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Comfortable rides in vehicles require active suspension systems to give road holding ability. Suspension systems have a long history from horse drawn carriages with leaf springs to today's automobiles with automation solutions. They need to absorb dips and bumps in the road and involve complex control strategies with many control parameters, multi objective and uncertain disturbances and can be passive or active systems. In the passive type dampers absorb shocks and springs act as energy storing elements. Performance of the passive suspension system is decided by the selection of springs and dampers and a compromise is usually required to give smooth movement. Active suspension systems use actuators to provide comfortable rides, where sprung mass and un-sprung mass are measured and sent to a controller. In this paper we propose a Fractional order PID controller which involves five parameters, three gains together with integrator and differentiator. As mathematical approaches to determine the parameters are complicated, we propose a novel use of Harmony search as a stochastic optimizer to auto-tune the Fractional order PID controller.

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