AUTONOMOUS UNDERWATER VEHICLE

Propulsion

Vertical Thrust

Controloverview

The AUV Team

Thrust Experiment

Pool Testing

The ultimate purpose of the AUV is to participate in theannual International Autonomous Underwater Vehicle Competition sponsored by ONR and AUVSI. It takesplace at the US Navy SPAWAR Systems Center in San Diego (pictured). The competitioninvolves such tasks as locating bins on the pool bottom, dropping markers, locating sonar pingers, and surfacing in an specified space. Participants in past competitions have includedschools such as MIT, Cornell, Virginia Tech, Duke, and many others. With continued Success for UMaine’s AUV team, the possibility of participating in the next one or two years is very good.

The solution to the problem of propulsion proved to betwo small Sevylor SBM 12 Volt trolling motors. The motors are light, and each provide about 15 pounds of thrust at full speed. The motors were modified to fit the AUV’s needs and mounted to the wings. Along with the control systems, these motors do a fantastic job propelling and turning the sub.

The submersion problem was solved with two Attwood12 Volt, 2.9 Amp, V750 bilge pumps. To get themaximum thrust out of the bilge pumps, an experiment was performed to find the optimal nozzle size. The idea is to get the AUV as close to neutrally buoyant as allowed by thecompetition rules. Once that has been achieved, the bilge pumps have the thrust it takesto submerge the AUV. The bilge pumps are located inside the hull, and have a combination of rubber tubing and PVC piping to connect them to the nozzles at the frontand back of the sub.

Derrick Brown Dominique Corriveau

The control system for the AUV has beenthe most important part of the work this year. The concept is essentially this: the motors are driven by motor controllers that run off a servo signal. This signal comes from a servo control board. The servo control board communicates with a computer (which can be on-board or not)via serial port. There are multiple programs which can easily send servo signals to the board, and it is also easy to do by programming. This gives the sub excellent proportional control, because the servo board gives 255 signals which would ideally give the motors 255 different possible speeds.

The computer used on board to control the AUV is the Prometheus LC PC/104. A PC/104 is essentially a PC with a different form factor. Due to the small size, PC/104s are widely used in applications where there is a need for an embedded programmable controller. The Prometheus LC is has a 486 MHz processor, 16 MB of RAM, and 1 MB of Flash memory. Because the AUV must becompletely autonomous and powered by on-board batteries, a feature whichmakes a PC/104 very desirable is the low power consumption. The Prometheus uses only 2 Watts total power consumption.

The bilge pump thrust experiment was performed to find the nozzlesize that would give the AUV the maximum thrust downward. Adigital scale was connected to the bilge pump, and a variety ofsignals were sent to the servo control board from the Lab PC. Thiscaused the bilge pump to thrust downward over the range that would potentially operate at. The rangeof signals was tested for each nozzle (ranging in diameters from ¼” to ¾”). The results gave a clearindication that the ½” nozzle was the right one to use.

Results

The final work done with the AUV was the pool tests. On the veryfirst time, the sub functioned marvelously, exceeding any expectationswe may have had for it. The thrust of both the motors and the bilge pumpswere more than enough for what the sub needed to move around quickly.It proved to have the capability to move forward quickly and turn sharply.This gives the teams who will take this project on in future years a greatstarting point. While the tests were not done with the computer on board,it was the PC/104 running them. This means that with a little more programming, the AUV will be able to run without a serial tether attached.