A. Pascoal

Trajectory Tracking for Autonomous Vehicles: An Integrated Approach to Guidance and Control

Abstract. This paper addresses the problem of in- tegrated design of guidance and control systems for au- tonomous vehicles (AVs). In fact, it introduces a new methodology for integrated design of guidance and control for such vehicles. The methodology proposed leads to an efficient procedure for the design of controllers for AVs to accurately track reference trajectories defined in an iner- tia! reference frame. The paper illustrates the application of this procedure on the design of a tracking controller for the Unmanned Air Vehicle Bluebird. The design phase is summarized, and the performance of the resulting con- troller is assessed in simulation using dynamic models of the vehicle and its sensor suite.

Brief paper: A velocity algorithm for the implementation of gain-scheduled controllers

A new method is proposed to implement gain-scheduled controllers for nonlinear plants. Given a family of linear feedback controllers designed for linearizations of a nonlinear plant about constant operating points, a nonlinear gain-scheduled controller is derived that preserves the input-output properties of the linear closed loop systems locally, about each equilibrium point. The key procedures in the proposed method are to provide integral action at the inputs to the plant and differentiate some of the measured outputs before they are fed back to the scheduled controller. For a fairly general class of systems, the nonlinear gain-scheduled controllers are easy to obtain, and their structure is similar to that of the original linear controllers.

Dynamic positioning and way-point tracking of underactuated AUVs in the presence of ocean currents

This paper addresses the problem of dynamic positioning and way-point tracking of underactuated autonomous underwater vehicles (AUVs) in the presence of constant unknown ocean currents and parametric modelling uncertainty. A non-linear adaptive controller is proposed that steers an AUV along a sequence of way-points consisting of desired positions (x, y) in a inertial reference frame, followed by vehicle positioning at the final target point. The controller is first derived at the kinematic level assuming that the ocean current disturbance is known. An exponential observer for the current is then designed and convergence of the resulting closed-loop system trajectories is analysed. Finally, integrator backstepping and Lyapunov based techniques are used to extend the kinematic controller to the dynamic case and to deal with model parameter uncertainty. Simulation results with a dynamic model of an underactuated autonomous underwater shuttle for the transport of benthic labs are presented and discussed.

3D path following for autonomous underwater vehicle

A new methodology is proposed for the design of path following systems for autonomous underwater vehicles. Global convergence to reference paths is achieved with a nonlinear control strategy that takes explicitly into account the dynamics of the vehicle. Formal convergence proofs are indicated. Simulation results with the model of a prototype autonomous underwater vehicle are presented to illustrate the performance of the path following system derived.

Combined trajectory tracking and path following: an application to the coordinated control of autonomous marine craft

The paper presents a solution to the problem of combined trajectory tracking and path following system design for autonomous marine craft. This problem is motivated by the practical need to develop control systems for marine craft that can yield good trajectory tracking performance while keeping some of the desired properties normally associated with path following. The solution described builds on and extends previous work by Hindman and Hauser (1992) on so-called maneuver modified trajectory tracking. An application is made to the problem of designing a control system for the coordinated operation of an autonomous surface craft (ASC) and an autonomous underwater vehicle (AUV). Simulations with nonlinear models of an underactuated marine craft and a fully actuated underwater vehicle illustrate the performance of the control system derived.

Coordinated Path-Following in the Presence of Communication Losses and Time Delays

This paper addresses the problem of steering a group of vehicles along given spatial paths while holding a desired time-varying geometrical formation pattern. The solution to this problem, henceforth referred to as the coordinated path-following (CPF) problem, unfolds in two basic steps. First, a path-following (PF) control law is designed to drive each vehicle to its assigned path, with a nominal speed profile that may be path dependent. This is done by making each vehicle approach a virtual target that moves along the path according to a conveniently defined dynamic law. In the second step, the speeds of the virtual targets (also called coordination states) are adjusted about their nominal values so as to synchronize their positions and achieve, indirectly, vehicle coordination. In the problem formulation, it is explicitly considered that each vehicle transmits its coordination state to a subset of the other vehicles only, as determined by the communications topology adopted. It is shown that the system that is obtained by putting together the PF and coordination subsystems can be naturally viewed as either the feedback or the cascade connection of the latter two. Using this fact and recent results from nonlinear systems and graph theory, conditions are derived under which the PF and the coordination errors are driven to a neighborhood of zero in the presence of communication losses and time delays. Two different situations are considered. The first captures the case where the communication graph is alternately connected and disconnected (brief connectivity losses). The second reflects an operational scenario where the union of the communication graphs over uniform intervals of time remains connected (uniformly connected in mean). To better root the paper in a nontrivial design example, a CPF algorithm is derived for multiple underactuated autonomous underwater vehicles (AUVs). Simulation results are presented and discussed.

Robotic ocean vehicles for marine science applications: the European ASIMOV project

The key objective of the ASIMOV project is the development and integration of advanced technological systems to achieve coordinated operation of an Autonomous Surface Craft (ASC) and an Autonomous Underwater Vehicle (AUV) while ensuring a fast communication link between the two vehicles. The ASC/AUV ensemble is being used to study the extent of shallow water hydrothermalism and to determine the patterns of community diversity at vents in the D. Joao de Castro (DJC) bank in the Azores.

Nonlinear path following with applications to the control of autonomous underwater vehicles

This paper derives a control law to steer the dynamic model of an autonomous underwater vehicle (AUV) along a desired path. The methodology adopted for path following deals explicitly with vehicle dynamics. Furthermore, it overcomes stringent initial condition constraints that are present in a number of path following control strategies described in the literature. Controller design builds on Lyapunov theory and backstepping techniques. The resulting nonlinear feedback control law yields convergence of the path following error trajectories to zero. Simulation results illustrate the performance of the control system proposed.

Path following for unmanned aerial vehicles using L1 adaptive augmentation of commercial autopilots

The paper presents a three-dimensional path-following control algorithm that expands the capabilities of conventional autopilots, which are normally designed to provide only guidance loops for waypoint navigation. Implementation of this algorithm broadens the range of possible applications of small unmanned aerial vehicles. The solution proposed takes explicit advantage of the fact that normally these vehicles are equipped with autopilots stabilizing the vehicles and providing angular-rate tracking capabilities. Therefore, the overall closed-loop system exhibits naturally an inner-outer (dynamics-kinematics) control loop structure. The outer-loop path-following control law developed relies on a nonlinear control strategy derived at the kinematic level, while the inner-loop consisting of the autopilot together with an L1 adaptive augmentation loop is designed to meet strict performance requirements in the presence of unmanned aerial vehicle modeling uncertainty and environmental disturbances. A rigorous proof of stability and performance of the path-following closed-loop system, including the dynamics of the unmanned aerial vehicle with its autopilot, is given. The paper bridges the gap between theory and practice and includes results of extensive flight tests performed in Camp Roberts, California, which demonstrate the benefits of the framework adopted for the control system design.

Study and implementation of an EKF GIB-based underwater positioning system

The paper addresses the problem of estimating the position of an underwater target in real time. In the scenario adopted, the target carries a pinger that emits acoustic signals periodically, as determined by a very high precision clock that is synchronized with GPS, prior to system deployment. The target is tracked from the surface by using a system of four buoys equipped with hydrophones and electronic circuitry that measures the times of arrival of the acoustic signals emitted by the pinger or, equivalently, the four target-to-buoy range measurements (a commercial version of this setup is the GIB system). Due to the finite speed of propagation of sound in water, these measurements are obtained with different latencies. The paper tackles the problem of underwater target tracking in the framework of extended Kalman filtering by relying on a purely kinematic model of the target. The paper further shows also how the differently delayed measurements can be merged using a back and forward fusion approach. A measurement validation procedure is introduced to deal with dropouts and outliers. Simulation as well as experimental results illustrate the performance of the filter proposed.