CONTROL AND ELIMINATION OF LEAD SCREW BACKLASH FOR ULTRA-PRECISION POSITIONING

This paper describes the principle of an active lead screw mechanism ( ALSM) and the basic characteristics of its experimental mechanism. The ALSM comprises a lead screw, two nuts, piezoelectric actuators (PEAs) and a movable table. The table is connected to the nuts through the P EAs so that the PEAs can control not only the backlash between the scr ew and nuts but also the position of the table. The ALSM has the follo wing capabilities: (1) when the table is to be driven at a high speed and positioned coarsely by a motor, sufficient backlash can be created between the screw and nuts, (2) after the table is positioned coarsel y, the backlash can be eliminated by the PEAs so as to give the ALSM a sufficient stiffness, and (3) while keeping the backlash at zero, the table can be positioned precisely by the PEAs. The experimental mecha nism achieved the following: (1) the maximum table speed of 130 mm/s, (2) the positioning accuracy of better than 10 nm, and (3) the positio ning time for a 10 mm step input of 369 ms.