Direct Adaptive Control of Systems with Actuator Failures: State of the Art and Continuing Challenges

In this paper, the problem of controlling systems with failures and faults is introduced, and an overview of recent work on direct adaptive control for compensation of uncertain actuator failures is presented. Actuator failures may be characterized by some unknown system inputs being stuck at some unknown (fixed or varying) values at unknown time instants, that cannot be influenced by the control signals. The key task of adaptive compensation is to design the control signals in such a manner that the remaining actuators can automatically and seamlessly take over for the failed ones, and achieve desired stability and asymptotic tracking. A certain degree of redundancy is necessary to accomplish failure compensation. The objective of adaptive control design is to effectively use the available actuation redundancy to handle failures without the knowledge of the failure patterns, parameters, and time of occurrence. This is a challenging problem because failures introduce large uncertainties in the dynamic structure of the system, in addition to parametric uncertainties and unknown disturbances. The paper addresses some theoretical issues in adaptive actuator failure compensation: actuator failure modeling, redundant actuation requirements, plant-model matching, error system dynamics, adaptation laws, and stability, tracking, and performance analysis. Adaptive control designs can be shown to effectively handle uncertain actuator failures without explicit failure detection. Some open technical challenges and research problems in this important research area are discussed

Direct Adaptive Control for Stability and Command Augmentation System of an Air-Breathing Hypersonic Vehicle

In this paper we explore a Direct Adaptive Control scheme for stabilizing a non-linear, physics based model of the longitudinal dynamics for an air breathing hypersonic vehicle. The model, derived from first principles, captures the complex interactions between the propulsion system, aerodynamics, and structural dynamics. The linearized aircraft dynamics show unstable and non-minimum phase behavior. It also shows a strong short period coupling with the fuselage-bending mode. The value added by direct adaptive control and the theoretical requirements for stable convergent operation is displayed. One of the main benefits of the Directive Adaptive Control is that it can be implemented knowing very little detail about the plant. The implementation uses only measured output feedback to accomplish the adaptation. A stability analysis is conducted on the linearized plant to understand the complex aero-propulsion and structural interactions. The multivariable system possesses certain characteristics beneficial to the adaptive control scheme; we discuss these advantages and ideas for future work

Nonlinear and adaptive control

The primary thrust of the research was to conduct fundamental research in the theories and methodologies for designing complex high-performance multivariable feedback control systems; and to conduct feasibiltiy studies in application areas of interest to NASA sponsors that point out advantages and shortcomings of available control system design methodologies

Design Of An Adaptive Autopilot For An Expendable Launch Vehicle

This study investigates the use of a Model Reference Adaptive Control (MRAC) direct approach to solve the attitude control problem of an Expendable Launch Vehicle (ELV) during its boost phase of flight. The adaptive autopilot design is based on Lyapunov Stability Theory and provides a useful means for controlling the ELV in the presence of environmental and dynamical uncertainties. Several different basis functions are employed to approximate the nonlinear parametric uncertainties in the system dynamics. The control system is designed so that the desire dresponse to a reference model would be tracked by the closed-loop system. The reference model is obtained via the feedback linearization technique applied to the nonlinear ELV dynamics. The adaptive control method is then applied to a representative ELV longitudinal motion, specifically the 6th flight of Atlas-Centaur launch vehicle (AC-6) in 1965. The simulation results presented are compared to that of the actual AC-6 post-flight trajectory reconstruction. Recommendations are made for modification and future applications of the method for several other ELV dynamics issues, such as control saturation, engine inertia, flexible body dynamics, and sloshing of liquid fuels

Active Fault Tolerant Control for Vertical Tail Damaged Aircraft with Dissimilar Redundant Actuation System

This paper proposes an active fault-tolerant control strategy for an aircraft with dissimilar redundant actuation system (DRAS) that has suffered from vertical tail damage. A damage degree coefficient based on the effective vertical tail area is introduced to parameterize the damaged flight dynamic model. The nonlinear relationship between the damage degree coefficient and the corresponding stability derivatives is considered. Furthermore, the performance degradation of new input channel with electro-hydrostatic actuator (EHA) is also taken into account in the damaged flight dynamic model. Based on the accurate damaged flight dynamic model, a composite method of linear quadratic regulator (LQR) integrating model reference adaptive control (MRAC) is proposed to reconfigure the fault-tolerant control law. The numerical simulation results validate the effectiveness of the proposed fault-tolerant control strategy with accurate flight dynamic model. The results also indicate that aircraft with DRAS has better fault-tolerant control ability than the traditional ones when the vertical tail suffers from serious damage. © 2016 Chinese Society of Aeronautics and Astronautic

Development of adaptive control methodologies and algorithms for nonlinear dynamic systems based on u-control framework

Inspired by the U-model based control system design (or called U-control system design), this study is mainly divided into three parts. The first one is a U-model based control system for unstable non-minimum phase system. Pulling theorems are proposed to apply zeros pulling filters and poles pulling filters to pass the unstable non-minimum phase characteristics of the plant model/system. The zeros pulling filters and poles pulling filters derive from a customised desired minimum phase plant model. The remaining controller design can be any classic control systems or U-model based control system. The difference between classic control systems and U-model based control system for unstable non-minimum phase will be shown in the case studies.Secondly, the U-model framework is proposed to integrate the direct model reference adaptive control with MIT normalised rules for nonlinear dynamic systems. The U-model based direct model reference adaptive control is defined as an enhanced direct model reference adaptive control expanding the application range from linear system to nonlinear system. The estimated parameter of the nonlinear dynamic system will be placement as the estimated gain of a customised linear virtual plant model with MIT normalised rules. The customised linear virtual plant model is the same form as the reference model. Moreover, the U-model framework is design for the nonlinear dynamic system within the root inversion.Thirdly, similar to the structure of the U-model based direct model reference adaptive control with MIT normalised rules, the U-model based direct model reference adaptive control with Lyapunov algorithms proposes a linear virtual plant model as well, estimated and adapted the particular parameters as the estimated gain which of the nonlinear plant model by Lyapunov algorithms. The root inversion such as Newton-Ralphson algorithm provides the simply and concise method to obtain the inversion of the nonlinear system without the estimated gain. The proposed U-model based direct control system design approach is applied to develop the controller for a nonlinear system to implement the linear adaptive control. The computational experiments are presented to validate the effectiveness and efficiency of the proposed U-model based direct model reference adaptive control approach and stabilise with satisfied performance as applying for the linear plant model

Model Reference Adaptive Control Laws: Application to Nonlinear Aeroelastic Systems

Nonlinear Aeroelastic Control has been a research topic of great interest for the past few decades. Dierent approaches has been attempted aiming to obtain better accuracy in the model dynamics description and better control performance. As far as the aeroelastic mathematical model is concerned, the scientic world converged in the use of a bi-dimension, two degree of freedom, plunging and pitching, wing section model, of which the bigger advantages are to be reproducible experimentally with an appropriate wind tunnel apparatus and to allow LCO (Limit Cycle Oscillation) exhibition at low values of wind speed, facilitating parametric studies of the nonlinear aeroelastic system and its control architecture. A parametric analysis of the linearized system, typical of aircraft ight dynamic studies, is employed to verify and validate the model dynamic properties dependency, focusing in particular to the eect of stiness reduction as means of failure simulation. In fact, despite of the recent years ourishing literature on aeroelastic adaptive controls, there is a noted lack of robustness and sensitivity analysis with respect to structural proprieties degradation which might be associated with a structural failure. Structural mode frequencies and aeroelastic response, including Limit Cycle Oscillations (LCOs) characteristics, are signicantly aected by changes in stiness. This leads to a great interest in evaluating and comparing the adaptation capabilities of dierent control architectures subjected to large plant uncertainties and unmodeled dynamics. Motivated by the constantly increasing diusion of the new L adaptive control theory, developed for the control of uncertain non-autonomous nonlinear systems, and by the fact that its application to aeroelasticity is in its infancy, a deep investigation of this control scheme properties and performance drew our attention. The new control theory is conceptually similar to the Model Reference Adaptive Control (MRAC) theory to which has often been compared indeed for performance evaluation purpose. In this dissertation, a comprehensive analysis of the new control theory is obtained by performance evaluation and comparison of four dierent control schemes, two MRAC and two L 1 , focusing the attention on the states and control input time response, adaptive law parameters' convergence, transient evolution and fastness, and robustness in terms of tolerance of uncertainties in o-design conditions. The objective is pursued by re- writing the aeroelastic model nonlinear equations of motion in an amenable form to the development of the four dierent control laws. The control laws are then derived for the appropriate class of plant which the system belongs to, and design parameter obtained, when necessary, following the mathematical formulation of the control theories developers. A simulation model is employed to carry out the numerical analysis and to outline pros and cons of each architecture, to obtain as nal result the architecture that better ts the nonlinear aeroelastic problem proposed. This methodology is used to guarantee a certain robustness in controlling a novel actuation architecture, developed for utter suppression of slender/highly exible wing, based on a coordinated multiple spoiler stripe, located at fteen percent of the mean aerodynamic chord. The control actuation system design, manufacturing and experimental wind tunnel test is part of the dissertation. Two dierent experimental setup are developed for two dierent purpose. First, a six-axis force balance test is carried out to validate the numerical aerodynamic results obtained during the validation process, and to collect the aerodynamic coecient date base useful for the development of the simulation model of the novel architecture. The second experimental apparatus, is a two degree of freedom, plunging/pitching, system on which the prototyped wing section is mounted to obtain LCO aeroelastic response during wind tunnel experiment. The nonlinear aeroelastic mathematical formulation is modied to take into account of the novel actuation architecture and, coupled with the more robust MRAC control laws derived for the previous model, serves as benchmark for properties assessment of the overall architecture, for utter suppression. The novel control actuation architecture proposed, is successfully tested in wind tunnel experimentation conrming the validity of the proposed solution. This dissertation provides a step forward to the denition of certain MRAC control schemes properties, and together provides a novel actuation solution for utter suppression which demonstrates to be a viable alternative to classical leading and/or trailing-edge ap architecture or to be used as redundancy to them

Status report #4 on nonlinear and adaptive control

Includes bibliographical references.Supported by NASA. NAG 2-297 MIT OSP no.95178prepared by Michael Athans, Gunter Stein, Lena Valavani ; submitted to NASA, Langley Research Center, Ames Research Center

Final report on robust stochastic adaptive control

Includes bibliographical references.Supported by the Office of Naval Research under contract N00014-82-K-0582 NR606-003 MIT OSP no.92775prepared by Lena Valavani, Michael Athans ; submitted to Office of Naval Research, Mathematical Sciences Division