Marlin V2’s hull is a 9.5-inch diameter acrylic tube, 25.875 inches long, and sealed by two anodized 6061-T6 aluminum endcaps.

The front endcap is sealed to the end of the tube with 3M DP420 epoxy, and the front camera dome is sealed using a face o-ring. An aluminum collar was added to the back of the tube, so that the back endcap seals to aluminum instead of casted acrylic, ensuring seal integrity.

The back end cap uses o-rings 3% smaller than normal specification and was given a 10-degree chamfer to ease insertion and extraction of the electronics rack. Four spring-draw latches on the rear of the hull help keep the back endcap seal watertight by preventing the endcap from sliding out of the tube. Thirteen waterproof SubConn connectors interface between Marlin V2’s internal and external electronics, and can be quickly detached and reattached without disrupting the watertight seal. Two aluminum mounts attach the hull to the horizontal plane of the submarine.

Marlin V2 maneuvers using eight BlueRobotics T200 thrusters, chosen for their high thrust and efficiency. Four are arranged horizontally at 45-degree angles and control strafe, yaw, forward, and reverse movements, while the other four are oriented vertically and control depth, pitch, and roll.

The frame is built from 6061-T6 aluminum components for high strength and relatively low weight. Sheets and L-pieces are machined using CNC machines and standard industrial tooling machines and then anodized with a type II coating. Modular Panels were also integrated to easily adapt the frame to changes that need to be needed. These panels can easily be printed or machined to fit our new needs. The top plane has five while the front and back legs each have one giving us a total of seven spots to mount components. With the 3D-printed modular panels, all of our new parts like grabbers and torpedoes can easily be added in multiple orientations and locations on the submarine creating more possibilities.

Marlin V2 runs two main code-bases at the same time. Boops-Boops is the higher level program that the Jetson executes, which controls sub trajectories, handles vision detections, and directs the AUV through the course. Maritime is the lower level program that is run on our Nucleo F767ZI microcontroller, which connects to our thrusters, Doppler-Velocity-Log, Attitude Heading and Reference System, pressure sensor, and kill switch. While Boops-Boops sends high level commands, maritime implements them by running a PID control program to send the appropriate thrusts to each thruster to move the submarine to its waypoint.

To monitor the submarine, we employ a web graphical user interface (GUI), that displays camera image feeds, the sub’s position and orientation in the water, and a remote control interface used when collecting images.

To control Marlin V2 with increased precision, the lower-level control program now implements a cascade PID loop, with position and velocity controllers. The output of the position controller on six axes, one for each degree of freedom, serves as the velocity setpoint. The output of the velocity controller or the velocity error, then becomes the force and torque setpoints on each degree of freedom. This array of force and torque setpoints is then mapped to provide a thrust value for each thruster to direct the submarine to correct its position and orientation error and converge on its waypoint.

To navigate through the course, Marlin V2 employs a vision pipeline of image acquisition, filtering and color correction, and OpenCV image processing to detect contours or PyTorch machine learning detection. Some tasks require OpenCV to determine the orientation, such as path markers, while others require machine learning for more complex image recognition. By combining in-house color correction algorithms with the upgraded YOLOv8s machine learning object detection model, this allows our AUV to see at increased distances with magnified accuracy and speed.

The simulator remains an integral tool for the software team, as it allows for instant debugging and validation of new navigation and vision detection code. By running Gazebo and designing the latest competition’s props according to their released specs, we can simulate competition settings to develop mission and motor control code to traverse the course. It serves as an invaluable tool, as it allows in-water pool testing sessions to be used primarily for validation of approaches that have already been tested in the simulator.

Mission generates a list of goals to complete throughout the competition run. Each goal contains a set of instructions, location, and point value. Based on the location of both the sub and the goal, we can compute an estimated time to completion for each goal, and visit the goals in an optimal fashion, maximizing our point value. Once we select a goal to complete, mission tells the corresponding vision function to start returning observations.