Wireless Underwater Power Transmission (WUPT) for Lithium Polymer Charging James D’Amato Shawn French Warsame Heban Kartik Vadlamani December 5, 2011 School of Electrical and Computer Engineering

2 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

3 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

4 Technical Specifications FeaturesProposed SpecificationsSpecifications Operating Frequency MHz kHz Phase Velocity1482 m/s Input Signal20 V Square Wave30 V Square Wave Distance to Transmit22” Matching Layer Thickness ”0.667” Transfer Efficiency10% Battery3.7 V, 160 mAh

5 WUPT System Transmitter Receiver Energy Harvesting Circuit Charging Circuit Battery

6 Transducer Dimensions 2.1” 2.5” Acrylic matching layer Stainless steel conduit sleeve Weight of 2.1 lbs

7 Piezo Electric Properties SM111 piezo material o PZT-4 50 mm diameter, 3 mm thickness 44 kHz +/- 3 kHz resonance 60% electromechanical coupling coefficient 8 Ω resonant impedance 7200 pF static capacitance Positive terminal Negative terminal

8 Transducer Cross Section Piezoelectric 30 MRayl Acrylic (0.67”) 3.67 MRayl Acrylic (0.67”) 3.67 MRay l Polyurethane 1.6 MRayl 5 minute epoxy (water-proofing) Stainless Steel Sleeve Water has an acoustic impedance of MRayl Polyurethane has high attenuation Stainless steel sleeve acts as heat sink Front Back

9 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

10 Energy Harvesting Profile 3 min. 30 sec charging time PGOOD goes high when V out is 92% of target value Buck Converter outputs constant voltage independent of V in

11 Battery Charging Circuit Low operating current (450 nA) 1% voltage accuracy 50 – 500 mA output current

12 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

13 WUPT Demo Configuration Distance of 22” between transmitting and receiving transducer Transmitter connected to function generator Receiver connected to energy harvesting circuit Receiver Transmitter

14 Results Input of 20 V pp square wave at kHz Output of 2.38 V pp sine wave at kHz Efficiency of 12% Specifications satisfied

15 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

16 Final Cost Analysis UnitPrice Nylon Sleeves$50 Epoxy$120 Small PiezoelectricsDonated Coaxial CableDonated Testing Apparatus$5 Lithium Polymer Battery$10 Circuit ComponentsDonated Large Piezoelectrics$36 Epoxy, Polyurethane, RTV, Caulk Gun$54 Acrylic Plexiglas$67 Total$342

17 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

18 Questions