In previous years, one of the largest problems with Marlin was the irregular horizontal cross-section that yielded balance problems. This caused Marlin to rollover and pitch forward when in motion. Changes to the overall shape of the hull were aimed at creating symmetrical cross-sections to increase control, leading to the design of two mounting planes that would evenly distribute the weight of the submarine. Now, Nemo’s center of gravity is geometrically centered and allows for easier maneuverability and an increase in the sub’s stability.

Almost all of the watertight enclosures on Nemo are made of carbon fiber. We chose carbon fiber due to its higher strength-to-weight ratio compared to other common materials such as aluminum alloy. Carbon fiber parts also have advantages in fabrication, allowing for unique shapes with virtually no material being "milled" away. Nemo's aluminum sealing lips are bonded to the carbon parts using an epoxy adhesive augmented with a bond line controller. O-rings were selected as the primary sealing device due to their proven reliability and low cost. ‍ Nemo’s main enclosure is pressurized to the equivalent underwater pressure at the maximum expected operating depth depending on its mission. This means that at depth, there are virtually no pressure forces acting on the hull. In addition, Nemo is able to detect a failed seal, indicated by a drop in air pressure, and make an emergency recovery without any ingress of water.

To meet our budget constraints and strict tolerances, the AVBotz team performed all of the carbon fiber enclosure production “in-house". Through discussions with experts in the composite industry and research into carbon fiber, the team used the wet-layup and vacuum-bagging technique.

In previous years, batteries were placed inside the main enclosure, making it difficult to replace batteries during pool tests and exposing fragile electronics to damage from handling or from water. Nemo has adopted external battery boxes, consisting of a lip, lid, and a carbon hull. The four draw latches allow for tight water sealing while also allowing quick access to the batteries. Since these enclosures will be accessed very frequently, an unique dovetail o-ring groove is added, negating any risk of an improperly seated o-ring causing a leakage. SubConn connectors allow for safe connection and disconnection even when the connectors are wet.

Nemo maneuvers using eight VideoRay M5 thrusters, chosen for their high thrust and efficiency: four are arranged on the horizontal plane at angles to control sway, yaw, and surge movements, while the other four are vertically mounted and control heave, pitch, and roll. The thrusters collectively use up to 750W and generate up to 23 lbs of thrust.

The electronics rack was modeled using a top-down design technique with the goal of fitting inside the smallest volume to minimize any extra buoyancy. The rack is made out of strong laser-cut plywood, allowing holes to be easily tapped for mounting any future electronics, or parts can be easily swapped due to plywood’s cheap cost and ease to work with.

The front camera enclosure consists of six o-ring seals and eighteen sealing washer, making it one of the single most complex parts. Four threaded rods are used to compress together the external lens and sealing flanges against Nemo’s internal pressure. Face-type o-ring seals are located in the interface between the lens and the forward flange and the interface between the back plate and Nemo’s main electronics enclosure.

The software stack consists of two programs. The first runs on a microcontroller and it is called Nautical. Its job is to maintain the state of the submarine and control its movement. Nautical also manages all manipulators on the submarine and it is the backbone of the sub. The second program runs on the main computer onboard the submarine, which is called Boops-Boops. This program issues commands to Nautical to complete the tasks. Porpoise runs in an Ubuntu installation using Robot Operating System (ROS) to communicate information between different subroutines that are used for controlling the sub.

Interface is the master program that starts each process on the software stack. Our low-level controller runs on the ATMega2560 microcontroller. It connects to the DVL, AHRS, and thrusters via serial UART pins. It uses six PID filters, one for each bearing of the submarine. The resulting PID vector is multiplied by the thruster matrix, mapping the effect of each thruster on each element of the state, to produce the desired thruster power. The state consists of the (X, Y) position, computed by integrating the DVL velocity readings, the depth determined from the analog pressure sensor, and the roll, pitch, and the yaw from AHRS. Because of the simple serial protocol, Nemo can be controlled manually through a terminal for testing and debugging use. Furthermore, all configuration variables (e.g. PID gains and the thruster matrix) can be adjusted over serial communication to empirically tune the optimal values on the fly.The software stack consists of two programs. The first runs on a microcontroller and it is called Nautical. Its job is to maintain the state of the submarine and control its movement. Nautical also manages all manipulators on the submarine and it is the backbone of the sub. The second program runs on the main computer onboard the submarine, which is called Boops-Boops. This program issues commands to Nautical to complete the tasks. Porpoise runs in an Ubuntu installation using Robot Operating System (ROS) to communicate information between different subroutines that are used for controlling the sub.

Competition tasks are associated with machine learning models that have been pre-trained with data from past competition logs, pool tests, and simulation runs. As we proceed through the tasks, our mission stack runs through a pipeline of image acquisition, downscaling, color correction, and finally PyTorch object detection. Our innovative approach of correcting and filtering out haze in the image using an in-house proprietary algorithm, coupled with our upgraded YOLOv5s model, allows our AUV to see at increased distances with magnified accuracy and speed.

The hydrophone code for our AUV is written in C and it runs on an FPGA which reads data from three hydrophones located outside the main hull. The analog-to-digital converter (ADC) receives amplified signals from the hydrophones at 500 thousand samples per second. Using each analog signal, we apply a Discrete Fourier Transform, with a variety of optimizations, to filter out undesirable frequencies. The time at which the ping is detected is recorded for each hydrophone, then the time difference of arrival (TDOA) is computed. Using a form of multilateration, the hydrophone code solves a system of equations to calculate an approximate angle to travel towards the source of sound. It then repeats this process at multiple points, and it uses calculus to identify the point that is closest to the lines formed by these angles. This deals with the potential error from physical measurements and from the DFT.

The simulator is a critical part of our submarine’s software stack. Throughout the development cycle, software engineers work inside the simulator to test,refine, and optimize, the subroutines used to complete the tasks. The sim consists of the competition’s TRANSDEC environment rendered with all the competition props stationed inside. Gazebo, which is the base software that our simulator is built on, synthesizes the visual conditions experienced underwater,also varying the position of props from run to run to prevent over optimization. With this we are able to rigorously and extensively test the Porpoise stack in depth, in essence repeating the competition thousands of times before ever arriving to TRANSDEC.