20 June 2013
Researchers in France working to increase the amount of energy that piezoelectric energy harvesters can glean have developed a pulse width modulation based electrical interface. The interface focuses on tuning the harvester’s output waveform to multiple mode vibrations rather than focusing on a single mode.
The purpose of an energy harvester’s electrical interface is to maximise the total harvested energy. In the last ten years the development of a technique called synchronised switch harvesting on inductor (SSHI) for switching-type interfaces has enabled a 1000% increase in harvested power for piezoelectric vibration energy harvesters.
In these harvesters the piezoelectric elements and structure are usually weakly coupled. This means that the energy extraction from piezoelectric elements doesn’t disturb the vibration of the structure, and the magnitude of velocity can be assumed to be unchanged. In this case, power depends on only two factors, the phase between voltage and velocity, and the piezoelectric voltage amplitude. To achieve a better power output, the piezoelectric voltage and vibration velocity need to be in phase and the voltage amplitude needs to be increased. SSHI alters the waveform of the piezoelectric voltage to achieve this.
However, real vibrations are often not a fixed single mode vibration. So to harvest as much energy as possible a piezoelectric structure with wide operational frequency range is needed. The output signal of such a structure will not be a single periodic signal but may be a periodic signal of multiple frequencies or a single frequency signal with noise. But many previous nonlinear electronic interfaces including SSHI interfaces concentrate on energy scavenging from a single mode vibration.
In the work reported in this issue a new approach to harvesting energy from wideband vibration is presented by researchers from the EPI (Integrated Power Electronics) group of the Universite de Cergy-Pontoise’s SATIE laboratory; part of France’s CNRS. To deal with multimode vibration the waveform of each harmonic of the piezoelectric voltage needs to be changed to follow the phase of each harmonic of velocity. To this end the interface uses pulse width modulation (PWM).
“The traditional interfaces only change the phase and increase the amplitude of piezoelectric voltage. The proposed PWM interface can also change the waveform of the voltage. As a result, the PWM interface is more adaptive to the vibration, especially in the case of multi-mode vibration,” said team member Dr Dejan Vasic.
The PWM voltage waveform is a pulse chain with different pulse widths. The average value of PWM voltage is made to follow the phase of the velocity. “More specifically, the piezoelectric voltage and velocity have equal polarities. The major difficulties are to achieve a totally self-powered interface and to keep switching losses small enough to harvest more energy compare to standard interface.” Said Vasic.
The results reported in the EPI team’s letter demonstrate the technique can improve electromechanical conversion efficiency when the piezoelectric energy harvester is excited under two-mode vibration. The energy harvesting device was based on the traditional cantilevered beam, which is efficient at resonance. In order to broaden the available vibration bandwidth the team are now using a bistable nonlinear beam with the proposed PWM interface (see bottom image). With this arrangement piezoelectric harvester can work more efficiently and more output power at a broadened frequency range can be gained.
The EPI group’s work in piezoelectric materials and power electronics covers both energy harvesting devices and the manufacture of transformers for a range of different application types. These include low-power low output-voltage DC/DC converters for space applications, a magnetic coupled non-linear piezoelectric energy harvesting device with power electronics stage, and power electronics to drive piezoelectric actuators for flapping wing micro-robots.
Looking to the future of this field the team believe the increased demands for high technology devices and the limitations of chemical batteries especially life span and the need for replacement will continue to drive demand for energy harvesting technology. “In this context, micropower harvesting from ambient energy will have wide application for tiny wireless sensor devices, implanted biomedical sensors etc. due to its potentially infinite lifetime,” said Vasic.
The twin challenges in the field are to increase the harvested power whilst also continuing to develop low-power integrated circuits to ‘narrow the gap’ and increase the range of devices that can be powered through energy harvesting. Vasic noted that: “Beside the development of electrical interfaces, a great effort has been made in the past few years in piezoelectric single crystals featuring giant piezoelectric properties. Nevertheless, the conception of such materials is complex and costly, and no industrial process exists up to now. Therefore, I would like to see in the next decade a real industrial application of piezoelectric energy harvesting for wireless sensors.”
This article is based on the Letter: PWM interface for piezoelectric energy harvesting (new window).
Integration in Power Electronics: http://www.satie.ens-cachan.fr/Equipes/IPEM/axes_en.html (new window)
Vasic Dejan: http://www.satie.ens-cachan.fr/php/cherchdet.php?id=35 (new window)
A PDF version (new window) of this feature article is also available.
Browse or search all papers in the latest or past issues of Electronics Letters on the IET Digital Library.