Neutral Buoyancy

February 12th 2023

My knowledge from working on my own personal project, BlueRay, inspired me to share my passions in school. As the chair of the robotics club, I lead our school's newly formed Singapore Autonomous Underwater Vehicle Competition (SAUVC) team this year, featuring the SEA-Dragon as seen in the above photograph. In this way, I can motivate others to get involved in the field of marine robotics and upskill my knowledge and experience in construction, physics, and electronics. 

Recently, the team and I completed construction of the first prototype of the robot following the BlueDOT ROV specification as a base. We were now excited to test the water proof seal and the neutral buoyancy of our robot. For those unfamiliar with the term, neutral buoyancy refers to the state where an object's weight is equal to the upward force exerted on it by the water. This is crucial to ensure that the robot can move without wasting thrust on maintaining it's current position.

To do so, we roughly calculated the mass required for the robot using archimedes principle of the displacement of water to estimate the buoyant force acting on the AUV. 

Using this formula, we predicted, conservatively, that the buoyant force that was approximately 3.5N and as a result we brought 700g of weight in screws, which added a weight of 7N would be able to more than sink the buoyant force convert the same density of the robot to be comparable to that of the water.

Our experiment involved placing weights inside the AUV and lowering it into the pool. We then observed its behavior underwater to determine if it was floating or sinking. If it was floating, we added more weights until it reached a state of neutral buoyancy, and if it was sinking, we removed some weights until it reached a state of neutral buoyancy. We found that on the 26th screw the robot began sinking, with approximately a mass of 14.6g per screw, we had an estimate of the mass required to weigh down the volume of the robot.

This experience was not only a lot of fun, but it also taught me about the importance of neutral buoyancy in underwater technology. Neutral buoyancy is critical for ensuring that an AUV does not sink or float uncontrollably, and it was exciting to see the effects of our modifications to the weights on the AUV's behaviour underwater.

This is a major milestone towards getting the robot functional underwater and obtaining the final design to continue with the simulation and optimize performance. With this, we have but to add the control system and make minor modifications to the frame structure to create a better thrust allocation matrix which will give us more freedom in the manoeuvres that the robot can undergo. Future updates will feature sensory equipment and how we can use them to analyse the world around the robot mathematically, and identify trends that hint at important information such as position and the correct action to take, given any particular scenario. I Look forward to seeing you all then!