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Wireless Power Scavenging

Wireless Power Scavenging. Chris Burgner and Will Kelleher. introduction. Wireless energy is everywhere! Common sources include: Cellular telephones Wireless internet ( WiFi ) AM and FM radio Can power be extracted from these signals?. Method.

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Wireless Power Scavenging

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  1. Wireless Power Scavenging Chris Burgner and Will Kelleher

  2. introduction • Wireless energy is everywhere! • Common sources include: • Cellular telephones • Wireless internet (WiFi) • AM and FM radio • Can power be extracted from these signals?

  3. Method • Absorb incident RF energy using a highly efficient antenna • Passively increase the voltage • Accumulate charge • Use charge to power a small sensor

  4. The Antenna

  5. Initial Antenna Design • Antenna is clearly an important system component • Initial design focused on “frequency independent” antennas • Constant pattern polarization, and impedance over a wide range of frequencies • Self Complementary Archimedean Spiral – one type of frequency independent antenna • Want to capture electromagnetic energy across many bands with the same circuitry

  6. Archimedean spiral design • Upper and lower frequency limits determined by smallest and largest diameters respectively • Initial target band: 800-2500MHz • Pros: usable over a wide range of frequencies, constant impedance • Cons: Large, hard to construct, must use a complicated Dyson balun

  7. Antenna fabrication • T-Tech Quick Circuit 5000 milling machine located in Professor Bernhard’s antenna lab • Duroid substrates donated by Rogers Corp • Machine takes an input CAD file, proceeds to mill off “unwanted” copper from the substrate • Learning curve exists

  8. Archimedean spiral design

  9. Change of strategy • Still wanted multiple widely used frequency bands, but needed a simpler and easier to fabricate antenna • Solution: Focus on 1850-2500 MHz, includes PCS CDMA, and Wi-Fi • New Antenna Design: Log Periodic Toothed • Similar to the spiral, longer features correspond to lower frequencies and vice-versa • Various arc-lengths refer to quarter wavelengths at those operating frequencies

  10. Log periodic toothed

  11. Strategy change once more • Rather than have an antenna perform “adequately” over a large frequency span, why not use an antenna which performs very well at a specific frequency (band of choice: 802.11, aka Wi-Fi) • Solution: Narrowband resonant antenna • Choice: Microstrip Patch • Pros: Extremely easy to fabricate, very durable, easily mountable • Cons: Very narrowband, not omni-directional • Designed with 50Ω impedance, and 5dB of gain

  12. Slight (final) strategy change • Microstrip patch designed for Wi-Fi was a good option • Weren’t getting enough power, options to remedy that: • Decrease frequency (not possible, no control) • Increase transmit power (no control) • Increase gain of transmitting antenna (no control) • Increase gain on receiving antenna (full control) • Final antenna: 2x2 Microstrip Patch Array • Designed for 50Ω (70Ω actual) impedance, and roughly 12dB gain

  13. Microstrip patch array

  14. Performance • Test setup: HP E4432B Signal Generator • Signal: 2.45GHz at 20dBm (100mW) transmit power, roughly 0.5 m from the signal analyzer • First test (control)

  15. performance • Log Periodic Tooth

  16. performance • Microstrip Patch

  17. performance • Microstrip PatchArray

  18. Analysis • Patch outperformed both the standard monopole, and the log period tooth • Log periodic tooth, however, still operated well at lower frequencies where the patch antennas struggled • Best choice: Patch array • Areas for improvement: • Larger array, more elements = more gain • Design our own power combiner for impedance match

  19. The voltage Multiplier

  20. Problem and solution • Captured RF signals will have a low voltage – on the order of milliwatts • The sensor requires 2.4-5 V to operate effectively • Active methods: • Charge pump • Boost converter • Passive methods: • Half-wave voltage multiplier (Villard cascade) • WE NEED A PASSIVE SOLUTION!

  21. Half-wave voltage multiplier • Uses diodes to direct current flow between successive capacitors • Cascading these sections can yield significant gains

  22. ADS Simulation of an 8-stage cascade The output voltage of the cascade requires some time to charge to its maximum level. Simulation details: Vin = 300 mV f = 10 MHz Vout ~= 2.4 V Multiplication -> 8x

  23. Prototype 1 - Design • Why? - Characterize higher frequency performance! • Start with a 4-stage cascade • Constructed using RF test boards • Components: • 470 pF thin-film capacitors • Schottky detector diodes

  24. Prototype 1 - Results Voltage Measurements Power Losses Approximately a 4 dBm power loss Impedance mismatch ~= 0.4

  25. Prototype 11 - Design • Increase from 4 stages to 8 stages • Very difficult to construct, likely cause of some strange behavior

  26. Prototype 1i - Results • The 8-stage cascade provides a range of multiplication levels: • 50 mV -> 9.4x • 150 mV -> 16.6x • 300 mV -> 22.2x • The voltage output appears to increase in a non-linear fashion

  27. Final prototype Design Temp. sensor From ant. Single stage

  28. Final PCB

  29. Final Results

  30. Multiplier summary • The multiplier requires a minimum of approximately 100 mV to achieve the desired output voltage to power the sensor • Adding extra stages increases the internal losses of the multiplier, resulting in a performance ceiling • Impedance matching was not necessary due to a close enough match to the 50 Ohm antenna

  31. Sensor • Any low-power load will suffice • Our circuit uses a simple temperature sensor • Requirements: • 2.4-5.5 V • 4.8-8 uA • Outputs a voltage which represents a temperature reading defined by the transfer function to the right

  32. Sensor test • Test setup: • Multiplier/sensor circuit fed from a signal generator (2.4 GHz, -2.5 dBm) • Sensor output read via multimeter • Reading compared to a separate digital thermometer to verify • Results: • Sensor voltage = 1.59 V • Calculated temperature = 74.5 deg. F • Test thermometer displayed 73.5 deg. F • These results are close within a reasonable margin of error

  33. Conclusion • Ambient power harvesting is still viable, but it is most useful in applications that do not require a “high” voltage • Efficient antenna design is critical to harvesting reasonable amounts of power • Energy accumulation, storage, and dispensing control is also very important when dealing with small amounts of power – our project failed at this aspect

  34. Thank you • Professor Bernhard • Michael Daly • Jessica Ruyle • Jim Kolodziej & the ECE 445 staff • Rogers Corp.

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