Fresh Ideas Flourish in Salt Water Environment

Sea, Sun, Sand… and Submarines

On a bright summer day off the southeast coast of Florida, the Ocean Explorer slips her moorings and heads north. One of a new breed of submarines, she quietly sinks beneath the surface, beginning a coastal search and survey mission. The mission is designed both to test the limits of the on-board instruments, and prepare for the day when her unique capabilities might be used to approach a more hostile coast in pursuit of military goals.

But whether engaged in hostilities or the more benign tasks of oceanographic exploration, this submarine offers one notable benefit that singles out her particular class of seagoing vessels: she carries neither passengers nor crew. For the Ocean Explorer is an autonomous underwater vehicle (AUV), a two meter, two hundred kilogram package of high tech wizardry with a range in excess of thirty-six nautical miles, a top speed of five knots, and a depth rating of three hundred meters. The Ocean Explorer is made practical by its central unifying element: a LonWorks technology-based control network.

When “All Wet” is a Design Criterion

Dr. Stanley Dunn, Director of the Advanced Marine Systems Laboratory at Florida Atlantic University (which shares Ocean Explorer development with the University of South Florida), elucidates the “three Cs” of AUV design: capability, cost-effectiveness, and certainty. Not only must such a vessel support the most advanced technologies in an economical manner, it must be absolutely certain in its function: reliable, survivable, and maintainable under challenging circumstances.

In addition to its own motion control, propulsion, navigation, and surface support systems, the AUV must also house a dizzying collection of sensors and actuators: multichannel fiber optic spectrophotometers, laser fluorometers, shear probes, electromagnetic velocity sensors, optical and acoustic particle counters, hyper-spectral imagers, radiometers, chirp sidescan sonar, bottom penetrating sonar, accelerometers, spread spectrum acoustic modems, and sonar arrays. These are selected and combined in endless ways to instrument the myriad missions for which the AUV is designed.

Traditional AUV designs have a single shared space for both dedicated (vessel specific) and payload (mission specific) subsystems; this requires custom mounting and rearrangement of components to reconfigure the craft. Conventional electrical and control systems are also centralized, with individual wires for the sensors and actuators; this means costly, difficult, and error-prone rewiring and reprogramming for different missions.

Reconfigurable Space, Interoperable Components

The Ocean Explorer circumvents these difficulties with two design innovations. The first is a structural philosophy that relegates most of the vessel’s dedicated components to the tail section of the craft, freeing the forward areas for payload packages or vessel sections without the need for retrofitting. The second is what Dr. Samuel Smith, technical director of the AUV project, terms an intelligent, distributed control system (IDCS). Aboard the AUV, this appears as a single, standardized network cable that delivers both power (12, 24, & 48 V) and communication services (1.25 Mbits/s) throughout the vessel; “smart” sensors and actuators tap into this cable using a common connector. Sensor level intelligence reduces the amount of data on the network, and minimizes programming and rewiring.

The search for an off-the-shelf solution that provided the interoper-ability support necessary to his vision of interchangeable components led Professor Smith to LonWorks technology. “Our system architecture demands a protocol that is medium independent, reliable, secure, and able to support thousands of nodes. Additionally, the ‘per node’ cost must be kept low. The IDCS network enables the connection of every subsystem, every vehicle in the fleet, and all support systems.” Thus, each network node incorporates a single Neuron Chip, containing the three microprocessors, program memory, and much of the communication and I/O device control circuitry necessary to perform its specialized task. One example, from the two dozen nodes included among the dedicated components, is the rudder servo motor control node, which provides closed-loop PID control of this critical motion control function. Another is the drop weight control node, which monitors a variety of activities and conditions to determine whether mission intervention is necessary (by assuming control, by powering to the surface, or by dropping a weight sufficient to force the vessel to positive buoyancy). Each such node, operating independently yet cooperatively within the system, contributes to the overall autonomy of the craft, and embodies the “plug and play” reconfigurability that permeates the LonWorks based architecture.

Bright Batteries on Board
“Intelligent components” measurably enhance AUV modularity and reliability. Such a component is the “smart battery pack,” one of eight aboard the Ocean Explorer, capable of operating individually or collectively to supply system power. Nominally rated at 5 ampere-hours @ 48 volts, each cylindrical, pressure sealed pack contains 42 NiCad batteries, controlled by a single Neuron Chip. The pack features 20 channels of 12-bit A/D, used to monitor battery voltages, currents, temperatures, leakage, and humidity; all information is available system-wide as standard network variables. The node also incorporates reverse voltage and current protection, damaged cell detection, and a battery charger (making daily operation practical). As a pack nears empty, it signals another to switch on (a damaged pack disqualifies itself). All packs completely discharge and charge in round-robin fashion, avoiding memory effects. Capacity is easily increased by adding packs to the payload section.

Two Generations of Silent Running with LonWorks Technology

The Ocean Explorer, first launched in July, 1995, was not the first AUV based on LonWorks technology. Her predecessor, the Ocean Voyager II, demonstrated the benefits of a control network based approach more than a year earlier (though lacked the structural philosophy that distinguishes the newer craft). And as Ocean Explorer continues to probe the Florida coastline, Dr. Smith is proclaiming his feelings about the technology in the most concrete way possible: he’s already hard at work on two sister vessels.

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