Comfort and road handling performance of a passenger car are mainly determined by the damping characteristic of the shock absorbers. Passive shock absorbers have a fixed damping characteristic determined by their design. Semi-active and active suspension systems offer the possibility to vary online the damping characteristic based on measurements of the car motions, e.g. by controlling the restriction of the shock absorber valves.

A flexible model-free control structure has been developed for a semi-active suspension system. The control structure is based on physical insights in the car and semi-active suspension dynamics which are used to linearise and decouple the system. It does not require a model of its dynamics, and consequently, is applicable to any semi-active or active suspension system and any type of car. A static decoupling is used to decouple the system into its modal motions heave, roll and pitch, which are then controlled by modal (diagonal) controllers, which consist of several feedback and feedforward modules, each tackling a specific comfort or handling issue. The feedback controller is based on the skyhook principle, with gains that are adapted automatically according to the road excitation. Steering angle, throttle, and break pedal feedforward control roll during cornering, and pitch during acceleration and deceleration, respectively. The control parameters are physically interpretable and can be easily tuned online according to guidelines given by test pilots and based on test results.