Patent ID: 8330097

Claim:
An angular correction method for a rotary encoder that includes: a dial having angular graduation slits formed therein; a detector, reading angular graduation slit image positions of light transmitted through the angular graduation slits, and a computing unit, obtaining a measured angle value by interpolation calculation from the angular graduation slit image positions, the angular correction method comprising: a step of determining the most probable values R′, A 0 ′, and P 0 ′ of R, A 0 , and P 0 , respectively, by regression analysis of P i 's and A i 's for the angular gradation slit image positions, based on a formula which is derived from a model used in the interpolation calculation that holds for a triangle P i OP 0 P i =R tan( A i −A o )+ P o where P i is the position (pixel number) of the i-th angular gradation slit image among successive angular gradation slit images, P 0 is the position (pixel number) of a foot of a perpendicular drawn from a center 0 of the dial ( 1 ) to the detector derived from the model used in the interpolation calculation, R is a distance between the center 0 of the dial and the detector, A 0 is the angle of position P 0 , A i is the angle of i-th angular gradation slit image, and of calculating residual r i for the i-th angular gradation slit image position defined by r i =R ′ tan( A i −A 0 ′)+ P 0 ′−P i ; a residual curve determining step of determining and recording a residual r between a position computed from the formula P i =R tan(A i −A o )+P o and an actually read position P, for each angular graduation slit image position and of fitting the residuals r to a polynomial f(x) which is a function of angular graduation slit image number x, whose order is set in advance, to determine a residual curve, r=f(x); a slope computing step of computing a tangential slope f′(x i ) of the residual curve at a point x=x i , with x i being the i-th angular graduation slit image number; and a step of calculating the value of (P k+1 −P 0 )/(P k+1 −P k ) during measurement, where P k+1 and P k are positions of two angular graduation slit images sandwiching the foot, Po, of the perpendicular drawn from the center 0 of the dial to the detector derived from the model used in the interpolation calculation, a step of calculating a correction quantity C i defined by C i =f ′( x i ){( P k+1 −P 0 )/( P k+1 −P k )−0.5 }+r i and calculating (P i +C i ) for each of the angular graduation slit image positions; and a step of performing the regression analysis using (P i +C i ) as the new angular graduation slit image position for the i-th image to compute the measured angle value by interpolation calculation.