Interrogation Distance Research Articles

PT-Symmetric LC Passive Wireless Sensing.

Parity-time (PT) symmetry challenges the long-held theoretical basis that only Hermitian operators correspond to observable phenomena in quantum mechanics. Non-Hermitian Hamiltonians satisfying PT symmetry also have a real-valued energy spectrum. In the field of inductor-capacitor (LC) passive wireless sensors, PT symmetry is mainly used for improving performance in terms of multi-parameter sensing, ultrahigh sensitivity, and longer interrogation distance. For example, the proposal of both higher-order PT symmetry and divergent exceptional points can utilize a more drastic bifurcation process around exceptional points (EPs) to accomplish a significantly higher sensitivity and spectral resolution. However, there are still many controversies regarding the inevitable noise and actual precision of the EP sensors. In this review, we systematically present the research status of PT-symmetric LC sensors in three working areas: exact phase, exceptional point, and broken phase, demonstrating the advantages of non-Hermitian sensing concerning classical LC sensing principles.

Volumetric quantification of peri-atrial fat inflammation in atrial fibrillation patients

Abstract Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): British Heart Foundation Centre for Research Excellence Award III (RE/18/5/34216). SEW is supported by the British Heart Foundation (FS/20/26/34952). MCW is supported by the British Heart Foundation (FS/ICRF/20/26002) Introduction Peri-atrial fat may have pro-inflammatory and pro-fibrotic effects on atrial myocardium and increase the risk of developing atrial fibrillation. Both peri-atrial fat volume and density have been proposed as markers for atrial fibrillation risk, but prior studies considered only single-plane analysis or peri-atrial fat attenuation limited to the posterior left atrium. Purpose We aimed to assess total peri-atrial fat volume and attenuation and to explore the relationship between both parameters and the occurrence of atrial fibrillation. Methods Contrast-enhanced cardiac computed tomography (CT) imaging was performed pre-ablation in atrial fibrillation patients (n=32) and in control patients (n=37) without atrial fibrillation undergoing investigation for chest pain. We developed a volumetric method for peri-atrial fat segmentation and quantification using seg3D2 and CEMRGapp. Peri-atrial fat volume and attenuation (Hounsfield units, HU) were compared between patients with and without atrial fibrillation in univariable and multivariable analyses. A sensitivity analysis was performed to assess the impact of fat proximity to the left atrial wall. Results Atrial fibrillation participants were older (64±11 years vs 58±7 years, P=0.004) and more likely to have a history of coronary artery disease (50% vs 25%, P=0.03). Participants with atrial fibrillation had greater left atrial volume (88±28 ml vs 69±19 ml, P=0.002) and mass (29±6 g vs 14±0.9 g, P=0.018) compared to controls. Peri-atrial fat volume was greater in those with than without atrial fibrillation (18.5±6.6 ml vs 14.2±7.0 ml, adjusted odds ratio (AOR) per ml 1.21; 95% confidence interval [CI] 1.03-1.48), after adjusting for age, body mass index, comorbidities, left atrial dimensions and mass. However, total peri-atrial fat attenuation was not significantly different between groups (-74.8 vs -73.0 HU; AOR per HU 1.01; 95% CI 0.82-1.22). Sensitivity analysis did not identify an interaction between interrogation distance from the left atrial wall and peri-atrial fat attenuation on prevalent atrial fibrillation (p=0.872). Conclusion Peri-atrial fat volume, but not attenuation, is associated with prevalent atrial fibrillation. Further mechanistic studies are needed to study the role of peri-atrial fat in atrial fibrillation pathogenesis.

VocalPrint: A mmWave-Based Unmediated Vocal Sensing System for Secure Authentication

With the continuing growth of voice-controlled devices, voice metrics have been widely used for user identification. However, voice biometrics is vulnerable to replay attacks and ambient noise. We identify that the fundamental vulnerability in voice biometrics is rooted in its indirect sensing modality (e.g., microphone). In this paper, we present <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VocalPrint</i> , a resilient mmWave interrogation system which directly captures and analyzes the vocal vibrations for user authentication. Specifically, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VocalPrint</i> exploits the unique disturbance of the skin-reflect radio frequency (RF) signals around the near-throat region of the user, caused by the vocal vibrations. The complex ambient noise is isolated from the RF signal using a novel resilience-aware clutter suppression approach for preserving fine-grained vocal biometric properties. Afterward, we extract the vocal tract and vocal source features and input them into an ensemble classifier for authentication. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VocalPrint</i> is practical as it allows the effortless transition to a smartphone while having sufficient usability due to its non-contact nature. Our experimental results from 41 participants with different interrogation distances, orientations, and body motions show that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VocalPrint</i> achieves over 96 percent authentication accuracy even under unfavorable conditions. We demonstrate the resilience of our system against complex noise interference and spoof attacks of various threat levels.

Ultrahigh-Resolution Wireless Capacitance Readout Based on a Single Real Mode in a Perturbed PT -Symmetric Electronic Trimer Sandwich

High-performance interrogation of an inductor-capacitor microsensor has been a long-standing challenge due to the limited size of the sensor. Parity-time ($PT$) symmetry, an intriguing concept originated from quantum physics, has been utilized to improve the spectral resolution and sensitivity of the conventional readout circuit, while the $PT$-symmetry condition has to be satisfied. In this work, a sandwich-type wireless capacitance readout mechanism based on perturbed $PT$-symmetric electronic trimer without manual tuning of the reader circuit is proposed. Theoretical eigenvalue analysis by solving the system equations shows that the system exhibits single real mode in weak coupling regime whose eigenfrequency changes in response to the capacitance of the neutral resonator. Furthermore, the proposed readout system exhibits wider readout capacitance range compared to the standard $PT$-symmetry-based system while retaining higher $Q$ factor compared to the conventional readout method, which has been validated with the experimental prototype based on printed circuit board. Our work not only enriches the underlying theory of non-Hermitian physics, but also shows potential applications in scenarios where longer interrogation distance is required, such as implanted medical devices, parameter detection in harsh environment, etc.

Structural Health Monitoring Cost Estimation of a Piezosensorized Aircraft Fuselage.

Guided waves-based SHM systems are of interest in the aeronautic sector due to their lightweight, long interrogation distances, and low power consumption. In this study, a bottom-up framework for the estimation of the initial investment cost and the added weight associated with the integration of a SHM system to an aircraft is presented. The framework provides a detailed breakdown of the activities and their costs for the sensorization of a structure using a fully wired approach or the adoption of the printed diagnostic film. Additionally, the framework considers the difference between configuring the system for Manual or Remote data acquisition. Based on the case study presented on the sensorization of a regional aircraft composite fuselage, there is a trade-off between and for the SHM options considered. The Wired–Manual case leads to the lowest with the highest , while the combination of diagnostic film with a Remote system leads to the highest and the lowest . These estimations capture the characteristics of each system and can be integrated into cost–benefit analyses for the final selection of a particular configuration.

Low-Cost Portable Readout System Design for Inductively Coupled Resonant Sensors

Spiral coil-based inductor–capacitor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> ) resonant sensors have seen many applications in agriculture, healthcare, biomanufacturing, and consumer electronics. Traditional techniques to interrogate such sensors using vector network analyzers (VNAs) are expensive and often not portable, whereas custom readouts suffer from high cost, low range of usable interrogation distances, or low frequency range. This article proposes a new simple, low-cost, and portable readout design based on the technique of coherent demodulation. A complete theoretical analysis examining how the interrogation distance is related to other circuit parameters is presented. Finally, a complete readout system was implemented using printed circuit board technology and commercially available off-the-shelf components. The system operates between 1 and 100 MHz and the fabricated system consumes 1.26 W. Measurements show that the system operates reliably and repeatably with interrogation distances up to 5 cm.

Anapole-enabled RFID security against far-field attacks

Abstract Radio frequency identification (RFID) is a widely used wireless technology for contactless data exchange. Owing to international standardization and one-way security nature of the communication protocol, RFID tags, holding sensitive information, may be a subject to theft. One of the major security loopholes is the so-called far-field attack, where unauthorized interrogation is performed from a distance, bypassing the user’s verification. This loophole is a penalty of using a dipole-like RFID tag antenna, leaking wireless information to the far-field. Here we introduce a new concept of anapole-enabled security, prohibiting far-field attacks by utilizing fundamental laws of physics. Our design is based on radiationless electromagnetic states (anapoles), which have high near-field concentration and theoretically nulling far-field scattering. The first property enables performing data readout from several centimeters (near-field), while the second prevents attacks from a distance, regardless an eavesdropper’s radiated power and antenna gain. Our realization is based on a compact 3 cm high-index ceramic core–shell structure, functionalized with a thin metal wire and an integrated circuit to control the tag. Switching scheme was designed to provide a modulation between two radiation-less anapole states, blocking both up and down links for a far-field access. The anapole tag demonstrates more than 20 dB suppression of far-field interrogation distance in respect with a standard commercial tag, while keeping the near-field performance at the same level. The proposed concept might significantly enhance the RFID communication channel in cases, where information security prevails over cost constrains.

Thermally Stable Wireless Patch Antenna Sensor for Strain and Crack Sensing.

Strain and crack are critical indicators of structural safety. As a novel sensing device, a patch antenna sensor can be utilized to wirelessly estimate structural strain and surface crack growth through resonance frequency shift. The main challenges for the sensor are other effects such as temperature fluctuation that can generate unwanted resonance frequency shift and result in large noise in the measurement. Another challenge for existing designs of patch antenna sensor is the limited interrogation distance. In this research, thermally stable patch antenna sensors are investigated for more reliable measurement. Fabricated on a substrate material with a steady dielectric constant, a new passive (battery-free) patch antenna sensor is designed to improve reliability under temperature fluctuations. In addition, another newly designed dual-mode patch antenna sensor is proposed to achieve a longer interrogation distance. Extensive experiments are conducted to characterize the patch antenna sensor performance, including thermal stability, tensile strain sensing, and emulated crack sensing. The two new patch antenna sensors are demonstrated to be effective in wireless strain and crack measurements and have potential applications in structural health monitoring (SHM).

Technique and Circuit for Contactless Readout of Piezoelectric MEMS Resonator Sensors.

A technique and electronic circuit for contactless electromagnetic interrogation of piezoelectric micro-electromechanical system (MEMS) resonator sensors are proposed. The adopted resonator is an aluminum-nitride (AlN) thin-film piezoelectric-on-silicon (TPoS) disk vibrating in radial contour mode at about 6.3 MHz. The MEMS resonator is operated in one-port configuration and it is connected to a spiral coil, forming the sensor unit. A proximate electronic interrogation unit is electromagnetically coupled through a readout coil to the sensor unit. The proposed technique exploits interleaved excitation and detection phases of the MEMS resonator. A tailored electronic circuit manages the periodic switching between the excitation phase, where it generates the excitation signal driving the readout coil, and the detection phase, where it senses the transient decaying response of the resonator by measuring through a high-impedance amplifier the voltage induced back across the readout coil. This approach advantageously ensures that the readout frequency of the MEMS resonator is first order independent of the interrogation distance between the readout and sensor coils. The reported experimental results show successful contactless readout of the MEMS resonator independently from the interrogation distance over a range of 12 mm, and the application as a resonant sensor for ambient temperature and as a resonant acoustic-load sensor to detect and track the deposition and evaporation processes of water microdroplets on the MEMS resonator surface.

Passive Wireless Pressure Sensing for Gastric Manometry.

We describe a wireless microsystem for gastrointestinal manometry that couples a microfabricated capacitive transducer to a dual-axis inductor, forming a resonant inductor-capacitor (LC) sensor within an ingestible 3D printed biocompatible capsule measuring ø 12 mm × 24 mm. An inductively coupled external telemetry unit wirelessly monitors the pressure dependent resonant frequency of the LC sensor, eliminating the need for integrated power sources within the ingested capsule. In vitro tests in saline show pressure response of −0.6 kHz/mmHg, interrogation distance up to 6 cm, and resolution up to 0.8 mmHg. In vivo functionality is validated with gastrointestinal pressure monitoring in a canine beagle over a 26-hour period.

Optimal Design of Passive Resonating Wireless Sensors for Wearable and Implantable Devices

Passive resonating wireless sensors consist of a variable capacitive ( $C_{s}$ ) sensing element in resonance with a coil ( $L_{s}$ ) that is inductively coupled with a proximal readout circuit. They have been utilized in a number of medical and contactless industrial applications for their simple, small, battery-less, and wireless capabilities. An external readout coil ( $L_{r}$ ) detects the changes in the impedance phase spectrum, resulting from the changes in the parameter of interest ( $\Delta P$ ), such as pressure, humidity, or permittivity. A method of optimization for this class of sensors is presented based on a new figure of merit (FoM) to help designers find optimal geometries for both the sensing and inductive link components of the resonating sensor, leading to maximum signal-to-noise ratio (SNR) and sensitivity at a desired reading range. To verify the proposed FoM and optimization algorithm, we fabricated an all-soft resonating wireless sensor prototype for chemical detection with optimal geometry. The phase-dip ( $\Delta \varphi _{dip}$ ) of the sensor at 5-mm interrogation distance is 43°, a 1380% improvement after optimization, at 313.5-MHz operating frequency. The maximum sensitivity, $\partial (\Delta f_{\mathbf {0}}/f_{\mathbf {0}})/\partial (\Delta P$ ), is 9200 ppm, a 24% improvement after optimization, when $100~\mu \text{L}$ of methanol (MeOH) is applied on the interdigitated capacitive sensor. The design methodology was also applied to an intraocular pressure (IOP) sensor and an implantable humidity sensor from literature and led to 661% and 41% improvement in the proposed FoM in simulations, respectively.

Electromagnetic Performance Evaluation of UHF RFID Tags With Power Discretization Error Cancellation

The rigorous electromagnetic characterization of passive UHF radio frequency identification (RFID) tags is a challenging task. Among various solutions, in a previous work, an automatic tool for the over-the-air evaluation of significant RFID tag metrics has been presented. Once the distance is fixed, the system, based on the estimation of the minimum power emitted by the off-the-shelf interrogating reader capable to energize the tag under test, exhibits an accuracy that, although adequate, is affected by the minimum power step at the reader side. In this communication, an evolution of a new platform whose working principle has already been introduced by the same authors is presented. It is based on an analytical model opportunely derived to overcome this limit while preserving flexibility and cost-effectiveness. The platform can automatically vary with continuity the interrogation distance—which becomes a new degree of freedom—and it is able to estimate the couple power distance for each frequency, which practically annuls the estimation error due to the power discretization. Moreover, the enhanced system now allows for a pretest phase that is useful to verify whether or not the implemented analytical model is valid in the specific environment, even if not electromagnetically anechoic.

Interrogation Techniques and Interface Circuits for Coil-Coupled Passive Sensors.

Coil-coupled passive sensors can be interrogated without contact, exploiting the magnetic coupling between two coils forming a telemetric proximity link. A primary coil connected to the interface circuit forms the readout unit, while a passive sensor connected to a secondary coil forms the sensor unit. This work is focused on the interrogation of sensor units based on resonance, denoted as resonant sensor units, in which the readout signals are the resonant frequency and, possibly, the quality factor. Specifically, capacitive and electromechanical piezoelectric resonator sensor units are considered. Two interrogation techniques, namely a frequency-domain technique and a time-domain technique, have been analyzed, that are theoretically independent of the coupling between the coils which, in turn, ensure that the sensor readings are not affected by the interrogation distance. However, it is shown that the unavoidable parasitic capacitance in parallel to the readout coil introduces, for both techniques, an undesired dependence of the readings on the interrogation distance. This effect is especially marked for capacitance sensor units. A compensation circuit is innovatively proposed to counteract the effects of the parasitic input capacitance, and advantageously obtain distance-independent readings in real operating conditions. Experimental tests on a coil-coupled capacitance sensor with resonance at 5.45 MHz have shown a deviation within 1.5 kHz, i.e., 300 ppm, for interrogation distances of up to 18 mm. For the same distance range, with a coil-coupled quartz crystal resonator with a mechanical resonant frequency of 4.432 MHz, variations of less than 1.8 Hz, i.e., 0.5 ppm, have been obtained.

Electronic technique and circuit topology for accurate distance-independent contactless readout of passive LC sensors

Contactless readout of passive LC sensors composed of a capacitance sensor connected to a coil can be performed through a readout coil electromagnetically coupled with the sensor coil. Resonant frequency and quality factor can be extracted from the impedance measured at the readout coil by a technique which results theoretically independent of the coupling, and, therefore, the distance between the readout and the sensor coils. However, the effect of the unavoidable parasitic capacitance connected to the readout coil introduces in practice an undesired dependence of the measured values on the distance between the coils, resulting in a decreased accuracy. In this paper, such dependence is predicted by the results of a numerical analysis and experimentally verified. To overcome this limitation, a novel electronic technique and circuit topology for the compensation of the readout parasitic capacitance is proposed. The experimental results show that the compensation technique allows to read the resonant frequency of a LC pair at around 5.3 MHz with a variation of less than 200 ppm across an interrogation distance between 2 and 24 mm.

Robust Reading Approach for Moving Chipless RFID Tags by Using ISAR Processing

A robust technique for reading moving chipless tags exploiting the inverse synthetic aperture radar data processing is presented. The procedure takes advantage from the known tag movement to improve the tag detection capability by integrating several received signals. The implicit advantage of the proposed detection scheme is that the background measurement, collected in the absence of the tag, can be easily performed before the tag passes in front of the reader antenna. The reading procedure allows for a significant increase of the interrogation distance and, more importantly, it greatly improves the detection probability in complex scenarios, where the tag is attached on arbitrary and highly scattering objects. A simplified model is used to demonstrate the capabilities of the presented procedure in detecting tags even in the presence of high clutter. Detection of tags with signal-to-noise ratio as low as −10 dB is experimentally demonstrated.

Applying Metamaterial-Based Repeater in LC Passive Wireless Sensors to Enhance Readout

Inductor-capacitor (LC) passive wireless sensors allow for a continuous, real-time monitoring for various parameters of interest in situations, where wired connections are difficult or even impossible. However, geometrical constraints reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Here, we propose a left-handed metamaterial as a repeater to enhance the readout of the LC passive wireless sensors. The left-handed metamaterial consists of double-side square spirals and possesses negative relative permeability. The operating frequency of the left-metamaterial is designed adjustable to cover the frequency spectrum of the sensor. By monitoring the relative humidity as an example, we have demonstrated that the readout distance with the left-handed metamaterial can achieve nearly 1.8 times as long as that without metamaterial, while keeping their signal strength and sensitivity the same. Overall, this strategy through the metamaterials-based repeaters represents an effective approach for enhancing the readout.

Analysis and Validation of Contactless Time-Gated Interrogation Technique for Quartz Resonator Sensors.

A technique for contactless electromagnetic interrogation of AT-cut quartz piezoelectric resonator sensors is proposed based on a primary coil electromagnetically air-coupled to a secondary coil connected to the electrodes of the resonator. The interrogation technique periodically switches between interleaved excitation and detection phases. During the excitation phase, the resonator is set into vibration by a driving voltage applied to the primary coil, whereas in the detection phase, the excitation signal is turned off and the transient decaying response of the resonator is sensed without contact by measuring the voltage induced back across the primary coil. This approach ensures that the readout frequency of the sensor signal is to a first order approximation independent of the interrogation distance between the primary and secondary coils. A detailed theoretical analysis of the interrogation principle based on a lumped-element equivalent circuit is presented. The analysis has been experimentally validated on a 4.432 MHz AT-cut quartz crystal resonator, demonstrating the accurate readout of the series resonant frequency and quality factor over an interrogation distance of up to 2 cm. As an application, the technique has been applied to the measurement of liquid microdroplets deposited on a 4.8 MHz AT-cut quartz crystal. More generally, the proposed technique can be exploited for the measurement of any physical or chemical quantities affecting the resonant response of quartz resonator sensors.

Development of a distance-independent wireless passive RF resonator sensor and a new telemetric measurement technique for wireless strain monitoring

We proposed and developed a novel wireless passive RF resonator scheme that enables telemetric strain sensing avoiding the need for calibration at different interrogation distances. The specific architecture of the proposed structure allows for strong inductive coupling and, thus, a higher wireless signal-to-noise ratio. Here, in operation, the frequency scan of the sensor impedance was used to measure simultaneously both the impedance amplitude and resonance frequency. Using this wireless sensor, we further introduced a new telemetric monitoring modality that employs full electrical characteristics of the system to achieve correct strain extraction at any interrogation distance. In principle, any deformation of the sensor structure results in the resonance frequency shift to track strain. However, changing of the interrogation distance also varies the inductive coupling between the sensor and its pick-up antenna at the interrogation distance. Therefore, at varying interrogation distances, it is not possible to attribute an individual resonance frequency value solely to an individual strain level, consequently resulting in discrepancies in the strain extraction if the interrogation distance is not kept fixed or distance-specific calibration is not used. In this work, we showed that by using both the proposed passive sensor structure and wireless measurement technique, strain can be successfully extracted independent of the interrogation distance for the first time. The experimental results indicate high sensitivity and linearity for the proposed system. These findings may open up new possibilities in applications with varying interrogation distance for mobile wireless sensing.

&lt;italic&gt;LC&lt;/italic&gt; Passive Wireless Sensors Toward a Wireless Sensing Platform: Status, Prospects, and Challenges

Inductor–capacitor ( $LC$ ) passive wireless sensors use a transformer with loose coupling between an external readout coil and an inductor that receives power through this inductive coupling. Changes in the sensor are wirelessly and remotely detected by the readout coil, which makes them highly useful in applications that require the sensor to be powered remotely and to occupy a small volume, such as harsh and sealed environments, where physical access to the sensor is difficult. Although the sensor to accomplish this function dates from the 1960’s, its rapid extension over the past decades has benefited from microelectromechanical systems. This paper provides an overview of the status and challenges in the $LC$ passive wireless sensor toward a wireless sensing platform. The basic sensing principles are first categorized into detecting changes of the sensor in response to the capacitance, resistance, inductance, or coupling distance due to the parameter of interest through monitoring the impedance magnitude and phase spectrum. The present state of the art in sensor applications for pressure, strain, temperature, humidity, biochemical, gas, and so on is then reviewed and compared. For emerging applications from many Internet of Things scenarios, geometrical constraints, such as small and non-invasive coils, reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Furthermore, an increasing number of applications also require the simultaneous measurement of multiple parameters. Recent efforts to increase the interrogation distance and to extend the measurement of single parameter to multiple parameters are finally outlined. [2016-0093]

A Time-gated Contactless Interrogation System for Frequency and Quality Factor Tracking in QCR to Investigate on Liquid Solution Microdroplets

In this work, a contactless interrogation via electromagnetic coupling based on time-gated techniques has been proposed for Quartz Crystal Resonator (QCR) to investigate on liquid solution microdroplets. The sensing of the resonator response is innovatively based on a heterodyne demodulation and autocorrelation analysis, providing high performances yet avoiding the use of expensive instruments and allowing to estimate both the sensor damped resonant frequency and the quality factor Q. The system works with interrogation distances up to few centimeters and has been experimentally validated by loading the QCR with microdroplets of different liquid microsamples.