Patent ID: 9651404
Date: 2017-05-16
CPC Classifications: G01D

Claim:
1. An optical encoder for measuring the relative displacement between two members, the optical encoder comprising: a scale grating extending along a measuring axis direction, the scale grating having a grating pitch P a readhead configuration that moves relative to the scale grating with the relative displacement, the readhead comprising: wherein: the scale grating is configured to receive the collimated source light and output diffracted scale light to the first lens; the first lens is configured to receive the scale light and focus it toward the aperture portion; the aperture portion is configured to receive the scale light, block a majority of zero order scale light, and transmit spatially filtered scale light comprising primarily +1 and −1 order scale light toward the second lens; the second lens is configured to receive the spatially filtered scale light and output the spatially filtered scale light toward the moiré grating to form a primary interference fringe pattern having fringes that extend along the direction of the optical axis and approximately parallel to bars of the moiré grating, the fringes having a primary fringe pitch P the moiré grating is configured to receive the primary interference fringe pattern and transmit a periodic spatially modulated fringe pattern to the detector portion, the spatially modulated fringe pattern having a modulation period P the detector portion comprises at least N respective detector elements which are positioned to detect N respective spatial phases of the periodic spatially modulated fringe pattern, where N is an integer that is at least three, and each respective detector element has a width dimension DW that is at least as large as the primary fringe pitch P