The patent badge is an abbreviated version of the USPTO patent document. The patent badge does contain a link to the full patent document.
The patent badge is an abbreviated version of the USPTO patent document. The patent badge covers the following: Patent number, Date patent was issued, Date patent was filed, Title of the patent, Applicant, Inventor, Assignee, Attorney firm, Primary examiner, Assistant examiner, CPCs, and Abstract. The patent badge does contain a link to the full patent document (in Adobe Acrobat format, aka pdf). To download or print any patent click here.
Patent No.:
Date of Patent:
Dec. 12, 1995
Filed:
Oct. 05, 1994
Nobuo Nabeshima, Tokyo, JP;
Sony Magnescale Inc., Tokyo, JP;
Abstract
A displacement measurement apparatus utilizing an astigmatism, comprises an objective lens, an optical system detecting an offset distance between a relative distance of the objective lens to a measured surface and its focal distance by utilizing a reflection beam from the measured surface, to generate a focus-error signal, an actuator connected to the lens for moving the objective lens in a direction of an optical axis of the lens on the basis of the focus-error signal, so that the relative distance is maintained at the focal distance, and a linear scale and a detecting head cooperative with each other for detecting a displacement of the linear scale to the head. One of the linear scale and the head is integrally connected to the actuator for moving together with the lens and coaxially aligned with the optical axis, and the other is stationary. A first servo control circuit is arranged for executing a first feedback control based on a linear-scale displacement signal so that the linear-scale displacement signal is adjusted to zero. A second servo control circuit is arranged for executing a second feedback control based on the focus-error signal so that the focus-error signal is adjusted to zero. The first feedback control is executed when a signal level of the focus-error signal is shifted to a dead zone in which the focus-error signal is unuseable as a servo signal necessary for the second feedback control.