This paper deals with a condition diagnosis technique to maintain a dynamic performance of electro-hydraulic servo system and to assure thickness accuracy. The diagnosis technique has following two features. One is to estimate the parameters of servo dynamics during rolling without specified input signals for testing. The other is to diagnose the degradation trend of the servo system and carry out the predictive maintenance based on the on-line monitoring of the estimated parameters, such as

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... resonant frequency and the damping coefficient. For the plate rolling, the servo signals at biting condition are used, because these signals have high frequency components enough for the estimation. The parameters are estimated such that the frequency characteristics of the model is optimally matched to the actual data. On the other hand, servo data at steady state rolling is used for the strip rolling. The parameters are calculated recursively by the least square estimation. These methods have been successfully applied to the actual rolling and the thickness variation caused by the servo degradation can be reduced by servo gain adjustment according to the estimated parameters.