Active control of precise vibration isolation table using two degrees of freedom controller

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Precise instruments such as an electron microscope and a micro machine currently interested have to be installed on a precision vibration isolation table generally comprising air springs. The table must provide with functions of both the vibration absorption and the fast and accurate positioning. The absorption requires the soft spring, the positioning does the hard one. To cope with such a mutually contradictory requirements, a conventional passive isolation system is not sufficient, which should be actively controlled.Considering the vibration absorption as a disturbance suppression and the positioning as a tracking to desired inputs, the concept of a two degrees of freedom control may be effective to reconcile both requirements. Further, as the table is a motion mechanism with multi-degrees of freedom, some problems of coupling among each degree of freedom appear in the control. Regarding coupling forces as a kind of disturbance, the decoupling owing to the disturbance suppression function may be expected. This study investigates the availability of two degrees of freedom controller to a pneumatic vibration isolation table, also of which a design approach is proposed. Comparing its control performance with that obtained from PI controller, the effectiveness of the two degrees of freedom control to the pneumatic vibration isolation table is assured.