Nemo: A High-fidelity Noninvasive Power Meter System for

Wireless Sensor Networks

Ruogu Zhou, Guoliang Xing

Department of Computer Science and Engineering,Michigan State University

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

2

Node1

Node2

Node3

Node4

Node5

Base Station

xx

x

Scarcity of Power

3

• 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

In-situ WSN Power Meters• SPOT[IPSN’07], iCount[IPSN’08]• Low sampling rate/resolution

– Cannot capture sleep power consumption or power transients

4SPOT mounts on MicaZ iCount with Telos

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

5

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-200mA dynamic range, >5 KHz sampling rate, <1uA

resolution

• Real-time communication with host– Enable real-time monitoring and energy-aware runtime

adaptation

6

TelosB node

Nemo+

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 mA current

7

Only connection

Outline

• Motivation• Challenges and System design

– Host-meter Communication– High Fidelity Measurement

• System evaluation• Case study• Conclusion

8

Diode

Switch

Voltage Modulator

Power Positive+

Power Ground

Modulation ControlADC

Sensor

Voltage Modulation (Meter->Host)

9

• 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

Current Modulation (Host->Meter)

10

• 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

Sensor

Nem

o

Power Positive+

Power Ground

Current

Measurement

Currrent Modulator

010011011100

Outline

• Motivation• Challenges and System design

– Host-meter Communication– High Fidelity Measurement

• System evaluation• Case study• Conclusion

11

Fidelity Requirements

12

• 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

-2 0 2 4 6 8 10 12 14 160

5

10

15

20

25

30

Time (ms)

Cur

rent

(mA

)

Power transients caused by radio on/off

Current Measurement 101

13

• 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

Sensor

Power Positive+

Power Ground

ADC

14

• 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!

Current Measurement 101

• 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 narrow measurement range, high resolution

Solution: Auto-ranging

15

Implementation of Auto-ranging

16

• 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

Sensor

Pwr Positive+

Pwr Ground

ADC

Input OutputResistors

Switches

Outline

• Motivation• Challenges and System design• System evaluation• Case study• Conclusion

17

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

18

Agilent 34410A benchtop DMM

Agilent DSO2024A Oscilloscope

Extech handheld DMM TelosB Nemo with battery

Measurement Fidelity (I)

19

• TelosB mote running a sense-and-send app as load

0 2 4 6 8 10 12 140

20

40

60

Time (ms)

Cur

rent

(mA

)

0 1 2 3 4 5 6 7 8 9 100

50

100

Relative Error (Percent)

CD

F

CDF

Ground-truthMeasurement

Match ground-truth closely

Average Error: 2.09%

Radio on ADC on

Radio RX on Radio TX on

Radio off ADC off

Measurement Fidelity (II)

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• 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

10-3

10-2

10-1

100

101

10210

-2

10-1

100

101

102

Current (mA)

Res

olut

ion

(uA

)

resolution

0.069 uA

0.68 uA

6.6 uA

48 uA

0.013 uA

Dynamic range0.8 uA to 202 mA

Resolution < 1uA

20 30 40 50 60 70 800

10

20

30

40

50

60

Temperature (Degrees Celsius)

Sle

ep C

urre

nt (u

A)

Mote1Mote2Mote3

Case Study: Sleep Power of Mote

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• 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

Conclusions

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• 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

Q/A

23