Lou Cattafesta

A MEMS acoustic energy harvester

This paper presents the development of a micromachined acoustic energy harvester for aeroacoustic applications. The acoustic energy harvester employs a silicon-micromachined circular, piezoelectric composite diaphragm for electroacoustic transduction. Lumped element modeling, design, fabrication and characterization of a micromachined acoustic energy harvester prototype are presented. Experimental results indicate a maximum output power density of 0.34 µW cm−2 at 149 dB (ref. 20 µPa) and suggest a potential output power density, for this design, of 250 µW cm−2 with an improved fabrication process.

Characterization of a Compliant-Backplate Helmholtz Resonator for An Electromechanical Acoustic Liner

Passive acoustic liners are currently used to reduce the noise radiated from aircraft engine nacelles. This study is the first phase in the development of an actively-tuned electromechanical acoustic liner that potentially offers improved noise suppression over conventional multi-layer liners. The underlying technical concept is based on the idea that the fundamental frequency of a Helmholtz resonator may be adjusted by adding degrees of freedom (DOF) via substitution of a rigid wall with a piezoelectric composite diaphragm coupled to a passive electrical shunt network. In this paper, a Helmholtz resonator containing a compliant aluminum diaphragm is investigated to provide a fundamental understanding of this two DOF system, before adding complexity via the piezoelectric composite material. Using lumped elements, an equivalent circuit model is derived, from which the transfer function and acoustic impedance are obtained. Additionally, a mass ratio is introduced that quantifies the amount of coupling between the elements of the system. The theory is then compared to experiment in a normal-incidence impedance tube. The experimental results confirm the additional DOF and overall acoustic behavior but also suggest the need for a more comprehensive analytical model to accurately predict the acoustic impedance. Nevertheless, the experiments demonstrate the potential benefits of this approach for the reduction of aircraft engine noise.

Modeling and Design of Piezoelectric Actuators for Fluid Flow Control

A theoretical and experimental investigation into the modeling and design of piezoelectric flap actuators is described. The motivation for this study is to develop design tools for piezoelectric actuators in active flow control systems. In line with this goal, structural dynamic models of varying complexity must first be assessed. Theoretical modeling of the flaps is carried out using finite element analysis. For comparison, a companion experimental parametric study is executed in which ten otherwise identical piezo flaps with varying piezo patch sizes are fabricated in the Dynamics and Controls Laboratory at the University of Florida. The flaps are characterized using a laser displacement sensor and a scanning laser vibrometer to obtain the frequency response functions between the input voltage signal and the tip displacement and velocity of the flaps. The DC response and natural frequency of the flaps are extracted from the frequency response functions, and these are compared with the theoretical values.

MIXED CONVECTION INDUCED BY MEMS-BASED THERMAL SHEAR STRESS SENSORS

The effect of buoyancy caused by heat generation from a microelectromechanical system (MEMS)-based thermal shear stress sensor is investigated. Due to the small size and relatively low power consumption of such sensors, the buoyancy effect on the overall flow structure is generally negligible. However, its impact on the flow variables such as shear stresses can be significant because such quantities are local and depend on the gradients of the velocity profile next to the sensor. Due to the small dimension of the MEMS sensor, a multiscale modeling approach is adopted to examine the effect of buoyancy on the velocity and wall shear stress profiles. Full-length channel computations are initially performed with finer resolution near the sensor region. Using the boundary conditions derived from the full-length computations, another simulation is performed concentrating on a small region near the shear stress sensor. Based on the temperature distribution in the region of the sensor, the effective thermal length scale is several times the streamwise dimension of the sensor. For a state-of-the-art MEMS sensor dimension of 200\,\mu {\bf m} , the effect of buoyancy on the accuracy of shear stress measurement can be noticeable.

Modeling and optimization of a side-implanted piezoresistive shear stress sensor

This paper presents the modeling and design optimization of a micromachined floating element piezoresistive shear stress sensor for the time-resolved, direct measurement of fluctuating wall shear stress in a turbulent flow. The sensor structure integrates side-implanted diffused resistors into the silicon tethers for piezoresistive detection. A theoretical nonlinear mechanical model is combined with a piezoresistive model to determine the electromechanical sensitivity. Lumped element modeling (LEM) is used to estimate the resonant frequency. Finite element modeling is employed to verify the mechanical models and LEM results. Two dominant noise sources, 1/f noise and thermal noise, were considered to determine the noise floor. These models were then leveraged to obtain optimal sensor designs for several sets of specifications. The cost function is the minimum detectable shear stress that is formulated in terms of sensitivity and noise floor. This cost function is subjected to the constraints of geometry, linearity, bandwidth, power and resistance. The results indicate the possibility of designs possessing dynamic ranges of greater than 85dB.

Extremum-Seeking Optimisation for Fluidic Jet-Noise Control

In this work we use an extremum-seeking algorithm to optimise a fluidic jet-noise controller. The device, which we call a fluidevron, has been shown to produce reductions in jet noise which are comparable with those achieved using conventional microjets, but the underlying flow-physics have been shown to be very different (see Laurendeau et al. for details). A negative effect produced by the control comprises a high-frequency noise increase. The extremum-seeking algorithm is used to optimise the control either for maximum low-frequency noise-reduction, or for maximum overall noise-reduction. This is achieved through a specification of the frequency-range over which noise reduction is sought. The extremum-seeking is shown to perform well, producing flows with integrated low-frequency gains of the order of 2.5dB when tuned for maximum low-frequency benefit, and flows where the high frequency penalty is virtually eliminated when tuned for maximum spectral range. The extremum-seeking is then implemented using metrics computed from farfield microphones at different polar stations; the control effect is thus found to be omnidirectional. This shows that there is no directional bias in the response of the source mechanisms to the actuation. Finally, the relationship between noise reduction and flow-rate is studied and found to be non-linear: the source mechanisms are most receptive at low flow-rate, a saturation point being reached after which the actuation no longer has any control authority over the source dynamics.

Development of MEMS-based Piezoelectric Vibration Energy Harvesters

In this paper, the development of a first generation MEMS-based piezoelectric energy harvester capable of converting ambient vibrations into storable electrical energy is presented. The energy harvester is designed using a validated analytical electromechanical Lumped Element Model (LEM) that accurately predicts the behavior of a piezoelectric composite structure. The MEMS device is fabricated using standard sol gel PZT and conventional surface and bulk micro processing techniques. It consists of a piezoelectric composite cantilever beam (Si/SiO 2 /Ti/Pt/PZT/Pt/Au) with a proof mass at one end. A prototype device packaged in a 5 mm2 area produces 0.98 µW rms power into an optimal resistive load when excited with an acceleration of 1 m/s2 at its resonant frequency of 129 Hz. Although the model predicts the general behavior of the device accurately, knowledge of the overall system damping is critical to accurately predict the power output, and therefore individual dissipation mechanisms in the system must be investigated. This effort lays the foundation for future development of MEMS piezoelectric energy harvester arrays as a potential power solution for self sustaining wireless embedded systems. The electromechanical model further enables intelligent and optimal design of these energy harvesters for specific applications minimizing prototype test runs.

An analytical model for the thermoelastic actuation of composite diaphragms

An analytical model for the thermoelastic actuation of a circular composite diaphragm is presented. The model incorporates several key aspects relevant to micromachined thermoelastic resonators including multiple material layers, in-plane heat conduction and fabrication-induced stresses. Using a Fourier heat conduction model, expressions for the 2-D temperature distribution in the composite diaphragm generated by the dynamic Joule heating of diffused resistors are obtained via Hankel transforms and Greens functions. The thermoelastically forced vibration of the composite diaphragm is analyzed using classical Kirchoff plate theory and solved to obtain expressions for the transverse diaphragm deflection. Comparisons of the analytical model with coupled thermal-mechanical finite element simulations show excellent agreement with significantly faster (80/spl times/) computation time.

Design-optimization of a broadband phased microphone array for aeroacoustic applications

Phased microphone arrays are commonly used in acoustic beamforming applications. While numerous beamforming algorithms have been proposed to alleviate deficiencies of the delay-and-sum approach, few studies have focused on the array design itself. In aeroacoustic applications, the most common designs are based on circularly symmetric spiral arrays devised by Underbrink (1995). The design of an array using such a method is complex and tedious due to the numerous design variables and corresponding trade-offs between resolution, sidelobe suppression, size, and cost. In this paper, a systematic design-optimization approach is described that offers several objective functions and constraints. Candidate arrays for use in the University of Florida Aeroacoustic Flow Facility (UFAFF) are designed for a broadband frequency range of 1 to 80 kHz. The results of these different cases will be compared to those of an existing array design currently used in the UFAFF. An optimized design is selected and fabricated for ch...

Analysis of source denoising techniques.

A denoising technique for acoustic signals is analyzed. Previous work derived a methodology where three microphones are assumed to view a single signal field, each under the influence of independent channel noise. Statistical relations are used to determine the coherent acoustic field at each microphone [J. Y. Chung, J. Acoust. Soc. Am. 62, 388–395 (1977)]. The technique is validated using a simulation of a directive acoustic source, specifically a baffled piston, with multiple microphones placed in an arc array in the far field. While the solution is analytically exact for truly incoherent noise, weakly coherent noise is shown to have a significant impact on the technique output. The sensitivity of the denoised output to signal‐to‐noise ratio and cross‐channel noise coherence is studied. In a high‐channel‐count array, different microphone combinations may be used for the technique. Different microphone selection algorithms are studied to find a rule set most likely to determine the true acoustic signal s...