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