1. Field of the Invention
This invention relates to an encoder such as a linear encoder or a rotary encoder for causing a light beam to enter a displaceable diffraction grating a plurality of times, and optically detecting any variation in a intensity of an interference light formed thereby, thereby detecting the displaced state such as the amount of displacement or the direction of displacement of the diffraction grating.
2.Description of the Related Background Art
In recent years, in precision machines such as NC machine tools and semiconductor printing apparatuses, there has been required a precise measuring machine which can measure the driving state and movement in units of 1 .mu.m or less (submicron). As a measuring machine which can accomplish measurement in submicron units, there is well known an encoder such as a rotary encoder or a linear encoder for causing a coherent light beam such as a laser beam to enter a diffraction grating formed on a moving object, causing diffracted lights of predetermined orders created from the diffraction grating to interfere with each other, counting the light and dark of the interference fringes and thereby finding a displaced state such as the amount of displacement or the direction of displacement of the moving object. For example, such rotary encoder is generally used in such a system that the number of revolutions of a motor is judged from the output of the encoder mounted on the rotary shaft of the motor and the driving of the motor is controlled on the basis thereof.
The applicant proposed such encoders, for example, in Japanese Laid-Open Patent Application No. 58-191907, Japanese Laid-Open Patent Application No. 62-163926, Japanese Laid-Open Patent Application No. 62-163924 and Japanese Laid-Open Patent Application No 62-200225.
Generally, in an encoder for forming interference fringes by the utilization of diffracted lights of predetermined orders created from a diffraction grating connected to a moving object, counting the light and dark stripes of the interference fringes by detecting means, i.e., detecting any phase variation in the interference fringes, and thereby detecting the displaced state of the moving object, it is sometimes the case that regular reflected light (0-order reflected diffracted light), when a coherent light beam is caused to enter the diffraction grating, enters the detecting means through various elements of the device.
The regular reflected light at this time does not cause any phase variation even if the diffraction grating is displaced, and is an unnecessary light that is unnecessary for the detecting means. If this unnecessary light enters the detecting means, there will arise a problem that it will become a flare light (a ghost light) and will reduce the S/N ratio of the output signal of the detecting means and thus, will reduce measurement accuracy.
In contrast, there is a method of preventing the incidence of the regular reflected light onto the detecting means by the use of a polarizing beam splitter and a wavelength plate, i.e., by the use of the construction of a so-called light isolator, but even if this method is used, it has been very difficult to completely prevent the regular reflected light from entering the detecting means.