This work presents a novel design for a micromachined, capacitively
sensed hydrophone. The design consists of a fluid-filled chamber
constrained by two sets of membranes. The "input" membranes are
arrayed around the outside of the circular chamber. Incoming sound
generates a trapped cylindrical wave, creating mechanically amplified
motion of the 1 mm diameter central "sensing" membrane. The membrane
material is a LPCVD nitride/oxide/nitride triple-stack with respective
film thickness 0.1/0.65/0.1 micron. The chamber is filled with 200
cSt viscosity silicone oil. Fluid-filling eases design constraints
associated with submerging the sensor, especially with respect to
exterior mass loading. Both silicon-glass anodic bonding and tin-gold
solder bonding are used to form the structure, including the 5 micron
sensing gap.
The fluid-structure system is computationally modeled using both
approximate analytic and numerical techniques. Model results indicate
a 28 dB displacement gain between the motion of the "input"
membranes and the "sensing" membranes. An off-chip charge
amplifier, with a 10 pF integrating capacitor, is used to convert
membrane motion into an electrical signal. Mean measured system
sensitivity is 0.8 mV/Pa (-180 dB re 1 V/microPa) from 300 Hz-15 kHz
with a 1.5 volt applied bias and a 26 dB preamplifier gain. The
predicted low frequency sensitivity is 0.3 mV/Pa. The measured
sensitivity exhibits considerable scatter below 7 kHz, with a standard
deviation of 80%. Laser vibrometry measurements indicate that this
scatter may be caused by compliance of the chip mounting scheme.
Above 10 kHz, the quiescent noise is -100 dB re 1 V/rtHz.
Noise characteristics exhibit a 1/f character below 10 kHz, rising to
a maximum of -50 dB re 1 V/rtHz at 100 Hz.
The mammalian cochlea achieves remarkable acoustic transduction characteristics in a compact and robust design. For this reason, its mechanics have been extensively studied, both mathematically and experimentally. Recently, a number of researchers have attempted to mimic the cochlear function of the basilar membrane in micromachined mechanical devices. This paper presents a design for a silicon cochlea which extends previous work by utilizing a micromachined liquid-filled two duct structure similar to the duct structure of the biological cochlea. Design issues related to both mechanical structure and electrical transduction will be discussed, particularly with regard to optimization of transducer performance. A parallel beam array structure is proposed as a model for an orthotropic membrane. Fabrication procedures and results are also presented. Challenging Fabrication issues related to through-wafer etching, adhesive wafer bonding, device release, and fluid injection are emphasized.
Cross-talk in transducer arrays is an important issue that affects the array performance. For therapeutic arrays and other arrays that are driven in continuous wave (CW) mode, the cross-talk between array elements reduce the acoustic power output and array directivity. This paper is devoted to the minimization of the cross-talk in air-backed arrays by optimizing the inter-element connections in the kerf. The kerf region is the design domain and the induced electric voltage on the passive element at a given operating frequency is the objective function. A density-based topology optimization problem is formulated and solved using the sequential linear programing (SLP). Preliminary optimization results for a three-element array are presented. The effectiveness of the design with respect to the cross-talk, far-field pressure level and directivity is discussed.
In commercial applications it is important to be able to either cancel noise, such as in the cabin of a combine, or to generate audible noise, such as in the case of an alarm. Piezoelectrics have demonstrated promise for active noise control and sound generation applications. In this investigation, the dynamic and acoustic capabilities of polymer piezoelectric semi-circular transducers were studied for such applications. The dynamic response of semi-circular piezoelectric transducers was determined numerically using the general purpose finite element code ABAQUS, then verified analytically and experimentally. The acoustic response was modeled using the commercial code COMET/Acoustics with the dynamic velocity response calculated by ABAQUS as initial boundary conditions. Experimental studies were performed on fixed-fixed polymeric piezoelectric curved transducers. The sensitivity of the sound generation capabilities of the transducer was investigated with respect to variations in radius, thickness and width parameters to demonstrate the potential of these devices for noise cancellation and audible sound applications.
In this paper, a design methodology for enhancing the acoustic power radiated from fluid-loaded piezoelectric transducers at a particular operating frequency is developed. For many applications the operating frequency is fixed by the absorption of the material and the desired depth of penetration (e.g., therapeutic ultrasound). For therapeutic ultrasound and other industrial applications, the acoustic power is the critical figure of merit. The acoustic power radiated from the transducer system is computed from a finite element formulation of the coupled acoustic, elastic, piezoelectric equations of motion. The sensitivities of the acoustic power to two design variables: the length of the piezoelectric element and the thickness of the matching layer, are derived. Using these sensitivities, a novel design methodology in which remeshing is avoided is developed and the effectiveness of the method is studied. Results from the application of this framework for transducer design demonstrate the dramatic increase in radiated power possible from this two member design space.
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