ARVP’s previous autonomous underwater robot, AquaUrsa, was retired after 4 years and three major rebuilds. Even though AquaUrsa had shown fantastic reliability within the last few years, signs of fatigue and material failure led the mechanical team to the retirement decision in 2016. A subteam was created early that year dedicated to designing a new robot for Robosub 2017. In December the robot’s design was finalized, the engineering calculations were completed, and the robot was named Auri.
Inspired by the fictional spaceships TIE Fighters from the movie franchise Star Wars, an octagonal outline was given to Auri. In Auri’s design process, the team aimed for keeping AquaUrsa’s best characteristics and improving the weaknesses. The aluminum frame and the transparent acrylic hull were two of the most important characteristics that were passed on to Auri. The battery enclosures and torpedo assemblies were also successfully integrated into Auri from when they were used on AquaUrsa. With six degrees of freedom compared to AquaUrsa’s five, a 30-pound drop in weight, and a significantly cheaper cost of manufacturing, Auri is without a doubt the most incredible underwater robot that ARVP has ever designed.
Auri Frame Redesign for 2018
Last year, Auri’s frame was redesigned to improve rigidity and balance, as well as to provide mounting locations for new components required for the 2018 Robosub competition. For better control, the thrusters are now positioned on the planes about the center of mass. By integrating the battery pods into the ribs of the frame at the midline, the center of mass and buoyancy are now closer together, allowing the robot to hold any orientation with very little effort. This frame is much easier to assemble and is self-standing without the central hull in place. Slots were integrated into the trays to allow for easy adjustment of weights to balance the robot. Additionally, the top trays now have torpedoes mounted on curved slots to easily adjust the aim of the torpedoes. This new frame has also been designed to mount a Nortek DVL to progress our robot into a new level of spatial awareness. Mounting locations for our new dropper system, hydrophone assembly, and much more are included as well.
The hull for Auri was completely redesigned from AquaUrsa. The best parts of the old design were kept in mind alongside new innovative ideas to solve issues from previous years. Hence, the final hull design is stronger and more practical than ever before. It is composed of a central aluminum section, from which the electronics trays are supported, and two acrylic tubes on either side which cover and house the trays. Two clear acrylic tubes allow for both visibility and easy access to front and back electronics trays. This means that the trays, although easily removed themselves, can be worked on without detaching them from the robot. The inner diameter of the tubes are only 7.5”, which is much smaller than that of the previous hull. This was chosen to reduce the buoyancy of the robot and thus reduce the weight required to sink it. The end caps are also clear acrylic disks to provide visibility for the onboard cameras. The cylindrical shape of the acrylic tubes allows for even pressure distribution to reduce stress while the octagonal shape of the aluminum section provides multiple flat surfaces for easy mounting of components.
Aluminum was chosen for the central section because it is not magnetic, meaning it would not cause a magnetic interference with any of the electrical components. It is also very easily machined, allowing for this custom octagonal shape to be made. The flat surfaces of the central section allow for subconn connectors and cable penetrators to be mounted for connection between the electrical boards inside the hull and the components outside of it. The cylindrical steps on either side of the aluminum allow for two sets of o-rings to be mounted, providing a reliably watertight seal for the hull. There is also versatility in the orientation of the electrical trays as they can slide into the hull either horizontally or vertically.
Auri’s frame was designed to be more compact, functional, and aesthetically pleasing than ARVP’s previous robot. The hull is friction-fit with two complete octagonal rings that are bolted to two lengths of aluminum angle. Attached to these bars are six aluminum ribs, making up the two wings. All components are located inside the wings, providing protection during transportation and testing. Moreover, portability is increased due to four rubber handles attached to the ribs. Bolted to the ribs are multiple trays and side panels. The trays and panels allow for the easy attachment and removal of a variety of components, including torpedoes, thrusters, and marker droppers. The 2018 redesign increased the size of the frame to allow for the mounting of a DVL and future mechanisms such as a cooling system or mechanical arm. The new frame is also more structurally stable and balanced. Finally, by integrating the battery pods into the ribs of the frame at the midline, the center of mass and buoyancy are now closer together, meaning the robot is able to hold any orientation with very little effort from the control system.
The vertical thrusters are mounted on the front and back of the hull through use of a gate mechanism that latches to the frame. This serves to mount the vertical thrusters centered and below the view of the forward facing camera, as well as to provide a mechanism to enclose the hull and seal the acrylic tubes inside the frame. The latches allow the acrylic tubes to be easily removed, granting quick access to the electronics inside. The redesign added guards around the thrusters to protect them against possible impacts in the pool or during transportation.
Due to the added components and the larger frame this year, the robot is heavier and the thus more buoyancy is required. With the addition of thin, 2-inch diameter PVC tubes strategically placed above and below the center of mass, the required buoyancy is easily achieved without significantly increasing the mass of the robot. Due to the versatility of these tubes, they can be used to easily move the center of buoyancy of the robot around as desired. When placed towards the top of the robot, the robot is very stable and has a strong tendency to stay upright in the water to allow for easier control. Since the tubes are placed above and below the midline, the center of mass and center of buoyancy are brought closer together, reducing the work required from the control system to hold certain orientations.
Auri has two separate battery enclosures, which are now slightly bigger to accommodate Auri’s new batteries. These enclosures are now embedded into the frame at the midline. The battery enclosures are made out of 3.5-inch acrylic cylinders, which are sealed at both ends with aluminum double o-ring flanges. The caps of the assembly are designed with 1/2-inch acrylic ends and have subconn electrical connections.
Marker Release Mechanism
Auri has been equipped with a marker dropper located underneath the front of the bottom left tray. The markers themselves are golf balls as per competition rules. The dropper is primarily made of 3D printed plastic and can hold two golf balls. The 3D printed nature of the dropper makes for a very lightweight design and a simpler machining process. When the target area is identified by the interior cameras, a waterproof servo will turn the arm of dropper allowing a golf ball to fall. Due to the shape of the arm, when the first ball is dropped, the other will still be held in place. This allows one ball to be released at a time as the arm will need to turn further to release the second ball.
The torpedo launching module was designed to occupy the least amount of space while delivering the required pressure to propel the torpedoes. This was accomplished using CO2 cartridges that are small when compared to an air tank and compressor. The torpedo launching module consists of a CO2 bucket changer that allows for the quick replacement of the cartridges. The bucket changer is directly mounted onto an on/off ASA regulator. This on/off regulator is connected through a 90-degree fitting to a variable pressure regulator that decreases the CO2 pressure from the cartridge to the desired value. The pressure regulator is threaded into an aluminum connection piece with a female end. A ¼” fitting is used to directly mount the aluminum piece to the 2-way solenoid valve. This design is rigidly mounted to Auri with a 3D printed mounting bracket that goes around the solenoid valve. This mounting bracket is screwed into curved slots in the upper tray of the frame to allow for the angle of the torpedoes to be easily adjusted. Mounted on both solenoid valves is a solenoid-to-tube connector that is press-fitted with a 123 mm long stainless steel rod. The torpedoes are held onto these pipes by o-rings. After the entire system is pressurized with CO2, an electrical signal is sent to the appropriate solenoid valve. This signal opens the valve allowing for CO2 to escape from the launch pipe to propel the torpedo forward. The torpedoes were 3D printed using PLA as the filament.
A sonar system was added to the design this year which includes three hydrophone sensors for detection and triangulation of the underwater pinger in the competition. This allows the robot to identify the correct sound frequency and locate the pinger to complete the associated tasks.