Patent ID: 8288709

Claim:
An optical encoder device comprising: a light emitting element; a light receiving element disposed to face the light emitting element; a movable slit plate disposed between the light emitting element and the light receiving element and including a first slit row in which a plurality of first light transmissive slits each having a predetermined slit width and a plurality of first light non-transmissive slits each having a predetermined slit width are alternately formed in a moving direction of the movable slit plate at a constant pitch P; and a stationary slit plate disposed between the light emitting element and the light receiving element and including one or more second slit rows in which a plurality of second light transmissive slits each having a predetermined slit width and a plurality of second light non-transmissive slits each having a predetermined slit width are alternately formed, wherein defining the number S of the plurality of second light transmissive slits formed in the stationary slit plate as S=nÃ—nâ€² where n is an integer of 2 or more and nâ€² is an integer of 2 or more except for a combination of n=2 and nâ€²=2 and a combination of n=3 and nâ€²=2, the value of a as an integer of 0â‰¦aâ‰¦nâˆ’1, and the value of b as an integer of 0â‰¦bâ‰¦nâ€²âˆ’1, the plurality of second light transmissive slits are formed in the stationary slit plate such that when one of the second light transmissive slits is optically coincident with one of the first light transmissive slits formed in the movable slit plate, the remaining S-1 second light transmissive slits are shifted in position from other first light transmissive slits corresponding to the remaining second light transmissive slits by S-1 phase differences represented by PÃ—[a/(3Ã—n)+b/(5Ã—nâ€²)]where the value of a and the value of b are in different combinations.