2009SensorsExpo2 2009Sensors Expo2

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Wireless Power for Battery-Free
Wireless Sensors
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Overview
• Powercast technology overview
• Applications for wireless sensors
• Battery-free reference design
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About Powercast
• Driving innovation and commercialization
of wireless power based on RF energy
• Custom engineering and components
•
Applications / Markets
•
Applications / Markets
– Wireless sensors and devices
– Defense
– Aerospace
– Manufacturing
– Others
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2009 Product
Showcase
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Wireless Power (over distance)
• Dedicated source transmits common radio waves
– Ambient sources augment when available: Cellular, TV, Radio, etc.
• Receiver
– Captures the RF energy with an antenna
– Converts the RF energy to the appropriate DC voltage
– Stores the DC energy
• Energy transfer is controllable and predictable by design
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Multiple forms of “wireless power”
Induction Micro-Contacts
Proximity
Wireless Laser / Infrared
Batteries
Solar
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Radio Waves
Harvesting
Vibration
Thermal
Radio Waves
Ambient Energy Harvesting
• “Free” Energy
• Multiple methods with
many sources
–
Solar
• Energy received is
dependent on the
source
• Sources may be
predictable but they
Benefits Drawbacks
–
Solar
– Vibration
– Thermal
– RF
predictable but they
are uncontrollable
• No source = No energy
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EH is fine for some applications, but not all.
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Dedicated Source – Controllable
by Design
System Parameters
• RF Power Level
• Frequency
• Transmit Antenna
•
Number of Transmitters
•
Number of Transmitters
• Distance
• Receive Antenna
• Device Duty Cycle
• Cost
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Wireless Power Transfer with
Radio Waves…
Governed by Friis Equation
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…Simplified
After parameter selection equation
simplifies to:
∴There are many parameters to adjust for system
optimization but after selection, calculations are straight
forward.
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Antennas have a significant
impact on power transfer
Patch
)))(((
Sleeve Dipole
Omni-directional
Patch
Directional
Loop
Bi-directional
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Sample Antennas Designs
2.45GHz Tx Array
2.45GHz Rx Array
7.26 x 5.78cm
2.45GHz Patch
Gain =4.9
Beam= 32deg
915 MHz
Sleeve Dipole
Beam = ~20deg
Beam=~90deg
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915 MHz Dipole
915 MHz Short
Dipole
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915 MHz Yagi
Gain = 6
System Comparison
Achieving Higher Performance with Lower Power
• Multiple Tx
• Any-to-Any
• Even coverage
• Lower total Tx power
• More robust
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• Single Tx
• One-to-Many
• Uneven coverage
• Higher Tx Power Not enough
power
Minimum
desired
power
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Wireless Power distribution is
similar to a cellular network
• Any to Any
• Redundancy
•
Enables Mobility
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•
Enables Mobility
• Distributed
• Area Coverage
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Gateway Access Points
Power
Data
Power Power
Vision: Unified Power and
Communications for a Ubiquitous
Sensor Network
Power
Data
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The Opportunity – Wireless Sensors
• Building automation
• Energy management
•
Location tracking
• Reduced wiring
• Sealed devices
•
Reduced maintenance
Applications Benefits of Wireless Power
•
Location tracking
• Condition monitoring
• Rotational Machinery
•
Reduced maintenance
• Controllable power
• Difficult locations
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Application – Building Automation
• Indoor sensors
• Low light areas
•
Behind walls
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•
Behind walls
• Above ceilings
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Application – Location Tracking
• Battery-Free Beacons
• Active Inside
• Inactive Outside
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• Longer range “RFID”
• Battery-Free “RTLS”
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Application – Industrial Monitoring
• Lack of vibration or
heat source
• Hazardous areas
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• Distance
• Battery trickle-charge
• Non-critical
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Application – Rotating Machinery
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Issues with Primary Batteries
in Wireless Sensor Networks
• Intentional constraints to save
power
– Design, Operation, Implementation
– Majority of energy consumed
sleeping
• Reliability
–
Retransmissions
Management
Effort
Battery-Powered
–
Retransmissions
– Lifetime vs. cell/pack size
– Shelf life?
– Temperature performance
• Battery replacement cost
• Device location / placement
• Ecology
• Limitations of scale
10s 100s 1000s
Management
Size of Sensor Network
Battery-Free
Battery
Replacement
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Majority of battery life is
consumed in sleep mode
60.0%
80.0%
100.0%
% Power Consumed in Sleep Mode
10uA
30uA
20uA
0.0%
20.0%
40.0%
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2
3
4
5
6
7
8
9
10
30
60
Daily
Wake-up Every X Minutes
Transmit current: 50mA
Other current: 30mA
Transmit time: 5 ms
Other time: 20ms
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Average energy ≈ Sleep energy
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Capacitor
Voltage
Power
Broadcast
Battery-Free Concept
Send power as needed - 1) On-Demand, 2) Scheduled, or 3) Continuously
VMIN
VMAX
Power Output
Sensor Active
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GND
Sensor Dormant
“Zero Stand-By” Power
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Battery-Free Reference System
P2100 Powerharvester™ Module
Front Back
Sleeve Dipole Antenna
Simple “2 wire” hardware integration for any RF module
Integrated, 915 MHz
High Efficiency
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TI eZ430-RF2500T
CAP-XX GZ115
Low Power
Small Form Factor
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Powerharvester™ Module
P2100 – 915MHz, Charge & Fire
• High Conversion Efficiency
• Internal Charge Management
• High Sensitivity
• Configurable Output Voltage
Features
• 50mA Output Current
• Capacitor Overvoltage Protection
• Internally Matched to 50 ohms
• Low Quiescent Current (<1µA)
• Simple Integration
• Small Footprint
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Energy Storage
Choosing the Supercap Value
1
2
3
4
Output Voltage (V)
Voltage Window (Hysteresis)
Energy Available
Capacitor Value
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0
1
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
Capacitor Voltage (V)
DC-DC conversion
efficiency
Capacitor Value
= required load energy GZ115 cap size = 0.16F (measured)
Stored energy = 22.7 mJ
VMIN VMAX
C = 7.02E/e e ≈0.82
C= 8.57E
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TI eZ430-RF2500T
Start-up and Data
3.70 mJ @ 3.3V
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Note: V1.5 Software
Initial start-up Data
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P2100 Charge Current
100
1000
10000
Output Current (uA)
P2100 Capacitor Charge Current at 1.1V
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1
10
-15 -10 -5 0 5 10 15
Output Current (uA)
Module Input Power (dBm)
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Energy Harvesting Performance
1000
10000
Charge Time (sec)
Charge Time vs. Distance
Measured @10ft
(152 sec)
Measured @ 15 ft
(103 sec)
3W EIRP Patch Antenna Transmitter
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1
10
100
5 10 15 20 25 30 40 45 50
Charge Time (sec)
Sleeve Dipole (G=1.5)
Air Dipole (G=4.1)
Yagi (G=6.1)
Distance (ft)
Measured @20ft
(145sec)
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Reference System Summary
• Stored energy = 22.7 mJ
• Usable energy = 18.6 mJ (current design)
• Initial start-up and data transmission = 3.7 mJ
• 20ft range (3W source, sleeve-dipole Rx antenna)
• Temperature and voltage sensing
• Extremely long life – NO BATTERIES!!!
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Reference System On-Going Efforts
Optimizing Performance
• Reduce the capacitor size by modifying the software start-
up sequence
– Target joule usage of 100uJ will require less than 1000uF capacitor
• Improve charge management efficiency
• Lower the harvester sensitivity to extend range
Targets
• 100uJ per activation
• Credit card form factor
• 100+ ft range (2-4X increase)
• 3Q09 timeframe
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Summary
• Wireless power via RF energy harvesting is capable
of powering wireless sensors over distance
• Capacitors offer an attractive alternative to
disposable batteries
•
Wireless power uniquely provides controllable
•
Wireless power uniquely provides controllable
power options: on-demand, scheduled, continuous
• Zero Stand-By operation eliminates design
concerns of driving down sleep current:
Average Current ≈ Sleep Current ≈ 0
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Thank You
Visit us in Booth #1026
www.powercastco.com
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