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

• 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
Node5
Base Station x Node1 x x
Node3
Node2
Node4
3

• 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
4

• 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
TelosB node
• Real-time communication with host
Nemo
–
+
6

• 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

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

• 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
Voltage
Modulator
Diode
Power Positive+
010011011100
Switch
Modulation Control
Power Ground
9

• 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
Currrent
Modulator
Power Positive+ 010011011100
Power Ground
10

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

• 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
30
25
20
15
10
5
0
-2
Power transients caused by radio on/off
0 2 4 6
Time (ms)
8 10 12 14 16
12

• 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
Power Positive+
Power Ground
13

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

• 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
15

• 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
Resistors
Input Output
Pwr Positive+
Switches
Pwr Ground
16

• 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
Agilent 34410A benchtop DMM
Agilent DSO2024A Oscilloscope
Extech handheld DMM TelosB Nemo with battery
18

• TelosB mote running a sense-and-send app as load
60
40
Radio on
ADC on
Match groundtruth closely
Ground-truth
Measurement
20
Radio off
ADC off
0
0 2 4
Radio RX on
6
Time (ms)
8
Radio TX on
10 12 14
100
50
0
0 1 2
Average Error: 2.09%
3 4 5 6
Relative Error (Percent)
7 8 9
CDF
10
19

• 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
2 resolution
10
1
Dynamic range
0.8 uA to 202 mA
10
0
0.68 uA
Resolution < 1uA
10
-1
10
-2
10
-3
0.013 uA
10
-2
0.069 uA
10
-1
10
0
Current (mA)
6.6 uA
10
1
48 uA
10
2
20

• 3 randomly selected TelosB motes running Null app
• Nemo is attached as power meter
• Surface of mote is heated to 80 o
C, then cooled down to 0 o
C
60
50
40
30
20
10
0
20
Mote1
Mote2
Mote3
Difference
<1uA
30 40 50 60
Temperature (Degrees Celsius)
70 80
5X difference
21

• 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
22

Q/A
23