Application Stories - Sensors and Instruments in Industrial Automation

Web processing combines linear motion (the web) with rotary motion (the tool) to print, cut or form the impression from the rotating tool on the moving web. For example, pressure sensitive labels used for mailing are first printed on a continuous web and then die cut to remove the unused margin and perforated to form individual labels. The first operation, printing, includes a registration mark (eye mark) used to register the subsequent two operations. Die cutting and perforation must be coordinated with printing so that the final product is correct.

The registration control system uses a registration mark scanner (Scanning DevicesSeries 410) to detect the position of the web and define a reference for the process. It uses a rotary encoder mechanically linked to the tool to determine the position of the tool. The controller establishes a registration target - the tool's rotational position when the eye mark is detected by the scanner.

The controller keeps track of the tool position by counting encoder pulses generated by the tool's rotation. When each eye mark is detected, the controller records the tool position, compares it to the target and generates a series of correction outputs to slow down (retard) or speed up (advance) the tool position.

The type and complexity of correction depend on the process, degree of precision required and capability of the press. Minimal registration controls provide a timed output each time registration position deviates from a defined acceptance band. More complex controls generate multiple proportional outputs, with outputs varying by the type and size of the registration error.