Dynamic Loop Research Articles

Ferroelectric switching studies in symmetric Ag/Poly(vinylidene fluoride-co-hexafluoropropylene)/Ag capacitor structures

In this article, ferroelectric switching properties of Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) thin films across symmetric Ag/PVDF-HFP/Ag capacitor structures have been investigated. The structural, microstructural and optical properties of the films are evaluated with grazing incidence X-ray diffraction, Atomic Force Microscopy, and UV visible spectrophotometer. Symmetric quasi-static current-voltage loops exhibit ferroelectric switching, and the corresponding electric displacement- electric field loops show an increase in the coercive field, with the increase in the amplitude of the applied electric field. Higher resistivity of (1.66 – 5.16) × 108 Ω-cm along with optical band gap of 4 eV indicated the good quality of the junctions. Dynamic Polarization-Electric field loops reveal a remnant polarization of 3.22 μC/cm2 and 3.16 μC/cm2 for 80 nm and 160 nm PVDF-HFP films. The increase in coercive field, remnant and saturation polarization parameters with rise in the amplitude of the applied electric fields at 1 Hz for 160 nm PVDF-HFP film are linked to the uniform domain growth phenomena.

Effect of CoFe2O4 mole percentage in the scaling behaviour of dynamic hysteresis loop in eco-friendly (1-x)(0.94Na0.5Bi0·5TiO3-0.06BaTiO3)–x(CoFe2O4) particulate composite

Many technological applications of ferroelectric materials are fundamentally connected to domain reversal under the electric field. Here the polarization reversible mechanism has been studied in (1-x)(0.94Na0·5Bi0·5TiO3-0.06BaTiO3)–x(CoFe2O4) (NBBT-CFO) (x = 0, 15, 25 and 35 mol%) particulate composites through the electric field and frequency-dependent scaling behavior of their hysteresis loop. Along with this, the role of the CFO on the polarization reversal processes of NBBT has been investigated systematically. Three-stage behaviour has been observed in the polarization reversal for all the systems. The observed scaling parameter β value for the composites is found to be less than parent NBBT, which has been discussed based on the pinning effect that arises due to the CFO phase. The induced pinning effect in the composites is due to the high leakage current which is confirmed by the leakage current density (J) measurement. The scaling relation of loop area <A> versus E0 and ƒ for the composites is followed by the power law <A>∝f−0.265E01.320 in the saturated region. The present findings may be useful in technological applications related to polarization reversal.

Robust Fault Estimation and Fault-Tolerant Control for Discrete-Time Systems Subject to Periodic Disturbances

To enhance the reliability of digital automation systems in the Industry 4.0 era, this paper investigates a robust fault-tolerant control scheme in the discrete-time domain subject to periodic disturbances, consisting of a fault estimator, dynamic disturbance compensation loop, and fault-tolerant controller. The fault estimator simultaneously estimates both the system states and actuator/sensor faults. The existence and stability conditions of the proposed estimator are given, and a robust design method is proposed to make the state estimates robust to disturbances. To further reduce the estimation errors caused by periodic disturbances, a novel disturbance compensation loop is introduced and is optimized by a joint zero-assignment and pole-optimization method to delicately compensate for the adverse impacts of periodic input disturbances. The proposed robust fault-tolerant controller uses fault estimation to ensure fast recovery in the event of bounded actuator/sensor faults. The proposed scheme is evaluated through simulations of a two-wheeled mobile robot subject to periodic disturbances and simultaneous abrupt inclination angular sensor and ramp actuator faults, where its performance is shown to exceed that of existing methods.

Beet leaf (beta vulgaris L.) extract attenuates iron-induced testicular toxicity: Experimental and computational approach

The purpose of this study was to investigate the protective effect of Beta vulgaris leaf extract (BVLE) on Fe2+-induced oxidative testicular damage via experimental and computational models. Oxidative testicular damage was induced via incubation of testicular tissue supernatant with 0.1 mM FeSO4 for 30 min at 37 °C. Treatment was achieved by incubating the testicular tissues with BVLE under the same conditions. The catalase (CAT), superoxide dismutase (SOD), glutathione (GSH), malondialdehyde (MDA), and nitric oxide (NO) levels, acetylcholinesterase (AChE), sodium-potassium adenosine triphosphatase (Na+/K + ATPase), ecto-nucleoside triphosphate diphosphohydrolase (ENTPDase), glucose-6-phosphatase (G6Pase), and fructose-1,6-bisphosphatase (F-1,6-BPase) were all measured in the tissues. We identified the bioactive compounds present using high-performance liquid chromatography (HPLC). Molecular docking and dynamic simulations were done on all identified compounds using a computational approach. The induction of testicular damage (p < 0.05) decreased the activities of GSH, SOD, CAT, and ENTPDase. In contrast, induction of testicular damage also resulted in a significant increase in MDA and NO levels and an increase in ATPase, G6Pase, and F-1,6-BPase activities. BVLE treatment (p < 0.05) reduced these levels and activities compared to control levels. An HPLC investigation revealed fifteen compounds in BVLE, with quercetin being the most abundant. The molecular docking and MDS analysis of the present study suggest that schaftoside may be an effective allosteric inhibitor of fructose 1,6-bisphosphatase based on the interacting residues and the subsequent effect on the dynamic loop conformation. These findings indicate that B. vulgaris can protect against Fe2+-induced testicular injury by suppressing oxidative stress, acetylcholinesterase, and purinergic activities while regulating carbohydrate dysmetabolism.

The source of unusual coronal upflows with photospheric abundance in a solar active region

Context. Upflows in the corona are of importance, as they may contribute to the solar wind. There has been considerable interest in upflows from active regions (ARs). The coronal upflows that are seen at the edges of active regions have coronal elemental composition and can contribute to the slow solar wind. The sources of the upflows have been challenging to determine because they may be multiple, and the spatial resolution of previous observations is not yet high enough. Aims. In this article, we analyse coronal upflows in AR 12960 that are unusually close to the sunspot umbra. We analyse their properties, and we attempt to determine if it is possible that they feed into the slow solar wind. Methods. We analysed the activity in the upflow region in detail using a combination of Solar Orbiter EUV images at high spatial and temporal resolution, Hinode/EUV Imaging Spectrometer data, and observations from instruments on board the Solar Dynamics Observatory. This combined dataset was acquired during the first Solar Orbiter perihelion of the science phase, which provided a spatial resolution of 356 km for two pixels. Doppler velocity, density, and plasma composition determinations, as well as coronal magnetic field modelling, were carried out to understand the source of the upflows. Results. We observed small magnetic fragments, called moving magnetic features (MMFs), moving away from the sunspot in the active region. Specifically, they moved towards the sunspot from the edge of the penumbra where a small positive polarity connects to the umbra via small-scale and very dynamic coronal loops. At this location, small dark grains are evident and flow along penumbral filaments in continuum images. The magnetic field modelling showed small low-lying loops anchored close to the umbral magnetic field. The high-resolution data of the Solar Orbiter EUV Imagers showed the dynamics of these small loops, which last on time scales of only minutes. The edges of these small loops are the location of the coronal upflow that has photospheric abundance.

Nonlinear analysis of vibration attenuation of ring-spring-resonator based meta-isolation system

Ring-spring-resonator (RSR) based nonlinear meta-isolation system (RSR-M) was proposed and its vibration attenuation characteristic was investigated experimentally and theoretically. The force–displacement hysteresis loops of the selected wire rope isolators (WRIs) exhibit softening under compression and hardening under extension, and are identified by a modified mathematical model combined with the normalized Bouc-Wen model. Frequency response curves of conventional isolation system and RSR-M−2 were calculated based on the single-degree-of-freedom (SDOF) and two-degree-of-freedom (TDOF) models, along with the fourth-order Runge-Kutta method. Shaking table experiments were carried out to evaluate the nonlinear dynamic response and vibration attenuation property of the RSR-M, which proves to be in good agreement with the numerical results of acceptable validity. Dynamic hysteresis loops and phase portraits were plotted to study the dynamic responses and stability of the RSR-M. The experimental results show that, no matter vertical or horizontal direction, the RSR-M possesses better vibration attenuation performance compared to conventional isolation system. In general, the meta-isolation vibration system provides a new reference and idea for the practice designing of the isolation system in some sense.

Contribution of the IGCR1 regulatory element and the 3′Igh CTCF-binding elements to regulation of Igh V(D)J recombination

Immunoglobulin heavy chain variable region exons are assembled in progenitor-B cells, from VH, D, and JH gene segments located in separate clusters across the Igh locus. RAG endonuclease initiates V(D)J recombination from a JH-based recombination center (RC). Cohesin-mediated extrusion of upstream chromatin past RC-bound RAG presents Ds for joining to JHs to form a DJH-RC. Igh has a provocative number and organization of CTCF-binding elements (CBEs) that can impede loop extrusion. Thus, Igh has two divergently oriented CBEs (CBE1 and CBE2) in the IGCR1 element between the VH and D/JH domains, over 100 CBEs across the VH domain convergent to CBE1, and 10 clustered 3'Igh-CBEs convergent to CBE2 and VH CBEs. IGCR1 CBEs segregate D/JH and VH domains by impeding loop extrusion-mediated RAG-scanning. Downregulation of WAPL, a cohesin unloader, in progenitor-B cells neutralizes CBEs, allowing DJH-RC-bound RAG to scan the VH domain and perform VH-to-DJH rearrangements. To elucidate potential roles of IGCR1-based CBEs and 3'Igh-CBEs in regulating RAG-scanning and elucidate the mechanism of the ordered transition from D-to-JH to VH-to-DJH recombination, we tested effects of inverting and/or deleting IGCR1 or 3'Igh-CBEs in mice and/or progenitor-B cell lines. These studies revealed that normal IGCR1 CBE orientation augments RAG-scanning impediment activity and suggest that 3'Igh-CBEs reinforce ability of the RC to function as a dynamic loop extrusion impediment to promote optimal RAG scanning activity. Finally, our findings indicate that ordered V(D)J recombination can be explained by a gradual WAPL downregulation mechanism in progenitor-B cells as opposed to a strict developmental switch.

Dynamic magnetic hysteresis features of a mixed spin (2, 5/2) Ising system on a hexagonal lattice under an oscillating magnetic field within the path probability method

Recently, we [J. Magn. Magn. Mater. 514 (2020) 167242] presented a study of the dynamic magnetic loop features of molecular-based magnetic materials and ferrimagnetism that were investigated by using a mixed spin 1/2 and spin-1 ferrimagnetic Ising system within the path probability method. We found several interesting dynamic magnetic hysteresis properties and observed that the obtained results are in qualitatively good agreement with some theoretical and experimental studies in magnetic materials. We extended our recent previous published work to study the dynamic magnetic hysteresis properties of molecular-based magnetic materials and ferrimagnetism utilizing the mixed spin (2, 5/2) ferrimagnetic Ising system on a hexagonal lattice employing the path probability method. The spin (2, 5/2) Ising system is one of the most suitable and most used prototypical systems to investigate many molecular-based magnetic materials and ferrimagnetic materials. We found more interesting dynamic magnetic hysteresis properties such as double, triple, quadruple and quintuple loop behaviors for the layer magnetization, and only double and triple loops for the total magnetization. We found that some of our obtained results are in qualitatively good agreement with some theoretical, experimental reported works, and the works on some magnetic materials as obtained by the melt-spinning technique.

Studying the rate-dependent specific absorption rate in magnetic hyperthermia through multiscale simulations

In this work, the issue of whether the dynamic magnetic properties of monodispersed magnetic colloids, modeled using micromagnetic simulations, can be extrapolated to analyze magnetic particle hyperthermia data, i.e., specific absorption rate (SAR) values acquired at high frequencies of excitation fields, is addressed. Micromagnetic finite difference simulations were performed using the Object Oriented Micromagnetic Framework (OOMMF) software package in order to obtain the dynamic hysteresis loops under a 24 kA/m alternating magnetic field amplitude and for various frequencies (50–765 kHz). In OOMMF, the finite difference method was used to find the solution of the nonlinear Landau–Lifshitz Gilbert (LLG) equation, which describes the nanoparticles’ magnetization motion when applying an effective magnetic field. To create a system of randomly oriented magnetite nanoparticles having a certain volume fraction (0.02%) that coincides with the experimentally utilized concentration of 1 mg/ml, we start with a perfect simple cubic lattice with a large lattice spacing so that the particle–particle distance is large enough to neglect dipolar interactions (non-interacting nanoparticles). The system under study is a set of 40-nm magnetite nanoparticles with a lognormal size distribution. The simulations were performed assuming quasistatic conditions, an approach that is reasonable for ferromagnetic-like behavior. It is worth noting that the code considers not only the uniaxial anisotropy Ku but also the cubic magnetocrystalline one Kc as well. Kc is usually neglected in literature because the uniaxial contribution dominates, but this is not the case for magnetite since Ku = 9 kJ/m3 and Kc = −11 kJ/m3. Moreover, such an inclusion seems quite reasonable since the magnetocrystalline anisotropy is always present yet with a relative contribution. The SAR values at each frequency were determined after calculating hysteresis losses via the area of the simulated hysteresis loops. Interestingly, SAR values at low frequencies followed an exponential increase trend with a frequency indicating a deviation from the linear behavior usually reported in the literature. To validate our approach, we employed a coupled electromagnetic-thermal model based on COMSOL Multiphysics simulations that provides an accurate estimation of the magnetic field and temperature distribution within the ferrofluid. The time-dependent temperature curves are obtained after 30 min of magnetic particle hyperthermia treatment for the same alternating magnetic field amplitude used in OOMMF simulations (30 mT) and for two representative frequency values. One in the low (300 kHz) and one in the high (765 kHz) frequency regimes. The numerical curves were compared to the corresponding experimental ones and found to be in good agreement. Our findings provide new insight into the validity of dynamic micromagnetic simulation to analyze the frequency behavior of SAR within the framework of LLG and indicate that anisotropy selection plays a key role in the reliability of simulations.

Comparative Stability Analysis of Synchronous Reference Frame Current Controllers Operated at High Fundamental Frequency

Synchronous reference frame (SRF) proportional-integral (PI) current controller (CC) is the most well-established solution for current regulation in AC machine drives and grid-connected voltage source converters. The design of high dynamic performance current control loop has several challenges in high-speed and high-power applications due to the effects of controller gain’s selection, non-linearities, parameters variations, disturbances, digital implementation, and time delays. These become more significant due to high operating frequencies, which severely degrade dynamics and stability of the current control system. Various structures of SRF PI CCs have been reported in the literature. However, the aforementioned effects on the dynamics at high frequency operation have not been thoroughly addressed. Therefore, a comparative analysis of different SRF PI CCs’ structures is proposed in this paper, which addresses the design principles and gains’ selection while also taking into account the heavy computational burden and PWM delays. Additionally, the paper thoroughly analyzes and evaluates the dynamics and stability of the system operating at high fundamental frequencies. The advantages and limitations of each SRF PI CC scheme are studied and reported. The performance of the SRF PI CCs is comprehensively tested to demonstrate the analytical outcome of this study.

LSIAN: Exploiting interval interests for session-based recommendation via sparse attention network

Session-based recommendation (SBR) recommends the next clicked item to anonymous users. Recent studies utilize graph attention networks for modeling. There exist various problems in these algorithms: First, most of these methods only consider the long-term interests of users, ignoring the short-term interest transfer of users; second, when the graph attention mechanism obtains item weights, irrelevant items are assigned weights, which reduces the weights of related items, resulting in insufficient weight discretization. Third, the last action is taken as the user's final preference, however, this assumption does not necessarily correspond to the user's actual interest. This paper proposes a long- and short-term interest-attention aware network (LSIAN) model. First, we propose a novel time-aware attention mechanism that learns users' short-term interests by considering context and time interval, and utilizes average pooling to represent users' long-term interests. Next, we introduce a context-based adaptive sparse attention mechanism to discover irrelevant items, down-weight irrelevant items through a dynamic loop, and reserve higher weights for related items. Furthermore, we introduce a gated fusion method adaptively integrate the user's long-term and short-term preferences, thereby alleviating the problem of excessive weighting of the last term. Experiments on two public datasets show that LSIAN outperforms state-of-the-art algorithms.

Dynamic Hysteresis Calculation of Silicon Steel Considering DC Hysteresis and Anomalous Eddy Current Loss

We had proposed a combined 3D solid core and 1D steel plate modeling method for accurate magnetic field and iron loss calculation of laminated iron cores with low computation cost. The effective permeability including the laminated effect can be obtained and fed back to the 3D model. However, only the initial <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BH</i> curve and classic eddy currents are considered in the 1D model, thus it is improved to consider the DC hysteresis property and anomalous eddy current loss in this paper. The improved 1D model is applied to the dynamic hysteresis loop calculation of non-oriented and grain-oriented silicon steels and confirmed to be effective by comparing with the measurement results.

Improving Dynamic Loop Response

Improving Dynamic Loop Response

Evolution of grades and social comparison concern within an introductory physics course

This study investigates the evolution and associations between exam grades and social comparison concern (SCC) among students in an introductory calculus-based physics course. We begin with a descriptive characterization of midterm and final exam scores as well as pre-post SCC scores, including the concurrent evolution of these scores during the course. We hypothesize a feedback loop in which changes in SCC scores are mediated by exam grades, and changes in exam scores are mediated by SCC scores. We employ a structural equation model to determine whether the data are consistent with these hypotheses. Results indicate that there were significant within-student changes in the relative grade standing from exam to exam and that changes in SCC scores depended on both the pre-SCC scores and scores on the first midterm exam. Further, we find evidence that exam scores partially mediate the association between pre- and post-SCC scores, and in turn, post-SCC scores partially mediate associations between midterm and final exam scores, though the mediation effects are somewhat small, comprising 5%–10% of the total effects between exam scores and SCC. We also find that while SCC scores are somewhat correlated with exam scores, they are only very weakly correlated with nonexam grade components, consistent with the idea that exam scores (rather than nonexam scores) are driving changes in SCC and vice versa. Overall, the results provide empirical, correlational evidence to motivate further experimental investigation into a hypothesized dynamic and iterative feedback loop in which student concern about ability or performance compared to others (SCC) can either negatively or positively interfere with student performance on exams.1 MoreReceived 5 January 2023Accepted 28 March 2023DOI:https://doi.org/10.1103/PhysRevPhysEducRes.19.010129Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDiversity & inclusionLearning environmentPhysics Education Research

Underwater dynamic hysteresis modeling and feedforward control of flexible caudal fin actuated by macro fiber composites

Underwater bionic robotics and vehicles that use flexible structures as power sources have attracted considerable attention. Thus, the underwater dynamic behaviors of the flexible structure of a variable cross-section actuated by smart actuators must be further investigated. In this study, underwater dynamic hysteresis modeling and feedforward compensation of a flexible caudal fin driven by macro fiber composites (MFC) at different frequencies were investigated. The MFC-actuated flexible caudal fin is considered a Euler–Bernoulli beam. A revised hydrodynamic function governed by the thickness of the viscous layer and the gap-to-width ratio of the cross-section was developed. Then, a characteristic-frequency-dependent extended transform function model is proposed for the hydrodynamic response of a non-uniform beam. An asymmetric hysteresis model is presented to characterize the rate-independent hysteresis behavior of the MFC actuators. Further, the underwater dynamic equation of the MFC-actuated caudal fin with hysteresis is obtained by connecting the two established models in series. Experiments demonstrate the dynamic behavior between the underwater responses of the caudal fin and the input voltages have an ellipse-like shape and vary in shape and size with oscillating frequency. The ellipse-like dynamic hysteresis loop first grows, reaching its maximum at the resonant frequency, and then decreases, because of the frequency-dependent dynamic behavior. Meanwhile, a phase shift of approximately 90°between the input voltage and underwater dynamic response was observed at the resonant state. Furthermore, the experimental results demonstrate that the proposed model can describe the nonlinear underwater dynamic hysteresis behavior of the proposed caudal fin at different frequencies. The trajectory tracking performances are remarkably improved over a broad frequency range with the proposed compensator. Consequently, the effectiveness of the proposed dynamic model and compensation method was demonstrated. These results are helpful for robotic fish or autonomous underwater vehicles propelled by oscillating foils actuated by smart actuators, including MFC.

Rheological properties of CO2 hydrate slurries in presence of Dioctyl sodium sulfosuccinate (AOT) in a dynamic loop for refrigeration application

Recent studies have shown real potential for using hydrate slurries as a secondary fluid in indirect refrigeration systems. However, pipeline clogging problems can appear due to hydrate agglomeration. Additives are generally used to counteract these problems. In this study, rheological properties of CO2 hydrate slurries in the presence of an additive Dioctyl sodium sulfosuccinate (AOT) are investigated. The capillary viscometer method is used to determine the fluid behaviour and the AOT impact on the slurry rheology. A dynamic loop is set up to collect experimental pressure drop and volume flow rate data. Based on the Rabinowitch and Mooney approach and Herschel-Bulkley model to determine the rheological properties, a shear-thinning behaviour was observed. The importance of the behaviour index choice in the procedure is discussed. Finally, the results obtained with AOT are compared with pre-existing studies. The present study highlights the AOT potential to significantly improve the CO2 hydrate slurries properties.

Dynamic Stabilization of Metastable States in Triple-well Ferroelectric Sn2 P2 S6.

Polarization dynamics in ferroelectric materials is governed by the effective potential energy landscape of the order parameter. The unique aspect of ferroelectrics compared to many other transitions is the possibility of more than two potential wells; this prospect leads to complicated energy landscapes with new fundamental and functional properties. In this work, we reveal direct dynamic evidence of a triple-well potential in the metal thiophosphate Sn2 P2 S6 compound using multivariate scanning probe microscopy combined with theoretical simulations. Our key finding is that the metastable zero polarization state can be accessed through a gradual switching process and is surprisingly stabilized over a broad range of electric fields. Simulations confirm that the observed zero polarization state originates from a kinetic stabilization of the non-polar state of the triple well, as opposed to domain walls. Dynamically, the triple well of Sn2 P2 S6 becomes equivalent to antiferroelectric hysteresis loop. Therefore, this material combines the robust and well-defined domain structure of a proper ferroelectric with dynamic hysteresis loops present in an antiferroelectric state. Moreover, the triple-well also enhances mem-capacitive effects in Sn2 P2 S6 which are forbidden for ideal double-well ferroelectrics. These findings provide a path to tunable electronic elements for beyond binary high-density computing devices and neuromorphic circuits based on three thermodynamically stable ferroelectric states. This article is protected by copyright. All rights reserved.

Experimental Study on Response of Non- and Externally-Gapped Metal-Oxide Arresters Excited by Nanosecond-Level Transient Electromagnetic Disturbances

Metal-oxide arresters (MOA), including the non-gapped MOAs (NG-MOA) and externally-gapped MOAs (EG-MOA), are the main equipment applied to protect the power system against conducted transient electromagnetic disturbances (TED). However, MOAs exhibit quite different electrical characteristics under microsecond-level and nanosecond-level transients. In order to figure out the response of MOAs under nanosecond-level TED, experimental platforms are built which contain a pulse generator with the amplitude of pulse current adjustable from 0 kA to 5 kA and the rise time of 20 ns. Then the dynamic hysteresis loops, the static V-I characteristics, the response time delay and the full response process of five types of 10-kV MOAs are investigated. From the dynamic hysteresis loops of NG-MOAs, the impedance conversion process can be seen clearly and can be explained by the grain-boundary theory. But EG-MOAs has not this conversion process due to the air-gap. And the experimental results demonstrate that there is no noticeable nanosecond-level conduction delay on NG-MOAs whereas the electric field non-uniformity of the air-gap has a significant influence on the time delay of the EG-MOAs. The response time delay of EG-MOAs changes ranging from tens of nanoseconds to hundreds of nanoseconds. This feature makes EG-MOAs more likely to raise insulation risks under nanosecond-level TED compared with NG-MOAs.

On Transient Instability Mechanism of PLL-Based VSC Connected to a Weak Grid

This paper analyzes the transient process of Phase Locked Loop (PLL) based grid-connected Voltage Source Converters (VSCs) under large disturbances by modeling the PLL dynamic model and active power loop model. With a focus on the effect of transient grid voltage changes on the stability of the PLL-based VSCs, it reveals that the mechanism of transient instability of PLL-based VSCs is different under various disturbance levels. It can be concluded that prior to the PLL losing synchronization, the power loop has already become unstable. In addition, the influence of the PLL prefilter used for asymmetrical faults on the stability criterion is analyzed. Moreover, based on the transient instability mechanism, two solutions are proposed to deal with different instability occasions, and the adaptability of these solutions is analyzed. Finally, hardware-in-the-loop experimental results validate the proposed transient instability mechanism and the proposed control strategies.

High Minimum Inter-Execution Time Sigmoid Event-Triggered Control for Spacecraft Attitude Tracking With Actuator Saturation

This paper investigates the attitude tracking problem for spacecraft with limited communication, network congestion, unknown model parameters, actuator saturation, and external disturbances. To alleviate communication load, a novel sigmoid event-triggered mechanism is proposed with the special ability to guarantee a high minimum inter-execution time to avoid network congestion like package loss or time delay effectively. A neural network-based adaptive control algorithm is designed to deal with unknown model parameters. Besides, the problem of actuator saturation is tackled by introducing a dynamic loop gain function-based approach. System stability is proved by Lyapunov stability analysis and the high minimum inter-event time is substantiated by Zeno Behavior analysis with explanatory remarks. Numerical simulation results also show that a high minimum inter-event time and a high average inter-event time can be realized on the premise of high attitude tracking accuracy. Compared with all the previous studies, the ratio of the minimum inter-execution time to the average minimum inter-execution time has been improved by nearly 10 times with the proposed approach. Note to Practitioners—This paper was motivated by the problem of spacecraft attitude takeover control, but it also applies to other aperiodic discrete-time control systems. Existing event-triggered control methods have solved the problem of limited communication for attitude takeover control of spacecraft by reducing the number of times wireless communication is required. This paper proposes a new method for realizing quasi-periodic control with a large time gap between any two consistent control impulses using a hyperbolic tangent function or another sigmoid function in an event-triggered mechanism. The proposed sigmoid event-triggered mechanism (ETM) can magnify the event-triggered threshold dozens of times in a short time to avoid unnecessary frequent triggering without decreasing the precision of control. It’s feasible to employ the proposed sigmoid ETM to deal with limited communication and network congestion in other network control systems that are constructed with similar dynamic structures. Furthermore, it would be marvelous if we could transform this sigmoid ETM into a high-efficiency decision-making mechanism in some control systems without using a wireless network. Now we are studying how to realize orbital-attitude (maneuver) control with a relatively average time interval and fewer times of orbit transfer by using the proposed sigmoid ETM. Besides, to further improve the system performance, we are also seeking to further increase the ratio of the minimum inter-execution time to the average inter-execution time. For the purpose of a more even distribution of the triggering events on the time axis, we may modify the proposed sigmoid event-triggered method or associate it with other methods.