James M. Buffington

Robust multivariable flight control

Manual flight control system design for fighter aircraft is one of the most demanding problems in automatic control. Fighter Aircraft dynamics generally have highly coupled uncertain and nonlinear dynamics. Multivariable control design techniques offer a solution to this problem. Robust Multivariable Flight Control provides the background, theory and examples for full envelope manual flight control system design. It gives a versatile framework for the application of advanced multivariable control theory to aircraft control problems. Two design case studies are presented for the manual flight control of lateral/directional axes of the VISTA-F-16 test vehicle and an F-18 trust vectoring system. They demonstrate the interplay between theory and the physical features of the systems.

Lyapunov Stability Analysis of Daisy Chain Control Allocation

A demonstration that feedback control of systems with redundant controls can be reduced to feedback control of systems without redundant controls and control allocation is presented. It is shown that control allocation can introduce unstable zero dynamics into the system, which is important if input/output inversion control techniques are utilized. The daisy chain control allocation technique for systems with redundant groups of controls is also presented. Sufficient conditions are given to ensure that the daisy chain control allocation does not introduce unstable zero dynamics into the system. Aircraft flight control examples are given to demonstrate the derived results.

Multiple Timescale Flight Control Using Reconfigurable Sliding Modes

Adual-timescaleaircrafte ight-controlproblemisaddressedviacontinuoussliding-modecontrol.Sliding-surface boundary-layer recone guration is used to account for actuator dynamics, dee ection limits, and rate limits. A recone gurable sliding-mode e ight controller is designed that achieves robust, high-accuracy tracking of outerloop command angles before and after damage to an aircraft. Angular rate commands are robustly tracked in an inner loop. The recone gurable e ight-control strategy is based on a continuous sliding-mode controller with direct boundary-layer adaptation for recone guration. On-line explicit system or damage identie cation is not required. Therecone gurablesliding-modee ight-controltechniqueisapplied to anonlineare ight-dynamicsmodelofan F-16 aircraft. Computer simulations demonstrate stability and high-accuracy tracking performance without violation of actuator limits.

Design of Nonlinear Control Laws for High-Angle-of-Attack Flight

High-angle-of-attack flight control laws are developed for a supermaneuvera ble fighter aircraft. The methods of dynamic inversion and structured singular value synthesis are combined into an approach which addresses both the nonlinearity and robustness problems of flight at extreme operating conditions. The primary purpose of the dynamic inversion control elements is to linearize the vehicle response across the flight envelope. Structured singular value synthesis is used to design a dynamic controller which provides robust tracking to pilot commands. The resulting control system achieves desired flying qualities and guarantees a large margin of robustness to uncertainties for high-angle-of-attack flight conditions. High-fidelity nonlinear simulation results show that the combined dynamic inversion /structured singular value synthesis control law achieves a high level of performance in a realistic environment.

Multiple time scale flight control using re-configurable sliding modes

A dual time scale aircraft flight control problem is addressed via sliding mode control theory. Actuator dynamics, deflection limits and rate limits are addressed via sliding mode controller re-configuration. The designed re-configurable sliding mode controller achieves a robust, high accuracy tracking performance to mission angles (such as attack, sideslip and roll angles) in an outer loop before and after damage to an aircraft. Angular rate commands are robustly tracked in an inner loop. The developed re-configurable control strategy is based on a continuous sliding mode controller with a boundary layer re-configuration. Online explicit damage identification is not required. A re-configurable sliding mode controller is designed for an F-16 jet fighter with a nonlinear flight dynamics model. Computer simulations demonstrate high accuracy tracking performance. Actuators deflection and deflection rate do not saturate.

Integration of On-line System Identification and Optimization-based Control Allocation*

An algorithm is presented for the identification of distributed aircraft control derivatives, in real-time. A distributed effector suite is a primary characteristic of tailless aircraft, since the requisite yaw power must come from several sources. The most commonly used control allocation algorithms correlate the effectors to such an extent that valid estimation is not possible. This paper addresses the effector identification problem by including decorrelating excitation into the control allocation cost function while still satisfying the desired control moment, and therefore does not introduce any residual perturbations into the motion variables. Results are shown for a stability axis roll maneuver with the control derivatives being identified for five differential lateral directional effectors.

Control Allocation and Zero Dynamics

Closed-loop stability for dynamic inversion controllers depends on the stability of the zero dynamics. The zero dynamics, however, depend on a generally nonlinear control allocation function that optimally distributes redundant controls. Therefore, closed-loop stability depends on the control allocation function. A sufe cient condition is provided for globally asymptotically stable zero dynamics with a class of admissible nonlinear control allocation functions. It is shown that many common control allocation functions belong to the class of functions that are covered by the aforementioned zero dynamics stability condition. Aircraft e ight control examples are given to demonstrate the utility of the results.

Continuous sliding mode control

We consider a continuous sliding mode controller as a specific type of a sliding mode controller, but not as implementation of a discontinuous high frequency switching control law. Using the finite-time differential equations analysis, a robust continuous sliding mode controller, which drives the systems trajectory to a sliding surface in a finite time, is designed. Computer simulations confirmed the theoretical results.

Anti-windup for an F-16's daisy chain control allocator

A recently developed anti-windup scheme is used to eliminate oscillations and instability that arise when a daisy chain control allocation scheme is applied to a linear, short period, high angle of attack model of the VISTA/MATV F-16 with magnitude and rate limits on the inputs.

Robust longitudinal axis flight control for an aircraft with thrust vectoring

A full conventional envelope longitudinal axis control design is presented for a fighter aircraft capable of thrust vectoring. An inner-outer loop modular control structure is used to provide good flying quantities in the presence of highly structured uncertainty across a wide flight envelope. Simple, low-order control laws are designed for a version of an F-18 aircraft model augmented with thrust vectoring nozzles. A minimal-order H∞ design algorithm is used to aid in the design of an inner loop equalization controller. Structured singular value synthesis is used to design outer loop implicit model-following controllers. Different control laws are found for high and low dynamic pressure conditions, and controller commands are blended for a small region of dynamic pressure. Daisy-chaining is used to blend elevator and thrust vectoring commands. Structured singular values are used to analyze stability robustness to structured parametric uncertainty, actuator and sensor unmodeled dynamics, and structured uncertainty corresponding to controller blending. A nonlinear simulation is used to show that the aircraft performs well across the flight envelope during outer loop controller blending and thrust vectoring actuation.