Semi-active damping control for vibration isolation of base disturbances
This thesis is concerned with semi-active damping control for vibration isolation of base disturbances. The aim is to investigate the effectiveness and suitability of semi-active damping control strategies for improving steady-state vibration isolation. A single-degree-of-freedom (SDOF) system, comprising a semi-active damper with a linear passive spring in parallel, is used to study the vibration isolation of base excitation. The semi-active control strategies investigated include on-off skyhook control, continuous skyhook control, on-off balance control and continuous balance control. Chatter and jerk problems are investigated, which can arise in numerical simulations and possibly in practice when using semi-active control strategies. Anti-chatter and anti-jerk control strategies are proposed. These control strategies are implemented numerically in Matlab/Simulink. Harmonic, periodic and random disturbances are considered in this thesis. The vibration isolation performance is evaluated in terms of Root-Mean-Square (RMS) acceleration transmissibility. The performance of these control strategies for the isolation of harmonic disturbances is firstly studied. The performance is compared with those of an adaptive-passive control strategy, a conventional and a skyhook passive damper. Results show that the semi-active control strategies can provide a better isolation than a conventional passive system with an equivalent damping level. The semi-active damper can provide isolation over the whole frequency range if the on-state damping of the semi-active damper is big enough. The fraction of time when the damper is turned on or off is found to be frequency dependent. The effects of secondary frequency, which is a harmonic or subharmonic of the fundamental frequency on switching time of the semi-active damper for isolation of the primary harmonic are examined. Upper bounds are derived for fraction of time when the switching time for the fundamental frequency may be affected by the presence of a secondary frequency. The performance of the semi-active isolation system for periodic and random disturbances, where there is more than one harmonic in the disturbance spectrum is investigated. The results for square wave and triangular wave disturbances suggest that semi-active control strategies are promising for periodic disturbance. Three special cases are considered for random disturbances when the acceleration, velocity and displacement inputs have flat spectra. The semi-active control strategies can provide some advantage in performance for random velocity and displacement disturbances with medium to high damping ratios. Only continuous skyhook control strategy can provide some benefit in isolation performance for random acceleration disturbances. Following on from the numerical simulations, experimental work is carried out to validate the simulation results. The experimental set-up incorporates an electromagnetic device as a semi-active damper. The on-off skyhook control algorithm is chosen to be implemented using an analogue circuit. The damping of the electromagnetic semi-active damper is achieved by opening and closing the magnet-coil circuit. Numerical predictions are confirmed by experimental observation. The performance of the electromagnetic damper is limited by the achievable damping level.