Self-adaptive MEMS vibration energy harvester targets low frequencies
CEA-Leti researchers are developing an innovative energy-harvesting technology that collects vibrations from the environment and converts them into electricity to power a variety of sensors.
A unique aspect of Leti’s energy-harvesting microsystem is its ability to use vibrations of varying frequency and amplitude. Although it is easier to convert stable, highfrequency vibrations (over 1 kHz) into electrical energy than low frequency unstable ones, Leti has chosen to focus on harvesting low-frequency vibrations with varying frequencies and amplitudes. The reason for this choice is that many of the vibrations in natural and man-made environments are relatively low frequency (under 120 Hz), and often depend on energy sources of varying activity levels such as engine vibrations, vehicle speed, wind level, etc.
The output power of a vibration-driven energy harvester is directly proportional to the vibration amplitude and frequency of the energy source and to the size (seismic mass weight) of the harvester. Output power is inversely proportional to the harvester’s frequency bandwidth. Consequently, it is much harder to efficiently harvest power from low-frequency sources with a large frequency band response and with a very small system size than from a stabilized high frequency vibration source.
Seeking better harvesters
With those challenges in mind, Leti (Laboratory for Electronic and Information Technologies of the French Atomic and Alternative Energies Commission) set out to find a better way to maximize the output power of energy harvesters. The laboratory developed an electrostatic micro-electromechanical system (MEMS) structure capable of efficiently converting both low- and highamplitude vibrations into electrical energy, thanks to a unique patterned electrode structure – see figure 1.
Fig. 1: All three elements shown separately – The SiO2-based electret (purple slice) is to be mounted or patterned directly onto the MEMS seismic mass (shown standing). The whole assembly is then mounted a few micrometers away from the electrode glass plate (shown at the bottom with several electrode patterns) to form the vibration energy harvester.
This patented electrostatic structure translates the input vibration into multiple capacitance variations, which are used to convert the input vibration energy into electrical energy. When a constant charge is placed in the variable capacitor, the voltage varies in inverse proportion to the capacitance variation (capacitor voltage = charge/ capacitance) and the associated energy varies in proportion to this voltage:
Energy = ½ charge x voltage
In other words, a capacitance variation induces an energy variation, and that energy variation is used to supply the output load.
This type of structure differs from a piezoelectric structure in that the energy level converted per cycle by the electrostatic structure can be adjusted by adjusting the charge value and is not linked to a specific material property. If no charges are placed on the structure, no electrostatic forces will be applied to the structure. The structure is then free to move and no mechanical work is converted into electrical energy. However, if a large charge is placed on the structure, a large electrostatic force appears in the structure, preventing it from moving. As a result, the capacitance value remains constant and no electrical energy is delivered.
To maximize the output power, an optimum charge value is required that matches the mechanical impedance of the vibration source and the converter input.
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.
Fig. 2: Different constant output power curves at 3V, measured in function of the input vibration amplitude and frequency for a 100 g seismic mass.
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 (firstname.lastname@example.org) 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 eeNews Europe.
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