No description available.

No description available.

This paper will discuss the use of force apportionment to isolate modes being excited by the sine-dwell technique. The effectiveness of the apportionment technique can be determined by examining the structural response as measured by laser vibrometry. First, the structure is investigated using impact-test-based modal extraction methods. Approximate mode shapes are determined by examining the phase resonance indicator function for the resonance responses at a number of reference points. By comparing condition numbers of submatrices of the approximate modal matrix, one can select the best positions for force application. The apportioned forces for a given mode are arrived at by requiring that the input energy excite only the mode of interest while the net amount of work on adjacent modes is zero. This method is illustrated on a 24 in. x 1.5 in. x 0.375 in. steel beam. The fourth bending mode is to be separated from the first torsional mode which is 26 Hz below the bending mode. The apportioned forces are applied and laser scans are acquired of the "modal" response. The laser allows detailed investigation of the deviations of the response from the theoretical fourth mode response. The scans reveal that the force apportionment technique used in this test case fails to reliably extract the theoretical modal response of a beam. A finite element model of the beam is created to verify that the apportionment technique works. Applying an apportioned force vector to the model shows that the method is capable of isolating the mode of interest. The interaction of the electrodynamic shaker, stinger, and force transducer with the structure is investigated as a possible explanation for the failure of the technique in experimentation. It is found that there exists axial and rotatory coupling which can influence the structural response of the test specimen and decrease the reliability of the apportionment technique. / Master of Science

Structural health monitoring (SHM) systems generally install low cost excitation component and/or sensors to the machines or buildings permanently to monitor the health of it. Non Destructive Evaluation (NDE) systems use high cost sensors to perform the inspection of structures. It would be advantageous to inspect the some aerospace structures and parts in vacuum environment by using the remotely monitoring systems such as laser vibrometer. In this study, a scanning laser vibrometer is used to detect the location of the problem at the structure. For data collection and analysis recently, developed surface response to excitation (SuRE) method was used. First the software of the scanning laser vibrometer was used to evaluate the capabilities of the system. The vibration of simple structures such as the cone of a speaker and a beam was tested when they were excited below 200 Hz and the mode shapes were studied. Later, the scanning laser vibrometer was used for implementation of the SuRE method. The surface vibration of beams and plates were monitored while their surfaces were excited at high frequencies with a piezoelectric exciter. External force was applied to one point of the surface of the considered structures. The scanning laser vibrometer evaluated the vibration of different points on the structure according to given program and obtained the frequency response before and after the external load was applied. The sum of the squares of the differences of the frequency responses were obtained for each point on the surface and presented with a contour plot. Use of the scanning laser vibrometer was convenient at the test conditions. The calculated mode shapes were very similar to the expected ones when the simple structures were tested. The locations of the external forces were identified correctly when the beams and plates were tested. The study indicated that the scanning laser vibrometer and SuRE method may be used for identification of defects and/or loose fasteners.

This thesis concerns the control of an adaptive passive vibration neutralizer and the feasibility of miniaturizing this type of tunable vibration neutralizer for small-scale applications. An analytical model for the adaptive passive vibration neutralizer is derived and compared to experimental results. A tuning algorithm is derived from a curve-fit of experimental tests on the specific neutralizer. A more generic tuning algorithm is also developed, which does not require testing of the neutralizer for optimal control. Both tuning algorithms are tested using a chirp forcing function to simulate drift in the excitation frequency of a host structure. Computer simulation and experimental results are given for these tests. A novel low-cost, small-scale vibration neutralizer is constructed from packing bubble-wrap. Analytical models for the stiffness are calculated, and experimental data is used to derive a damped mass-spring model. Miniaturization of vibration neutralizers is described, and many of the pitfalls in design are discussed. Theoretical tuning frequencies of possible adaptive passive vibration neutralizers at different scales are included. The goal for these miniaturized vibration neutralizers is vibration control in computer hard drives. A hard drive is analyzed for vibration problems. Included are plots of the velocities of the read-write head and spindle. Limitations of the measurement equipment are discussed, and directions for future work on small-scale tunable vibration neutralizers are outlined. / Master of Science

No description available.

This thesis presents fundamental research in the field of laser vibrometry for the application to vibration measurements. A key concern for laser vibrometry is the effect of laser speckle which appears when a coherent laser beam scatters from an optically rough surface. The laser vibrometer is sensitive to changes in laser speckle which result from surface motions not in the direction of the incident beam. This adds speckle noise to the vibrometer output which can be indistinguishable from the genuine surface vibrations. This has been termed ‘pseudo-vibration' and requires careful data interpretation by the vibration engineer. This research has discovered that measurements from smooth surfaces, even when no identifiable speckle pattern is generated, can produce noise and therefore reference to speckle noise, in such circumstances, is inappropriate. This thesis has, therefore, adopted the more general term of pseudo-vibration to include noise generated from any surface roughness or treatment, i.e. including but not limited to speckle noise. This thesis develops and implements novel experimental methods to quantify pseudovibration sensitivities (transverse, tilt and rotation sensitivity) with attention focussed on commercially available laser vibrometers and consideration is given to a range of surface roughnesses and treatments. It investigates, experimentally, the fundamental behaviour of speckles and attempts to formulate, for the first time, a relationship between changes in intensity to pseudo-vibration sensitivity levels. The thesis also develops and implements models for computational simulation of pseudo-vibrations using the fundamental behaviour of speckles. The combination of experimentation and simulation improves current understanding of the pseudo-vibration mechanisms and provides the vibration engineer with a valuable resource to improve data interpretation. Two experimental methods of quantifying pseudo-vibration sensitivity are developed and successfully applied in the evaluation of transverse, tilt and rotation sensitivity for two models of commercial laser vibrometer. These evaluations cover both single beam (translational vibration measurement) and parallel beam (for angular vibration measurement) modes. The first method presented requires correction of the vibrometer measurement with an independent measurement of genuine velocity to produce an iii apparent velocity dominated by the required noise components. The second method requires a differential measurement using two vibrometers to cancel common components such as genuine velocity, leaving only uncorrelated noise from each measurement in the resulting apparent velocity. In each case, a third measurement is required of the surface motion component causing pseudo-vibration and this is used to normalise the apparent velocity. Pseudo-vibration sensitivity is then presented as a map showing the spectral shape of the noise, as a mean and standard deviation of harmonic peaks in the map and as a total rms level across a defined bandwidth. The simulations employ a novel and effective approach to modelling speckle evolution. Transverse and tilt sensitivity are predicted for the first time and are verified by the experimental study. They provide the vibration engineer with the potential to estimate pseudo-vibrations using a simple piece of software. The laser beam spot diameter has a large influence on the pseudo-vibration sensitivity. Transverse sensitivity has been quantified as around 0.03% and 0.01% (per order) of the transverse velocity of the surface for beam spot diameters of 100 μm and 600 μm respectively. Larger beam spots have been shown to significantly reduce transverse sensitivity and measurements from smoother surfaces have also shown a reduced level of transverse sensitivity. Tilt sensitivity has been quantified at about 0.1 μms-1/degs-1 and 0.3 μms-1/degs-1 (per order) of angular velocity of the surface for beam spot diameters of 100 μm and 600 μm respectively. Smaller beam spot diameters significantly reduce tilt sensitivity. The surface roughness or treatment has been shown to have little effect on the level of tilt sensitivity. Rotation sensitivity has been quantified at approximately 0.6 μms- 1/rads-1 and 1.9μms-1/rads-1 (per order) of rotation velocity of the rotor for 90 μm and 520 μm. Smaller beam spot diameters have shown a significant reduction in rotation sensitivity and measurements on smoother surfaces have shown a reduced rotation sensitivity. Focussing the laser beam approximately on the rotation axis has also shown a significant reduction in rotation sensitivity. Parallel beam rotation sensitivity has been quantified at 0.016 degs-1/rads-1 and it is demonstrated that this can adequately be estimated using the single beam rotation sensitivity.

No description available.