Three main converters allow mechanical-to-electrical transduction: piezoelectric, electromagnetic and electrostatic devices (Figure 5
Figure 5: Basic converters a) piezoelectric, b) electromagnetic and c) electret-based electrostatic devices
(Click on image to enlarge)
Piezoelectric converters (Figure 5a
) use piezoelectric materials that generate charges under stress or strain. Electromagnetic converters (Figure 5b
) are based on Lenz’s law: the movement of a magnet in a coil generates a current. Finally, electret-based electrostatic converters (Figure 5c
) use electrets to induce charges on electrodes; a relative displacement of an electrode compared to an electret generates a variation of electret charges’ influence on the electrode and charge circulation.
Nevertheless, whatever the converter, VEh output power is limited by physics and will not exceed Pth (except in certain circumstances; e.g. non-linear behaviors). VEh output power is therefore proportional to mobile mass and acceleration squared and inversely proportional to the harvester’s frequency bandwidth.
is the mobile mass, A
the acceleration amplitude and BWHz
the frequency bandwidth)
Each of these converters presents both pros and cons that are summed up in Table 1
Table 1: Pros and cons of the different converters
For reasons given in Table 1
, our choice fell on piezoelectric and electrostatic devices (Figure 6
) that present high output voltages simple to rectify with a diode bridge and lower resistive losses.
Figure 6: Electrostatic VEh a) scheme and b) prototype
For these devices, up to 10µW/g of mobile mass can be harvested from ambient vibrations (0.1G@50Hz) and when VEh resonant frequency is tuned to ambient vibration frequency.
Actually, the resonant effect is probably both the main advantage and the main drawback of VEh as they can harvest much power when ambient vibration frequency fits their resonant frequency but have a tight frequency bandwidth that does not exceed some Hz in the best case.