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A novel MEMS-based piezoelectric multi-modal vibration energy harvester concept to power autonomous remote sensing nodes for Internet of Things (IoT) applications

The Internet of Things (IoT) urges for development and pervasive deployment of energy-autonomous integrated remote sensing nodes. Microsystems-based (i.e. MEMS) Energy Harvesting (EH) seems to be a key-enabling technology to face such a challenge, and environmental vibrations are a source of energy commonly available in domestic, industrial and public contexts. A major limitation of standard vibration Energy Harvesters (EHs) is the power conversion performance typically pronounced just around the fundamental mechanical resonant frequency (narrowband device). In this work, we introduce a resonator design concept named Four-Leaf Clover (FLC). Having several mechanical Degrees Of Freedom (DOFs), the FLC EH-MEMS exhibits several resonant modes in the frequency range from around 200 Hz up to several kHz. Thereafter, it IS capable to convert mechanical into electric energy efficiently in a wide frequency range of vibrations (wideband device). The FLC EH-MEMS design concept discussion is corroborated by simulations and experimental measurements that verify its performance and, in turn, that provide support to the contribution it can bring to self-powered integrated sensing nodes in the IoT scenario.

Design of a novel tri-axial force sensor for optimized design of prosthetic socket for lower limb amputees

Realizing a prosthesis for lower limbs amputee is a time consuming and costly process. The socket design results particularly challenging due the need of tailoring its shape on the residual limb of the amputee. Socketmaster is an European project aiming to improve this process by integrating several micro sensors in a master socket, allowing a fast customization of prosthetic socket for lower limb. One of the most important parameter that the master socket has to monitor is the force interaction between the leg and the socket. This paper describe a low cost tri-axial force sensor designed to be implemented in a master socket.

Study on the performance of tailored spring elements for piezoelectric MEMS energy harvesters

Energy harvesting has recently attracted much attention of the research community as a key enabling technology in applications such as autonomous Wireless Sensors Network (WSN), Internet of Things (IoT), e-health and more in general all the applicative scenarios requiring an autonomous low power distributed system. Among the various energy harvesting techniques, vibrational piezoelectric energy harvesters have several advantages compare to other solutions (e.g. high output density power, high output voltage). One of the main constraints in the exploitation of such a technology is the limited bandwidth of the devices, intrinsic to the mechanical resonator typically used. In this contribution, different resonators based on a cantilever-like structure are studied both by FEM simulation and by measurements of physical samples. The goal of this preliminary study is to verify the effectiveness of those whip designs for energy harvesting purposes.

An analytical model for the optimization of toggle-based RF-MEMS varactors tuning range

This paper presents an analysis of the actuation mechanism of the push-pull toggle varactor design concept for radio frequency applications, based on RF-MEMS technology. The focus of this analysis is to maximize the tuning range. In particular the influence of the dimension and the distance of the fixed electrode on the tuning range has been investigated. The results of the analytical formulation have been then compared with multi-physical simulations in order to assess the accuracy of the prediction showing promising results.

An Energy Harvester concept for electrostatic conversion manufactured in MEMS surface micromachining technology

In this work we present the concept of a MEMS-based Energy Harvester (EH) for the conversion of vibration into electrical energy. The employed electrostatic conversion mechanism of the device is sensitive both to vertical (out-of-plane) and horizontal (in-plane) displacements, thanks to the presence of buried planar and interdigitated fixed electrodes, respectively. The proposed EH is inexpensively manufactured in the MEMS/RF-MEMS surface micromachining process available at Fondazione Bruno Kessler (FBK) in Italy, and, thereby, does not require any specific technology modification to be realized. Modeling of the EH concept (Finite Element Method based and analytical) is reported and discussed, and validated against preliminary experimental measurements. The structure exhibits resonant frequencies in the range up to 10-12 kHz, it being compatible with vibration sources typically available in the surrounding environment, like busy street, car engine, industrial and domestic appliance, and so on. Preliminary estimates of the power conversion capability seem to address rather low levels (in the range of pW), despite, on the other hand, the EH design as well as the fabrication process admit significant margins of performance improvement.

Characterization of quartz-based package for RF MEMS

In the last decade Micro-Electro-Mechanical Systems (MEMS) technology experienced a significant development in various fields of Information and Communication Technology (ICT). In particular MEMS for Radio Frequency (RF) applications have emerged as a remarkable solution in order to fabricate components with outstanding performances. The encapsulation of such devices is a relevant aspect to be addressed in order to enable wide exploitation of RF-MEMS technology in commercial applications. A MEMS package must not only protect fragile mechanical parts but also provide the interface to the next level of the packaging hierarchy in a cost effective technology. Additionally, in RF applications the electromagnetic impact of the package has to be carefully considered. Given such a scenario, the focus of this work is the characterization of a chip capping solution for RF-MEMS devices. Such solution uses a quartz cap having an epoxy-based dry film sealing ring. Relevant issues affecting RF-MEMS devices once packaged, e.g. the mechanical strain induced by the cap and the hermeticity of the sealing ring, are worth investigating. This work focuses on the study of induced strain, as a function of different bonding parameters. Dimensional features of the sealing ring (i.e. the width), and process parameters, like temperature and pressure, have been considered. The package characterization is performed by using basic test vehicles, such as strain gauges, designed to be integrated inside the internal cavity of the package itself. Polysilicon piezoresistors are used as strain gauges, whereas aluminum resistors are used as thermometers to assess the impact of temperature changes on strain measurements. Experimental data are reported including calibration of the sensors as well as environmental measurements with and without cap. In addition measurements of the shear stress of the proposed packaging solution are also reported.

Study on the effectiveness of different electrode geometries for sputtered Aluminium Nitride-based MEMS energy harvesters

IoT (Internet of Things) paradigm depicts a scenario where thousands of elements are connected in a network accessible worldwide. To realize such ambitious scenario different research fields have to converge. One of the issues of such high scale network is its power demand typically solved by means of batteries. A promising approach to such a problem consists in harvesting energy from mechanical vibrations present in the environment. To efficiently convert the mechanical energy into useful electrical energy the design of the conversion element is fundamental. The electrodes have to be designed accordingly to the piezoelectric crystal orientation implemented in the device. This paper analyzes the performances of different electrode designs for Aluminum Nitride deposited by a magnetron sputtering process and presenting a strong c-axis orientation.

Experimental verification of a novel MEMS multi-modal vibration energy harvester for ultra-low power remote sensing nodes

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.

From MEMS to macro-world: a micro-milling machined wideband vibration piezoelectric energy harvester

In this work, we discuss a novel mechanical resonator design for the realization of vibration Energy Harvester (EH) capable to deliver power levels in the mW range. The device overcomes the typical constraint of frequency narrowband operability of standard cantilevered EHs, by exploiting a circular-shaped resonator with an increased number of mechanical Degrees Of Freedom (DOFs), leading to several resonant modes in the range of vibrations of interest (i.e. multi-modal wideband EH). The device, named Four-Leaf Clover (FLC), is simulated in Ansys Worbench™, showing a significant number of resonant modes up to vibrations of around 2 kHz (modal eigenfrequencies analysis), and exhibiting levels of converted power up to a few mW at resonance (harmonic coupled-field analysis). The sole FLC mechanical structure is realized by micro-milling an Aluminum foil, while a cantilevered test structure also including PolyVinyliDene Fluoride (PVDF) film sheet is assembled in order to collect first experimental feedback on generated power levels. The first lab based tests show peak-to-peak voltages of several Volts when the cantilever is stimulated with a mechanical pulse. Further developments of this work will comprise the assembly of an FLC demonstrator with PVDF pads, and its experimental testing in order to validate the simulated results.

Smoothing the way towards miniaturized MEMS AlN-based piezoelectric transformers

This work presents the electromechanical characterization and design of a Piezoelectric Transformer (PT) made on a Silicon-On-Insulator (SOI) structure through a bulk micromachining process exploiting low-frequency flexural-modes. The footprint area of the device is less than 3.5 mm2. The purpose of this work is to show the enhancement obtained with respect to prior state of the art in terms of quality factor and voltage gain for the fundamental mode. As matter of fact, the presented device, achieves 2-factor ∼71 at environmental pressure, being almost 6 times greater with respect to the one of the same membrane with double radius and almost 4X footprint area. The step-up ratio at environmental pressure is ∼25mV/V, but it increases at 250mV/V when the device is operated in a vacuum chamber, thus being more than 20X times higher the step-up ratio achieved for the same membrane with double radius in vacuum environment.