Toshiharu Tanaka, Jiro Otsuka, Ikuro Masuda, Yasuaki Aoyama, and Asuka Inagaki
We have developed an ultra-precision positioning device that has the following characteristics: 1) The 210 mm strokes stage is driven by a new type of linear motor called “Tunnel Actuator (TA).” 2) The stage has very rigid structure so as not to cause vibration and to achieve high resolution for its feed-back system. 3) The stage is supported by linear ball guideways that have nonlinear spring behavior in the small stroke range. 4) Much attention has been paid to the time lag of the electric control system in the PID control using a linear encoder of 0.034 nm resolution for the feed-back system. The electric control system compensates for the disturbance of induced electromotive voltage that is generated in proportion to the stage velocity. We have studied how the equivalent time constant T of the control system affects the stage displacement deviation Δx when the command of stage displacement xr is kept at zero. The following results have been obtained: 1)With a decrease in time constant T of the current control system, the change in the motor current Io becomes smaller, and, at the same time, the change in stage deviation Δx becomes smaller. 2) At the smallest time constant T of the current system, a displacement resolution of 0.2 nm has been obtained under the nonlinear spring behavior of linear ball guideways. 3) There is a possibility of obtaining a displacement resolution of less than 0.1 nm with a further decrease in T.
Keywords: linear motor, sub-nanometer positioning, electric current control system, time constant, displacement resolution