This paper presents adaptive robust controllers for force tracking application in a quarter-car active suspension system. In previous publications (Chantranuwathana and Peng 1999a, 1999b), an active suspension architecture was presented. The overall active suspension system was decomposed into two loops. At the main-loop, the desired force signal is calculated while the sub-loop force tracking controller tries to keep the actual force close to this desired force. An Adaptive Robust Control (Yao and Tomizuka 1997) design technique was used to achieve good force tracking performance in a robust manner under plant uncertainties. It was found that force-tracking of up to 5Hz can be reliably achieved. It is, however, found to be unreliable in experiments, especially when high frequency disturbances are present. In this paper, we will show that unmodeled dynamics and especially, the delay (first order lag) in implementing the control signal is a main cause of the problem. With this insight, three controller modifications are proposed to reduce the effect of the unmodeled dynamics, 1) include the actuator dynamics in the ARC design, 2) cancellation of the actuator dynamics and 3) online-adaptation of an ARC parameter. A number of simulation results will be presented to show the effect of these remedies. The last two modifications were found to be promising for actual implementations.

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