7/9/2007AIIT Summer Course - D# 1

Wireless Embedded Systems and Networking

Foundations of IP-based Ubiquitous Sensor Networks

Micro-Power Systems

David E. CullerUniversity of California, Berkeley

Arch Rock Corp.

July 13, 2007

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Micro-Power System Architecture

• Evaluation Metrics– Effsolar = Pon / PmaxP

– Effsystem = (EL1+ … + ELn + Econs) / Esol

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An Example

• Solar energy scavenging system for Telos

– Super capacitors buffer energy– Lithium rechargeable battery as a

backup– Uses MCU to manage charge

cycles to extend system lifetime– Manage limited recharges– Simple, carefully developed

design

• Redesigned for TRIO deployment

– Boosting and current limiting

• Developed reactive power management software architecture

• Demonstrated in REALITY

Duty Cycle Light Required System Lifetime

1% 5 hrs / 1 mo 43 years

10% 5 hrs / 4 days 4 years

100% 10 hrs / 1 day 1 year

Prometheus Design estimates

Perpetual Environmentally Powered Sensor Networks, Jiang, Polastre, Culler, IPSN/SPOTS, 2005

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Facts

• E = P * T

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Energy Storage

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Energy and Power Density

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Battery Chemistry

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Energy Stroage

• Requirements:– Lifetime, Capacity, Current draw, Size/Weight

• Types of storage:– NiMH: capacity and cost

– Li+: energy density and capacity

– Supercap: lifetime

• Storage configuration:– Combination of battery and supercap provides good lifetime as

well as capacity.

• Charging mechanisms:– HW vs. SW, Complexity vs. Efficiency

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The Load

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Load (Sensor Node):Estimating Node Consumption

• Energy consumption with radio comm:– Iest = R*Iawake + (1-R) * Isleep

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The Ambient Source

• Solar

• Vibration

• Movement

• Flow

• Heat transfer

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External Environment:Estimating Solar Radiation• Statistical Model

• Mathematical Model

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Solar Collector:Solar-cell Characteristics

• Solar-cell I-V curve • Regulator

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Charging to Energy Storage Element

• Supercap for primary, lithium-ion for secondary.– Reduces battery charging frequency.

• Software-controlled battery charging.– Unlike other batteries, Li+ battery should be charged only

when there is sufficient charge in the supercap.

– Pros: Simple hardware: micro-controller, DC-DC converter, analog switch.

– Cons: Requires correct software for charging control.

Energy StorageController

Energy StorageElement

SolarCell

Sunlight

Super-capacitor

Power Selection

SW

RegulatingCircuit

Li-ionBattery

DC-DCConverter

WirelessSensorNode

(Micro-controller

&Radio)

Solar Cell Circuit

Solar Energy Harvesting Unit

VCC

SetCharge

SetPower

Charging Characteristic

2.800

3.000

3.200

3.400

3.600

3.800

4.000

4.200

4.400

0.0 20.0 40.0 60.0 80.0 100.0

Time

Vo

ltag

e (V

)

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

Cu

rren

t (A

)

Cell Voltage (V)

Charge Current (A)

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Consideration of other types of storage element

(1) Trio [DHJ+06] (2) Heliomote[RKH+05]

(3) Everlast[SSC05]

Storage One Li+ batterywith one 22F cap Two AA NiMH batteries One 100F capacitor

Capacity Ebat = 2625mWh Ebat = 4320mWh Ecap = 86.8mWh

Bday14.5 days at 10%6.7 days at 25%

23.9 days at 10%11.0 days at 25%

0.48 days at 10%0.22 days at 25%

Chargingcontrol

Software, pulse charging

Hardware,trickle charging

Hardware,trickle charging

overcast days?

YES YES NO

• Battery is needed during overcast days.– Supercap-only method doesn’t have sufficient capacity.

• Comparison of charging efficiency is not available yet.

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Comparative Study:Solar-Collector Operation

• Compare Pon with PmaxP

a. solar-cell operating point

b. maximum possible value

• Trio– Pon – PmaxP

= 4.83mW (5.3%)

• Heliomote– Pon – PmaxP

= -16.75mW (-23.2%)

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Comparative Study:Energy flow and efficiency

• Compare mote consumption (Econs) and stored energy (Ebat and Ecap) with solar energy income (Esol).

• Trio: up to 33.4%, Heliomote: up to 14.6%

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Solar-Collector Operation: Trio

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Solar-Collector Operation: Heliomote

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Energy flow and efficiency (Heliomote)- Energy loss due to regulator

• Solar energy income: 08:00 to 17:00.

• Clipped after 12:00.

• Two-third loss in daily energy income.

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Related Work on Solar Powered Sensor Network• Trio [DHJ+06]

– Real deployment of large sensor nodes.

– Multi-hop routing.

– Operate only for several hours with full radio cycle.

• Other Previous Works– RF transmit beacon [ROC+03], Prometheus [JPC05]

Heliomote [RKH+05], ZebraNet [ZSLM04]

RF TX beacon

Prometheus

Heliomote

ZebraNet

Trio

Trio[DHJ+06]

RF TX beacon[ROC+03]

Prometheus[JPC05]

Heliomote[RKH+05]

ZebraNet[ZSLM04]

Multi-hop Yes No No No No

SustainableOperation No No

(No battery) Yes Yes Yes

Duty-cycling On-offduty-cycle

On-offduty-cycle

On-offduty-cycle

On-offduty-cycle

GPS assistedtime-sync

Deployment ~ 500 Lab bench Lab bench Lab bench ~ 10

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Energy Management Architecture