Abstract: A position encoder and a method for estimating absolute position using two or more diffractive grating tracks of differing periods to generate interference fringe patterns on a multi-track sensor. Detectors, corresponding to the diffractive grating tracks, detect the interference fringes. A first processing circuitry coupled to the detectors extract phase signals from the signals from the detectors. A second processing circuitry then estimates the cycle counts of the track signals based on the phase signals from the first processing circuitry. The absolute position is estimated by combining the cycle count, the fractional fringe value, and the grating period.

Abstract: An index structure and method are disclosed which employ, in addition to a main diffractive grating track of an encoder, a short diffractive grating track (the vernier or index track) having a different frequency than that of the main track. The phase of the index track is used to provide windows for the main track in a manner so that a preselected phase condition of the main track occurs in only one of the windows provided by the index track.

Abstract: A displacement measuring apparatus is disclosed, one embodiment of which employs two diffractive code disks, attached to a shaft, and a sensing head which need not contact the shaft. The head includes a light source with beam shaping optics, which creates two individual beamlets and pre-cranks the beamlets so that they reach the first grating at proscribed angles. Two diffracted orders from the first grating are allowed, preferably through free space propagation, to strike the second grating and to be rediffracted by the second grating. A phased-array detector, or equivalent array of detecting elements, are positioned beyond the second grating and in a region of natural interference between rediffracted orders of the incident diffracted orders, to detect the interference fringes which are generated there. The change in phase of the fringes is proportional to the relative displacement between the first and second gratings.

Abstract: The apparatus disclosed herein employs a grating (13) which concentrates light at a preselected wavelength into the positive (33) and negative (35) first orders while minimizing the zeroth order (31). The grating (13) is illuminated with monochromatic light of the selected wavelength and a poly-phase periodic detector (25) has its sensing plane spaced from the grating a distance less than ##EQU1## where W is the width of the illuminated region of the grating. The period of the poly-phase detector is equal to P/2 so that each detector element (51) or phase responds principally to the natural interference between the positive and negative first orders without requiring magnification or redirection of the diffracted light. Preferably, the distance of the sensing plane from the grating (13) is greater than ##EQU2## so that the detector response does not include substantial components from diffraction orders higher than the first.

Abstract: The apparatus disclosed herein employs a grating or scale which concentrates light at a preselected wavelength into the positive and negative first orders while minimizing the zeroth order. The scale is illuminated with monochromatic light of the selected wavelength and a poly-phase periodic detector has its sensing plane spaced from the scale a distance less than ##EQU1## where W is the width of the illuminated region of the scale. The period of the poly-phase detector is equal to P/2 so that each detector element or phase responds principally to interference between the positive and negative first orders without requiring magnification or redirection of the diffracted light. Preferably, the distance of the sensing plane from the scale is greater than ##EQU2## so that the detector response does not include substantial components from diffraction orders higher than the first.