  Thanks to miniaturisation, it is today possible to imagine self-powered
systems that use vibrations to produce their own electrical energy. Many
energy-harvesting systems already exist. Some of them are based on the use of
electrets: electrically charged dielectrics that can keep charges for years.
This paper presents an optimisation of an existing system and proves that
electret-based electrostatic energy scavengers can be excellent solutions to
power microsystems even with low-level ambient vibrations. Thereby, it is
possible to harvest up to 200\muW with vibrations lower than 1G of acceleration
(typically 50\mumpp at 50Hz) using thin SiO2 electrets with an active surface
of 1 cm^{2} and a mobile mass of 1g. This paper optimises such a system
(geometric, electrostatic and mechanical parameters), using FEM (Finite Element
Method) software (Comsol Multiphysics) and Matlab to compute the parameters and
proves the importance of such an optimisation to build efficient systems.
Finally, it shows that the use of electrets with high surface potential is not
always the best way to maximise output power.
