Effect Of Actuator Saturation Research Articles

Robust adaptive tracking control for a class of mechanical systems with unknown disturbances under actuator saturation

SummaryA robust adaptive tracking control scheme is presented for a class of multiple‐input and multiple‐output mechanical systems with unknown disturbances under actuator saturation. The unknown disturbances are expressed as the outputs of a linear exogenous system with unknown coefficient matrices. An adaptive disturbance observer is constructed for the online disturbance estimation. An actuator saturation compensator is introduced to attenuate the adverse effects of actuator saturation. The adaptive backstepping method is then applied to design the robust adaptive tracking control law. It is proved that the designed control law makes the system outputs track the desired trajectories and guarantees the global uniform ultimate stability of the closed‐loop control system. Simulations on a two‐link robotic manipulator verify the effectiveness of the proposed control scheme.

Robust Control Allocation in Attitude Fault-Tolerant Control for Combined Spacecraft Under Measurement Uncertainty

Attitude control of combined service-target system in the post-capture phase has received great attention. A new attitude dynamics of the combined spacecraft with reaction wheels has been established in the author's former work, however, the measurement uncertainty in attitude and angular velocity and uncertainty in the reconfiguration matrix of reaction wheels have not been considered, which may cause huge impact on the system performance. In this paper, a novel combination of disturbance-observer-based dynamic surface control under measurement uncertainty and robust control allocation due to uncertain mass center is investigated for attitude stabilization of the combined spacecraft. Firstly, considering measurement uncertainty, inertia uncertainty, actuator fault and actuator saturation, a new attitude dynamics of combined spacecraft is established. Then, a virtual controller is designed and all the states in the closed-loop system converge to a small neighborhood of zero, where the lumped disturbance is compensated by two stable nonlinear disturbance observers and adverse effect of actuator saturation is addressed by a stable compensator. Finally, in consideration of uncertain location of mass center in the reconfiguration matrix, a LMI-based robust control allocation is employed to deal with the problem of distributing the three axis torques over the reaction wheels. Numerical simulations are presented to illustrate the effectiveness of the proposed method.

Adaptive fractional-order control of electrical flexible-joint robots: Theory and experiment

Real-time fractional-order control of electrically driven flexible-joint robots has been addressed in this article. An important contribution of this article is that the control law is designed based on the Fourier series that eliminates the need for computation of regressor matrix. Moreover, the nonlinear effects of actuator saturation are considered in the control law. The lumped uncertainty can be approximated using Fourier series with unknown coefficients. Then, the unknown coefficients are estimated using the adaptation law obtained in the stability analysis. The overall closed-loop system is proven to be robust and bounded-input bounded-output stable. In addition, it has been shown that the joint-position errors are uniformly bounded based on Lyapunov’s stability concept. The satisfactory performance of the proposed control scheme is verified by experimental results. To highlight the superiority of the proposed method, experimental results of two voltage-based controllers are also presented.

Nonlinear sampled-data ESO-based active disturbance rejection control for networked control systems with actuator saturation

This paper proposes a framework of anti-windup active disturbance rejection control for the networked control systems (NCSs) subjected to actuator saturation. The sensor-to-controller network is considered where only one sensor can report its measurements at each transmission instant. Both the round-robin and try-once-discard protocols are applied, respectively, to determine which sensor should be given the access to the network at a certain instant. To reflect the impact of communication constraints, a nonlinear sampled-data extended state observer (NSESO) is employed to estimate the states and ignored nonlinearities of the addressed system. Then, a composite control strategy with an anti-windup compensator is designed based on the NSESO, and the effects of actuator saturation is eliminated by the anti-windup compensator. The sufficient conditions to guarantee the convergence of the NSESO are provided, and then the input-to-state stability of the overall NCSs is given as well. Finally, a numerical example is introduced to demonstrate the effectiveness of the proposed design technique.

Delay-dependent supplementary damping controller of TCSC for interconnected power system with time-delays and actuator saturation

In this paper, a delay-dependent state feedback based supplementary damping controller of Thyristor Controlled Series Capacitor (TCSC) is designed to enhance the damping of inter-area oscillations of an interconnected power system. Although wide-area feedback signals are advantageous for damping enhancement of inter-area modes, but its usage as feedback signals in designing of a damping controller together with physical limitation of the actuator introduces time-delay and actuator saturation in the feedback loop. This results in degradation of the power system performance and even cause instability. The paper focuses on designing a damping controller considering the aforesaid time-delay and actuator saturation effects. Residue approach and Schur balanced truncation model reduction methods are used to obtain input-output control signals and reduced order power system model. The nonlinear effects of the actuator saturation are consider in the closed-loop system by using generalized sector condition. Based on Lyapunov–Krasovskii functional approach, a delay-dependent stability and stabilization conditions via linear matrix inequalities (LMIs) formulation are derived to guarantee the asymptotic stability of the closed-loop system. Then the problem of designing a state feedback control law which maximizes the region of attraction is formulated and solved as an optimization problem with LMI constraints. Simulation studies in the two-area four-machine power system demonstrate the effectiveness of the proposed controller for damping enhancement of the inter-area oscillations and compensating the effect of time-delays and actuator saturation.

Robust spacecraft attitude tracking control with integral terminal sliding mode surface considering input saturation

This paper studies the finite time spacecraft attitude tracking control problem, while considering modeling uncertainty, external disturbances and control input saturation. A novel integral terminal sliding mode surface (ITSMS) is designed by combining the fast terminal sliding mode surface (FTSMS) with a low pass filter to achieve a fast finite time convergence rate for the control system, without input singularity. An auxiliary signal is used to compensate for the effects of actuator saturation. The basic controller is first formulated based on the ITSMS, fast-TSM-type reaching law and auxiliary system, in the presence of an external disturbance and input saturation. Then, an adaptive control procedure is introduced, which simultaneously handles modeling uncertainty and external disturbance, thereby creating an adaptive attitude tracking controller. The proposed controller provides a fast finite time convergence rate for the control system, based on the newly designed ITSMS, while simultaneously compensating for modeling uncertainty, external disturbances and input saturation, without restricting the parameter selection process nor requiring repeated differentiation of nonlinear functions. Finally, digital simulation results are presented and demonstrate the effectiveness of the proposed controllers.

Adaptive backstepping control for a class of MIMO uncertain underactuated systems with input constraints

This paper presents a backstepping methodology-based semi generic adaptive controller scheme for a class of multi-input multi-output (MIMO) uncertain underactuated systems in presence of actuator constraints. To develop a feasible controller scheme for multi-input multi-output underactuated systems, (n – p + 1) dimensions of the n dimensional configuration space are stabilised by using one dimension of the input space. This control term is developed by applying hierarchical methodology whereas as remaining p – 1 input dimensions are assigned as dedicated control terms to solve the control problem of remaining dimensions of the configuration space. Backstepping technique is used to develop the classical control terms whereas wavelet networks are used to approximate the uncertain dynamics as well as the nonlinear effects of actuator saturation. The proposed scheme, thus, relaxes the constraint of completely and accurately known system dynamics as well as the requirement of measuring the clipped portion of the control effort. A robust control term is used to attenuate the approximation error to a prescribed level. Uniform ultimate boundedness (UUB) stability of the closed loop system is verified in the Lyapunov sense. Simulation results illustrate the effectiveness of theoretical development.

Observer‐based fault‐tolerant control of hypersonic scramjet vehicles in the presence of actuator faults and saturation

SummaryAn adaptive sliding mode observer (SMO)–based fault‐tolerant control method taking into consideration of actuator saturation is proposed for a hypersonic scramjet vehicle (HSV) under a class of time‐varying actuator faults. The SMO is designed to robustly estimate the HSV states and reconstruct the fault signals. The adaptive technique is integrated into the SMO to approximate the unknown bounds of system uncertainties, actuator faults, and estimation errors. The robust SMO synthesis condition, which can be formulated as a set of linear matrix inequalities, is improved by relaxing structure constraints to the Lyapunov matrix. An anti‐windup feedback control law, which utilizes the estimated HSV states and the fault signals, is designed to counteract the negative effects of actuator saturation induced by actuator faults. Simulation results demonstrate that the proposed approach can guarantee stability and maintain performance of the closed‐loop system in the presence of HSV actuator faults and saturation.

Robust adaptive passivity‐based control of open‐loop unstable affine non‐linear systems subject to actuator saturation

Considering the effect of actuator saturation, a control design approach is presented for open-loop unstable affine non-linear systems. A feedback control law with some degrees of freedom along with a virtual output function is utilised to make the system passive. In order to achieve a fast convergence, an analytical discussion is presented to opt for the free parameters. The procedure is further developed for systems with unknown parameters, where some adaptation laws are utilised for parameter estimation. In order to enhance the convergence performance, the gains of the adaptation laws are obtained based on sensitivity analysis. Using projection technique to prevent parameter drift, it is analytically shown that the proposed scheme is robust against bounded disturbances. The effectiveness of the proposed controller is assessed through extensive simulations.

Adaptive fault-tolerant control of air-breathing hypersonic vehicles robust to input nonlinearities

ABSTRACTThis paper designs a fault-tolerant adaptive controller for air-breathing hypersonic vehicles (AHVs) subject to modelling parameter uncertainties, external disturbances, and actuator nonlinearities of saturation and backlash. The proposed adaptive control scheme is able to compensate the effects of actuator saturation by utilising the states of five auxiliary dynamic systems, which are driven by the differences between the nominal and saturated input signals. Additionally, the effects of control surfaces on the aerodynamic force and moment, which are commonly neglected by the existing adaptive control designs, can be well handled. Transient tracking performance is explicitly derived in terms of norms of the tracking errors. The final control scheme is obtained in a direct form, which makes its implementation more practical compared with other adaptive controllers for AHVs. The effectiveness of the proposed control scheme is demonstrated by numerical simulations.

Adaptive Fuzzy Trajectory Tracking Control of an Under-Actuated Autonomous Underwater Vehicle Subject to Actuator Saturation

This paper focuses on vertical-plane trajectory tracking of an under-actuated autonomous underwater vehicle (AUV) subject to actuator saturation and external disturbances. A successive guidance and control frame is designed to avoid the cascade analysis between the kinematics guidance and the dynamics control, and the complete Lyapunov function is chosen to analyze the asymptotic stability of trajectory tracking system. In the guidance loop, the line-of-sight guidance law is applied to trajectory tracking of AUVs, which transforms the depth tracking error into the elevation angle tracking error and solves the problem of the under-actuated configuration in heave. In the control loop, direct adaptive fuzzy control is adopted to compensate for the effect of actuator saturation, which guarantees the system stability of trajectory tracking in the presence of actuator saturation. Finally, comparative numerical simulations are provided to illustrate the robust and bounded performance of the designed trajectory tracking control system.

Finite-Time Control of Teleoperation Systems With Input Saturation and Varying Time Delays

This paper develops a finite-time control approach for nonlinear teleoperation systems, which is capable of unifying the study of model uncertainties, actuator saturation, and asymmetric time-varying delays in the same framework. First, a novel anti-windup compensator is designed to analyze the effect of actuator saturation. To achieve the finite-time tracking, a nonsmooth generalized switched filter is also investigated. By introducing the anti-windup compensator and the generalized switched filter into the adaptive fuzzy control torque design, a novel finite-time controller is developed. By using the multiple Lypaunov–Krasovskii functionals method, the resulting closed-loop system is state-independent input-to-output practical stable (SIIOpS) and based on this, it is proved to be finite-time SIIOpS. It is shown that the asymptotic convergence of the adaptive estimation errors and the finite-time convergence of the position tracking errors are obtained, whether the robots contact with the human operator/environment or not. Finally, the effectiveness is demonstrated by the simulation results.

Reduction of the effect of actuator saturation with periodic servo-constraints

Saturation is an undesired event in trajectory tracking control of mechanical systems. When the actuators of a robotic device saturate, the solution of the inverse dynamics problem cannot fully be realized, which results in deviations from the desired trajectory and loss of performance. It is generally hard to consider the limited actuator torques and the corresponding nonlinear effects in the control design. The most common way to handle the problem is recalculating the control forces and trying to adjust the desired trajectory such that saturation will not happen. In contrast we propose a switched control approach, where, upon saturation, different sets of inputs are varied periodically to keep the reference point of the robot on the desired trajectory. For this, the desired motion is formulated by means of servo-constraints, and the periodic switching of these constraints is adjusted according to the variation of a new, manipulability type performance measure. It is demonstrated that the proposed controller can effectively reduce the trajectory following error due to actuator saturation. A typical robotic benchmark example is provided to show the application of the method, and to compare it with other approaches taken from the literature.

Adaptive neural network control for active suspension system with actuator saturation

This study investigates adaptive neural network (NN) state feedback control and robust observation for an active suspension system that considers parametric uncertainties, road disturbances and actuator saturation. An adaptive radial basis function neural network is adopted to approximate uncertain non-linear functions in the dynamic system. An auxiliary system is designed and presented to deal with the effects of actuator saturation. In addition, since it is difficult to obtain accurate states in practice, an NN observer is developed to provide state estimation using the measured input and output data of the system. The state observer-based feedback control parameters with saturated inputs are optimised by the particle swarm optimisation scheme. Furthermore, the uniformly ultimately boundedness of all the closed-loop signals is guaranteed through rigorous Lyapunov analysis. The simulation results further demonstrate that the proposed controller can effectively suppress car body vibrations and offers superior control performance despite the existence of non-linear dynamics and control input constraints.

Guaranteed Cost Control of Uncertain Discrete Time Systems Subjected to Actuator Saturation via Homogenous Polynomial Lyapunov Function

Estimation of an optimal controller is a fundamental problem in control engineering and is widely known as Optimization. Numerous computation and numerical techniques have been evolved during the past years for the estimation of the optimal solution. Optimal control of a discrete-time system is concerned with optimizing a given objective function using “Homogenous Polynomial Lyapunov Function (HPLF)”. This research focuses upon the design of optimal guaranteed cost controller (GCC) for discrete-time uncertain system using HPLF. The uncertainties are assumed to be norm bounded uncertainties. The effect of actuator saturation is also incorporated in the system. Sufficient conditions for the existence of HPLF are derived in term of Linear Matrix Inequalities (LMI). The LMI approach has the advantage that it can be solved efficiently using Convex Optimization. LMI’s combined with HPLF helps to design the guaranteed cost controller which minimizes the cost by minimizing cost function. Furthermore, the state trajectories and the invariant set is also shown to observe the overall performance.

A Simplified Adaptive Backstepping Control of Aircraft Lateral/Directional Dynamics

Abstract A simple approach is proposed in this paper to control lateral/directional dynamics of Unmanned Aerial Vehicles (UAVs). The approach uses backstepping technique to design a lateral/directional controller. As the aerodynamics coefficients are not accurately available for the whole flight regime, we propose an adaptive design to learn and control unknown dynamics. The proposed approach provides satisfactory performance in tracking reference in roll rate using aileron and in maintaining sideslip angle to zero value using rudder. The design is validated on the six degrees of freedom model with the effect of actuator saturation.

Simultaneous Blade–Vortex Interaction Noise and Vibration Reduction in Rotorcraft Using Microflaps, Including the Effect of Actuator Saturation

The effectiveness of a sliding microflap to simultaneously reduce rotorcraft vibrations and noise was examined at a descending flight condition of advance ratio 0.15 with significant blade–vortex interactions (BVI). two configurations, namely dual and a five-microflap configuration, were considered. Closed-loop control studies were conducted on a hingeless rotor configuration resembling MBB BO-105, using the adaptive higher harmonic control algorithm. The performance of the microflap was also compared to a conventional trailing-edge plain flap. The results demonstrate the effectiveness and control authority of the microflap for simultaneous BVI noise and vibration reduction in rotorcraft. Finally, a new saturation control algorithm is developed for limiting the microflap or flap deflections such that the best utilization of on-blade controllers implemented through multiple control surfaces is achieved. The performance of the new algorithm is compared to the existing ones on the dual and five-microflap configurations.

Anti-unwinding constrained attitude control for flexible spacecraft with actuator saturation

In this paper, two novel anti-unwinding constrained attitude controllers for flexible spacecraft are designed based on inverse optimal approach and unit quaternion representation of attitude subjected to inertia matrix uncertainty and external disturbance. One accounts for actuator amplitude constraint and the other considers actuator amplitude and rate constraints at length. A new saturation compensator is designed and embedded into the controllers to eliminate the effect of actuator saturation. The proposed controllers can avoid the issue of attitude slewing unwinding due to the double values of unit quaternions. The stability of the resulting closed-loop systems by the proposed controllers is guaranteed by Lyapunov-based approach. Simulation results are presented to assess the performance of the proposed methods.

Composite fuzzy control of a class of uncertain nonlinear systems with disturbance observer

In this paper, a novel composite control, using fuzzy logic system (FLS) and disturbance observer, is proposed for a class of uncertain nonlinear systems with actuator saturation and external disturbances. FLS is employed to approximate the unknown nonlinearities and a serial–parallel identification model is introduced to construct the composite updating law. The disturbance observer is developed to estimate the unknown compounded disturbance composed of the unknown external disturbance, the unknown fuzzy approximation error and the effect of actuator saturation. The uniformly ultimate boundedness of the closed-loop tracking error can be guaranteed rigorously via Lyapunov stability analysis. Simulation results are presented to demonstrate the effectiveness of the proposed method.

Robust adaptive neural network control for a class of uncertain nonlinear systems with actuator amplitude and rate saturations

An adaptive controller which is designed with a priori consideration of actuator saturation effects and guarantees H∞ tracking performance for a class of multiple-input–multiple-output (MIMO) uncertain nonlinear systems with extern disturbances and actuator saturations is presented in this paper. Adaptive radial basis function (RBF) neural networks are used in this controller to approximate the unknown nonlinearities. An auxiliary system is constructed to compensate the effects of actuator saturations. Furthermore, in order to deal with approximation errors for unknown nonlinearities and extern disturbances, a supervisory control is designed, which guarantees that the closed loop system achieves a prescribed disturbance attenuation level so that H∞ tracking performance is achieved. Steady and transient tracking performance are analyzed and the tracking error is adjustable by explicit choice of design parameters. Computer simulations are presented to illustrate the efficiency of the proposed controller.