22-08-2014, 03:04 PM
OPTIMAL POWER FLOW OF PIEZOELECTRIC DEVICE
. OPTIMAL.docx (Size: 881.61 KB / Downloads: 18)
INTRODUCTION
The 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. A foot powered radio “tag’’, and a Pico Radio. 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 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. Finally, the paper presents experimental results that validate the technique.
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
. EXPERIMENTAL SETUP
A Quickpack® QP20W purchased from Active Control Experts (ACX), Cambridge, MA, was used as the piezoelectric energy source. It is a two-layer device that generates an ac voltage when vibrated in a direction perpendicular to its mid plane. Device specifications and diagram are shown in Fig. 6 along with the piezoelectric element properties.
The piezoelectric device is secured to an electric powered shaker, which provides variable mechanical excitation in response to a sine wave input. The magnitude of the mechanical excitation of the piezoelectric element will be characterized by the open circuit voltage that is measured across the unloaded rectifier capacitor, . A small mass was added to the free tip of the bi morph to enhance the external stress and increase the tip deflection, thus providing a larger open-circuit voltage.
The step down converter consists of a MOSFET switch with a high breakdown voltage rating, a custom wound inductor with inductance of 10.03 mH, a Schottky diode, and a filter capacitor. The voltage across the current-sense resistor is amplified with a precision op amp (powered by the 3 V battery), and then sampled by the A/D converter on the controller card. The controller card then generates the PWM signal at the calculated duty cycle that is fed to a high-side MOSFET driver. The driver was powered by an external dc power supply. Due to the low power levels expected from the piezoelectric element, it is assumed that the converter will operate in discontinuous current conduction mode at the chosen switching frequency of 1 kHz. Such a low switching frequency was chosen because switching losses in the experimental setup comprised a significant fraction of the power flow from the element.
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.