motion energy harvesting: myths & opportunities seminar, sept 24, 2007 motion energy harvesting:...
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NPL Seminar, Sept 24, 2007
Motion Energy Harvesting: Myths & Opportunities
Eric M. Yeatman
Department of Electronic & Electrical EngineeringImperial College London
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!
NPL Seminar, Sept 24, 2007
Let’s get rid of this:
NPL Seminar, Sept 24, 2007
And this:
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
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
NPL Seminar, Sept 24, 2007
1 cc wireless sensor node, IMEC
NPL Seminar, Sept 24, 2007
Sensor Node Power Requirements:
• Sensing Element
• Signal Conditioning Electronics
• Data Transmission
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.
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!
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.
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
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
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.
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
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
NPL Seminar, Sept 24, 2007
Heliomotes, UCLA
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
NPL Seminar, Sept 24, 2007
US Dept of Energy
NPL Seminar, Sept 24, 2007
Seiko Thermic
NPL Seminar, Sept 24, 2007
Ambient Electromagnetic Radiation
Graph: Mantiply et al.
≈ 10 V/m needed for reasonable power: not generally available
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
NPL Seminar, Sept 24, 2007
Motion Energy Scavenging
• Direct force devices
• Inertial devices
NPL Seminar, Sept 24, 2007
Direct Force: Heel Strike
Heel strike generator: Paradiso et al, MIT
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
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
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
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
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
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
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
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
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
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
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
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
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
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)
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)
NPL Seminar, Sept 24, 2007
Typical Inertial Generators
Piezoelectric
Ferro solutionsWright et al, Berkeley
NPL Seminar, Sept 24, 2007
Typical Inertial Generators
Magnetic
Southampton U. CUHK
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)
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
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
NPL Seminar, Sept 24, 2007
Assembled generator Detail of deep-etched moving plate
Prototype MEMS Device
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
NPL Seminar, Sept 24, 2007
Power Conditioning Circuit: Challenges
Power convertorefficiency is highly sensitive to parasitics!
NPL Seminar, Sept 24, 2007
Other Options: Rotating Mass
Example #1: traditional self-winding watch
NPL Seminar, Sept 24, 2007
Example #2: Seiko Kinetic
NPL Seminar, Sept 24, 2007
Large Inertial Generators
Backpack: U Penn
• 7 watts!
NPL Seminar, Sept 24, 2007
Large Scavenging Applications
East Japan Railway Co.
• Energy scavenging ticket gates
NPL Seminar, Sept 24, 2007
How else can rotating motion be used in inertial generation?
Proposal: Gyroscopic power generation
NPL Seminar, Sept 24, 2007
Gyroscopic power generationCan also be implemented in silicon!
Georgia Tech / u Mich
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
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 ]
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)
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
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