Safety of a multi-vehicle system in mixed communication environments

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2007.Includes bibliographical references (leaves 131-138).Recent news events and statistics demonstrate the frequent occurrence of pile-up crashes on highways. A predominant reason for the occurrence of such crashes is that current vehicles (including those equipped with an Automatic Cruise Control system) do not provide the driver with advance information of events occurring far ahead of him/her. The use of inter-vehicular communication to provide advance warnings to enhance automotive safety is therefore being actively discussed in the research community. In this thesis, we investigate scenarios wherein only a subset of the vehicles in a multi-vehicle stream, are equipped with such advance warning capabilities. These vehicles (equipped with the capability to receive far-ahead information) are arbitrarily distributed among other unequipped vehicles that are capable of receiving only local, near-neighbor information. It is seen that there are conditions wherein even a partial equipment of the system can be beneficial (to both the equipped and the unequipped vehicles in a mixed vehicle stream). We demonstrate this through both simulations and a theoretical analysis. Towards this end, two distinct modeling approaches are adopted: microscopic and macroscopic.(cont.) The microscopic modeling approach uses ordinary differential equations to model each driver-vehicle unit and its interactions with its neighbors. A single-lane model is employed; and the problem is formulated as a collision avoidance problem. Sufficient conditions on the number of equipped vehicles, as well as their distributions in a mixed vehicle string are obtained; under these conditions, it is guaranteed that collisions do not occur. The macroscopic modeling approach, on the other hand, uses partial differential equations that govern the average behavior of groups of vehicles. In this approach, a multi-lane formulation is employed. This thesis examines the influence of partial equipment of the advance warning system on some of the wave effects that are known to exist in traffic flows, in particular, shocks and large negative velocity gradient waves that travel unattenuated or get amplified as they pass through the traffic. We examine the influence of the equipped vehicles in attenuating such waves. The resulting velocity gradients are parametrized as a function of the percentage of equipped vehicles. A prototype of an advance warning system was also developed and road tests were conducted to test the concept. These road tests have demonstrated the system's performance to be satisfactory, subject to good communication links, for the class of scenarios tested.by Animesh Chakravarthy.Ph.D

Control Law Design for Perching an Agile MAV with Articulated Wings

This paper explores the use of variable wing dihedral and variable wing twist (in conjunction with a conventional horizontal elevator) to control an aircraft performing a perching maneuver. A choice of controller architecture wherein the dihedral is employed in the forward path and the elevator and twist are employed in the feedback path, is considered. The aircraft is modeled as a multivariable linear time-varying system. A specific perching trajectory is considered; and the open-loop aircraft is longitudinally unstable for a segment of this perching trajectory and lateral-directionally unstable for the entire perching trajectory. A multivariable time-varying controller is designed to efficiently stabilize the aircraft as well as reject longitudinal-lateral-directional wind disturbances, while closely tracking the reference perching trajectory

Dynamics and Performance of Tailless Micro Aerial Vehicle with Flexible Articulated Wings

The purpose of this paper is to analyze and discuss the performance and stability of a tailless micro aerial vehicle with flexible articulated wings. The dihedral angles can be varied symmetrically on both wings to control the aircraft speed independently of the angle of attack and flight-path angle, while an asymmetric dihedral setting can be used to control yaw in the absence of a vertical tail.Anonlinear aero-elastic model is derived, and it is used to study the steady-state performance and flight stability of the micro aerial vehicle. The concept of the effective dihedral is introduced, which allows for a unified treatment of rigid and flexible wing aircraft. It also identifies the amount of elasticity that is necessary to obtain tangible performance benefits over a rigid wing. The feasibility of using axial tension to stiffen the wing is discussed, and, at least in the context of a linear model, it is shown that adding axial tension is effective but undesirable. The turning performance of an micro aerial vehicle with flexible wings is compared to an otherwise identical micro aerial vehicle with rigid wings. The wing dihedral alone can be varied asymmetrically to perform rapid turns and regulate sideslip. The maximum attainable turn rate for a given elevator setting, however, does not increase unless antisymmetric wing twisting is employed

Controller design for a nonlinear morphing UAV

In this work, we provide details of the control design of a morphing UAV capable of variable wing sweep. This morphing capability is assumed to be superimposed on a GENMAV (Generic Micro Air Vehicle) configuration originally developed by the Air Force Research Laboratory (AFRL). The nonlinear aerodynamic model for this aircraft is obtained using the Athena-Vortex Lattice (AVL) method with a quasi-steady state assumption

Collision Cones for Quadric Surfaces

The problem of collision prediction in dynamic environments appears in several diverse fields, which include robotics, air vehicles, underwater vehicles, and computer animation. In this paper, collision prediction of objects that move in 3-D environments is considered. Most work on collision prediction assumes objects to be modeled as spheres. However, there are many instances of object shapes where an ellipsoidal or a hyperboloid-like bounding box would be more appropriate. In this paper, a collision cone approach is used to determine collision between objects whose shapes can be modeled by general quadric surfaces. Exact collision conditions for such quadric surfaces are obtained in the form of analytical expressions in the relative velocity space. For objects of arbitrary shapes, exact representations of planar sections of the 3-D collision cone are obtained

Generalization of the collision cone approach for motion safety in 3-D environments

Avoidance of collision between moving objects in a 3-D environment is fundamental to the problem of planning safe trajectories in dynamic environments. This problem appears in several diverse fields including robotics, air vehicles, underwater vehicles and computer animation. Most of the existing literature on collision prediction assumes objects to be modelled as spheres. While the conservative spherical bounding box is valid in many cases, in many other cases, where objects operate in close proximity, a less conservative approach, that allows objects to be modelled using analytic surfaces that closely mimic the shape of the object, is more desirable. In this paper, a collision cone approach (previously developed only for objects moving on a plane) is used to determine collision between objects, moving in 3-D space, whose shapes can be modelled by general quadric surfaces. Exact collision conditions for such quadric surfaces are obtained and used to derive dynamic inversion based avoidance strategies

Obstacle Avoidance in a Dynamic Environment: A Collision Cone Approach

A novel collision cone approach is proposed as an aid to collision detection and avoidance between irregularly shaped moving objects with unknown trajectories. It is shown that the collision cone can be effectively used to determine whether collision between a robot and an obstacle (both moving in a dynamic environment) is imminent. No restrictions are placed on the shapes of either the robot or the obstacle, i.e., they can both be of any arbitrary shape. The collision cone concept is developed in a phased manner starting from existing analytical results-available in aerospace literature-that enable prediction of collision between two moving point objects. These results are extended to predict collision between a point and a circular object, between a point and an irregularly shaped object, between two circular objects, and finally between two irregularly shaped objects. Using the collision cone approach, several strategies that the robot can follow in order to avoid collision, are presented. A discussion on how the shapes of the robot and obstacles can be approximated in order to reduce computational burden is also presented. A number of examples are given to illustrate both collision prediction and avoidance strategies of the robot

Guidance for Precision Three-Dimensional Maneuvers Through Orifices Using Safe-Passage Cones

This paper proposes a safe-passage cone-based guidance strategy developed in a relative velocity framework to enable a generic aerial robot execute a precision three-dimensional maneuver through a narrow orifice in a wall. The relative sizes of the robot and the orifice are such that the orifice is only slightly larger than the robot. The orifice is approximated as an elliptical shape, and analytical nonlinear guidance laws incorporating state-based switching are derived. It is demonstrated that the guidance laws developed can be used to maneuver the robot through both fixed and moving orifices as well as in scenarios when the orifice is closing with time

Active wing shaping control of an elastic aircraft

Click on the DOI link to access the article (may not be free).In recent years, lightweight materials have been adopted in airframe design as a means to reduce aircraft weight. These lightweight materials provide adequate strength and durability, but at the expense of reduced structural rigidity. This increase in structural flexibility leads to a significant increase in the effects of aero-elastic interaction forces and moments, which can lead to important aircraft stability and control issues. In this paper, the optimal control design of an elastically shaped aircraft that has highly flexible wings is discussed. The aircraft has the capability to actively change the wing twist and bending in flight so as to achieve a local angle of attack distribution that is optimal for the specific flight condition. The aircraft has twenty three control surfaces, distributed along the trailing edge and leading edge of each wing. A multi-objective performance index that includes an explicit drag minimization term is considered, and a static output feedback controller design is performed using this performance index. Simulation results demonstrate the optimal wing shape change trajectories as well as the validity of the designed controller in stabilizing this elastic aircraft. The drag reduction achieved using static output feedback is compared with that obtained using full state feedback