Vibration Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 110

3

P

F

(6)

- -

- -

Q or V Q or V

2

P + P +

(5)

+ F

+

(4)

1

Labelling of Reference
Axes and Planes

Parallel Compression Generator
(mode 33)

Transverse Tension Generator
(mode 31)

FIGURE 4.1
Parallel and transverse tension modes of operation for a piezoelectric generator. (From J.L.
Gonzalez, A. Rubio, and F. Moll, “Human-powered piezoelectric batteries to supply power
of wearables electronic devices,” International Journal of the Society of Materials Engineering for
Resources , vol. 10, no. 1, pp. 34-40, 2002 [79].)

the mechanical stress X applied, and the electrical field E is generated. The
proportionality constants are the d coefficient and the dielectric constant

X
measured at constant stress. As for the expression in Equation 4.2 , it relates
the mechanical strain developed when an electrical field E is applied with the
field, and the mechanical stress X is developed. The proportionality constants
are the same d coefficient and the elastic compliance s E measured at a constant
electrical field. For the third relationship, according to Gonzalez et al. [79],
the piezoelectric materials are visualized as 3D (three-dimensinal) structures
where the mechanical and electrical magnitudes can be applied or measured
in any of the three axes. The axes are defined in Figure 4.1 where the impact-
based piezoelectric generators in parallel and transverse modes of operation
are illustrated.
In this VEH research, two types of piezoelectric generators that are de-
signed to harvest impact or impulse forces are explored: (1) the piezoelectric
push-button igniter described in Section 4.1 and (2) the prestressed piezoelec-
tric diaphragm material described in Section 4.2. With reference to Figure 4.1 ,
the piezoelectric igniter operates in the parallel compression mode or 33-mode
of operation where the electrical field is generated (i.e., axis- 3) across the
same axis where an external mechanical force F is applied to create a me-
chanical resonance in the piezoelectric element [79]. As for the other type
of piezoelectric generator, which uses a prestressed piezoelectric diaphragm
material, a transverse mode or 31-mode of operation as shown in Figure 4.1 is
considered [102]. The mechanical strain applied on the piezoelectric material
is perpendicular to its output electrodes, so the surface where the charge is
collected and the surface where the force is applied are independent. When
the piezoelectric material is excited in 31-mode operation, elongation or com-
pression normal to the 1 axis is induced; hence, an electric field normal to the
3 axis is generated.
Impact-based piezoelectric energy harvesting has been widely discussed
in the literature to harvest waste kinetic energy from human motions, ammu-
nitions, and so on for powering low-power electronic devices. According to