In this work, we discuss the verification and preliminary experimental characterization of a MEMS-based vibration Energy Harvester (EH) design. The device, named Four-Leaf Clover (FLC), is based on a circular-shaped mechanical resonator with four petal-like mass-spring cascaded systems. This solution introduces several mechanical Degrees of Freedom (DOFs), and therefore enables multiple resonant modes and deformation shapes in the vibrations frequency range of interest. The target is to realize a wideband multi-modal EH-MEMS device, that overcomes the typical narrowband working characteristics of standard cantilevered EHs, by ensuring flexible and adaptable power source to ultra-low power electronics for integrated remote sensing nodes (e.g. Wireless Sensor Networks – WSNs) in the Internet of Things (IoT) scenario, aiming to self-powered and energy autonomous smart systems. Finite Element Method simulations of the FLC EH-MEMS show the presence of several resonant modes for vibrations up to 4-5 kHz, and level of converted power up to a few μW at resonance and in closed-loop conditions (i.e. with resistive load). On the other hand, the first experimental tests of FLC fabricated samples, conducted with a Laser Doppler Vibrometer (LDV), proved the presence of several resonant modes, and allowed to validate the accuracy of the FEM modeling method. Such a good accordance holds validity for what concerns the coupled field behavior of the FLC EH-MEMS, as well. Both measurements and simulations performed at 190 Hz (i.e. out of resonance) showed the generation of power in the range of nW (Root Mean Square – RMS values). Further steps of this work will include the experimental characterization in a full range of vibrations, aiming to prove the whole functionality of the FLC EH-MEMS proposed design concept.
In an optomechanical cavity the optical and mechanical degree of freedom are strongly coupled by the radiation pressure of the light. This field of research has been gathering a lot of momentum during the last couple of years, driven by the technological advances in microfabrication and the first observation of quantum phenomena. These results open new perspectives in a wide range of applications, including high sensitivity measurements of position, acceleration, force, mass, and for fundamental research. We are working on low frequency pondero-motive light squeezing as a tool for improving the sensitivity of audio frequency measuring devices such as magnetic resonance force microscopes and gravitational-wave detectors. It is well known that experiments aiming to produce and manipulate non-classical (squeezed) light by effect of optomechanical interaction need a mechanical oscillator with low optical and mechanical losses. These technological requirements permit to maximize the force per incoming photon exerted by the cavity field on the mechanical element and to improve the element’s response to the radiation pressure force and, at the same time, to decrease the influence of the thermal bath. In this contribution we describe a class of mechanical devices for which we measured a mechanical quality factor up to 1.2 × 106 and with which it was possible to build a Fabry-Perot cavity with optical finesse up to 9 × 104. From our estimations, these characteristics meet the requirements for the generation of radiation squeezing and quantum correlations in the ∼ 100kHz region. Moreover our devices are characterized by high reproducibility to allow inclusion in integrated systems. We show the results of the characterization realized with a Michelson interferometer down to 4.2K and measurements in optical cavities performed at cryogenic temperature with input optical powers up to a few mW. We also report on the dynamical stability and the thermal response of the system.
Energy Harvesters (EH) are devices that convert environmental energy (i.e. thermal, vibrational, solar 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 and so the device 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 behavior. In Ultra Low Power (ULP) applications the aspect of integration is of utmost importance and so MEMS-based (micro electro-mechanical systems) EHs are preferable. Within this scenario the multimodal solution is the more suitable considering the technological constraints imposed by the micro machining manufacturing process.
In this paper we optimize a given multimodal mechanical geometry in order to maximize the number of resonances within a certain frequency band. In the particular case of piezoelectric energy harvesting, the strain distribution of each modes is critical and has to be taken into consideration for the designing of efficient device. The proposed optimization is FEM-based and it uses modal and harmonic simulations for both select the useful modes and then to design the device in a way that presents those modes within a predefined frequency range. This mechanical optimization could be considered the first step for maximizing the output power of a multimodal piezoelectric energy harvester. The second step focuses on the geometry optimization of the piezoelectric transducer element, starting from the desired resonant mode configuration defined in the first stage. The number of modes stimulated applying a vertical acceleration increases in number in the desired frequency range (i.e. around 1 kHz). As the output power is proportional to the stress of the flexible device, these promising results show clearly how an optimization of the geometry could significantly boost the performance of such devices.
The aim of this contribution is to report and discuss a preliminary study and rough optimization of a novel concept of
MEMS device for vibration energy harvesting, based on a multi-modal dynamic behavior. The circular-shaped device
features Four-Leaf Clover-like (FLC) double spring-mass cascaded systems, kept constrained to the surrounding frame
by means of four straight beams. The combination of flexural bending behavior of the slender beams plus deformable
parts of the petals enable to populate the desired vibration frequency range with a number of resonant modes, and
improve the energy conversion capability of the micro-transducer. The harvester device, conceived for piezoelectric
mechanical into electric energy conversion, is intended to sense environmental vibrations and, thereby, its geometry is
optimized to have a large concentration of resonant modes in a frequency range below 5-10 kHz. The results of FEM
(Finite Element Method) based analysis performed in ANSYSTM Workbench are reported, both concerning modal and
harmonic response, providing important indications related to the device geometry optimization. The analysis reported in
this work is limited to the sole mechanical modeling of the proposed MEMS harvester device concept. Future
developments of the study will encompass the inclusion of piezoelectric conversion in the FEM simulations, in order to
have indications of the actual power levels achievable with the proposed harvester concept. Furthermore, the results of
the FEM studies here discussed, will be validated against experimental data, as soon as the MEMS resonator specimens,
currently under fabrication, are ready for testing.
The interaction of the radiation pressure with micro-mechanical oscillators is earning a growing interest for its
wide-range applications (including high sensitivity measurements of force and position) and for fundamental
research (entanglement, ponderomotive squeezing, quantum non-demolition measurements). In this contribution
we describe the fabrication of a family of opto-mechanical devices specifically designed to ease the detection of
ponderomotive squeezing and of entanglement between macroscopic objects and light. These phenomena are not
easily observed, due to the overwhelming effects of classical noise sources of thermal origin with respect to the
weak quantum fluctuations of the radiation pressure. Therefore, a low thermal noise background is required,
together with a weak interaction between the micro-mirror and this background (i.e. high mechanical quality
factors). The device should also be capable to manage a relatively large amount of dissipated power at cryogenic
temperatures, as the use of a laser with power up to a ten of mW can be useful to enhance radiation pressure
effects. In the development of our opto-mechanical devices, we are exploring an approach focused on relatively
thick silicon oscillators with high reflectivity coating. The relatively high mass is compensated by the capability
to manage high power at low temperatures, owing to a favourable geometric factor (thicker connectors) and
the excellent thermal conductivity of silicon crystals at cryogenic temperature. We have measured at cryogenic
temperatures mechanical quality factors up to 105 in a micro-oscillator designed to reduce as much as possible
the strain in the coating layer and the consequent energy dissipation. This design improves an approach applied
in micro-mirror and micro-cantilevers, where the coated surface is reduced as much as possible to improve the
quality factor. The deposition of the highly reflective coating layer has been carefully integrated in the micromachining
process to preserve its low optical losses: an optical finesse of F = 6×104 has been measured in a
Fabry-Perot cavity with the micro-resonator used as end mirror.