Magneto-rheological fluid-based devices for vibration control of structures : development and testing
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Since 1970’s, the idea to use energy dissipative means in protecting building structural systems against un-wanted vibrations caused by extreme events such as strong winds; waves; destructive earthquakes; excessive man-made excitations, etc., has captured the imagination of many researchers. To benefit from the advantages of passive and active systems, semi-active systems, possessing the characteristics of passive and active systems, such as: low power input requirements; adjustable dynamic characteristics; no spill-over phenomenon; structural motion triggered operations; need for a suitable control algorithm requirements; no time delay problems if using electric or magnetic fluid devices; cost effective manufacturing and minor maintenance requirements; having fail-safe characteristics is selected in this research. This thesis presents a theoretical study of basic principles of controllable fluid flows for device modellings; the development of semi-active fluid control devices, whose dynamic range and desired damping level are investigated; and ‘proof-of-concept-tests’ of the prototype fluid control devices using sudden release and shake table tests. The performance of developed fluid control devices: prototype shear damper and smart pin damper, are verified using experimental results. Numerical simulations using MATLAB SIMULINK or tailor made ‘m’ code programs are furthermore used to gain insight into the damper control characteristics. The research scope involves the following five areas: 1. Studying basic principles and mechanism of controllable fluid flows treated as single fluid (Non-Newtonian fluids), magnetic fluid and two fluid models with magnetic forces. The study led to developing fluid dynamics-based device models that were then verified with experimental results. 2. Design, fabrication and testing prototype Magneto-rheological (MR) control devices: prototype shear (blade-column) damper and smart pin damper. 3. Characterization of damper performance in quasi-static and dynamic fashions using curve fitting, model mapping and system identification methods for modelling of devices. 4. Tailoring control algorithms to suit specific characteristics of devices. 5. Validation of device performance using numerical simulation and ‘proof-of-concept’ experiments of single storey plane frame models subjected to free vibration, sinusoidal and earthquake excitations, conducted using sudden release, impulse and shake table tests. The research, commenced with mathematically studying the duct flows through a parallel plate model for electro-rheological (ER) or magneto-rheological (MR) fluids, treated as non-Newtonian fluids, such as Bingham, Herschel-Bulkley and author proposed multi-piecewise linear viscous, which were introduced to develop the dashpot damper force-velocity characteristics. The use of single fluid models, so far, cannot analytically explain the physical mechanism behind electric or magnetic field-effects to the fluid rheological properties. The field of magneto-hydrodynamics, once attracting researchers in early 1960’s, can partly explain the fluid rheological property changes due to the presence of Lorentz force, which can accelerate or de-accelerate fluid flows in a duct. The accuracy of numerical or mathematical models for one dashpot damper MRD- 105 and two in-house designed and fabricated prototype shear and smart pin dampers is assessed by verification and validation of numerical simulation results with corresponding experimental results, tested in quasi-static and dynamic modes in the UTS Structures laboratory. Results were characterised using curve fitting, model mapping and system identification method in MATLAB package to determine the relationship between damper force and velocity or DC current level input. Good agreement for dashpot damper was obtained between the experimental results and proposed mathematical models: multi-piece wise linear and Bingham-Maxwell models. The prototype fluid dampers basically constituting three main components, namely, electromagnetic circuit system, controllable fluid and mechanical part were studied to elaborate the two important aspects in designing this kind of device: dynamic range and desired damping level. The ‘proof-of concept-tests’ of these prototype dampers: shear and pin dampers, respectively, mounted to a single-storey plane steel frame using an inverted ‘V’ brace truss, and as a beam-column connector, were later conducted using shake table for performance investigation. The dynamic performance of both models was studied using the experimental results of various free and forced vibration cases tested using sudden release and shake table, respectively. While, the free vibration results were used to characterise dynamic properties of the integrated structural systems using curve-fitting method for static stiffness coefficient of bare frame and brace, Fast Fourier Transform (FFT) for frequency, decay function and logarithmic decrement methods for static friction and damping ratios. Together with the control algorithms, state feedback and clipped-optimal controls employing Linear Quadratic Regulator (LQR) optimal and experimentally-tuned gains, discrete-time state space models were simulated using SIMULINK of MATLAB to validate sudden release and shake table experimental results of the shear damper-braced-frame system. In this case, two benchmark earthquake excitations: El-Centro N-S 1940 and Northridge N-S 1994, representing far- and near-field earthquakes, respectively, were selected as two standard random excitations for six dynamic system conditions: no damper (ND), passive-off (Poff), passive-on (Pon), State feedback control (Cl) and Clippedoptimal control (C2). Experimental results confirmed that the prototype shear damper is capable of suppressing the vibration responses of tested physical model, whose peak displacement responses were reduced as exceeding 50% when compared with no damper condition. Furthermore, good agreements for both free and forced vibration cases were achieved between numerical simulations and experimental results. Unlike, the smart pin-frame model, having non-linear DC current-based dynamic properties, can alter its system frequency in accordance with the supplied DC current level to the smart pin, The available linear control algorithms, therefore, cannot be implemented; as the feedback term by the damper is not a control force, but pin rotational stiffness, The sweep tests using sinusoidal excitations, conducted by the shake table, proved that the efficiency of displacement response reductions could be enhanced by properly switching the system frequency. Even though, the system performed moderately well when subjected to earthquake excitations, further research to this preliminary investigation is absolutely required to tailor the appropriate control strategy to this system. Finally, the results of the analytical and experimental studies demonstrated the advantages and potential use of semi-active MR fluid control devices: shear and smart pin dampers. This research is expected to facilitate and accelerate the implementation of these kinds of dampers in earthquake resistant structures of the future.
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