Proc. SPIE. 10172, A Tribute Conference Honoring Daniel Inman
KEYWORDS: 3D acquisition, Aerospace engineering, Mechanics, Resistance, 3D modeling, Wave propagation, Solids, 3D metrology, Mechanical engineering, Acoustics, Structural dynamics, Commercial off the shelf technology, Systems modeling
The problem of free vibration of a linear uniform axial bar, fixed at one end and connected to ground at the other end through a linear viscous damper has been carefully studied by several researchers. It’s known that, for a fixed set of bar parameters and the special case of the damping coefficient λ equal to <i>EA / c</i> (<i>c</i> being the speed of sound in the continuum), no eigenvalues exist. Thus, energy imparted to the bar via harmonic motion of the fixed support will propagate through the bar and be fully dissipated in the damper, in effect making the bar appear to be semi-infinite. I will show some recent results by the present authors in which this phenomenon has been exploited in several other nondispersive media, the taut string and the circular acoustic duct, incorporating viscoelastic supports or absorbers to produce responses to harmonic motion at one or both boundaries that exhibit complete separation of traveling and standing waves, in effect localizing the vibration over a portion of the domain.
In this paper physical quantities were measured using by the fiber optic sensor when a pendulum ball collides to the fixed ball on the wall. Both steel ball dimensions are 1 inch in diameter. The fiber optic Sagnac interferometer was used to detect the impact time. It is made a 1mm penetrated hole and optical fiber in the Sagnac loop passed through the hole. A ball was welded on the steel plate wall as a fixed ball. When the another ball collides to the fixed ball the fiber optic sensor detects the impact force in between them. Based on the experimental result impact time was measured about 0.14ms at the angle of 30 degree. This fiber optic sensor technique can be expanded to the moving bodies.
The Hilbert-Huang transform (HHT) has been shown to be effective for characterizing a wide range of nonstationary
signals in terms of elemental components through what has been called the empirical mode decomposition. The HHT
has been utilized extensively despite the absence of a serious analytical foundation, as it provides a concise basis for the
analysis of strongly nonlinear systems. In this paper, we attempt to provide the missing link, showing the relationship
between the EMD and the slow-flow equations of the system. The slow-flow model is established by performing a
partition between slow and fast dynamics using the complexification-averaging technique, and a dynamical system
described by slowly-varying amplitudes and phases is obtained. These variables can also be extracted directly from the
experimental measurements using the Hilbert transform coupled with the EMD. The comparison between the
experimental and analytical results forms the basis of a nonlinear system identification method, termed the slow-flow
model identification method, which is demonstrated using numerical examples.
Assessment of vehicle tire forces is important in problems related to the structural health monitoring of highway bridges, damage to road pavements, design of suspensions, and road safety issues. In this paper, the effects of using semi-active control strategies, such as MR dampers, in vehicle suspensions on the dynamic tire forces are examined for the development of smart suspension systems for pavement- and bridge-friendly vehicles. The vehicle dynamics is described by a general linear MDOF model with multiple contacts (i.e., a multiple-axle vehicle) with the road. It is assumed that the tires are always in contact with the road surface. In particular, we are interested in the evaluation of the tire forces due to a harmonic excitation. A technique is developed to analytically assess the magnitude of the resulting tire force in the case of a passive suspension. Although the technique discussed cannot directly be applied to the calculation of tire forces in vehicles with controlled suspensions, it can efficiently be used for design purposes, which is demonstrated by an example of a semi-active suspension based on the sky-hook control. The discussion is amply illustrated by numerical examples.
In this paper, the effects of using semi-active control strategy (such as MR dampers) in vehicle suspensions on the coupled vibrations of a vehicle traversing a bridge are examined in order to develop various designs of smart suspension systems for bridge-friendly vehicles. The bridge-vehicle coupled system is modeled as a simply supported beam traversed by a two-degree-of-freedom quarter-car model. The surface unevenness on the bridge deck is modeled as a deterministic profile of a sinusoidal wave. As the vehicle travels along the bridge, the system is excited as a result of the surface unevenness and this excitation is characterized by a frequency defined by the speed of travel and the wavelength of the profile. The dynamic interactions between the bridge and the vehicle due to surface deck irregularities are obtained by solving the coupled equations of motion. Numerical results of a passive control strategy show that, when the lower natural frequency of the vehicle matches with a natural frequency (usually the first frequency) of the bridge and the excitation frequency, the maximum response of the bridge is large while the response of the vehicle is relatively smaller, meaning that the bridge behaves like a vibration absorber. This is undesirable from a bridge design viewpoint. Comparative studies of passive and semi-active controls for the vehicle suspension are performed. It is demonstrated that skyhook control can significantly mitigate the response of the bridge, while ground-hook control reduces the tire force impacted onto the bridge.
We present a new technique for identifying the dynamics of bolted joints. The technique relies on the comparison of the overall dynamics of the bolted structure to that of a similar but unbolted one. The difference in the dynamics of the two systems can be attributed solely to the joint; modeling this different in the dynamics enables us to construct a nonparametric model for the joint dynamics. Noncontacting, laser vibrometry is utilized to experimentally measure the structural responses with increased accuracy and to perform scans of the structural modes at fixed frequency. A numerical algorithm is then developed to post-process the experimental data and identify the joint force. Theoretical calculations are first used to validate the technique, which is then utilized to identify a practical joint. Experimental force-displacement plots at the joint reveal clear hysteresis loops which, in turn, can be used to estimate the damping dissipation at the joint. Moreover, experimental frequency responses and scans of the mode shapes of the bolted structure reveal nonproportional damping and nonlinear effects due to micro-impacts of the connected beams at the bolted joint.
A 100-Mbit/s FDDI network interface unit (NIU) is described that supports real-time data, voice and video. Its high-speed interrupt-driven hardware architecture efficiently manages stream and packet data transfers to the FDDI network. Other enhancements include modular single-mode laser-dioce fiber optic links to maximize node spacing, optic bypass switches for increased fault tolerance, and a hardware performance monitor to gather real-time network diagnostics.
Following a brief review of the radiation environment encountered by NASA spacecraft, we present examples of the use of fiber optic and optoelectronic components in this environment. Initial results of the fiber optic experiments on the recently retrieved Long Duration Exposure Facility (LDEF) will be presented. Very little radiation induced attenuation was observed during the LDEF flight. Next, we discuss the application of a Fiber Optic Rotation Sensor (FORS) on the JPL CRAF/Cassini missions. For these relatively long missions, reliability is expected to be more of an issue than radiation damage. Finally, we briefly discuss the application of fiber optic data busses to NASA spacecraft. Because of the short fiber lengths required, radiation is not expected to be a serious problem with data link applications.