Patent ID: 6552802
Filing Date: 2003-04-22
Classification: G01D

Abstract:
A method for sensing the position of a first member relative to a second member, movement of the first member relative to the first member being characterized by lateral displacement and roll, comprising the steps of:providing one or more diffractive grating tracks on a diffractive grating scale attached to the first member; directing a first pair of light beams to a first location on the one or more diffractive grating tracks, said first pair of beams reflectively diffracting from said diffractive grating scale to form a first set of optical fringes of a first periodicity P1; directing a second pair of light beams to a second location on the one or more diffractive grating tracks, said second pair of beams reflectively diffracting from said diffractive grating scale to form a second set of optical fringes of a second periodicity P2, P2 being different from P1; detecting a displacement of said first set of optical fringes DfM to generate a first signal representative of the displacement; detecting a displacement of said second set of optical fringes DfA to generate a second signal representative of the displacement; and processing said first and second signals to combine them according to the equation S1D1âˆ’S2D2 wherein D1 is a displacement of the diffractive grating scale estimated from DfM and D2 is a displacement of the diffractive grating scale estimated from DfA, and S1 and S2 are scale factors, to estimate the lateral displacement or roll of the diffractive grating scale or both.