The performance of vehicle suspensions fitted with controllable dampers
First, techniques for modelling the vehicle and road surface are discussed, and the standard linear and nonlinear analysis methods are reviewed. Then, using the quarter car model and a single idealised road surface, a brief analysis of the passive and active suspensions, including full and limited state feedback schemes, is presented. The performance in terms of ride comfort, road holding ability and suspension travel for both systems is established, providing a yardstick against which the controllable damper systems can be compared. Three suspensions fitted with controllable dampers are then analysed. In order of increasing complexity these are; a three-state adaptive system, a two-state switcliable system, and a continuously variable system. After a performance comparison of the ideal system, the practical limitations present in real hardware are included in the damper model. Their effect on performance is quantified and realistic response targets are set. The model is then extended to a two dimensional “bicycle” model, which enables control laws to be generated which take into account the correlation between front and rear wheel inputs. Using these laws to drive the active and continuously variable damper system, the advantages of a correlated law are identified. The accelerations and suspension displacements of a passively suspended production vehicle are measured during actual runs over three roads of varying roughness. These are used to estimate the surface roughness properties of the roads. Using this range of typical conditions, the idea of system adaptation is then considered. The performance of each controllable damper system lies between that of the active and conventional passive systems. The continuously variable system generally offers the best ride comfort, but worthwhile improvements are also possible with the two-state switchable system. The adaptive system offers only marginal improvements for the conditions considered.