This paper considers actuator redundancy management for a class of overactuated nonlinear systems. Two tools for distributing the control effort among a redundant set of actuators are optimal control design and control allocation. In this paper, we investigate the relationship between these two design tools when the performance indexes are quadratic in the control input. We show that for a particular class of nonlinear systems, they give exactly the same design freedom in distributing the control effort among the actuators. Linear quadratic optimal control is contained as a special case. A benefit of using a separate control allocator is that actuator constraints can be considered, which is illustrated with a flight control example.

Resolving actuator redundancy: optimal control vs. control allocation

In this paper, a novel finite time fault tolerant control (FTC) is proposed for uncertain robot manipulators with actuator faults. First, a finite time passive FTC (PFTC) based on a robust nonsingular fast terminal sliding mode control (NFTSMC) is investigated. Be analyzed for addressing the disadvantages of the PFTC, an AFTC are then investigated by combining NFTSMC with a simple fault diagnosis scheme. In this scheme, an online fault estimation algorithm based on time delay estimation (TDE) is proposed to approximate actuator faults. The estimated fault information is used to detect, isolate, and accommodate the effect of the faults in the system. Then, a robust AFTC law is established by combining the obtained fault information and a robust NFTSMC. Finally, a high-order sliding mode (HOSM) control based on super-twisting algorithm is employed to eliminate the chattering. In comparison to the PFTC and other state-of-the-art approaches, the proposed AFTC scheme possess several advantages such as high precision, strong robustness, no singularity, less chattering, and fast finite-time convergence due to the combined NFTSMC and HOSM control, and requires no prior knowledge of the fault due to TDE-based fault estimation. Finally, simulation results are obtained to verify the effectiveness of the proposed strategy.

Finite Time Fault Tolerant Control for Robot Manipulators Using Time Delay Estimation and Continuous Nonsingular Fast Terminal Sliding Mode Control

Motivated by several accidents, attitude control of a spacecraft subject to faults/failures has gained considerable attention in a wider range of aerospace engineering and academic communities. This paper is concerned with industrial practices and theoretical approaches for fault tolerant control (FTC) and fault detection and diagnosis (FDD) in spacecraft attitude control system. An overview on recent development of spacecraft attitude FTC system design is presented. The basis of a FTC system is introduced. The existing engineering FTC techniques and theoretical methodologies, including their advantages and disadvantages, are discussed. Moreover, closely associated with the reliability-relevant issues, recent progress in attitude FTC design strategies is reviewed. A brief review of some open problems in the general area of spacecraft attitude control design subject to components faults/failures is further concluded.

A Review on Recent Development of Spacecraft Attitude Fault Tolerant Control System

The science objectives of a spacecraft mission place stringent performance requirements on the spacecraft attitude control system. However, it remains an open problem how to guarantee consistent control performance necessary to meet these requirements, especially in the event of actuator faults and input saturation. Motivated by this fact, in this paper, we address the problem of attitude tracking control with prescribed performance guarantees for a rigid spacecraft subject to unknown but constant inertia parameters, unexpected disturbances, actuator faults, and input saturation. First, certain performance functions specified a priori by the designer are adopted to impose desired performance metrics on the attitude tracking errors. Then, the original attitude tracking error dynamics with performance constraints is transformed into an equivalent “state-constrained” one whose robust stabilization is shown to be sufficient to solve the stated problem via a novel error transformation. Subsequently, based on the transformed system, an adaptive fault-tolerant controller is derived by incorporating backstepping control, the barrier Lyapunov function, and Nussbaum gains. It is proved that the designed controller is able to guarantee the satisfaction of the prespecified constraints on the transformed errors, as well as the boundedness of all other closed-loop signals, without resorting to a judicious selection of the control parameters. Finally, the effectiveness of the proposed control scheme is evaluated by means of simulation experiments carried out on a microsatellite.

Adaptive Fault-Tolerant Attitude Tracking Control of Spacecraft With Prescribed Performance

A smooth attitude-stabilizing control law is derived from which known limits on the control authority of the system are rigorously enforced. Unknown disturbance torques, assumed to be of lesser magnitude than the control limits, are included in the formulation. A smooth control signal containing hyperbolic tangent functions that rigorously obeys a known maximum-torque constraint is introduced. The controller can be viewed as a smooth analog of the variable-structure approach, with the degree of sharpness of the control permitted to vary with time according to a set of user-defined parameters. Lyapunov analysis is employed to ensure global stability, and asymptotic convergence of the angular velocity is guaranteed via the Barbalat lemma. Attitude errors, expressed as Euler parameters, are shown via simulation to vanish whenever certain design parameters are selected appropriately, and guidelines for selection of those parameters are provided in depth.

Globally stabilizing saturated attitude control in the presence of bounded unknown disturbances

This paper investigates rest-to-rest attitude reorientation problem for a rigid spacecraft in the presence of attitude and angular velocity constraints. Based on a potential function, a nonlinear attitude controller is developed to avoid the undesired celestial objects autonomously and limit the spacecraft rotation speed while achieving asymptotic attitude stabilization. To improve the agility of spacecraft, control moment gyro (CMG) is considered as torque-generating actuators for attitude control. General singular robust (GSR) steering logic is employed to determine the CMG gimbal rate commands. The proposed attitude control scheme has simple structure, which is of great interest for aerospace industry when onboard computing power is limited. Finally, simulation results for a CMG-based spacecraft are presented to show the effectiveness of the proposed attitude control systems.

Spacecraft reorientation control with attitude and velocity constrains

In this paper, a neural network based terminal iterative learning control (NNTILC) method is proposed for a class of discrete time linear run-to-run systems to track run-varying reference point with initial state disturbance. An iterative training radial basis function (RBF) neural network is developed to estimate the effect of initial state on terminal output and to learn the changes in initial state iteratively at the same time. By involving these information in the control scheme, the proposed NNTILC can drive the system to track run-varying reference point fast and precisely beyond the initial disturbance and reference change. Stability and convergence of this NNTILC method is proved and computer simulation results confirm its effectiveness further.

Neural network based terminal iterative learning control for tracking run-varying reference point

This paper presents a solution for rest-to-rest attitude reorientation for a rigid body under angular velocity constraints and external disturbances. Assuming that external disturbances are unknown but bounded, the sliding mode technique is used in attitude controller to reject disturbances. Then, a potential function in terms of sliding vector is proposed with a largest potential placing at the maximal angular rate. Based on the potential function, an adaptive sliding mode controller is developed to stabilize the closed-loop attitude control system. Finally, the efficiency of the proposed attitude control scheme is demonstrated by numerical simulations.

Angular rate constrained attitude reorientation of rigid body

Momentum exchange devices like reaction wheel (RW) and control moment gyro (CMG) are always used as actuators in spacecraft attitude control systems. However, the unexpected faults in these momentum exchange devices may result in mission failures or even the spacecraft breakup. To enhance the reliability and safety of the attitude control systems, this paper addressed the CMG fault modeling and estimation problems in attitude control systems. We model the CMG as a combination of two EM-VSD (electrical motor (EM) and variable speed drive (VSD)) systems. All potential faults in an EM and a VSD are analyzed and mathematically modeled. Based on the fault model of a EM-VSD system, the CMG fault model consisting of faults in two EM-VSD systems are developed. An observer based fault estimation method is then proposed, in which the past gimbal angle and commanded gimbal rate are utilized to estimate the total CMG fault effects. It is proved that the gimbal and fault estimation errors converge to small compact sets containing origin. To verify the proposed fault estimation method, numerical simulations are carried out based on a model of a rigid spacecraft.

Fault Modeling and Estimation for CMG

This paper presents a dynamic state observer design for discrete-time linear time-varying systems that robustly achieves equalized recovery despite delayed or missing observations, where the set of all temporal patterns for the missing or delayed data is modeled by a finite-length language. By introducing a mapping of the language onto a reduced event-based language, we design a state estimator that adapts based on the history of available data at each step, and satisfies equalized recovery for all patterns in the reduced language. In contrast to existing equalized recovery estimators, the proposed design considers the equalized recovery level as a decision variable, which enables us to directly obtain the global minimum for the intermediate recovery level, resulting in improved estimation performance. Finally, we demonstrate the effectiveness of the proposed observer when compared to existing approaches using several illustrative examples.

Qiang Shen

Papers

Spacecraft reorientation control with attitude and velocity constrains

Neural network based terminal iterative learning control for tracking run-varying reference point

Angular rate constrained attitude reorientation of rigid body

Fault Modeling and Estimation for CMG

Equalized Recovery State Estimators for Linear Systems with Delayed and Missing Observations