Nathan Slegers

Aspects of Control for a Parafoil and Payload System

A parafoil controlled by parafoil brake dee ection offers a lightweight and space-efe cient control mechanism for autonomous placement of air-dropped payloads to specie ed ground coordinates. The work reported here investigates control issues for a parafoil and payload system with left and right parafoil brakes used as the control mechanism.Itisshownthatparafoilandpayloadsystemscanexhibittwobasicmodesoflateralcontrol,namely,roll and skidsteering.Thesetwo modesoflateralsteeringgeneratelateralresponseinoppositedirections.Forexample, a roll steer cone guration turns left when the right parafoil brake is activated, whereas a skid steer cone guration turnsright under the samecontrol input. In transition between roll and skid lateral steering, the lateral responseis zero, and the system becomes uncontrollable. Angle of incidence, canopy curvature of the parafoil, and magnitude of brake dee ections are important design parameters for a controllable parafoil and payload system and greatly effect control response, including whether the basic lateral control mode is roll or skid steering. It is shown how the steering mode switches when fundamental design parameters are altered and as the magnitude of the brake dee ection increases. The mode of directional control transitions toward roll steering as the canopy curvature decreases or the angle of incidence becomes more negative. The mode of directional control transitions away from the roll steering mode as the magnitude of the brake dee ection increases, and for “ large” brake dee ections most parafoils will always skid steer.

Model Predictive Control Of A Parafoil And Payload System

A model predictive control strategy is developed for an autonomous parafoil and payload system. Since the technique requires a linear dynamic model of the system, a reduced state linear model based on a nonlinear 6 degree-of-freedom parafoil and payload model is established and validated. In order to use the reduced state linear model for model predictive control the desired trajectory in the x-y plane is mapped into a desired heading angle using Lagrange interpolating polynomials. Flight test results demonstrate that this model predictive control strategy is a natural and effective method of achieving trajectory tracking in a parafoil and payload system.

Nonlinear Model Predictive Control Technique for Unmanned Air Vehicles

A nonlinear model predictive control strategy is developed and subsequently specialized to autonomous aircraft that can be adequately modeled with a rigid 6-degrees-of-freedom representation. Whereas the general air vehicle dynamic equations are nonlinear and nonaffine in control, a closed-form solution for the optimal control input is enabledby expandingboth the output and control in a truncatedTaylor series. The closed-form solution for control is relatively simple to calculate and well suited to the real time embedded computing environment. An interesting feature of this control law is that the number of Taylor series expansion terms can be used to indirectly penalize control action. Also, ill conditioning in the optimal control gain equation limits practical selection of the number of Taylor series expansion terms. These claims are substantiated through simulation by application of the method to a parafoil and payload aircraft as well as a glider.

Use of Variable Incidence Angle for Glide Slope Control of Autonomous Parafoils

Strictly speaking, most autonomous parafoil and payload systems possess only lateral control, achieved by right and left parafoil brake deflection. An innovative technique to achieve direct longitudinal control through incidence angle changes is reported. Addition of this extra control channel requires simple rigging changes and an additional servoactuator. The ability of incidence angle to alter the glide slope of a parafoil and payload aircraft is demonstrated through a flight-test program with a microparafoil system. Results from the flight-test program are synthesized and integrated into a six degree-of-freedom simulation. The simulation model is subsequently used to assess the utility of glide slope control to improve autonomous flight control system performance. Through Monte Carlo simulation, impact point statistics with and without glide slope control indicate that dramatic improvements in impact point statistics are possible using direct glide slope control.

Development and Testing of the Miniature Aerial Delivery System Snowflake

Proceedings of the 20th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar, Seattle, WA, May 4-7, 2009.

Optimal Control for Terminal Guidance of Autonomous Parafoils

This paper deals with the development of guidance, navigation and control algorithms for a prototype of a miniature aerial delivery system capable of high-precision maneuvering and high touchdown accuracy. High accuracy enables use in precision troop resupply, sensor placement, urban warfare reconnaissance, and other similar operations. Specifically, this paper addresses the terminal phase, where uncertainties in winds cause most of the problems. The paper develops a six degree-offreedom model to adequately address dynamics and kinematics of the prototype delivery system and then reduces it to a two degrees-of-freedom model to develop a model predictive control algorithm for reference trajectory tracking during all stages. Reference trajectories are developed in the inertial coordinate frame associated with the target. The reference trajectory during terminal guidance, just prior to impact, is especially important to the final accuracy of the system. This paper explores an approach for generating reference trajectories based on the inverse dynamics in the virtual domain. The method results in efficient solution of a two-point boundary-value problem onboard the aerial delivery system allowing the trajectory to be generated at a high rate, mitigating effects of the unknown winds. This paper provides derivation of the guidance and control algorithms and present analysis through simulation.

Effects of Canopy-Payload Relative Motion on Control of Autonomous Parafoils

An eight-degree-of-freedom model is developed that accurately models relative pitching and yawing motion of a payload with respect to a parafoil. Constraint forces and moments are found analytically rather than using artificial constraint stabilization. A turn rate controller common in precision placement algorithms is used to demonstrate that relative yawing motion of the payload can result in persistent oscillations of the system. A model neglecting relative payload yawing failed to predict the same oscillations. It is shown that persistent oscillations can be eliminated by reduction of feedback gains; however, resulting tracking performance is poor. A reduced-order linear model is shown to be able to adequately predict relative payload dynamics for the proposed turn rate controller on the full eight-degree-of-freedom system.

Terminal Guidance of Autonomous Parafoils in High Wind-To-Airspeed Ratios

Autonomous precision placement of parafoils is challenging because of their limited control authority and sensitivity to winds. In particular, when wind speed is near the airspeed, guidance is further complicated by the parafoils inability to penetrate the wind. This article speci-fically addresses the terminal phase and develops an approach for generating optimal trajectories in real-time based on the inverse dynamics in the virtual domain. The method results in efficient solution of a two-point boundary-value problem using only a single optimization parameter allowing the trajectory to be generated at a high rate, mitigating effects of the unknown winds. It is shown through simulation and experimental results that the proposed algorithm works well even in strong winds and is robust to sensor errors and wind uncertainty.

Specialized System Identification for Parafoil and Payload Systems

There are a number of peculiar aspects to parafoil and payload systems thatmake it difficult to apply conventional system identification procedures used for aerospace systems. Parafoil and payload systems are unique because typically there is very little sensor information available, the sensors that are available are separated from the canopy by a complex network of flexible rigging, the systems are very sensitive to wind and turbulence, the systems exhibit a number of nonlinear behaviors, and the systems exhibit a high degree of variability from flight to flight. The current work describes a robust system identification procedure developed to address the specific difficulties posed by airdrop systems. By employing a two-phase approach that separately considers atmospheric winds estimation and aerodynamic coefficient estimation, a nonlinear, 6-degree-of-freedom dynamic simulation model is generated using only Global Positioning System data from the flight test. The key to this approach is the use of a simplified aerodynamic representation of the canopy, which requires identification of only the steady-state response to control input to completely define the dynamic model. The proposed procedure is demonstrated by creating a simulation model usingGlobal Positioning Systemdata fromactual flight tests. To validate the procedure, the dynamic response of the simulationmodel is then compared to inertial measurement unit data that were not used in any way to develop the simulation model, with excellent results.

On the Benefits of In-Flight System Identification for Autonomous Airdrop Systems

A unique feature of airdrop systems is the inherent and large variability in flight dynamic characteristics. The same physical article dropped on two different occasions will exhibit significantly different dynamic response. The problem only becomes worse for different test articles. Control systems for autonomous airdrop systems explicitly or implicitly assume knowledge of the flight dynamic characteristics in some way, shape, or form. A question facing autonomous airdrop designers is whether to use precomputed dynamic characteristics inside the control law, or to compute the needed flight dynamic characteristics in-flight and subsequently employ them in the control law. This paper establishes conditions when in-flight identified characteristics, with a focus on the turn rate dynamics, should be used, and when it is better to use precomputed results. It is shown that with expected levels of system variability, sensor noise, and atmospheric wind, in-flight identification generally produces significantly more accurate dynamic behavior of the lateral dynamics than a precomputed model of the nominal system, even when the in-flight identification is performed with highly inaccurate sensor data. The only exception to this rule observed in this work is the situation where atmospheric winds are high and a direct heading measurement is not available. In this situation, a precomputed estimate of the time constant of the lateral dynamics is more accurate than an in-flight estimate. These conclusions are reached though a comprehensive simulation study using a validated airdrop flight dynamic model applied to both a small and large parafoil.