PROMETHEUS Intelligent Multi-Stage Energy Transfer System for Near Perpetual Sensor Networks Xiaofan JiangJoseph PolastreDavid Culler Electrical Engineering Department University of California, Berkeley Computer Science Department University of California, Berkeley

2 IPSN 2005 A Typical Solar Powered Sensor Node Charging Circuit Voltage Regulation DAYNIGHT A Batterys Perspective ENERGY DAY Shallow Depth of Discharge Medium Depth of Discharge

3 IPSN 2005 Energy Storage Element Designed for Pulsing Applications Is it enough? Supercapacitor Medium Capacity High Leakage Infinite recharge cycles Rechargeable Battery High Capacity Low Leakage recharge cycles vs

4 IPSN 2005 Multi-Stage Inefficient and complex hardware? NO! Simple and efficient hardware Complete and powerful control using software AND Intelligent

5 IPSN 2005 Architecture

6 IPSN 2005 Environmental Energy Solar Characteristics Sizing considerations Typically 18mW/cm2 under direct sunlight Vibration / kinetics Sound / wave Heat

7 IPSN 2005 Wireless Sensor Node Duty cycle Intelligence – self-aware of power levels and able to exert control Ultra-low power – Telos Low operating voltage

8 IPSN 2005 The Larger the Better? NO!

9 IPSN 2005 Secondary Buffer Li+ vs NiMH Direct from environment vs from primary buffer Dedicated charging chip vs software+simple hardware

10 IPSN 2005 Case Study: Prometheus

11 IPSN 2005 Control Loop and Charging by 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

12 IPSN 2005 Temperature Compensation

13 IPSN 2005 Driver PROMETHEUS DRIVER piggy-backed on application active duty cycle 1. if TempV > BattV = BattV (TempV 2.2) 3. if CapV < SwitchCap = FALSE 5. if CapV > SwitchCap = TRUE 7. if CapV > 4.4 and BattV < ChargeBatt = TRUE 9. if CapV < ChargeBatt = FALSE 11. call Radio.send(STATS)

14 IPSN 2005 Duty-Cycle Adaptation

15 IPSN 2005 Results

16 IPSN 2005 Conclusion Multi-stage + simple hardware + software == Perpetual operation

17 IPSN 2005 Future Work Deployment of 1500 nodes equipped with Prometheus for long term observation Questions

18 IPSN 2005 Energy - Fundamental Limitation of Sensor Networks How long could current sensor networks survive without human intervention? 2 to 3 years Why? Limiting factor storage device

19 IPSN 2005 Multi-Stage and Intelligent - Environmental energy unpredictable buffer battery recharge cycles - Sensor nodes pulses storage designed for pulsing applications not batteries - Sensor nodes uC brain can adapt All these points to … Reduce stress on critical path components Intelligent

20 IPSN 2005 Primary Buffer Medium Capacity High Leakage Infinite recharge cycles High Capacity Low Leakage recharge cycles vs