1. Wireless Power Transfer Via Inductive Coupling SENIOR DESIGN GROUP 1615 RYAN ANDREWS, MICHAEL DONOHUE, WEICHEN ZHANG

2. Objective Approach:  Use a closed loop microcontroller system.  Voltage across load will provide feedback to source.  Frequency and voltage will be modulated to alter Power. Statement: Design, build and optimize a Wireless Power Transfer (WPT) system utilizing inductive coupling. Explore power modulation to a receiver under changing conditions such as distance and pitch that a free floating sensor may experience. Model efficiency in air and water to determine the validity of inductive coupling in water as a means of power transfer.

3. Transmitter

4. Transmitter Overview:  Arduino Due generates high frequency sin wave using on board DAC from 0 to 3.3V.  High pass filter removes DC component, sin wave now +/- 1.65V.  Buffer amp isolates sub circuits.  Low pass filter removes high frequency components of signal.  Series LC circuit, containing primary coil transmits AC to receiver.  Frequency of signal from Arduino controllers power transmitted.

5. Arduino Due  32bit ARM core processor  CPU clock speed 84MHz  Runs at 3.3V  Sin Wave generation of up to 1MHz at 0 to 3.3V  Device to device communication via  SPI  I2C  UART  2 on board DAC  Write Rate of 1.74MHz

6. Reconstruction \ Low Pass Filter  Converts impulse of DAC into smooth wave form.  Cutoff Frequency = 200kHz High Pass Filter  Filters out DC component of sin wave.  Cutoff Frequency = 482Hz Graphs Courtesy of http://sim.okawa-denshi.jp/

7. OpAmp Buffer Use:  Isolates high pass, low pass filter and primary LC sub circuits.  Filter will not influence resonant frequency of coil.  Protects microcontroller from high current generated in primary LC. Specifications:  LM7171  Low cost - $2.81  Attenuation at 220MHz allowing high frequency signals to pass without attenuation.

8. Primary Coil  Litz wire having 30 strands of 0.1 mm diameter.  Shielding directs field lines outwards and protects the rest of the electronics from field.  Strong coupling up to 7cm. D = Diameter of Larger Coil Graph Courtesy of Wireless Power Consortium

9. Resonance and Voltage Magnification Simulated Voltage Across Primary Coil vs Frequency

10. Q Factor (Magnification Factor)

11. Coupling Coefficient k  A measure of the percentage of power from the EM field of the primary coil is induced in the secondary.  Ranges from 0 to 1  K>.5 is considered strongly coupled  Inversely proportional to distance.  Decreases when out of coils are out of alignment.

12. Power at Resonance Series LC

13. Receiver

14. Receiver Overview:  Secondary LC with same topography as transmitter to resonate at same frequency.  Full bridge rectifier plus smoothing capacitor for AC to DC conversion.  IC Chip DF04M used for bridge rectifier.  10uF smoothing cap chosen experientially. No visible ripple present.  Voltage Divider to scale down from 40Vmax to 5Vmax for ADC conversion.  Buffer protects microcontroller from high input current.  Microcontroller takes ADC conversion.

15. Micro-controller Interaction:  Performs ADC conversion every 1ms.  Target Voltage Specified in Advance  Transmits voltage to Arduino Due via SPI communication.  Arduino steps down frequency by 500Hz if Voltage is too large.  Steps up voltage by 500Hz if too low.  Start frequency selected to be 10KHz off resonance  Max frequency at resonance.  Voltage Delivered kept constant under changing distance / pitch.

16. Underwater Application In the waterIn the air Speed of Sound at T=20° C1484 m/s343 m/s Relative Permittivity80 F/m1 F/m Relative Permeability1 H/m 1.257 x 10 ^- 6 H/m Conductivity10^-3 S/m 3x 10 ^-15 to 8x 10^ -15 S/m  Predicted efficiency in salt water is approximately 20% as efficient as air.  Based on independent research from the WiTricity Corporation.  A device could be charged with no external ports using an internal coil. Allowing for underwater devices to be fully water proof.  Device would have to be kept within transmission distance to avoid large loss.  Best to mechanically hold the device stationary.

17. Alternate Design Considerations  Larger coil for larger transmission distance.  Gain applied at transmitter buffers for greater power delivery.  Stepping down DAC amplitude rather than frequency.  Another Atmega328p was considered as a signal generator until the DAC sample rate was determined to be too slow to produce proper frequencies.