Project BOb [Buoyancy Object]

BOb's purpose is to navigate a water column by controlling its buoyancy with a ballast tank, and to collect both video and water temperature measurements. Remotely controlled via an attached surface tether, BOb is remotely powered but also has on-board backup power. Project BOb explores the principles of fluid mechanics and underwater operation.

• Arduino controlled
• PVC pipe main shell with acrylic flanges/caps
• Dual-chamber construction isolates electronics from possible water ingress
• Ballast tank for buoyancy control
• Electro-mechanical valves
• Pressure sensors for ballast tank and outside pressures
• Temperature sensors for interior electronics compartment and exterior water temperatures
• Custom fabricated capacitive water level sensor for ballast tank water level measurements
• On-board IP camera for live video streaming with LED lighting
• Powered via PoE (Power over Ethernet) with battery backup in the event of tether damage
• On-board web server for monitoring and control

Construction

Flanges were fabricated from 6mm acrylic sheet and bonded to the PVC tubing. Acrylic was also used to mount the electronics to the baffles between compartments, and water tight connectors pass sensor and tether wiring through the baffle.

For measuring the water level in the ballast tank, a capacitive level sensor was fabricated using flat pieces of aluminum supported by acrylic. This approach was used since ultrasonic and photoelectric sensors have a blind range approaching the size of the tank itself; the sensing range required falls within the blind range. The sensor must also operate immersed in water within a sealed pressurized vessel.

Pressure Testing

To ensure safe operation, the ballast tank was pressure tested to determine the maximum sustained pressure. Failure occurred at 4.5 bar, providing a safety factor greater than 4 over the target operating pressure of 1 bar. The seals were leak-tested up to 1.4 bar.

Programming

BOb's software consists of an operator dashboard for control and monitoring, as well as a logic control program running on an Arduino UNO R3. In addition, an IP camera provides live video feedback.