A. J. Healey

Multivariable sliding mode control for autonomous diving and steering of unmanned underwater vehicles

A six-degree-of-freedom model for the maneuvering of an underwater vehicle is used and a sliding-mode autopilot is designed for the combined steering, diving, and speed control functions. In flight control applications of this kind, difficulties arise because the system to be controlled is highly nonlinear and coupled, and there is a good deal of parameter uncertainty and variation with operational conditions. The development of variable-structure control in the form of sliding modes has been shown to provide robustness that is expected to be quite remarkable for AUV autopilot design. It is shown that a multivariable sliding-mode autopilot based on state feedback, designed assuming decoupled modeling, is quite satisfactory for the combined speed, steering, and diving response of a slow AUV. The influence of speed, modeling nonlinearity, uncertainty, and disturbances, can be effectively compensated, even for complex maneuvering. Waypoint acquisition based on line-of-sight guidance is used to achieve path tracking. >

Adaptive sliding mode control of autonomous underwater vehicles in the dive plane

The problem of controlling an autonomous underwater vehicle (AUV) in a diving maneuver is addressed. Having a simple controller which performs satisfactorily in the presence of dynamical uncertainties calls for a design using the sliding mode approach, based on a dominant linear model and bounds on the nonlinear perturbations of the dynamics. Nonadaptive and adaptive techniques are considered, leading to the design of robust controllers that can adjust to changing dynamics and operating conditions. The problem of using the observed state in the control design is addressed, leading to a sliding mode control system based on input-output signals in terms of drive-phase command and depth measurement. Numerical simulations using a full set of nonlinear equations of motion show the effectiveness of the proposed techniques. >

Command, control, and navigation experimental results with the NPS ARIES AUV

Provides an overview of the Naval Postgraduate School ARIES autonomous underwater vehicle and its control and navigation. An attempt is made to highlight its current operational capabilities and provide a description of future enhancements for greater mission utility and flexibility. An overview of the vehicle design along with descriptions of all major hardware components and sensors is given. A major discussion of the implementation of a modular, multirate, multiprocess software architecture for the ARIES is provided. The architecture is designed to operate using either a single computer processor or two independent, cooperating processors linked through a network interface for improved load balancing. A dual computer implementation is presented since each processor assumes different tasks for mission operation. Also included is a section on the underwater navigation method using a real-time extended Kalman filter that fuses all sensor data and computes the real time position, orientation and velocity. Experimental results for navigational accuracy using a DGPS/IMU/Doppler-aided navigation system are presented with DGPS pop-up maneuvers. Navigational accuracy is a requirement for the use of ARIES as a mobile communications network node. This work provides some examples of missions possible with such a node and the current state of its command and control system.

Testing and evaluation of an integrated GPS/INS system for small AUV navigation

A Small Autonomous Underwater Vehicle Navigation System (SANS) is being developed at the Naval Postgraduate School. The SANS is an integrated Global Positioning System/Inertial Navigation System (GPS/INS) navigation system composed of low-cost and small-size components. It is designed to demonstrate the feasibility of using a low-cost strap-down inertial measurement unit (IMU) to navigate between intermittent GPS fixes. The present hardware consists of a GPS/DGPS receiver, IMU, compass, water speed sensor, water depth sensor, and a data processing computer. The software is based on a 12-state complementary filter that combines measurement data from all sensors to derive a vehicle position/orientation estimate. This paper describes hardware and software design and testing results of the SANS. It is shown that results from tilt table testing and bench testing provide an effective means for tuning filter gains. Ground vehicle testing verifies the overall functioning of the SANS and exhibits an encouraging degree of accuracy.

Enhancement of the inertial navigation system for the Morpheus autonomous underwater vehicles

This paper presents the design and development of an enhanced inertial navigation system that is to be integrated into the Morpheus autonomous underwater vehicle at Florida Atlantic University. The inertial measurement unit is based on the off-the-shelf Honeywell HG1700-AG25 3-axis ring-laser gyros and three-axis accelerometers and is aided with ground speed measurements obtained using an RDI Doppler-velocity-log sonar. An extended Kalman filter has been developed, which fuses together asynchronously the inertial and Doppler data, as well as the differential Global Positioning System positional fixes whenever they are available. A complementary filter was implemented to provide a much smoother and stable attitude estimate. Thus far, preliminary study has been made on characterizing the inertial navigation system-based navigation system performance, and the corresponding results and analyzes are provided.

Autonomous underwater vehicles: Hybrid control of mission and motion

This paper provides an experimental implementation and verification of a hybrid (mixed discrete state/ continuous state) controller for semi-autonomous and autonomous underwater vehicles in which the missions imply multiple task robot behavior. An overview of some of the missions being considered for this rapidly developing technology is mentioned including environmental monitoring, underwater inspection, geological survey as well as military missions in mine countermeasures.The functionalities required of such vehicles and their relation to ‘intelligent control’ technology is discussed. In particular, the use of Prolog as a computer language for the specification of the discrete event system (DES) aspects of the mission control is proposed. The connections between a Prolog specification and the more common Petri Net graphical representation of a DES are made. Links are made between activation commands, transitioning signals, and the continuous state dynamic control system (DCS) responsible for vehicle stabilization.Details are given of the NPS Phoenix vehicle implementation at the present time, together with experimental validation of the concepts outlined using a simplified example mission. The paper ends with a listing of questions and concerns for the evaluation of software controllers. A list of references is given for readers interested in this subject.

Application of formation control for multi-vehicle robotic minesweeping

In an effort to find low cost solutions to Naval minesweeping, a fleet of robot minesweepers equipped with detection sensors and acoustic/magnetic devices is proposed. To ensure maximum sweeping all vehicle movements are coordinated through a supervisor vehicle that determines if vehicles are lost to mine detonation, and re-tasks, as needed, the remaining vehicles to follow tracks left by lost vehicles. A computer program has been developed to evaluate the control logic linking the supervisor and worker vehicles. The algorithms for tracking control and vehicle ID reassignment are given and example results are shown.

Asynchronous data fusion for AUV navigation via heuristic fuzzy filtering techniques

This paper presents a heuristic fuzzy position estimation technique for autonomous underwater vehicle navigation. The heuristic estimator performs asynchronous data fusion of all sensor measurements based on their relative confidence levels, and then nonlinearly combines the fused information with the INS estimates via fuzzy filtering techniques. In this paper, the basis and implementation of the estimator will be described, and navigation results will be presented based on the heuristic estimator. In addition, performance comparison based on the heuristic estimator and those based on extended Kalman filters will be reported in our companion paper, and the results are expected to provide insights into the pros and cons of individual methods in terms of computational cost, steady-state and convergence characteristics for bias estimation.

AUV Control and Communication using Underwater Acoustic Networks

Underwater acoustic networks can be quite effective to establish communication links between autonomous underwater vehicles (AUVs) and other vehicles or control units, enabling complex vehicle applications and control scenarios. A communications and control framework to support the use of underwater acoustic networks and sample application scenarios are described for single and multi-AUV operation.

Online compensation of heading sensor bias for low cost AUVs

Presents a study of the effects of compass bias on navigational accuracy of autonomous underwater vehicles. Low cost vehicle systems utilize a magnetometer, and Doppler sonar for dead reckoning, and a DGPS fix when surfaced. By learning the compass bias from a few DGPS fixes, the navigational errors can be bounded with only a small number of fixes. The study is conducted using the Florida Atlantic University OEX vehicle and a large set of data obtained from a 3.5 Km run including several segments at different headings so that the dependency of compass error on actual heading could be evaluated.