1. A method for self-calibrating a rotary encoder (130) comprising a single read-head (110) and a circular scale (100), comprising the steps of:
acquiring calibration samples (150) by the read-head (110) for rotational angles of the circular scale (100);
estimating (160) spatial frequency and spatial distortion parameters of the rotary encoder from the calibration samples (150) for self-calibrating the rotary encoder (130),
measuring c bits between two successive zero-crossings on the circular scale; and
modeling the zero-crossings as a cubic model of c(i)
wherein z(i) is the zero-crossing, k(i) is the number of bits between two successive zero-crossings z(i) and z(i+l), i is an integer parameter, P is a phase value, F is the spatial frequency, and a and /? are the spatial distortion parameters,
wherein the spatial frequency and the spatial distortion parameters are modeled by accounting for the error due to non-uniform spacing of scale lines on the circular scale (100),
wherein the spatial frequency and spatial distortion parameters are estimated by using zero-crossings and the number of bits between the zero-crossings, wherein the zero-crossings indicate edge locations in a scan-line of the read-head (110).
2. A method for self-calibrating a rotary encoder (130) comprising a single read-head (110) and a circular scale (100), comprising the steps of:
acquiring calibration samples (150) by the read-head (110) for rotational angles of the circular scale (100);

estimating (160) spatial frequency and spatial distortion parameters of the rotary encoder from the calibration samples (150) for self-calibrating the rotary encoder (130),
modeling (170) variations in the spatial frequency and the spatial distortion parameters using a parametric function (171);
acquiring test samples (151) of the circular scale (100); and
determining a phase angle of the rotary encoder (130) using the modeled frequency and distortion parameters,
wherein the spatial frequency and the spatial distortion parameters are modeled by accounting for the error due to non-uniform spacing of scale lines on the circular scale (100),
wherein the spatial frequency and the spatial distortion parameters are modeled as a function of an actual angle of rotation,
wherein the spatial frequency parameters is F(6), and the spatial distortion parameters are a(6) and /?(#), and wherein a fourth degree polynomial is
where 6 is an angle of rotation, and t\, t2, h, t4, and t$ are parameters of the fourth degree polynomial estimated using the least square fitting.
3. The method as claimed in 1 or 2, comprising:
acquiring test samples of the circular scale (100);
determining a phase angle of the rotary encoder (130) using the spatial frequency and spatial distortion parameters.
4. The method as claimed in 1 or 2, wherein the rotary encoder (130) is an absolute rotary encoder having a non-periodic sequence.
5. The method as claimed in 1 or 2, wherein the circular scale (100) is a form of a de Bruijn sequence.

6. The method as claimed in 1 or 2, wherein marks on the circular scale (100) are arranged as sectors, and the read-head (110) is centered tangentially at an offset (115) with respect to a center (116) of rotation of the circular scale (100).
7. The method as claimed in 1 or 2, wherein the read-head data is obtained for a rotation angle of 360 degrees or less.
8. The method as claimed in 1 or 2, comprising:
storing the spatial frequency and spatial distortion parameters in a memory (180) as a look-up table.
9. The method as claimed in 1 or 2, wherein the spatial frequency and spatial distortion
parameters are acquired during real-time operation of the rotary encoder (130).
10. The method as claimed in 1 or 2, wherein the read-head (110) comprises a linear array
of pixels, and comprising:
measuring intensities of the pixels to obtain a maximal intensity as a scaling factor, and a minimal intensity as an offset factor,
wherein the pixel intensities are modified using the scaling and offset factor.
11. The method as claimed in 10, wherein the pixel intensities i(p) are modified according
to
i{p)