Measuring gauge with air bearing and resistant to tilt

Claim 1

In an apparatus for measuring a property of a material sheet, including a member connected to a mount means allowing movement of said member relative to said sheet, said member comprising a sensor means for responding to said sheet property and a substantially flat surface which is to be maintained at a constant distance from said sheet by means including a gas film interposed between said surface and said sheet, said gas film being maintained by a flow of gas from gas-directing means in said member connecting a gas-confining chamber in said member with said surface and maintaining said member substantially in a state of equilibrium with respect to said sheet but subject to tilting movement relative to said sheet, the improvement wherein said gas-directing means comrises at least one gas-discharging opening in said surface located at a position that is slightly greater than halfway towards the periphery of said surface along a line between the center and the periphery of said surface, the distance across said opening along said line being at least an order of magnitude smaller than the length of said line, said surface being impermeable to the flow of said gas except at the location of said opening, said gas-directing means comprising at least one channel inclined relative to said sheet for directing the flow of gas through said opening towards said sheet in the direction of said surface periphery with a resultant momentum comprising a first momentum component normal to said sheet and a second momentum component parallel to said sheet in the direction of the periphery that is adjacent said opening, said flow of gas being of a magnitude sufficient to maintain said member at said constant distance from said sheet when said member is in said state of equilibrium relative to said sheet, the inclination of said at least one channel and the location of said at least one opening being so related to the flow of gas from said opening to the periphery that substantially maximum variations in gas pressure occur at different points on said surface when said member is tilted relative to said sheet to produce a substantially maximum restorative action tending to restore said member to said state of equilibrium, while for the same inclination and location the amount of said restorative action first increases in a substantially linear relationship with an increase in the angle at which said member is tilted, tilted relative to said sheet and then increases more markedly in a substantially nonlinear relationship as said member further approaches contact with said sheet.