Large External Disturbances Research Articles

A novel TDE error compensation method based on manipulator time-delay control

Abstract The classic model-free controller, the time delay controller (TDC), operates under complex conditions with large external disturbance changes and suffers from time delay estimation (TDE) errors, which can lead to reduced controller performance. To compensate for the TDE error, a novel compensator is proposed in the TDC framework by considering the TDE error as a separate disturbance term. This compensator innovatively combines a gradient error compensator and a disturbance observer to correct fast-time-varying TDE errors using the gradient error compensator and slow-time-varying TDE errors using the disturbance observer. Furthermore, a high-order time delay observer (HOTDEO) is designed based on the first-order disturbance observer. Compared to the first-order and second-order observers, the HOTDEO has a lower steady-state error. The designed high-order time delay compensator is used as a supplementary term for traditional TDC and applies to various TDC frameworks. Additionally, the stability theory for both the observer and controller is analyzed in detail, and their convergence conditions are verified. Finally, simulation and experimental results demonstrate that the proposed high-order time delay compensator significantly improves the tracking accuracy and robustness of the control algorithm, particularly in terms of TDE error compensation. This research provides valuable contributions to the further development of TDC in the field.

Compensation Function Observer-Based Backstepping Sliding-Mode Control of Uncertain Electro-Hydraulic Servo System

Observer-based control is the most commonly used method in the control of electro-hydraulic servo system (EHSS) with uncertainties, but it suffers from the drawback of low accuracy under the influence of large external load forces and disturbances. To address this problem, this paper proposes a novel compensation function observer-based backstepping sliding-mode control (BSMC) approach to achieve high-accuracy tracking control. In particular, the model uncertainties, including nonlinearities, parameter perturbations and external disturbances are analyzed and treated together as a lumped disturbance. Then, a fourth-order compensation function observer (CFO) is constructed, which fully utilizes the system state information to accurately estimate the lumped disturbance. On this basis, the estimate of the lumped disturbance is incorporated into the design of a backstepping sliding-mode controller, allowing the control system to compensate for the disturbance effect. The stability of the closed-loop control system under the CFO and BSMC is rigorously proven through the use of the Lyapunov theory, which guarantees that all the tracking error signals converge exponentially to the origin. Comparative simulations are carried out to show the effectiveness and efficiency of the proposed approach, i.e., compared with PID and ESO-based BSMC methods, the tracking accuracy is respectively improved by 94.86% and 88.19% under the influence of large external load forces and disturbances.

A novel virtual synchronous generator control scheme of DFIG-based wind turbine generators based on the rotor current-induced electromotive force

This paper focuses on the research of virtual synchronous generator (VSG) control technology for the doubly-fed induction generator (DFIG)-based wind turbines. The key for DFIG VSG operation is to construct an internal electromotive force (EMF) with the virtual rotor motion characteristic in the stator. The existing control schemes use the rotor or mutual flux-induced EMF as the internal EMF. Since the flux corresponds to a composite magnetic field generated together by the stator and rotor current. Any variations in the stator current caused by external disturbances will reflect on the rotor current to keep the flux constant. The rotor overcurrent is inevitable under a large external disturbance, threatening the safety of the rotor-side converter (RSC). Moreover, the flux is immeasurable. The accurate flux control depends on flux observers, increasing the complexity; while the control without observers cannot ensure the effectiveness, leading to control failure. To address these problems, a novel DFIG VSG control scheme with the rotor current-induced EMF as the internal EMF is proposed in this paper. The internal EMF is corresponded to the single magnetic field generated only by the rotor current. This can lead to the following benefits: the rotor current will not vary with the stator current and can be limited under large external disturbances, ensuring the safety of the RSC; the rotor current can be directly measured without the need for extra observers, reducing the difficulty of control implementations; the underlying controlled vector is unified with the traditional grid-following DFIG, favoring the retrofit of the installed DFIGs. Compared to the existing schemes, this scheme has a larger equivalent internal impedance. An additional rotor current-terminal voltage magnitude droop control is added to effectively restrain the problems brought by this larger impedance. Simulation and analysis results demonstrate that the proposed DFIG VSG control scheme effectively addresses problems in the existing schemes. Moreover, the results also exhibit that the scheme has a robust adaptability across a wide range of short-circuit ratios and possesses the grid-forming ability independent of SGs.

Robocentric Model-Based Visual Servoing for Quadrotor Flights

This article proposes a robocentric formulation for quadrotor visual servoing. This formulation presents the task-specific state dynamics of the quadrotor in its body reference frame. Compared with other visual servoing methods, our method allows tightly and integrated state estimation and control on the same robocentric model, and allows a faster system response in aggressive quadrotor flights. On the theory level, we prove the controllability and observability of the proposed robocentric model. Then, we design an on-manifold Kalman filter for the estimation and an on-manifold iterative model predictive control for motion planning and control. We verify our proposed formulation and controller in two crucial quadrotor flight tasks: 1) hovering; and 2) dynamic obstacle avoidance. Experiment results show that the quadrotor is able to resist large external disturbances and recover its position and orientation from two reference visual features. Moreover, the quadrotor is able to avoid dynamic obstacles reliably at a relative speed up to 7.4 m/s, demonstrating the effectiveness of our visual servoing methods in agile quadrotor flights.

Neural Approximation-Based Adaptive Control Using Reinforced Gain for Steering Wheel Torque Tracking of Electric Power Steering System

In practice, steering wheel torque (SWT) control performance for an electric power steering (EPS) system may degrade owing to various reasons, such as model/parameter uncertainties and unpredictable large external disturbances (e.g., reaction force). Furthermore, the desired SWT suddenly varies and its sign also changes because the direction of the steering wheel angle (SWA) often changes according to the driver. In this article, a neural network (NN) approximation-based adaptive nonlinear control (ANC) using reinforced gain (RG) for an EPS system is proposed to resolve the above-mentioned problems using an SWT model. The proposed control method employs an NN approximator, ANC, and RG. The NN approximator is designed to estimate the unknown complex nonlinear function in EPS modeling. The ANC is designed via backstepping to compensate for parametric uncertainties and external disturbances. Further, the RG is designed as a positive nonlinear function of the error to sufficiently suppress the error according to variations in the desired SWT and external disturbance. The RG increases to suppress the error at a rapid transient response, owing to the sudden change in the sign of the desired SWT and the external disturbance. Thus, a high gain (increased RG) is used only when necessary so that an unnecessarily high gain can be avoided.

Control of spacecraft by using multiple model algorithms

Abstract. An effective scheme for fault detection and identification (FDI) and adaptive reconfigurable control (ARC) for controlling a spacecraft under conditions of actuator failures, saturation of control inputs and large external disturbances is proposed. The obtained results increase significantly the autonomy of the spacecraft. The control scheme, based on a general methodology of multiple models, switching and tuning, is designed in the context of nonlinear spacecraft dynamics. The results obtained by using the proposed scheme are illustrated by using numerical simulation algorithms. Keywords spacecraft control, multi-model approach, nonlinear dynamics of spacecraft, numerical simulation algorithm

Control of Unstable Systems Using a 7 DoF Robotic Manipulator

Robotic manipulators are widely used in industrial applications, and their rigidity and flexibility are very important factors during their deployment. However, their usage is not limited to repetitive point-to-point tasks and can be used for more real-time control of various processes. This paper uses a 7-degrees-of-freedom manipulator to control an unstable system (Ball and Plate) as a proof of concept. The Ball and Plate system is widely used for testing algorithms designed for unstable systems, and many recent works have dealt with robotic manipulators as a control motion system. Robots are not usually used to control unstable systems, but bipedal robots are an exception. This paper aims to design a controller capable of stabilizing an unstable system with solid robustness while keeping actuator action values as low as possible because these robots will be indented to work for a prolonged time. An algorithm for an LQ polynomial controller is described and designed, and the whole setup is tested for ball stabilization in the center. The results show that the designed controller stabilizes the ball even with large external and internal disturbances while keeping the controller effort as low as possible.

Visual servoing control of omnidirectional mobile robots based on disturbance observer

This study studies a critical issue that linear velocity and steering angular velocity of the wheels of omni-directional mobile robot are easily affected by the unknown external disturbances such as wind direction and road flatness during the visual servoing stabilization task. To overcome such challenging problems, this study proposes a disturbance observer based visual servoing control strategy. In configuration, the controller is divided into two parts: the quasi-min-max model predictive controller is designed to accommodate the state constraint and input constraint of the system, and complete the visual stabilization task without disturbance. A nonlinear disturbance observer is designed to estimate the external disturbance and feedback compensate the effect of the actual disturbance on the wheel speed. The simulation results and real experiments show that the proposed control strategy can stably accomplish the visual servoing stabilization task even with a large external disturbance.

Adaptive Fault-tolerant and Anti-disturbance Attitude Control of the Reconfigurable Modular CubeSat

This paper addresses the dynamical coupling and attitude control problems caused by the large-scale abrupt changes of the system mass characteristics and the relative motion of the sub-body during the autonomous reconfiguration of the modular satellite in orbit, the attitude dynamics equations of the reconfigurable modular CubeSat system with time-varying mass characteristics parameters are derived. An improved adaptive fault-tolerant anti-disturbance control algorithm that does not rely on an accurate dynamics model and converges quickly in finite time is also proposed based on the composite control idea. In the attitude control of modular CubeSat with large external disturbances and large range of moment of inertia time variation, the method has a faster convergence speed and higher accuracy than the traditional sliding mode control. Without any fault detection and diagnostic information, the method is able to achieve fast and highly accurate attitude adaptive fault-tolerant control in the presence of large disturbances and actuator time-varying, stochastic-type faults through real-time observation of total disturbances inside and outside the system. The final simulation results verify the effectiveness and reliability of the method.

Dynamic Landing Control of a Quadrotor on the Wave Glider

Aiming at the problems of difficult attitude stabilization, low landing accuracy, large external disturbance and slow dynamic response during the quadrotor dynamic landing on the wave glider, an improved series active disturbance rejection control method for the quadrotor is proposed. The quadrotor controller with inner-loop attitude angular velocity control and outer-loop position control based on the active disturbance rejection controller (ADRC) is designed by analyzing the dynamic model of the quadrotor. A tracking differentiator (TD) is adopted to track the input signal, and an expansive state observer (ESO) is used to estimate the total disturbance. Moreover, a nonlinear law state error feedback (NLSEF) is used to generate the virtual control volume of the system to realize the control of the quadrotor, and the stability of the cascaded self-turbulent controller is verified by Lyapunov’s theory. The simulation verifies that the proposed controller can accurately control the attitude and the position with better anti-interference capability and faster tracking speed. According to the final sea trial, a combination of an active disturbance rejection controller optimized with improved crow search algorithm (ICADRC) and April Tag visual reference system is used to land the quadrotor efficiently and successfully even under the surface float attitude uncertainty.

Position estimation of synchronous electrostatic film motors under pulse-voltage operation by using driving currents

Synchronous electrostatic film motors under pulse-voltage operation require position estimation, as it is unavoidable to step out from synchronization due to unexpected large external disturbance. However, the existing position estimation method using driving currents can only work well under sine-voltage operation and fails with a huge error for the pulse-voltage operation due to the impulsive currents at the switching time of the pulse-voltage. This work proposes a new position estimation method dedicated for the pulse-voltage operation, which is mainly used to track the slider position at out-of-step. The impulse currents is removed directly in this method, however, the removal period affects the estimation performance since some essential information of position can also be removed. Theoretically, as long as the information loss is less than a certain level, the proposed method can always work well. Simulation results confirmed that the slider position can be estimated correctly with a resolution of an electrode pitch if the duration of the impulsive currents is short. Experiments under various conditions were carried out by using a real motor, and the results verified that the proposed method can not only successfully track the slider position at out-of-step, but also during synchronous operation.

Fast Terminal Sliding Control of Underactuated Robotic Systems Based on Disturbance Observer with Experimental Validation

In this study, a novel fast terminal sliding mode control technique based on the disturbance observer is recommended for the stabilization of underactuated robotic systems. The finite time disturbance observer is employed to estimate the exterior disturbances of the system and develop the finite time control law. The proposed controller can regulate the state trajectories of the underactuated systems to the origin within a finite time in the existence of external disturbances. The stability analysis of the proposed control scheme is verified via the Lyapunov stabilization theory. The designed control law is enough to drive a switching surface achieving the fast terminal sliding mode against severe model nonlinearities with large parametric uncertainties and external disturbances. Illustrative simulation results and experimental validations on a cart-inverted pendulum system are provided to display the success and efficacy of the offered method.

Finite-time switched LPV control of quadrotors with guaranteed performance

This paper presents a novel finite-time switched linear parameter-varying (LPV) control method for quadrotors subject to large attitude angles and external disturbances. For controller design purpose, both the nonlinear attitude and position dynamics are modeled as a switched LPV system without the idealized assumption of linear approximation. A family of polytopic LPV controllers are designed and each of them is available for a specific parameter subspace of the quadrotor system. The persistent dwell time (PDT) switching logic is adopted to govern the switching behavior among these controllers, which is capable of addressing more general parameter variations than typically used dwell time or average dwell time. By applying an extended cost function, the established switched LPV control method ensures a guaranteed finite-time H∞/LQR performance for the error-tracking system of quadrotors. The advantages of the proposed method are demonstrated via simulations.

Shock-induced persistent contact and synchronous re-levitation control in an AMB-rotor system

Active magnetic bearings (AMBs) have limited dynamic load capacity due to magnetic saturation. Hence, large external disturbances (such as shock loads) may cause contact between the rotor and touchdown bearings (TDBs), which may evolve into complex dynamic behaviour and damage the machine. This paper considers the shock responses of a rotor and viable re-levitation control options when the AMB is still functional. Bi-stable responses and shock-induced persistent forward rubbing were observed in an experimental AMB-flexible rotor facility and its numerical model. The analytical solution for steady synchronous motions with rubbing of a general AMB-flexible rotor system was proposed. The standard control action for a contact-free rotor state would not be appropriate due to phase changes and the displacement amplitude differences in the frequency responses. To destabilise the persistent contact responses and restore contact-free levitation, open-loop phase search based synchronous compensation (PSSC) control and synchronous motion compensation (SMC) control are designed, which are activated when a persistent contact is detected. Stability of the control system and the effectiveness of these two re-levitation control methods are verified by simulation and experimental results. It is also found by comparison that the efficiency of PSSC depends on the phase difference (incorrect phases may degrade rotor response), while the SMC consumes more computing effort.

Terminal sliding mode‐based tracking control with error transformation for underwater vehicles

AbstractOcean currents and waves can cause the initial position of an underwater vehicle to deviate from the initial state of the desired trajectory. For such cases, this paper investigates the finite‐time tracking control problem for a vehicle in the presence of large initial tracking errors and external disturbances. A continuous finite‐time tracking control scheme is developed for this scenario based on an improved nonsingular terminal sliding mode surface modified by a piecewise function. Compared to the alternative approaches, a method based on error transformation is added to the sliding mode surface, resulting in improved tracking accuracy in the steady state. Then, to reduce the effect of large initial tracking error on the control input signals, an exponential decay function about the initial location of the vehicle is developed to obtain the control law by combining it with a strictly generalized saturation function. In contrast to alternative approaches, a nonlinear structure is used to counteract the approximation error of the neural network used in the modeling of the dynamics. Moreover, it is shown that the tracking error converges to a small neighborhood of zero in a fixed time. Finally, the new control scheme's effectiveness is verified by simulation studies on an underwater vehicle model and the results compared to other existing control designs.

Shock Resistance Design of High-Speed Maglev Motor–Stability Considerations and Quantitative Shock Tests

Large external disturbances (such as shock loads) can cause contact between the rotor and touchdown bearings (TDBs). Hence, maintaining the stability of systems with active magnetic bearings (AMBs) is a major challenge for mobile applications such as on-board steam turbines or vehicle turbochargers. In this paper, two key factors (power bandwidth and bi-stable characteristic) that affect the shock stability of AMB-rotor systems are considered in the design of a high-speed maglev motor. Insufficient power bandwidth can induce current saturation leading to destabilization, while a bi-stable characteristic can cause persistent contact between the rotor and TDBs under external disturbances. Theoretical analyzes and criteria are provided, and the influence of these two factors is investigated by base shock experiments of a high-speed maglev motor. These experiments involved mounting a high-speed maglev motor on a shock table then subjecting it to shock loads of different amplitudes. The designed motor continued to operate stably under shock loads up to 20 G, verifying the correctness of the stability considerations.

A Robust Noise-Free Linear Control Design for Robot Manipulator with Uncertain System Parameters

In robot control, the sliding mode control is known for its robustness against external disturbances and system uncertainties. However, it has the disadvantage of control chattering, which can damage the actuator and degrade system performance. With a new stability proof, this paper presents an alternative simple linear feedback control that can cope with large system uncertainties and suppress large external disturbances, doing so as effectively as sliding mode control does. The advantage of using linear control is that the control law is simple and control chattering can be avoided. Moreover, a noise-free control scheme is proposed as an improvement of the feedback control; the modified design preserves the advantages of linear control and generates a chattering-free control signal even in a noisy environment.

Virtual tracking control of underwater vehicles based on error injection and adaptive gain

An improved virtual tracking control scheme is proposed based on error injection and adaptive gain for underwater vehicles in the presence of a large initial tracking error and external disturbances. To relieve the effect caused by a large initial tracking error, the developed control scheme is achieved based on two closed-loop systems. Specifically, a virtual closed-loop system is constructed based on an approximate dynamic model of an underwater vehicle, while an actual closed-loop system is built with a real underwater vehicle. Firstly, in order to improve the tracking precision of the virtual tracking control scheme, an auxiliary variable produced by a first-order filter is injected into a virtual tracking error in the virtual closed-loop system. And then, the virtual trajectory provided by the virtual closed-loop system is followed by the actual closed-loop system. In the actual closed-loop system, a modified sliding mode surface is designed to achieve the finite-time stability, while the control gains can be on-line adjusted based on the tracking performance. Finally, the effectiveness and feasibility of the proposed control scheme are demonstrated by case studies on an underwater vehicle subject to different external disturbances.

A dynamic gait stabilization algorithm for quadrupedal locomotion through contact time modulation

In this paper, we propose a stabilization method for dynamic gaits of quadrupedal walking robots covering a wide range of speeds and various types of gait. Our stabilization method is based on adjusting the contact time between the four legs and ground. By modulating the contact time, the impact applied to the body can be controlled and stabilized. The stability provided by the proposed algorithm was proved in the sense of Lyapunov. The proposed algorithm also demonstrated robust performance under large external disturbances, and the performance was compared with other algorithms through simulations. Simulation results of bounding gaits under different ground conditions were compared, and the various types of stable gait implemented by the proposed algorithm are also presented.

Modeling and Nonlinear Control of a Two-Wheeled Self Balancing Human Transporter

The two-wheeled self-balancing human transporters (TW-SBHT) are being widely used in transportation nowadays owing to their advantages such as energy saving, environmental protection, simple structure, and flexible operation. The modelling and control of TW-SBHT have emerged as one of the trending research areas in the field of control system design of mobile robots. Being a complex and nonlinear system, the control problem of TW-SBHT is a challenging task and needs to be effectively tackled to achieve the control objectives of maintaining uniform speed and dynamic stability. Though linear control strategies for TW-SBHT have been already proposed in the literature, they cannot offer an effective control for large external disturbances. Therefore, in this work, a non-linear control i.e. State-Dependent Riccati Equation (SDRE) has been implemented for effective control of TW-SBHT and its performance is compared with the linear controls including Proportional-Integral-Derivative (PID) and Linear-Quadratic Regulator (LQR) techniques. Initially, the more accurate model of TW-SBHT has been derived by applying the suitable modifications in existing models. Then, the application of SDRE for control of TW-SBHT has been presented and its performance is compared with linear control strategies.