| 摘要 |
Two detectors are used, each including an emitter directing a laser beam against the web (2) and a receiver (15) for detecting a laser beam shone at right angles through formaliner perforations (13) along the web edges. The receiver is capable of detecting a beam shone through perforations aligned with a pre-determined axis (LV1) extending through the perforation, or a beam shone through perforations aligned with a second axis (LV1) laterally displaced from the pre-determined axis due to lateral displacement of the moving web. Web length is calculated by measuring the distances between two perforations and comparing this with a pre-selected required web length. Any deviations from the required length results in a signal being sent to a unit used to control the web and ensure the right web length is obtained. |
