motion energy harvesting: myths & opportunities seminar, sept 24, 2007 motion energy harvesting:...
TRANSCRIPT
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NPL Seminar, Sept 24, 2007
Motion Energy Harvesting: Myths & Opportunities
Eric M. Yeatman
Department of Electronic & Electrical EngineeringImperial College London
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NPL Seminar, Sept 24, 2007
Energy Scavenging
• Collecting energy available in the environment, wirelessly
• Wireless means freedom, mobility, simplicity• Wireless doesn’t just mean data!
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NPL Seminar, Sept 24, 2007
Let’s get rid of this:
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NPL Seminar, Sept 24, 2007
And this:
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NPL Seminar, Sept 24, 2007
How Much Power?
Power station 100 megawatts
House 10 kilowatts
Person, lightbulb 100 watts
Laptop, heart 10 watts
Cellphone 1 watt
Wristwatch, sensor node 1 microwatt
Cellphone signal 1 nanowatt
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NPL Seminar, Sept 24, 2007
Main Application Area: Wireless Sensor Nodes• self-contained and self-powered, mm-scale• large numbers of nodes, must be low maintenance
Biomedical Application: Body Sensor Networks• low data rates, short transmission range• on body or implanted
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NPL Seminar, Sept 24, 2007
1 cc wireless sensor node, IMEC
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NPL Seminar, Sept 24, 2007
Sensor Node Power Requirements:
• Sensing Element
• Signal Conditioning Electronics
• Data Transmission
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NPL Seminar, Sept 24, 2007
Sensing Element
Simple signals - temperature, pressure, motion – require electrical power above thermal noise limit.
NT ≈ 10-20 W/Hz
For most applications, this is negligible.
For some, e.g. video, power is very high.
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NPL Seminar, Sept 24, 2007
Signal Conditioning
Likely principal function: A/D Converter
Recent results: Sauerbrey et al., Infineon (’03)
Power < 1 μW possible for low sample rates!
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NPL Seminar, Sept 24, 2007
16 bits/min.1 / min.16 bitsBlood oxygen
16 bits/min.1 / min.16 bitsTemperature
32 bits/min.1 / min.16 bitsBlood pressure
80 bits/min.10 / min.8 bitsHeart rate
Data RateRateDepthSignal
Data Transmission: Required Rates
Conclusion: average rates at or below 1 bit/s sufficient for many sensor types.
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NPL Seminar, Sept 24, 2007
Data Transmission: Required Power
Conclusions:
Power independent of bit-rate for low bit-rate
-40 dBm (0.1 μW) feasible for body-scale transmission range
1000100101
50
40
30
20
10
0
-10
-20
-30
-40
-50
Range (m)
Tran
smit
Pow
er (d
Bm
)
Ideal free-space propagation
Typical indoorLoss exponent(3.5)
Figure: F. Martin, Motorola
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NPL Seminar, Sept 24, 2007
Estimated Total Power Needs
• Peak power 1 – 100 uW
• Average power can be below 1 uW
Batteries: Present Capability
• 1 μW⋅yr for 0.1 cm3 battery feasible
• Not easy to beat!
• Useful energy reservoir for energy scavenging
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NPL Seminar, Sept 24, 2007
Fuel-Based Power Sources
• Energy density much higher than for batteries, ≈ 10 kJ/ cm3
• Technology immature, fuel cells most promising
Micro fuel cell, Yen et al.Fraunhofer Inst.
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NPL Seminar, Sept 24, 2007
Magnetic (induction)Piezoelectric Electrostatic
KineticVolume flow (liquids or gases)Movement and vibration
Magnetic induction (induction loop)Antennas
Magnetic and Electro-magnetic Electro-magnetic waves
Thermoelectric or Heat EngineThermal Temperature gradients
Solar CellsLight Ambient light, such as sunlight
Conversion MechanismEnergy Source
Energy Scavenging : Sources
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NPL Seminar, Sept 24, 2007
Solar Cells
• highly developed
• suited to integration
• high power density possible:
100 mW/cm2 (strong sunlight)
• but not common:
100 μW/cm2 (office)
• Need to be exposed, and oriented correctly
Solar cell for Berkeley Pico-Radio
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NPL Seminar, Sept 24, 2007
Heliomotes, UCLA
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NPL Seminar, Sept 24, 2007
Thermo-Electric
• need reasonable temperature difference (5 – 10°C) in short distance
• ADS device ≈ 10 μW for 5°C
• even small ΔT hard to achieve
Heat engine, Whalen et al,
Applied Digital Solutions
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NPL Seminar, Sept 24, 2007
US Dept of Energy
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NPL Seminar, Sept 24, 2007
Seiko Thermic
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NPL Seminar, Sept 24, 2007
Ambient Electromagnetic Radiation
Graph: Mantiply et al.
≈ 10 V/m needed for reasonable power: not generally available
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NPL Seminar, Sept 24, 2007
Body Sensor Nodes : Power Scavenging
• Light not generally available
• Temperature differences small
• Motion sources attractive
• Body motion or organ motion (heart, lungs)
• Inertial scavenging safest method
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NPL Seminar, Sept 24, 2007
Motion Energy Scavenging
• Direct force devices
• Inertial devices
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NPL Seminar, Sept 24, 2007
Direct Force: Heel Strike
Heel strike generator: Paradiso et al, MIT
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NPL Seminar, Sept 24, 2007
Body Motion Power Sources
• Low and varying frequency (1 – 10 Hz)
• Large variation in amplitude
Body Sensor Size Limits
• Below 1 cc desirable
• Less for implanted nodes
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NPL Seminar, Sept 24, 2007
m
zo
y = Y cos( t)o ω
damper implements energy conversion
Available Power from Inertial Scavengersassume:• source motion amplitude Yo and frequency ω• Proof mass m, max internal displacement zo
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NPL Seminar, Sept 24, 2007
m
zo
y = Y cos( t)o ω
damper implements energy conversion
Available Power from Inertial Scavengers
• Peak force on proof mass F = ma = mω2Yo
• Damper force < F or no movement
• Maximum work per transit W = Fzo = mω2Yozo
• Maximum power P = 2W/T = mω3Yozo/π
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NPL Seminar, Sept 24, 2007
0.1
1
10
100
1 10 100frequency (Hz)
pow
er (u
W)
How much power is this?
Plot assumes:
• Si proof mass (higher densities possible)
• max source acceleration 1g (determines Yo for any f)
10 x 10 x 2 mm
3 x 3 x 0.6 mm
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NPL Seminar, Sept 24, 2007
Performance Trends – progress with time
0
0.02
0.04
0.06
0.08
0.1
0.12
1996 1998 2000 2002 2004 2006
year
norm
alis
ed p
ower
electromagnetic electrostatic piezo
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NPL Seminar, Sept 24, 2007
Performance Trends – progress with time
0
0.02
0.04
0.06
0.08
0.1
0.12
1996 1998 2000 2002 2004 2006
year
norm
alis
ed p
ower
electromagnetic electrostatic piezo
Imperial
UC Berkeley
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NPL Seminar, Sept 24, 2007
Performance Trends – progress with time
0
0.02
0.04
0.06
0.08
0.1
0.12
1996 1998 2000 2002 2004 2006
year
norm
alis
ed p
ower
electromagnetic electrostatic piezo • There is an upward trend
• No particular correlation of performance with transducer type
• All of the normalised power values are low – there is room for improvement
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NPL Seminar, Sept 24, 2007
Performance Trends – as a function of volume
0
0.02
0.04
0.06
0.08
0.1
0.12
0.01 0.1 1 10
volume (cc)
norm
alis
ed p
ower
electromagnetic electrostatic piezo
• There is an upward trend – easier to make a large device than a small device
• No particular correlation of device volume with transducer type
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NPL Seminar, Sept 24, 2007
Performance Trends – as a function of frequency
0
0.02
0.04
0.06
0.08
0.1
0.12
1 10 100 1000
frequency (Hz)
norm
alis
ed p
ower
electromagnetic electrostatic piezo • Downward trend –parasitic damping becomes more dominant with increased frequency
• No particular correlation of transducer type with operating frequency
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NPL Seminar, Sept 24, 2007
How does this compare to applications?
Plot assumes:
• proof mass 10 g/cc
• source acceleration 1g
0.001
0.01
0.1
1
10
100
1000
10000
100000
0.01 0.1 1 10 100 1000
volume (cc)
pow
er (m
W)
f = 1 Hzf = 10 Hz
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NPL Seminar, Sept 24, 2007
How does this compare to applications?
Plot assumes:
• proof mass 10 g/cc
• source acceleration 1g
0.001
0.01
0.1
1
10
100
1000
10000
100000
0.01 0.1 1 10 100 1000
volume (cc)
pow
er (m
W)
f = 1 Hzf = 10 Hz
Sensor node
watch
cellphone
laptop
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NPL Seminar, Sept 24, 2007
Implementation Issues: Resonance
Why use resonant device?• Allows use of full internal range for low Yo
Why not use resonant device?• For body sensor application, Yo > zo likely • Low resonant frequency hard to achieve for small device• Varying source frequency bad for resonant devices
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NPL Seminar, Sept 24, 2007
Response Comparison: Harmonic Drive
Resonant devices better for large generators / small displacements, but only if operated near resonance
Non-resonant good for high displacements, wide input frequency ranges
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NPL Seminar, Sept 24, 2007
Response Comparison: True Body Motion
• Non-resonant device wins for small generators
• Data obtained in collaboration with ETH Zurich (T. von Buren)
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NPL Seminar, Sept 24, 2007
Implementation Issues: Mechanism
Piezoelectric?• Difficult integration of piezo material• Possible leakage time issues for low frequency use
Electromagnetic?• Needs high dφ/dt to get damper force (φ = flux)• dφ/dt = (dφ/dz )(dz/dt )• Low frequency (low dz/dt) needs very high flux gradient• Efficiency issues (coil current)
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NPL Seminar, Sept 24, 2007
Typical Inertial Generators
Piezoelectric
Ferro solutionsWright et al, Berkeley
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NPL Seminar, Sept 24, 2007
Typical Inertial Generators
Magnetic
Southampton U. CUHK
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NPL Seminar, Sept 24, 2007
Implementation Issues: Mechanism
Electrostatic?• Simple implementation, no field gradient problem• Damping force can be varied via applied voltage• But needs priming voltage (or electret)
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NPL Seminar, Sept 24, 2007
Chosen Approach: Constant Charge
Input phase Output phase
inputinputVCQ = outputoutputVCQ =
inputoutput
inputouput V
CC
V =
222
21
21
21
outputoutputinputinputouputoutput VCVCVCE ≈−=Δ
inputoutput VV >>
∴
Q
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NPL Seminar, Sept 24, 2007
Electro-mechanical Design
Mass
Top plate (silicon)
Base plate (quartz)
Mass
Mass
Input phase
Output phase
Gap
COM
Vout
Vin
Polyimide suspension
Buck converter
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NPL Seminar, Sept 24, 2007
Assembled generator Detail of deep-etched moving plate
Prototype MEMS Device
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NPL Seminar, Sept 24, 2007
Device Operation
posi
tion
time
time
trajectory of moving plate
volta
ge
t2 t3t1
voltage on moving plate
upperlimit
lower limit
moving plate/ proof mass
fixed plate
discharge contact
charging contact
Output > 2 μW
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NPL Seminar, Sept 24, 2007
Power Conditioning Circuit: Challenges
Power convertorefficiency is highly sensitive to parasitics!
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NPL Seminar, Sept 24, 2007
Other Options: Rotating Mass
Example #1: traditional self-winding watch
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NPL Seminar, Sept 24, 2007
Example #2: Seiko Kinetic
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NPL Seminar, Sept 24, 2007
Large Inertial Generators
Backpack: U Penn
• 7 watts!
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NPL Seminar, Sept 24, 2007
Large Scavenging Applications
East Japan Railway Co.
• Energy scavenging ticket gates
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NPL Seminar, Sept 24, 2007
How else can rotating motion be used in inertial generation?
Proposal: Gyroscopic power generation
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NPL Seminar, Sept 24, 2007
Gyroscopic power generationCan also be implemented in silicon!
Georgia Tech / u Mich
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NPL Seminar, Sept 24, 2007
Motion Energy Harvesting for Sustainability• Power levels modest – energy saving not key motivator
• Human powered – mW levels likely practical limit
• Battery elimination: yes, except need local storage
• Pervasive sensing: major possibilities for efficient buildings, machines, processes
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NPL Seminar, Sept 24, 2007
Motion Energy Harvesting for Sustainability• Power levels modest – energy saving not key motivator
• Human powered – mW levels likely practical limit
• Battery elimination: yes, except need local storage
• Pervasive sensing: major possibilities for efficient buildings, machines, processes
• [ plug: Imperial College Centre for Pervasive Sensing ]
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NPL Seminar, Sept 24, 2007
Interaction with Energy Source• Typical energy scavenging implies no significant effect on
source
• i.e. effectively infinite source, power limited only by scavenger
• Otherwise not really “scavenging”
• In practice, loading of source at least power level extracted
• Alternative: collecting wasted power
• Key example: combined heat and power (CHP)
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NPL Seminar, Sept 24, 2007
Conclusions• Power levels in the microwatt range are enough for many
wireless sensor nodes
• Inertial devices driven by body motion can achieve these levels
• Nonlinear devices are suitable for low and variable input frequencies, and for high displacement to device size ratios
• Prototype devices have been demonstrated at useful power levels
• Future challenges include associated power electronics
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NPL Seminar, Sept 24, 2007
Thanks to:
at Imperial:Andrew Holmes, Paul Mitcheson, Tim Green
others:Joe Paradiso, MIT
Paul Wright, UC BerkeleyThomas von Büren, ETH Zurich
Contact me:[email protected]
www.imperial.ac.uk/ee