david e. culler university of california, berkeley arch rock corp. july 13, 2007
DESCRIPTION
Wireless Embedded Systems and Networking Foundations of IP-based Ubiquitous Sensor Networks Micro-Power Systems. David E. Culler University of California, Berkeley Arch Rock Corp. July 13, 2007. Micro-Power System Architecture. Evaluation Metrics Eff solar = P on / P maxP - PowerPoint PPT PresentationTRANSCRIPT
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
7/9/2007AIIT Summer Course - D# 2
Micro-Power System Architecture
• Evaluation Metrics– Effsolar = Pon / PmaxP
– Effsystem = (EL1+ … + ELn + Econs) / Esol
7/9/2007AIIT Summer Course - D# 3
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
7/9/2007AIIT Summer Course - D# 6
Energy and Power Density
7/9/2007AIIT Summer Course - D# 8
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
7/9/2007AIIT Summer Course - D# 10
Load (Sensor Node):Estimating Node Consumption
• Energy consumption with radio comm:– Iest = R*Iawake + (1-R) * Isleep
7/9/2007AIIT Summer Course - D# 11
The Ambient Source
• Solar
• Vibration
• Movement
• Flow
• Heat transfer
7/9/2007AIIT Summer Course - D# 12
External Environment:Estimating Solar Radiation• Statistical Model
• Mathematical Model
7/9/2007AIIT Summer Course - D# 13
Solar Collector:Solar-cell Characteristics
• Solar-cell I-V curve • Regulator
7/9/2007AIIT Summer Course - D# 14
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)
7/9/2007AIIT Summer Course - D# 15
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.
7/9/2007AIIT Summer Course - D# 16
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%)
7/9/2007AIIT Summer Course - D# 17
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%
7/9/2007AIIT Summer Course - D# 20
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.
7/9/2007AIIT Summer Course - D# 21
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