Web inspection system having enhanced video signal preprocessing

Abstract

A web inspection system that inspects a moving web for defects has an enhanced video signal preprocessing capability. The enhanced video signal preprocessing is provided by a preprocessing circuit which has a maximum signal-to-noise (S/N) ratio that is no greater than an inherent maximum S/N ratio of a raw video signal as generated by a charge-coupled device CCD camera. The preprocessing circuit is preferably DC-coupled to the CCD camera to receive the raw video signal and generate an analog filtered video signal using an adaptive feedback loop that preserves a fidelity of the raw video signal in generating the filtered video signal. By improving the video signal preprocessing, the overall defect detection consistency of the web inspection system is improved.

I claim: 
     
       1. A web inspection system for inspecting a moving web for defects comprising: a charge-coupled device (CCD) camera that uses a scan array comprised of a plurality of pixel photosites to produce an analog raw video signal representative of the moving web, the raw video signal including a sync pulse for each scan period and having an inherent maximum signal-to-noise (S/N) ratio for a pixel resolution of the raw video signal during each scan period;   a preprocessing circuit operatively coupled to the CCD camera to receive the raw video signal and generate an analog filtered video signal for each scan period that has a preprocessing maximum S/N ratio at the pixel resolution of the raw video signal that is no greater than the inherent maximum S/N ratio of the raw video signal; and   a post processing system operatively coupled to the preprocessing circuit to receive the filtered video signal and evaluate the filtered video signal to detect video signals representative of defects in the moving web that are as small as a scan area in the moving web that is scanned by a single one of the plurality of pixel photosites.   
     
     
       2. The web inspection system of claim 1 wherein the maximum preprocessing S/N ratio of the analog filtered video signal as compared to the raw video signal is less than 5%. 
     
     
       3. The web inspection system of claim 1 wherein the maximum preprocessing S/N ratio of the analog filtered video signal as compared to the raw video signal is less than 2.5%. 
     
     
       4. The web inspection system of claim 1 wherein the scan area is less than 40.32 mm 2  (0.0625 in 2 ) and the consistency of the web inspection system is less than one pixel variance/billion pixels scanned. 
     
     
       5. The web inspection system of claim 4 wherein the scan area is less than 1 mm 2  (0.0016 in 2 ) and the consistency of the web inspection system is less than one pixel variance/twenty five billion pixels scanned. 
     
     
       6. The web inspection system of claim 1 wherein the moving web comprises generally planar material which has a velocity relative to the CCD camera, the planar material being, selected from the group consisting of a substantially continuous web of rolled material or a series of discontinuous sheets of flat material. 
     
     
       7. The web inspection system of claim I wherein the CCD camera is selected from a group consisting of an area scan camera and a line scan camera. 
     
     
       8. A web inspection system for inspecting a moving web for defects comprising: a charge-coupled device (CCD) camera that uses a scan array comprised of a plurality of pixel photosites to produce an analog raw video signal representative of the moving web, the raw video signal including a sync pulse for each scan period;   a preprocessing circuit DC-coupled to the CCD camera to receive the raw video signal and generate an analog filtered video signal for each scan period using an adaptive feedback loop that preserves a fidelity of the raw video signal in generating the filtered video signal; and   a post processing system operatively coupled to the preprocessing circuit to receive the filtered video signal and evaluate the filtered video signal for the existence of video signals representative of defects in the moving web,   such that the web inspection system provides for consistent detection of defects in the moving web.   
     
     
       9. The web inspection system of claim 8 wherein at least one pixel photosite is internally masked within the CCD camera and the adaptive feedback loop utilizes a portion of the raw video signal corresponding to the at least one pixel photosite that is internally masked to remove a dark level DC offset in the raw video signal. 
     
     
       10. The web inspection system of claim 9 wherein the adaptive feedback loop comprises: a summing node that receives the raw video signal and combines an error signal with the raw video signal to produce a zeroed video signal;   means for sampling selected portions of the zeroed video signal corresponding to the least one pixel photosite that is internally masked; and   a difference integrator that compares the selected portion of the zeroed video signal with a zero voltage reference to produce the error signal.   
     
     
       11. The web inspection system of claim 10 wherein the moving web comprises generally planar pieces of material which have a velocity relative to the CCD camera, the planar pieces of material being selected from the group consisting of a substantially continuous web of rolled material or a series of discontinuous sheets of flat material and wherein the CCD camera is selected from a group consisting of an area scan camera and a line scan camera. 
     
     
       12. The web inspection system of claim 8 wherein the preprocessing circuit includes a video gain circuit within the adaptive feedback loop. 
     
     
       13. The web inspection system of claim 8 wherein the raw video is comprised of an even pixel video signal and an odd pixel video stream and wherein the preprocessing circuit includes a pair of adaptive feedback loops, a first adaptive feedback loop to process the even pixel video signal and generate a corrected even pixel video signal and a second adaptive feedback loop to process the odd pixel video signal and generate a corrected odd pixel video signal. 
     
     
       14. The web inspection system of claim 8 wherein the raw video is comprised of an even pixel video signal and an odd pixel video stream and wherein the preprocessing circuit further comprises: an automatic pixel balancing circuit that balances the even pixel video signal with the odd pixel video signal.   
     
     
       15. The web inspection system of claim 14 wherein the automatic pixel balancing circuit comprises: a first analog-to digital (A/D) converter for converting the even pixel video signal to a digital sequence of even pixel values using a fixed reference voltage value;   a second analog-to digital (A/D) converter for converting the odd pixel video signal to a digital sequence of odd pixel values using a variable reference voltage value;   a first digital-to-analog (D/A) converter for converting the digital sequence of even pixel values to an even feedback signal;   a second digital-to-analog (D/A) converter for converting the digital sequence of odd pixel values to an odd feedback signal; and   a difference integrator circuit that compares the odd feedback signal to the even feedback signal and generates the variable reference voltage.   
     
     
       16. The web inspection system of claim 8 wherein the raw video is comprised of an even pixel video signal and an odd pixel video stream and wherein the preprocessing circuit further comprises: a pixel recombining circuit for multiplexing the even pixel video signal and the odd pixel video signal to generate the filtered video signal.   
     
     
       17. The web inspection system of claim 16 wherein the pixel recombining circuit includes: a first analog-to digital (A/D) converter for converting the even pixel video signal to a digital sequence of even pixel values   a second analog-to digital (A/D) converter for converting the odd pixel video signal to a digital sequence of odd pixel values;   a multiplexer for sequentially combining the digital sequence of even pixel values with the digital sequence of odd pixel values to produce a digital sequence of even and odd pixel values; and   a digital-to-analog (D/A) converter for converting the digital sequence of even and odd pixel values to the analog filtered video signal.   
     
     
       18. The web inspection system of claim 8 wherein the raw video is comprised of an even pixel video signal and an odd pixel video stream and at least one pixel photosite for the even pixel video signal and one pixel photosite for the odd pixel video signal is internally masked within the CCD camera and wherein the preprocessing circuit comprises: a pair of separate adaptive feedback loops, a first adaptive feedback loop to process the even pixel video signal and a second adaptive feedback loop to process the odd pixel video signal, each adaptive feedback loop utilizing a portion of the raw video signal corresponding to the at least one pixel photosite that is internally masked to remove a dark level DC offset in the raw video signal and generate a zeroed video signal;   an automatic pixel balancing circuit that balances a zeroed even pixel video signal with a zeroed odd pixel video signal to generate a balanced even pixel video signal and a balanced odd pixel video signal; and   a pixel recombining circuit for multiplexing the balanced even pixel video signal and the balanced odd pixel video signal to generate the filtered video signal.   
     
     
       19. The web inspection system of claim 18, wherein: each adaptive feedback loop includes: a summing node that receives the raw video signal and combines an error signal with the raw video signal to produce a zeroed video signal;   a video gain circuit that selectively amplifies the zeroed video signal to produce an amplified and zeroed video signal;   means for sampling selected portions of the amplified and zeroed video signal corresponding to the at least one pixel photosite that is internally masked; and   a difference integrator that compares the selected portion of the amplified and zeroed video signal with a zero voltage reference to produce the error signal;     the automatic pixel balancing circuit comprises: a first analog-to digital (A/D) converter for converting a zeroed and amplified even pixel video signal to a digital sequence of even pixel values using a fixed reference voltage value;   a second analog-to digital (A/D) converter for converting a zeroed and amplified odd pixel video signal to a digital sequence of odd pixel values using a variable reference voltage value;   a first digital-to-analog (D/A) converter for converting the digital sequence of even pixel values to an even feedback signal;   a second digital-to-analog (D/A) converter for converting the digital sequence of odd pixel values to an odd feedback signal; and   a difference integrator circuit that compares the odd feedback signal to-the even feedback signal and generates the variable reference voltage; and     the pixel recombining circuit includes: a multiplexer for sequentially combining the digital sequence of even pixel values from the first A/D converter with the digital sequence of odd pixel values from the second A/D converter to produce a digital sequence of even and odd pixel values; and   a digital-to-analog (D/A) converter for converting the digital sequence of even and odd pixel values to the analog filtered video signal.     
     
     
       20. A preprocessing circuit for web inspection system that inspects a moving web for defects, the web inspection system including a charge-coupled device (CCD) camera that uses a scan array comprised of a plurality of pixel photosites to produce an analog raw video signal representative of the moving web, the raw video signal being comprised of an even pixel video signal and an odd pixel video stream and at least one pixel photosite for the even pixel video signal and one pixel photosite for the odd pixel video signal being internally masked within the CCD camera, each of the even pixel video signal and the odd pixel video signal including a sync pulse for each scan period and having an inherent maximum signal-to-noise (S/N) ratio, and a post processing system that receives a filtered video signal and evaluates the filtered video signal to detect video signals representative of defects in the moving web that are as small as a scan area in the moving web that is scanned by a single one of the plurality of pixel photosites, the preprocessing circuit comprising: a pair of separate adaptive feedback loops, a first adaptive feedback loop DC-coupled to the CCD camera to process the even pixel video signal and a second adaptive feedback loop DC-coupled to the CCD camera to process the odd pixel video signal, each adaptive feedback loop utilizing a portion of the raw video signal corresponding to the at least one pixel photosite that is internally masked to remove a dark level DC offset in the raw video signal and generate a zeroed video signal;   an automatic pixel balancing circuit that balances a zeroed even pixel video signal with a zeroed odd pixel video signal to generate a balanced even pixel video signal and a balanced odd pixel video signal; and   a pixel recombining circuit that multiplexes the balanced even pixel video signal and the balanced odd pixel video signal to generate the filtered video signal for each scan period,   such that the filtered video signal has a preprocessing maximum S/N ratio no greater than the inherent maximum S/N ratio of the raw video signal.