Battery-less IoT Devices Battery-Less IoT Device

1. Battery-less IoT Devices Battery-Less IoT Device SD-DEC19 Team 21 Advisor: Dr. Henry Duwe Clients: Dr. Nathan Neihart Dr. Daji Qaio http://sddec19-21.sd.ece.iastate.edu/

2. Member Contributions • Antenna design and testing • Matt & Mohamed G • Power System & Rectifier design and testing • Derek & Adi • Microcontroller functionality, communications, and testing • Mohammed Al-Mukhaini & Brad • Website design/maintenance • Mohammed Al-Mukhaini & Derek • Documentation • All members • Outreach/ meeting coordination • Mohammed Al-Mukhaini

3. PROJECT PLAN

4. Problem Statement • General Problem statement: • Harvest RF energy and convert it into a form useable by a microcontroller • General Solution Approach: • Harvest and convert ambient RF waves into DC • Gradual charge and storage (capacitor) • Low Power Mode Microcontroller

5. Conceptual Sketch

6. Functional Requirements Energy Harvesting • Harvest RF signal from WiFi router Power • Long term low voltage storage • Regulated Voltage supply to microcontroller Rectifier • Rectified & Multiplied voltage ≥ 1.8 V Embedded Systems • MSP430 microcontroller • Record temperature via internal ADC • Store and transmit data at a later time

7. Non-functional Requirements • Portability • Prototype reasonable size • Compatible • Efficiency • Reasonable operation and charging time • Testability • Reliable for demonstration • Reasonable cost • Under $100

8. Market Survey • Not many other manufacturers • Powerspot kit • www.powercastco.com • Focused on harvesting • Not communicating • HUGE potential • Network of batteryless devices • Self-sustaining • Remotely located • Dependable

9. Potential Risks Device Safety • ESD • Fall height Financial Safety • Unfeasible to implement • Parts are too expensive Public Safety • Device is above head height • Touching may induce a shock

10. Resource Cost Estimate

11. SYSTEM DESIGN

12. Functional Modules

13. Antenna Circuit Platforms • Patch Antenna radiating at 2.4GHz • Easy to manufacture, low-profile • Generating low power • Moderate antenna efficiency Figure 01: 2.4 GHz Patch Antenna

14. Antenna Circuit- Simulation • Simulation, tuning, and validation done through HFSS • dB gain values, radiation pattern and S parameters

15. Power Circuit Platforms • Cockcroft-Walton Voltage Multiplier (CW) • Rectify & Multiply • 2 Stage multiplier • Primary Component • Schottky diode (Skyworks Schottky Diode SC-79) • Low forward voltage drop • Negligible leakage current Figure 04: Schematic of CW voltage multiplier Figure 5: PCB design of CW voltage multiplier

16. Power Circuit- Simulation • Using ADS (Advanced Design Systems) • Tests include: S-parameter, transient, and harmonic balance Figure 06: Rectifier circuit simulation Figure 07: Rectifier Schematic on ADS

17. Embedded Systems Platforms (MSP430FR2100) • MSP430FR2100 MCU • Voltage Range: 1.8v - 3.6 v • Low Power Modes • Different current supply demand • System clock up to 16MHz • FRAM - 1KB • Unified memory program, constants, and storage • 10-bit ADC • Integrated UART functionality

18. Embedded Systems- Testing and Simulation • MSP-EXP430FR5994 • Code Composer Studio • Energia Current Developments • Temperature data recorded and stored in FRAM • Previously recorded data is not overwritten • Display and measure energy profile

19. Building Block Implementations Figure 8: Hardware flow Implementation Figure 9: Software flow control Implementation

20. CONCLUSION

21. Project Milestones & Schedule • MCU code working on development board - May 2019 • Rectifier circuit built - Mid September 2019 • Energy harvesting circuit built - October 2019 • Prototype board assembled - October 2019 - early December 2019 • Final product assembled - Mid December 2019

22. Current Status

23. Plan for next semester • Finalize software and flash code onto FR2100 MCU • Incremental tests, culminating in full test • Delivery of final product

24. QUESTIONS?

25. Energy Storage - Capacitor • Capacitor to store charge • Power demand • X = 1.8V = min voltage for MSP430 • W = ideal steady operation power (watts) • t = operating time • Vs = starting voltage (output of rectifier) • VC = Vs * e-t / (RC) • W = X2 / R => R = W / X2 • VC = Vs * e(-t*X^2) / (WC) • X≤ Vs * e(-t*X^2) / (WC) • With all other variables found, solve for C

26. Justification - Rectifier V0 = 2nVMAX - ⃤ V0 Ideally, ⃤ V0 = 0. But due to not-fully-charged capacitors, we have: Larger C = smaller ⃤ V0 loss Proof: https://www.hindawi.com/journals/jece/2017/4805268/ (plan to test with several different types of capacitors empirically)

27. Justification- Schottky Diodes • No depletion region → low forward voltage drop • Electrons present on both sides of the junction • Current conduction due to electron movement only → negligible leakage current • No reverse recovery time • Instantaneous switching action

28. MSP430 Energy Consumption • Current draw drops rapidly when switching to lower power modes • Efficiency increases when operating at higher frequencies and lower voltages

29. FR 4 Parasitics Calculation • Capacitance Parasitics • Trace Inductance Parasitics • Inductance Parasitics

30. Potential Risks Headache Device Safety • ESD • Fall height Financial Safety • Unfeasible to implement • Parts are too expensive Public Safety • Device is above head height • Touching may induce a shock Electric Shock Electrostatic Discharge Short Circuit LOW RISK FACTOR