Final Design

The final design of the AUV is a bioinspired robotic jellyfish, taking advantage of the low forward hydrodynamic footprint and quiet tentacle propulsion system of the real animal. Controlled by a Raspberry Pi 2 Model B, the robot can take continuous 1080p video footage through its transparent camera dome, communicate via WiFi at the water’s surface, and actuate its tentacles and steering mechanism for some mobility. With a maximum runtime of several hours, the robot serves as prototype platform for the future implementation of the functional requirements originally proposed with the project.

The AUV housing is composed of a 3D printed acrylic housing with a 15.25 cm (6 in) transparent acrylic dome sealed on top, as seen in the figure above previously. All electronics and motors are contained inside, with a double O-ring dynamic seal allowing for the transmission of mechanical energy into the water to propel the device. The AUV moves utilizing a single stepper motor to actuate six tentacles attached around the rim of the circular housing. In a single movement cycle, the stepper motor first quickly turns, using polypropylene string to pull and compress the tentacles together, forming a jet of water. This power stroke propels the robot forwards. In order to return to the open tentacle position without moving backwards, the motor slowly rotates back, taking advantage of the elasticity of the tentacles to actuate the recovery stroke. By varying the speed of these two strokes, the mechanism can generate a small net motion.

In order to control the direction of the AUV, two planetary gears weighted with 375g rotate inside the housing, changing the AUV’s center of mass. Since this mechanism is powered by two small geared stepper motors, the Raspberry Pi is theoretically able to dictate the angle of both gears. When the weights are at 3 o’clock and 9 o’clock, the robot is upright due to the balance of moments across the center of mass. When both weights are moved to the same angle, the robot tilts at ~30° from the z-axis in that direction, allowing for steering in that direction.

The Raspberry Pi runs using the Raspbian OS, and can utilize open source computer vision (OpenCV) for control and tracking. A 128GB microSD card was installed for data storage, and two motor driver boards are mounted on top of the module for safe and easy control of the stepper motors in the design. A universal battery elimination circuit (UBEC) step-down converter is utilized in order to supply a safe and constant 5V to the controller, while also allowing usage of the full 14.8V of the battery to power the motors. A inertial measurement unit (IMU) could be connected directly to the general purpose input output (GPIO) pins of the controller, and the Wifi module used for surface communication is plugged directly into one of its USB ports.

All parts, with the exception of the laser cut gaskets, 3D printed housing, and machined double O-ring seal, are off-the-shelf and open source items. The final size of the robot is 18x18x28cm, and the total cost of raw materials for a single unit is less than $500. The robot is can be used in both open bodies of water and closed aquariums, and is fully functional in salty ocean water. The SD card installed in the Raspberry Pi can hold enough video data to last an entire mission, in addition to the operating system, code, and sensor data needed to run the robot.