What is a Float?

A vertically profiling float is an underwater vehicle that uses a buoyancy engine to move itself through a column of water. A buoyancy engine is an emerging technology that changes its density so it can float or sink at will.

The main benefit of using a buoyancy engine over a more traditional propulsion method is energy efficiently. A buoyancy engine only needs to input energy for a small portion of the time that it is moving.

Vertical profiling floats are mainly used in the real world for surveying underwater environments and the atmosphere. When wings are attached to the floats they become able to glide horizontally. Floats are able to be deployed for months and travel thousands of miles on one charge.

Floats often have a suite of sensors that they use to collect data. Weather surveying floats often use a combination of sensors including temperature and salinity in order to predict weather patterns before they make landfall. The Sunk Robotics float currently only has a depth sensor, but it is built on a modular platform that could easily be upgraded with more sensors.

The Sunk Robotics Float

The Sunk Robotics float is our answer to the 2024 MATE ROV mission task. We designed and built a custom float fully in house. Rather than using a more traditional design where oil is moved from an internal hard bladder to an external hard bladder, we are using a simpler and safer pneumatic design where air is compressed and decompressed in a hard cylinder that houses the electronics.

The core of our float is our two foot long acrylic electronics tube. It acts as the housing for all of our other components and acts as a pressure vessel.

On either end of the electronics tube we are using custom seals to keep water our. On the top of our float we have a static seal that we made using our resin 3D Printers. On the bottom we have our dynamic seal that we machined out of aluminum using our lathe.

The dynamic seal was one of the most challenging elements of our float to create. Dynamic seals have very narrow margins for error. They need to be effective at keeping water out, while also limiting the amount of energy used to move the piston. This was further complicated because acrylic tubes are produced with a large margin of error, so we needed to account for that in our design.

To move our seal we are using a Pololu GB37-150 motor to move the dynamic seal. We chose this motor because it has enough torque to move the seal while under the pressure created by being12 feet underwater.

This motor is controlled using the electronics on own custom made PCB (Printed Circut Board). Our board is using an Adafruit QT Py ESP32-S2 as our microcontroller (a tiny programmable computer) and a Pololu md26a motor driver (a small device that allows us to control how much power our motor is getting).

The whole robot is powered with 8 AA batteries in order to comply with MATE rules for non-ROV devices. We are putting them in series and drawing current at two points: 3.3v for logic (the power going into our microcontroller and motor driver) and 12v for powering the motor. We made a revolver-cylinder style custom battery holder that taps power at both voltage levels.

We wrote our own firmware using the Arduino Programming Language (a superset of C++). Our electronics use an external WiFi Antenna that is attached through the top seal. Using the antenna for communication, our board hosts a Websockets server.

Our custom topside client connects to the Websockets server. We designed the interface to be simular to interacting with a computer through the shell.

Our float is able to process a few commands:

  • get_data - Uploads data from the depth sensor that has been stored over time. The client will then process the data into a graph using matplotlib.
  • profile - Does one vertical profile, descending the water column, then ascending it
  • break - Closes the connection

With all of our components working together, our float is able to successfully traverse the water column while using very little power. We intentionally designed the platform our float is built on to be easily modifiable. If there are more tasks added to the Float Mission Task in 2025, we hope that we can do small modifications to our preexisting to complete them.