Consider using PVDF shoe inserts for recovering some of the power in
the process of walking. There are many advantages to this tactic.
First, a 40 ply pile would be only (28 
m)(40) = 1.1 mm thick
(without electrodes). In addition, the natural flexing of the shoe
when walking provides the necessary deflection for generating power
from the piezoelectric pile (see Figure 3). PVDF is easy
to cut into an appropriate shape and is very durable
[AMP, 1995,Fraden, 1993]. In fact, PVDF might be used as a direct
replacement for normal shoe stiffeners. Thus, the inserts could be easily put
into shoes without moving parts or seriously redesigning the shoe.
A small women's shoe
has a footprint of approximately 116 cm
. Knowing that the maximum
effective force applied at the end of a user's step increases the
apparent mass by 30%, the user needs only 52 kg (115 lbs) of mass to
deflect the PVDF plate a full 5 cm. While the
numbers given in the last section were for a 15.2 cm by 15.2 cm
triangular 40 ply pile, the value can be used to approximate the
amount of power an appropriately shaped piezoelectric insert could
produce. Thus, scaling the previous 1.5 W at 0.6 deflections per
second to 2 steps per second,
of electrical power could be generated by a 52 kg user at a brisk walking pace.