AbstractVibration dampers are very important for vibration mitigation of dynamic structures to avoid excessive vibration or induced noise. For a simple single-degree-of-freedom (SDOF) vibration system, larger amount of damping implies better vibration suppression effect. However, vibration isolation for such SDOF system requires higher damping at lower frequency and lower damping at higher frequency. For an auxiliary dynamic vibration absorber (DVA), specific damping ratio is a pre-requisite for achieving and maintaining optimum performance of the DVA. Moreover, the system parameters and environment of a vibration damper may be changed or varied during operation. The amount of damping in the vibration damper in many applications requires to be adjusted accordingly for best vibration control performance.
The aim of this research is to design tunable dampers with both high tunable damping range and precision. Two novel tunable damper designs are proposed and tested to solve vibration problems which required precise control of damping. Design I: electromagnetic shunt damper (EMSD) with opposing magnets configuration, Design II: hybrid damper with Coulomb friction and electromagnetic shunt damping.
An EMSD with opposing magnets configuration (Design I) to provide a tunable damping force is proposed first for vibration damping applications. The proposed EMSD configuration allows a significant reduction in size in comparison with other similar designs of EMSD found in the literature. In particular, an ESMD comprising six opposing magnets is designed and tested on a SDOF vibration system. The damping coefficient of the proposed EMSD offers a large tunable range with maximum damping coefficient about nine times or 900% of the minimum damping coefficient.
However, the higher number of opposing magnets pairs will reduce the radial magnetic flux density, an optimum number of opposing magnets pairs can provide maximum damping force for the best performance of Design I. The proposed damper is applied in a DVA system to achieve optimum performance of the DVA following the design procedure of the fixed-points theory. To the knowledge of the author, this is the first experimental proof of the fixed-points theory with a real DVA. In the optimum DVA implementation, the mass ratio is calibrated to find the fixed points of optimum DVA with different damping coefficients by tuning the external resistance of EMSD. H∞ optimal DVA is implemented with the tunable EMSD in four, six and twelve opposing magnets pairs. The damping coefficient of EMSD can be easily tuned in real time to keep the DVA in optimum status even the external parasitic damping is changing.
Moreover, EMSD is also widely applied for energy harvesting from vibrating structures owing to its electromechanical energy conversion capability. A bi-objective optimization methodology of Design I is proposed to achieve minimum resonant vibration of a vibrating structure and maximum energy harvesting at the same time. The proposed EMSD is designed such that its electromechanical transduction factor can be easily tuned to achieve both objectives simultaneously. The proposed EMSD is tested in a SDOF system and a DVA for the simultaneous realization of the two functions. Maximum harvested energy and minimum resonant vibration amplitude of the controlled mass are both achieved in the tested DVA system when the proposed EMSD is properly tuned.
Nonetheless, Design I is still hard to accomplish large improvement of the tunable damping range. A hybrid damper (Design II) combining the advantages of both Coulomb friction damping and electromagnetic shunt damping is therefore proposed to provide a larger range of tunable damping. A friction damper (FD) with adjustable damping with advantages of small size and at low cost is designed. The tunable friction damper can provide coarse tuning of damping force with a large tunable range, while the tunable EMSD functions as a fine tuner of damping force with higher precision. The achieved tunable range of the damping force is significantly improved with decent tuning precision by the proposed hybrid damper. The method for adjusting the damping forces in both dampers is simple and can be done in real time. A prototype of the proposed hybrid damper is tested and applied to a DVA system to verify its tunability by experiments. H∞ optimal damping of the DVA is readily achieved experimentally by using the proposed hybrid damper.
Through a combination of theoretical, numerical, and experimental studies, this thesis explores several innovative strategies to improve the tunable damping range with high precision. The outcome facilitates the potential applications of these devices in future applications of smart structures.
|Date of Award||2022|
|Supervisor||Wai On Wong (Chief supervisor) & Li Cheng (Co-supervisor)|