Apparatus for controlled perforation of moving webs with fixed focus laser beam

Claim 1

Apparatus for controllably perforating a moving web by a laser beam, comprising: a laser assembly for producing a pulsed beam in response to a pulsed excitation signal and for directing said beam to the web at a perforating station; a porosity feedback arrangement producing an error signal representative of the difference between actual web porosity as measured and desired web porosity as preset, said feedback arrangement including porosity sensing means for sensing the porosity of the perforated web and for providing a porosity signal corresponding to said sensed porosity, preset means for producing a preset signal corresponding to a desired web porosity, and a comparator circuit connected to said porosity sensing means and to said preset means for producing the error signal as a function of the difference between said porosity signal and said preset signal; velocity sensing means for producing a sensed velocity signal corresponding to actual web velocity; and a control circuit for said laser assembly, said control circuit connected to receive both the error signal and the sensed velocity signal, and operable to produce a continuous pulse train in response, and said control circuit connected to said laser assembly to provide a replica of said pulse train as said laser pulsed excitation signal, the repetition rate of said pulse train varying directly in response to the sensed velocity signal and the duration of each pulse in said pulse train varying in response to the error signal, whereby a constant number of elongated perforations per unit length are produced, with the length of the perforations dependent upon the porosity feedback error signal.