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Abstract—This paper describes an approach to harvesting electrical
energy from a mechanically excited piezoelectric element.
A vibrating piezoelectric device differs from a typical electrical
power source in that it has a capacitive rather than inductive source
impedance, and may be driven by mechanical vibrations of varying
amplitude. An analytical expression for the optimal power flow
from a rectified piezoelectric device is derived, and an “energy harvesting”
circuit is proposed which can achieve this optimal power
flow. The harvesting circuit consists of an ac–dc rectifier with an
output capacitor, an electrochemical battery, and a switch-mode
dc–dc converter that controls the energy flow into the battery. An
adaptive control technique for the dc–dc converter is used to continuously
implement the optimal power transfer theory and maximize
the power stored by the battery. Experimental results reveal
that use of the adaptive dc–dc converter increases power transfer
by over 400% as compared to when the dc–dc converter is not used
INTRODUCTION
T HE need for a wireless electrical power supply has spurred
an interest in piezoelectric energy harvesting, or the
extraction of electrical energy using a vibrating piezoelectric
device. Examples of applications that would benefit from
such a supply are a capacitively tuned vibration absorber [1],
a foot-powered radio “tag” [2], [3], and a PicoRadio [4]. A
vibrating piezoelectric device differs from a typical electrical
power source in that its internal impedance is capacitive rather
than inductive in nature, and that it may be driven by mechanical
vibrations of varying amplitude and frequency. While there have
been previous approaches to harvesting energy generated by a
piezoelectric device [2], [3], [5], [6] there has not been an attempt
to develop an adaptive circuit that maximizes power transfer
from the piezoelectric device. The objective of the research described herein was to develop an approach that maximizes the
power transferred from a vibrating piezoelectric transducer to
an electrochemical battery. The paper initially presents a simple
model of a piezoelectric transducer. An ac–dc rectifier is added
and the model is used to determine the point of optimal power
flow for the piezoelectric element. The paper then introduces an
adaptive approach to achieving the optimal power flow through
the use of a switch-mode dc–dc converter. This approach is
similar to the so-called maximum power point trackers used to
maximize power from solar cells [7]–[10]. Finally, the paper
presents experimental results that validate the technique.
II. OPTIMAL POWER FLOW OF PIEZOELECTRIC DEVICE
To determine its power flow characteristics, a vibrating piezoelectric
element is modeled as a sinusoidal current source
in parallel with its internal electrode capacitance . This model
will be validated in a later section. The magnitude of the polarization
current varies with the mechanical excitation level of
the piezoelectric element, but is assumed to be relatively constant
regardless of external loading. A vibrating piezoelectric
device generates an ac voltage while electrochemical batteries
require a dc voltage, hence the first stage needed in an energy
harvesting circuit is an ac–dc rectifier connected to the output
of the piezoelectric device, as shown in Fig. 1. In the following
analysis, the dc filter capacitor is assumed to be large
enough so that the output voltage is essentially constant;
the load is modeled as a constant current source ; and the
diodes are assumed to exhibit ideal behavior.
The voltage and current waveforms associated with the circuit
are shown in Fig. 2. These waveforms can be divided into two
intervals. In interval 1, denoted as , the polarization current is
charging the electrode capacitance of the piezoelectric element.
During this time, all diodes are reverse-biased and no current
flows to the output. This condition continues until the magnitude
CONCLUSIONS
This paper presents an adaptive approach to harvesting
electrical energy from a mechanically excited piezoelectric
element. The dc–dc converter with an adaptive control algorithm
harvested energy at over four times the rate of direct
charging without a converter. Furthermore, this rate is expected
to continue to improve at higher excitation levels.
The flexibility of the controller allows the energy harvesting
circuit to be used on any vibrating structure, regardless of excitation
frequency, provided a piezoelectric element can be attached.
Also, external parameters such as device placement,
level of mechanical vibrations or type of piezoelectric devices
will not affect controller operation. The control algorithm can
also be applied to other dc–dc converter topologies. This would
allow the development of optimized system designs based upon the expected excitation or the electronic load that is to be powered.
Future work will focus on the design of an optimized
system design using standalone control circuitry