# Energy Harvesting from Mechanical Vibrations

Topics: Resonance, Vibration, Damping Pages: 8 (2042 words) Published: August 22, 2013
Human power-based energy harvesting strategies for mobile
electronic devices
a. Describe the application (10%)

1.2.3 Wearable or implantable medical devices
Implantable or wearable medical devices refer to any
device that could help monitor metabolic parameters, assist
defective physical function or cure diseases. Structural
devices such as artificial joints, vascular grafts and artificial valves are called passive devices and their working does
not need external power. But active devices can consume
energy with different orders of magnitude from microwatts
to several watts, as shown in Table 2.

b. Identify how the mechanical vibration is induced (20 %)

2.3.1 Characterization of vibration system for motion
harvesting
Displacement driven generators are typically inertial
mechanism-based, second-order vibration systems excited
by periodical human body movement of the legs, limbs or
feet. Ideally, these systems can be described as spring-mass systems , as shown in Fig. 5. The frame is attached to the
moving body. A proof mass (m) is suspended inside. A
spring (with stiffness of k) and a damper (with damping
coefficient of c) couple the relative movement (Z1)
between these two parts. Z1 induces electricity by the
transduction mechanism of the damper. Assuming that the
mass of the vibration source is significantly larger than that of the seismic mass and therefore not affected by its
presence, and that the external excitation is harmonic, then the differential equation of motion is described as
(5)
The standard steady-state solution for the mass displacement is given by
(6)
where f is the angle phase given by
: (7)
Maximum energy can be extracted when the excitation
frequency matches the natural resonant frequency of the
generator system ωn, given by
: (8)
The vibration structure for which resonant frequencies
range from 10 kHz to 1 MHz is good at converting the high
frequency energy of machine vibration with small amplitude
to electrical energy. However, the human body moves
at a low frequency of less than 10 Hz [49] and at high
amplitude. For the human being as the excitation source, a
specific design must be developed. The state-of-the-art
technique to harvest vibration energy from low frequency
excitation is especially reviewed in this paper.
It should be noted that the damping coefficient [50] is
comprised of parasitic losses, cp, and electrical energy
extracted by the transduction mechanism, ce. As the
extracted energy is characterized by the transduction
mechanism or the coupling efficiency of ce, a comprehensive
review of existing transduction mechanisms and their
specific characteristic equations are presented and compared in the following section.
c. Describe how the operation utilises the source of mechanical vibration to achieve the desired outcomes (30%)

2.2.3 Electromagnetic induction
Based on Faraday’s law on electromagnetic induction, the
variation in magnetic flux through an electrical circuit
yields an open circuit voltage. Almost all traditional
conductors take the form of a coil and the electricity is
generated by either the relative movement of the magnet
and coil, or by changes in the magnetic field.
The most natural method to innocuously tap human
activity power is by placing sets of coil and magnet to
extract their relative activities [43,44]. Low transduction
efficiency yet high power output [45] due to cumbersome
mounting and bulk scale has been described in many
applications. Their power generation ability is characterized by direct proportion to the scale of coils or length of
stroke. However, innovative designs [46] particularly
engineered by human activity outperform their predecessors
several times
2.3.2 Piezoelectric property
An expression for the piezoelectric damping coefficient is
[51]
, (9)
where k is the piezoelectric material electromechanical