Patent ID: 7385179
Filing Date: 2008-06-10
Classification: G01D

Abstract:
1. An optical encoder comprising: a scale including a first diffraction grating, the scale configured to move relative to a light source, the first diffraction grating configured to diffract a portion of a light beam emitted from the light source; a light receiving unit including a plurality of light-receiving elements, the light receiving unit configured to receive interference fringes formed by the overlap of diffracted light beams diffracted by the first diffraction grating and configured to generate an output signal; the light receiving unit disposed between the first diffraction grating and the light receiving unit; and a computing unit configured to compute an amount of movement of the scale relative to the light source on the basis of the output signal; wherein, when λ denotes the wavelength of the light source, P denotes the pitch of the first diffraction grating, x denotes a direction of the grating arrangement, AR (=d/P) denotes the optical effective aperture ratio of the first diffraction grating, d denotes the optical effective aperture width of the first diffraction grating, GAP (=z) denotes an effective optical gap between the first diffraction grating and the light receiving unit, and PO denotes a period of a fundamental harmonic of interference fringes overlapped on the second diffraction grating, AR and GAP are determined so that the following evaluation equation D including a relational expression I(x, z, d, P, λ) has a minimal value: when the first diffraction grating is illuminated with the portion of the light beam which is a substantially parallel light beam portion, and when the first diffraction grating is illuminated with the portion of the light beam which is a substantially divergent light beam portion.