PROMETHEUSIntelligent Multi-Stage Energy Transfer System for Near Perpetual Sensor Networks

Xiaofan Jiang Joseph Polastre David CullerElectrical Engineering DepartmentUniversity of California, Berkeley

Computer Science DepartmentUniversity of California, Berkeley

Computer Science DepartmentUniversity of California, Berkeley

2IPSN 2005

A Typical Solar Powered Sensor Node

Charging Circuit

Voltage Regulation

DAY NIGHT

Stress on Battery

EN

ER

GY

DAY NIGHT NIGHTDAY

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Energy Storage Element Designed for Pulsing Applications

Is it enough?

Supercapacitor Medium Capacity High Leakage Infinite recharge cycles

Rechargeable Battery High Capacity Low Leakage 300-500 recharge cycles

vs

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Multi-Stage

DAY NIGHT

EN

ER

GY

DAY

DAY NIGHT

Stress on Battery

EN

ER

GY

DAY NIGHT NIGHTDAY

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Inefficient and complex hardware? NO!

Simple and efficient hardware

Complete and powerful control using software

Intelligent

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Architecture

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Environmental Energy

SolarCharacteristicsSizing considerationsTypically 18mW/cm2 under direct sunlight

Vibration / kinetics Sound / wave Heat

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Wireless Sensor Node

Duty cycle Intelligence – self-aware of power levels

and able to exert control Ultra-low power – Telos

Low operating voltage

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The Larger the Better?

NO!

Without Load With Load

MINIMUM OPERATING VOLTAGE

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Secondary Buffer

Li+ vs NiMH Dedicated charging chip vs

software+simple hardware

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Case Study: Prometheus

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Control Loop and Chargingby SoftwareSensing ADC Piggy-back Voltage divider

trade-offActuation uC I/O Digital switch

Charging Dedicated charging

chip? MOS switch vs Digital

Switch DC/DC current

limiting

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Temperature Compensation

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Driverpiggy-backed on application active duty cycle

1. if TempV > 2.22. BattV = BattV + 1.45 (TempV − 2.2)3. if CapV < 2.24. SwitchCap = FALSE5. if CapV > 3.56. SwitchCap = TRUE7. if CapV > 4.4 and BattV < 3.58. ChargeBatt = TRUE9. if CapV < 3.810. ChargeBatt = FALSE11. call Radio.send(STATS)

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Duty-Cycle Adaptation

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Test 1 / Data Reported by TelosScenario: Supercapacitor is less than half full Battery is half full 1% duty cycle No light t0: Source = Cap; Vref = 2.5

Expect: t1: Vref = 1.5

t2: Source = Batt; Vref = 2.5

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Result 1

Supercapacitor Battery

t0: Source = Cap

t1: Vref = 1.5

t2: Source = Batt; Vref = 2.5

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Test 2 / Actual Measured DataScenario: Supercapacitor very low Battery is half full 1% duty cycle From dark to sunrise t0: Source = Batt

Expect: t1: Source = Cap

t2: Charge = True

t2+dt: Charge = False

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Result 2

Supercapacitor Battery

t0: Source = Battt1: Source = Cap

t2: Charge

T2+dt: !Charge

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Conclusion

Intelligence + Multi-stage + Simple Hardware

==

Perpetual operation

*

* Direct sunlight