wireless underwater power transmission (wupt) for lithium polymer charging

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