Building a Sea Glider Part 1 - The Plan

2023-10-20 | By Antonio Velasco

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Building a Sea Glider Part 1 - The Plan

I’ve mentioned a couple of times throughout my previous blogs that I’ve built an Autonomous Underwater Vehicle–a Sea Glider that utilizes the principles of buoyancy and gliding to traverse the ocean. It’s been a couple of years since that project ended, and I’ve always wanted to get back to it. That day has finally come, and with the renewed knowledge that I’ve picked up along the way, I have a lot of improvements that I believe will make it more efficient and allow me to gather a lot more data. It’ll probably be a long(ish) process as I traverse school and my extracurriculars, but I’m determined to embark on this journey to both revisit one of my very first projects and make progress on it.

Some Context

An Autonomous Underwater Vehicle (AUV) is, as the name suggests, an underwater vehicle that moves autonomously. This entails a multitude of requirements: waterproofing, onboarding computers and electronics, and propulsion methods. The latter is addressed by the type of AUV, a Sea Glider.

Here's the one that my friend and I worked on for a Science Fair project:

The way that a Sea Glider works is through its buoyancy engine: a mechanism that changes the vehicle’s density and center of gravity, allowing it to dive or surface. The Sea Glider is calibrated to maintain a neutral buoyancy at rest. This means when it increases its density, it would sink or dive, and when it decreases its density, it would surface. The buoyancy engine is designed to take advantage of this. It works by intaking or expelling water into the Sea Glider, allowing it to increase or decrease its buoyancy at will. A higher buoyancy (and thus lower density) allows the glider to surface, and a lower buoyancy (and thus higher density) allows the glider to sink. This mechanism, coupled with wings that guide the AUV to “glide” forward, creates an undulating motion like so:

The wings provide lift for the AUV to glide forward, allowing it to move only on the energy required for the buoyancy engine to operate–which is typically very low as it just needs to intake water and expel water, as opposed to a propeller. This creates an extremely energy-efficient method of propulsion.

Sea Gliders are perfect for following ocean currents and collecting data with any onboard sensors. It is also important for oceanic exploration as they can cover a large distance with very little human input or adjustments. I’ve been working on them for this purpose and to possibly create a very low-cost AUV using everyday materials, allowing anybody to collect oceanic data and explore the vast seas.

My Previous Build

My first iteration was a heavily modified version from a SeaGlide kit. It utilized a water bottle as a chassis and had acrylic wings lined with a pink coating to make it smoother. The external parts were attached through 3D printed parts from the SeaGlide kit, secured by zip ties. pH and light sensors were also added to the outside of the structure to record data.

Moving onto the internal structure, a syringe was connected to the sipping part of the water bottle and waterproofed with epoxy. The syringe plunger would be pushed in or out, expelling or taking water by a servo-controlled with an Arduino Mini. The electronics system was powered by a 9V battery and featured a potentiometer to remotely control the time that the Sea Glider would dive.

The syringe plunger was attached to a cylindrical ballast, a heavy mass that would shift the center of gravity greatly, provided in the SeaGlide kit. When water was taken in, the density would increase and thus the glider would sink. To ensure that the wings would guide it forward, the glider would need to be tipped down. The ballast comes into play here when the syringe pulls in water, the ballast would also be moved to the front of the AUV and thus tip it forward.

What’s Next?

My previous AUV worked well in the short term. Eventually, I had to put the project on halt as the electronics system started to fail, and the waterproofing became undone. I was able to collect meaningful data with it and learn a lot about how it could be improved, but it was simply not designed to last very long and, as a result, started to fail after a year. Now that I’m returning to this project, I have a ton of ideas to implement.

• Onboard Sensors/Cameras. My old AUV didn’t have the pH sensor connected to the Arduino, it was all external. This also meant that we had to waterproof any sensors we placed onto the AUV, leading to more complications. Integrating the sensors with the AUV would allow the Arduino to collect or possibly transmit the data and would likely lead to greater redundancy. I’ve also been meaning to attach a camera to it because getting underwater videos of reefs would be totally sick.

• Larger Chassis. What I had initially from the kit was a typical water bottle. While this worked well for my applications, I didn’t have very much space to add any other components or the area to innovate. Having a larger chassis would give me room to play with and, if waterproofed properly, would last longer than a plastic water bottle. A larger chassis would also likely result in a larger buoyancy engine, so testing out the balancing would likely take the most time here.

• Improved Wings and Rudder. With the old AUV, my friend and I completed a Science Fair project where we tested how the wingspan would affect its dive cycle and distance underwater. Since then, I’ve found an appropriate wing aspect ratio for the glider. It’d be similar to current iterations where the wings appear stubbier as opposed to a very long one. I’d also like to be able to change the rudder’s direction, giving the AUV the capability of turning.

• Transmittance Capability. The final goal that I’d like to meet is to add the ability for the AUV to transmit its data so that I can see where it is, location-wise, and get data without a tether. This would allow me to also potentially send commands to it, whether that’s the rudder or changing the dive cycle. Admittedly, this may be very hard as it is underwater, but exploring this possibility would provide a fun challenge.

Being from Hawaii, there are so many beautiful features in the waters that would be fun to traverse and send an AUV with a camera into. I’m excited to get back onto this project, which could provide a sustainable option for oceanic exploration. Best of all, it shouldn’t run my bank account into the ground (fingers crossed!)

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