On line electromagnetic web thickness measuring apparatus incorporating a servomechanism with optical distance measuring

Apparatus for measuring the thickness of a travelling sheet of nonconductive material including in combination a first head, a second head, means mounting said heads in spaced relationship on opposite sides of said sheet with the first head in contact with one surface of the sheet and the second head out of contact with the sheet, a ferromagnetic core, a winding carried by said core, means mounting said core on one of said heads, a body of ferromagnetic material, and a noncontacting servomechanism mounting said body on the other head, said noncontacting servomechanism automatically maintaining said body a predetermined distance from said sheet whereby the reluctance of said coil is a measure of the distance between said body and said coil, said servomechanism comprising optical means for measuring the distance between said body and the surface of said sheet adjacent to said other head and means responsive to said optical means for positioning said body whereby the reluctance of said coil is a measure of the thickness of the sheet.

Apparatus for measuring the thickness of a sheet of nonconductive material including in combination interacting first and second electromagnetic means for producing an indication which is a measure of the distance therebetween, said first and second electromagnetic means being disposed on opposite sides of said sheet, a first noncontacting servomechanism for automatically maintaining said first electromagnetic means at a constant distance from one surface of said sheet, said first servomechanism comprising optical means for producing a first positioning signal as a measure of the distance between said first electromagnetic means and said one surface of said sheet and means responsive to said first positioning signal for positioning said first electromagnetic means, and a second noncontacting servomechanism for automatically maintaining said second electromagnetic means at a constant distance from the other surface of said sheet whereby the indication produced by said first and second electromagnetic means is an indication of the thickness of said sheet, said second servomechanism comprising optical means for producing a second positioning signal as a measure of the distance between said second electromagnetic means and said other surface of said sheet and means responsive to said second positioning signal for positioning said second electromagnetic means.

A method for measuring the thickness of a nonconductive sheet having opposite surfaces with a pair of interacting elements adapted to produce a sensible indication of the distance therebetween including the steps of positioning said interacting elements respectively adjacent to said opposite sides of said sheet, maintaining one of said interacting elements at a fixed distance from one surface of said sheet, optically measuring the distance between the other surface of said sheet and the other interacting element and maintaining said other interacting element at a constant distance from said other sheet surface in response to said measuring step whereby said indication is a measure of sheet thickness.