David O. Sigthorsson

Robust Linear Output Feedback Control of an Airbreathing Hypersonic Vehicle

This paper addresses issues related to robust output-feedback control for a model of an airbreathing hypersonic vehicle. The control objective is to provide robust velocity and altitude tracking in the presence of model uncertainties and varying flight conditions, using only limited state information. A baseline control design based on a robust full-order observer is shown to provide, in nonlinear simulations, insufficient robustness with respect to variations of the vehicle dynamics due to fuel consumption. An alternative approach to robust output-feedback design, which does not employ state estimation, is presented and shown to provide an increased level of performance. The proposed methodology reposes upon robust servomechanism theory and makes use of a novel internal model design. Robust compensation of the unstable zero dynamics of the plant is achieved by using measurements of pitch rate. The selection of the plants output map by sensor placement is an integral part of the control design procedures, accomplished by preserving certain system structures that are favorable for robust control design. The performance of each controller is comparatively evaluated by means of simulations of a full nonlinear model of the vehicle dynamics and is tested on a given range of operating conditions.

Wingbeat Shape Modulation for Flapping-Wing Micro-Air-Vehicle Control During Hover

Abstract : A new method of controlling a flapping-wing micro air vehicle by varying the velocity profiles of the wing strokes is presented in this manuscript. An exhaustive theoretical analysis along with simulation results show that this new method, called split-cycle constantperiod frequency modulation, is capable of providing independent control over vertical and horizontal body forces as well as rolling and yawing moments using only two physical actuators, whose oscillatory motion is defined by four parameters. An actuated bobweight is introduced to enable independent control of pitching moment. A general method for deriving sensitivities of cycle-averaged forces and moments to changes in wingbeat kinematic parameters is provided, followed by an analytical treatment for a case where the angle of attack of each wing is passively regulated and the motion of the wing spar in the stroke plane is driven by a split-cycle waveform. These sensitivities are used in the formulation of a cycle-averaged control law that successfully stabilizes and controls two different simulation models of the aircraft. One simulation model is driven by instantaneous aerodynamic forces derived from bladeelement theory, while the other is driven by an empirical representation of an unsteady aerodynamic model that was derived from experiments.

Reference Command Tracking for a Linearized Model of an Air-Breathing Hypersonic Vehicle

The focus of this paper is on control design and simulation for an air-breathing hypersonic vehicle. The challenges for control design in this class of vehicles lie in the inherent coupling between the propulsion system, and the airframe dynamics, and the presence of strong exibilit y eects. Working from a highly nonlinear, dynamically-coupled simulation model, control designs are presented for velocity, angle-of-attack, and altitude command input tracking for a linearized version of a generic air-breathing hypersonic vehicle model linearized about a specic trim condition. Control inputs for this study include elevator deection, total temperature change across the combustor, and the diuser area ratio. Two control design methods are presented, both using linear quadratic techniques with integral augmentation, and are implemented in tracking control studies. The rst approach focuses on setpoint tracking control, whereas in the second, a regulator design approach is taken. The eectiv eness of each control design is demonstrated in simulation on the full nonlinear model of the generic vehicle.

Dynamics and Control of a Biomimetic Vehicle Using Biased Wingbeat Forcing Functions

A wingbeat forcing function and control method are presented that allow six-degree-of-freedom control of a flapping-wing micro air vehicle using only two actuators, each of which independently actuate a wing. Split-cycle constant-period frequency modulation with wing bias is used to produce nonzero cycle-averaged drag. The wing bias provides pitching-moment control and, when coupled with split-cycle constant-period frequency modulation, requires only independently actuated wings to enable six-degree-of-freedom flight. Wing bias shifts the cycle-averaged center-of-pressure locations of the wings, thus providing the ability to pitch the vehicle. Implementation of the wing bias is discussed, and modifications to the wingbeat forcing function are made to maintain wing position continuity. Instantaneous and cycle-averaged forces and moments are computed, cycle-averaged control derivatives are calculated, and a controller is developed. The controller is designed using a simplified aerodynamic model derived with blade-element theory and cycle averaging. The controller is tested using a simulation that includes blade-element-based estimates of the instantaneous aerodynamic forces and moments that are generated by the combined motion of the rigid-body fuselage and the flapping wings. Simulations using this higher-fidelity model indicate that the cycle-averaged blade-element-based controller is capable of achieving controlled flight.

Dynamics and Control of a Minimally Actuated Biomimetic Vehicle: Part II - Control

A control strategy is proposed for a minimally-actuated flapping-wing micro air-vehicle (FWMAV). The proposed vehicle is similar to the Harvard RoboFly that accomplished the first takeoff of an insect scale flapping wing aircraft, except that it is equipped with independently actuated wings and the vehicle center-of-gravity can be manipulated for control purposes. Using the results from the derivation of the aerodynamic forces and moments from Part I, a control allocation strategy and a feedback control law are designed that enables the vehicle to achieve untethered, stabilized flight about a hover condition. The control laws are designed to make use of three actuators, two of which control the angular position of the wing in the stroke plane, and one that moves a bob-weight that manipulates the vehicle center-of-gravity. The Split-Cycle Constant-Period Frequency Modulation technique, introduced in Part I, is used to allow each wing to generate nonzero cycle-averaged rolling and yawing moments. The technique achieves this objective by varying the frequency of the oscillators, that drive each wing throughout the wing-beat cycles, such that the dynamic pressure acting on each wing during the upstroke is different from that which acts on the

Dynamics and Control of a Minimally Actuated Biomimetic Vehicle: Part I - Aerodynamic Model

An aerodynamic model for the forces and moments acting on a minimally actuated flapping wing micro air vehicle (FWMAV) are derived from blade element theory. The proposed vehicle is similar to the Harvard RoboFly that accomplished the first takeoff of an insect scale flapping wing aircraft, except that it is equipped with independently actuated wings and the vehicle center-of-gravity can be manipulated for control purposes. Using a blade element-based approach, both instantaneous and cycle-averaged forces and moments are computed for a specific type of wing beat motion that enables nearly decoupled, multi-degree-of-freedom control of the aircraft. The wing positions are controlled using oscillators whose frequencies change once per wing beat cycle. A new technique is introduced, called Split-Cycle Constant-Period Frequency Modulation, that has the desirable property of providing a high level of control input decoupling for vehicles without active angle-of-attack control. Like the RoboFly, the wing angle-ofattack variation is passive by design, and is a function of the instantaneous angular velocity of the wing in the stroke plane. A control-oriented dynamic model of the vehicle is derived, which is based on a cycle-averaged � Senior Aerospace Engineer, Control Design and Analysis Branch, 2210 Eighth Street, Ste. 21, Air Force

Tracking Control For An Overactuated Hypersonic Air-Breathing Vehicle With Steady State Constraints

The development of an air-breathing hypersonic vehicle employing scramjet propulsion is an ongoing research endeavor. Because of high velocity (>Mach 5), length and positioning of the engine, and relative sleekness, the flexibility of the vehicle is significant and there are strong couplings between thrust and pitch . As a result, control design for such a vehicle is a challenge. In previous works, linear controllers have been designed for a model of the longitudinal dynamics of a specific air-breathing vehicle possessing the same number of inputs and outputs. In this paper we consider a control design for the same vehicle model, but we restrict our attention to controlling only two outputs, namely the altitude and velocity, while we employ as control inputs, the elevator deflection, total temperature change across the combustor and the diffuser area ratio of the combustor. The specific control problem addressed in the paper is the design of a controller that ensures asymptotic tracking of altitude and velocity reference trajectories, while using the redundancy in the inputs to optimize the performance in steady-state. As a matter of fact, since the system is not square, the steady state solutions that enforce perfect tracking are nonunique. The controller employs a parameterization of all possible steady state trajectories that is used for optimization of the steady state input while providing perfect tracking and fulfilment of constraint on the magnitude of the control input. Simulations results are provided to validate the proposed approach.

Output feedback control and sensor placement for a hypersonic vehicle model

This paper addresses issues related to output feedback control, including sensor placement, for a model of an air-breathing hypersonic vehicle. The model presents a number of control challenges, in particular because of strong couplings between the propulsive and aerodynamic forces. Because of the vehicle’s low weight, slenderness, and length, the vehicle’s flexibility has a large impact on stability and control of the vehicle. Two output feedback control methods are developed. One applies reconstruction of the flexible body system states, toward applications of state feedback control. The other uses a robust design that does not rely on an observer to ensure stabilization and performance throughout a given flight envelope. A rate gyroscope and an accelerometer have been modeled, incorporating the flexible effects, and strategies for sensor placement have been developed for the hypersonic vehicle model to enhance observability or to preserve certain system structures that are favorable for robust control design. Simulation results are provided to demonstrate the sensor placement strategies and output feedback control performances.

Dynamics and Control of a Biomimetic Vehicle Using Biased Wingbeat Forcing Functions: Part II - Controller

A control strategy is proposed for a minimally-actuated flapping-wing micro air-vehicle (FWMAV). The proposed vehicle is similar to the Harvard RoboFly that accomplished the first takeoff of an insect scale flapping wing aircraft, except that it is equipped with independently actuated wings. Using the derivation of the aerodynamic forces and moments from Part I, a control allocation strategy and a feedback control law are designed that enable the vehicle to achieve untethered, stabilized flight about a hover condition. Six degree-of-freedom maneuvers near hover are demonstrated as well. The control laws are designed to make use of two actuators that control the angular position of the wing in the stroke plane. The SplitCycle Constant-Period Frequency Modulation with Wing Bias technique, introduced in Part I, is used to allow each wing to generate non-zero cycleaveraged aerodynamic forces and moments. This technique modifies the frequencies of the up and down strokes to yield non-zero cycle-averaged drag due to the flapping motion of a wing. Additionally, the midpoint of the wingbeat profile can be modified by use of a wing bias. The bias is introduced to primarily provide pitching moment control. In this work, the sensitivities of cycle-averaged forces and moments with respect to the

Dynamics and Control of a Biomimetic Vehicle Using Biased Wingbeat Forcing Functions: Part 1 - Aerodynamic Model (Postprint)

Abstract : An aerodynamic model, for a minimally actuated flapping wing micro air vehicle (FWMAV), is derived from blade element theory. The vehicle considered in this work is similar to the Harvard RoboFly, except that it is equipped with independently actuated wings. A blade element-based approach is used to compute both instantaneous and cycle-averaged forces and moments for a specific type of wingbeat motion that enables nearly decoupled, multi-degree-of-freedom control of the aircraft. The wing positions are controlled using oscillators whose frequencies change once per wingbeat cycle. A technique is introduced, called Split-Cycle Constant-Period Frequency Modulation with Wing Bias, that provides a high level of control input decoupling for vehicles without active angle of attack control. This technique allows the frequencies of the upstroke and downstroke of each wing to differ such that non-zero cycleaveraged drag can be generated. Additionally, a wing bias term has been added to the wingbeat waveform and is utilized to provide pitching moment control. With this technique, it is possible to achieve five degree-of-freedom control using only two physical actuators. The present paper is concerned with the derivation of the instantaneous and cycle-averaged forces and moments for the Split- Cycle Constant-Period Frequency Modulation with Wing Bias technique. Implementation of the wing bias is discussed and modifications to the wingbeat forcing function, which are necessary to maintain a continuous wing position, are made.