Detection of marks repetitively placed at lengthwise intervals along a web

A method of detecting a predetermined mark made at nominally known intervals along a web of material, comprising the steps of:

• moving the web lengthwise past a detection apparatus;
• initiating a mark detection procedure to capture a virtually instantaneous image of a portion of the web at predetermined moments;
• comparing the position in the lengthwise direction of the web at the relevant moment for each mark detected to a position predicted from the detection of a previously detected mark to determine the difference between the detected and the predicted positions;
• and
• predicting the moment of initiating the detection procedure to capture an image of each detected mark using the difference between the detected and predicted positions of said previously detected mark.

A method of generating discrete signals representative of the positions of a series of marks spaced at nominally known intervals along a paper web that is moving lengthwise, comprising the steps of:

• a) capturing an image of a given mark of said series at a predetermined moment;
• b) processing the captured image to compare the lengthwise position of said given mark at said predetermined moment to a lengthwise position predicted therefor as a result of processing of a previously captured image of a predecessor mark in said series, and generating a signal representative of the difference between the captured and predicted positions of said given mark;
• and
• c) predicting the moment for capturing an image of a subsequent mark of said series using said signal representative of the difference between the captured and predicted lengthwise positions of the given mark;
• wherein said predetermined moments are established in terms of length of web run, and said predicted moments and detected positions are determined in terms of distance along the web;
• wherein each predicted position is established as a count value, and said method further comprising adjusting said count value in accord with difference between the predicted position and detected position, and in which said predetermined moment of capturing said image of said given mark of said series is determined by counting a number of length increment pulses to a value determined by a current adjusted count value;
• and
• wherein the counting of length increment pulses re-starts at each predetermined moment and the next predetermined moment is that at which the number of length increment pulses counted equals the current adjusted count value.

A method of generating discrete signals representative of the positions of a series of marks spaced at nominally known intervals along a paper web that is moving lengthwise, comprising the steps of:

• a) capturing an image of a given mark of said series at a predetermined moment;
• b) processing the captured image to compare the lengthwise position of said given mark at said predetermined moment to a lengthwise position predicted therefor as a result of processing of a previously captured image of a predecessor mark in said series, and generating a signal representative of the difference between the captured and predicted positions of said given mark;
• and
• c) predicting the moment for capturing an image of a subsequent mark of said series using said signal representative of the difference between the captured and predicted lengthwise positions of the given mark;
• wherein said predetermined moments are established in terms of length of web run, and said predicted moments and detected positions are determined in terms of distances along the web;
• wherein each predicted position is established as a count value and said method further comprising adjusting said count value in accord with difference between the predicted position and detected position, and in which said predetermined moment of capturing said image of said given mark of said series is determined by counting a number of length increment pulses to a value determined by a current adjusted count value;
• and
• wherein the step of adjusting said count value is performed during a next, or a subsequent, predetermined moment.

A method of acquiring and tracking a series of predetermined marks made at nominally known intervals along a web of material in which:

• A) moving the web lengthwise past a detection apparatus;
• B) initiating a first mark acquisition procedure by:
• i) capturing an image of a first predetermined area in the plane of the surface to thereby image the portion of the web within that area at the moment of capture;
• ii) said predetermined area having a dimension in the lengthwise direction sufficient to ensure that one of said series of marks is within the captured image;
• iii) determining the position of said one mark in the lengthwise direction of the web with respect to a reference position within said predetermined area;
• and
• iv) predicting the position of a succeeding mark of said series using the offset of said one mark with respect of said reference position;
• C) initiating a second mark acquisition procedure by:
• i) capturing an image of a second predetermined area in the plane of the web to thereby image the portion of the web within that area at the moment of capture and defining the moment of capture in accord with the predicted position;
• ii) said second predetermined area having a dimension in the lengthwise direction less than that of said first predetermined area such that not more than one of said series of marks can be located within said second predetermined area at the moment of capture;
• iii) determining the position of said succeeding mark in the lengthwise direction with respect to a reference position within said second predetermined area;
• and
• iv) predicting the position of another succeeding mark of said series using the knowledge of the offset of the first-mentioned succeeding mark from the reference position in the second predetermined area;
• and
• D) repeating said second mark acquisition procedure for the other succeeding mark to predict the position of yet another succeeding mark and so on.

A method of acquiring a series of predetermined marks made at nominally fixed intervals along a web of material and interleaved with a second series of the same marks that are not equidistant between adjacent marks of the first series comprising the steps of:

• A) moving the web lengthwise past a detection apparatus;
• B) initiating a first mark acquisition procedure by:
• i) capturing an image of a predetermined area in the plane of the surface to thereby image the portion of the web within that area at the moment of capture;
• ii) said predetermined area having a dimension in the lengthwise direction sufficient to ensure that one of said series of marks is within the captured image;
• iii) determining the position of said one mark in the lengthwise direction of the web with respect to a reference position within said predetermined area;
• and
• iv) establishing the moment of image capture of a succeeding mark in the other series using the offset of said one mark with respect of said reference position, said moment being established such that the succeeding mark of the other series is offset to one side or the other of said reference position in dependence on whether said succeeding mark is in the first or second series;
• C) initiating a second mark acquisition procedure by:
• i) capturing, at said established moment, an image of a predetermined area in the plane of the web to thereby image the portion of the web within that area at the moment of capture;
• and
• ii) determining the offset of the position of said succeeding mark in the lengthwise direction with respect to said reference position to determine to which series the succeeding mark belongs.