Inductive Power System for Autonomous Underwater Vehicles Tim McGinnis University of Washington, Applied Physics Lab

ALOHA-MARS Mooring Sensor Network Major Components Features Seafloor Secondary Node with Sensors Subsurface float at 200m depth with Secondary Node and Sensor Suite Mooring profiler with sensor suite that can “dock” for inductive battery charging 1700m electro-optical seafloor extension cable with E-O Converters 800m electro-optical mooring cable with E-O Converters Features Cable connection provides high power and real-time communications Mooring profiler uses inductive modem for continuous comms MARS compatible ROV-mateable Science Connectors on Float & Seafloor Nodes ROV servicing and installation of sensors Deployments 2007 in Puget Sound at Seahurst Observatory in 30m water depth 2008 on MARS in 950m water depth

Inductive Power System Requirements Needed to transfer several hundred watts to profiler Could not generate enough mate/unmate force for connectors Preferred non-conductive technique due to conductive medium Decided to use inductive transfer

S&K Engineering Principals had worked in the Electric Vehicle (EV) industry Working with US Navy on underwater inductive power transfer technology

Block Diagram Powered from 375VDC from MARS Node Driver chops input at 50kHz AC signal transferred across couplers AC signal rectified & regulated to 16.4VDC Microcontroller manages Lithium-Ion charging profile

Maximum Output Power vs. Gap Maximum Output Power at 14.6Vdc 50 100 150 200 250 300 350 1 2 3 4 5 6 Physical Gap mm Output Power Watts Maximum Output Power vs. Gap

Efficiency vs. Gap and Output Voltage

4 cell Lithium-Ion battery pack charging profile 5 packs in parallel require 15A charging current

Li-Ion Battery Charging Profile - 2 4 6 8 10 12 14 16 18 0:00 0:05 0:32 1:20 1:54 3:20 5:30 5:44 6:03 Elapsed Time Volts/ Amps Voltage Current Li-Ion Battery Charging Profile - constant current to 16.4V then constant voltage

Battery discharge profile at different loads voltage is reasonable indicator of capacity

Limit switch to indicate profiler in dock Spring shock absorber

Preparing for deployment in Puget Sound

Profiler with inductive couplers (docked) in the water

Operation Profiler is programmed with minimum voltage (15V) – allowing 25-30% battery capacity remaining When profiler detects voltage below minimum, it returns to charging dock IPS driver turns on every minute – if current is flowing it continues to operate, if no current it turns off Profiler is programmed with minimum current (3A) When minimum current is reached, profiler resumes profiling IPS turns off at 1.5A to prevent over-charging if profiler present

Results of 2 month Deployment Deployed June 26, 2007 Driver FET Failure on Aug 27, 2007 Completed 4984 profiles Charge interval was approximately 7-8 days Currently investigating reason for FET failure

Current Status

HOT Profiling Mooring Hawaii Ocean Time-series (HOT) site No cable to shore – large battery pack Iridium, FreeWave, Acoustic Modem comms 5000m water depth Test deployment in Puget Sound 2008 HOT deployment in 2009

Future Plans Implement “fuel gauge” – challenging to do with multiple charge/discharge cycles Utilize Smart Battery Data (SBD) available over System Management Bus (SMB) Improve efficiency by: Modulate switching frequency with current Optimize gap Make profiler & coupler ROV removeable

Questions?