Web alignment system

A system for maintaining lateral alignment of web travelling along a path comprising

• A. a light beam source,
• B. means for scanning the light beam from the source along a track across the web path so that the beam is intercepted by web only on those portions of the track where web is present, said scanning means including
• 1. a parabolic reflector positioned opposite the web path,
• 2. a mirror positioned at the focal point of the reflector and oriented to direct the light beam from the light beam source toward the reflector, and
• 3. means for rotating the mirror at a constant rate so that the circular sweep of light from the mirror is converted by the reflector to rectilinear light that scans along said track at a constant incident angle
• C. means positioned to detect the beam scanning along the track and producing an output signal depending upon the presence or absence of web,
• D. means for detecting when the beam scans through a selected position on the track and producing a second output signal in response thereto, and
• E. means for producing a web position error signal dependent upon the time interval between the first and second signals.