14-08-2012, 02:41 PM
Piezoelectric Power Scavenging of Mechanical Vibration
Energy
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ABSTRACT
The process of acquiring the energy surrounding a system and converting it into usable
electrical energy is termed power harvesting. With piezoceramic materials, it is possible
to harvest power from vibrating structures. It has been proven that micro- to milliwatts
of power can be generated from vibrating systems. The project targets the
transformation of mechanical vibration into electrical energy using piezoelectric
material. In some mining applications, eg water jet drilling; large high frequency
vibrations may be present. If successfully harvested, this energy could be used to
eliminate batteries in wireless sensors. This article presents a model of a piezoelectric
transducer, a mechanical vibration spectrum, the simulation of the model, prototype of
the power scavenging circuit, experimental results and its future perspectives.
INTRODUCTION
With the recent advances in wireless and microelectromechanical systems technology, the demand for
portable electronics and wireless sensors is growing rapidly. Because these devices are portable, it
becomes necessary that they carry their own power supply. In most cases this power supply is the
conventional battery; however, problems can occur when using batteries because of their finite
lifespan. For portable electronics, replacing the battery is problematic because the electronics could die
at any time and replacement of the battery can become a tedious task. Supplying power through a long
cable can be impractical, hard to install and maintain. Advances in integrated circuit manufacturing and
low power circuit design have reduced the total power requirements of a wireless sensor node to well
below 1 mW. The requirement of a power supply for small power electronics, a sensor in this case in the
mining environment, is the main motivating factor of this project.
GENERAL POWER SCAVENGING
The following are several common energy conversion techniques (see Figures 1 - 3):
1. piezoelectric: strain in piezoelectric material causes a charge separation (voltage across capacitor);
2. electromagnetic/inductive: coil moves through magnetic field causing current in wire;
3. capacitive: change in capacitance causes either voltage or charge increase; and
4. thermoelectric: change in temperature between two physical locations can be used as a thermal
source, which can be converted into electrical energy.
PIEZOELECTRIC GENERATION
In this project, we are particularly interested in application to sensors for water jet assisted drilling. There
are high frequency vibrations present in the nozzle in water jet assisted drilling. Generating electricity
locally at the nozzle itself can solve many of the above problems. Piezoelectric material can be used to
harvest energy from such vibration, and use the harvested energy as a power supply for the sensor.
The unique physical properties of piezoelectric material is that when a piezoelectric material is
subject to stress/strain it produces electrical charge on its surface, and vice versa, ie when charge flows
through such material, the material goes through physical deformation. Since vibrations cause stress
and strain on to the piezoelectric material, the charge produced moves in one direction under positive
stress and in the opposite direction under negative stress.
Vibration source
Water jet assisted drilling gives rise to large mechanical vibrations because of the natural pulsation of
positive displacement high pressure pumps. Vibration from this source was measured with an
accelerometer. The acceleration magnitude of the vibrations was plotted against frequency over the log
scale. There are two main resonant peaks to consider and those peaks at about 400 Hz and 1400 Hz
were chosen. Figure 5 shows the graphs of recorded spectral data supplied by CRCMining derived
from the Tight Radius Drilling Project.