Eric W. Frew

Coordinated Standoff Tracking of Moving Targets Using Lyapunov Guidance Vector Fields

This paper presents a control structure for cooperative stand-off line-of-sight tracking of a moving target by a team of unmanned aircraft based on a Lyapunov guidance vector field that produces stable convergence to a circling limit cycle behavior. A guidance vector field is designed for a stationary target and then modified with a correction term that accounts for a moving target and constant background wind. Cooperative tracking by multiple unmanned aircraft is achieved through additional phasing, also with a Lyapunov stability analysis. Convoy protection, in which the unmanned aircraft must scout an area ahead of the moving target, is performed by extending the cooperative stand-offline-of-sight limit cycle attractor along the direction of travel. Simulation results demonstrate the behavior of the algorithms as well as the improvement that results from cooperation. Finally, simulations of a larger cooperative search, acquisition, and tracking scenario are described that illustrate the use of the cooperative standoff line-of-sight and convoy protection controllers in a realistic application.

Vision-based road-following using a small autonomous aircraft

This paper describes the vision-based control of a small autonomous aircraft following a road. The computer vision system detects natural features of the scene and tracks the roadway in order to determine relative yaw and lateral displacement between the aircraft and the road. Using only the vision measurements and onboard inertial sensors, a control strategy stabilizes the aircraft and follows the road. The road detection and aircraft control strategies have been verified by hardware in the loop (HIL) simulations over long stretches (several kilometers) of straight roads and in conditions of up to 5 m/s of prevailing wind. Hardware experiments have also been conducted using a modified radio-controlled aircraft. Successful road following was demonstrated over an airfield runway under variable lighting and wind conditions. The development of vision-based control strategies for unmanned aerial vehicles (UAVs), such as the ones presented here, enables complex autonomous missions in environments where typical navigation sensor like GPS are unavailable.

Airborne Communication Networks for Small Unmanned Aircraft Systems

This paper explores the role of meshed airborne communication networks in the operational performance of small unmanned aircraft systems. Small unmanned aircraft systems have the potential to create new applications and markets in civil domains, enable many disruptive technologies, and put considerable stress on air traffic control systems. We argue that of the existing networked communication architectures, only meshed ad hoc networking can meet the communication demands for the large number of small aircraft expected to be deployed in future. Experimental results using the heterogeneous unmanned aircraft system are presented to show that meshed airborne communication is feasible, that it extends the operational envelope of small unmanned aircraft at the expense of increased communication variability, and that net-centric operation of multiple cooperating aircraft is possible. Additionally, the ability of airborne networks of small unmanned aircraft to exploit controlled mobility to improve performance is discussed.

Lyapunov Vector Fields for Autonomous Unmanned Aircraft Flight Control

General techniques for constructing vector fields for unmanned aircraft guidance are provided that incorporate Lyapunov stability properties to produce simple, globally stable vector fields in three dimensions. Use of these fields to produce circular loiter pattern attractors is illustrated, along with a simple switching algorithm to enable following of arbitrary way point sequences. Alternatively, attractor shape variations are developed by warping the circular loiter, preserving global stability guarantees, and accurate path tracking. An example of this technique is provided that produces a racetrack loiter pattern, and three different variations in the warping technique are compared. Finally, tracking of the vector field is considered, using Lyapunov techniques to show global stability of heading and path position for several types of tracking control laws that are compatible with low cost unmanned aircraft avionics.

Maintaining optimal communication chains in robotic sensor networks using mobility control

This paper presents a decentralized mobility control algorithm for the formation and maintenance of an optimal cascaded communication chain between a lead sensor-equipped robot and a control station, using a team of robotic vehicles acting as communication relays in an unknown and dynamic RF environment. The gradient-based controller presented uses measurements of the signal-to-noise ratio (SNR) field of neighbor communication links, as opposed to relative position between nodes, as input into a localized performance function. By using the SNR field as input into the control system, the controller is reactive to unexpected and unpredictable changes in the RF environment that is not possible with range-based controllers. Since the operating environment is not known a priori to deployment of a robotic sensor network, an adaptive model-free extremum seeking (ES) algorithm, that uses the motion of the relays to estimate the performance function gradient, is presented to control the motion of 2D nonholonomic vehicles acting as communication relays using the gradient-based controller. Even without specific knowledge of the SNR field, simulations show that the ES decentralized chaining controller using measurements of the SNR field, will drive a team of robotic vehicles to locations that achieve the global objective of maximizing capacity of a cascaded communication chain, even in the presence of an active jamming source.

Networking Issues for Small Unmanned Aircraft Systems

This paper explores networking issues that arise as a result of the operational requirements of future applications of small unmanned aircraft systems. Small unmanned aircraft systems have the potential to create new applications and markets in civil domains, enable many disruptive technologies, and put considerable stress on air traffic control systems. The operational requirements lead to networking requirements that are mapped to three different conceptual axes that include network connectivity, data delivery, and service discovery. The location of small UAS networking requirements and limitations along these axes has implications on the networking architectures that should be deployed. The delay-tolerant mobile ad-hoc network architecture offers the best option in terms of flexibility, reliability, robustness, and performance compared to other possibilities. This network architecture also provides the opportunity to exploit controlled mobility to improve performance when the network becomes stressed or fractured.

Lyapunov Guidance Vector Fields for Unmanned Aircraft Applications

This paper presents results implementing Lyapunov vector fields for the guidance of unmanned aircraft. The vector fields yield globally stable tracking of circular loiter patterns. These loiter patterns are used in several unmanned aircraft applications including hierarchical micro air vehicle control for cooperative plume tracking, extremum seeking for electronic chaining, and cooperative tracking of moving targets. Extensions of the basic LGVF approach are made for each application including: warping the circular pattern to form other closed orbit patterns; driving the center of the LGVF orbit using virtual dynamics; and spacing multiple unmanned aircraft around a circular orbit. Hardware-in-the- loop simulation results and flight data are given to validate performance.

Cooperative Target Localization with a Communication-Aware Unmanned Aircraft System

This paper presents an approach to generating information-gathering trajectories for a cooperative unmanned aircraft system that takes into account the reliability of multihop networked communication. The algorithms presented here implicitly consider the communication limitations of the aircraft within a path-planning algorithm in which there is a position dependency on both the sensing and communication capabilities of the network. A communication-aware performance metric is derived by combining the extended information filter with a packet- erasure channel model and stochastic model of packet routing. Informative trajectories that maximize this metric are generated by a distributed, hierarchical planning algorithm. Flight results are used to characterize the communication channel model for use in a multivehicle simulation. Finally, simulation studies are used to evaluate the path-planning approach through the specific application of stationary WiFi source localization. These studies show that the approach leads to improved estimation performance relative to path-planning that does not consider communication reliability. Further, the approach leads to the emergence of natural roles whereby some of the aircraft contribute largely to the sensing portion of the objective, while others improve communication reliability by serving mainly as communication relays.

Optimizing Cascaded Chains of Unmanned Aircraft Acting as Communication Relays

This work presents a decentralized mobility control algorithm for an optimal end-to-end communication chain using a team of unmanned aircraft acting solely as communication relays. The chaining controller drives the location of a virtual control point, using estimates of the communication objective function gradient calculated using stochastic approximation techniques, to locations of improved relaying for unmanned aircraft. The gradient estimate is derived from observation data of the communication objective function taken along a path generated by the aircraft orbiting about the control point. Flight experiments show that an unmanned aircraft can measure the signal-to-noise-and-interference ratio fields from IEEE 802.11b/g (WiFi) communication links; generate estimates of the field gradients using the least-squares gradient estimation method; and use the gradient estimates to drive a control point to a location that improves communication capacity.

Lyapunov Vector Fields for Autonomous UAV Flight Control 1

General techniques for construct ing vector fields for UAV guidance are provided that incorporate Lyapunov stability properties to produce simple, globally stable vector fields in 3D. Use of these fields in circular loiter patterns is illustrated, along with simple switching algorithms fo r circular loiter vector fields to enable following of arbitrary way point paths or loops. Another variation is also developed, where a simple circular loiter is warped into other shapes, preserving global stability guarantees and accurate path tracking. An example of this technique is provided that produces a “racetrack” loiter pattern, and three different variations in the warping technique are compared. Finally, tracking of the vector field is considered, using Lyapunov techniques to show global stabili ty of heading and path position for several types of tracking control laws that are compatible with low cost UAV avionics.