Nemo : A High-fidelity Noninvasive Power Meter System for Wireless Sensor Networks

1. Nemo: A High-fidelity Noninvasive Power Meter System forWireless Sensor Networks Ruogu Zhou, Guoliang Xing Department of Computer Science and Engineering, Michigan State University

2. Wireless Sensor Networks Platforms • Microscopic and inexpensive devices • Densely deployed to increase sensing fidelity • Ad-hoc deployment • Powered by battery; transmit wirelessly • Various form factors

3. Scarcity of Power • Small energy reservoir on node • Usually 2 AA batteries • Energy-efficiency is crucial for WSN • Many energy-efficient protocols are proposed • Their effectiveness is hard to verify • Power outages are common in deployment • Greatly impair sensing fidelity • Exact reasons are usually unknown Node1 x x Node2 x Node3 Node4 Node5 Base Station

4. In-situ WSN Power Meters • SPOT[IPSN’07], iCount[IPSN’08] • Low sampling rate/resolution • Cannot capture sleep power consumption or power transients SPOT mounts on MicaZ iCount with Telos

5. In-situ WSN Power Meters • SPOT[IPSN’07], iCount[IPSN’08] • Low sampling rate/resolution • Cannot capture sleep power consumption or power transients • Invasive to host node • Require host CPU, RAM , I/O and timer • Installation requires wiring and soldering

6. Nemo: Noninvasive High Fidelity Power Meter • Retrofit with after-market platforms w/ power metering • Noninvasive to host node • Standalone meter, plug &play, work with virtually any platform • High measurement fidelity • 2uA-200mAdynamic range, >5 KHz sampling rate, <1uA resolution • Real-time communication with host • Enable real-time monitoring and energy-aware runtime adaptation + TelosBnode Nemo

7. Challenges • Noninvasiveness and real-time communication? • Only connection b/w meter and host is power rail • No dedicated data wires • High fidelity and low power consumption? • High fidelity usually results in high power consumption • Ex: ADC w/ high dynamic range consumes > 10 mAcurrent Only connection

8. Outline • Motivation • Challenges and System design • Host-meter Communication • High Fidelity Measurement • System evaluation • Case study • Conclusion

9. Voltage Modulation (Meter->Host) • Modulate supply voltage of host to transmit measurements • Modulator: A Schottky diode controlled by a switch • Host decodes by sampling supply voltage • Most built-in ADCs can be programmed to measure supply voltage • Host cannot modulate supply voltage • Cannot be applied to host-> meter link 010011011100

10. Current Modulation (Host->Meter) • Modulate own current draw to transmit data to meter • Modulator: Any component that can be switched fast, e.g. LED • Meter decodes by measuring host current draw 010011011100

11. Outline • Motivation • Challenges and System design • Host-meter Communication • High Fidelity Measurement • System evaluation • Case study • Conclusion

12. Fidelity Requirements • Wide dynamic range • Sleep (~2uA) to Active (~200mA), 5 orders of difference • High sampling rate • > 5kHz to capture power transients • High resolution • Monitor sleep power (< 1uA) which determines system life Power transients caused by radio on/off

13. Current Measurement 101 • Shunt resistor (current sensing resistor) • Convert current intensity to voltage signal • Pre-amplifier • Amplify voltage signal to a proper level • ADC • Convert analog signal to digital signal • 2uA to 200mA dynamic range and < 1uA resolution  18-bit ADC

14. Current Measurement 101 • Shunt resistor (current sensing resistor) • Convert current intensity to voltage signal • Pre-amplifier • Amplify voltage signal to a proper level • ADC • Convert analog signal to digital signal • 2uA to 200mA dynamic range requires an 18-bit ADC • High dynamic range ADCs are expensive and power hungry!

15. Solution: Auto-ranging • High resolution needed only when measuring small current • Small current does not need 0-200mA dynamic range • Wide dynamic range needed only when measuring large current • Large current does not need <1uA resolution • Adjust measurement range and resolution dynamically • Large current -> use wide measurement range, low resolution • Small current -> use narrowmeasurement range, high resolution

16. Implementation of Auto-ranging • Adjust shunt resistor to change measurement range& resolution • Wide (narrow) range, low (high) resolution -> small (large) shunt resistor • Use low dynamic range low power ADC • Adjust measurement range according to ADC reading • Shunt resistor switch • A series of electrically controlled shunt resistors • Adjust resistance by shorting one or more resistors

17. Outline • Motivation • Challenges and System design • System evaluation • Case study • Conclusion

18. Implementation & Experiment Setup • PCB area 1.5 inch by 2.5 inch • System software implemented in C and assembly • Nemo is calibrated using a set of resistors • Agilent 34410A Bench-top digital multi-meter as reference

19. Measurement Fidelity (I) • TelosB mote running a sense-and-send app as load Radio on ADC on Match ground-truth closely Radio off ADC off Radio RX on Radio TX on Average Error: 2.09%

20. Measurement Fidelity (II) • Sampling rate: constant 8.192 KHz • Dynamic range: 0.8 uA to 202 mA • Resolution < 1 uA when current is less than 2.5 mA Dynamic range 0.8 uA to 202 mA Resolution < 1uA

21. Case Study: Sleep Power of Mote • 3 randomly selected TelosB motes running Null app • Nemo is attached as power meter • Surface of mote is heated to 80oC, then cooled down to 0oC Difference <1uA 5X difference

22. Conclusions • A noninvasive in-situ power meter for WSN • Plug and play, high measurement fidelity • Novel communication scheme for host-meter comm. • Voltage&current modulation for communication over power rails • Auto-ranging technique for high measurement fidelity • Dynamically configure meter according to measurement requirements • Evaluation in real experiments • High dynamic range, high sampling rate, high resolution, low error

23. Q/A