Energy Scavenging

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Energy Scavenging for Wireless Sensor Nodes with
a Focus on Vibration-to-Electricity Conversion
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Motivation and Objective

• Scaling of IC technology is far outpacing that of
  power supplies.
• Example At an average power consumption of 100
  mW, you need more than 1 cm3 of lithium battery
  volume for 1 year of operation.
• Goal To investigate power scavenging
  technologies that can provide an average of 100
  mW/cm3 indefinitely.

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Batteries, Solar, and Vibrations
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Simple Model for Conversion
z transducer displacement y magnitude of input
Power in terms of magnitude and frequency of
input.
Power assuming w wn.
Power in terms of acceleration magnitude.
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Three Ways to Convert Vibrations
Capacitive Change in capacitance causes either
voltage or charge increase.
Inductive Coil moves through magnetic field
causing current in wire.

• Piezoelectric
• Strain in piezoelectric
• material causes a charge
• separation (voltage across
• capacitor)

Amirtharajah et. al., 1998
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Three Types of MEMS Converters
In-plane, gap closing type Capacitance changes
by changing gap between fingers.
In-plane, overlap type Capacitance changes by
changing overlap area of fingers.
Out-of-plane, gap closing type Capacitance
changes by changing gap two large plates.
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MEMS Capacitive Converters
MEMS in-plane gap-closing test structures. Built
to test the minimum allowable gap between
fingers, and the viability of assembling JFETS or
diodes to the die.
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MEMS Capacitive Converters
Another full size test device with tungsten proof
mass attached
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Piezoelectric Conversion
33 Mode
F
Constitutive Equations
31 Mode
d strain s stress Y Youngs modulus d
piezoelectric coeff. D electrical
displacement e dielectric constant E electric
field
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Piezoelectric Benders
System Equations
Where k equivalent spring stiffness of beam
m attached proof mass bm damping
coefficient a1 geometric constant a2
geometric constant d31 piezoelectric
coefficient tc thickness of one
piezo-ceramic layer VR voltage across load
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Test Results and Model Verification
Optimized Prototype - overall size lt 1cm3 -
total length lt 1.5cm
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Test Results and Model Verification
Optimized Prototype - overall size lt 1cm3 -
total length lt 3cm
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Real Circuit for Piezo Converters
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Test Results Capacitive Load Circuit
Power vs. time
Voltage vs. time
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Piezo Generator with Power Circuit
Piezo Generator
Super Capacitor
Simulated Load
Power circuit
Vibrometer
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Generators to be Embedded in Tires
4mm
Clamp
Proof Mass
Piezoelectric Bimorph
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Test Results with a Resistive Load
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Power Output vs. Load Resistance
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Test Results Capacitive Load
Voltage Across 3.3 mF (top) and 0.8 mF (bottom)
Load Capacitor vs. Time
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Solar Power Train Circuitry
Wires to load
2 cm
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Solar Power Circuit Schematic
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Solar Power Train Test Results
Charge up profile of battery from solar cell.
Solar cells place 12 inches under a 60 watt desk
lamp.
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Conclusions

• Scavenging the power from commonly occurring
  vibrations for use by electronics is both
  feasible and attractive for certain applications.
• Piezoelectric converters appear to be the most
  attractive for meso-scale devices with a maximum
  demonstrated power density of approximately 200
  mW/cm3 vs. 80 mW/cm3 for capacitive MEMS devices.
• Both solar powered and vibration powered systems
  are being actively pursued and will be up and
  running shortly.