PROMETHEUS Intelligent Multi-Stage Energy Transfer System for Near Perpetual Sensor Networks

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PROMETHEUSIntelligent Multi-Stage Energy
Transfer System for Near Perpetual Sensor Networks
Xiaofan Jiang Joseph Polastre David Culler
Electrical Engineering DepartmentUniversity of California, Berkeley Computer Science DepartmentUniversity of California, Berkeley Computer Science DepartmentUniversity of California, Berkeley
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A Typical Solar Powered Sensor Node
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Energy Storage Element Designed for Pulsing
Applications

• Is it enough?

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Multi-Stage
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Inefficient and complex hardware?
NO!
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Architecture
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Environmental Energy

• Solar
• Characteristics
• Sizing considerations
• Typically 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!
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Secondary Buffer

• Li vs NiMH
• Dedicated charging chip vs softwaresimple
  hardware

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Case Study Prometheus
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Control Loop and Chargingby Software

• Sensing
• ADC
• Piggy-back
• Voltage divider trade-off
• Actuation
• 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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Driver
piggy-backed on application active duty cycle 1.
if TempV gt 2.2 2. BattV BattV 1.45 (TempV -
2.2) 3. if CapV lt 2.2 4. SwitchCap FALSE 5. if
CapV gt 3.5 6. SwitchCap TRUE 7. if CapV gt 4.4
and BattV lt 3.5 8. ChargeBatt TRUE 9. if CapV
lt 3.8 10. ChargeBatt FALSE 11. call
Radio.send(STATS)
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Duty-Cycle Adaptation
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Test 1 / Data Reported by Telos

• Scenario
• 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
t2 Source Batt Vref 2.5
t0 Source Cap
t1 Vref 1.5
Supercapacitor
Battery
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Test 2 / Actual Measured Data

• Scenario
• Supercapacitor very low
• Battery is half full
• 1 duty cycle
• From dark to sunrise
• t0 Source Batt

• Expect
• t1 Source Cap
• t2 Charge True
• t2dt Charge False

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Result 2
t0 Source Batt
t1 Source Cap
t2 Charge
T2dt !Charge
Supercapacitor
Battery
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Conclusion

• Intelligence Multi-stage Simple Hardware
• Perpetual operation

Direct sunlight