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 Microscopic and inexpensive devices – Densely deployed to increase sensing fidelity Ad-hoc deployment Ad-hoc deployment – Powered by battery; transmit wirelessly Various form factors Various form factors 2

Node1 Node2 Node3 Node4 Node5 Base Station x x x Scarcity of Power 3 Small energy reservoir on node Small energy reservoir on node – Usually 2 AA batteries Energy-efficiency is crucial for WSN Energy-efficiency is crucial for WSN – Many energy-efficient protocols are proposed – Their effectiveness is hard to verify Power outages are common in deployment 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] SPOT[IPSN’07], iCount[IPSN’08] Low sampling rate/resolution Low sampling rate/resolution – Cannot capture sleep power consumption or power transients 4 SPOT mounts on MicaZiCount with Telos

In-situ WSN Power Meters SPOT[IPSN’07], iCount[IPSN’08] SPOT[IPSN’07], iCount[IPSN’08] Low sampling rate/resolution Low sampling rate/resolution – Cannot capture sleep power consumption or power transients Invasive to host node 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 Retrofit with after-market platforms w/ power metering Noninvasive to host node Noninvasive to host node – Standalone meter, plug &play, work with virtually any platform High measurement fidelity High measurement fidelity – 2uA-200mA dynamic range, >5 KHz sampling rate, <1uA resolution Real-time communication with host Real-time communication with host – Enable real-time monitoring and energy-aware runtime adaptation 6 TelosB node Nemo +

Challenges Noninvasiveness and real-time communication? 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 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

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

Current Modulation (Host->Meter) 10 Modulate own current draw to transmit data to 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 Meter decodes by measuring host current draw

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

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

Current Measurement Shunt resistor (current sensing resistor) Shunt resistor (current sensing resistor) – Convert current intensity to voltage signal Pre-amplifier Pre-amplifier – Amplify voltage signal to a proper level ADC ADC – Convert analog signal to digital signal – 2uA to 200mA dynamic range and < 1uA resolution  18-bit 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 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 Wide dynamic range needed only when measuring large current – Large current does not need <1uA resolution Adjust measurement range and resolution dynamically 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 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 Use low dynamic range low power ADC – Adjust measurement range according to ADC reading Shunt resistor switch Shunt resistor switch – A series of electrically controlled shunt resistors – Adjust resistance by shorting one or more resistors

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

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

Measurement Fidelity (I) 19 TelosB mote running a sense-and-send app as load TelosB mote running a sense-and-send app as load 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) 20 Sampling rate: constant KHz Sampling rate: constant KHz Dynamic range: 0.8 uA to 202 mA Dynamic range: 0.8 uA to 202 mA Resolution < 1 uA when current is less than 2.5 mA Resolution < 1 uA when current is less than 2.5 mA Dynamic range 0.8 uA to 202 mA Resolution < 1uA

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

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

Q/A 23

Power Outage in Deployment 24 Small energy reservoir on node – Usually 2 AA batteries Disasterous impact on deployment – Compromise monitoring fidelity – Interrupt data delivery – Replacement can be expensive Complex reasons – Expose to severe environment – Software bugs – RF Interference Example: [Szewczyk et al., SENSYS’04] – Due to no real-time power usage data, exact reason remains unknown How to diagnose and prevent power outage?

Power Meter in WSN 25 Track real-time power usage in deployment phase – Enable online power usage monitoring – Enable energy-aware runtime adaptation Validate design in prototype phase – Detect and diagnose abnormal power usage caused by software and hardware bugs

Communication Protocol 26 A preamble sequence is always sent before data frame – Fixed pattern: notifies receiver the incoming of a frame – Receiver is trained by preamble to learn receiving parameters Communication is always initiated by host node – Host node polls meter; meter does not transmit unless it was polled Support modulation rate up to 16 kbps – Sufficient for transmitting large chunk of power monitoring data

Key observation: high resolution is useful only when measuring small current – Don’t need 1uA resolution when measuring 100 mA current Use low dynamic range ADC ( < 12bit ) – Low power consumption; inexpensive; often integrated in MCU Adjust measurement range and resolution dynamically – Large current -> use large measurement range: lower resolution – Small current -> use small measurement range: higher resolution Solution: Auto-ranging Technique 27 12bit ADC uA Full scale 1bit=0.024uA Satisfy measurement range and resolution requirements in an economic way!

Measurement Accuracy: Resistor as Load 28

Host-meter Communication: Throughput 29