V20W Datasheet, PDF (7/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

POWER MEASUREMENTS
Piezoelectric material produces mechanical strain
under the influence of an externally applied electrical
field, and conversely produces electrical potential in
response to applied mechanical strain. Products such
as the Volture™ piezo energy harvester are typically
used in a cantilevered-beam configuration, in which the
piezoelectric beam is clamped at one end and the other
end allowed to oscillate freely in response to vibration
normal to the flat surface of the beam, converting
these vibrations to in-plane material strain. The beam
dimensions and tip mass determine the resonant
frequency of the beam, which is tuned to match the
dominant vibrational frequency of its environment,
mechanically amplifying this typically small vibration.
Power Measurements
The power output capability of the Volture™ products
was measured in the following manner. In the
cantilevered beam configuration above, the Volture was
mounted to a shaker capable of generating vibrations of
varying frequency and amplitude. Tip masses (four for
each product) were added to alter the natural frequency
of the Volture™ products. The vibration frequency being
generated by the shaker was then matched to the
frequency of the Volture™ product to provide resonant
and therefore optimized energy harvesting. Four
different amplitudes were tested (0.25, 0.375, 0.5, and
1.00g) at each of these frequencies. The piezo’s output
was rectified and then placed across a purely capacitive
load. The capacitor value was chosen using the
following equation for average power, where C is the
capacitance in Farads, V is the piezo’s open circuit
voltage, and Δt is a reasonable time interval (~ 10
seconds), and solving for C:
Yielding:
1
2
C
V²
P_AVG
t
C ^{2 PAVG t}
V²
The figure below shows the voltage (operating voltage)
on the capacitor and instantaneous power into capacitor
vs. time for a representative vibration level and
frequency. The V25W product was used, demonstrating
that the power increases until it peaks when the
operating voltage is at about half its open circuit value.
After that, it decreases.
See Application Note: Load Isolation Example.
Typical Power & Voltage vs Time
P_PEAK
V_{OPEN CIRCUIT}
V_{OPEN CIRCUIT}
0
Time (Seconds)
REVISION N0. 002 REVISION DATE: 01-23-2013
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