Self-adaptive MEMS vibration energy harvester targets low frequencies

New electret material
To maintain an optimal electric charge in the electrostatic structure, Leti developed an electret material able to keep its charge over many years, even when built into very small electrodes less than 20µm. The electret in that case is a silicon oxide-based dielectric material compatible with microelectronic manufacturing processes and is able to permanently store an electric charge, or polarization.

This polarized material behaves very much like a permanent magnet in close proximity to a coil. When an electret changes of position relative to two electrodes, it induces a new charge distribution on the electrodes. If an electrical load is connected between these electrodes then the electret movement will generate electrical energy. Because the structure is electrostatic, there are no resistive losses, unlike in small electromagnetic systems with low operating frequencies where the losses generated by the coil grow drastically as the size and frequency decrease.

At vibrations of less than 0.2 grams at 50 Hz, Leti’s system was able to output 3V, reaching an output power of about 10 microwatts per gram of seismic mass. The resulting mechanical-to-electrical conversion efficiency was an impressive 60 percent. Thus, the main limitation is no longer the converter, but the available mechanical input energy. The largest piece of the system, which represents 80 percent of the system’s total mass, is not the mechanical-to-electric converter but the seismic mass component required to provide more mechanical energy to the converter.

Increasing the frequency range

To further increase the energy harvester’s vibration amplitude and frequency range, Leti has developed a patented mechanical non-linear spring, which limits the relative displacement of the seismic mass without dissipating energy and helps keep the relative displacement fairly constant over a wide range of frequencies.

To make the most of resonant effects over a wide frequency band, Leti is also working on two solutions that would allow the electrostatic structure to resonate at any frequency. The first one consists of electrically modifying a mechanical parameter of the structure to adjust its resonant frequency so that it can follow the main vibration frequency, for example when harvesting energy in a car, a train or an airplane whose engine is speeding up or slowing down. A piezoelectric effect is used to electrically modify the resonant frequency of the energy harvester.

Experiments suggest that with such as solution, the system’s resonant frequency can be tuned over a range representing up to 30 percent of its main resonant frequency. The feedback loop that automatically tunes the system’s resonant frequency to the vibration source’s frequency is expected to draw less than 5µW.

The second patented solution consists in amplifying a random vibration by synchronizing a mechanical rebound with the vibration source acceleration. This allows the energy harvester system to absorb the maximum possible input vibration energy and translate it into electrical energy. To achieve this, the seismic mass displacement direction is changed when the vibration source reaches its peak speed by making a short elastic rebound (which lasts a short time compared to the vibration source period).

This mechanical rebound induces a speed increase in the seismic mass of about twice the vibration source’s maximum speed at each rebound. The speed increase at each rebound induces a kinetic energy increase of the seismic mass.

Finally, one part of the kinetic energy increase is proportional to the square of the vibration source speed, while the other part is proportional to the input speed times the previous speed. Hence, the higher the previous speed is, the higher is the kinetic energy increase.

This energy amplification process enables this system to reach high mechanical energy levels from the vibration source at each rebound, which are then converted into electrical energy when the amplification level is sufficiently high. The second solution was tested using piezoelectric actuators to create the rebound, and a relative average displacement gain six times greater than that of a simple resonant system was obtained, over more than one octave.

Ghislain Despesse (ghislain.despesse@cea.fr) is a researcher at the Grenoble-based CEA-Leti,  a French research institute focused on micro- and nanotechnologies and their applications.

The is article was published in the March issue of EE Times Europe.