Inductive Power System for

Autonomous Underwater VehiclesTim McGinnis

University of Washington, Applied Physics Lab

Inductive Power System for

Autonomous Underwater VehiclesTim McGinnis

University of Washington, Applied Physics Lab

• Major Components– 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

ALOHA-MARS Mooring Sensor Network

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

DC-HFAC Driver (in Float Pressure Housing)

HFAC-DC Rectifier (in Vehicle Pressure

Housing)

Vehicle Battery Housing

EMI Filter/Transient Protection

Input150-400Vdc

Primary SideUnderwater

Coupler

Resonant Converter

BoostRegulator

In-Rush Current

Limit

Secondary Side

UnderwaterCoupler

400Vac50kHz

Driver Side Controller

Active Rectifier

Secondary Side

Controller/Li-Ion

Battery Charger

16.4VdcLithium Battery

Pack

Loads

Voltage Regulator

12V

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

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6Physical Gap mm

Ou

tpu

t P

ow

er

Wa

tts

0.64

0.66

0.68

0.7

0.72

0.74

0.76

0.78

0.8

0 1 2 3 4 5 6

Physical Gap mm

Eff

icie

nc

y

14Vout 140 Vin

16Vout 140Vin

18Vout 140Vin

14Vout 240Vin

16Vout 240Vin

18Vout 240Vin

20Vout 240Vin

Efficiency vs. Gap and Output Voltage

4 cell Lithium-Ion battery pack charging profile5 packs in parallel require 15A charging current

0

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

Vo

lts/

Am

ps Voltage

Current

Li-Ion Battery Charging Profile - constant current to 16.4V then constant voltage

Battery discharge profile at different loadsvoltage is reasonable indicator of capacity

Vout(t)

VISEC(t)

Gate Drive

Active Rectifier

V1

ISEC

PICMicrocontrollerIout(t)

PWMLowPass Filter

+

Vout(t)

-

Iout(t)

+

--

+

Synchronous Ramp

Reference

Vout(t)

VISEC(t)

Gate Drive

Active Rectifier

V1

ISEC

PICMicrocontrollerIout(t)

PWMLowPass Filter

+

Vout(t)

-

Iout(t)

+

--

+

Synchronous Ramp

Reference

Limit switch to indicate profiler in dockSpring 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?