Fixed-time Control Scheme Research Articles

Fixed-time control of teleoperation systems based on adaptive event-triggered communication and force estimators

Abstract A fixed-time control strategy based on adaptive event-triggered communication and force estimators is proposed for a class of teleoperation systems with time-varying delays and limited bandwidth. Two force estimators are designed to estimate the operator force and environment force instead of force sensors. With the position, velocity, force estimate signals, and triggering error, an adaptive event-triggered scheme is designed, which automatically adjusts the triggering thresholds to reduce the access frequency of the communication network. With the state information transmitted at the moment of event triggering while considering the time-varying delays, a fixed-time sliding mode controller is designed to achieve the position and force tracking. The stability of the system and the convergence of tracking error within a fixed time are mathematically proved. Experimental results indicate that the control strategy can significantly reduce the information transmission, enhance the bandwidth utilization, and ensure the convergence of tracking error within a fixed time for teleoperation systems.

Resilient Neuroadaptive Distributed Fixed-Time Attitude Coordination Control for Multiple Spacecraft.

This work studies the attitude coordination tracking problem for multiple spacecraft with consideration of unintended faults (communication link faults and actuator faults), inertial uncertainties, and external disturbances under a directed communication graph. A resilient neuroadaptive distributed fixed-time control scheme is investigated to solve this challenging problem. First, an improved adaptive distributed observer is established for followers to estimate the states of the leader when considering communication link faults. The proposed observer improves the resilience against communication link faults. Subsequently, to further cope with the problem of actuator faults, inertial uncertainties, and external disturbances, based on the proposed observer and the technique of adding a power integrator, a neuroadaptive distributed fixed-time attitude coordination controller is developed. Unlike the existing controllers, the proposed one requires less information when dealing with faults and lumped uncertainties, and has a lower-computational cost. Moreover, the fixed-time stability of the closed-loop system is ensured under the designed resilient neuroadaptive distributed control scheme. Finally, comparative simulations are carried out to manifest the effectiveness of the investigated coordination control method.

Dynamic surface control for satellite attitude of the chained three-body tethered system during deployment

A fixed-time attitude stabilization scheme based on dynamic surface control (DSC) is proposed for the attitude of the three-body chained tethered system during tether deployment. Considering that the angular velocity of the satellite is difficult to measure, an extended state observer (ESO) is introduced to simultaneously compensate for the adverse effects of flexible panel vibrations and uncertain inertia tensors. On this basis, a fixed-time convergence attitude control scheme using DSC is designed, accounting for filtering error compensation. To offset the limitations of traditional fixed-time control strategies that ignore actuator saturation, an auxiliary system is introduced to incorporate control input constraints directly into controller design. The stability of the closed system is analyzed based on Lyapunov theory. Finally, the effectiveness and superiority of the proposed algorithm are verified by numerical simulation.

Adaptive fixed-time tracking control for nonlinear systems subject to asymmetric input saturation

For nonlinear systems with asymmetric input saturation, an adaptive fixed-time control strategy via multi-dimensional Taylor network (MTN) is proposed to effectively solve the tracking control problem. Firstly, the Gaussian error function and the differential mean value theorem are used to simulate the asymmetric input saturation model, that is, the nonlinear model is transformed into a linear model with a bounded perturbation. Secondly, MTNs are applied to control the design process, which approximate the nonlinear structures with linear combination of polynomials. Thirdly, in addition to the tracking error converging to an arbitrarily small neighborhood around the origin within a fixed time, the proposed control scheme not only guarantees that any signal in the controlled system remains bounded, but also avoids the problem of finite-time control relying on the initial state. Finally, three examples are used to validate the feasibility and superiority of the scheme.

Neuro-Adaptive-Based Fixed-Time Composite Learning Control for Manipulators With Given Transient Performance.

This article investigates an adaptive neural network (NN) control technique with fixed-time tracking capabilities, employing composite learning, for manipulators under constrained position error. The first step involves integrating the composite learning method into the NN to address the dynamic uncertainties that inevitably arise in manipulators. A composite adaptive updating law of NN weights is formulated, requiring adherence solely to the relaxed interval excitation (IE) conditions. In addition, for the output error, instead of knowing the initial conditions, this article integrates the error transfer function and asymmetric barrier function to achieve the specific performance for position error in both steady and transient states. Furthermore, the fixed-time control methodology and Lyapunov stability criterion are synergistically employed in order to guarantee the convergence of all signals in the manipulators to a compact neighborhood around the origin within a fixed-time. Finally, numerical simulation and experiments with the Baxter robot results both determine the capability of the NN composite learning technique and fixed-time control strategy.

Disturbance observer-based fixed time control for bidirectional DC-DC converter with input voltage and load uncertainty

The bidirectional DC-DC converter serves a vital role in the energy recovery system of new energy vehicles. However, external interference and parameter uncertainty can easily affect its dynamic performance. A disturbance observer-based fixed time control strategy for bidirectional DC-DC is proposed to address the impact of input voltage and load uncertainty on bidirectional converters. Firstly, the bidirectional converter model undergoes coordinate transformation, and disturbance observers are designed to compensate for input voltage and load disturbances. Secondly, based on the fixed time control theory, a fixed-time controller is devised for the bidirectional DC-DC converter to facilitate fast tracking and adjustment of output voltage without requiring measurement of input voltage and load current. Finally, simulation and experiment evaluations are conducted to assess the proposed scheme’s effectiveness in output voltage regulation.

Fixed-time tracking control of manipulators based on non-singular fast terminal sliding surface

As for the problem of trajectory tracking of a multi-joint serial manipulator, a novel fixed-time control scheme is proposed based on non-singular fast terminal sliding mode control. By employing a fast terminal sliding mode surface, we solve the singularity problem existed in traditional terminal sliding mode surface. In the meantime, in order to improve the rapidity of the system, the fixed-time control is incorporated with the fast terminal sliding mode surface control. Theoretical analysis proves that the proposed control scheme guarantees that better tracking performance is obtained, and its convergence time upper limit is not affected by the initial states. In addition, a reaching law with the exponential approach characteristic is added to the control law, which effectively reduces the chattering phenomenon in the controller design. Finally, the effectiveness and feasibility of the designed controller are verified through a numerical simulation.

Nonlinear system controlled using novel adaptive fixed-time SMC

<abstract><p>This work introduces a novel adaptive fixed-time control strategy for nonlinear systems subject to external disturbances. The focus pertains to the introduction of the fixed-time terminal sliding mode control (FxSMC) technique. The proposed scheme exhibits rapid convergence, chatter-free and smooth control inputs, and stability within a fixed time. The utilization of an adaptive methodology in combination with the FxSMC yields the proposed strategy. This approach is employed to address the dynamic system in the presence of external disturbances. The results obtained from the Lyapunov analysis will provide insights into the stability of the closed-loop system in a fixed time. In the end, the simulation results are presented in order to assess and demonstrate the effectiveness of the methodology.</p></abstract>

Distributed fixed-time control strategy for islanded microgrids considering perturbations and switching topologies

Distributed fixed-time control strategy for islanded microgrids considering perturbations and switching topologies

Distributed fixed-time control strategy for islanded microgrids considering perturbations and switching topologies

Distributed fixed-time control strategy for islanded microgrids considering perturbations and switching topologies

Adaptive Fixed-Time Control Strategy of Generator Excitation and Thyristor-Controlled Series Capacitor in Multi-Machine Energy Systems

Adaptive Fixed-Time Control Strategy of Generator Excitation and Thyristor-Controlled Series Capacitor in Multi-Machine Energy Systems

Event-triggered attitude control for hypersonic morphing vehicles using fixed-time control technique

This research discusses the attitude tracking issue for hypersonic morphing vehicles (HMVs) subject to multisource uncertainty and limited system resources. Hypersonic vehicles with variable-span wings can change the aerodynamic configuration according to various requirement segments. However, A morphing process will cause significant changes in aerodynamic coefficients, mass distribution, and inertial characteristics. Moreover, the additional forces and moments exist in the HMV model, impacting the attitude control. Therefore, a robust and fast flight control system is necessary for HMVs to stabilize the attitude angles in the presence of multisource uncertainty, which always consists of modeling inaccuracies, external disturbances, and coefficient perturbances. Considering such factors, the fixed-time control theory is employed to develop the flight control system with strong robustness and adaptation. Compared to the existing fixed-time control strategies with complex expressions to indicate the convergence time, the setting time of the proposed controller is more explicit and can be exactly preassigned in the design procedure. Fast attitude tracking can be achieved and accurately evaluated with the help of the proposed technique. Furthermore, to avoid excessive occupation of limited aircraft resources, a novel switched dynamic event-triggered technique is constructed, leading to a desired level of control performance with low resource consumption. The proposed event-triggering mechanism will strike a better balance between tracking accuracy and resource occupation than the commonly used triggered control methods, such as the fixed-threshold and relative-threshold event-triggering strategy. Finally, the event-triggered fixed-time controller provides an outstanding tracking performance, with the Zeno behavior avoided. The stability of the closed-loop system is proved by introducing the Lyapunov theory. Simulation results and comparisons validate the effectiveness and advantages of the developed control scheme.

Non-singular fixed-time robust containment control for autonomous surface vehicles

For the containment control problem of autonomous surface vehicles with external disturbances, a novel non-singular fixed-time control scheme is developed, where the multi-ship system consists of real leaders and followers. In contrast with traditional containment cases studied in most of the existing literature, real leaders are required to track the desired trajectories, and the followers need to get into the convex hull spanned by the real leaders in a fixed time. The method for solving the fixed-time containment control problem is divided into the following steps: Firstly, a fixed-time disturbance observer is designed to estimate the external disturbance for the leaders and the followers, and the estimation result is used to compensate for the input of the control system; Second, combined with fixed-time stability theory and a switch function, a non-singular fixed-time trajectory tracking controller for leaders is designed, such that the position tracking error could be bounded within a fixed time; Third, to achieve formation efficiently, a non-singular fixed-time containment controller strategy incorporating with graph theory is proposed for coordinating followers, such that the fixed-time stability can be ensured. Finally, simulation examples are given to demonstrate the effectiveness of the proposed method.

Fast Output Consensus Tracking of Heterogeneous Multi-Agent Systems Under Directed Graphs via a Fixed-Time Control Strategy

The brief is concerned with the fast output consensus tracking problem of heterogeneous multi-agent systems (MASs) under the digraph with a directed spanning tree. A novel fixed-time control protocol is proposed within the hierarchical framework, which not only endows a feature of faster convergency but also offers a more flexible estimation on the settling time comparing with asymptotic and finite-time protocols. Within the developed protocol, the challenge and complexity caused by heterogeneities of agents’ dynamics and generalities of communication topologies can be fairly handled by recasting the fixed-time output consensus tracking problem into the problems of cooperative fixed-time estimation and local fixed-time tracking, separately. In upper layer, an elaborate potential function is designed to construct the fixed-time dynamic cooperative compensator to realize fast estimation of leader’s states. In lower layer, based on the implicit Lyapunov function approach, a local fixed-time tracking controller is designed for each agent to track the compensator’s states. Based on the incorporation of above two layers, each agent can track leader’s output within a fixed time.

Vision-based adaptive fixed-time uncooperative target tracking for QUAV with unknown disturbances

This article investigates the problem of the vision-based uncooperative target tracking control for a quadrotor unmanned aerial vehicle (QUAV) with unknown external disturbances. Note that the uncooperative target information cannot be obtained directly and need to be measured by a monocular sensor installed on the QUAV. In the light of the monocular sensor, a vision-based adaptive fixed-time control scheme is constructed to deal with the uncooperative target tracking problem. To efficiently observe unknown external disturbances, a fixed-time disturbance observer (FTDO) is designed by using the super-twisting (ST) algorithm. Finally, with the help of the Lyapunov stability theory, which guarantees the boundedness of all closed-loop signals, the QUAV is capable of tracking the uncooperative target in fixed time. The results of the simulation demonstrate the effectiveness of the proposed control strategy.

Global fixed-time adaptive fuzzy path following control for unmanned surface vehicles subject to lumped uncertainties and actuator saturation

A global fixed-time control strategy is presented for addressing the path following problem of underactuated unmanned surface vehicles (USVs) with lumped uncertainties and actuator saturation in this paper. Firstly, a novel fixed-time dual adaptive line-of-sight (FTDALOS) guidance law is proposed, which provides the desired heading angle and surge velocity. Subsequently, considering the complex marine environment, a novel fixed-time adaptive fuzzy controller (FTAFC) is developed. Additionally, to solve the issues of lumped uncertainties and actuator saturation, a fixed-time adaptive fuzzy control strategy and a fixed-time dynamic auxiliary saturation system are proposed. It is demonstrated that the proposed strategy can drive the USV along the desired path smoothly and rapidly, and all errors of the closed-loop system can converge to arbitrary small neighborhoods of the origin within fixed-time intervals. Finally, the effectiveness and superiority of FTDALOS guidance law and FTAFC are verified by theoretical analysis and simulation studies.

Adaptive Fixed-Time Event-Triggered Fuzzy Control for Time-Delay Nonlinear Systems With Disturbances and Quantization

This work investigates the adaptive fixed-time disturbance rejection control issue for a class of time-delay nonlinear systems subject to event-triggered and quantized input signals. First, a disturbance observer is proposed to promote the controller design. Compared with the related disturbance observer design, the observer designed in this paper can be used to construct the fixed-time control scheme. Then, combined with the backstepping technique and fuzzy logic systems (FLSs), an adaptive fuzzy event-triggered and quantized fixed-time control scheme is designed. Moreover, to deal with the unknown time delays, the Lyapunov-Krasovskii functional method is utilized to achieve the system stability. The proposed control algorithm reduces the transmission burden of data channel with a new combined event-based sampling and quantization, and improves the robustness of the time-delay nonlinear system. Finally, a numerical example and a practical example are given to validate the feasibility of the control scheme.

Observer based fixed-time integral sliding mode tracking control for underactuated AUVs via an event-triggered mechanism

In this paper, a fixed-time control scheme is presented for trajectory tracking of underactuated autonomous underwater vehicles (AUVs) subject to external disturbances. An adaptive fixed-time disturbance observer (AFDO) is developed to estimate the external disturbances in fixed time without the prior knowledge of external disturbances. An event-triggered mechanism is introduced to save the bandwidth resources and reduce the communication burden. By synthesizing the developed AFDO and the event-triggered mechanism, an AFDO based event-triggered fixed-time integral sliding mode control (AFDO-EFISMC) scheme is proposed to force the AUV to track the reference trajectory in fixed time. With the aid of the developed AFDO, the gain of discontinuous control part of the designed control scheme is reduced, and therefore the control chattering is alleviated. Stability analysis is given to show that the designed AFDO-EFISMC scheme can make the tracking errors of the AUV converge to arbitrarily small neighborhood of the origin within a fixed-time independent of the initial conditions. The effectiveness of the proposed control scheme is proven by comparative numerical simulations.

Adaptive fixed-time robust control for function projective synchronization of hyperchaotic economic systems with external perturbations

In today's sophisticated world, where financial systems play an increasingly important role, the application of controllers through management strategies is of crucial importance, given their vital role. An analysis of the synchronization of economic systems can provide a valuable insight into the financial synchronous development of different countries or regions around the world. In this paper, an adaptive fixed-time control strategy is presented for the function projective synchronization of drive-response hyperchaotic economic systems with unknown parameters and external perturbations. The developed controller is constructed by combining the fixed-time control approach and parametric adaptation technique. Specifically, the adaptive adjusting mechanisms are designed to estimate the unknown parameters and external perturbations, respectively. Stability investigation shows that the function projective synchronization errors under the developed controller can stabilize to the minor fields about zero in fixed time. A key feature of the developed controller is its high insensitive to unknown parameters and strong robustness against external perturbations. Finally, the effectiveness and merits of the presented control strategy are examined by simulation scenarios.

Disturbance observer-based fixed-time control for nonlinear systems with multiple disturbances

The fixed-time control is researched for the nonlinear system subject to multiple disturbances, which contain an external disturbance with unknown frequency and an unknown bounded disturbance. An adaptive nonlinear disturbance observer is devised to estimate the unknown frequency disturbance generated by an exogenous system. On this foundation, a composite fixed-time control scheme is proposed by combining the [Formula: see text] control and the fixed-time theory. It guarantees that the expected control performances of system can be achieved under the existence of multiple disturbances. Eventually, the validity of the control scheme is proved via simulation examples.