Arvind A. de Menezes Pereira

USC CINAPS Builds Bridges

More than 70% of our earth is covered by water, yet we have explored less than 5% of the aquatic environment. Aquatic robots, such as autonomous underwater vehicles (AUVs), and their supporting infrastructure play a major role in the collection of oceanographic data. To make new discoveries and improve our overall understanding of the ocean, scientists must make use of these platforms by implementing effective monitoring and sampling techniques to study ocean upwelling, tidal mixing, and other ocean processes. Effective observation and continual monitoring of a dynamic system as complex as the ocean cannot be done with one instrument in a fixed location. A more practical approach is to deploy a collection of static and mobile sensors, where the information gleaned from the acquired data is distributed across the network. Additionally, orchestrating a multisensor, long-term deployment with a high volume of distributed data involves a robust, rapid, and cost-effective communication network. Connecting all of these components, which form an aquatic robotic system, in synchronous operation can greatly assist the scientists in improving our overall understanding of the complex ocean environment.

Risk-aware Path Planning for Autonomous Underwater Vehicles using Predictive Ocean Models

Recent advances in Autonomous Underwater Vehicle (AUV) technology have facilitated the collection of oceanographic data at a fraction of the cost of ship-based sampling methods. Unlike oceanographic data collection in the deep ocean, operation of AUVs in coastal regions exposes them to the risk of collision with ships and land. Such concerns are particularly prominent for slow-moving AUVs since ocean current magnitudes are often strong enough to alter the planned path significantly. Prior work using predictive ocean currents relies upon deterministic outcomes, which do not account for the uncertainty in the ocean current predictions themselves. To improve the safety and reliability of AUV operation in coastal regions, we introduce two stochastic planners: (a) a Minimum Expected Risk planner and (b) a risk-aware Markov Decision Process, both of which have the ability to utilize ocean current predictions probabilistically. We report results from extensive simulation studies in realistic ocean current fields obtained from widely used regional ocean models. Our simulations show that the proposed planners have lower collision risk than state-of-the-art methods. We present additional results from field experiments where ocean current predictions were used to plan the paths of two Slocum gliders. Field trials indicate the practical usefulness of our techniques over long-term deployments, showing them to be ideal for AUV operations.

Autonomous Underwater Vehicle trajectory design coupled with predictive ocean models: A case study

Data collection using Autonomous Underwater Vehicles (AUVs) is increasing in importance within the oceanographic research community. Contrary to traditional moored or static platforms, mobile sensors require intelligent planning strategies to maneuver through the ocean. However, the ability to navigate to high-value locations and collect data with specific scientific merit is worth the planning efforts. In this study, we examine the use of ocean model predictions to determine the locations to be visited by an AUV, and aid in planning the trajectory that the vehicle executes during the sampling mission. The objectives are: a) to provide near-real time, in situ measurements to a large-scale ocean model to increase the skill of future predictions, and b) to utilize ocean model predictions as a component in an end-to-end autonomous prediction and tasking system for aquatic, mobile sensor networks. We present an algorithm designed to generate paths for AUVs to track a dynamically evolving ocean feature utilizing ocean model predictions. This builds on previous work in this area by incorporating the predicted current velocities into the path planning to assist in solving the 3-D motion planning problem of steering an AUV between two selected locations. We present simulation results for tracking a fresh water plume by use of our algorithm. Additionally, we present experimental results from field trials that test the skill of the model used as well as the incorporation of the model predictions into an AUV trajectory planner. These results indicate a modest, but measurable, improvement in surfacing error when the model predictions are incorporated into the planner.

Cooperative Autonomous Marine Vehicle motion control in the scope of the EU GREX Project: Theory and Practice

This paper describes the core of the research work done by the ISR/IST team on cooperative Autonomous Marine Vehicle (AMV) motion control in the scope of the EU Project GREX - coordination and control of cooperating heterogeneous unmanned systems in uncertain environments. The first part of the paper affords the reader a concise introduction to the general problem of cooperative motion control of fleets of AMVs by highlighting illustrative mission scenarios developed collectively by the GREX partners and summarizing the main challenges that they pose to system engineers. This is followed by the description of a general architecture for cooperative autonomous marine vehicle control in the presence of time-varying communication topologies and communication losses that is rooted in a solid control-theoretic framework. The results of simulations with the NetMarSyS - Networked Marine Systems Simulator - of ISR/IST are presented and show the efficacy of the algorithms developed for cooperative motion control. The last part of the paper focuses on practical issues and describes the results of tests at sea in the Azores, in the Summer of 2008. The paper concludes with a critical review of the work done and a discussion of theoretical and practical implementation issues that warrant further research and development effort.

An experimental study of station keeping on an underactuated ASV

Dynamic positioning is an important application for marine vehicles that do not have the luxury of anchoring or mooring themselves. Such vehicles are usually large and have arrays of thrusters that allow for controllability in the sway as well as the surge and yaw axes. Most smaller boats however, are underactuated and do not possess control in the sway direction. This makes the control problem significantly more challenging. We address the station keeping problem for a small autonomous surface vehicle (ASV) with significant windage. The vehicle is required to hold station at a given position. We describe the design of a weighted controller that uses wind feed-forward to complement a line-of-sight guidance controller to achieve satisfactory performance under slow-varying moderate wind conditions. We test the control system in simulation and in field trials with a twin-propeller ASV. Experiments show that the controller works very well in moderate wind conditions allowing the ASV to keep station with a position error of approximately one vehicle length.

Cooperative Control of Multiple Marine Vehicles Theoretical Challenges and Practical Issues

Abstract This paper is a brief overview of some of the theoretical and practical issues that arise in the process of developing advanced motion control systems for cooperative multiple autonomous marine vehicles (AMVs). Many of the problems addressed have been formulated in the scope of the EU GREX project, entitled Coordination and Control of Cooperating Heterogeneous Unmanned Systems in Uncertain Environments. The paper offers a concise introduction to the general problem of cooperative motion control that is well rooted in illustrative mission scenarios developed collectively by the GREX partners. This is followed by the description of a general architecture for cooperative autonomous marine vehicle control in the presence of time-varying communication topologies and communication losses. The results of simulations with the NetMar SyS (Networked Marine Systems Simulator) of ISR/IST are presented and show the efficacy of the algorithms developed for cooperative motion control. The paper concludes with a description of representative results obtained during a series of tests at sea in the Azores, in 2008.

Toward risk aware mission planning for Autonomous Underwater Vehicles

Long range and high endurance Autonomous Underwater Vehicles such as gliders enable sustained oceanographic sampling at larger time-scales and much lower operational costs compared to traditional ship-based sampling methods. While most path-planning methods for AUVs optimize paths with respect to efficiency, obstacle avoidance, and control they do not explicitly address the issue of finding the safest possible path when considering risks such as shipping traffic and bathymetry. In coastal regions with high shipping traffic, reducing collision risk at the path planning stage, at the expense of efficiency, is a worthwhile trade-off. We propose a method of building risk maps using historical data from the Automated Information System. These are used to plan minimum risk paths between a specified start and goal location, while avoiding obstacles, using an algorithm based on A* search. Our planner incorporates the uncertainty in dead-reckoning without explicitly considering the effect of ocean currents. We compare the relative risk of paths produced by our method when compared to a shortest-path planner which does not take risk into account, and show that our methods performs significantly better, while producing competitive paths lengths.

A Small Autonomous Surface Vehicle for Ocean Color Remote Sensing

This paper provides a study on the development and the use of a small autonomous surface vehicle (ASV) that automatically follows programmed mission transects, while measuring sensor outputs along the tracks. It discusses the mechanical construction of the ASV, the distributed architecture of controller area network (CAN)-based nodes for science and vehicle payloads, high-speed radio-frequency (RF) communications, the performance of the heading autopilot, global positioning system (GPS)-based guidance algorithm, and the mission programming technique. The field trials of the ASV, performed off the coast of Goa, India, are focused on retrieving the 2-D spatial distribution of surface chlorophyll, which is one of the useful parameters in characterizing the nature of calibration-validation (CALVAL) sites for ocean remote sensing needs. A further benefit of ASVs is that they can be built at a low cost and used in monitoring applications of diverse coastal ecosystems.

Multi-Robot Collaboration with Range-Limited Communication: Experiments with Two Underactuated ASVs

We present a collaborative team of two under-actuated autonomous surface vessels (ASVs) that performs a cooperative navigation task while satisfying a communication constraint. Our approach is based on the use of a hierarchical control structure where a supervisory module commands each vessel to perform prioritized elementary tasks, a behavior-based controller generates motion directives to achieve the assigned tasks, and a maneuvering controller generates the actuator commands to follow the motion directives. The control technique has been tested in a mission where a set of target locations spread across a planar environment has to be visited once by either of the two ASVs while maintaining a relative separation less than a given maximum distance (to guarantee inter-ASV wireless communication). Experiments were carried out in the field with a team of two ASVs visiting 22 locations on a lake surface (approximately 30000m 2) with static obstacles. Results show a 30% improvement in mission time over the single-robot case.

A Communication Framework for Cost-effective Operation of AUVs in Coastal Regions

Autonomous Underwater Vehicles (AUVs) are revolutionizing oceanography. Most high-endurance and long-range AUVs rely on satellite phones as their primary communications interface during missions for data/command telemetry due to its global coverage. Satellite phone (e.g., Iridium) expenses can make up a significant portion of an AUV’s operating budget during long missions. Slocum gliders are a type of AUV that provide unprecedented longevity in scientific missions for data collection. Here we describe a minimally-intrusive modification to the existing hardware and an accompanying software system that provides an alternative robust disruption-tolerant communications framework enabling cost-effective glider operation in coastal regions. Our framework is specifically designed to address multiple-AUV operations in a region covered by multiple networked base-stations equipped with radio modems. We provide a system overview and preliminary evaluation results from three field deployments using a glider.We believe that this framework can be extended to reduce operational costs for other AUVs during coastal operations.