Healthy Actuators Research Articles

Intermittent monitoring‐based adaptive fault‐tolerant control for uncertain nonlinear systems with actuator switching

AbstractIn this paper, an adaptive fault‐tolerant control (FTC) scheme is proposed for a class of uncertain nonlinear systems with actuator failure, in which a healthy actuator is available as a backup. Instead of monitoring the failure in real‐time, an intermittent monitoring mechanism (IMM) is proposed by designing a set of monitoring functions operating only at discrete time instants, which makes the proposed scheme more applicable for networked control systems for which the actuator status has to be monitored via network. An adaptive backstepping controller is designed with the errors being modified to include shifting functions. Once actuator failure is detected by the monitoring functions, the actuator is switched to its backup and the controller is reconfigured by updating the shifting functions so that the tracking error can be driven into the prescribed performance in a given time.

Fault tolerant control of an octorotor UAV using sliding mode for applications in challenging environments

This paper presents the development of fault-tolerant controller and their application for multirotor unmanned aerial vehicles – specifically an octorotor – in challenging environments e.g. nuclear power plant inspection or other dull, dirty and dangerous applications. This paper considers a combination of sliding mode control robustness properties (to deal with actuator faults) and control allocation (to automatically redistribute the control signals to healthy actuators, especially in the event of actuator failures). The resultant controller has the ability to operate in both fault-free and fault/failure conditions without reconfiguring the main baseline controller. The proposed controller also has the ability to operate for up to six rotor failures which represent an under-actuation condition i.e., a case when only two rotors are available. The under-actuation scenarios are conditions when most FTC schemes are not able to operate due to the lack of redundancy. The simulation results conducted on the nonlinear model with wind/gusts and sensor noise, show a good tracking performance under various fault-free and fault/failure scenarios (over-actuation, sufficient actuation and under-actuation conditions).

Adaptive Actuator Failure Compensation Control: A New Scheme Based on Fault Detection and Isolation

In flight control systems, multiple actuator stuck (or hardover) failures would cause saturation of the remaining healthy actuators and result in system instability. In this article, a new adaptive actuator failure compensation control scheme is proposed for a class of nonlinear systems with multiple redundant inputs. Under a mild assumption on the time interval between two successive failures, an online fault detection and isolation (FDI) algorithm is introduced, with which an actuator failure can be exactly identified in a given finite time without using the persistent excitation (PE) condition. The main advantage of the proposed scheme is that with the help of the FDI algorithm, an actuator with stuck failure (SF) can be deactivated before a new failure occurs and therefore, the drawback of the current adaptive failure compensation control approach that may cause more control effort or even saturation with multiple actuators being stuck can be overcome.

Locking underactuated robots by shrinking their manifolds of free-swinging motion

When a robot suffers a Free Swing Joint Failure, it becomes underactuated since some of its actuated joints become passive. If the joints that remain actuated are locked, the robot can perform uncontrollable free-swinging motions that are dangerous due to the risk of collision. This is typically corrected by using brakes or redundant actuators, which are costly solutions. In this paper, we propose an alternative method to secure underactuated robots after Free Swing Joint Failure without brakes or redundant actuators, by taking the healthy actuated joints to what we call “locking configurations”. At these configurations, the manifolds along which the underactuated robot performs free-swinging motions degenerate into points, suppressing such uncontrollable motions. We present a discretization and clustering method to find locking configurations by detecting the degeneracy of free-swinging manifolds into points, introducing two criteria to confirm if these configurations are stable, by checking if passive joint velocities are forbidden or if any external wrench can be balanced by the healthy actuators. Several examples illustrate the method, including serial and parallel underactuated robots with varying numbers of failed actuators.

Adaptive active fault-tolerant dynamic surface load following controller for a modular high-temperature gas-cooled reactor

The control of nuclear reactors is one of the most challenging requirements in performing the load following operation mode; however, most of the previous studies either do not consider the adverse effects of the lumped uncertainty or rely on the assumption of possessing healthy actuators without any faults. Besides, the recently published backstepping load following controllers may result in the problem of “explosion of complexity” due to repeatedly differentiating the virtual control laws. To this end, this paper provides an adaptive active fault-tolerant dynamic surface control strategy for the load following problem of a modular high-temperature gas-cooled reactor suffering from lumped uncertainty and actuator faults. The main contributions of this paper are threefold. (i) a radial basis function neural network is introduced to real-timely learn the lumped uncertainty that contains model uncertainties, parameter variations, external disturbances, and unmeasured states; (ii) a dynamic surface control technique is employed to cope with the problem of “explosion of complexity” induced by traditional backstepping approach; and (iii) an active fault-tolerant controller, incorporated with a fault detection observer and a fault estimator, is developed such that the reliability and safety of the modular high-temperature gas-cooled reactor system could be enhanced. With Lyapunov stability theory, both the stability of the closed-loop system and the error convergence are proved. At the end, comparative simulation studies and analysis using quantitative performance indices indicate the proposed control strategy’s superiority in dealing with lumped uncertainty and actuator faults.

A sliding mode observer-based robust fault-tolerant control allocation for descriptor systems

The problem of designing an observer-based fault-tolerant control allocation mechanism for a multi-input multi-output (MIMO) uncertain descriptor system (DS) subjected to actuator faults/failures and external uncertainties is discussed in this paper. The proposed approach guarantees system performance and closed-loop stability using the integral sliding mode control (ISMC) and a fixed control allocation (CA) strategy. A robust sliding mode observer (SMO) is designed to estimate the states of the DS in a realistic setting. The sliding surface is constructed with the estimated states, following the ISMC principle. To meet the admissibility conditions of DS, a multivariable super-twisting sliding mode control (STSMC) algorithm is designed. The CA technique reallocates the control effort to the remaining healthy actuators on the occurrence of any fault/failure. Lyapunov-based stability is established for the closed-loop system with the controller–observer pair. Finally, the proposed design is validated by evaluating various actuator fault/failure situations.

Fault Tolerant Control Based on Thau Observer of a Reconfigurable Quadrotor with Total Loss of Actuator

In this paper, we propose a Fault Tolerant Control (FTC) strategy for a reconfigurable quadrotor with foldable arms, to deal with a total loss of one of its rotors. The proposed strategy allows adapting the configuration of the quadrotor when one of its rotors is lost. It consists of: (i) a Fault Detection and Isolation (FDI) module, (ii) a robust controller, and (iii) a reconfiguration module. The FDI module is designed based on the nonlinear Thau observer in order to detect and identify the rotors faults. Once a rotor fault is detected based on the residual analysis, the quadrotor changes its shape by rotating the two adjacent arms in the direction of damaged one in order to form a trirotor configuration. This transformation induces a variation of vehicle’s center of gravity (CoG), inertia, and consequently the control matrix. To deal with this issue, a sliding mode controller (SMC) is designed based on the control allocation matrix, where the control efforts are redistributed among healthy actuators. Numerical simulations are carried out to verify the effectiveness of the proposed approach. The obtained results show that the proposed strategy is successful in controlling the damaged quadrotor by trajectory tracking.

Fault-tolerant adaptive PID switched control of robot manipulator based on average dwell time

The multi-mode tracking control of robot manipulators with unknown dynamics is investigated in this paper. In view of varying loads, robot manipulators are modeled as multi-mode switched systems. Adaptive PID (proportion–integration–differentiation)–like switched controllers are designed via the error transformation and multiple Lyapunov function method, which can make the error system practical stability within prescribed time under a class of switching signals satisfying average dwell time. Furthermore, two cases under the healthy actuator and faulty actuator are discussed. For the case of the partial failure of the actuator, the fault-tolerant adaptive PID-like controller is presented, which consists of two parts. Finally, a simulation example is given to demonstrate the effectiveness of the proposed method.

Collision-avoiding formation for multiple Euler–Lagrange systems against external disturbances and actuator faults

In this paper, for multiple Euler–Lagrange systems embodying external disturbances and unknown uncertainties, the problems of collision-avoiding formation (CAF) are investigated. With regard to Euler–Lagrange systems under healthy actuator condition and under actuator failures, two distributed collision-avoiding formation (DCAF) control laws are proposed. In one case, which the systems are under healthy actuator condition, firstly, a robust continuous term with adaptive variable gain is utilized to reduce the influence of external disturbances under unknown range. In addition, in order to handle the uncertainties of dynamical systems and collision avoidance, both the estimations for uncertain terms and repulsive potential functions are established in design of algorithms. For the other case, the systems under actuator failures, by utilizing the Lyapunov function and relevant adaptive updating laws, the effects subjected to partial loss of actuator effectiveness can be eliminated. Eventually, two distributed algorithms are proposed to achieve the expected formation configuration with no collision occurred. Numerical simulations are conducted to illustrate the validities of the presented control methodologies.

Robust Adaptive Control Allocation Schemes for Overactuated Underwater Vehicles under Actuator Faults

In this paper, we investigate the fault-tolerant path following control problem for an overactuated underwater vehicle subject to input saturation and rate of change constraints, actuator faults and parametric (mass and inertia) uncertainty. A robust adaptive control allocation strategy is proposed capable to reconfigure the distribution of the control effort among the remaining healthy actuators in the case some of them failed. The online estimation of actuators' effectiveness and bias fault parameters is integrated with the control allocation and reconfiguration unit. The robustness and performance of the scheme are shown by evaluating the results of simulations and pool experiments of different path-following control problems using overactuated underwater vehicles.

Control allocation based fault tolerant control of descriptor system with actuator saturation

This paper deals with the design of active fault-tolerant control (FTC) system based on control allocation (CA) technique for the uncertain descriptor system (DS) with actuator saturation. This work assumes that a fault detection unit is available to estimate the actuator fault. The proposed CA technique is responsible to redistribute the control effort to the healthy actuators taking the advantage of actuator redundancy of the DS. The so-called virtual control law is designed based on a variable gain super twisting sliding mode algorithm (VGSTSMA) to meet the admissibility criteria of continuous control input for the DS. This VGSTSMA based control strategy is designed based on the generalized regular form (GRF) of DS ensuring the robustness against the actuator faults, error in the fault estimation, and the external uncertainties. The CA scheme developed here is applicable to the DS even with a full rank input matrix which is different from the traditional design where it generally deals with the system with rank deficient input matrix. Furthermore, a static control redistribution mechanism is introduced to handle the actuator saturation which is responsible to re-allocate the excess control efforts to the available healthy actuators in the face of actuator saturation. Finally, a dual-pipe heat exchanger unit, modeled as DS, is simulated with proposed CA-based FTC such that the system can follow the time varying and constant reference trajectories in a situation of actuator faults and saturation. The simulation results are compared with the CA-based FTC where virtual control is designed based on super-twisting algorithm (STA) and with the traditional pseudo inverse-CA-based FTC to establish the superiority of the proposed scheme.

Adaptive output feedback fault tolerant control for uncertain nonlinear systems based on actuator switching

In this paper, an adaptive output feedback fault tolerant control (FTC) based on actuator switching is proposed for a class of single-input single-output (SISO) nonlinear systems with uncertain parameters and possible actuator failures, for which a set of healthy actuators are available as backups. While high-gain K-filters are utilized to estimate the unmeasured states, an adaptive control law is designed to compensate for the parameter uncertainties and certain actuator failures, an actuator switching strategy based on a set of appropriately designed monitoring functions (MFs) is proposed to tackle those serious actuator failures, make tracking error satisfy prescribed transient and steady-state performance and guarantee closed-loop signal boundedness.

Accelerated Landweber iteration based control allocation for fault tolerant control of reusable launch vehicle

This paper presents a novel Fault Tolerant Control (FTC) scheme based on accelerated Landweber iteration and redistribution mechanism for a horizontal takeoff horizontal landing reusable launch vehicle (RLV). First, an adaptive law based on fixed-time non-singular fast terminal sliding mode control (NFTSMC), which focuses on the attitude tracking controller design for RLV in the presence of model couplings, parameter uncertainties and external disturbances, is proposed to produce virtual control command. On this basis, a novel Control Allocation (CA) based on accelerated Landwber iteration is presented to realize proportional allocation of virtual control command among the actuators according to the effective gain as well as the distance from the current position of actuator to corresponding saturation limit. Meanwhile a novel redistribution mechanism is introduced to redistribute oversaturated command among healthy actuators (non-faulty or redundant). The proposed method can be applied to a real-time FTC system so that the controller reconfiguring is not required in case of actuator faults. Finally, the effectiveness of the proposed method is demonstrated by numerical simulations.

Adaptive model-parameter-free fault-tolerant trajectory tracking control for autonomous underwater vehicles

This paper provides a model-parameter-free control strategy for the trajectory tracking problem of the autonomous underwater vehicle exposed to external disturbances and actuator failures. Two control architectures have been constructed such that the system states could be forced to the desired trajectories with acceptable performance. By combining sliding mode control (SMC) technology and adaptive algorithm, the first control architecture is developed for tracking missions under healthy actuators. Taking actuator failures scenario into account, system reliability is improved considerably by the utilization of a passive fault-tolerant technology in the second controller. Benefitting from properties of Euler–Lagrange systems, the nonlinear dynamics of the underwater vehicles could be handled properly such that the proposed controllers could be developed without model parameters. Finally, the validity of the proposed controllers is demonstrated by theoretical analysis and numerical simulations.

Fault-tolerant attitude control for flexible spacecraft subject to input and state constraint

This paper considers the problem of fault-tolerant attitude control for a flexible spacecraft subject to input and state constraint. Particularly, a new sliding mode-based attitude control with fixed-time convergent for the flexible spacecraft is developed in which the convergence rate of the system state is improved both far from and at close range of the origin. In contrast to the existing complicated prescribed performance controls (PPC), the proposed PPC possesses a much simpler structure due to the use of a novel constraint concept without employing error transformation. It also introduces a modified prescribed performance function (MPPF) to explicitly determine the settling time. It is rigorously proved that the attitude variable is kept within the predefined constraint boundaries even when the actuator saturation is taken into account. Moreover, the proposed controller is inherently continuous and the chattering is effectively reduced. An adaptive mechanism is developed in which no prior knowledge of the lumped uncertainties is required. Finally, numerical simulations are presented to demonstrate that the proposed controller is able to successfully accomplish attitude control with high attitude pointing accuracy and stability. More specifically, it provides faster convergence (improvement percentage of convergence time (IP_CT) is about 18%) and more accurate control (improvement percentages of MRPs (IP_MRPs) and angular velocity (IP_AV) are about 60% and 80%, respectively) under healthy actuators. Values of IP_CT, IP_CT, and IP_AV are 50%, 99.9% and 99.9% under faulty actuators, respectively.

Fault-tolerant control based on 2D game for independent driving electric vehicle suffering actuator failures

This paper proposes a cooperative game-based actuator fault-tolerant control strategy for a four-wheel independent drive electric vehicle with an active front steering system. For achieving fault-tolerant control and targets cooperation, a two-dimensional game strategy is proposed to balance the stability and economy. The first-dimensional game is utilized to determine the dominant control target of the actuator, then the second-dimensional game is employed to assign the fault-tolerant control task for the remaining healthy actuators. The two dimensions are integrated based on the linear quadratic differential game theory, and a hybrid weighted Pareto frontier is thus established. A Shapley value based weight calculation method is proposed to obtain a set of fair and unique weights according to the importance of each player, which makes the solution of the optimal control problem more easily obtained. The effectiveness and real-time performance of the control strategy are tested under different scenarios. The simulation results demonstrate that the proposed strategy can balance the stability and economy well, outperforms the traditional method in terms of target tracking performance. For special case, the response of the yaw rate could be improved up to 39.83% comparing to that of the linear quadratic regulator method.

Flight evaluation of a sliding mode online control allocation scheme for fault tolerant control

This paper develops a sliding mode fault tolerant control scheme based on a Linear Parameter Varying (LPV) system representation of the plant. The scheme includes a control allocation component, which is capable of utilizing the available healthy actuators in the face of actuator faults/failures, in an effort to retain close to nominal fault free performance. The proposed scheme is validated using the Japan Aerospace Exploration Agency’s Multi-Purpose Aviation Laboratory (MuPAL-α) research aircraft. The flight test results demonstrate good lateral-directional state tracking performance with no visible performance degradation in the presence of rudder and aileron faults.

Fault-tolerant control with state and disturbance observers for vehicle active suspension systems

This paper deals with the design of a fault-tolerant control scheme of active suspension systems for vehicle ride comfort. Unknown actuator failures from a variety of reasons cause performance deterioration of the active suspension controller. The proposed fault-tolerant control algorithm consists of two parts: a compensation for actuator failure and a fault mode selector. The main function of the fault compensation strategy is to estimate and compensate for the loss of effectiveness of the actuators. A suspension state observer and a disturbance observer operate simultaneously to determine the feedback control input. The controller and observer have been developed based on a reduced full-car dynamic model that contains only the vehicle body dynamics. The main advantage of the proposed observer is that an easily accessible and inexpensive measurement is only required and the effect of unknown road disturbance on the estimation error is completely removed. To cope with complete failure cases, the fault mode selector is also designed to redistribute the control input to the remaining healthy actuators. Tracking of the loss of effectiveness of the actuators is used for the fault model identification. The performance of the proposed approach has been evaluated via simulation studies. It is shown that the vehicle ride comfort in the presence of actuator faults can be improved by the proposed combined strategy of the fault compensation method and the fault mode selector.

Active versus passive fault-tolerant control of a redundant multirotor UAV

ABSTRACTThis paper considers actuator redundancy management for a redundant multirotor Unmanned Aerial Vehicle (UAV) under actuators failures. Different approaches are proposed: using robust control (passive fault tolerance), and reconfigurable control (active fault tolerance). The robust controller is designed using high-order super-twisting sliding mode techniques, and handles the failures without requiring information from a Fault Detection scheme. The Active Fault-Tolerant Control (AFTC) is achieved through redistributing the control signals among the healthy actuators using reconfigurable multiplexing and pseudo-inverse control allocation. The Fault Detection and Isolation problem is also considered by proposing model-based and model-free modules. The proposed techniques are all implemented on a coaxial octorotor UAV. Different experiments with different scenarios were conducted for the validation of the proposed strategies. Finally, advantages, disadvantages, application considerations and limitations of each method are examined through quantitative and qualitative studies.

Control allocation based fault-tolerant strategy for a bio-ethanol processor system integrated to a PEM fuel cell

This paper presents a control approach to obtain actuator fault tolerance in a bio-ethanol processor system coupled with a proton exchange membrane fuel cell. The proposed strategy is based on the development of two cascaded modules. First, a high-level controller consisting on single-input single-output conventional proportional-integral control loops is designed for computing virtual actions which represent the overall control effort. Second, a control allocation module is proposed for the on-line redistribution of the virtual commands onto the available healthy actuators. This scheme is able to compensate from actuator position/rate saturations, to severe abnormal events such as loss of effectiveness and lock-in-place actuator faults. Some attractive features of the proposal are: (i) the control structure design is exclusively based on a steady-state model of the process, (ii) different actuator faults can be efficiently handled without the need of reconfiguring the high-level controller, (iii) the control and optimization tasks are efficiently integrated demanding a reduced computation time, (iv) the on-line controller re-allocation allows to manage not predefined actuator faults, (v) a considerable set of disturbances can be rejected with remarkable performance. Dynamic simulations performed on a rigorous nonlinear model of the process show the benefits of the proposed strategy for both fault-free as well as different fault situations comprising partial and total actuator faults.