In this thesis, we examine the Linear DC Motor in a configuration such that it provides the forces necessary to damp vibrations in a large flexible structure. The design is broken down into three steps where in each step, a feedback loop is placed around the actuator and/or the structure. The first loop is a motor compensation loop which effectively decouples the motor model from the structure model by removing the effect of the velocity of the structure on the motor's performance. The second loop stabilizes the relative position response of the combined actuator/structure model. This loop also shapes the magnitude response of the system, thus determining the bandwidth of the actuator. Two designs are developed: a narrow bandwidth design and a wide bandwidth design. The third loop is the vibration suppression design loop and can be designed a number of ways. In this thesis, we develop two decentralized designs and a centralized design. The final system is simulated to check design results. The various nonlinearities of the proof mass actuator are considered and their effect on results noted. These nonlinearites, the stroke and current limits, determine the effectiveness of each vibration suppression design. The linear model is checked for robustness to parameter uncertainty. Results for the various designs are tabulated and discussed. / M.S.

In order to provide a realistic and challenging experimental test for active vibration control concepts applicable to large space structures, an experimental model is required that simulates the complicated dynamic characteristics of such structures. This study presents the design, theoretical analysis, and experimental testing of such a model - a large, flexible plane grillage, with an adjustable skew angle, free to rotate on knife edges. The plane grillage model was shown, by theory and experiment, to have high modal density at low frequencies (twelve modes below 11 Hz). It was also demonstrated, by analogy with published results for a cantilevered skew plate, that the model would have a pair of closely spaced modes, with distinct mode shapes, at a particular skew angle. By using an ana1ogy with a simple rigid bar model, the pendulum mode of the plane grillage was shown to have a frequency that could be driven towards zero, thereby simulating a rigid body mode. The theoretical analysis was conducted, for one skew angle, using MSC/NASTRAN, and included the effect of gravity. Experimental tests were conducted on the model, with the same skew angle, using frequency and transient response techniques. The theoretical and experimental results were compared, with good quantitative agreement for the natural frequencies (first ten modes within 10%), and reasonable qualitative agreement for the lower mode shapes. / M.S.

The vibration of and wave propagation in periodic cellular structures are analyzed. Cellular structures exhibit a number of desirable multifunctional properties, which make them attractive in a variety of engineering applications. These include ultra-light structures, thermal and acoustic insulators, and impact amelioration systems, among others. Cellular structures with deterministic architecture can be considered as example of periodic structures. Periodic structures feature unique wave propagation characteristics, whereby elastic waves propagate only in specific frequency bands, known as "pass band", while they are attenuated in all other frequency bands, known as "stop bands". Such dynamic properties are here exploited to provide cellular structures with the capability of behaving as directional, pass-band mechanical filters, thus complementing their well documented multifunctional characteristics. This work presents a methodology for the analysis of the dynamic behavior of periodic cellular structures, which allows the evaluation of location and spectral width of propagation and attenuation regions. The filtering characteristics are tested and demonstrated for structures of various geometry and topology, including cylindrical grid-like structures, Kagom and eacute; and tetrhedral truss core lattices. Experimental investigations is done on a 2-D lattice manufactured out of aluminum. The complete wave field of the specimen at various frequencies is measured using a Scanning Laser Doppler Vibrometer (SLDV). Experimental results show good agreement with the methodology and computational tools developed in this work. The results demonstrate how wave propagation characteristics are defined by cell geometry and configuration. Numerical and experimental results show the potential of periodic cellular structures as mechanical filters and/or isolators of vibrations.