The invention relates to a positioning device comprising a carriage displaceable in at least two coordinate directions (X, Y) with respect to a base and intended for displacement of a table which is displaceable with respect to the carriage in the said coordinate directions.
In a known positioning device of the kind mentioned above (see "Bulletin of the Japan Society of Precision Engineering", Vol. 22, No. 1, March, '88, pp. 13-17), the table is coupled by four flexible rods to the carriage. The displacement of the table with respect to the carriage is obtained by three so-called piezo-actuators, which are each secured near one end to the carriage and are secured near the other end to the table. The carriage constitutes an upper support of a so-called cross table, which further has a lower support. The lower and upper supports can be displaced in the X and Y coordinate directions, respectively, the displacement of the lower support in the X coordinate direction also resulting in a displacement of the upper support in this direction. The displacement of an object lying on the table is effected by a coarse displacement of the carriage (upper support) followed by a fine displacement of the table with respect to the carriage.
A disadvantage of the known two-stage positioning device described is that the mechanical coupling between the table and a carriage acting as the upper support in a cross table in a controlled system results in bandwidth-limiting natural (Eigen) frequencies. The friction in the guides of the supports in the cross table moreover considerably limits the accuracy of the positioning device.