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a uniquely assembled cantilever design to mount and excite the commercially
available piezoelectric transducer material for harvesting vibrational energy
generated from human pressing.
Referring to Figure 4.14 , whenever a user depresses the commercially avail-
able switch equipped with the energy harvesting feature, the harvested impact
force is not directly applied to the energy harvesting material (i.e., piezoelec-
tric PZT material), instead it is transferred from the human depressing point
through an in-built mechanical cantilevered structure to indirectly activate
the material. The custom design mechanical structure translates the applied
mechanical force onto the edge of the cantilevered piezoelectric transducer
material. As the mechanical force is applied onto the piezoelectric material, it
bends until the mechanical structure releases the piezoelectric material; thus,
mechanical oscillating vibration is generated. The vibration energy is then
converted into electrical energy using the cantilevered piezoelectric mate-
rial. In the LightningSwitch design, the whole energy conversion process
involves many different stages from human depressing force to harvested
electrical energy. During these conversion stages, a certain portion of the har-
vested mechanical energy is lost due to an energy efficiency drop at each
of the individual stages. In addition, the uniquely assembled cantilever de-
sign used in LightningSwitch products, which employs the extra compli-
cated mechanical structure design, would also incur more cost to develop
and manufacture it.
Unlike the commercially available products and research prototypes, which
employ a complicated energy harvesting mechanism to excite its 31-mode
piezoelectric generator, a relatively new concept of energy harvesting from
depressing a prestressed piezoelectric diaphragm material has been proposed
in this research to generate electrical power for a wireless RF transmitter and
its power management circuit to switch on/off electrical appliances such as
lighting, fans, and so on, in a wireless manner. The main objective of this
research is to fulfill a self-powered switch with a less-complicated as well
as less-costly energy harvesting technique. This is achieved by removing the
excessive components of the energy harvesting mechanism.
4.2.1
Description of Prestressed Piezoelectric Diaphragm Material
The impact-based energy harvesting mechanism used in this research is sim-
ply a prestressed piezoelectric diaphragm material manufactured by Face ®
International Corporation based on the THUNDER (THin layer UNimorph
ferroelectric DrivER and sensor) technology originally developed by NASA
in conjunction with the RAINBOW (reduced and internally biased oxide
wafer) design effort [116]. The prestressed piezoelectric diaphragm material,
as shown in Figure 4.15 , is electrically poled in the 31 coupling mode, and it
is initially curved, arc shaped, and rectangular, which elongates when a force
is applied to the top of the arc. The elongation causes strain in the active ma-
terial, which produces a voltage. The device is simply supported and allows
for movement only in the lateral direction.
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