Feature ArticleOcean Observation With Coordinated Robotic Platforms
By Nuno A. Cruz • Bruno M. Ferreira • Aníbal C. Matos
The MARES AUV.
The coordinated operation of marine vehicles has been a particularly active research topic during the last decade, with the realization that the new scenarios for underwater monitoring and intervention require new paradigms of operation, based on heterogeneous robotic assets. In order to exploit their complementary competencies in an efficient way, such operations require the development of robust formation-control strategies and a dependable communications infrastructure to exchange reliable information in a timely manner. When using AUVs, the communications channel is the water, and the formation-control strategy has to encompass the hindrances associated to underwater communications.
A long-term program for using multiple heterogeneous vehicles for marine operations is being developed at the Ocean Systems Group at INESC TEC, University of Porto, in Portugal. The overall goal is to integrate a fleet of AUVs, autonomous surface vehicles (ASVs) and moored systems. Present work involves use of the MARES AUV, Zarco and Gama ASVs, and navigation and instrumentation buoys (NIBs).
The MARES is a small, torpedo-shaped AUV 1.5 meters long and 32 kilograms in weight in the basic version. In a typical configuration, a PC/104 computational system manages the entire mission, including communications with other devices and a control station. Navigation is provided by the fusion of data from an inertial measurement unit (IMU) and an acoustic system for long baseline localization (LBL), complemented by a small GPS receiver, when the vehicle is at the surface. Four thrusters provide the capability to move as fast as 5 knots and to hover in the water column, with a set of lithium-ion batteries ensuring 10 hours of operation. MARES is a highly modular vehicle, with the ability to integrate a great variety of payload sensors, and it has been operating since 2007, mainly in environmental-monitoring missions.
Zarco and Gama are ASVs that use IMU measurements and GPS data to navigate. Their open platforms allow an easy integration of new sensors, and all data can be stored onboard and transmitted in real time via radio. These vehicles operate with two independent thrusters, capable of holding position or navigating up to 4 knots. Zarco and Gama have been in operation since 2006, either independently (for bathymetry, for example) or coordinated, to provide a moving baseline for AUV navigation.
The NIBs are auxiliary equipment commonly used as static beacons responding to acoustic pings, using a two-way travel time scheme, or transmitting synchronized pings for one-way travel time positioning, therefore providing ranging capabilities to underwater vehicles. They are equipped with GPS receivers and radio-communication devices. Whenever communication with AUVs is available, the geolocation of the NIBs are fed into the vehicles for more precise localization.
The Ocean Systems Group has tackled the technical challenges associated with the operation of these marine systems, such as relative and absolute positioning systems for multiple vehicles; sensor fusion for precise navigation, including underwater navigation in a moving acoustic network; and guidance and control of individual vehicles, both ASVs and AUVs. The current research builds upon earlier results to address the challenges associated with coordinated operations, mainly formation control under the constraints of communications and networking. These are particularly severe when using the underwater channel, with high latency, low data rate and high probability of transmission error.
The goal is for the communications network to be adaptive according to the instantaneous conditions of the acoustic channel and to the capabilities of the moving nodes. In particular, some of the nodes may have to move to increase the overall throughput, while, at the same time, the network routing will deal with the transmission conditions between the available nodes to ensure a proper transmission of information. To continue this article please click here.
Nuno A. Cruz is a researcher at INESC TEC and a lecturer at Porto University in Portugal. During the last 15 years, he has been involved in the design, development and operation of small size AUVs and ASVs. His current research interests include the implementation of adaptive sampling strategies in AUVs.
Bruno M. Ferreira holds a master's in electrical and computer engineering. Currently, he is a Ph.D. candidate at the University of Porto. He has been working under the Robotics Unit at INESC TEC and his main research interests include cooperation, nonlinear control of autonomous marine vehicles and localization.
Aníbal C. Matos is a researcher in the Robotics Unit at INESC TEC and an assistant professor at Porto University. His research interests include control and navigation of marine robots. He has participated in the development of MARES, an AUV for environmental monitoring, and TriMARES, an autonomous underwater inspection vehicle.
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