miniaturising motion energy harvesters: limits and ways...
TRANSCRIPT
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Miniaturising Motion Energy Harvesters:
Limits and Ways Around Them
Eric M. Yeatman
Imperial College London
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Inertial Harvesters
• Mass mounted on a spring within a frame
• Frame attached to moving “host” (person, machine…)
• Host motion vibrates internal mass
• Internal transducer extracts power
transducer
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Available Power from Inertial Harvesters
assume:
• source motion amplitude Yo and frequency w
• Proof mass m, max internal displacement zo
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• Peak force on proof mass F = ma = mw2Yo
• Damper force < F or no movement
• Maximum work per transit W = Fzo = mw2Yozo
• Maximum power P = 2W/T = mw3Yozo/p
Ref: Mitcheson, P. et al. “Architectures for vibration-driven micropower
generators”, J. Microelectromechanical Systems 13(3), pp. 429-440
(2004).
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• Maximum power P = mw3Yozo/p
• For length dimension L, m scales as L3
• Zo scales as L
• So power scales as L4
• Power density falls as size reduces
Implications for Scaling
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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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Achievable Power Relative to Applications
Sensor node
watch
cellphone
laptop
Plot assumes:
• proof mass 10 g/cc
• source acceleration 1g
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Possible Power Relative to Batteries
Plot assumes:
• proof mass 10 g/cc
• source acceleration 1g
0.0001
0.001
0.01
0.1
1
10
100
0.001 0.01 0.1 1 10
pow
er
(mW
)
volume (cc)
f = 1 Hz
f = 10 Hz
Battery (1 mo)
Battery (1 yr)
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Power Density
• Depends on geometry: highest P/Vol for travel along long axis
• MEMS devices typically use plate geometry – not ideal
• In-plane motion: hard to achieve optimal travel range
• Off-axis travel can be a problem
BLOCKPIN SHUTTLE
SHUTTLE PLATE AXIAL PLATE
0.001
0.01
0.1
1
10
1 10 100 1000
frequency (Hz)M
ax. P
ow
er
(mW
)
0.01 cc axial 0.1 cc axial
0.01 cc shuttle 0.1 cc shuttle
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Implementation Issues: Resonance
Why use resonant device?
• Allows use of full internal range for low Yo
Why not use resonant device?
• For low frequency application, Yo > zo likely
• Low resonant frequency hard to achieve for small devices
• Not suitable for broadband or varying source frequency
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Implementation Issues: Resonance
Input displacement vs frequency: low frequency range
0.01
0.1
1
10
100
1000
1 10 100
Yo (mm) (0.1 g)
Yo (mm) (1 g)
Frequency (Hz)
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Transduction: Electromagnetic
Advantages:
• Well understood system
• No source voltage needed
(with permanent magnets)
Disadvantages
• Limited number of winding
turns in MEMS: low voltages
• Low damping forces in low
frequency operation Example:
Southampton/Tyndall Inst.
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Transduction: Electrostatic
Advantages:
• No special materials
• Suitable for MEMS scale
Disadvantages
• Needs priming voltage, or
electret
• high output voltages typical
Example: MIT
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Transduction: Piezoelectric
Advantages:
• High voltage even at low
frequency
• Simple geometries
Disadvantages
• Low coupling coefficient
• integration of material
Example: UC Berkeley
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A Non-Resonant Electrostatic Harvester
• Si proof mass: whole wafer etching
• Polyimide suspension: low stiffness
• Wide frequency range of operation: suitable
for body motion
• Self-synchronous: physical contact to
charging and discharging terminals
• Size ≈ 12 × 12 × 1.5 mm
detail of moving plate assembled generator
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po
sit
ion
time
time
trajectory of moving plate
vo
ltag
e
t2 t3t1
voltage on moving plate
upperlimit
lower limit
Mass
Top plate (silicon)
Base plate (quartz)
Mass
Mass
Input phase
Output phase
Gap
COM
Vout
Vin
Polyimide
suspension
Non-Resonant Electrostatic Harvester 2
Ref: Miao, P. et al. “MEMS inertial power generators for biomedical
applications”, Microsystem Techn. 12 (10-11), pp.1079-1083 (2006).
• Measured output > 2 μW at 20 Hz excitation
• Wide operating frequency range
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Non-Resonant Electrostatic Harvester: Problems
• Si density low – reduces m
• Travel range limited – movement
is in short dimension
• Whole wafer etching expensive
and limits integration potential
• Output in inconvenient large
impulses
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External Mass Electrostatic Harvester
• Proof mass rolls on substrate
• Multiple charge-discharge cycles per transit
• No deep etching: fabrication simplicity
• Large mass and internal travel range
But:
• Very low capacitances & capacitance ratios
• Thus, low power for given priming voltage
Rolling mass on prototype device
Schematic illustrating concept
Electrostatic simulation
Ref:
M. Kiziroglou, C. He and E.M. Yeatman, “Rolling Rod Electrostatic Microgenerator”, IEEE Trans.
Industrial Electronics 56(4), pp. 1101-1108 (2009).
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Overcoming Low Electro-mechanical Coupling: Frequency Up-Converting Piezoelectric Harvester
• External rolling proof mass
• Distributed transduction by series
of piezo beams
• Proof mass “plucks” beams by
magnetic interaction
• Energy extracted as beams ring
down: high electrical damping not
needed
Ref:
P. Pillatsch, E.M. Yeatman & A.S. Holmes, “Piezoelectric Impulse-Excited Generator for Low
Frequency Non-Harmonic Vibrations ”, Proc. PowerMEMS 2011, Seoul, Nov. 2011, pp. 245-
248.
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Frequency Up-Converting Piezoelectric Harvester
• Operation over a wide frequency range
(6:1) demonstrated at higher acceleration
• Effectiveness reasonable for first design
• Power density of 4-13 mW/cm3 for lighter
proof mass
• Scalable design
4 test configurations:
• a1 = 2.72 m/s2
• a2 = 0.873 m/s2
• m1 = 0.285 kg
• m2 = 0.143 kg
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Overcoming Low Electro-Mechanical Coupling: Active Interface Circuits
• Piezo devices limited by high output
capacitance
• Difficult to match load impedance –
leads to weak damping factor
• Concept is to synchronously
precharge the piezo cell to increase
damping force
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Re-designing Sensor Architecture for Harvester-Powered Operation
• Harvester power density inherently
low for low frequency (e.g. human
powered) applications
• Traditional architecture based on
separate power and other modules
• Data processing and transmission
modules most power intensive
• Solution: new approach to node
architecture, mixing modules
together
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New Architecture
• Harvester connected between
sensor output and transmitter
• Sensor acts as priming voltage,
harvester as pulse former and
energy amplifier
• Output pulses transmitted
directly without further
processing
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Fully Assembled Device
• Input from voltage supply representing
output of sensors
• RF frequency determined by size of
antenna loop: in this case 350 MHz
• Commercial off-the-shelf TV receiver
employed for its broad bandwidth
• Higher frequency ( > 1 GHz) will allow
antenna loop close to harvester size (5
mm)
Ref:
C. He, M. Kiziroglou, D. Yates and E.M. Yeatman, “A MEMS Self-Powered Sensor and RF
Transmission Platform for WSN Nodes”, IEEE Sensors 11(12), pp.3437-3445 (2011).
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Inertial Harvesters: power is limited by proof mass and travel range:
Maximum power = mw3Yozo/p
Any alternatives?
yes, rotating proof mass:
limited motion range not inherent
Overcoming Displacement Limit: Rotational Harvesters
Ref:
E.M Yeatman, "Energy Harvesting from Motion Using Rotating and Gyroscopic Proof Masses",
J. Mechanical Engineering Science 222 (C1), pp. 27-36 (2008).
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Rotating Mass Inertial Generator
Example #1: traditional self-winding watch
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Rotating Mass Inertial Generator
Example #2: Seiko Kinetic
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Rotating mass generator – two possible modes:
• driven by linear motion
• driven by rotating motion
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Rotating mass generator – two possible modes:
• driven by linear motion
• driven by rotating motion
Semi-circle design of watch proof masses allows the former:
• Theoretically achievable power is similar to linear motion device: relative
direction of mass and frame motion reverses on each half turn
• Advantage is in implementation practicalities.
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Rotating mass generator driven by rotating motion
Potential advantage: resonant enhancement
• Allows benefit of “unconstrained” internal amplitude
• Actual constraint is the need for a spring
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Proposal : Rotating mass resonant generator
source motion amplitude qo , frequency w
proof mass m, radius R
Achievable power:
QmR
P o
8
322
max
wq
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Compare: Rotating vs Linear resonant generator
Example: upper limb swinging at 1 Hz
• Linear: Yo = 5 cm
• Rotating: qo = 25 deg
• Use mass of 1 g, radius = travel range = 0.5 cm
QmR
P o
8
322
max
wq
p
w3
maxooZmY
P vs.
Result:
Plin = 13 uW Prot = 0.2 uW √Q
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Rotating vs Linear resonant generator
Plin = 13 uW Prot = 0.2 uW √Q
Prot higher for Q > 4000
Technical Challenge:
• High Q for resonant rotating device requires spring with
very high number of turns
Practical Challenge:
• High Q means high drive frequency dependence
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How else can rotating motion be used in inertial generation?
Overcoming the Mass Limit
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How else can rotating motion be used in inertial generation?
What about driving the rotation actively?
Overcoming the Mass Limit
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Proposal: Gyroscopic power generation
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Gyroscopic power generation
Basic principle: for moment of inertia I rotating at ws and tipped at wp :
torque T = Iwswp
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Gyroscopic power generation
Mechanism: couple the rocking frame to the gyroscopic body by the energy
extracting damper (electrostatic…)
For disk spun at ws and rocked at wo,
achievable power:
soogyr mRP wwq 222
4
1
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Gyroscopic power generation
Opportunity: power output rises with spin speed
Limitation: need to subtract drive power
• Depends on drive speed; optimum drive speed thus determined by Q
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Gyroscopic power generation
Net power:
About 4x resonant rotating (passive) case
QmRP ogyr
322
3
32wq
p
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Gyroscopic power generation
0
50
100
150
200
1 10 100 1000 10000 100000
Q of gyroscopic device
Pow
er
(uW
)
Linear device
gyroscopic
device
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Gyroscopic power generation
How to implement in MEMS? High quality spinning bearings not really available.
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Gyroscopic power generation
How to implement in MEMS? High quality spinning bearings not really available.
• Solution: well known format for MEMS gyros
• Vibrating gyro
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Gyroscopic power generation
• Proposed format: linear vibration on two axes, one for drive, one for pick-
off;
• Same as gyro sensor except pick-off extracts energy, not signal
Vertical spring
Anchor
Frame Lateral spring
Drive comb
after Fedder et al
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Conclusions
• Basic mechanics sets strict limits on achievable power from inertial harvesters
• Ultimate power density drops as devices shrink
• Form factor, resonance and choice of transduction are important considerations
• Rotating harvesters can offer some ways around the basic limits
Thanks:
Paul Mitcheson, Andrew Holmes, Tzern Toh, Peng Miao, Michalis Kiziroglou, Cairan He, David
Yates, Pit Pillatsch
EPSRC, European Commission
Contact: [email protected]
Review Paper: P. D. Mitcheson, E. M. Yeatman, G.K. Rao, A. S. Holmes & T. C. Green, “Energy Harvesting from Human and
Machine Motion for Wireless Electronic Devices”, Proc. IEEE 96(9), pp. 1457-1486 (2008).