Adaptable resolution optical encoder having structured illumination and spatial filtering

A device for measuring the relative displacement between two members, the device comprising:

• a scale grating extending along a measuring axis direction and having a scale pitch PSF;
• an illumination source comprising a light source that outputs light having a wavelength λ, and a structured illumination generating portion that inputs the light and outputs structured illumination to the scale grating where the structured illumination comprises an illumination fringe pattern oriented transversely to the measuring axis direction and that have an illumination fringe pitch PMI along the measuring axis direction at a plane coinciding with the scale grating;
• an imaging portion positioned to receive operable spatially modulated image light output from the scale grating and output an operable periodic image of the spatially modulated image light when the scale grating is illuminated by the structured illumination from the illumination source;
• and
• a detector portion comprising a set of respective optical detectors positioned to respectively receive different phases of the operable periodic image, the set of respective optical detectors having a detector pitch Pd along the measuring axis direction, the detector pitch Pd corresponding to a pitch between detector elements corresponding to a particular detector signal phase;
• wherein:
• the imaging portion comprises:
• the detector portion positioned a distance Z from the imaging portion along an optical path of the imaging portion;
• a first lens positioned to receive the spatially modulated image light transmitted from the scale grating, the first lens having a focal length F that defines a focal point located between the first lens and the detector portion;
• and
• an aperture positioned approximately at the focal length F between the first lens and the detector portion;
• the spatially modulated image light comprises fringes formed from the interference of two diffractive orders which differ by a value Δn;
• the aperture is configured with a dimension W along the measuring axis direction such that ΔW=Z*λ*(a*(Δn+1)/(PMIPSF/((PMI−PSF)*M)), where M is a magnification value of the imaging portion, and the value of a is greater than 0.5 and less than 4.0;
• and
• the scale grating inputs the structured illumination and outputs the spatially modulated image light comprising the structured illumination modulated by an intensity modulation envelope that has a spatial wavelength PIMESF that depends on the scale pitch PSF and the illumination fringe pitch PMI, and PSF and PMI are selected to cooperate with the detector pitch Pd such that ΔnPMIPSF/(ΔnPMI-PSF=PIMESF=m*Pd/k when the light source outputs incoherent light and ΔnPMIPSF/(2ΔnPMI−PSF)=PIMESF=m*Pd/k when the light source outputs coherent light, wherein m is a number of phase signals output by the detector portion and k is an odd integer, and wherein the spatial wavelength PIMESF is larger than the scale pitch PSF.