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G. Sordo, E. Serra, U. Schmid, J. Iannacci:
"Optimization method for designing multimodal piezoelectric MEMS energy harvesters";
Microsystem Technologies - Micro- and Nanosystems - Information Storage and Processing Systems, 22 (2016), 1811 - 1820.

Energy harvesters (EH) are devices that convert
environmental energy (i.e. thermal, vibrational or electromagnetic)
into electrical energy. One of the most promising
solutions consists in transforming energy from vibrations
using a piezoelectric material placed onto a mechanical resonator.
The intrinsic drawback of this solution is the typically
high quality factor of the device which works effectively
only within a narrow bandwidth. To overcome this
limitation it is possible to tune the mechanical resonance of
the device, to introduce non-linear elements (e.g. magnets)
or to design the mechanical resonator with a multimodal
behaviour. In ultra low power applications the aspect of
integration is of utmost importance and so micro electromechanical
systems (MEMS)-based EHs are preferable.
Within this scenario the multimodal solution is the more
suitable considering the technological constraints imposed
by the micromachining manufacturing process. In this
paper, we describe the optimization of a given multimodal
mechanical geometry in order to maximize the number of
resonances within a certain frequency band. The proposed
optimization is finite element method (FEM)-based and it
uses modal and harmonic simulations for both selecting the
useful modes and then designing the device in a way that
presents those modes within a predefined frequency range.
This mechanical optimization is the first step for maximizing
the output power of a multimodal piezoelectric energy
harvester. The second step focuses on the optimization of the piezoelectric transducer geometry targeting the resonant
modes defined in the first step. The optimization procedure
is applied to an array of cantilever used as a case
study.

Energy harvesters (EH) are devices that convert
environmental energy (i.e. thermal, vibrational or electromagnetic)
into electrical energy. One of the most promising
solutions consists in transforming energy from vibrations
using a piezoelectric material placed onto a mechanical resonator.
The intrinsic drawback of this solution is the typically
high quality factor of the device which works effectively
only within a narrow bandwidth. To overcome this
limitation it is possible to tune the mechanical resonance of
the device, to introduce non-linear elements (e.g. magnets)
or to design the mechanical resonator with a multimodal
behaviour. In ultra low power applications the aspect of
integration is of utmost importance and so micro electromechanical
systems (MEMS)-based EHs are preferable.
Within this scenario the multimodal solution is the more
suitable considering the technological constraints imposed
by the micromachining manufacturing process. In this
paper, we describe the optimization of a given multimodal
mechanical geometry in order to maximize the number of
resonances within a certain frequency band. The proposed
optimization is finite element method (FEM)-based and it
uses modal and harmonic simulations for both selecting the
useful modes and then designing the device in a way that
presents those modes within a predefined frequency range.
This mechanical optimization is the first step for maximizing
the output power of a multimodal piezoelectric energy
harvester. The second step focuses on the optimization of the piezoelectric transducer geometry targeting the resonant
modes defined in the first step. The optimization procedure
is applied to an array of cantilever used as a case
study.

http://dx.doi.org/10.1007/s00542-016-2848-9