Jordi Agustí

Inducing bistability with local electret technology in a microcantilever based non-linear vibration energy harvester

A micro-electro-mechanical system based vibration energy harvester is studied exploring the benefits of bistable non linear dynamics in terms of energy conversion. An electrostatic based approach to achieve bistability, which consists in the repulsive interaction between two electrets locally charged in both tip free ends of an atomic force microscope cantilever and a counter electrode, is experimentally demonstrated. A simple model allows the prediction of the measured dynamics of the system, which shows an optimal distance between the cantilever and the counter electrode in terms of the root mean square vibration response to a colored Gaussian excitation noise.

Top-down silicon microcantilever with coupled bottom-up silicon nanowire for enhanced mass resolution

A stepped cantilever composed of a bottom-up silicon nanowire coupled to a top-down silicon microcantilever electrostatically actuated and with capacitive or optical readout is fabricated and analyzed, both theoretically and experimentally, for mass sensing applications. The mass sensitivity at the nanowire free end and the frequency resolution considering thermomechanical noise are computed for different nanowire dimensions. The results obtained show that the coupled structure presents a very good mass sensitivity thanks to the nanowire, where the mass depositions take place, while also presenting a very good frequency resolution due to the microcantilever, where the transduction is carried out. A two-fold improvement in mass sensitivity with respect to that of the microcantilever standalone is experimentally demonstrated, and at least an order-of-magnitude improvement is theoretically predicted, only changing the nanowire length. Very close frequency resolutions are experimentally measured and theoretically predicted for a standalone microcantilever and for a microcantilever-nanowire coupled system. Thus, an improvement in mass sensing resolution of the microcantilever-nanowire stepped cantilever is demonstrated with respect to that of the microcantilever standalone.

An analytical model for the opto-thermo-mechanical conversion mechanisms in a MOEMS based energy harvester

The use of optical excitation in MEMS elements triggers several mechanisms that can be properly used for enhancing its mechanical response. This is particularly interesting when MEMS components are used as the transducer element on energy harvesting applications involving opto-mechanical conversion schemes. One of the pathways is based on the thermal response of the vibrating structures. In this contribution we have analyzed how a MEMS structure consisting on a clamped-clamped beam responds mechanically to the heating of the element. This heating is produced by the partial absorption of an incident radiation at the IR band. A thin metal layer evaporated on top of the suspended beam acts as the infrared light absorber. The gap forms an optical interferometer which couples the light absorption to the mechanical deflection of the CC-beam. This effect can be enhanced by a proper design of the whole mechanical geometry. Both, the optical absorbance and the energy conversion to the thermal domain of the MOEMS device are analyzed. Additionally, the transduction to the mechanical domain in the form of beam vibrations of the optical energy absorbed by the structure and transformed into heat is also modeled. This paper focus on the analytical model that is necessary to understand the involved physical mechanisms and the results obtained from the simulation of the device.

Material dependence of the distributed bolometric effect in resonant metallic nanostructures

Optical antennas and passive resonant structures, as frequency selective surfaces, configure a new kind of optical systems that can be classified as belonging to the resonant optics area. Typical antenna-coupled detectors using microbolometers as transducers have included materials with the largest temperature coefficient of resistance (TCR) value. These materials are located at the feed point of the antenna where the electric current is the largest and the Joule effect dissipates the best. At the same time, the signal delivered to the external circuit is also depending on the resistivity value. This two-material configuration requires al least two e-beam fabrication steps. Although the resistivity values of metals changes substantially, the actual range of TCR values for most of metals is quite narrow. In this contribution we analyze how the choice of the material involved in the fabrication of resonant structures may enhance the bolometric effect. This analysis is made taking into account the electromagnetic interaction of light with the resonant element. The generated heat changes temperature and this variation produces the signal. Finite element package Comsol has been used to properly simulate the situation and predict the effect of changing the fabrication to an unique material, simplifying the manufacturing. Besides, the performance of the structure is depending on the used material.

Self-suspended vibration-driven energy harvesting chip for power density maximization

This work introduces a new concept to integrate energy-harvesting devices with the aim of improving their throughput, mainly in terms of scavenged energy density and frequency tunability. This concept, named energy harvester in package (EHiP), is focused on the heterogeneous integration of a MEMS die, dedicated to scavenging energy, with an auxiliary chip, which can include the control and power management circuitry, sensors and RF transmission capabilities. The main advantages are that the whole die can be used as an inertial mass and the chip area usage is optimized. Based on this concept, in this paper we describe the development and characterization of a MEMS die fully dedicated to harvesting mechanical energy from ambient vibrations through an electrostatic transduction. A test PCB has been fabricated to perform the assembly that allows measurement of the resonance motion of the whole system at 289 Hz. An estimated maximum generated power of around 11 μW has been obtained for an input vibration acceleration of ~10 m s−2 when the energy harvester operates in a constant-charge cycle for the best-case scenario. Therefore, a maximum scavenged power density of 0.85 mW cm−3 is theoretically expected for the assembled system. These results demonstrate that the generated power density of any vibration-based energy harvester can be significantly increased by applying the EHiP concept, which could become an industrial standard for manufacturing this kind of system, independently of the transduction type, fabrication technology or application.

Diffractive characterization of the vibrational state of an array of microcantilevers

Abstract. The vibrational state of an array of microcantilevers piezoelectrically excited has been characterized analyzing the far-field diffraction pattern produced by the structure. A model based on the definition of a complex reflectivity window has been developed to fully understand the dynamic of the elements and the diffraction pattern produced by them. This model is parameterized by the driving voltage of the excitation, the vibration mode, and the phase correlation among cantilevers. An experimental set-up has been developed to register the far-field diffraction patterns, avoiding some undesired reflections from the surrounding structures. The experimental results have been fitted with those obtained from the simulation. This fitting allows for identifying the vibration mode and the phase relation among cantilevers.

Integration of piezoelectric energy scavengers with FBAR resonators for the miniaturization of autonomous wireless sensors nodes

In this paper, the transduction part of a vibration-driven piezoelectric energy harvester has been designed, fabricated and characterized. The fabrication technology used allows the monolithic integration of Film Bulk Acoustic Resonators (FBARs) in the same wafer. This piezoelectric-based system together with an integrated circuitry can contain all the elements of a node that can belong to a wireless sensor network (WSN). A scavenged power of around 0.2 μW, i.e. an power density of around nd 0.13 mW/cm3, can be extracted at a resonance frequency of 515 Hz and an input acceleration of 0.64 m/s2.