Upper Bound Of External Disturbances Research Articles

Adaptive backstepping and sliding mode control of a quadrotor

This article proposes a simple robust adaptive control architecture with minimum parameter estimation laws for trajectory tracking of a quadrotor subjected to parametric variations and environmental disturbances. This simple control architecture aims to achieve highly accurate, robust, and fast trajectory tracking of the quadrotor in a short time with low computational cost. Firstly, the quadrotor model is divided into altitude, attitude, and position subsystems for which appropriate control methods are designed without prior knowledge of the upper bound of external disturbances. A simple adaptive fractional-order sliding mode control (AFSMC) is designed to enhance the tracking of the altitude subsystem and estimate the upper bound of the disturbances. Then, a simple adaptive backstepping control (ABC) is developed for the horizontal position to generate the required roll and pitch orientations. The adaptation laws not only estimate the upper bound of the disturbances but also adjust the controller gains thereby enhancing the robustness of the ABC. A nonsingular fast terminal sliding mode control (NFTSMC) is incorporated with a finite-time disturbance observer (FDO) to accurately suppress the disturbances, and follow the target rotation angles within a short finite-time. Simulation results showed that the compounded control structure ensures accurate, fast, and robust tracking. The AFSMC can achieve the desired altitude with a settling time of 0.31 s and root mean square error (RMSE) of 0.0454 m. The ABC can attain the target horizontal position coordinates with a settling time of (0.47 s, 0.47 s) and RMSE of (0.0370 m, 0.0518 s). The NFTSMC-FDO can achieve the desired attitude angles with a settling time of (0.11 s, 0.13 s, 0.52 s) and RMSE of (0.0098 rad, 0.0092 rad, 0.0935 rad). Performance comparisons with existing control methods in terms of settling time and RMSE demonstrated that the proposed control architecture is superior.

The nonlinear model reference adaptive impedance control for underwater manipulator operation objects in bilateral teleoperation system

The bilateral teleoperation system is susceptible to model parameter uncertainty and unknown disturbances in both the master and slave manipulators, resulting in instability and inaccuracies in the force and position tracking performance. To address these issues, a novel nonlinear model reference adaptive impedance controller has been designed to achieve coordinated force and position synchronization of dual manipulators. The adaptive control laws, based on sliding mode functions, have been designed to compensate for the uncertainty of the manipulator model. Furthermore, an adaptive estimation law has been employed to appraise the unknown upper bound of external disturbances. This ensures that the closed-loop model parameters of the dual manipulator converge to the reference impedance model respectively. Simultaneously, it enables the position error between the reference model response and the end-effector task space position to asymptotically converge to zero. To verify the effectiveness of the proposed controller, simulations have been conducted on the MATLAB platform and experiments on a single degree of freedom teleoperation system have been performed. The results demonstrate that the controller exhibits strong robustness and has the capability of force-position tracking ability.

Adaptive optimal ℓ∞‐induced robust stabilization of minimum phase SISO plant under bounded disturbance and coprime factor perturbations

AbstractThis paper addresses the problem of optimal robust stabilization of a discrete‐time minimum‐phase plant in the framework of robust control theory in the setup and under poor a priori information. Coefficients of the transfer function of the plant nominal model with stable zeros are unknown and belong to a known bounded polyhedron in the space of coefficients. The gains of coprime factor perturbations of the plant and the upper bound of external disturbance are also unknown. The problem under consideration is to design adaptive controller that minimizes, with the prescribed accuracy, the worst‐case asymptotic upper bound of the output. Solution of the problem is based on set‐membership estimation of unknown parameters and treating the control criterion as the identification criterion. A hard nonconvex problem of on‐line computation of optimal estimates is reduced, under additional nonrestrictive assumption, to a linear‐fractional programming via a nonlinear transformation of estimated parameters. Despite the non‐identifiability of the unknown parameters, the proposed adaptive controller guarantees, with the prescribed accuracy, the same optimal asymptotic upper bound of the output of adaptive system as the optimal controller for the plant with known parameters. In addition to the optimality of adaptive control, the proposed solution provides on‐line verification/validation of current estimates and a priori assumptions.

Command filtered-based neuro-adaptive robust finite-time trajectory tracking control of autonomous underwater vehicles under stochastic perturbations

In this paper, the problem of finite-time trajectory tracking control is studied and addressed for a 6 degree of freedom (DOF) autonomous underwater vehicle (AUV) subjected to unknown dynamic model, stochastic perturbations, external disturbances (matched and mismatched) and saturation input nonlinearities. Based on the backstepping control approach, novel finite-time control inputs are designed and proposed. Artificial neural networks (ANNs) and finite-time adaptation laws are exploited to approximate the nonlinear dynamics of AUV, the stochastic perturbations and the upper bound of external disturbances. To handle the destructive effects of saturation input nonlinearities, finite-time auxiliary system method is utilized. To overcome the explosion of complexity problem of backstepping control strategy, compensator-based finite-time command filter approach is exploited. By utilizing the Lyapunov stability theorem, it is mathematically proven and demonstrated that the suggested nonlinear control inputs are able to guarantee the semi-global finite-time stability in probability (SGFSP) of the closed-loop AUV system. Finally, numerical simulations are carried out to illustrate and depict the effectiveness and performance of the proposed neuro-adaptive robust finite-time control scheme.

Control of a Digital Galvanometer Scanner Using a Discrete-Time Sliding-Mode Variable-Structure Controller Based on a Decoupled Disturbance Compensator

Laser processing plays an important role in industrial manufacturing, in which a galvanometer scanner (GS) functions as the core component of the laser processing equipment. With the development of the digital system, the GS based on the digital system finds a broader range of potential application. In order to address the slow step-responses of a GS with disturbance and parameter perturbation, the mathematical model of the motor of the GS is derived and a discrete-time sliding-mode variable-structure controller (DSVC) based on a decoupled disturbance compensator (DDC) (DSVC+DDC) is designed. The step-response performance of a GS is the key factor affecting the quality evaluation of laser processing. Experiments are conducted on the step responses of the motor of the GS on a digital experimental platform. The experiment results show that when guaranteeing a steady-state error within 20 urad and an overshoot of less than 5%, the rise time for step-responses in 1% and 10% of the whole stroke is 1 and 2 ms, decreasing by 23% and 58% compared with the reference performance index, which indicates the effectiveness of the proposed method. The proposed approach can not only compensate for the external disturbance online and improve the step-response speed of the GS, but also relax the traditionally assumed limit of the upper bound of external disturbance to the limit of the change rate of external disturbance, which reduces the difficulty of a practical application. This method has great significance for further applications in high precision machining.

Robust Control of Nonlinear Fractional-Order Systems with Unknown Upper Bound of Uncertainties and External Disturbance

This paper investigates the asymptotic stabilization of nonlinear fractional-order systems under unknown upper bounds on external disturbances and uncertainties. The upper bound of uncertainties is a nonlinear function of the pseudostates norm with unknown coefficients. Based on the robust fractional-order sliding mode control, the states of the nonlinear fractional-order system under unknown upper bounds on external disturbances and uncertainties have been stabled. Adaptive control laws estimate the upper bound of external disturbances and uncertainties. In all cases, the stability proof is obtained using the Lyapunov theorem to show the convergence of the sliding surface to zero. Also, by introducing a suitable sliding surface, the chattering phenomenon is eliminated. Finally, the effectiveness of the proposed fractional-order controller is demonstrated using practical examples. The simulation results show the proposed controller has a proper performance in the presence of external disturbance and uncertainties.

Adaptive Terminal Sliding Mode Control for Attitude and Position Tracking Control of Quadrotor UAVs in the Existence of External Disturbance

In this paper, a 6-Degrees-of-Freedom (6-DOF) Unmanned Aerial Vehicle (UAV) system with external disturbance corresponding to sensor failure is considered. The control method is presented in two parts. In the first part, the upper bound of external disturbance is known and a Proportional-Integral-Derivative (PID) Sliding Mode Control (SMC) technique is planned for maintaining the desired position in the finite time. Whereas, the upper bound of the external disturbance is considered unknown in the second part and the adaptive PID-SMC method is offered for stability and position tracking control of UAV systems. Using the Lyapunov stability notion, the offered control method proves that the quadrotor's states can be tracked and stabilized in the finite time. Moreover, for the approximation of unknown bound of the external disturbances which are entered in the quadrotor dynamic model at any moment, adaptive control laws have been applied. Finally, simulation outcomes are provided to display the efficiency of the recommended technique.

Minimum-learning-parameter-based anti-unwinding attitude tracking control for spacecraft with unknown inertia parameters

This paper investigates the finite-time attitude tracking control problem for rigid spacecraft exposed to the external disturbance and unknown inertial parameters. Initially, the basic controller is developed under the assumption that the inertial parameters and the upper bound of external disturbances are available to the designer. However, this assumption is inapplicable in practical applications. Considering this limitation, adaptive laws and Minimum-learning- parameter (MLP) algorithm are employed in the second controller to estimate the unknown system dynamics. Finite-time stability for tracking errors is achievable under the proposed methods. Though theoretical analysis and numerical simulations, effectiveness of the proposed controllers has been validated.

Adaptive prescribed performance terminal sliding mode attitude control for quadrotor under input saturation

This study investigates the attitude tracking problem of quadrotor in presence of external disturbances and the constraint of input saturation. An adaptive prescribed performance terminal sliding mode control scheme is proposed. Terminal sliding mode control algorithm guarantees all state variables to converge to their desired value in a short time while prescribed performance function limits the convergence speed, maximum overshoot and steady-state error. Adaptive law is adopted to estimate the unknown upper bound of external disturbances. To further address input saturation problem encountered in practical system, an auxiliary system is designed. Lyapunov stability approach is applied to guarantee the stability of the closed-loop system. Finally, Numerical simulations with non-linear model are presented to demonstrate the effectiveness and superiority of the proposed control scheme.

Chattering-free Fault-tolerant Attitude Control with Fast Fixed-time Convergence for Flexible Spacecraft

This paper is mainly dedicated to the challenging issue of fixed-time attitude control for a flexible spacecraft in the presence of actuator faults, external disturbances and coupling effect of flexible modes. The attitude controller is developed by employing a fixed-time nonsingular terminal sliding mode under which the convergence time is bounded and independent of the initial states. This robust attitude controller is able to provide superior properties such as fast fixed-time attitude manoeuvring with high pointing accuracy, singularity avoidance and chattering free. More specifically, a new reaching law is employed to provide convergence rate improvement as well as chattering alleviating simultaneously. The actuator fault problem is also considered and the attitude control is achieved even when the actuators experience severe faults. The proposed controller ensures that the closed-loop attitude system is stable in the sense of fixed-time stability concept. Furthermore, since the upper bound of external disturbances and flexible vibrations acting on the spacecraft is not available, an adaptation mechanism is presented. Numerical simulations demonstrate that the proposed controller is able to successfully accomplish attitude control with high attitude pointing accuracy and stability in spite of the actuator faults, flexible structures vibrations and external disturbances.

Adaptive flight control for unmanned autonomous helicopter with external disturbance and actuator fault

A backstepping-based robust adaptive fault tolerant control (FTC) scheme is proposed for the altitude and attitude dynamic model of the medium-scale unmanned autonomous helicopter (UAH) subjected to external disturbance and actuator fault. The external disturbance and actuator fault are integrated into the medium-scale UAH altitude and attitude dynamic model. Since the actuator fault is uncertain in occurring time instant, the adaptive control approach is adopted to compensate for its negative effect. Meanwhile, the upper bound of external disturbance is estimated and the estimation is employed to design the controller. Based on the backstepping control technique, a robust adaptive FTC law is developed, which guarantees the desired system stability as well as the satisfactory tracking property. Simulation studies verify the effectiveness of the designed controller.

Adaptive control for hypersonic vehicle with input saturation and state constraints

The tracing control problem of hypersonic vehicle subject to external disturbances, parametric uncertainties, input saturation and state constraints. Firstly, the longitudinal model of the hypersonic vehicle is converted to the velocity subsystem and altitude subsystem based on the functional decomposition. Secondly, two adaptive anti-saturation controllers are proposed for the velocity subsystem and altitude subsystem with the unknown upper bound of external disturbances. By using the asymmetric barrier Lyapunov function, the two controllers can make flight path angle, angle of attack and pitch angle rate keep within the certain ranges. Meanwhile, the auxiliary system is introduced to deal with the problem of input saturation and a low-pass filter is designed to avoid the “explosion of terms” in traditional backstepping control caused by the complicated differentiations of the virtual control signals. Finally, the effectiveness of the presented control strategy is verified by the Lyapunov stability theory and numerical simulations results.

Finite time control scheme for robot manipulators using fast terminal sliding mode control and RBFNN

Based on terminal sliding mode control, a finite time tracking control scheme, which utilize the advantage of fast terminal sliding mode control and neural network, is presented for robot manipulators. A modified form of sliding surface is considered by introducing two nonlinear terms in the sliding surface. Then a novel robust control scheme is proposed, which shows the strong robustness towards uncertainties and disturbance and as a result the finite time convergence of the tracking error is achieved. The radial basis function neural network is utilized to estimate the nonlinearity of the robot dynamics using update laws. Furthermore the adaptive compensator eliminates the need of knowledge about the upper bound of external disturbances and neural network reconstruction error. The numerical simulation result shows the effectiveness of proposed controller for the case of microbot type robot manipulator in a comparative manner.

Adaptive Dynamic Surface Control for Spacecraft Terminal Safe Approach with Input Saturation Based on Tracking Differentiator

This thesis studies the spacecraft terminal safe approach control problem considering input saturation. Based on the spacecraft relative motion model and sphere collision avoidance potential function, an anti-saturation controller and an adaptive finite-time anti-saturation controller using dynamic surface control(DSC) are presented for the situations of known and unknown upper bound of external disturbances respectively, which can guarantee that no collisions happen in the tracking process. The second-order tracking differentiator is introduced to design the controllers, which avoids the differential of the virtual control signal and ensures the tracking performance of system output signals. Meanwhile, the auxiliary system is introduced to handle input saturation. Lyapunov stability theory is adopted to prove that the states of system under the designed controllers are uniformly ultimately bounded and practical finite-time stable respectively, and the chaser spacecraft can approach to the desired position without collision. The numerical simulation results demonstrate that the chaser spacecraft using the designed controllers can realize terminal safe approach to target spacecraft, which further illustrate the effectiveness of the proposed controllers.

Finite-time antisaturation control for spacecraft rendezvous and docking with safe constraint

This thesis studies control schemes for spacecraft safe rendezvous and docking considering input saturation. Based on the spacecraft attitude and orbit coupled model, by using fast terminal sliding mode method, a finite-time antisaturation controller and an adaptive finite-time antisaturation controller are designed for the situations of known and unknown upper bound of external disturbances, respectively. In controller design, a novel continuous and differentiable collision avoidance potential function is presented to restrict motion area and guarantee the safety of the spacecrafts. Meanwhile, the saturation function and auxiliary system are introduced to deal with the input saturation. Lyapunov theory is utilized to prove that the error states of the system under the proposed controllers are finite-time convergent, and the rendezvous and docking without collision can be accomplished. The numerical simulation results indicate that the chaser can realize the rendezvous and docking with input saturation and safe constraint, which can further illustrate the effectiveness of the designed controllers.

Linear-parameter-varying-based adaptive sliding mode control with bounded L2 gain performance for a morphing aircraft

Large-scale morphing aircraft can adaptively alter configuration according to the different flight conditions or a variety of missions to ensure the optimal aerodynamic performance. However, this will certainly put forward a higher request to modeling method and controller performance. In this paper, we propose an adaptive sliding mode control strategy with bounded L2 gain performance based on linear-parameter-varying methodology for the robust control of a morphing aircraft with variable span and variable sweep angle. Using the Kane method, the longitudinal dynamic model of the morphing aircraft is derived. Moreover, the linearized linear-parameter-varying model of the aircraft in the wing varying process is formulated for controller synthesis. The adaptive sliding mode control synthesis for this uncertain linear-parameter-varying system consists of two steps. Firstly, based on the full block S-procedure, the sufficient condition in form of linear matrix inequality constrains is derived for the existence of a reduced-order sliding mode dynamics. Then, the synthesized adaptive sliding mode control is proved to drive the linear-parameter-varying system trajectories onto the predefined switching surface without the information of upper bound of external disturbances. The performance and robustness of the proposed controller are verified by the numerical results.

An adaptive backstepping sliding mode method for flight attitude of quadrotor UAVs

To overcome nonlinear and 6-DOF (degrees of freedom) under-actuated problems for the attitude and position of quadrotor UAVs, an adaptive backstepping sliding mode method for flight attitude of quadrotor UAVs is proposed, in which an adaptive law is designed to online estimate the parameter variations and the upper bound of external disturbances and the assessments is utilized to compensate the backstepping sliding mode control. In addition, the tracking error of the design method is shown to asymptotically converge to zero by using Lyapunov theory. Finally, based on the numerical simulation of quadrotor UAVs using the setting parameters, the results show that the proposed control approach can stabilize the attitude and has hover flight capabilities under the parameter perturbations and external disturbances.

Fault-tolerant sliding mode attitude tracking control for flexible spacecraft with disturbance and modeling uncertainty

This article investigates the attitude tracking control problem for a class of flexible spacecraft with a redundant four reaction wheels’ setting. In this study, inertia uncertainties, external disturbance, wheel torque saturation, and configuration misalignment are taken into account simultaneously. Two types of sliding mode controllers are presented to solve this design problem. First, supposing the norm upper bound of external disturbances, bounds of inertia, and input uncertainties are available, a terminal sliding mode attitude tracking control strategy is developed, where the controller and steering laws are synthesized together. Second, supposing these norm upper bounds are unavailable, an adaptive control law is designed to cope with attitude tracking problem. Under both control schemes, the system trajectory can be ensured to arrive on the specified sliding surface in finite time. Finally, an illustrative example is given to verify the effectiveness of the proposed attitude tracking control methodologies.

Anti-disturbance autopilot design for missile system via finite time integral sliding mode control method and nonlinear disturbance observer technique

In this paper, a composite anti-disturbance autopilot design for a missile system is proposed based on finite time integral sliding mode control scheme and nonlinear disturbance observer technique. First, a nonlinear disturbance observer is developed to estimate the external disturbances with partial known information. Second, in order to obtain good disturbance rejection performance and reject the other type of disturbances, a finite time integral sliding mode control strategy is employed to the design of a feedback controller. Third, an improved adaptive composite anti-disturbance autopilot is given to avoid the inaccuracy of the upper bound of external disturbances. Finally, simulation results are employed to demonstrate the effectiveness of the proposed method.

Adaptive neural sliding control of MEMS gyroscope with robust feedback compensator

A novel robust neural sliding control using the radial basis function (RBF) neural network for a micro-electro-mechanical system (MEMS) z-axis gyroscope is proposed in this paper. A robust feedback compensator is incorporated into the normal adaptive sliding control in order to eliminate the effect of model uncertainties and external disturbances, and improve the robustness of the whole control system. An adaptive RBF is also adopted to learn online the upper bound of external disturbances, relaxing the requirement of prior knowledge of the upper bound of model uncertainties. By using the proposed neural control, satisfactory dynamic characteristics, strong robustness with respect to external disturbances and parameter uncertainties and fast convergence of tracking errors to zero can be all obtained. In addition, the proposed adaptive neural controller can estimate the angular velocity and all the gyroscope parameters including damping and stiffness coefficients. All the adaptive laws are derived in the same Lyapunov framework, so that the stability of the closed-loop system can be guaranteed. Numerical simulations for MEMS gyroscope are implemented to verify the effectiveness of the proposed control scheme.