Nanometer positioning of a linear motion stage under static loads

A standard lead-screw-driven linear translation table was fitted with a secondary voice coil actuator, which can apply forces of /spl plusmn/0.5 N on the translating stage. The DC servomotor-driven lead screw is capable of positioning the table to a nominal precision of /spl plusmn/0.5 /spl mu/m. The secondary actuator then positions the table to /spl plusmn/1 mm. Both actuators utilize closed-loop feedback algorithms. This configuration has significant advantages over the more common "piggyback" arrangement, which often uses piezoelectric actuators. The advantages include preservation of the original load surface and position sensors and an easier to control actuator. Its ability to operate under static load and a model of the microdynamic behavior are addressed in this paper. When static loads of up to 36 N were applied to the translating stage, the secondary actuator could still displace the table to /spl plusmn/500 nm and control the position down to its zero-load precision. The microdynamic behavior in this region is nonlinear and was modeled using the modified Dahl model of friction. The key parameters in the model were identified using a 2 Hz sinusoidal input force with varying amplitudes from the secondary actuator. For a constant loading force, the parameters did not vary significantly with different excitation forces. The friction parameter relating the initial slope of the friction force versus displacement curve did change with different loading forces. Also, for smaller input signals causing displacements of less than 200 nm, the friction parameters would tend to drift, depending on input magnitudes.