Luis G. Crespo

Sensitivity of the Generic Transport Model upset dynamics to time delay

Bifurcation analysis has previously been applied to the NASA Generic Transport Model (GTM) to provide insight into open-loop upset dynamics and also the impact on and sensitivity of such behaviour to closing the loop with a flight controller. However, these studies have not considered time delay in the system: this arises in all feedback controllers and has specific relevance when remotely piloting a vehicle such as the NASA AirSTAR GTM with ground-based controllers. Developments in the AirSTAR programme, in which a sub-scale generic airliner model will be tested for loss-of-control conditions over long ranges, raise the prospect of increased adverse effects of time delay relative to previous testing. This paper utilises bifurcation analysis, supplemented with time histories, on the GTM numerical model with a LQR-PI controller to evaluate the sensitivity of the closed-loop system stability to time delay. In this paper, the impact of time delays in both a fixed-gain and a gain-scheduled version of the controller is presented in terms of stability of nominal and off-nominal solutions.

Adaptive control of hypersonic vehicles in the presence of modeling uncertainties

This paper proposes an adaptive controller for a hypersonic cruise vehicle subject to aerodynamic uncertainties, center-of-gravity movements, actuator saturation, failures, and time-delays. The adaptive control architecture is based on a linearized model of the underlying rigid body dynamics and explicitly accommodates for all uncertainties. It also includes a baseline proportional integral filter commonly used in optimal control designs. The control design is validated using a high-fidelity HSV model that incorporates various effects including coupling between structural modes and aerodynamics, and thrust pitch coupling. An elaborate comparative analysis of the proposed Adaptive Robust Controller for Hypersonic Vehicles (ARCH) is carried out using a control verification methodology. In particular, we study the resilience of the controller to the uncertainties mentioned above for a set of closed-loop requirements that prevent excessive structural loading, poor tracking performance and engine stalls. This analysis enables the quantification of the improvements that result from using and adaptive controller for a typical maneuver in the V - h space under cruise conditions.

Brief Stochastic optimal control via Bellman's principle

This paper presents a strategy for finding optimal controls of non-linear systems subject to random excitations. The method is capable to generate global control solutions when state and control constraints are present. The solution is global in the sense that controls for all initial conditions in a region of the state space are obtained. The approach is based on Bellmans principle of optimality, the cumulant neglect closure method and the short-time Gaussian approximation. Problems with state-dependent diffusion terms, non-closeable hierarchies of moment equations for the states and singular state boundary condition are considered in the examples. The uncontrolled and controlled system responses are evaluated by creating a Markov chain with a control dependent transition probability matrix via the generalized cell mapping method. In all numerical examples, excellent controlled performances were obtained.

The NASA Langley Multidisciplinary Uncertainty Quantification Challenge

NASA missions often involve the development of new vehicles and systems that must be designed to operate in harsh domains with a wide array of operating conditions. These missions involve high-consequence and safety-critical systems for which quantitative data is either very sparse or prohibitively expensive to collect. Limited heritage data may exist, but is also usually sparse and may not be directly applicable to the system of interest, making uncertainty quantification extremely challenging. NASA modeling and simulation standards require estimates of uncertainty and descriptions of any processes used to obtain these estimates. The NASA Langley Research Center has developed an uncertainty quantification challenge problem in an effort to focus a community of researchers towards a common problem. This challenge problem features key issues in both uncertainty quantification and robust design using a discipline-independent formulation. While the formulation is indeed discipline-independent, the underlying model, as well as the requirements imposed upon it, describes a realistic aeronautics application. A few high-level details of this application are provided at the end of this document. Additional information is available at: http://uqtools.larc.nasa.gov/nda-uq-challenge-problem-2014/.

Reliability-Based Control Design for Uncertain Systems

This paper presents a robust control design methodology for systems with probabilistic parametric uncertainty. Control design is carried out by solving a reliability-based multi-objective optimization problem where the probability of violating design requirements is minimized. Simultaneously, failure domains are optimally enlarged to enable global improvements in the closed-loop performance. To enable an ecient numerical implementation, a hybrid approach for estimating reliability metrics is developed. This approach, which integrates deterministic sampling and asymptotic approximations, greatly reduces the numerical burden associated with complex probabilistic computations without compromising the accuracy of the results. Examples using output-feedback and full-state feedback with state estimation are used to demonstrate the ideas proposed.

Design and Verification of an Adaptive Controller for the Generic Transport Model

This paper focuses on the development, implementation, and verification of An Adaptive Control Technology for Safe Flight (ACTS). In particular, we design a controller for the Generic Transport Model (GTM) and evaluate the robustness improvements resulting from adaptation when various uncertainties and failures occur. The ACTS architecture consists of three major components (i) a baseline controller that provides satisfactory performance under nominal flying conditions, (ii) an adaptive controller that accommodates for anomalous flying conditions resulting from uncertainty and failure, and (iii) a nonlinear reference model customized according to the GTM dynamics. While the baseline controller uses anti-wind up devices and a control allocation scheme that correlates inputs of the same class, the adaptive controller accommodates for control saturation and integration wind-up without enforcing any allocation, thereby enabling the generation of independent inputs. The effectiveness of the ACTS controller is studied by evaluating its performance for a set of damages in the aircraft’s structure, and by carrying out control verification studies that evaluate the degradation in closed-loop performance resulting from failures of increasing levels of severity.

Optimal performance, robustness and reliability based designs of systems with structured uncertainty

This paper presents a study on the optimization of systems with structured uncertainty, whose inputs and outputs can be exhaustively described in the probabilistic sense. By propagating the uncertainty in the space of the probability density functions and the moments, optimization problems that pursue performance, robustness and reliability based designs are studied. Applications to static optimization and stability control are used to illustrate the relevance of incorporating uncertainty in the early stages of the design. Several examples that admit a full probabilistic description of the output in terms of the design variables and the statistics of the uncertain inputs are used to elucidate the features of the generic problem and its solution.

Analysis of Control Strategies for Aircraft Flight Upset Recovery

This paper proposes a framework for studying the ability of a control strategy, consisting of a control law and a command law, to recover an aircraft from ight conditions that may extend beyond the normal ight envelope. This study was carried out (i) by evaluating time responses of particular ight upsets, (ii) by evaluating local stability over an equilibrium manifold that included stall, and (iii) by bounding the set in the state space from where the vehicle can be safely own to wings-level ight. These states comprise what will be called the safely recoverable ight envelope (SRFE), which is a set containing the aircraft states from where a control strategy can safely stabilize the aircraft. By safe recovery it is implied that the tran- sient response stays between prescribed limits before converging to a steady horizontal ight. The calculation of the SRFE bounds yields the worst-case initial state corresponding to each control strategy. This information is used to compare alternative recovery strategies, determine their strengths and limitations, and identify the most e ective strategy. In regard to the control law, the authors developed feedback feedforward laws based on the gain scheduling of multivariable controllers. In regard to the command law, which is the mechanism governing the exogenous signals driving the feed- forward component of the controller, we developed laws with a feedback structure that combines local stability and transient response considera- tions. The upset recovery of the Generic Transport Model, a sub-scale twin-engine jet vehicle developed by NASA Langley Research Center, is used as a case study.

Design of an Adaptive Controller for a Remotely Operated Air Vehicle

This paper presents an augmented control architecture for safe flight. This architecture consists of a nominal controller that provides satisfactory performance under nominal flying conditions and a direct model reference adaptive controller that provides robustness to parametric uncertainty. The design, implementation, tuning, and robustness analysis procedures of both the nominal and augmented controllers are presented. The aim of these procedures, which encompass both theoretical and practical considerations, is to develop a controller suitable for flight. The architecture proposed is applied to the NASA generic transport model. This is a model of a transport aircraft for which both a dynamically scaled flight-test article and a high-fidelity simulation are available. A robustness analysis framework, which bounds the set of adverse flying conditions for which all closed-loop requirements are met, indicates some advantages and drawbacks of adaptation. The adverse conditions considered are grouped into four categories: aerodynamic uncertainties, structural damage, unknown time delays, and actuator failures. These failures include partial and total loss of control effectiveness, locked-in-place control surface deflections, and engine-out conditions. The requirements are fast pilot-command tracking, bounded structural loading, satisfactory transient response, bounded flight envelope, and satisfactory handling/riding qualities. A computational approach that integrates this robustness analysis framework and a design-optimization technique is proposed. This approach enables the systematic search for the controller’s parameters that yield the best robustness characteristics allowed by the control structure.

Nonlinear Dynamics of Aircraft Controller Characteristics Outside the Standard Flight Envelope

In this paper, the influence of the flight control system over the offnominal behavior of a remotely operated air vehicle is evaluated. Of particular interest is the departure/upset characteristics of the closed-loop system near and beyond stall. The study vehicle is the NASA Generic Transport Model, and both fixed-gain and gain-scheduled versions of a linear quadratic regulator controller with proportional and integral components are evaluated. Bifurcation analysis is used to characterize spiral and spin behavior of the aircraft in closed-loop form and yields an understanding of the underlying vehicle dynamics outside the standard flight envelope. The use of a “gain parameter” to scale the controller gains provides information on the sensitivity of stability to gain variation, along with tracking how the controller modifies the open-loop steady states. Hence, this provides a means of assessing the effectiveness of the controller and evaluating the upset tendencies of the aircraft.