A Ashwin Seshia

Ultrasensitive mode-localized micromechanical electrometer

We report a highly sensitive prototype micromechanical electrometer that employs the phenomena of mode-localization and curve veering for monitoring minute charge fluctuations across an input capacitor. The device consists of a pair of weakly coupled, nearly identical single crystal silicon, double-ended tuning fork (DETF) resonators. An addition of charge across an input capacitor on one of the coupled resonators induces a differential axial strain on that resonator relative to the other consequently perturbing the structural symmetry of the nearly periodic system. The resulting shifts in the eigenstates for the same magnitudes of charge input are theoretically and experimentally demonstrated to be nearly three orders of magnitude greater than corresponding resonant frequency variations. The topology chosen may also be adapted for force or strain monitoring thereby widening the relevance of the results reported here to precision inertial sensing as well.

Optimisation of a piezoelectric system for energy harvesting from traffic vibrations

Piezoelectric systems are viewed as a promising approach to energy harvesting from environmental vibrations. The energy harvested from real vibration sources is usually difficult to estimate analytically. Therefore, it is hard to optimise the associated energy harvesting system. This work investigates the optimisation of a piezoelectric cantilever system using a genetic algorithm based approach with numerical simulations. The genetic algorithm globally considers the effects of each parameter to produce an optimal frequency response to scavenge more energy from the real vibrations while the conventional sinusoidal based method can only optimise the resistive load for a given resonant frequency. Experimental acceleration data from the vibrations of a vehicle-excited manhole cover demonstrates that the optimised harvester automatically selects the right frequency and also synchronously optimises the damper and the resistive load. This method shows great potential for optimizing the energy harvesting systems with real vibration data.

Common mode rejection in electrically coupled MEMS resonators utilizing mode localization for sensor applications

Measuring shifts in eigenstates due to vibration localization in an array of weakly coupled resonators offer two distinct advantages for sensor applications as opposed to the technique of simply measuring resonant frequency shifts: (1) orders of magnitude enhancement in parametric sensitivity and (2) intrinsic common mode rejection. In this paper, we experimentally demonstrate the common mode rejection capabilities of such sensors. The vibration behavior is studied in pairs of nearly identical MEMS resonators that are electrically coupled, and subjected to small perturbations in stiffness under different ambient pressure and temperature. The shifts in the eigenstates for the same parametric perturbation in stiffness are experimentally demonstrated to be over three orders of magnitude greater than corresponding resonant frequency variations. They are also shown to remain relatively constant to variations in ambient temperature and pressure. This increased relative robustness to environmental drift, along with the advantage of ultra-high parametric sensitivity, opens the door to an alternative approach to achieving higher sensitivity and stability in micromechanical sensors.

Voltage programmable dual-band bandpass/bandstop filter response in a single micro-electro-mechanical device

This paper reports on a switchable multi-band filter response achieved within a single micro-electro-mechanical device. A prototype device fabricated in a SOI process demonstrates a voltage programmable and tunable, dual-band, band-pass/band-stop response. Both analytical and finite element models are introduced in this paper to elucidate the operating principle of the filter and to guide filter design. Voltage programmability of the filter characteristic is demonstrated with the ability to independently tune the centre frequency and bandwidth for each band. A representative measurement shows that the minimum 3 dB-bandwidth (BW) is 155 Hz, 140Hz, and 20 dB-BW is 216 Hz, 203Hz for the upper-band and lower-band center frequencies located at 131.5 kHz and 130.7 kHz, respectively.

Topology dependence of mass sensitivities in mode-localized sensors

Vibration mode localization has been employed as an ultrasensitive approach for mass detection and identification in recent years. Sensitivity enhancements of nearly three orders of magnitude relative to the more conventional resonant frequency shift approach have been experimentally demonstrated using this sensing paradigm, by either exploiting the critical dependence of the parametric sensitivity on the strength of internal coupling or by increasing the number of degrees of freedom by arraying multiple resonators with weak coupling springs. We propose here, for the first time, an additional approach to sensitivity enhancement in such mode-localized mass sensors by utilizing the sensitivity dependence on the operating frequency and the stiffness of the resonator topology. We experimentally demonstrate the sensitivity dependence on the topology, by comparing and contrasting the vibration behaviour of three pairs of electrically coupled microelectromechanical (MEM) resonators of different structural configurations and operating frequencies. The shifts in the eigenstates for the same relative mass additions are experimentally demonstrated to be over three orders of magnitude greater than corresponding resonant frequency shifts. They are also shown to increase proportionally with the square of the resonant frequencies of the coupled resonator platforms (with the stiffer configurations of higher operating frequencies yielding a further one order of magnitude improvement over the more compliant topologies of nearly equal resonator masses). This topology dependence while providing a systematic approach to the design of mode-localized mass sensors within a given design space, also suggests an alternate route to improving the mass sensitivity by designing stiffer topologies with weak coupling instead of the more conventional route of scaling down system dimensions in traditional resonant mass sensors.