Displacement measurement apparatus having first and second servo control circuits and a zone decision circuit

A displacement measurement apparatus utilizing an astigmatism, comprising:

• an objective lens focusing a laser beam on a measured surface;
• an optical system detecting an offset distance between a relative distance of said objective lens to said measured surface and a focal distance of said lens by utilizing a reflection beam reflected from said measured surface, to generate a focus-error signal indicative of said offset distance;
• an actuator integrally connected to said objective lens for moving said objective lens in a direction of an optical axis of said lens on the basis of said focus-error signal, so that said relative distance is maintained at said focal distance;
• a linear scale and a detecting head cooperative with each other, for detecting a displacement of said linear scale to said head;
• one of said linear scale and said head being integrally connected to said actuator for moving together with said lens and coaxially aligned with said optical axis, and the other being stationary;
• said head generating a linear-scale displacement signal indicative of said displacement of said linear scale;
• a first servo control circuit arranged for executing a first feedback control based on said linear-scale displacement signal so that said linear-scale displacement signal is adjusted to zero;
• a second servo control circuit arranged for executing a second feedback control based on said focus-error signal so that said focus-error signal is adjusted to zero;
• and
• a zone decision circuit connected to said first and second servo control circuits, for selecting said first feedback control among the two feedback controls when a signal level of said focus-error signal is shifted to a dead zone in which said focus-error signal is unusable as a servo signal necessary for said second feedback control.