Development

The NU MONET group is developing a framework for underwater acoustic communications research. Our most recent hardware establishes the ability to control Teledyne Benthos acoustic modems from the shore.

Additionally, we can remotely generate and capture arbitrary acoustic waveforms using a high-quality audio interface inside our smart buoy. MATLAB/Simulink or any ASIO-compatible software can be used to access the hydrophones and transducers.

Below is a summary of our most recent hardware as well as the steps we took in developing it.

In 2016, we designed and deployed a first prototype for a smart buoy. That buoy met most of its design goals, but had deployment issues and structural concerns when exposed to the elements and wave action in the ocean. With these results in mind, we overhauled the design in Summer 2017 and created a more robust smart buoy with a smaller form factor.

This is the Meduza smart buoy. In addition to weighing 63% less than its predecessor, the Meduza buoy is equipped with advanced high-speed wireless communications devices that allow remote control of USB devices inside the buoy. This means that a person can control equipment (an audio interface, a webcam, etc.) from the shore without having a physical connection to the buoy.

Electrical Design

XBee/Relay Board Power Management and Monitoring

XBee Pro S3B radio modules are embedded systems that operate in the 900 MHz frequency band. XBee has a maximum transmit power of 250 mW (24 dBm) and with an additional 6 dBi antenna gain and line of sight, communication is theoretically possible with 10 km separation between antennae. Power management is performed through configuration of the XBee’s GPIO pins. The state of these pins controls a relay board, which routes the power flow of four other main devices: The Bullet M2 Titanium, BeagleBone Black, QuadCapture, and SM-975 acoustic modem. The XBee also provides analog input pins, which are used to monitor battery voltage and other important data such as a water sensor.

Ubiquiti Bullet M2

The Bullet M2 is a high power Wi-Fi device that acts as part of a point-to-multipoint network between the shore and all deployed Meduza buoys. A Bullet M2 Titanium module at the shore acts as a base station to which many Bullets can connect, allowing control of multiple Meduza buoys from the shore simultaneously. The network provides transparent links between a laptop at the shore and the BeagleBone Ethernet port.

BeagleBone Black Processing Unit

The BeagleBone Black is a small embedded Linux board similar to a Raspberry Pi. It is tasked with controlling the Teledyne Benthos SM-975 acoustic modems and creating a USB over IP link from the Bullet Wi-Fi module to the QuadCapture audio interface. The BeagleBone Black replaces the Silex USB over Wi-Fi module from the first version of our smart buoy.

Roland QuadCapture Audio Interface

The Roland QuadCapture audio interface is an essential part of the arbitrary waveform generation capability in Meduza buoys. The QuadCapture provides 192 kSPS audio input and output for a maximum achievable bandwidth of 96 kHz. The device allows the preamp gain to be adjusted through software, so if a particularly strong/weak signal is detected, the gain in the preamp can be turned down/up. USB data from the QuadCapture is routed through several layers to facilitate remote control: USB over IP software running on the BeagleBone Black creates data packets that are output on an Ethernet port; the Bullet Wi-Fi module transmits this data to the shore; and a laptop at the shore decodes USB over IP packets, sending decoded USB data to host software. The link is fully transparent such that the QuadCapture appears to be connected directly to the computer and can interface with Simulink.

NiMH Batteries

Three parallel sets of ten NiMH D-size batteries are used to power the buoy. NiMH cells were chosen for the Meduza design because of their high power to weight ratio (250 J/g), improved fire risk over lithium ion cells, and easy rechargeability. Our NiMH assembly has a mass of 5 kg and provides 350 watt-hours of power at an output voltage of 12 V. In comparison to the lead-acid battery used in the previous design, the new NiMh configuration weighs 69% less while providing the same energy output, and the NiMH cells are recoverable if overdrained whereas lead-acid batteries are not.

Mechanical Design

Casing

An aluminum casing contains the battery compartment and electronics sections of Meduza buoys. The aluminum casing is capped at both ends by plastic pieces. O-rings waterproof the connections between the caps and casing, and a flexible plastic rod that sits in a groove secures the caps in place. The bottom end piece has eye bolts for attachment to shackles and a four-pin waterproof electrical connection for a hydrophone.

The buoy’s aluminum casing has a one-inch diameter hole drilled into its side 20 cm from the top. The cable for the acoustic modem is routed through this hole; waterproofing is performed by a cord grip with a custom 3D-printed washer that curves around the casing.

Battery Compartment

A 40 cm tall battery compartment holds three parallel sets of ten batteries. With each NiMH D-size battery weighing about 160 g, the compartment has a mass of roughly 5 kg when fully assembled. The physical arrangement of the batteries is six rows of five cells; a PCB at the top and bottom of the compartment make necessary electrical connections and provide space for a fuse. Additionally, there is a cylindrical channel running through the compartment to provide space for wiring between the bottom of the compartment and the electronics section.

Electronics Compartment
 
The Meduza electronics compartment is based on a metal plate that runs down the center of the buoy. A laser cut acrylic sheet is fixed to this aluminum plate, and components are mounted to the acrylic sheet by screws. Devices with mounting holes are secured to the acrylic sheet directly using standoff screws; FDM (fused deposition modeling) 3D printing technology was used to fabricate holders for smaller components. The 3D-printed parts are secured to the acrylic sheet with screws.
 
Antenna Assembly
 
Meduza buoys contain two antennae, one optimized for reception at 900 MHz and the other for 2.4 GHz (Wi-Fi).
Since the antennae would be extremely ineffective inside the aluminum casing, 3D printed pieces were fabricated to hold the antennae in place on top of the smart buoy. The 900 MHz antenna is 12 cm above the aluminum casing and the Wi-Fi antenna sticks out the top of the 3D-printed assembly. The Wi-Fi antenna fits snugly inside the 3D-printed top piece and shrink-wrap over the connection helps make the assembly splashproof. The assembly is secured by a plastic holder, which screws into the large plastic top piece and is made waterproof by an O-ring.

There were two major devices involved in our first prototype system: the Teledyne Benthos SM-975 acoustic modems and our own custom built Smart Buoy.

Teledyne Benthos SM-975 Acoustic Smart Modems

The SM 975 modem’s electronics are housed in a vacuum sealed glass sphere nestled within a two piece polyethylene hardhat. The vacuum seal allows the modem to descend to depths of up to 6,700 meters. An omnidirectional transducer extends from the top glass hemisphere and through a cutout in the hardhat. The bottom of the hardhat has an electrolytic dissolving “burn wire” which allows the remote release of the modem from the sea floor. Users can interact and control the modem via a proprietary serial port/power connection near the modem base. The modem contains a 28 Ah battery allowing the modem to run on battery life for up to one year between charges depending on usage.

Submerged modems present two major complications for testing and long term deployment: servicing and communication. The SM-975’s limited battery life requires the arduous process of regularly servicing the modems. Every time a battery needs to be replaced, the modems must be uninstalled, shipped out for servicing, and then re-installed on the seafloor. This process is both expensive and time consuming. Furthermore, though our research deals with communication over the acoustic channel, we also need a reliable method of communication to control our network and run underwater experimentation. This poses something of an issue as we are in the process of developing a reliable form of underwater communication.

In order to address these issues, we developed a Smart Buoy to provide power to the modem and reliable wireless (radio) communication to shore.

Smart Buoy

Our prototype Smart Buoy was based on a five foot, 12″ diameter PVC pipe, permanently capped on one end. The other end had a removable plug that allowed insertion and removal of the buoy’s internal frame and mounted electronics. Inside the buoy, a 55 Ah lead acid battery supplied power to the buoys mounted electronics. The total height of the buoy measured 7.5 feet.

Electronics: An Xbee radio module managed communications to and from the shore. The Xbee also controlled a four-socket relay board that distributed power to the other onboard electronics. These include a BeagleBone Black small form factor (SFF) computer, a Silex USB Device Server, a Wi-Fi enabled webcam (to be used in conjunction with the Silex), and a powerful LED capable of illuminating the buoy’s interior.

Design: The modem’s internal frame had several benefits. The removable frame provided easy access to all the modem’s internal components and allows us to fasten and organize the onboard layout. PVC piping is both highly modular and very inexpensive, so updates to the frame could be made quickly and cheaply.

However, the sheer size and weight of the buoy made transportation and management difficult. A complete redesign of the Smart Buoy led to the creation of the Meduza Smart Buoys currently deployed at Nahant.

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