Underwater Capsule

Fall 2022

In my Senior Design Class, a classmate and I were tasked with creating a device to collect pressure and depth data at the bottom of a swimming pool.

Through the design process with the constraints we were given, my teammate Elliot Rosen and I decided on a cylindrical body to contain all of our electronics. A flanged lid would allow for easy attachment for fishing line to raise and lower the capsule, along with a mount for the exterior of the pressure sensor. The fishing line would be used in tandem with a pulley system so the capsule can be lowered smoothly into the pool. I used Fusion 360 to generate the assembly.

To measure pressure, we decided to utilize the Bar 02 10m pressure sensor from Blue Robotics. The sensor had a built-in watertight seal and threads for easy attachment, in addition to the fact that it was compatible with an Arduino when used jointly with a level converter. The Bar 02 also yielded more accurate data than other sensors available to us.

To provide a seal between the capsule and the lid, we opted for the AS568-229S o-ring. This o-ring has a rectangular cross-section, which is useful for both static and high-pressure situations and fits our purpose. We were looking for an o-ring that would compress to 20% of its height and stretch about 3% of its inner diameter. M5 machine screws would tighten the lid to the capsule.

We used a solid ABS rod as the material for the capsule. The chamber in the ABS was machined out using a Tormach CNC Machine. Using a CNC was important as we needed a high-precision slot to house our o-ring so it would stretch and compress the perfect amount. ABS was chosen over Aluminum due to ease of machinability, and ABS had a high enough density to sink in the pool at our desired velocity. The CAM was done in Fusion 360.

We decided to sand cast our lid out of aluminum, as it provided bonus points for the project and allowed us to create an irregular shape out of metal.

We generated a 3D print of the lid such that we could then sand cast it for the final capsule. After casting, we trimmed the surface of the lid using the CNC Tormach to create a tighter waterproof seal with the o-ring. In addition, we machined out the holes for the M5 screws since the sand casting made it very hard to accurately create them.

For the electronics, we soldered an assembly together containing a 9V battery, an Arduino nano, a 5V-3.3V level converter, a MicroSD card reader, and the Bar 02 Pressure sensor. We edited code provided online by Blue Robotics so that all of the components worked within our assembly, along with adding a function to calibrate the sensor to the current altitude.

Below is our final design, which shows the electronics fitting inside the capsule. After initial testing, we were worried having too small of a density due to the air pocket inside the capsule. To solve this, we bolted a scrap piece of steel to the bottom.

For the project, the capsule had to follow a specific path of descending at 1ft/sec, staying stationary at the bottom of the pool for 10 sec, then ascending at 1ft/sec. Initially, we acquired a motor and wrote code to power our spool at the correct speed to follow the mission profile accurately. Unfortunately, the motor was significantly too weak, and we decided to lower the capsule by hand. After collecting data for 3 trials, our best trial is shown to the right and compared to the desired mission profile (graphed in MatPlotLib).

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