PURPOSE:To improve the working efficiency of works by driving a work to be controlled so that its excess drive at the time of driving a servo system is offset when the detecting position of the work to be controlled exceeds a previously determined allowable driving range in a coordinate system at the time of starting the servo system. CONSTITUTION:When the current position of a working head 2 is on the outside of a Z axis stroke range is judged, the excess value of the current position from the boundary value of the Z stroke range is calculated as a stroke excess value So and whether the value So is included in an allowable excess range Sh or not is judged. When the value So is included in the range Sh, a driving signal for correcting the value So so as to offset the value So is outputted to a Z axis feeding motor 9 to move the head 2 and the routine is ended. If the excess value So exceeds the range Sh, an alarm signal is outputted and an alarm lamp (not shown figure) is turned on to end the routine.
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