Strategies for Road Profile in Adaptive Suspension Control

Abstract: Semi-active suspension systems has become an increasingly popular alternative to the passive suspensions in recent time. By varying the spring stiffness and damping coefficient in the system, different characteristics can be achieved depending on driver preferability and road disturbances. The difference in damping coefficient will however lead to a trade-off between comfort and road holding,which means that in order to improve one of the areas, performance in the other will need to be reduced. This trade-off will also be different depending on the underlying road disturbances. This thesis is conducted on behalf of Volvo Cars, who are looking for a strategy in order to analyze the trade-off for different ISO classified roads which has been done through a literature study, theoretical analysis and comparison of control methods for different roads. A skyhook based controller was built with added modifications in order to reduce the jerk in the vehicle. Tests with the controller were carried out on a quarter car model as well as a full car model in IPG Carmaker. The simulations with the quarter car model showed that the controllerimproved the peak jerk values as well as the comfort, compared to both skyhook control and passive suspension. The full car simulations consisted of test runs on four roads, all with different road profiles and frequency content. For each road, tests were conducted with two different damper control methods as well as with a varying spring stiffness.The tested damper control methods were force gain control, which modulates the force request from the skyhook controller, as well as rate limitation control, which modulates the rate at which current is applied to the damper. The results showed that the trade-off between road holding and comfort appeared differently when the road profile changed. All results were compared to Janeway’s comfort criterion in order to validate the comfort of the vehicle. The outcome of the thesis was a strategy that involved various tuning for the semi-active damper and air suspension depending on the primary and secondary ride nature of the road. More specifically, a stiff damping is preferred during general primary ride conditions and a soft damping is preferred during general secondary ride conditions. For cases when the low frequency disturbances are small, the trade-off will be small and comfort can be improved with little to no cost of road holding. When the high frequency disturbances increase in amplitude, the damper should be tuned depending on preferability. For improved road holding ability, a stiff damping is preferred and for improved comfort, a soft damping is preferred. A soft spring will, for most cases, be preferred in terms of both road holding and comfort.