Daniel Moerder

An Overview of the NASA Flying Test Platform Research

A methodology for improving attitude stability and control for low-speed and hovering air vehicle is under development. In addition to aerodynamically induced control forces such as vector thrusting, the new approach exploits the use of bias momenta and torque actuators, similar to a class of spacecraft system, for its guidance and control needs. This approach will be validated on a free-flying research platform under development at NASA Langley Research Center. More broadly, this platform also serves as an in-house testbed for research in new technologies aimed at improving guidance and control of a Vertical Take-Off and Landing (VTOL) vehicle. 1 Research Motivation This paper gives an overview of the ongoing research in precision guidance and robust control based on the NASA Flying Test Platform (NFTP) research vehicle currently under development at NASA Langley Research Center. The research is motivated by core GN&C objectives that include optimal guidance and navigation, and robust attitude and position stabilization under uncertain exogenous disturbances and model variations. A key goal of this research is to investigate novel technologies to improve attitude stability particularly during hovering or at low airspeed flight wherein conventional control effectors become ineffective. This particular need arises from current limitations in attitude stabilization performance for ∗Senior Research Engineer, Guidance and Controls Branch, k.b.lim@larc.nasa.gov †Staff Scientist, NIA, j.y.shin@larc.nasa.gov ‡Senior Research Engineer, Systems Integration Branch, e.g.cooper@larc.nasa.gov §Senior Research Engineer, Guidance and Controls Branch, d.d.moerder@larc.nasa.gov ¶Research Engineer, Guidance and Controls Branch, t.h.khong@larc.nasa.gov ‖Technician, Guidance and Controls Branch, m.f.smith@larc.nasa.gov vector-thrusted air vehicles such as Osprey, Harrier VTOL, and for helicopters with sling loads. The basic and common limitation in the above control problem appears to be a lack of an accurate dynamical model suited for response prediction and controller design to attain precision and reliable performance under unsteady aerodynamics. In retrospect, this apparent performance limitation in the use of a vector-thrusting approach for dynamical stabilization of the vehicle is not surprising since stability is fundamentally an unsteady aerodynamics phenomenon. This phenomenon is the current limiting factor in predicting loads and responses on the vehicle (see for example, [1], [2]), which are necessary ingredients for robust and precise stabilizing feedback control. 2 Test Platform Description 2.1 System Configuration Figure 1 shows the NFTP system, which consists of a square rigid platform levitated and propelled by a set of four battery-powered ducted fans each with a pair of control vanes. The vehicle employs a sensing system that fuses Inertial Measurement Unit (IMU) sensors and an optically based 6-DOF target tracking inertial position/attitude sensing system. The rigid platform is about 1.2 meters wide and weighs about 12 kg. The NFTP is a free flying vehicle designed to fly within a flight envelope box which is approximately 5 meters wide, indoors. Figure 2 is a schematic of the hardware architecture and major components for the basic NFTP system. The PC104 is used as the onboard flight control computer, which will implement an inner-loop controller for stability augmentation and has the capability of a wireless datalink to a ground control computer. A dSPACE system is used as the ground computer whose primary function is guidance from an operator, data logging, and communication with a vision system which tracks the flying vehicle. The flight control system will be capable of using all sensor measurements which include inertial measure-

Variable Speed CMG Control of a Dual-Spin Stabilized Unconventional VTOL Air Vehicle

This paper describes an approach based on using both bias momentum and multiple control moment gyros for controlling the attitude of statically unstable thrustlevitated vehicles in hover or slow translation. The stabilization approach described in this paper uses these internal angular momentum transfer devices for stability, augmented by thrust vectoring for trim and other “outer loop” control functions, including CMG stabilization/desaturation under persistent externanl disturbances. Simulation results show the feasibility of (1) improved vehicle performance beyond bias momentum assisted vector thrusting control, and (2) using control moment gyros to significantly reduce the external torque required from the vector thrusting machinery.

Bias Momentum Sizing for Hovering Dual-Spin Platforms

Abstract An atmospheric flight vehicle in hover is typically controlled by varying its thrustvector. Achieving both levitation and attitude control with the propulsion systemplaces considerable demands on it for agility and precision, particularly if the vehicleis statically unstable, or nearly so. These demands can be relaxed by introducingan appropriately sized angular momentum bias aligned with the vehicle’s yaw axis,thus providing an additional margin of attitude stability about the roll and pitchaxes. This paper describes a methodical approach for trading off angular momen-tum bias level needed with desired levels of vehicle response due to the designdisturbance environment given a vehicle’s physical parameters. It also describesseveral simplifications that provide a more physical and intuitive understanding ofdual-spin dynamics for hovering atmospheric vehicles. This approach also mitigatesthe need for control torques and inadvertent actuator saturation difficulties in try-ing to stabilize a vehicle via control torques produced by unsteady aerodynamics,thrust vectoring, and unsteady throttling. Simulation results, based on a subscalelaboratory test flying platform, demonstrate significant improvements in the atti-tude control robustness of the vehicle with respect to both wind disturbances andoff-center of gravity payload changes during flight.1

Control Laws for a Dual-Spin Stabilized Platform

This paper describes two attitude control laws suitable for atmospheric flight vehicles with a steady angular momentum bias in the vehicle yaw axis. This bias is assumed to be provided by an internal flywheel, and is introduced to enhance roll and pitch stiffness. The first control law is based on Lyapunov stability theory, and stability proofs are given. The second control law, which assumes that the angular momentum bias is large, is based on a classical PID control. It is shown that the large yaw-axis bias requires that the PI feedback component on the roll and pitch angle errors be cross-fed. Both control laws are applied to a vehicle simulation in the presence of disturbances for several values of yaw-axis angular momentum bias. It is seen that both control laws provide a significant improvement in attitude performance when the bias is sufficiently large, but the nonlinear control law is also able to provide improved performance for a small value of bias. This is important because the smaller bias corresponds to a smaller requirement for mass to be dedicated to the flywheel.

Toward n-Ship Computation of Trajectories for Shared Airspace

This paper considers an approach for modelling transport aircraft trajectories that can facilitate their rapid evaluation and modification, either en route or in terminal control areas, with the goal of efficiently making use of airspace and runways by a large population of vehicles without pairwise violation of separation criteria.