Sliding mode control of active suspension for transit buses based on a novel air-spring model

Pneumatic suspensions, or air suspensions, are commonly used in railway and road vehicles because of their practical advantages, such as road friendliness, a constant suspension frequency and a constant ride height regardless of the vehicle load. On the other hand, it is difficult either to model or to control such suspension systems due to the presence of nonlinearity and model parameter uncertainty in air springs, namely preload dependence. The goal of this study is to design a robust active suspension that is capable of handling the combination of suspension nonlinearities, uncertainty in the sprung mass, and preload-dependent variation in the suspension characteristics. Since loading condition of transit buses often varies from trip to trip, robustness of the active suspension system with respect to the model parameter uncertainty becomes especially important. The effort of this study consists of two parts: one is to develop a nonlinear air-spring model based on experimental data; and the other is to design a sliding mode controller to improve suspension system performance, more specifically, ride quality.