Method of synchronizing a line scan camera
A method of synchronizing a line scan camera. The method comprises: obtaining line scan data of a region of interest (ROI) of a travelling surface from the line scan camera, the line scan camera being oriented perpendicular to a direction of travel of the travelling surface, the line scan data comprising a plurality of lines; identifying occurrences of a major frequency of a repeated texture on the travelling surface using characterized line scan data for each line in the plurality of lines of the line scan data; determining a period of the major frequency; and changing a line rate of the line scan camera when the determined period is different than a reference period.
1. A method of synchronizing a line scan camera, comprising:
obtaining line scan data of a region of interest (ROI) of a travelling surface from the line scan camera, the line scan camera being oriented perpendicular to a direction of travel of the travelling surface, the line scan data comprising a plurality of lines;
identifying occurrences of a major frequency of a repeated texture on the travelling surface in the direction of travel of the travelling surface using characterized line scan data for each line in the plurality of lines of the line scan data, wherein the characterized line scan data for each line is a statistical measure of the pixel values of each line of the line scan data, each of the pixel values being a digital number;
determining a period of the major frequency; and
changing a line rate of the line scan camera in response to a determination that the determined period is different than a reference period.
2. The method of claim 1 , wherein the major frequency of the repeated texture is determined using autocorrelation.
3. The method of claim 1 , wherein the major frequency of the repeated texture comprises:
performing a Fast Fourier Transform (FFT) on the characterized line scan data to obtain an FFT spectrum;
multiplying the FFT spectrum by a complex conjugate of the FFT spectrum to obtain a product;
performing an inverse FFT (IFFT) on the product to obtain an IFFT sequence; and
determining local peaks in the IFFT sequence.
4. The method of claim 3 , wherein determining the period of the major frequency comprises:
determining the difference between the local peaks as occurrences of the major frequency of the repeated texture on the travelling surface; and
determining the mean of the differences as the period of the major frequency.
5. The method of claim 1 , wherein the ROI is a portion of a field of view (FOV) of the line scan camera.
6. The method of claim 5 , wherein the ROI is located towards an edge of the FOV of the line scan camera.
7. The method of claim 1 , further comprising:
maintaining the line rate of the line scan camera in response to a determination that the determined period is the same as the reference period.
8. The method of claim 1 , wherein changing the line rate of the line scan camera in response to a determination that the determined period is different than a reference period comprises:
increasing the line rate of the line scan camera in response to a determination that the determined period is lower than the reference period; and
decreasing the line rate of the line scan camera in response to a determination that the determined frequency is higher than the reference period.
9. The method of claim 1 , further comprising:
characterizing the line scan data obtained from the line scan camera.
10. The method of claim 9 , wherein characterizing the line scan data comprises applying a noise reduction filter.
11. The method of claim 9 , wherein characterizing the line scan data comprises determining a mean value of the digital number of the pixel values of each line of line scan data.
12. The method of claim 1 , further comprising:
before characterizing the line scan data, performing edge enhancement on the line scan data obtained from the line scan camera.
13. The method of claim 1 , wherein the reference period is the period of the major frequency in horizontal direction if the textures are equal in horizontal and vertical directions, or the period of the major frequency in vertical direction if the textures are not equal in horizontal and vertical directions.
14. The method of claim 1 , wherein the reference period is determined in real-time or in training prior to real-time operations.
15. The method of claim 1 , wherein the major frequency is a vertical frequency of the repeated texture on the travelling surface in the direction of travel of the travelling surface.
16. The method of claim 1 , further comprising:
identifying occurrences of a major horizontal frequency of the repeated texture on the travelling surface in the direction of perpendicular to the travel of the travelling surface using characterized line scan data for each line in the plurality of lines of the line scan data;
determining a period of the major horizontal frequency; and
setting the period of the major horizontal frequency as the reference period.
17. A line scan camera, comprising:
a line scan sensor comprising one or more lines of pixels;
a controller coupled to the line scan sensor and configured to cause the line scan camera to:
obtain line scan data of a region of interest (ROI) of a travelling surface from the line scan camera, the line scan camera being oriented perpendicular to a direction of travel of the travelling surface, the line scan data comprising a plurality of lines;
identify occurrences of a major frequency of a repeated texture on the travelling surface in the direction of travel of the travelling surface using characterized line scan data for each line in the plurality of lines of the line scan data, wherein the characterized line scan data for each line is a statistical measure of the pixel values of each line of the line scan data, each of the pixel values being a digital number;
determine a period of the major frequency; and
change a line rate of the line scan camera in response to a determination that the determined period is different than a reference period.
18. The line scan camera of claim 17 , wherein the line scan camera is a single line, line scan camera.
19. The line scan camera of claim 17 , wherein the controller is a field-programmable gate array.
20. A non-transitory machine readable medium having tangibly stored thereon executable instructions for execution by a processor of a line scan camera, wherein the executable instructions, in response to execution by the processor, cause the line scan camera to:
obtain line scan data of a region of interest (ROI) of a travelling surface from the line scan camera, the line scan camera being oriented perpendicular to a direction of travel of the travelling surface, the line scan data comprising a plurality of lines;
identify occurrences of a major frequency of a repeated texture on the travelling surface in the direction of travel of the travelling surface using characterized line scan data for each line in the plurality of lines of the line scan data, wherein the characterized line scan data for each line is a statistical measure of the pixel values of each line of the line scan data, each of the pixel values being a digital number;
determine a period of the major frequency; and
change a line rate of the line scan camera in response to a determination that the determined period is different than a reference period.