Presence Of External Disturbances Research Articles

Robust predictive control of perturbed switched linear systems with stable and unstable subsystems in the presence of uncertainties

In this article, robust model predictive control is proposed for discrete-time switched linear systems in the presence of external disturbances, model uncertainties and constraints on the states and input. To solve the problem, a mode-dependent average dwell time switching strategy is constructed in controller design procedure, with less limitations than the existing average dwell time switching. Using some mathematical tools, the model predictive control problem is turned into a feasible linear matrix inequality. The control algorithm is presented via two theorems, and the stability analysis is analytically presented, based on multiple Lyapunov function method. Compared with the previous studies, the underlying switched system may consist of stable and unstable subsystems, perturbed by model uncertainties and external disturbances. The simulation results are also given to study the effectiveness of the proposed robust model predictive control algorithm, from a comparison viewpoint.

Five-degree-of-freedom robust control of a magnetic endoscopic capsule using an electromagnetic system

Capsule endoscopy is a minimally invasive gastrointestinal diagnostic procedure. The active endoscopic capsule enables the physician to steer the device to any desired position and direction, resulting in a better diagnosis of the disease. Due to the model uncertainties and the existence of disturbances in the gastrointestinal system, traditional control methods cannot achieve robust performance and stability. To overcome this issue, in this paper, a robust sliding mode controller is proposed for 5 Degrees Of Freedom (DOF) control (3-DOF closed-loop control for position and 2-DOF open-loop control for orientation) of the capsule endoscope using an electromagnetic actuation system. The efficiency of the proposed control method is compared with a Proportional–Integral–Derivative (PID) controller considering the model uncertainties and external disturbances. The results of simulations considering 5% of uncertainty in the model parameters show that the suggested approach can precisely and efficiently actuate the capsule to follow the desired trajectory, with position and orientation accuracy of 0.53 mm and 0.69 degree, respectively. Experiments demonstrate the efficiency of the proposed controller, revealing a tracking error of about 0.73 mm in the capsule position. Furthermore, it has been proven by experiments that even in the presence of external disturbances, the proposed control system performs very well and keeps the capsule on the desired trajectory.

On the Development of a Data-Driven-Based Fractional-Order Controller for Unmanned Aerial Vehicles

Proper control is necessary for ensuring that UAVs successfully navigate their surroundings and accomplish their intended tasks. Undoubtedly, a perfect control technique can significantly improve the performance and reliability of UAVs in a wide range of applications. Motivated by this, in the current paper, a new data-driven-based fractional-order control technique is proposed to address this issue and enable UAVs to track desired trajectories despite the presence of external disturbances and uncertainties. The control approach combines a deep neural network with a robust fractional-order controller to estimate uncertainties and minimize the impact of unknown disturbances. The design procedure for the controller is outlined in the paper. To evaluate the proposed technique, numerical simulations are performed for two different desired paths. The results show that the control method performs well in the presence of dynamic uncertainties and control input constraints, making it a promising approach for enabling UAVs to track desired trajectories in challenging environments.

Robust wave compensation controller design for an active hexapod platform with time-varying input delays

In this paper, we address the problem of wave motion compensation for a Stewart platform with gangway. The kinematics and dynamics of the Stewart-Gangway system are established by employing the Kane’s method. Building on the derived model, a robust controller is designed to guarantee that the Stewart top platform keeps motionless in the presence of external disturbance and time-varying input delays. A pre-compensation for the input delays is integrated in the devised controller, achieving uniformly ultimate boundedness (UUB). In order to ensure that the designed actuation signals are bounded with respect to position and attitude errors, a saturation function is used. A smooth projection operator is introduced to ensure that the estimated disturbance remains within a prescribed bound. Simulation results are presented and analyzed, validating the efficacy and performance of the proposed controller.

Prescribed performance-based robust inverse optimal control for spacecraft proximity operations with safety concern

This paper explores and analyzes the robust proximity control problem for spacecraft in the presence of external disturbance, model uncertainty, and safety concern. To fulfill the safety proximity requirement, a novel two-stage time-depended performance function is constructed to guarantee the transient and steady-state proximity performance. Then, by assimilating the advantages of the prescribed performance control methodology and inverse optimal control approach, a neuroadaptive prescribed performance-based inverse optimal control scheme is proposed, which ensures all system signals comply with the preassigned performance function. The developed controller shows optimality concerning convergence performance and energy consumption, whilst satisfying the well-designed safety corridor. Finally, simulation scenarios are provided to demonstrate the effectiveness of the presented control scheme.

Distributed Event-Triggered Secondary Control for Islanded Microgrids With Disturbances: A Hybrid Systems Approach

This paper focuses on the event-triggered secondary control problem of islanded microgrid with nonlinear dynamics and unknown external disturbances. Distributed secondary control law is proposed to compensate for frequency and voltage deviation caused by primary control, and to realize accurate active power sharing. In order to save communication resources, a novel hybrid event-triggering mechanism (ETM) is proposed to schedule the information transmission for each distributed generator. A hybrid model is constructed to describe the closed-loop dynamics and incorporate the ETM into flow and jump sets. Based on this model, Lyapunov stability conditions and ETM design are developed. Compared with the existing results, the proposed hybrid event-triggered control strategy not only has potential to reduce triggering number, but also guarantee a strictly positive minimum event-triggering interval even in the presence of external disturbances, that is important for physical implementation. Finally, simulation results are provided to illustrate the proposed method.

Resilient Consensus for Networked Lagrangian Systems With Disturbances and DoS Attacks

In this article, we consider the resilient consensus control problem of multiple Euler–Lagrange systems in the presence of external disturbances and denial-of-service (DoS) attacks, which have bounded duration and frequency. Unlike the most existing methods for linear multiagent systems, the strong nonlinearity, and unknown disturbances of multiple Euler–Lagrange systems make the resilient control problem under DoS attacks be more challenging and still not well explored. To this end, an adaptive control scheme is first proposed with a robust sliding mode-like term to handle the unknown bounded external disturbances. In addition, an auxiliary system is designed by using the sampled neighboring information to achieve resilience to DoS attacks. Sufficient conditions are presented by using Lyapunov and small-gain approaches. Finally, simulation results are given to verify the effectiveness of the proposed resilient consensus control approach.

Multi-Objective Optimisation-based Robust H∞ Controller Design Approach for a Multi-Level DC-DC Voltage Regulator

In case an analytical approach to the selection of any weighting function is not possible, the selection process is usually a random and time-consuming process. In robust H∞ control theory, the selection of scalar, time, or frequency-dependent weighting functions is the main issue to shape the amplitude-frequency characteristic curve of the feedback controller. Therefore, we propose a robust H∞ control approach which utilises the multi-objective grey wolf optimisation algorithm (MOGWO) to obtain the optimal performance weighting functions in the presence of right half-plane zeros and limited bandwidth constraints. A trade-off design flowchart is proposed, providing Pareto optimal solutions to choose the optimal configuration of the robust feedback controller. The control method is structured by combining the robust H∞ optimal technique and the multi-objective algorithm. The effectiveness of the approach is compared with the non-convex single-objective heuristic solutions like the multi-verse optimisation algorithm (MVO), whale optimisation algorithm (WOA), and grey wolf optimisation algorithm (GWO). The focus of this design is to track and stabilise the output voltage of the DC-DC converter in the presence of external disturbances and parameter uncertainties. The optimised controllers are implemented using a digital signal processor (DSP) on a 200 W interleaved boost converter. The simulation results and experimental findings show that the proposed control method provides supreme disturbance rejection along with maintaining the stability of the system.

Adaptive Control Algorithm for Trajectory Tracking of Underactuated Unmanned Surface Vehicle (UUSV)

The ability to track the trajectory or path on the sea surface remains a key measurement in the control system of an unmanned surface vehicle (USV). In this research, the designed algorithm defines the path and minimizes the disturbances to zero. Underactuated USVs are ships or boats, which operate on the water surface without a crew and the underactuated system has low actuators than its degree of freedom (DOF). The adaptive control strategy is applied to the unbounded system as the ocean due to its high performance, while the robust control system attains high performance in the bounded system. To consider the trajectory tracking of UUSV and provide an optimal control strategy for a ship in the presence of external disturbances, the study presents a controller-based on model reference adaptive control with an integrator (MRACI), which guarantees the stability of a closed-loop system. The vehicle experiences variations in the system response due to external disturbance. This abovementioned scheme reduces the variations to nearly equal to zero making the vehicle stable. First, use computer-based simulation to verify the proposed controller under two different scenarios. Then, simulation results show that the designed scheme lowers the errors and performs well.

Anti-disturbance fault-tolerant control with flexible performance for combined spacecraft attitude tracking under limited thrusts

This paper proposes a novel anti-disturbance fault-tolerant control scheme for prescribed performance attitude tracking of combined spacecraft equipped with thrusters. In practice, the output magnitude of each thruster is limited. When severe actuator faults and/or strong disturbances occur, the remaining active thrusters under limited thrusts may not provide sufficient actuating torques to ensure the satisfaction of prescribed performance. To fill this gap, a tactfully constructed auxiliary system is introduced to generate a series of modification signals, whereby the performance envelops can be actively relaxed when the commanded inputs exceed the thrust limits, while recovering to the nominal level when input saturations disappear. With the modified performance functions (MPFs), a robust adaptive controller is designed to achieve stable attitude tracking of combined spacecraft with flexible performance. Stability analysis shows that the proposed controller can ensure that both the attitude and angular velocity tracking errors evolve strictly within the MPFs, and that all closed-loop signals are uniformly ultimately bounded, despite the presence of external disturbances, thruster faults, and input saturation. Finally, simulation results are provided to show the effectiveness of the proposed method.

Fixed-time regulation of spacecraft orbit and attitude coordination with optimal actuation allocation using dual quaternion.

On-orbit service spacecraft with redundant actuators need to overcome orbital and attitude coupling when performing proximity maneuvers. In addition, transient/steady-state performance is required to fulfill the user-defined requirements. To these ends, this paper introduces a fixed-time tracking regulation and actuation allocation scheme for redundantly actuated spacecraft. The coupling effect of translational and rotational motions is described by dual quaternion. Based on this, we propose a non-singular fast terminal sliding mode controller to guarantee fixed-time tracking performance in the presence of external disturbances and system uncertainties, where the settling time is only dependent on user-defined control parameters rather than initial values. The unwinding problem caused by the redundancy of dual quaternion is handled by a novel attitude error function. Moreover, optimal quadratic programming is incorporated into null space pseudo-inverse control allocation that ensures the actuation smoothness and never violates the maximum output capability of each actuator. Numerical simulations on a spacecraft platform with symmetric thruster configuration demonstrate the validity of the proposed approach.

Model-free super-twisting terminal sliding mode controller using sliding mode disturbance observer for n-DOF upper-limb rehabilitation exoskeleton with backlash hysteresis

This paper proposes a model-free super-twisting terminal sliding mode controller (MF-STTSMC) for the n-DOF upper-limb rehabilitation exoskeleton in the presence of external disturbance and backlash hysteresis. This ultra-local model-based control scheme consists of a sliding mode disturbance observer (SMDO) and a super-twisting non-singular fast terminal sliding mode controller (ST-NFTSMC). The ultra-local model is used to approximate the origin complex exoskeleton system in a short sliding time window, which avoids the requirement for accurate modelling and decreases the difficulty of controller design. To compensate for the lumped uncertainties, the SMDO is constructed to achieve finite-time observation and zero estimation error. Besides, to realise finite-time convergence of sliding surface and tracking error with less chattering, the ST-NFTSMC is proposed by employing a super-twisting algorithm. Furthermore, the stability of a closed-loop system with MF-STTSMC is ensured by using the Lyapunov theorem. Finally, to validate the proposed strategy, co-simulations of 7-DOF iReHave upper-limb exoskeleton are realised by using SolidWorks, SimMechanics, and Matlab/Simulink. Compared to time-delay estimation-based intelligent proportional-derivative control (TDE-iPD), α-variable adaptive model-free controller (α-AMFC), and inclusive and enhanced time delay controller (IETDC), the higher performances of the proposed MF-STTSMC are demonstrated.

A constrained model predictive controller for two cooperative tripod mobile robots

A constrained model predictive controller for two tripod mobile robots performing cooperative transportation of an object through a pre-defined trajectory in the presence of external disturbances has been developed and validated in this paper. The robots are designed to hold an object through their end effectors while applying controlled forces to the object and transitioning along the pre-defined reference trajectory. In this collaborative transportation task, a constrained model predictive controller to control the applied forces and a sliding-mode controller to control the motion of the systems are implemented. A load-sharing algorithm allowed for decentralizing the control system to determine the share of the force applied to the object from each end effector. The system and control algorithms are modeled and simulated in a computational environment using MATLAB software. The results showed that the cooperative system’s position tracking and the force control on the object are successfully achieved using the developed algorithms with minimum deviation from the desired trajectory. In addition, robustness to a continuously increasing external disturbance exerted on the system was achieved using the proposed force control strategy.

Predefined-Performance-Based Full-Process Control for Ultra-Close and High-Precision Formation Flying

The prescribed performance robust control method for the leader/follower (L/F) formation is proposed in this paper to solve the problem of spacecraft formation flying (SFF) full-process control (FPC). The objective of FPC is to establish an ultra-close formation with the constraint of collision avoidance between two spacecraft, and then to maintain the formation configuration with high-precision accuracy in a period of time. The main contribution of this paper lies in the following three aspects. Firstly, the six-degree-of-freedom (DOF) error dynamics model of SFF is developed to describe the synchronization motion of the L/F system. Secondly, the prescribed performance bound that comprehensively considers transience and transient performance is designed, which is key for the realization of collision avoidance and high-precision accuracy requirements. Finally, combing prescribed performance control and robust control theories, based on the backstepping method, the predefined performance robust controller is designed, and the tracking errors are proven to converge to the predefined performance bounds in the presence of external disturbances by using the predefined performance robust controller. Illustrative simulations are performed to verify the proposed theoretical results.

Decoupled Control Design of Aerial Manipulation Systems for Vegetation Sampling Application

A key challenge in the use of drones for an aerial manipulation task such as cutting tree branches is the control problem, especially in the presence of an unpredictable and nonlinear environment. While prior work focused on simplifying the problem by modeling a simple interaction with branches and controlling the system with nonlinear and non-robust control schemes, the current work deals with the problem by designing novel robust nonlinear controllers for aerial manipulation systems that are appropriate for vegetation sampling. In this regard, two different potential control schemes are proposed: nonlinear disturbance observer-based control (NDOBC) and adaptive sliding mode control (ASMC). Each considers the external disturbances and unknown parameters in controller design. The proposed control scheme in both methods employs a decoupled architecture that treats the unmanned aerial vehicle and the manipulator arm of the sampler payload as separate units. In the proposed control structures, controllers are designed after comprehensively investigating the dynamics of both the aerial vehicle and the robotic arm. Each system is then controlled independently in the presence of external disturbances, unknown parameter changes, and the nonlinear coupling between the aerial vehicle and robotic arm. In addition, fully actuated and underactuated aerial platforms are examined, and their stability and controllability are compared so as to choose the most practical framework. Finally, the simulation findings verify and compare the performance and effectiveness of the proposed control strategies for a custom aerial manipulation system that has been designed and developed for field trials.

Backstepping Control Merged with Disturbances Observer for Quadrotor with Rotating Arms

Controlling nonlinear systems is an important and serious task, especially in the presence of external disturbances and uncertain parameters. The main aim of this paper is to design a robust controller that ensures good trajectory tracking of a new quadrotor with rotating arms subjected to external disturbances. Before proceeding to the quadrotor control, it is necessary to first develop a dynamic model of the studied system that takes into account the variation of: the Center of Gravity (CoG), the inertia, and the allocation matrix. Then, based on the finite time Lyapunov stability theory, the theoretical basis of backstepping control integrated with disturbance observer is explained. The observer estimates online the external disturbances and compensates them in the internal loop that contains the attitude and the position backstepping controllers. Numerical simulations are performed to illustrate the efficiency of the proposed control technique, where the controller’s parameters are tuned using a Genetic Algorithm (GA). Finally, qualitative and quantitative comparison of the suggested controller with the conventional backstepping controller is carried out. Overall, the findings show that the proposed control technique outperforms in terms of accuracy and robustness.

Learning-based robust optimal tracking controller design for unmanned underwater vehicles with full-state and input constraints

In this article, the optimal tracking control problem for unmanned underwater vehicles (UUVs) with full-state and input constraints under the presence of external disturbances and internal dynamic uncertainties is addressed. To achieve preassigned state constraints on UUVs, the traditional UUVs model is transformed into an unconstrained one by using two different nonlinear mappings (NMs). Then the robust tracking control problem of traditional UUVs model under position/Euler angles and velocity constraints is transformed to an optimal control problem of the transformed system without any constraints. A learning-based optimal control method is designed to solve the optimal control problem of the transformed system by employing the optimized backstepping (OB) paradigm and reinforcement-learning (RL) technique, achieving uniformly ultimately boundedness (UUB) subject to optimal cost. To deal with lumped disturbances for the velocity control loop, a neural-network (NN) identifier is employed and incorporated into actor–critic architecture, attaining robust tracking performance. Due to the adopted nonquadratic cost function with respect to the control input, the optimal control solution is established in the form of a hyperbolic tangent function to handle the input constraints. Compared with traditional PID method and MPC approach, the proposed controller can improve tracking performance of UUV by 32.04% and 79.64%, respectively.

Fractional‐order integral terminal sliding‐mode control for perturbed nonlinear systems with application to quadrotors

AbstractIn this article, a novel fractional‐order recursive integral terminal sliding mode (FORITSM) control is proposed for nonlinear systems in the presence of external disturbances with unknown bounds. The proposed control approach provides an easy‐to implement solution capable of zeroing the sliding variable in a finite‐time (FnT) by adding a fractional‐order command filter. Moreover, the reaching phase is eliminated, and FnT convergence of the system states is proved. The proposed technique has also a chattering alleviation property, which is beneficial for practical cases, as the control of quadrotor UAVs presented in the article. Finally, a simulation case study on a quadrotor system is illustrated to show the effectiveness of the proposed FORITSM control, also with respect to classical methods.

Spacecraft attitude estimation under attitude tracking maneuver during close-proximity operations

This paper presents a real-time relative three-axis attitude estimation using a camera and two gyros under momentum changes during close-proximity operations. The camera sensor provides quaternion measurements that relate the body frame of the deputy spacecraft with respect to the body frame of the chief spacecraft. The quaternion measurements are coupled with gyro measurements and attitude dynamics model in a multiplicative extended Kalman filter to determine relative attitude and gyro biases under momentum changes. The relative quaternion kinematics is augmented with spacecraft relative motion dynamics to represent the filter process dynamics. The quaternion measurements from a camera sensor and inertial measurement units (IMU) are utilized to the filter measurement model. The fault-tolerant attitude controller actuated by four reaction wheels, which has no unwinding problem is used for attitude tracking maneuvers during close-proximity operations in the presence of external disturbances, uncertain inertia parameter and actuator faults. This controller is combined with the extended Kalman filter to filter noisy measurements and to estimate gyro biases, which leads to a full-state feedback control system. Numerical simulations are performed to verify the effectiveness of the attitude estimation and control systems of the spacecraft with four reaction wheels during close-proximity operations of spacecraft in low-Earth orbit.

A High-Order Sliding-Mode Adaptive Observer for Uncertain Nonlinear Systems

A high-order sliding-mode adaptive observer is proposed to solve the problem of an adaptive estimation, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i.e.,</i> the simultaneous estimation of the state and parameters, for a class of uncertain nonlinear systems in the presence of external disturbances, which do not need to satisfy a relative degree condition equal to one. This approach is based on a high-order sliding-mode observer and a nonlinear parameter identification algorithm. The practical, global, and uniform asymptotic stability of the adaptive estimation error, despite the external disturbances, is guaranteed through the small-gain theorem. The convergence proofs are developed based on the Lyapunov and input-to-state stability theories. Some simulation results illustrate the performance of the proposed high-order sliding-mode adaptive observer.