An MR(Magneto-Rheological) fluid damper is designed and applied to a semi-active suspension system of a quarter car model. The damping constant of the MR damper changes according to the input current and the time delay of the damper is included in the system dynamics. The passive method, LQ control and frequency shaped LQ control are compared in experiments. The advantage of the proposed frequency shaped LQ control is that the ride comfort improves in the frequency range of 4 and 8 Hz where a human is most sensitive and the driving safety improves around the resonance frequency of unsprung mass, 11 Hz. The experiments using a quarter car model show the effectiveness of the algorithm.
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