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The crystal structure uniformity is numerically estimated as the standard deviation of the crystal space quantizer ⟨G3⟩. This criterion has been applied to explore the uniformity of ionic sublattices in 21465 crystal structures of inorganic ionic compounds. In most cases, at least one kind of sublattice (whole ionic lattice, cationic or anionic sublattice) was found to be highly uniform with a small ⟨G3⟩ value. Non‐uniform structures appeared to be either erroneous or essentially non‐ionic. As a result, a set of uniformity criteria is proposed for the estimation of the stability of ionic crystal structures. [ABSTRACT FROM AUTHOR]

The stability of complex systems is profoundly affected by underlying structures, which are often modeled as networks where nodes indicate system components and edges indicate pairwise interactions between nodes. However, such networks cannot encode the overall complexity of networked systems with higher-order interactions among more than two nodes. Set structures provide a natural description of pairwise and higher-order interactions where nodes are grouped into multiple sets based on their shared traits. Here we derive the stability criteria for networked systems with higher-order interactions by employing set structures. In particular, we provide a simple rule showing that the higher-order interactions play a double-sided role in community stability—networked systems with set structures are stabilized if the expected number of common sets for any two nodes is less than one. Moreover, although previous knowledge suggests that more interactions (i.e. complexity) destabilize networked systems, we report that, with higher-order interactions, networked systems can be stabilized by forming more local sets. Our findings are robust with respect to degree heterogeneous structures, diverse equilibrium states and interaction types. [ABSTRACT FROM AUTHOR]

This paper explores the influence of engine modernization on the dynamic stability and performance of a general aviation aircraft. Utilizing an integral mathematical model, the study conducts a comparative analysis of the I-23 Manager piston-engine aircraft and its modified I-31T turboprop version, examining changes in aircraft dynamics depending on the power plant type. The modernization necessitated a redesign of the nose section of the fuselage, resulting in alterations to the external shape and flight properties of the aircraft. The research evaluates various dynamic stability parameters, including phugoid, short period, Dutch roll, roll, and spiral modes, under different flight conditions. Results indicate minimal changes in aerodynamic characteristics due to the engine type, yet significant improvements are observed in efficiency, noise reduction, and operational costs. The impact of the propulsion unit on the dynamic stability of the light aircraft was assessed as insignificant, suggesting that the strategy of modernizing an existing piston-driven aircraft by switching to a turboprop drive is indeed promising. With appropriate initial design assumptions, a modern turbine aircraft with strong flight qualities can be efficiently modernized in this way, without compromising the good flying properties of the existing plane. The outcomes are validated against flight tests, reinforcing the viability of integrating more sustainable and efficient propulsion systems into light aircraft. This study may therefore inform future design and regulatory decisions, providing a perspective on the implications of engine upgrades in the general aviation sector. [ABSTRACT FROM AUTHOR]

Abstract In power systems, voltage collapse during overload can be a significant threat. Accurate forecasting of critical operational conditions within power grids is crucial for preventing such situations. Precise predictions of voltage collapse enable operators to monitor the system closely and implement necessary corrective measures promptly, avoiding potential issues. However, monitoring networks can be costly due to the numerous loads and transformers in the distribution system. A comprehensive approach known as the voltage stability index (VSI) forecast without measurement buses (VFWMB) has been introduced to address this challenge. This approach involves innovative methods, including the seeking observation zone with weight least square (SOZWLS) technique for determining the number and location of measurements in the network based on its topology. Additionally, short‐term load forecasting is performed using the long short‐term memory (LSTM) forecasting method, followed by voltage estimation for buses without measurements. Finally, the proposed method calculates the modern voltage stability index for distribution systems (MVSIDS) for upcoming hours. All indicators and techniques in the VFWMB method have been validated. The algorithm has been thoroughly tested on various networks, including small and large, balanced and unbalanced, and both real and test networks, showing high efficiency in the electricity industry.

Abstract This paper is dedicated to proposing improved input‐to‐state stability (ISS) criteria for a class of continuous‐time time‐varying switched singular systems (CTSSS) under slow/fast/slow‐fast switching signals. Specifically, for a CTSSS with time‐varying singular matrices, the authors start with an improved sufficient condition for verifying its ISS under slow switching signals based on the average dwell time method and a relaxed multiple Lyapunov function. Then, considering several or all subsystems may destroy the overall ISS, relaxed sufficient conditions are presented to ensure the ISS of the CTSSS under fast switching signals and slow‐fast switching signals, respectively. Note that the improvement of the proposed ISS criteria is embodied in the derivative of the required multiple Lyapunov functions along with the state trajectories of CTSSS can be positive or negative. Finally, three numerical examples are provided to illustrate the feasibility of the obtained results.

In power systems, voltage collapse during overload can be a significant threat. Accurate forecasting of critical operational conditions within power grids is crucial for preventing such situations. Precise predictions of voltage collapse enable operators to monitor the system closely and implement necessary corrective measures promptly, avoiding potential issues. However, monitoring networks can be costly due to the numerous loads and transformers in the distribution system. A comprehensive approach known as the voltage stability index (VSI) forecast without measurement buses (VFWMB) has been introduced to address this challenge. This approach involves innovative methods, including the seeking observation zone with weight least square (SOZWLS) technique for determining the number and location of measurements in the network based on its topology. Additionally, short‐term load forecasting is performed using the long short‐term memory (LSTM) forecasting method, followed by voltage estimation for buses without measurements. Finally, the proposed method calculates the modern voltage stability index for distribution systems (MVSIDS) for upcoming hours. All indicators and techniques in the VFWMB method have been validated. The algorithm has been thoroughly tested on various networks, including small and large, balanced and unbalanced, and both real and test networks, showing high efficiency in the electricity industry. [ABSTRACT FROM AUTHOR]

This paper is dedicated to proposing improved input‐to‐state stability (ISS) criteria for a class of continuous‐time time‐varying switched singular systems (CTSSS) under slow/fast/slow‐fast switching signals. Specifically, for a CTSSS with time‐varying singular matrices, the authors start with an improved sufficient condition for verifying its ISS under slow switching signals based on the average dwell time method and a relaxed multiple Lyapunov function. Then, considering several or all subsystems may destroy the overall ISS, relaxed sufficient conditions are presented to ensure the ISS of the CTSSS under fast switching signals and slow‐fast switching signals, respectively. Note that the improvement of the proposed ISS criteria is embodied in the derivative of the required multiple Lyapunov functions along with the state trajectories of CTSSS can be positive or negative. Finally, three numerical examples are provided to illustrate the feasibility of the obtained results. [ABSTRACT FROM AUTHOR]

A new class of nonlocal boundary value problems consisting of multi-term delay fractional differential equations and multipoint-integral boundary conditions is studied in this paper. We derive a more general form of the solution for the given problem by applying a fractional integral operator of an arbitrary order $ \beta_{\xi} $ instead of $ \beta_{1} $; for details, see Lemma 2. The given problem is converted into an equivalent fixed-point problem to apply the tools of fixed-point theory. The existence of solutions for the given problem is established through the use of a nonlinear alternative of the Leray-Schauder theorem, while the uniqueness of its solutions is shown with the aid of Banach's fixed-point theorem. We also discuss the stability criteria, icluding Ulam-Hyers, generalized Ulam-Hyers, Ulam-Hyers-Rassias, and generalized Ulam-Hyers-Rassias stability, for solutions of the problem at hand. For illustration of the abstract results, we present examples. Our results are new and useful for the discipline of multi-term fractional differential equations related to hydrodynamics. The paper concludes with some interesting observations.

A new class of nonlocal boundary value problems consisting of multi-term delay fractional differential equations and multipoint-integral boundary conditions is studied in this paper. We derive a more general form of the solution for the given problem by applying a fractional integral operator of an arbitrary order βξ instead of β1; for details, see Lemma 2.2. The given problem is converted into an equivalent fixed-point problem to apply the tools of fixed-point theory. The existence of solutions for the given problem is established through the use of a nonlinear alternative of the Leray-Schauder theorem, while the uniqueness of its solutions is shown with the aid of Banach's fixed-point theorem. We also discuss the stability criteria, icluding Ulam-Hyers, generalized Ulam-Hyers, Ulam-Hyers-Rassias, and generalized Ulam-Hyers-Rassias stability, for solutions of the problem at hand. For illustration of the abstract results, we present examples. Our results are new and useful for the discipline of multi-term fractional differential equations related to hydrodynamics. The paper concludes with some interesting observations. [ABSTRACT FROM AUTHOR]

Parametric excitation is a unique type of excitation that arises from the time-varying parameters of a dynamical system, typically induced by the periodic movements of the system components. However, the structure-moving subsystems problem creates a new type of parametric excitation where the subsystems do not have exact periodic movements. Under the circumstances, dimension of the mathematical model of such a system exhibits time dependency, which prohibits the employment of conventional analytical methods to predict its stability. In this paper, we propose a novel Generalized Sequential State Equation Method that provides a distinct solution framework for analysis of the subsystem-induced parametric resonance. With the time-domain system partition and state mapping, the proposed method reconfigures the coupled system with a sequence of state space equations and solves them chronologically. The proposed method relaxes the single-subsystem-assumption in its original form and applies to a general scenario where the subsystems travel with arbitrary characteristic period. With the proposed method, occurrences of the subsystem-induced parametric resonance are precisely predicted via a set of analytical stability criteria and the steady state response is determined in an exact analytical form. • Novel generalized sequential state equation method for parametric resonance with arbitrary characteristic period. • The proposed method advances the existing method by lifting the constraints on the inter-distance between subsystems. • Determining the stability of a moving subsystem induced parametric vibration via analytical stability criteria. • Delivering the exact waveform of the steady state of a parametrically excited structure in a general manner. • Investigating the characteristics of the subsystem induced parametric resonance in a multidimensional parameter space. [ABSTRACT FROM AUTHOR]

This paper concentrates on presenting a reliable procedure to compute the stabilizing region of fractional‐order proportional integral derivative (FOPID) controllers for interval fractional‐order plants having an interval time delay. An interval fractional‐order plant is defined as a fractional‐order transfer function whose denominator and numerator coefficients are all uncertain and lie in specified intervals. Also, an interval time delay points to a delay term whose value varies in a specific interval. The D‐decomposition technique and the value set concept are employed to determine the stabilizing region of FOPID controllers. In this study, first, a theorem is presented to compute the boundary of the value sets of systems having interval time day. Then, a lemma is provided for robust stability analysis of the given closed‐loop control system. For a convenient use of the paper results, an algorithm is proposed to solve the problem of robustly stabilizing interval fractional‐order plants with an interval time delay using FOPID controllers. Finally, four examples are provided to illustrate the proposed procedure. [ABSTRACT FROM AUTHOR]

This study addresses the problem of distributed optimization in control of heterogeneous linear multi‐agent systems. Instead of assuming a perfect communication medium as in many existing approaches, two important issues arising from communication networks are considered. First, the existence of communication delays is assumed when each agent receives information from its neighbors. Second, since communication networks are generally unreliable, it is assumed that each agent interacts with other agents through random digraphs. Finally, delay‐dependent sufficient conditions are derived in the form of linear matrix inequalities (LMIs) to prove convergence to optimal solutions. Numerical simulation results are analyzed to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Ultra-high strength low alloy (UHSLA) steels are one of pivotal materials used in industrial sector. Its diversified compositions and characteristic mechanical properties as per service requirement have created an inevitable place for this steel class. The current study has been carried out on the deformation behavior and plastic instability of newly developed (UHSLA steel) XF1700 over a wide temperature range T r to near T m (298-1673 K) and at quasi-static (0.01-1/s) as well as dynamic (10-104/s) conditions. Such a wide range of data were obtained by utilizing chemical composition of this alloy system in JMatPro (an efficient software based on CALPHAD module). The effect of temperature with increasing strain and strain rate has been quantified in terms of a four-parameter exponential model based on two Z (Zener–Hollomon) parameters Z m and Z n , as such to predict cold, warm and hot deformation mechanism by a subsection method. The novelty of this work lies in its simple mathematical model that can describe the deformation behavior of a material for temperature range T r to near T m , considering the temperature compensated strain and strain rate factors. EBSD analysis was carried out for microstructural analysis to support the deformation behavior and plastic instability mechanism. Based on four-parameter exponential model, the deformation stability criteria have also been elucidated for all the three zones: Zone I, cold forming zone (298-673 K); Zone II, warm forming zone (673 < T ≤ 973 K) and Zone III, hot forming zone (973 < T ≤ 1673 K). Through non-linear regression model in SPSS software, material parameters describing deformation behavior and plastic instability were segregated in parts and the four-parameter exponential model has been verified which is found to be in excellent agreement with available data. The AARE obtained was 2.03% (zone I), 7.17% (zone II) and 4.07% (zone III), respectively, while the overall AARE was 4.42%. [ABSTRACT FROM AUTHOR]

In microwave electronics, the power gains of a linear two-port are customarily defined as the ratio of an output port and input port power, where such powers are intended either as operational or as available. Two input and two output powers are thus introduced, with four possible combinations of output/input power ratios, but only three are practically exploited, the well-known operational power gain, available power gain, and transducer power gain. In the present paper, we provide a comprehensive review of gain definitions (including the less commonly exploited added-power gains) and finally consider the missing fourth element (defined as the ratio of the output available power and of the input operational power), derive a few mathematical properties of it, both in the general and in the unilateral case, and ultimately justify the reason why this fourth gain G 4 which, following the suggestion of an anonymous reviewer, we will call apparent power gain, G app , has little interest in the optimization of the power transfer between the generator and the load. Nevertheless, the definition and analysis of G app , besides being formally useful to complete the gain family, may yield a deeper insight into the very nature of power transfer optimization in a loaded two-port. [ABSTRACT FROM AUTHOR]

Remote sensing image registration is one of the crucial steps in remote sensing image processing, where ground control information is essential. Maintenance of control point databases is complex and expensive. Consequently, lightweight feature databases are emerging. Lightweight feature databases need to store stable and reproducible features. In this context, region features exhibit a distinct advantage. In feature registration methods, the reproducibility of regional features is typically stronger than with individual points. A popular feature region matching method is currently the combination of maximally stable extremal regions (MSER) and scale-invariant feature transform (SIFT). However, the direct combining of MSER and SIFT has difficulties primarily due to redundancy and overlap in regions extracted by MSER, as well as the conflict in applying texture descriptors on homogeneous regions. In this research, we first suggest a salient MSER detection approach that combines frequency-tuned salient region detection and effective nonmaximum suppression filtering to get rid of redundant information and enhance the stability and dependability of the feature region; afterward, we describe the feature region using the unique, enhanced segment distance-weighted gradient location-orientation histogram, which aims to comprehensively describe the feature regions by incorporating more information about the gradient at the edges of the regions. In the experimental phase, we validate the proposed method using multiple remote sensing images. The experimental results confirm the superiority of the proposed method and demonstrate the significant potential and advantages of feature region matching in the context of lightweight feature databases and remote sensing image registration. [ABSTRACT FROM AUTHOR]

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This article focuses on the stability problem of sampled-data systems, which is addressed using a new integral inequality. A method for systematic analysis is presented, then applied to the electric power market. The sampling point from tk to tk+1 assume to be a sampling interval but bounded. In addition, we develop a novel closed-loop Lyapunov functional that considers intervals from the sampling point t to tk and from t to tk+1. This new functional is utilized to derive a stability criterion that is less conservative than previous works based on novel integral inequalities for sampled-data systems. Numerical examples are proposed to demonstrate the efficacy and decreased conservatism of the suggested approach. Additionally, the method addresses the stability issue in electric power markets and explores the importance of reducing conservatism. [ABSTRACT FROM AUTHOR]

Abstract As the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation

This paper proposes a simplified and analytical method for designing fractional order lead compensator (FOLC). In the proposed method, the structure of fractional order lead compensator considered is more generalized. The fractional operator of the FOLC is approximated to its equivalent rational form using binomial expansion to lower the computation complexity of the designing approach. Then, steady-state gain, gain cross over frequency and phase margin are used as the design specifications of the FOLC. The proposed method also considers the magnitude and phase of the plant to be controlled. Phase margin criteria ensure the stability of the designed compensator. The compensator is designed strictly adhering to gain and phase margin criteria. The proposed methodology is validated with four different illustrative examples through simulation. Integral absolute error (IAE) and integral square error (ISE) are considered as the performance indices. The simulation results demonstrate that the proposed scheme gives a better transient response, IAE, and ISE as compared to the existing FOLC design techniques. [ABSTRACT FROM AUTHOR]

This article proposes a discrete proportional-integral-derivative (PID) load frequency control (LFC) scheme to investigate the dissipative analysis issue of restructured wind power systems via a non-fragile design approach. Firstly, by taking the different power-sharing rates of governors into full consideration, a unified model is constructed for interconnected power systems containing multiple governors. Secondly, unlike existing LFC schemes, a non-fragile discrete PID control scheme is designed, which has the performance of tolerating control gain fluctuation and relieving the huge computational burden. Further, by constructing a discrete-type Lyapunov–Krasovskii functional, improved stability criteria with a strict dissipative performance index are established. Finally, numerical examples are presented to demonstrate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar (GPR) attenuation compensation is an indispensable part of radar data processing. However, traditional compensation methods based on physical mechanisms rely heavily on quality factor ($Q$) and are noise-sensitive, such as the inverse $Q$ -filter method. Inspired by the nonlinear mapping capabilities of deep learning (DL), a series of DL-based attenuation compensation methods have been proposed to circumvent the limitations of the traditional attenuation compensation methods. Among the existing DL-based methods, convolutional neural networks (CNNs) are widely used. However, the electromagnetic wave propagation in the subsurface is characterized by dual dynamic changes in space and time. CNN only extracts local morphological features and cannot ensure the integrity of the GPR data in the time dimension, which increases the instability of the attenuation compensation results. Therefore, we build a network, spatio-temporal neural network (STNN), for attenuating compensation of GPR data. The bidirectional long short-term memory (Bi-LSTM) layer is added to the CNN-based network to simultaneously capture both spatial and temporal characteristics of electromagnetic wave propagation, thus giving clear physical meaning to the DL-based attenuation compensation network. Numerical and real data experiments show that the attenuation compensation results of the proposed method have better continuity and stability than those of CNN-based and the stabilized inverse $Q$ -filter method. [ABSTRACT FROM AUTHOR]

As the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation [ABSTRACT FROM AUTHOR]

In the design of amplifier stages based on unconditionally stable linear active two-ports, the amplifier gain can be maximized through simultaneous conjugate matching with passive loads at the input and output ports. Conversely, the optimization of linear amplifiers based on conditionally stable active devices requires a trade-off between gain, stability margin, input/output port mismatch and (for low-noise amplifiers) noise figure. Exploiting potentially in-band unstable devices can be advantageous in the design of open-loop low-noise amplifiers, since the in-band stabilization with input resistors is well known to negatively affect the amplifier minimum noise figure. Within this framework, the article derives a lower bound to the input and output mismatch of non unconditionally stable linear two-ports. The minumum mismatch is shown to only depend, in a simple way, on the stability factor $K$ and on the assumed mismatch ratio between the two ports. The minimum mismatch condition can be implemented by cascading the active, potentially in-band unstable two-port with two (input and output) reactive matching sections. The application of the theory to the design of low-noise amplifier open-loop stages based on conditionally stable active devices is discussed through CAD examples.

The control of a rotary inverted pendulum (RIP) is challenging because it is an underactuated, highly sensitive, and unsteady system. Sliding mode control (SMC) is a nonlinear control method with high-frequency switching control. Designing a proportional integral derivative (PID) controller for a RIP is challenging due to its nonlinearity and instability in open-loop characteristics. The primacy of the SMC over the PID is the stability of the closed-loop. Hybrid control of a PID-SMC controller can provide better performance because this technique demonstrates less chatter, higher precision, no oscillation, and adequate gain tuning. To achieve gain tuning, the PID and SMC parameters must be optimized. Thus, this paper proposed a congruently tuned control strategy (CTCS) to fine-tune the controller parameters. The proposed strategy uses an improved whale optimization algorithm (WOA), i.e., the modified Manhattan distance updated WOA (MMD-WOA) to identify effective coefficient values for the sliding surface to reduce tracking errors while reaching the desired position. The proposed CTCS for a RIP with the MMD-WOA was implemented, and the results are very promising.

Flow assurance has been a major challenge in petroleum production systems especially in offshore operations where severe slugging is a dominant aching issue. This is characterized by pulsating flow behavior which impacts the reservoir productivity, pipeline-riser systems, and surface facilities (separator). The only means so far, of delivering reservoir fluids from offshore wells to the separator at the surface is by pipeline-riser systems where slugging is most prominent and oftentimes unavoidable. Though passive and active measures have been deployed over time to mitigate the associated problem, there is the need to develop a predictive tool for mitigating offshore slugging in the riser system. This study involves the use of a model and analysis approach to eliminate severe slugging based on the balance of forces acting on the pipeline-riser system. Modified stability criteria which consider accumulation and frictional pressure drops have been developed. The current approach is solely physicsdriven, to predict the corresponding liquid and gas velocities at which the fluctuations would be severe when producing multiphase fluids through pipeline-riser systems. By balancing the forces acting on a riser-pipeline system and accounting for frictional and accumulation effects, two hydraulic flow modeling approaches have been used in this study to improve the criteria for predicting the condition of severe slugging. The developed criteria were computed using MATLAB/Simulink and a severe slugging flow map was generated. The “new study 1” and “new study 2” showed a 16% and 13% increase respectively in predicted unstable cases than the Malekzadeh criteria. The criteria were tested at different separator pressures. The Taitel and Jansen criteria could not predict severe slugging at elevated separator pressure. Notwithstanding, the developed flow maps using the “new study 1” are more reliable than the “new study 2”. This is because the “new study 2” approach over defines the physics of severe slugging. The criteria were also tested at varying accumulation times. This was done by step increments in the order of magnitude of time. The separator pressure affects the optimal time for modeling the accumulation at the riser-base. The criteria are therefore useful for predicting optimal operating conditions for flow assurance for the pipeline-riser system in consideration.

Wind energy production mainly depends on atmospheric conditions. The atmospheric stability can be described through different parameters, such as wind shear, turbulence intensity, bulk Richardson number, and the Monin–Obukhov length. Although they are frequently used in micrometeorology and the wind industry, there is no standard comparison method. This study describes the atmospheric stability of a coastal region of Yucatan, Mexico, using these four parameters. They are calculated using six-month data from a meteorological mast and a marine buoy to determine atmospheric stability conditions and compare their results. The unstable atmospheric condition was predominant at the site, with an 80% occurrence during the measurement period, followed by 12% in neutral and 6% in stable conditions. Wind speed estimations were performed for each atmospheric stability scenario, and the variation in the energy produced was derived for each case. Unstable atmospheric conditions deliver up to 8% more power than stable conditions, while neutral conditions deliver up to 9% more energy than stable conditions. Therefore, considering a neutral state may lead to a considerably biased energy production estimation. Finally, an example calculation indicates that atmospheric stability is a crucial parameter in estimating wind energy production more accurately. [ABSTRACT FROM AUTHOR]

Under fault conditions, virtual synchronous generators (VSGs) are prone to lose transient stability, similar to synchronous generators. However, the existing studies on VSG transient stability are not comprehensive. The transient characteristics of VSGs are highly affected by control parameters, but the transient models of VSGs in existing studies are overly simplified. In addition, the critical clearing angle (CCA) and critical clearing time (CCT) are two indices used to measure the transient stability of a system, but there are few studies that consider both CCA and CCT. In this article, we analyze the CCA and CCT quantitatively to describe the transient stability boundary of a VSG under different control parameters and fault conditions. Then, an equal proportional area criterion is proposed to provide guidance for the control of virtual inertia in the VSG. Based on a theoretical analysis, a transient control method for a VSG is proposed, which can improve the transient stability of the VSG in terms of both CCA and CCT. Finally, simulation and experimental tests are performed to validate the correctness of the theoretical analysis and the proposed method. [ABSTRACT FROM AUTHOR]

Grasp stability recognition based on tactile perception has attracted increasing attention in the robotics community. In this article, we extract tactile features from multimodal tactile signals and propose a novel ensemble approach named genetic algorithm-based ensemble hybrid sparse extreme learning machine (GA-EHSELM) for the grasp stability recognition task. In contrast to traditional ensemble extreme learning machines (ELMs), the proposed approach takes the sparsity of the random weights assigned to the connection between the input layer and the hidden layer for each base learner into consideration, in order to avoid the problem of overfitting. In addition, The diversity of base learners is increased by constructing two types of sparse ELM (SELM). The random weights of the base SELMs are sampled from two different distributions with a certain probability. Furthermore, we utilize the genetic algorithm (GA) for the optimization of tactile features and the base sparse ELMs. To be specific, GA is employed for feature selection as well as the optimization of the sparsity of the random weights associated with different types of base learners and the number of each type of base learner. The effectiveness of the proposed approach has been demonstrated on a public tactile grasp stability dataset. [ABSTRACT FROM AUTHOR]

This article presents an improved equivalent-input-disturbance (EID) approach to actively rejecting exogenous disturbances for a plant. A control system based on the approach includes a new EID estimator constructed by embedding integrals to the conventional EID estimator. These integrals enable the estimator to improve the disturbance-estimation precision without amplifying the effect of measurement noise. Stability conditions of the control system are obtained using separation theorem. The system design ensures that the control system satisfies the stability conditions. Experiments of a rotational control system demonstrate the validity of the approach. [ABSTRACT FROM AUTHOR]

In parallel three-level inverters, to eliminate the undesired internal common-mode (CM) resonance circulating currents (ICMRCCs) and external CM resonance circulating currents (ECMRCCs) introduced by the improved LCL, the inverter-side CM current feedback (ICMCF) control is presented. However, since the switching frequency of the inverters is generally low in high-power applications to reduce power loss, the stability of the ICMCF control will be threatened by the delays of digital controller. In this case, the existing methods are not capable of ICMRCC and ECMRCC suppression any more. To deal with the situation, an improved CM resonance circulating current control method for high-power parallel inverters is proposed. First, the model of ICMCF with control delay is established. Second, the influences of control delay with different sampling modes on the stability of CM current control are investigated. Based on this, the allowable range of control delays and sampling frequency to keep the system stable is analyzed and deduced. Third, the multisampling mode is further extended to CM current control in the parallel inverters. The analysis results show that the stability margin and control bandwidth of the ICMCF control are significantly improved. Finally, the theoretical analysis and proposed control method are verified by simulation and experiment. [ABSTRACT FROM AUTHOR]

This letter proposes a self-healing called automatic monitoring architecture (AMA), which can help RISC-V server self-heal from hardware errors. AMA reduces redundant error classification, and only monitors hardware devices that have a relatively large proportion of errors in RISC-V server, thereby reducing the resource consumption of AMA on the basis of ensuring performance. In addition, AMA uses the Correctable-error Dynamic Threshold technology to further reduce the probability of serious uncorrectable hardware errors. Compared with an RISC-V server without a self-healing system, this architecture consumes very few hardware resources, reducing server downtime by about 80% each year. Compared with other self-healing architectures, such as Intel’s machine check architecture, AMA can reduce server downtime by an additional 5% per year. Therefore, AMA is highly efficient with little resource consumption. [ABSTRACT FROM AUTHOR]

This article addresses a fundamental issue in power converter control. Namely, the direct output-voltage control of power converters with higher order dynamics. We argue that despite of the burst in the development of new topologies, legacy control design principles—inherited from second-order converter analysis— have been unduly adopted as de facto rules of procedure. In particular, we refer to the minimum-phase requirement on the feedback variable, as the underlying stabilizing mechanism. In this article, we demonstrate that the requirement of minimum-phaseness for higher order converters and the legacy ad hoc solution via ancillary current control, can be bypassed without compromising closed-loop performance. To prove this, we unveil important differences between stability properties of second-order and higher order converters. Then, we introduce new linear and nonlinear control design strategies that permit direct output-voltage control. Experimental results are provided to validate the theory. [ABSTRACT FROM AUTHOR]

Unbalanced grid faults are very harmful to the network operation. Distributed static synchronous compensator (D-STATCOM) can help to support the grid voltage by injecting the reactive power into the inductive power grid. However, this approach turns to be ineffective for a low-voltage grid, where the network impedance is mainly resistive. In this article, a dynamic voltage equalization control (DVEC) scheme is proposed to suppress the negative sequence voltage, and therefore, reduce the voltage unbalance during the faults in the resistive network. This effect is achieved through transferring proper active power from the negative sequence channel to the positive sequence channel. And the dual-sequence active currents are controlled by DVEC without exchanging extra active power with the grid except the little circuit loss compensation. By flexibly adjusting the control parameter of DVEC, current peak and active power oscillation can be limited within a preset range, respectively, during the process of maximizing the voltage control. As a result, the DVEC scheme can ensure the safe operation of D-STATCOM and the stability of the power grid. Moreover, experimental results are reported to validate the correctness and effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This article reveals that the saturation block (SB) used with the power-angle control loop can enhance the transient stability of grid-forming (GFM) inverters. Three methods of using the SB in the power-angle loop are examined based on power-angle curves. It is found that all can enhance the transient stability of GFM inverters by either decreasing the acceleration area or increasing the deceleration area. At the same time, the threshold value selection of SB in the three schemes is also explained, considering the tradeoff between the transient stability and the response speed. A holistic comparison of the three schemes recommends the scheme with the SB used for power error adopted by the GFM inverters. Finally, experimental tests confirm the effectiveness and correctness of the analysis. [ABSTRACT FROM AUTHOR]

This article investigates the impacts of different current limiting strategies on the transient stability of the virtual synchronous generator (VSG). The power–angle curve of the VSG under different operating conditions is theoretically characterized in detail and experimentally verified through tests conducted on hardware-implemented lab-scale VSGs. It is shown that the reference current saturation approaches prioritizing the current vector angle, the d-axis current, and the q-axis current reshape the VSG power–angle curve in different ways. The resulting impacts on the VSG transient stability are comprehensively investigated through time-domain simulation of a battery energy storage system that is operated as a VSG and connected to a medium-voltage Canadian distribution feeder. The transient stability margin of the VSG is evaluated by determining the critical clearing time (CCT) of various fault scenarios. The studies conducted in the PSCAD/EMTDC software environment utilizing a detailed switching model of the VSG indicate that 1) the current limit of the VSG significantly impacts its power output during and after faults; and 2) the q-axis priority current limiting strategy provides a larger transient stability margin (CCT) as compared with the current vector angle and d-axis current priority approaches. [ABSTRACT FROM AUTHOR]

A synchronization mechanism for three motors coupled with a tensile spring in a nonlinear vibrating system is proposed to apply in drilling fluid shaker. The purpose of this paper is to investigate the synchronization mechanism of the system. Firstly, the dynamic equations of the system are established by using the Lagrange's equation, and the steady-state solutions of the system are solved. Then, synchronous conditions and stability criteria of the system in a steady state are obtained. Next, the influences of various structure parameters on the synchronous state of the system are numerically discussed. Additionally, we use the Runge–Kutta algorithm to build an electromagnetic coupling model, and the synchronous behavior of the system is explored. Finally, an experimental strategy including the synchronous tests of the system is implemented to verify the correctness of theoretical analysis, numerical discussion, and simulation. It is concluded that the steady synchronous motion of three motors can be achieved by selecting an appropriate stiffness of the tensile spring. All these study results will provide a strong support for developing the design of large-scale vibration machinery with high efficiency and energy-saving in engineering. [ABSTRACT FROM AUTHOR]

Unsupervised person re-identification (re-ID) remains a challenging task. While extensive research has focused on the framework design and loss function, this paper shows that sampling strategy plays an equally important role. We analyze the reasons for the performance differences between various sampling strategies under the same framework and loss function. We suggest that deteriorated over-fitting is an important factor causing poor performance, and enhancing statistical stability can rectify this problem. Inspired by that, a simple yet effective approach is proposed, termed group sampling, which gathers samples from the same class into groups. The model is thereby trained using normalized group samples, which helps alleviate the negative impact of individual samples. Group sampling updates the pipeline of pseudo-label generation by guaranteeing that samples are more efficiently classified into the correct classes. It regulates the representation learning process, enhancing statistical stability for feature representation in a progressive fashion. Extensive experiments on Market-1501, DukeMTMC-reID and MSMT17 show that group sampling achieves performance comparable to state-of-the-art methods and outperforms the current techniques under purely camera-agnostic settings. Code has been available at https://github.com/ucas-vg/GroupSampling. [ABSTRACT FROM AUTHOR]

Nowadays, due to many limitations in reality, the optimization of tracking precision and convergence time has attracted the attention of researchers in robotics community. In this article, a fixed-time adaptive neural network (NN) controller is proposed for unknown robot manipulators. A switching mechanism is integrated into the control design such that the semiglobal stability of the conventional NN control systems can be extended to global stability. The time-varying barrier Lyapunov function and the fixed-time control technique are incorporated into the controller design to guarantee the prescribed motion constraints and the fixed-time convergence simultaneously. Compared with some existing fixed-time NN control algorithms, the assumption that NN weight should be upper bounded can be relaxed in our work. Finally, the simulation and experiment studies are respectively carried out based on an unknown 2-degree of freedom robot and a Baxter robot to demonstrate the effectiveness and superiority of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This article presents a new method that employs the active disturbance rejection control (ADRC) paradigm to handle the control problem for a class of nonlinear systems with unknown disturbances and measurement noise. An improved extended state observer (ESO) is devised to separate disturbance estimation from state reconstruction and a new parameter is used to adjust disturbance-rejection performance independently. This increases the flexibility of system design and makes the gain of the improved ESO (IESO) small compared to that of a conventional ESO in the ADRC. A low-pass filter is added to the IESO. This changes the slope of the magnitude curve of the Bode diagram from $-20$ dB/dec to $-40$ dB/dec at high frequencies for measurement noise, which improves the noise-suppression performance. Comparison with a conventional ESO reveals the superiority of the IESO in disturbance rejection and noise suppression. The presented method is validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

If one cannot obtain the accurate prior information of the controlled object in advance, it will take much effort to get good dynamic performance. To this end, this article proposes an online estimation method to deal with disturbances and uncertainties for a class of systems with uncertain control gain. Based on the extended-state observer scheme, the estimation problem of disturbance and uncertain control gain is transformed to the convex optimization problem and solved by the sign-projected gradient flow, then the estimation can be applied to compensate disturbance and design controller. Moreover, the exponential and ultimate boundedness is obtained for the derived closed-loop system, together with rigorous theoretical analysis and proof. Finally, both the numerical simulations and the experiment on a brushless dc motor demonstrate that the proposed design has a better tracking performance and stronger robustness to deal with disturbances and uncertainties compared with the existing methods. [ABSTRACT FROM AUTHOR]

To cope with future goal of efficiency, next generation of more electrical aircraft is likely to embed reconfigurable microgrid for the primary electrical distribution. The stability analysis of such time-varying structure is challenging. Conventional linear and nonlinear approaches failed to produce proper answers from a practical point of view. Under conditions, passivity property of each equipment is sufficient to ensure stability of the overall microgrid. But with improper passivity-based control (PBC) design, the passivity property may not be propagated. The main contribution of this article is to propose a modified IDA-PBC procedure that will ensure the passivity properties of the system regarding its electrical inputs and outputs involved in the interconnection. Thus, for electrical microgrids resulting from the interconnection of controllable equipment, input/output filters, and electrical lines, if all the equipment embed the proposed control, the whole microgrid have passive properties and the global stability of the system is ensured. Moreover, this article will also present how to achieve the proposed control with high transient and robust performances thanks to nonlinear parameters’ estimator preserving the passivity proper. Experimental validation of the proposed control in representative electrical conditions is provided. [ABSTRACT FROM AUTHOR]

Quantitative evaluation of human stability using foot pressure/force measurement hardware and motion capture (mocap) technology is expensive, time consuming, and restricted to the laboratory. We propose a novel image-based method to estimate three key components for stability computation: Center of Mass (CoM), Base of Support (BoS), and Center of Pressure (CoP). Furthermore, we quantitatively validate our image-based methods for computing two classic stability measures, CoMtoCoP and CoMtoBoS distances, against values generated directly from laboratory-based sensor output (ground truth) using a publicly available, multi-modality (mocap, foot pressure, two-view videos), ten-subject human motion dataset. Using Leave One Subject Out (LOSO) cross-validation, experimental results show: 1) our image-based CoM estimation method (CoMNet) consistently outperforms state-of-the-art inertial sensor-based CoM estimation techniques; 2) stability computed by our image-based method combined with insole foot pressure sensor data produces consistent, strong, and statistically significant correlation with ground truth stability measures (CoMtoCoP r = 0.79 p < 0.001, CoMtoBoS r = 0.75 p < 0.001); 3) our fully image-based estimation of stability produces consistent, positive, and statistically significant correlation on the two stability metrics (CoMtoCoP r = 0.31 p < 0.001, CoMtoBoS r = 0.22 p < 0.043). Our study provides promising quantitative evidence for the feasibility of image-based stability evaluation in natural environments. [ABSTRACT FROM AUTHOR]

Grid-forming converters (GFMCs) prevail for their capability of providing voltage and frequency support in modern power grids with large integration of intermittent distributed power sources. As GFMCs behave as voltage sources, the current limiting of GFMCs under large disturbances requires special efforts. Furthermore, a reliable GFMC should also be capable of remaining synchronized with the grid during and after large voltage disturbances. To avoid the instability caused by phase-locked loops during low-voltage faults, GFMCs usually retain the active power synchronization under low-voltage faults when the current saturation is triggered. When voltage sags are severe, the transient stability highly depends on the fault duration. This article proposed a robust fault ride through strategy for GFMCs based on reactive power synchronization (Q-Syn) under voltage sags. The Q-Syn method ensures transient stability of GFMCs regardless of the fault duration or level. Furthermore, fast postfault recovery can be achieved with the Q-Syn method. The effectiveness of the proposed method is verified by experiments. [ABSTRACT FROM AUTHOR]

Inertia and damping are the core index of the estimation of power system stability. Existing analysis to the support of inertia and damping are focused on the improvement of system stability as well as parameters tuning of virtual inertia and virtual damping but rarely involved in the quantitative calculation of level of inertia and damping under a certain condition. Based on this background, this article puts forward a unified calculation method to identify the equivalence of inertia and damping under various ratios of Rg/Xg. In addition, the quantity of improved inertia and damping support enabled by virtual inductance control are estimated by the derived calculation model of equivalence. The calculated mathematical model of equivalence of inertia and damping is well verified by the overlapped eigenvalues distribution. Besides, the negative synchronous power will inevitably influence the aperiodic loss of synchronization (ALoS) of power system caused by the large virtual inductance. Therefore, a solution should be derived to make a compromise between oscillation damping and ALoS by tuning the value of virtual inductance. Finally, the simulation and experiment results in various cases verify the effectiveness of the proposed equivalent damping and inertia calculation method as well as virtual inductance control. [ABSTRACT FROM AUTHOR]

For single-phase grid-connected inverters, the second-order generalized integrator-based phase-locked loop (SOGI-PLL) is widely used for good harmonics attenuation and frequency adaptability. However, existing studies usually neglect the frequency feedback loop in the modeling and design process, and thus, the potential instability caused by the frequency feedback loop is not well explained. Besides, the improper design of SOGI-PLL can lead to the inverter instability both under ideal and weak grid conditions. Therefore, by fully considering the coupling of the frequency feedback loop, this article establishes an accurate model for SOGI-PLL. Furthermore, effects of different PLL parameters on behaviors of both the SOGI-PLL itself and the inverter are studied, based on which a graphical method for PLL parameters design is proposed. As proved, the frequency feedback loop can deteriorate the stability and performance of both SOGI-PLL itself and inverter system, especially if rapid transient is expected. The modeling method is also extended to the low-pass filter-SOGI-PLL and the improved parameters design is discussed. Experimental results verify the correctness of the proposed PLL model and the abovementioned analysis results. The proposed model is helpful to deeply reveal the instability mechanism and provides a reference for the PLL design under nonideal grid conditions. [ABSTRACT FROM AUTHOR]