The development of a novel, biologically inspired acoustic sensor is presented. The primary goal of this effort is to construct a miniature device that is capable of detecting the orientation of an incident sound source with an accuracy of 2°. The design approach follows from our investigation of the mechanics of directional hearing in the parasitoid fly, Ormia ochracea. This animal has been shown to be able to detect changes in the line of bearing of an incident sound that are as small as 2°. The tympanal structures of the ears of this animal suggest a novel approach to designing very small directionally sensitive microphones. A microphone diaphragm design is presented that has been fabricated using silicon microfabrication technology. Measurements of the static deflection due to intrinsic stress and of the response to sound are shown to be in excellent agreement with predictions. Predicted results indicate that this microphone concept could lead to a practical differential microphone with self-noise as low as 20 dBA.
KEYWORDS: Fourier transforms, Chemical elements, Finite element methods, Computing systems, Systems modeling, Data modeling, Optical simulations, Information operations, Matrices, Motion models
A procedure is presented for computing the transient response of a multiple degree of freedom finite element model of a beam system containing a viscoelastic material. A complex, frequency and temperature dependent shear modulus is used in representing the properties of this material. The beam is struck with an arbitrary transient input pulse, which is transformed to the frequency domain via the fast Fourier transform (FFT) algorithm. The frequency dependent response of the beam may then easily be computed. Applying the inverse fast Fourier transform to this result then yields the transient, damped response of the complete beam system. This `approximate' approach is compared against an exact, modal solution for a system with viscous damping and excellent correlation is observed between the two. Finally, a procedure is presented to incorporate the finite element code ANSYS into the prediction procedure. Through the use of this code, a model constrained layer damped beam may be analyzed to obtain its transient response to an applied load.
Viscoelastic adhesives are commonly used as vibration damping treatments for mechanical systems. When using these adhesives, knowledge of the shear modulus and loss factor at a given temperature is essential. The standard method for determining shear modulus and loss factor yields values at only a few frequency points for any given temperature. The method presented in this report offers a continuous curve for the two material properties over a broad frequency range.
A non-destructive evaluation technique based on full-field dynamic strain distribution information is presented in this paper. It is well known that the fatigue damage in advanced composites typically manifests itself in the form of a gradual reduction in local stiffness. The reduction of local stiffness will change the strain distribution at the corresponding area. By comparing the strain distribution over the surface of the structure, the fatigue damage in the structure can be detected and located. In the present study, a numerical simulation has been carried out to show that the fatigue damage in the structure can be detected and located by observing the changes in the strain distribution over the structural surface. The changes of the strain distribution are usually related to the damage pattern. It is found that when the damage is in the form of local stiffness reduction, strain distributions for all strain components will change in the same order of magnitude. When the damage is in the form of a crack, only the strain component which is perpendicular to the crack has a significant change. This suggests that it is possible to determine the damage pattern by examining strain distribution changes in all strain components.
The characteristics of a new twin-track absolute position optical encoder are presented. One tract containing a pseudorandom code and the second a periodic pattern are synchronously engraved on a glass slab. The optical reading head design composed of a LED, a gradient index lens and a linear CCD array, with simple intensity-based signal processing to read the code pattern is described in detail. This design is combined with a standard Moire method to enhance resolution. Performance enhancement of the optical reading head is studied in terms of track illumination versus code density, detector geometry versus the lens point-spread function (Airy disk); and reduction of extrinsic spatial noise by the application of thick films. Each parameter was considered in terms of the optimal field of view and depth of field of the system.
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