Wireless Underwater Power Transmission (WUPT) for Lithium Polymer Charging
James D’AmatoShawn French
Warsame HebanKartik Vadlamani
December 5, 2011
School of Electrical and Computer Engineering
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Project Overview
• Goal: Provide wireless solution to recharge submerged battery cells
• Target Customer: Upstream oil exploration industry• Motivation: Increase longevity of submerged acoustic
sensors• Target Cost: Prototype < $350
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Design Objectives
• Convert an electrical signal to an acoustic signal
• Transmit acoustic signal through water
• Generate a voltage from the acoustic signal
• Rectify and amplify voltage
• Charge a lithium-ion battery
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Technical Specifications
Features Proposed Specifications SpecificationsOperating Frequency 2.1-2.3 MHz 41 - 47 kHz
Phase Velocity 1482 m/s 1482 m/s
Input Signal 20 V Square Wave 30 V Square Wave
Distance to Transmit 22” 22”
Matching Layer Thickness
0.0008” 0.667”
Transfer Efficiency 10% 10%
Battery 3.7 V, 160 mAh 3.7 V, 160 mAh
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WUPT System
Transmitter
Receiver
Energy HarvestingCircuit
ChargingCircuit
Battery
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Transducer Dimensions
2.1”
2.5”
• Acrylic matching layer
• Stainless steel conduit sleeve
• Weight of 2.1 lbs
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Piezo Electric Properties
• SM111 piezo materialo PZT-4
• 50 mm diameter, 3 mm thickness
• 44 kHz +/- 3 kHz resonance
• 60% electromechanical coupling coefficient
• 8 Ω resonant impedance
• 7200 pF static capacitancePositive terminal
Negative terminal
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Transducer Cross Section
Piezoelectric 30 MRayl
Acrylic (0.67”)3.67 MRayl
Acrylic (0.67”)3.67 MRayl
Polyurethane1.6 MRayl 5 minute epoxy
(water-proofing)
Stainless Steel Sleeve
• Water has an
acoustic impedance of 1.438 MRayl
• Polyurethane has high attenuation
• Stainless steel sleeve acts as heat sink
Front
Back
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Energy Harvesting Circuit
Piezoelectric• 2.7 – 20 V Input Operating Range
• Low-loss Full-Wave Bridge Rectifier
• 100 mA Output Current
• Buck DC/DC Converter
• Selectable Output Voltages of 1.8 V, 2.5 V, 3.3 V, 3.6 V
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Energy Harvesting Profile
• 3 min. 30 sec charging time
• PGOOD goes high when Vout is 92% of target value
• Buck Converter outputs constant voltage independent of Vin
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Battery Charging Circuit
• Low operating current (450 nA)
• 1% voltage accuracy• 50 – 500 mA output
current
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Lithium Polymer Charging Profile
• LTC4070 adheres to this charge profile
• Li-po battery is 3.7 V, 160 mA
• Icc is 0.7C Icc = 112 mA
• Itc is 0.1C Itc = 16 mA
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WUPT Demo Configuration
• Distance of 22” between transmitting and receiving transducer• Transmitter connected to function generator• Receiver connected to energy harvesting circuit
ReceiverTransmitter
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Results
• Input of 20 Vpp
square wave at 46.77 kHz
• Output of 2.38 Vpp sine wave at 46.77 kHz
• Efficiency of 12%
• Specifications satisfied
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Problems
• Initial transducers were operating at too high of a frequency
• Matching layer was not a precise thickness nor was effectively impedance matched
• Backing layer was not acoustically matched to transmission medium
• Nylon sleeves were reflecting heat• Energy harvesting circuit currently not matching output
profile
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Final Cost Analysis
Unit PriceNylon Sleeves $50
Epoxy $120
Small Piezoelectrics Donated
Coaxial Cable Donated
Testing Apparatus $5
Lithium Polymer Battery $10
Circuit Components Donated
Large Piezoelectrics $36
Epoxy, Polyurethane, RTV, Caulk Gun $54
Acrylic Plexiglas $67
Total $342
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Future Work
• Implement piezoelectric transducers with more suitable internal acoustic impedance for better matching
• Develop polymer matching layer that can meet desired requirements
• Implement charging and end-of-charge feedback signals to charging source
• Increase effective range
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Questions