In the context of discrete-time adaptive control of plants subject to parameter variations, direct adaptive control (despite various attempts: model reference control, minimum variance self-tuning) cannot effectively be used in practice because the discrete-time plant models feature generically unstable zeros. There is however another extremely important area for application of adaptive control: rejection of unknown disturbances. The paper shows that in this case it is possible to develop

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... adaptive control schemes, which are very effective in practice. These schemes perform better and they are simpler than the indirect adaptive control schemes. This is illustrated by their application to adaptive rejection of unknown narrow band disturbances in an active suspension system. * Aurelian Constantinescu is currently with Ecole de Technologie Supérieure, Montréal, Canada.