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Robust Control of Active Vehicle Suspension Systems Using Sliding Modes and Differential Flatness with MATLAB

The main control objectives of active vehicle suspension systems are to improve the ride comfort and handling performance of the vehicle by adding degrees of freedom to the system and/or controlling actuator forces depending on feedback and feedforward information of the system obtained from sensors. Passenger comfort is provided by isolating the passengers from undesirable vibrations induced from irregular road disturbances, and its performance is evaluated by the level of acceleration which vehicle passengers are exposed. Handling performance is achieved by maintaining a good contact between the tire and the road to provide guidance along the track. The topic of active vehicle suspension control system, for linear and nonlinear models, in general, has been quite challenging over the years and we refer the reader to some of the fundamental work in the area which has been helpful in the preparation of this chapter. Control strategies like Linear Quadratic Regulator (LQR) in combination with nonlinear backstepping control techniques are proposed in (Liu et al., 2006). This strategy requires information about the state vector (vertical positions and speeds of the tire and car body). A reduced order controller is proposed in (Yousefi et al., 2006) to decrease the implementation costs without sacrificing the security and the comfort by using accelerometers for measurements of the vertical movement of the tire and car body. In (Tahboub, 2005) a controller of variable gain that considers the nonlinear dynamics of the suspension system is proposed. It requires measurements of the vertical positions of the car body and the tire, and the estimation of other states and of the road profile. On the other hand, many dynamical systems exhibit a structural property called differential flatness. This property is equivalent to the existence of a set of independent outputs, called flat outputs and equal in number to the control inputs, which completely parameterizes every state variable and control input (Fliess et al., 1995). By means of differential flatness