Bi-directional registration of servo indexed webs

Abstract From the Canadian patent

A bag machine fabricates plastic bags by transversely cutting and sealing, at regularly spaced preselected locations, an elongate plastic web having graphic material repetitively printed thereon. In operation, the bag machine advances the web by a calculated draw length, and an optical sensor sensitive to the passage of eyemarks printed in regularly spaced locations on the web, functions to ensure that the web is cut and sealed only at the desired locations. To avoid inaccuracies resulting from a gradual variance between the nominal draw length and the actual distance between eyemarks, a control system monitors the actual distance between eyemarks and sets the nominal draw length to the …