Impedance Variance Research Articles

Improved Grid Impedance Compensation for Phase-Locked Loop to Stabilize the Very-Weak-Grid Connection of VSIs

Voltage source inverters (VSIs) with vector control based on phase-locked loop (PLL) suffer instability when connecting to a very weak AC grid (short circuit ratio (SCR) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &lt; $</tex-math></inline-formula> 1.3). The conventional inductive grid impedance compensation for the PLL by virtually reducing the grid impedance can stabilize this connection. However, the analysis in this paper indicates that its stabilization effectiveness is sensitive to grid impedance variance and, indeed, overcompensation causes the PLL instability. Therefore, in this paper, an improved grid impedance compensation for the PLL is proposed to achieve the same stabilization for very-weak-grid connection and possess a good tolerance of grid impedance variance and overcompensation. A comprehensive small-signal model of the VSI using the proposed PLL’s grid impedance compensation is derived for stability analysis and parameter design. The time-domain simulation for this VSI is built to validate the stability analysis. Comparison studies for both proposed and conventional PLL’s grid impedance compensation are conducted including the stability effectiveness, VSI performance and grid impedance variance.

Learning-Based Multifunctional Elbow Exoskeleton Control

We propose a learning-based model for multifunctional elbow exoskeleton control, i.e., assist- and resist-as-needed (AAN and RAN). The model consists of online iterative learning and impedance adaptation mechanisms for predictive and variable compliant joint control. The model was implemented on a lightweight (0.425 kg) and portable elbow exoskeleton (i.e., POW-EXO) worn by three subjects, respectively. The implementation relies only on internal pose (e.g., joint position) feedback, rather than physical compliant mechanisms (e.g., springs) and external sensors (e.g., electromyography (EMG) or force) typically required by conventional exoskeletons and controllers. The proposed model provides a novel technique to achieve multifunctional exoskeleton control with minimal mechatronics and sensing. Interestingly, its RAN control and POW-EXO as a quantification means, may reveal interactive (mechanical) impedance variance and invariance in human motor control.

Towards Estimating Arterial Diameter Using Bioimpedance Spectroscopy: A Computational Simulation and Tissue Phantom Analysis.

This paper improves the accuracy of quantification in the arterial diameter-dependent impedance variance by altering the electrode configuration. The finite element analysis was implemented with a 3D human wrist fragment using ANSYS Electronics Desktop, containing fat, muscle, and a blood-filled radial artery. Then, the skin layer and bones were stepwise added, helping to understand the dielectric response of multi-tissues and blood flow from 1 kHz to 1 MHz, the current distribution throughout the wrist, and the optimisation of electrode configurations for arterial pulse sensing. Moreover, a low-cost wrist phantom was fabricated, containing two components: the surrounding tissue simulant (20 wt % gelatine power and 0.017 M sodium chloride (NaCl) solution) and the blood simulant (0.08 M NaCl solution). The blood-filled artery was constricted using a desktop injection pump, and the impedance change was measured by the Multi-frequency Impedance Analyser (MFIA). The simulation revealed the promising capabilities of band electrodes to generate a more uniform current distribution than the traditional spot electrodes. Both simulation and phantom experimental results indicated that a longer spacing between current-carrying (CC) electrodes with shorter spacing between pick-up (PU) electrodes in the middle could sense a more uniform electric field, engendering a more accurate arterial diameter estimation. This work provided an improved electrode configuration for more accurate arterial diameter estimation from the numerical simulation and tissue phantom perspectives.

Enhanced dynamic performance of grid feeding distributed generation under variable grid inductance

&lt;p&gt;Controlling weak grid-connected systems is very challenging. In transient, frequency and voltage oscillations may lead to voltage and/or frequency stability problems and finally lead to system collapse. During steady-state operation and at the point of common coupling (PCC), voltage degradation and grid voltage background harmonics restrict the inverter's functionality, reduce the power flow capability and cause poor power quality. With weak grid connection, grid impedance variance will contaminate the voltage waveform by harmonics and augment the resonance, destabilizing the inverter operation. In this paper, complete mathematical modeling is carried out and state feedback-plus-integral control is implemented to support the stabilization of the system. The proposed controller is adopted to provide a smooth transient under sudden load change by controlling the injected grid current under different grid inductance values. Furthermore, the proposed control is used to reduce the order and size of the inverter output filter while maintaining system stability. The proposed control has been compared with the conventional proportional integral (PI) controller under different scenarios to validate its effectiveness and to strengthen its implementation as a simple controller for distributed generator applications.&lt;/p&gt;

High-resolution seismic acoustic impedance inversion with the sparsity-based statistical model

Seismic acoustic impedance inversion plays an important role in subsurface quantitative interpretation. Due to the band-limited property of the seismic record and the discretization of the continuous elastic parameters with a limited sampling interval, the inverse problem suffers from serious ill-posedness. Various regularization methods are introduced into the seismic inversion to make the inversion results comply with the prespecified characteristics. However, conventional seismic inversion methods can only reflect fixed distribution characteristics and do not take into account discretization challenges. We have adopted a new poststack seismic impedance inversion method with upsampling and adaptive regularization. The adaptive regularization is constructed with two trained dictionaries from the true model and upsampled model-based inversion result to capture the features of high- and low-resolution details, and a sparsity-based statistical model is proposed to build the relationship between their sparse representations. The high-resolution components can be recovered based on the prediction model and low-resolution sparse representations, and the parameters of the statistical prediction model can be obtained effectively with conventional optimization algorithms. The synthetic and field data tests show that the model-based inversion is dependent on the sample interval, and our method can reveal more thin layers and enhance the extension of the strata compared with conventional inversion methods. Moreover, the inverted impedance variance of our method matches well with the borehole observations. The tests demonstrate that the interpolated model-based inversion result combined with the sparsity-based prediction model can effectively improve the resolution and accuracy of the inversion results.

A 0.5-V Sub-10-μW 15.28-mΩ/√Hz Bio-Impedance Sensor IC With Sub-1° Phase Error

Continuous monitoring of fluid status through bio-impedance (Bio-Z) measurement of thoracic magnitude or phase is critical in reducing the mortality of chronic heart failure (CHF) patients. However, the stringent power constraints of implantable devices force the design of Bio-Z sensors to be highly challenging. We present a sub-10- $\mu \text{W}$ Bio-Z sensor IC operating from a 0.5 V-supply, sustaining a sub-1° of phase error even under 10% of supply and 0 °C–70 °C of temperature variations. Fabricated in a 65-nm CMOS process, this sensor IC exhibits a 15.28- $\text{m}\Omega /\surd $ Hz of input-referred impedance noise performance, occupying a 4.83 mm2. Its linearity performance shows that the magnitude and the phase of Bio-Z can be measured within the error of 2% and 0.4°, respectively, given that the magnitude of load impedance is less than 126 $\Omega $ , and thus, it is suitable for fluid status monitoring applications. Experimental measurement on the human chest demonstrates its capabilities of thoracic impedance variance (TIV) and impedance cardiography (ICG) monitoring.

The Daily Variance in Impedance at Acupuncture Points

The Daily Variance in Impedance at Acupuncture Points

Method for Recovering Lost Ultrasonic Information Using the Echo-intensity Mean.

The axial resolution of a B-mode (or intensity) image is limited by the bandwidth of the pulse envelope. In this report, we investigate the source of this limitation by examining the transfer of high-resolution information from the tissue impedance variance throughout the imaging process. For that purpose, we express the mean and variance of the echo-intensity signal as a linear system to track the flow of object information along the image-formation chain. The results reveal how demodulation influences the available information by discarding high spatial-frequency information. This analysis further points to a simple way to recover lost information with only a minor addition to signal processing. Software phantoms are used to show that under ideal conditions, information from small-scale high-contrast reflectors, such as microcalcifications, can be significantly enhanced with this simple change to echo processing.

Intracardiac impedance response during acute AF internal cardioversion using novel rectilinear and capacitor-discharge waveforms

Intracardiac impedance (ICI) is a major determinant of success during internal cardioversion of atrial fibrillation (AF). However, there have been few studies that have examined the dynamic behaviour of atrial impedance during internal cardioversion in relation to clinical outcome. In this study, voltage and current waveforms captured during internal cardioversion of acute AF in ovine models using novel radiofrequency (RF) generated low-tilt rectilinear and conventional capacitor-discharge based shock waveforms were retrospectively analysed using a digital signal processing algorithm to investigate the dynamic behaviour of atrial impedance during cardioversion. The algorithm was specifically designed to facilitate the simultaneous analysis of multiple impedance parameters, including: mean intracardiac impedance (ZM), intracardiac impedance variance (ICIV) and impedance amplitude spectrum area (IAMSA) for each cardioversion event. A significant reduction in ICI was observed when comparing two successive shocks of increasing energy where cardioversion outcome was successful. In addition, ICIV and IAMSA variables were found to inversely correlate to the magnitude of energy delivered; with a stronger correlation found to the former parameter. In conclusion, ICIV and IAMSA have been evidenced as two key dynamic intracardiac impedance variables that may prove useful in better understanding of the cardioversion process and that could potentially act as prognostic markers with respect to clinical outcome.

Impedance and Scattering Variance Ratios of Complicated Wave Scattering Systems in the Low Loss Regime

Random matrix theory (RMT) successfully predicts universal statistical properties of complicated wave scattering systems in the semiclassical limit, while the random coupling model offers a complete statistical model with a simple additive formula in terms of impedance to combine the predictions of RMT and nonuniversal system-specific features. The statistics of measured wave properties generally have nonuniversal features. However, ratios of the variances of elements of the impedance matrix are predicted to be independent of such nonuniversal features and thus should be universal functions of the overall system loss. In contrast with impedance variance ratios, scattering variance ratios depends on nonuniversal features unless the system is in the high loss regime. In this paper, we present numerical tests of the predicted universal impedance variance ratios and show that an insufficient sample size can lead to apparent deviation from the theory, particularly in the low loss regime. Experimental tests are carried out in three two-port microwave cavities with varied loss parameters, including a novel experimental system with a superconducting microwave billiard, to test the variance-ratio predictions in the low loss time-reversal-invariant regime. It is found that the experimental results agree with the theoretical predictions to the extent permitted by the finite sample size.

Scattering versus intrinsic attenuation in the vadose zone: A VSP experiment

We studied scattering versus intrinsic attenuation estimates in the vadose zone from a shallow VSP experiment conducted in the Lawrence Livermore National Laboratory (LLNL) facility. Using permanent downhole geophones and a vertical impact source, we estimated effective attenuation of the downgoing transmitted P-wave. We compared theoretical scattering attenuation estimates and finite-difference synthetics to the measured field [Formula: see text] values ([Formula: see text] being a measure of attenuation). Using a selected range of impedance profiles of variance typical for a sedimentary basin, our estimates of [Formula: see text] are in the order of 20–85. Given the short propagation pathlengths involved ([Formula: see text]), we show that attenuation due to lateral heterogeneity is not significant. We analyzed additional distorting factors, including near-field presence, local impedance effects, and interference from reflections originating beneath receivers, and found that they may significantly impact attenuation measurement in near-surface studies, and result in biased [Formula: see text] values. From a comparison of the analyzed ranges of [Formula: see text] to the measured [Formula: see text] values, we deduced [Formula: see text], which is consistently low, but whose inferred frequency dependence depends strongly on the scattering model assumed. The ranges for the [Formula: see text] factor resulting from scattering and distorting factors, and the intrinsic [Formula: see text] value were estimated as, respectively, 4–7, and 7–4. We identified two potential mechanisms which could lead to low [Formula: see text] values in the vadose zone: patchy saturation and squirt flow. We found through viscoelastic 3D synthetic modeling using a standard linear solid (SLS), that the field [Formula: see text] frequency dependence can be reproduced, although nonuniquely, for the studied range of impedance variance.

Electroencephalography Leads Placed by Nontechnologists Using a Template System Produce Signals Equal in Quality to Technologist-Applied, Collodion Disk Leads

The purpose of this study was to compare the quality of the electroencephalographic (EEG) data obtained with a BraiNet template in a practical use setting, to that obtained with standard 10/20 spaced, technologist-applied, collodion-based disk leads. Pairs of 8-hour blocks of EEG data were prospectively collected from 32 patients with a Glasgow coma score of ≤9 and clinical concern for underlying nonconvulsive status epilepticus over a 6-month period in the Neurocritical Care Unit at the Duke University Medical Center. The studies were initiated with the BraiNet template system applied by critical care nurse practitioners or physicians, followed by standard, collodion leads applied by registered technologists using the 10/20 system of placement. Impedances were measured at the beginning and end of each block recorded and variance in impedance, mean impedance, and the largest differences in impedances found within a given lead set were compared. Physicians experienced in reading EEG performed a masked review of the EEG segments obtained to assess the subjective quality of the recordings obtained with the templates. We found no clinically significant differences in the impedance measures. There was a 3-hour reduction in the time required to initiate EEG recording using the templates (P < 0.001). There was no difference in the overall subjective quality distributions for template-applied versus technologist-applied EEG leads. The templates were also found to be well accepted by the primary users in the intensive care unit. The findings suggest that the EEG data obtained with this approach are comparable with that obtained by registered technologist-applied leads and represents a possible solution to the growing clinical need for continuous EEG recording availability in the critical care setting.

A 3.9 mW 25-Electrode Reconfigured Sensor for Wearable Cardiac Monitoring System

A low power highly sensitive Thoracic Impedance Variance (TIV) and Electrocardiogram (ECG) monitoring SoC is designed and implemented into a poultice-like plaster sensor for wearable cardiac monitoring. 0.1 Ω TIV detection is possible with a sensitivity of 3.17 V/Ω and SNR > 40 dB. This is achieved with the help of a high quality (Q-factor > 30) balanced sinusoidal current source and low noise reconfigurable readout electronics. A cm-range 13.56 MHz fabric inductor coupling is adopted to start/stop the SoC remotely. Moreover, a 5% duty-cycled Body Channel Communication (BCC) is exploited for 0.2 nJ/b 1 Mbps energy efficient external data communication. The proposed SoC occupies 5 mm × 5 mm including pads in a standard 0.18 μm 1P6M CMOS technology. It dissipates a peak power of 3.9 mW when operating in body channel receiver mode, and consumes 2.4 mW when operating in TIV and ECG detection mode. The SoC is integrated on a 15 cm × 15 cm fabric circuit board together with a flexible battery to form a compact wearable sensor. With 25 adhesive screen-printed fabric electrodes, detection of TIV and ECG at 16 different sites of the heart is possible, allowing optimal detection sites to be configured to accommodate different user dependencies.

Comportamento da impedância elétrica dos tecidos biológicos durante estimulação elétrica transcutânea

OBJECTIVE: To analyze the electrical impedance of biological tissues during electrical stimulation in relation to different segments, surfaces and current frequencies, with increasing distance between electrodes. METHOD: 20 female volunteers of mean age 23 ± 2.25 years and mean body mass index 20.65 ± 1.44 kg/m2 were positioned in decubitus with one electrode placed proximally to the wrist and ankle joint lines, anteriorly and posteriorly, or on the posterosuperior iliac spine, and the other electrode was placed at distance of 10, 20, 30 and 40 cm, sequentially. Two currents (100 us and 10 mA) were applied: one at 100 Hz (LF) and the other at 2000 Hz modulated at 100% of the amplitude for 100 Hz (MF), with a minimum interval of seven days. The impedance was calculated indirectly using Ohm's Law, from the applied intensity and the electrical voltage picked up by a system consisting of a digital oscilloscope (TDS 210, Tektronix®) and a direct current generator (Dualpex 961, Quark®). For statistical analysis, Anova-F and Kruskal-Wallis were applied, with post hoc (SNK), Friedman test and Spearman correlation coefficient, taking p< 0.05. RESULTS: Despite similar electrical impedance behavior with increasing distance between electrodes for the two currents, there was a reduction in impedance under MF stimulation. In the limbs, approximately 50% of the impedance variance was explained by the increase in electrode separation, although this relationship was not observed on the posterior surface of the trunk. Independent of the current type, the trunk presented the lowest electrical impedance, followed by the lower limbs. CONCLUSION: The electrical impedance of the tissues was influenced by current frequency and the positioning and distance between electrodes, thus presenting a non-uniform pattern in the different segments.