Theory of Active Suspension Design

Active suspension design has been developed as the application of optimal control theory. However, optimal control theory is only suitable for the design of regulator, where transient responses starting from any initial state are required to converge to zero. The active suspension system is not a simple regulator because road surface unevenness acts only as disturbance in the low frequency range, while it acts not only as disturbance but also as reference signal in the high frequency range. Thus, optimal control theory is not considered suitable for active suspension design. As an alternative to optimal control theory, a new design theory based on exact model matching (EMM) with a disturbance predictor is developed in this paper. One of the peculiarities of this problem is the need to prepare a separate control law for each frequency range. The other is that the outer signal is inaccessible. The former problem is solved by introducing a weighing rational function. The latter problem is fortunately settled by the fact that disturbance and outer signal have a simple relation to each other.