Aaron J. Ostroff

RECONFIGURABLE FLIGHT CONTROL USING NONLINEAR DYNAMIC INVERSION WITH A SPECIAL ACCELEROMETER IMPLEMENTATION

This paper presents an approach to on-line control design for aircraft that have suffered either actuator failure, missing effector surfaces, surface damage, or any combination. The approach is based on a modified version of nonlinear dynamic inversion. The approach does not require a model of the baseline vehicle (effectors at zero deflection), but does require feedback of accelerations and effector positions. Implementation issues are addressed and the method is demonstrated on an advanced tailless aircraft. An experimental simulation analysis tool is used to directly evaluate the nonlinear systems stability robustness.

Techniques for Accommodating Control Effector Failures on a Mildly Statically Unstable Airplane

Commercial airplanes are becoming increasingly more sophisticated, placing an increasing burden on pilots to detect and resolve the exhaustive set of possible control effector failures. Automatic techniques are needed to either reconfigure an existing control law or restructure a new control law after failure. A discrete control law has been designed for the longitudinal channel of a mildly statically unstable commercial airplane, to track the glideslope during the approach to landing phase of flight. Single effector failures with time delays for failure detection and identification are analyzed for both the reconfigured and restructured control laws, and results are compared with those from previous research using a statically stable airplane. Strategies considered include reconfiguration and restructuring with new flight conditions. Validation of all cases is made using a 6 DOF nonlinear airplane simulation.

Reconfigurable NDI controller using inertial sensor failure detection & isolation

A modified derivation of nonlinear dynamic inversion provides the theoretical underpinnings for a reconfigurable control law for aircraft that have suffered combinations of actuator failures, missing effector surfaces, and aerodynamic changes. The approach makes use of acceleration feedback to extract information pertaining to any aerodynamic change and thus does not require a complete aerodynamic model of the aircraft. The control law does require feedback of effector positions to accommodate actuator dynamics. Both accelerometer and rate gyro failure detection and isolation (FDI) systems are implemented, allowing up to three independent failures for each FDI system as long as they are in different axes. Nonlinear simulation results show that the FDI systems improve the robustness to accelerometer/rate gyro uncertainties. An advanced tailless aircraft model is used to demonstrate the concepts. The simulation includes accelerometer and rate gyro noise and bias, failures due to accelerometers, rate gyros, and actuators, and modeled missing surfaces that cause airplane aerodynamic changes.

Force and Moment Approach for Achievable Dynamics Using Nonlinear Dynamic Inversion

This paper describes a general form of nonlinear dynamic inversion control for use in a generic nonlinear simulation to evaluate candidate augmented aircraft dynamics. The implementation is specifically tailored to the task of quickly assessing an aircraft’s control power requirements and defining the achievable dynamic set. The achievable set is evaluated while undergoing complex mission maneuvers, and perfect tracking will be accomplished when the desired dynamics are achievable. Variables are extracted directly from the simulation model each iteration, so robustness is not an issue. Included in this paper is a description of the implementation of the forces and moments from simulation variables, the calculation of control effectiveness coefficients, methods for implementing different types of aerodynamic and thrust vectoring controls, adjustments for control effector failmes, and the allocation approach used. A few examples illustrate the perfect tracking results obtained.

Enhanced NDI strategies for reconfigurable flight control

A recently proposed method of on-line control design for aircraft reconfiguration is modified to mitigate the effects of effector rate/position saturation and sensor noise in critical measurements while preserving some, perhaps reduced, level of flying qualities. The on-line control design, based on an incremental version of nonlinear dynamic inversion, does not require a complete aerodynamic model of the aircraft, but does require the local control derivatives along with feedback of accelerations and effector positions. Recovery from a variety of failure (stuck or missing effectors) is possible under the original design as long as the working effectors do not enter saturation for extended periods and critical measurements are relatively noise free - an unlikely situation. Here, an improved control allocator minimizes both effector rate and position, utilizing a multi-pass strategy to restore lost control power due to saturation using the remaining unsaturated controls. Command model flying parameters are adaptively manipulated online to comply with reduced levels of control power further reducing saturation. A classically designed compensator placed around each actuator underpins strategy to reduce jitter due to sensor noise in the control variable responses while preserving decoupling of original control. Improvements due to these modifications are demonstrated on an advanced tailless fighter.

Nonlinear dynamic inversion reconfigurable controller utilizing a fault tolerant accelerometer

A modified derivation of nonlinear dynamic inversion provides the theoretical underpinnings of a reconfigurable control law for aircraft that have suffered combinations of actuator failures, missing effector surfaces, and surface damage. The approach does not require a model of the baseline vehicle, but does require feedback of effector positions to accommodate actuator dynamics and accelerations, which contain information pertaining to any aerodynamic change. An accelerometer failure detection and isolation (FDI) system is implemented, allowing up to three independent failures as long as they are in different axes. Experimental results show that the FDI system also improves the robustness to accelerometer uncertainties. An advanced tailless aircraft is used to demonstrate the concepts. The simulation includes accelerometer noise and bias as well as accelerometer and actuator failures.

Figure control of flexible structures: Optimal surfaces of thin deformable primary

Application of a modal control design technique to achieve discrete control of distributed parameter systems is considered. Results are presented for application of the design technique to achieve diffraction limited performance from the primary mirror of a space telescope and to provide flutter suppression for an aircraft wing.