Practical Stability Research Articles

The United European Gastroenterology green paper-climate change and gastroenterology.

Climate change, described by the World Health Organization (WHO) in 2021 as 'the single biggest health threat facing humanity', causes extreme weather, disrupts food supplies, and increases the prevalence of diseases, thereby affecting human health, medical practice, and healthcare stability. Greener Gastroenterology is an important movement that has the potential to make a real difference in reducing the impact of the delivery of healthcare, on the environment. The WHO defines an environmentally sustainable health system as one which would improve, maintain or restore health while minimizing negative environmental impacts. Gastroenterologists encounter the impacts of climate change in daily patient care. Alterations in the gut microbiome and dietary habits, air pollution, heat waves, and the distribution of infectious diseases result in changed disease patterns affecting gastrointestinal and hepatic health, with particularly severe impacts on vulnerable groups such as children, adolescents, and the elderly. Additionally, women are disproportionally affected, since climate change can exacerbate gender inequalities. Paradoxically, while healthcare aims to improve health, the sector is responsible for 4.4% of global carbon emissions. Endoscopy is a significant waste producer in healthcare, being the third highest generator with 3.09kg of waste per day per bed, contributing to the carbon footprint of the GI sector. Solutions to the climate crisis can offer significant health co-benefits. Steps to reduce our carbon footprint include fostering a Planetary Health Diet and implementing measures for greener healthcare, such as telemedicine, digitalization, education, and research on sustainable healthcare practices. Adhering to the principles of 'reduce, reuse, recycle' is crucial. Reducing unnecessary procedures, which constitute a significant portion of endoscopies, can significantly decrease the carbon footprint and enhance sustainability. This position paper by the United European Gastroenterology aims to raise awareness and outline key principles that the GI workforce can adopt to tackle the climate crisis together.

Fixed-time active fault-tolerant control for a class of nonlinear systems with intermittent faults and input saturation

In this paper, the problem of fixed-time active fault-tolerant control is studied for systems with sector-bounded nonlinearities, intermittent faults, and input saturation. Since intermittent faults appear and disappear randomly, a fixed-time scheme is considered in the active fault-tolerant control algorithm, composed of detection, isolation, estimation, and the control unit. Utilizing homogeneity-based observers, the fixed-time state estimation is available in the presence of unknown but bounded disturbances, and a fault diagnosis unit is proposed. An input saturation compensator is introduced to analyze the effect of input saturation, and its auxiliary variables are used in the reconfigurable control law. The fault-tolerant controller, which is constructed via the information provided by the fault diagnosis unit and saturation compensator, has two switching modes. As a consequence, intermittent faults are compensated via the designed active fault-tolerant control method and the system reaches practical stability with the entire convergence time bounded in a fixed time. Finally, the example of a heat control system is exploited to demonstrate the effectiveness of the developed active fault-tolerant control scheme.

Adaptive Free‐Will Arbitrary Time Tracking of a Class of Uncertain SISO Nonlinear Systems

ABSTRACTThis paper investigates the free‐will arbitrary time (FWAT) tracking problem for a class of single‐input single‐output (SISO) strict‐feedback nonlinear systems without/with parametric uncertainties. First, we design an FWAT backstepping tracking controller for strict‐feedback nonlinear systems without uncertainties in a recursive manner. It is shown that the proposed virtual controllers and their derivatives are continuously differentiable for the backstepping design, and the tracking error converges to zero within the pre‐specified settling time independent of design parameters and initial conditions and remains at zero after the pre‐specified settling time. Second, we address the adaptive FWAT tracking problem of SISO strict‐feedback nonlinear systems with parametric uncertainties in the practical stability sense. The continuously differentiable and nonsingular time‐varying adjustment functions for practical FWAT tracking are developed to transform error variables for the recursive backstepping design. Then, the adaptive FWAT backstepping tracking design and stability analysis strategy are established to ensure practical FWAT convergence within the pre‐specified settling time independent of design parameters and initial conditions. The designed adaptive virtual and actual controllers are continuous for all . Furthermore, a practical adaptive prescribed‐time tracking scheme is provided using the proposed practical adaptive FWAT tracking scheme. Finally, the effectiveness of the theoretical approaches is confirmed through a simulation example and an experimental result.

Filter‐Based Average Dwell‐Time Tuning Approach for Adaptive Prescribed‐Time Tracking of Uncertain Switched Nonlinear Systems

ABSTRACTThis paper addresses neural‐network‐based adaptive prescribed‐time (PT) tracking for uncertain switched systems with unmatched nonlinearities. A continuously switched adaptive tuning mechanism for neural network learning is developed by applying the average dwell time (ADT). First, a neural‐network‐based PT tracking control design strategy using the ADT‐based adaptive tuning mechanism is established for switched nonlinear systems in strict‐feedback form. A novel adaptive dynamic surface controller is designed recursively using a practical finite‐time scaling function and continuously switched tuning parameters. The switched adaptive tuning laws for neural networks are structured to reduce the conservatism associated with common adaptive laws. Then, a filter‐based tuning approach is employed to ensure the continuity of switched adaptive parameters with ADT in the designed controller. The practical PT stability of the closed‐loop system is demonstrated based on the boundedness of the adaptive parameters. Building upon this foundation, the proposed PT design approach is extended to control switched pure‐feedback nonlinear systems, even in cases where control directions are unspecified. The unknown sign problem encountered with switched virtual and actual control coefficient functions is resolved in the PT control framework. It is shown that the PT performance bound of the tracking error can be reduced by selecting the design parameter of the scaling function. Finally, simulation results illustrate the merits of the proposed theoretical approach.

Finite-Time Stability Analysis and Stabilization of Switched Affine Systems via an Event-Triggered Strategy.

This article investigates the finite-time control problem of the switched affine systems via an event-triggered strategy. It is well known that the existence of affine terms brings great difficulties in analysis of the finite-time property of such systems. Furthermore, the design of the globally feasible event-triggered mechanism (ETM) under a finite-time control framework is challenging. Thus, a two-step hybrid control scheme is proposed in this article. The first step focuses on the event-triggered finite-time control for practical stability, while the second step aims to achieve finite-time stabilization. Particularly, in step one, by constructing the intersection between the affine term's threshold and feasible state region of the established ETM, it is verified that the Zeno behavior can be excluded. Thereafter, an affine state-dependent switching law and sufficient conditions are provided for achieving practical stability. Meanwhile, an estimation for the practical settling time to enter the bounded set is provided. In step two, the criteria for finite-time stabilization of the considered systems are further presented, and an overall settling-time upper bound is derived. Finally, a numerical example is illustrated to demonstrate the effectiveness of our proposed method.

Observer‐based control for nonlinear Hadamard fractional‐order systems via SOS approach

AbstractPractical stability refers to the notion that the origin is not an equilibrium point (EP) and that the system states tend to converge toward a sphere centered at the origin. The first goal of this paper is to analyze the concept of “practical stability” in Caputo–Hadamard fractional‐order derivative (CHFOD) systems. Then, using the Lyapunov approach, a polynomial fuzzy (PF) observer‐based controller for stabilizing CHFOD PF systems is created. The observer‐based control is innovative since it was created and proven using the sum‐of‐squares (SOS) method. In conclusion, a numerical illustration is provided to corroborate the theoretical findings.

On the Novel Auxiliary Lyapunov Function and Uniform Asymptotic Practical Stability of Nonlinear Impulsive Caputo Fractional Differential Equations via New Modelled Generalized Dini Derivative

In this paper, the uniform asymptotic practical stability of nonlinear impulsive Caputo fractional differential equations with fixed moments of impulse is examined using an auxiliary Lyapunov functions which are analogues of vector Lyapunov functions. Together with comparison results, sufficient conditions for the uniform practical stability as well as the uniform asymptotic practical stability of the impulsive Caputo fractional order systems are established. An illustrative example is given to confirm the suitability of the obtained results.

Finite-time SMC-based admittance controller design of macro-micro robotic system for complex surface polishing operations

In the field of robotic polishing, achieving uniform material removal typically involves addressing the issue of constant contact force control. However, multi-source external disturbances in the polishing scenarios of complex workpiece surfaces can severely affect the robot’s force control accuracy. To enhance the responsiveness and disturbance rejection capabilities of robots in the compliant polishing process, this paper proposes an adaptive admittance controller with practical finite-time stability. A virtual control input is introduced into the basic admittance control framework in light of the state space theory, aiming to provide flexibility for common adaptive law designs. On this basis, a robust sliding mode control (SMC) algorithm is proposed to suppress external disturbances. The force tracking error is theoretically proven to achieve finite-time convergence when applying the proposed control strategy. Experimental results across various polishing scenarios demonstrate that, compared with the existing admittance control strategies, the proposed method can reduce fluctuations of the polishing force and improve the surface quality, thus verifying its effectiveness.

Efficient structural and compositional modulation of vanadium oxides for high-areal-capacity zinc-ion batteries

Although rechargeable aqueous zinc-ion batteries (RAZIBs) have been revitalized as competitive candidates for large-scale energy storage, the development of this technology is stalled by the underutilization of cathode materials due to sluggish reaction kinetics, resulting in low areal capacities that cannot meet the practical requirements. Herein, this study reveals a simple, efficient, and low-energy synthetic route to effectively convert relatively inactive α-V2O5 (131 mAh g−1 at 0.1 A g−1) into highly active MxV8O20·nH2O (MVO, M = Li, Na, and K) by liquid-phase dissolution-recrystallization reaction and chemical preintercalation under low-temperature conditions. Particularly, NaVO presents the most expansive diffusion channel, the highest specific surface area, and the smallest band gap, allowing faster electron/ion transport kinetics and better material utilization. With these features, NaVO accommodates electrolyte Zn2+ and H+ with good reversibility, showing high capacity (383 mAh g−1 at 0.1 A g−1) and rate-performance (207 mAh g−1 at 8 A g−1). More importantly, high-areal-capacity of 3.2 mAh cm−2 and remarkable cycle stability over 1500 cycles can be achieved from free-standing high-mass-loading electrode (∼14 mg cm−2) made of NaVO and multi-walled carbon nanotubes (MWCNTs). This work enriches the synthetic methods for obtaining high-performance vanadium-based host materials with practical areal capacity and cycle stability.

Crosslinking assembly of Propenyl-Calix[4]-Crown-6 interpenetrating functional mesoporous carbon Spheres: High selectivity and stability to Remove Cs(I) from high acid solution

The calix[4]-crown-6 ligand shows high selectivity for Cs(I) over other alkali and alkaline earth metal cations, making it a promising candidate for radiocesium separation. However, its high viscosity and poor adhesion limit its practical use and stability. In this study, an alkenyl-functionalized calix[4]-crown-6 (p-C[4]C6) was developed. Using a mesoporous structure and post-assembly techniques, a composite (p-C[4]C6/MMCs) was created via crosslinking within mesoporous matrix channels. The p-C[4]C6/MMCs exhibited high stability and practicality. The p-C[4]C6/MMCs demonstrated high selectivity, good reusability, and significant adsorption capacity (47.76 mg/g) for Cs(I) in 3.0 M HNO3. Kinetic, isotherm, and thermodynamic studies suggest a spontaneous chemical adsorption mechanism. This research provides insights into the design of calix[4]-crown-6 based materials and offers a viable approach for Cs(I) separation from high-level liquid waste (HLW).

Practical fixed-time non-singular sliding mode control of second order nonlinear dynamic systems with chattering and overshooting avoidance

This paper introduces a Practical Fixed-Time Stabilization technique (PFxT) designed for a specific class of nonlinear second-order systems. The closed-loop systems, when appropriately parameterized, exhibit convergence within a predetermined time frame to a confined region near the origin. This outcome is achieved by amalgamating a nonlinear sliding mode approach with a practical fixed-time tracking virtual trajectory. The control algorithmś design incorporates considerations for overshoot reduction and chattering cancellation. Furthermore, the Lyapunov method is employed to establish the practical fixed-time stability of the proposed solution, providing insights into the limits within which the algorithm can be effectively deployed. To complete the algorithm specification, a parameter selection approach is introduced, enabling customization of the desired settling time. Comprehensive simulations are conducted to validate the effectiveness and viability of the proposed PFxT technique.

Fixed-time active fault-tolerant control for dynamical systems with intermittent faults and unknown disturbances

In this article, the problem of fixed-time active fault-tolerant control is investigated for dynamical linear systems with intermittent faults and unknown disturbances. Unlike traditional active fault-tolerant control, fixed-time control is taken into account in this article since intermittent faults appear and disappear within a certain period of time. The entire active fault-tolerant control framework is composed of fault detection, fault isolation, fault and state estimation as well as the reconfigurable controller. Using the homogeneity-based observers, states and faults are well estimated and a fault diagnosis scheme is proposed for the sake of detecting and isolating intermittent faults in a fixed time. The fault-tolerant controller, which provides global practical fixed-time stability of the closed-loop system, has two switching states corresponding to the appearance and disappearance of intermittent faults. As a consequence, intermittent faults are compensated via the designed active fault-tolerant control method and the system reaches practical stability with the entire convergence time bounded in a fixed time. Finally, two examples are exploited to demonstrate the effectiveness of theoretical results.

Uniform Practical Asymptotic Stability for Position Control of Underwater Snake Robots

Uniform Practical Asymptotic Stability for Position Control of Underwater Snake Robots

Robust stabilisation of uncertain nonlinear systems with matched and unmatched uncertainties

This paper considers the problem of robust stabilisation of uncertain nonlinear systems with both matched and unmatched uncertainties. A new robust controller is proposed to guarantee practical or asymptotic stability under non-vanishing or vanishing unmatched uncertainties. First, the system uncertainties are decomposed optimally into the matched and unmatched portions with an objective of the minimum norm for the unmatched part. Next, instead of a robust control design based on only the matched part of uncertainties commonly used in the previous works, a robust controller is synthesised based on both matched and mismatched portions of the uncertainties. It is shown that in uncertain systems whose state cannot simultaneously stay in a so-called null set and diverge to infinity, the proposed controller guarantees stability no matter how large the input-unrelated unmatched uncertainties are. Numerical examples illustrate that the proposed controller is more robust against uncertainties, and offers improved damping and stabilisation features.

Practical Exponential Stability of Positive Switched Linear System and Its Application in Practical Consensus of Multi-agent System

Practical Exponential Stability of Positive Switched Linear System and Its Application in Practical Consensus of Multi-agent System

Adaptive Finite-Time-Based Neural Optimal Control of Time-Delayed Wheeled Mobile Robotics Systems.

For nonlinear systems with uncertain state time delays, an adaptive neural optimal tracking control method based on finite time is designed. With the help of the appropriate LKFs, the time-delay problem is handled. A novel nonquadratic Hamilton-Jacobi-Bellman (HJB) function is defined, where finite time is selected as the upper limit of integration. This function contains information on the state time delay, while also maintaining the basic information. To meet specific requirements, the integral reinforcement learning method is employed to solve the ideal HJB function. Then, a tracking controller is designed to ensure finite-time convergence and optimization of the controlled system. This involves the evaluation and execution of gradient descent updates of neural network weights based on a reinforcement learning architecture. The semi-global practical finite-time stability of the controlled system and the finite-time convergence of the tracking error are guaranteed.

Enhanced trajectory tracking control algorithm for ROVs considering actuator saturation, external disturbances, and model parameter uncertainties

In this study, an augmented model-based predictive control (AMPC) algorithm was designed for a work-class remotely operated vehicles (ROV) with actuator saturation constraints, model parameter uncertainty, and external disturbances. Firstly, model parameter uncertainties and external disturbances (primarily umbilical cable forces) were considered in the kinematic model of the ROV. The ROV's actuator saturation constraints were introduced when designing the optimal control problem (OCP). Secondly, Bellman's principle of optimality was applied to the solution process of the OCP, decomposing it into multiple easily solvable sub-optimization problems to reduce computational complexity. Then, the recursive feasibility and input-to-state practical stability (ISpS) of the AMPC algorithm were proven in the presence of model parameter uncertainties and external disturbance forces. It was also demonstrated that the state convergence region of the system employing the AMPC algorithm was smaller than that of the system using the min-max MPC algorithm when reaching a steady state. Finally, numerical simulations verified that under the influence of external disturbance forces, model parameter uncertainties, and positioning system noise, the ROV using the AMPC algorithm achieved higher accuracy in tracking the desired trajectory compared to the min-max MPC algorithm.

Damp and droop coefficient stability region analysis for interlinking converters with virtual asynchronous machine controllers in virtual energy storage systems

Although virtual energy storage systems (VESSs) based on virtual asynchronous machine (VAM) control strategy have been widely applied to microgrids to achieve power balance between supply and demand sides, damp and droop coefficients are variable due to the system operation state. It is prone to causing wideband oscillation, such as low-frequency and sub/super-synchronous oscillation. Thus, this paper proposes one damp and droop coefficient stability region identification approach based on the Guardian map theory for the interlinking converter (IC) of VESSs. Firstly, the VAM control method of IC is derived through the traditional asynchronous machine, and its small-signal model is further built. The aforesaid process is the important modeling basic. Secondly, the concept of the practical small signal stability region (P-SSSR) is proposed to make up for the deficiency of the traditional small signal stability region (SSSR) that cannot ensure system robustness. Thirdly, the mapping method based on translation and rotation mapping is introduced to ensure that the identification of the P-SSSR problem is switched into the Hurwitz matrix identification problem. Furthermore, the high-dimensional parameter stability region can be obtained through the Guardian map theory. Finally, the performance of the proposed method can be verified through simulation and experiment results.

Disturbance observer-based event-triggered impulsive control for nonlinear systems with unknown external disturbances

Input-to-state practical stability (ISpS) of a kind of nonlinear systems suffering from unknown exogenous disturbances is explored in this article, where a disturbance observer is established to estimate the information of the exogenous disturbances. To achieve ISpS of the system, the impulsive controller as well as state-feedback controller are both considered to regulate the discrete and continuous dynamics of the system, respectively. Especially, a novel disturbance observer-based event-triggered mechanism is devised to decide the release of impulsive control signal. Furthermore, several adequate conditions are given for excluding the occurrence of Zeno phenomenon. To confirm the feasibility of the proposed results, two numerical instances and their corresponding simulation results are presented.

A novel finite element limit equilibrium method and its application in instability analysis for 3-D high core rockfill dams

The safe operation of high rockfill dams is an important issue related to human life and property safety. However, the stability analysis of high core rockfill dams with complex spatial morphology is mostly confined to two-dimensional, and appropriate stability analysis methods are scarce. Therefore, it is necessary to develop the three-dimensional stability analysis method for high core rockfill dams and study the spatial stability mechanism after impoundment. Firstly, a finite element limit equilibrium framework suitable for stability analysis of high rockfill dams is introduced, and the horn-shaped three-dimensional failure mechanism is presented for optimization. Then, according to the statistical results of the actual core rockfill dam above 100 m, 15 high dam models with actual Duncan-Chang parameters between 100 m and 300 m are established to study the three-dimensional instability mechanism. Meanwhile, grey relation analysis (GRA) is used for quantitative analysis of stability sensitive factors. Finally, the stability analysis of the Quxue core rockfill dam (173.2 m) was completed within the established stability framework. The results show that the combined failure mechanism with the horn-shaped is suitable for dam stability analysis, and with the increase of height and slope ratio of dam, the most dangerous sliding surface evolves to shallow layer. In addition, GRA results show that slope ratio is the most sensitive parameter compared to dam height and material parameters. The practical engineering stability results of Quxue show that the research results can provide a reliable reference for the stability of the high core rockfill dam.