good sensing ability of the designed probe in the 10-800 kHz frequency range, with peak-signal-to-noise ratios higher than 36 dB at 10 kHz (and 62 dB at 800 kHz) for defects featuring dimensions as small as 0.4 mm × 0.2 mm × 0.2 mm. Furthermore, a staggered row arrangement of the sensing array was proposed so as to significantly reduce the error due to the sampling step resulting from the pickup coils geometry (from 35% to less than 9% in the worst case). The experimental evaluation of the probe provides promising prospects for the accurate characterization of defects, by means of advanced multifrequency signal processing algorithms.

A high resolution approach is proposed to quantitatively estimate the depth of defects buried in planar metallic structures. This approach associates a multiple signal characterization (MUSIC) algorithm with an original eddy current imager. The interactions of the eddy currents and the defects are modelled by a set of virtual magnetic sources propagating in a spherical manner up to the surface of the structure. The defect localization is carried out using the MUSIC algorithm, applied to the multi-frequency observation of the magnetic field distribution at the surface of the structure. Accurate results obtained on simulated and experimental data relative to defects buried down to 5.4 mm in an aluminium layered structure validate the approach.