Self-calibrating single track absolute rotary encoder

Patent metadata

Field	Value
Publication number	US-9423281-B2
Application number	US-201313899025-A
Country	US
Kind code	B2
Filing date	May 21, 2013
Priority date	Feb 7, 2012
Publication date	Aug 23, 2016
Grant date	Aug 23, 2016

Abstract

A rotary encoder includes a single read-head and a circular scale. The encoder is self-calibrated by acquiring calibration samples with the read-head for rotational angles of the circular scale, and estimating spatial frequency and spatial distortion parameters of the encoder from the calibration samples.

First claim

We claim:

1. A method for self-calibrating a rotary encoder including a single read-head having an imaging sensor and a circular scale having a circle disk, wherein the single read-head reads the circular scale at positions in the circumferential direction of the circle disk and is in communication with a processor, the method comprising the steps of:
positioning the single read-head at a distance and parallel to a plane of the circular scale;
acquiring calibration samples by rotating the circular scale past the read-head to obtain calibration samples at different positions along the circumference of the circle disk, wherein each calibration sample by the read-head includes measurements of pixels at positions in the circumferential direction of the circle disk;
obtaining from each calibration sample a set of edge locations on the circular scale; and
estimating spatial frequency and spatial distortion parameters of the encoder from the sets of edge locations of the calibration samples for self-calibrating the rotary encoder, wherein the steps are performed in the processor.

2. The method of claim 1 , further comprising:
determining a phase of the encoder using the estimated frequency and distortion parameters.

3. The method of claim 1 , wherein estimating spatial frequency and spatial distortion parameters of the encoder from the sets of edge locations or zero-crossings of the calibration samples for self-calibrating the rotary encoder, further comprises:
modeling variations in the frequency and the distortion parameters using a parametric function.

4. The method of claim 3 , wherein the parametric function is a spline.

5. The method of claim 3 , wherein the parametric function uses least squares fitting between the parametric function and the zero-crossings.

6. The method of claim 5 , further comprising:
measuring c bits between two successive zero-crossing on the circular scale; and
modeling the zero-crossings as
z ( i )= P+Fc ( i )+α c ( i ) 2 +βc ( i ) 3 ,

wherein P is a phase value, F is the spatial frequency, and α and β are the spatial distortion parameters.

7. The method of claim 6 , wherein the spatial frequency parameters is F(θ), and the spatial distortion parameters are α and β, and wherein a fourth degree polynomial is
α(θ)= t 1 +t 2 θ+t 3 θ 2 +t 4 θ 3 +t 5 θ 4 ,

where t 1 , t 2 , t 3 , t 4 , and t 5 are parameters of the fourth degree polynomial estimated using the least square fitting.

8. The method of claim 3 , wherein the parametric function is a fourth degree polynomial with respect to the rotational angle.

9. The method of claim 1 , wherein marks on the circular scale are arranged as sectors, and the read-head is centered tangentially at an offset with respect to a center of rotation of the circular scale.

10. The method of claim 1 , wherein the read-head data is obtained for a rotation angle of 360 degrees or less.

11. The method of claim 1 , further comprising: storing the frequency and distortion parameters in a memory as a look-up table.

12. The method of claim 1 , wherein the frequency and distortion parameters correct for eccentricity of the circular scale.

13. The method of claim 1 , wherein the frequency and distortion parameters correct for wobble of the circular scale.

14. The method of claim 1 , wherein the frequency and distortion parameters correct for a change in a distance between the read-head and the circular scale.

15. The method of claim 1 , wherein the frequency and distortion parameters correct for a temperature or mechanical vibration during operation of the encoders.

16. The method of claim 1 , wherein the frequency and distortion parameters are acquired during real-time operation of the encoder.

17. The method of claim 1 , wherein the read-head includes a linear array of pixels, and further comprising:
measuring intensities of the pixels to obtain a maximal intensity as a scaling factor, and a minimal intensity as an offset factor.

18. The method of claim 17 , wherein the pixel intensities are modified using the scaling and offset factor.

19. The method of claim 17 , wherein the pixel intensities i(p) are modified according to
i ( p )←255*( i ( p )− m 2 ( p ))/( m 1 ( p )− m 2 ( p ))

wherein m 1 (p) is the maximal intensity and m 2 (p) is the minimal intensity.

20. The method of claim 1 , wherein the circular scale is a form of a de Bruijn sequence.

Assignees

• Mitsubishi Electric Res Laboratories Inc

Inventors

• Agrawal Amit
• Thornton Jay

Classifications

• G01D5/2455

  with incremental and absolute tracks on the same encoder · CPC title

• G01D5/34792 Primary

  with only digital scales or both digital and incremental scales · CPC title

• G01D18/00 Primary

  Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00 · CPC title

• G01D5/24452

  Physics · mapped topic

• G01D18/001

  Calibrating encoders · CPC title