Optical displacement sensing device with reduced sensitivity to misalignment

摘要

An optical displacement sensing device is provided for determining the relative displacement of a diffraction grating scale along a measuring axis. The grating may be reflective and the grating pitch may be less than the wavelength of the light of the displacement sensing device. The sensing device includes a split light beam input portion for inputting two split light beams along respective light paths, and light beam directing elements for directing the two split beams to converge proximate to a first zone on the scale grating to give rise to two diffracted beams which diverge proximate to the first zone. The sensing device further includes retroreflector elements for receiving and retroreflecting the two diffracted beams to converge proximate to a second zone on the scale grating to give rise to two later-diffracted light beams which diverge and are directed to a shared zone. An optical detector detects at least one illumination characteristic arising from the shared zone, thus sensing displacement of the grating scale along the measuring axis.