Search Results (343)

Conventional active disturbance rejection control (ADRC) schemes for grid-connected inverters (GCIs) suffer from performance degradation under grid frequency fluctuations and voltage distortions, primarily due to the limited bandwidth of conventional extended state observers (ESOs) and their reliance on fixed-frequency internal models. To address these issues, this paper proposes a second-order repetitive ADRC (SRC-ADRC) strategy for LCL-type single-phase GCIs. The proposed method integrates a second-order repetitive extended state observer (SRC-ESO) and a frequency-adaptive proportional-quasi resonant (FPQ) controller. By cascading an additional internal model into the repetitive control-based ESO, the SRC-ESO achieves higher internal model gain and wider resonant bandwidth, significantly improving periodic disturbance estimation accuracy under off-nominal grid frequencies. The FPQ controller ensures reference current tracking without phase error and amplitude attenuation, while the control law and observer are decoupled for independent parameter design. The robustness and frequency adaptability of the SRC-ADRC are verified through theoretical stability analysis and frequency-domain evaluation. Experiments on a 1.5 kW LCL-type single-phase GCI platform show that SRC-ADRC provides better frequency robustness, stronger harmonic mitigation, and improved current-tracking performance than the comparison methods. Full article

To address head-end three-phase current unbalance and terminal power-quality deterioration caused by uneven three-phase load allocation in a low-voltage transformer area (LVTA), this paper proposes a DC-link-voltage-control-based phase-wise unbalanced power compensation strategy for a head-to-tail flexible interconnection structure embedded in the LVTA. The proposed structure consists of two three-phase four-leg converters sharing a common DC bus and connected to the head end and tail end of the LVTA, respectively. Different from conventional phase-wise compensation methods in which the DC side mainly acts as a power-transfer channel, the proposed strategy uses the DC-link voltage control of the head-end converter as the core of compensation power generation. Specifically, the outer DC-link voltage loop generates the total active compensation power, which is then allocated among the three phases according to the measured phase-power unbalance of the LVTA, thereby yielding the phase-wise compensation current references. Combined with phase-wise quasi-proportional-resonant current control, the compensation currents of different phase legs can be regulated without explicit positive-, negative-, and zero-sequence decomposition. Meanwhile, the tail-end converter adopts PQ control to support terminal power regulation and improve the terminal voltage quality of the LVTA. To provide a theoretical basis for the proposed method, a switching-cycle averaged model of the three-phase four-leg converter is established, and the leg-level phase-wise control characteristics are analyzed under the assumptions of a stiff DC link and symmetrical converter parameters. A control-oriented equivalent LVTA model is developed in MATLAB/Simulink. The proposed strategy is validated under steady-state unbalanced, RL load, load-disturbance, and equivalent feeder-impedance conditions. In addition, a conventional positive-, negative-, and zero-sequence compensation method is introduced as a benchmark for quantitative comparison. The simulation results demonstrate that the proposed method can effectively suppress the head-end three-phase current unbalance, maintain the DC-link voltage around its reference value, and improve the terminal voltage quality of the LVTA. Compared with the conventional sequence-component-based compensation method, the proposed strategy achieves effective unbalance mitigation while avoiding explicit sequence extraction and reducing the complexity of the compensation-current generation process. This study provides a feasible control framework for three-phase unbalance mitigation in flexible low-voltage transformer areas. Full article

Reservoir effectiveness in marine shales is controlled not only by pore volume but also by pore-fluid occurrence, pore–throat connectivity, and mineral–organic matter coupling. In this study, the Permian Gufeng Formation shales from the Enshi area, western Hubei, South China, were investigated through an integrated analysis of total organic carbon (TOC), X-ray diffraction (XRD)-based mineral composition and lithofacies, low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), micro-computed tomography (Micro-CT), and entropy-weighted technique for order preference by similarity to an ideal solution (TOPSIS) evaluation. The TOC content ranges from 1.60% to 21.38% and shows clear vertical differentiation, with moderate but variable enrichment in the lower interval, reduced organic matter abundance in the middle interval, and pronounced organic enrichment in the upper interval. Mineral compositions demonstrate an upward transition from a mixed siliceous–carbonate system to a dominantly siliceous shale system. NMR results reveal strong heterogeneity in porosity, NMR-derived permeability, T2cutoff, bound-fluid saturation, and free-fluid saturation. Based on saturated and centrifuged T2 spectra, four descriptive reservoir response types were identified: short-T2-dominated micropore-bound response, intermediate-T2-dominated movable-fluid response, long-T2-enriched but low-efficiency response, and NMR-inferred enhanced mobility composite response. SEM observations show diverse pore types, including organic-matter-related pores, dissolution pores, interparticle pores, mineral-edge pores, pyrite intercrystalline pores, and local microfracture-like pores. Micro-CT results indicate that micrometer-scale pore bodies are commonly isolated, demonstrating that pore abundance or pore size alone cannot determine reservoir effectiveness. TOC mainly controls pore generation potential, whereas siliceous minerals, pore–throat connectivity, movable fluid proportion, and local fractures exert stronger controls on effective reservoir development. The most favorable reservoir responses are concentrated in the upper high-organic siliceous shale interval from A33 to A42, with local enhanced responses in A16 and A21. These results provide an integrated framework for evaluating reservoir heterogeneity and favorable intervals in complex marine shale systems. Full article

To address the issues of pronounced resonance, limited control bandwidth, and insufficient trajectory tracking accuracy in piezoelectric-driven elliptical vibration-assisted cutting (EVC) systems under high-frequency vibration, this paper proposes a trajectory tracking control strategy combining damping control with frequency-domain shaping. First, a damping-control strategy is integrated into the control system to refine the plant’s inherent dynamic properties, suppressing the resonance peak and elevating the system’s stability margin. Second, to enhance the system bandwidth and dynamic response, a high-gain PID controller is designed via frequency shaping. Additionally, given that the nominal model becomes high-order after implementing the damping controller, proportional gain is used for approximate equivalence with the system transfer function, lowering the model order and streamlining controller design. Next, a disturbance observer (DOB) is introduced to estimate and compensate for the unmodeled dynamics in the feedforward path in real time, further improving the trajectory tracking accuracy. Finally, taking the designed piezoelectric-driven EVC device as the controlled plant, the system frequency response is obtained through sweep excitation experiments, based on which the nominal model is identified, and the controller parameters are determined. The experimental results demonstrate that the proposed control strategy effectively suppresses resonance effects, increases system bandwidth, and reduces the trajectory tracking error. In the complex harmonic superposition trajectory tracking experiment, the steady-state tracking error is maintained within ±0.09 μm. These results demonstrate that the proposed approach markedly improves the system’s dynamic response and trajectory tracking performance, thereby providing technical support for high-precision fabrication of micro/nano-structured surfaces. Full article

Deep tight sandstone reservoirs are characterized by strong microscopic pore structure heterogeneity and commonly exhibit a high-porosity, low-permeability profile, posing significant challenges for effective reservoir evaluation and “sweet spot” prediction. The microscopic pore structure of 209 tight sandstone samples from the deeply buried Huagang Formation in the Xihu Depression, East China Sea Basin, was systematically characterized by integrating multiple analytical techniques, including casting thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and high-pressure mercury injection (HPMI). The results indicate that the reservoir space is dominated by mesopores (55.48%) and transition pores (32.39%), with macropores (2.09%) and micropores (10.04%) being relatively underdeveloped. A significant vertical heterogeneity in reservoir quality is observed. The H4 member exhibits superior properties, characterized by a higher average movable fluid saturation (averaging 46%) and better pore connectivity. In contrast, the H5 member is more compact, with a notably higher proportion of bound fluid (averaging 47%). The differences in reservoir quality are controlled by a sedimentary–diagenetic coupling mechanism. High-energy, coarse-grained facies underwent a constructive pathway involving chlorite coating protection and dissolution enhancement, forming high-quality pore networks. In contrast, low-energy, fine-grained facies experienced a destructive pathway dominated by intense compaction and cementation, leading to the deterioration of pore structure. The petrophysical properties of the deep reservoirs are primarily governed by the three-dimensional connectivity and spatial distribution of effective “pore-throat assemblages” composed of dominant throats. Accordingly, a “binary” pore structure development pattern is established for the deep tight sandstone reservoirs in the study area. This pattern posits that the reservoir space is heterogeneously composed of a minority of connected “effective percolation assemblages” and a majority of isolated “ineffective assemblages”. Full article

Background: Er:YAG laser (λ = 2940 nm) biomodification of the implant osteotomy site removes the smear layer after rotary preparation and may enhance bone-implant contact. This randomized controlled clinical study evaluated implant stability dynamics following Er:YAG laser biomodification using resonance frequency analysis (RFA). Methods: Ninety patients were randomized 1:1 into a case group (n = 45; rotary osteotomy + Er:YAG biomodification; 400 mJ, 17 Hz) and a control group (n = 45; rotary osteotomy alone). Implant stability quotient (ISQ) was measured by RFA in vestibulo-oral (VO) and mesiodistal (MD) directions at placement, days 10, 20, 30, and month 3. Results: The case group showed significantly higher ISQ values at all time points in both directions (t-test, p < 0.05). Repeated measures ANOVA revealed a significant time × group interaction in the MD direction (F = 14.461, p < 0.001, partial η² = 0.341). Primary VO ISQ: 75.04 ± 4.27 (cases) vs. 72.29 ± 3.38 (controls); primary MD ISQ: 76.49 ± 4.29 vs. 72.89 ± 2.29. The proportion achieving ISQ ≥ 70 was consistently higher in the case group. Conclusions: Er:YAG laser biomodification combined with rotary osteotomy yields higher, more stable ISQ values throughout early healing in mandibular D2/D3 bone, potentially supporting shorter healing intervals and early loading in selected clinical situations. Full article

To investigate the mechanism by which a combination of desert sand (DS) and recycled coarse aggregate (RA) affects the sustainability of recycled concrete, multiple mix proportions with varying replacement ratios were designed in this study. Macroscopic performance tests and microscopic analyses were performed using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and X-ray diffraction (XRD), and the results were combined with grey relational analysis to reveal the intrinsic relationships between pore parameters and macroscopic properties. Additionally, technical and economic evaluations were conducted. The results indicate that incorporating either type of aggregate individually has a nonlinear effect on the compressive strength and impermeability of concrete. The optimal compressive strength is achieved when both aggregates are used at 20% replacement, whereas the best impermeability occurs at 10% replacement for each. The proportions of transitional pores and capillary pores, along with T₂ relaxation parameters, serve as key microstructural indicators for controlling performance. Economically, the use of both aggregates together significantly reduces material costs—reaching a cost savings rate of 3.51% with a recycled aggregate replacement level of 30%. Further substitution of 30% desert sand for river sand under the same replacement ratio can reduce costs by an additional 1.56%. This mix proportion achieves optimal synergy among mechanical performance, cost control, and low-carbon benefits. The findings provide theoretical guidance and practical support for mix design, durability enhancement, and the promotion of such green, low-cost concrete in engineering applications. Full article

Background: Vitamin D deficiency (VDD) may be associated with chronic health issues, including markers of liver disease; however, evidence in American Indian adolescents is limited. Therefore, we aimed to evaluate the relationship between VDD in American Indian adolescents and markers of liver disease, including both serum and magnetic resonance imaging (MRI) markers. Methods: The Strong Heart Family Study (SHFS) is a multicenter, family-based, prospective cohort study among American Indians. We evaluated SHFS participants who were <20 years old at baseline (2001–2003; n = 308), defined VDD as hydroxyvitamin-D (25[OH]D) ≤ 20 ng/mL and calculated the Hepatic Steatosis Index (HSI). In 2006–2009, we measured follow-up serum markers (n = 269, median follow-up = 5.8 years, range = 3.0–8.5), and in 2018–2020, we collected MRI markers of hepatic steatosis and fibrosis (n = 33, median follow-up = 16.7 years, range = 15.3–18.1). Results: At baseline, a greater proportion of those with VDD had HSI > 36 (64.5% with VDD; p = 0.002). For participants who reported consuming alcohol, serum alkaline phosphate (ALP) increased at follow-up, which was higher for those with versus without VDD (beta = 12.24, 95% CI = 1.23–23.24). For participants who reported not consuming alcohol, ALP decreased but was not different between the VDD groups. The MRI-proton density fat fraction was higher in those with (median = 11.0%, IQR = 7.7–19.2%) versus without (median = 4.4, IQR = 1.8–6.3%) VDD at baseline (p < 0.001). Conclusions: VDD may be associated with markers of liver disease in American Indian adolescents. After 5.5 years, VDD was associated with increasing ALP in those who consume alcohol, while controlling for adiposity and other covariates. After 17 years of follow-up, VDD was associated with increased liver MRI-PDFF. Full article

The paper presents a fast adaptive power control with implicit predictive behavior for an on-board power converter operating in support of a 400 Hz aircraft electrical network. Accurate control of active and reactive power in such high-frequency networks requires precise estimation of the network voltage phase, frequency, and amplitude. Therefore, a proposed adaptive phase-locked loop (PLL) algorithm is integrated with a proportional resonant current controller (PR). The adaptive PLL continuously estimates the instantaneous phase, frequency, and amplitude of the fundamental voltage component, enabling fast synchronization and dynamic adjustment of the PR controller resonant frequency. Consequently, the combination familiarises anticipatory response characteristics with the control loop without the need for computationally intensive model predictive control algorithms. The simulation results demonstrate that the proposed method significantly reduces the synchronization time, maintains high accuracy under frequency variations and harmonic distortion, and exhibits robustness against measurement noise. Furthermore, the modular and computationally efficient structure of the algorithm makes it suitable for real-time implementation of FPGA. The proposed approach provides an effective solution for high-performance power management in aircraft electrical systems, ensuring precise power control under hard dynamic conditions. Full article

This paper presents a model-based control strategy for a power factor correction system that employs a high conversion-ratio DC–DC converter. The proposed system consists of two stages. In the first stage, a full-bridge diode rectifier is connected to the grid through a passive filter to improve the quality of the injected current. Two passive AC input filters, namely L and $LCL$ configurations, are evaluated to analyze their impact on grid current quality and overall system performance. The second stage is a high-step-up DC–DC converter based on the switched-inductor technique, which provides a high voltage conversion ratio. A model-based approach is employed to derive the control design from the averaged system model. The resulting control structure consists of a current tracking loop and a voltage regulation loop. A proportional-resonant controller is used to ensure current tracking and achieve a near-unity power factor, while a proportional-integral controller regulates the output voltage. Experimental validation is carried out using a low-power laboratory-scale prototype to assess the effectiveness of the proposed approach. The results demonstrate adequate current tracking and satisfactory dynamic performance within the tested operating conditions. Full article

To address fault-period DC-link overvoltage, the reduction in grid-side active-power regulation margin caused by reactive-current-priority operation, and the double-frequency current fluctuation induced by negative-sequence components under asymmetrical grid faults in a flywheel-assisted permanent-magnet direct-drive wind power system, this paper proposes a coordinated low-voltage ride-through (LVRT) strategy based on DC-link-voltage-threshold partitioning. According to the DC-link voltage level, the operating process is divided into a normal regulation region, a grid-side saturation region, and a flywheel activation region, thereby enabling coordinated regulation between grid-side reactive-current support and flywheel-side active-power absorption. To improve transient smoothness, an anti-windup mechanism together with a bumpless transfer scheme is incorporated into the coordinated control process to suppress integrator saturation and mitigate mode-transition disturbances. In addition, a grid-side proportional–integral–vector resonant controller (PI-VRC) is introduced to improve the suppression of double-frequency current fluctuation under asymmetrical faults and enhance converter capacity utilization. Simulation results show that the proposed strategy can effectively restrain fault-period DC-link voltage rise, improve three-phase current symmetry and grid power quality, and strengthen transient reactive-power support, thereby enhancing the asymmetrical-fault LVRT capability of the system. Full article

The increasingly prominent “double-high” characteristics (high penetration of renewable energy and high proportion of power electronic devices) in modern power systems pose severe challenges to secure and stable operation, especially due to wideband oscillations induced by grid-connected inverters. In view of the fact that existing impedance modeling for grid-forming control often neglects the decoupling effect of the LC filter capacitor and the dynamics of inner voltage/current loops, leading to inaccurate characterization of mid-to-high frequency impedance, this paper aims to establish more accurate impedance models for grid-connected inverters and to develop effective oscillation mitigation methods accordingly. First, the harmonic linearization method is adopted to derive refined positive- and negative-sequence impedance analytical models for NPC inverters under both grid-following and grid-forming control. Second, simulation-based frequency scanning is conducted to validate the accuracy of the proposed models, and the differences in system resonance characteristics under the two control modes are comparatively analyzed. Finally, oscillation suppression strategies based on active damping and virtual impedance are, respectively, designed. The results show that the proposed models can accurately characterize mid-to-high frequency impedance, reveal the distinct resonance mechanisms of different control modes, and the proposed suppression strategies can effectively attenuate wideband oscillations. These findings provide theoretical foundations and practical technical pathways for stability analysis and optimization design of inverter-grid systems in high-renewable-penetration scenarios. Full article

Pitch control is the standard approach to regulating the rotational speed of large wind energy systems when the wind speed goes over its rated value. However, the pitch control system can also be used to reduce blade loads. In this last case, it is necessary to extend the classic collective pitch control system by including a complicated mechanism, which involves a Coleman or a Clarke transformation. This extension is known as the individual pitch control (IPC). While the performance of the IPC is satisfactory regarding the load alleviation, its dynamics remain insufficiently comprehended, especially due to the previously mentioned embedded transformations. Hence, the tuning of the IPC is sometimes challenging, and the controller can exhibit unexpected behaviours. The idea of this work is to formulate the IPC as a multivariable controller in the input/output representation such that the classic tools for the analysis and control of linear systems can be applied. As a result, some lesser-known properties as well as limitations are disclosed. Specifically, the approach makes apparent the existence of proportional-resonant controllers, which are crucial for dynamical behaviour. This additional knowledge can assist in the design of control systems and the tuning of controllers. A simulation study completes the presentation, including qualitative and quantitative analysis. Full article

Alzheimer’s disease (AD) is characterized by pronounced spatial heterogeneity and complex neurodegenerative patterns, which pose significant challenges for representation learning on three-dimensional brain images. Conventional convolutional neural networks relying on regular grid sampling struggle to align localized structural degeneration, and their performance is further compromised by class imbalance and a high proportion of weakly discriminative samples, leading to suboptimal optimization dynamics and reduced generalization ability. To address the aforementioned challenges, this study proposes Deformable Attention and Risk-aware 3D Network (DAR-3DNet), a modeling framework for 3D magnetic resonance imaging (MRI) classification. The proposed method incorporates deformable sampling and sampling-point modulation into spatial attention generation, enabling the attention estimation process to better adapt to the non-rigid spatial patterns associated with brain structural degeneration. On this basis, an instance-adaptive label smoothing loss with composite risk, termed Instance-wise Adaptive Label Smoothing Loss with Composite Risk (IASLCR), is further introduced to dynamically adjust supervision strength based on sample-specific risk, thereby alleviating optimization bias caused by class imbalance and weakly discriminative samples. Experiments conducted on 1749 structural MRI scans from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) dataset show that the proposed method achieves accuracy values above 0.93 on the AD vs. normal control (NC), mild cognitive impairment (MCI) vs. NC, and AD vs. MCI classification tasks, while yielding better overall performance than the evaluated baseline models. These results suggest that the proposed framework has considerable potential for structural MRI-based AD/MCI classification. Full article

The Ordos Basin hosts abundant lacustrine shale oil resources. Adequately retained hydrocarbons in source rocks, together with favorable mobility, are prerequisites for large-scale shale oil exploitation. Therefore, the quantitative characterization of retained hydrocarbon content and mobility is a core research focus in shale oil exploration and development. This study investigates Chang 7 shale with varying lithofacies and geochemical characteristics. Stepwise pyrolysis and pyrolysis gas chromatography–mass spectrometry (GC–MS) were applied to analyze retained hydrocarbons in different occurrence states, their compositions, and biomarkers. In addition, nuclear magnetic resonance (NMR) combined with CO₂ flooding experiments was conducted, and the collected products under different displacement pressures were analyzed using GC–MS. The aim was to quantitatively examine the variations in expelled oil volume, compositional differences during migration, and occurrence features of shale oil within reservoir micro-pores. The results show the following: (1) Organic-rich shale is characterized by higher proportions of light and medium hydrocarbons, lower heavy fractions, and elevated aromatic hydrocarbon content. In contrast, low-organic-carbon mudstone or siltstone contains more medium and heavy hydrocarbons, with lower light and aromatic fractions. The C13−/C14+ ratio increases with total organic carbon (TOC). (2) In black shale, oil displacement is mainly contributed by mesopores. At low pressures, oil expulsion is difficult and dominated by heavy hydrocarbons. When pressure reaches a threshold, the capillary-bound oil in micropores is released, increasing production and improving oil quality. Muddy siltstone shows higher displacement efficiency than black shale, with contributions from pores of all sizes. At low pressures, its expelled oil volume is larger and lighter than that of black shale. With increasing pressure, the oil yield rises significantly, and medium–large pores produce heavier fractions compared with micropores, likely because light hydrocarbons preferentially enter micropores and are less prone to dissipation. (3) The main controlling factors for shale oil enrichment include retained hydrocarbon content, mobile hydrocarbon fraction, fluidity, and engineering-related parameters. Thick shale layers with high organic matter abundance, high proportions of light–medium hydrocarbons, and favorable porosity–permeability conditions, as well as interbedded siltstone, are enriched in mobile hydrocarbons. Full article