Nanometer level positioning using three kinds of lead screws

A one-dimensional table unit with a laser feedback control system is manufactured, aiming at ultraprecision positioning of nanometer level accuracy by step response, with the use of one of the three kinds of lead screws (a precision sliding screw, a precision ball screw, and a rolled ball screw). The following items are made clear. (i) A quantified error (offset error) is caused by the proportional gain and D/A converter. (ii) In an effort to overcome this obstacle, the author devises a 'bit control'. This control method proves to be very effective in dealing with the quantified errors for three kinds of lead screws allowing one to obtain nanometer accuracy for all three. (iii) The characteristics of these three kinds of lead screws in the very short stroke range (0.1 mu m) are clarified. The precision sliding screw has a stick-slip between the screw and nut. The precision ball screw behaves elastically between them. The rolled ball screw shows a nanometer level positioning in spite of its large variation in friction torque and it having lowest accuracy.