System for measuring the relaxed length of a moving web

Apparatus for measuring the relaxed repeat length L.sub.R of a moving web, comprising:

• a frame;
• path defining means connected to said frame for defining a web path;
• means for advancing the web along said web path;
• mark forming means for forming a plurality of detectable marks along the length of the web, said marks being formed at a nominal repetitive length interval along the web, whereby a nominal value for said relaxed repeat length may be selected such that said nominal relaxed repeat length is an integer multiple of said length interval;
• first and second detectors mounted to said frame and located along said web path, said detectors separated by a known distance S, each detector sensitive to movement there past of one of said detectable marks;
• means for alternatively applying to the moving web a first tensioning force and a second tensioning force of a known proportional relationship X;
• means for measuring the speed of the moving web V;
• control means connected to said first and second detectors, said tension applying means, and said web speed measuring means, said control means operating to:
• cause said tension applying means to apply said first tensioning force to the moving web;
• in response to detection by said first detector the movement of a mark there past, and detection by said second detector the next movement of a mark there past, measure the time t.sub.1 ;
• separating said detections and calculate a first tensioned repeat length L.sub.1 ;
• cause said tension applying means to apply said second tensioning force to the moving web;
• in response to detection by said first detector the movement of a mark there past, and detection by said second detector the next movement of a mark there past, measure the time t.sub.2 separating said detections and calculate a second tensioned repeat length L.sub.2 ;
• and
• calculate a relaxed repeat length from said first tensioned repeat length, said second tensioned repeat length, and said proportional relationship between said first and said second tensioning forces.