60,929 research outputs found
Stability Metrics for Simulation and Flight-Software Assessment and Monitoring of Adaptive Control Assist Compensators
Due to a need for improved reliability and performance in aerospace systems, there is increased interest in the use of adaptive control or other nonlinear, time-varying control designs in aerospace vehicles. While such techniques are built on Lyapunov stability theory, they lack an accompanying set of metrics for the assessment of stability margins such as the classical gain and phase margins used in linear time-invariant systems. Such metrics must both be physically meaningful and permit the user to draw conclusions in a straightforward fashion. We present in this paper a roadmap to the development of metrics appropriate to nonlinear, time-varying systems. We also present two case studies in which frozen-time gain and phase margins incorrectly predict stability or instability. We then present a multi-resolution analysis approach that permits on-line real-time stability assessment of nonlinear systems
Adaptive Control for Uncertain Nonlinear Multi-Input Multi-Output Systems
Systems and methods of adaptive control for uncertain nonlinear multi-input multi-output systems in the presence of significant unmatched uncertainty with assured performance are provided. The need for gain-scheduling is eliminated through the use of bandwidth-limited (low-pass) filtering in the control channel, which appropriately attenuates the high frequencies typically appearing in fast adaptation situations and preserves the robustness margins in the presence of fast adaptation
Robust, Practical Adaptive Control for Launch Vehicles
A modern mechanization of a classical adaptive control concept is presented with an application to launch vehicle attitude control systems. Due to a rigorous flight certification environment, many adaptive control concepts are infeasible when applied to high-risk aerospace systems; methods of stability analysis are either intractable for high complexity models or cannot be reconciled in light of classical requirements. Furthermore, many adaptive techniques appearing in the literature are not suitable for application to conditionally stable systems with complex flexible-body dynamics, as is often the case with launch vehicles. The present technique is a multiplicative forward loop gain adaptive law similar to that used for the NASA X-15 flight research vehicle. In digital implementation with several novel features, it is well-suited to application on aerodynamically unstable launch vehicles with thrust vector control via augmentation of the baseline attitude/attitude-rate feedback control scheme. The approach is compatible with standard design features of autopilots for launch vehicles, including phase stabilization of lateral bending and slosh via linear filters. In addition, the method of assessing flight control stability via classical gain and phase margins is not affected under reasonable assumptions. The algorithm s ability to recover from certain unstable operating regimes can in fact be understood in terms of frequency-domain criteria. Finally, simulation results are presented that confirm the ability of the algorithm to improve performance and robustness in realistic failure scenarios
Adaptive control design with guaranteed margins for nonlinear plants
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references (p. 139-142).Adaptive control is one of the technologies that improve both performance and safety as controller parameters can be redesigned autonomously in the presence of uncertainties. Considerable research has been accomplished in adaptive control theory for several decades and a solid foundation has been laid out for stability and robustness of adaptive systems. However, a large gap between theory and practice has been an obstacle to transition theoretical results into applications and it still remains. In order to reduce the gap, this thesis presents a unified framework for design and analysis of adaptive control for general nonlinear plants.An augmented adaptive control architecture is proposed where a nominal controller is designed in the inner-loop with an adaptive controller in the outer-loop. The architecture is completed by addressing three separate problems. The first problem is the design of adaptive control in the presence of input constraints. With a rigorous stability analysis, an algorithm is developed to remove the adverse effects of multi-input magnitude saturation. The second problem is the augmentation of adaptive control with a nominal gain-scheduling controller. Though adaptive controllers have been employed with gain-scheduling to various applications, no formal stability analysis has been developed. In the proposed architecture, adaptive control is combined with gain-scheduling in a specific manner while stability is guaranteed. The third problem is the development of analytic stability margins of the closed-loop plant with the proposed adaptive controller. A time-delay margin is derived using standard Lyapunov stability analysis as an analytic stability margin.The overall adaptive control architecture as well as the analytically derived margins are validated by a 6-DoF nonlinear flight dynamics based on the NASA X-15 hypersonic aircraft. Simulation results show that the augmented adaptive control is able to stabilize the plant and tracks desired trajectories with uncertainties in the plant while instability cannot be overcome only with the nominal controller. The time-delay margins are validated based on a generic transport model and they are compared with margins obtained from simulations studies. We utilize numerical methods to find less conservative time-delay margins.by Jinho Jang.Ph.D
Robustness margins and high performance for an adaptive flight control system with application to hypersonic vehicles
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (leaves 59-61).The design tools developed for use with linear controllers such as gain and phase margins do not apply to nonlinear control architectures such as adaptive control. For decades, flight control engineers have used these tools extensively to measure the robustness of their linear control systems and make guarantees on the performance of the closed-loop system in the presence of uncertainties. Stringent demands on performance for safety-critical flight systems, as in the case of hypersonic vehicles, make advanced control methods such as adaptive control increasingly attractive. The major obstacle in the widespread application of adaptive control to such applications is the lack of guarantees on performance and robustness. This thesis presents robustness margins, adaptive control analogs to the linear control notions of gain and phase margins, which can be used to make those guarantees. This paves the way for a systematic Verification and Validation (V&V) approach for adaptive controllers. The operation of an adaptive controller can be broken down into two distinct phases: the adaptation mode, in which the adaptive parameters are varying, and the steady-state mode, in which the adaptive parameters have converged to their steady-state values.(cont.) During the steady-state mode, the nonlinear adaptive controller converges to a linear time-invariant (LTI) system, and many tools exist for the calculation of the requisite margins. However, during the adaptation mode, which is arguably a more crucial mode of operation for the aircraft, no such tools exist. This thesis provides the tools for the numerical calculation of robustness margins during the adaptation mode. Robustness with respect to a range of uncertainties including parametric uncertainties, disturbances, time-delays, unmodeled dynamics, and actuator saturation is derived. The robustness of the adaptive controller is then demonstrated on a fully nonlinear model of a high-performance hypersonic aircraft. The importance of theoretically justified adaptive controllers is illustrated using the historical example of the NASA X-15 research airplane. NASA's three X-15 aircraft together flew nearly 200 flights, acting as test beds for many bleeding-edge technologies, including the nonlinear adaptive controller implemented on the X-15-3. The application of this controller demonstrated the advantages of adaptive control including improved performance and a shorter design cycle.(cont.) However, when the X-15-3 crashed in 1967, one of the severe disadvantages of this early adaptive control was highlighted: the lack of guaranteed stability and performance. Using modern adaptive control theory and the tools developed in this thesis, the control design of the X-15 is revisited and it is demonstrated that had the X-15 controllers been implemented today, all of the 200 flights, without a single exception, would have been performed safely, without incident.by Zachary Thompson Dydek.S.M
Analysis of an On-Line Stability Monitoring Approach for DC Microgrid Power Converters
An online approach to evaluate and monitor the stability margins of dc microgrid power converters is presented in this paper. The discussed online stability monitoring technique is based on the Middlebrook's loop-gain measurement technique, adapted to the digitally controlled power converters. In this approach, a perturbation is injected into a specific digital control loop of the converter and after measuring the loop gain, its crossover frequency and phase margin are continuously evaluated and monitored. The complete analytical derivation of the model, as well as detailed design aspects, are reported. In addition, the presence of multiple power converters connected to the same dc bus, all having the stability monitoring unit, is also investigated. An experimental microgrid prototype is implemented and considered to validate the theoretical analysis and simulation results, and to evaluate the effectiveness of the digital implementation of the technique for different control loops. The obtained results confirm the expected performance of the stability monitoring tool in steady-state and transient operating conditions. The proposed method can be extended to generic control loops in power converters operating in dc microgrids
Robust Stability of Iterative Learning Control Schemes
A notion of robust stability is developed for iterative learning control in the context of disturbance attenuation. The size of the unmodelled dynamics is captured via a gap distance, which in turn is related to the standard H2 gap metric, and the resulting robustness certificate is qualitatively equivalent to that obtained in classical robust Hâ theory. A bound on the robust stability margin for a specific adaptive ILC design is established
PID control system analysis, design, and technology
Designing and tuning a proportional-integral-derivative
(PID) controller appears to be conceptually intuitive, but can
be hard in practice, if multiple (and often conflicting) objectives
such as short transient and high stability are to be achieved.
Usually, initial designs obtained by all means need to be adjusted
repeatedly through computer simulations until the closed-loop
system performs or compromises as desired. This stimulates
the development of "intelligent" tools that can assist engineers
to achieve the best overall PID control for the entire operating
envelope. This development has further led to the incorporation
of some advanced tuning algorithms into PID hardware modules.
Corresponding to these developments, this paper presents a
modern overview of functionalities and tuning methods in patents,
software packages and commercial hardware modules. It is seen
that many PID variants have been developed in order to improve
transient performance, but standardising and modularising PID
control are desired, although challenging. The inclusion of system
identification and "intelligent" techniques in software based PID
systems helps automate the entire design and tuning process to
a useful degree. This should also assist future development of
"plug-and-play" PID controllers that are widely applicable and
can be set up easily and operate optimally for enhanced productivity,
improved quality and reduced maintenance requirements
An investigation study on model reference adaptive techniques as applied to attitude control system for launch vehicles Final report
Self adaptive control techniques for attitude control on Saturn 5 launch vehicle
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