Robust control of active suspensions for high performance vehicles

This paper aims to outline the main steps of the design of a slow-active suspension controller able to deal with parametric uncertainties of the plant. The most relevant noise inputs to a suspension system are caused by road surface roughness (fast varying inputs) and by cornering and fore and aft acceleration (slow varying inputs). A previous paper was devoted to the design of a semiactive suspension which consisted of a backup passive system managing fast-varying inputs and of an active one managing slow-varying inputs. The suspension design was developed assuming a relatively simple model-the two-degree-of-freedom quarter-car model-and replacing the conventional components, spring and damper, with a simple hydraulic device. A proper control law, designed by means of the linear quadratic frequency-shaped (LQ) methodology, achieved the result of minimizing the sprung mass movements caused by a slow varying downforce acting on it. In practice, however, all suspension states are not directly measurable, thus a Kalman filter has to be introduced for state estimation: it yields a linear quadratic Gaussian (LQG) controller. Both the fact that the suspension dynamics depends on the static load and the fact that some parameters may vary while the suspension works, have suggested comparison of the LQ and the LQG active suspension from the point of view of stability robustness. This paper demonstrates that the robustness properties of the LQG active suspension are not necessarily bad and depend strongly on the design of the backup passive suspension.