Realization of a wideband three-axis horizontal vibration isolator with adjustable stiffness and damping

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Horizontal vibration isolators (HVIs) designed to be on the verge of elastic instability offer many opportunities for isolating a variety of vibration-sensitive instruments, such as atomic force microscopes and laser/optical systems from ambient excitations at very low frequencies (0.5 – 5 Hz). These HVI systems are designed to have very low natural frequencies and can achieve quasi-zero-stiffness (QZS) in the horizontal axes when they support payloads close to their maximum payload-carrying capacities. Payloads of different sizes and weights necessitate to have adjustable stiffness and damping to operate in very wide bandwidths. To address these issues, a QZS HVI using axially compressed elastic columns is designed, optimized and fabricated. The system has adjustable natural frequencies in three axes (two translational and one torsional) via a string tensioning mechanism. Besides, the system enables the torsional natural frequency to be adjusted independently of the two translational natural frequencies by means of novel column sliding mechanisms that can change the radial positions of the elastic columns from the center axis of the system. Amplitude-dependent damping and stiffness characteristics of this variable natural frequency system are determined under various axial preload conditions. The system also involves an adjustable eddy current damper to effectively suppress low frequency resonance peaks. Finally, a methodology is proposed to modify the ideally clamped boundary conditions of the elastic columns, resulting in very good agreement between the analytical, numerical, and experimental results. The results show that the proposed HVI can achieve bandwidths between 1.6 – 311 Hz in the translational axes and 0.7 – 311 Hz in the torsional axis for payloads between 0 – 45 kg, providing a very large isolation bandwidth in three axes for a wide range of payload masses.