Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1996-11-04
1998-06-23
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
73159, 162263, 324231, G01B 706, G01B 1306, G01B 1312, D21F 706
Patent
active
057709491
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device for on-line measurements of the thickness of a continuously produced sheet. It is known that the industries producing continuous sheet material such as paper, rubber, plastics encounter an increasing need of a measuring device able to precisely indicate without direct contact the variations of thickness from edge to edge of the sheet while it is fabricated. In fact the increasing speed of production makes it not adequate the traditional off line tests which are normally done at the end of the roll with a micrometer on a cross machine strip.
For such a reason the world leading producers of on-line measurement systems for sheet material have been engaged for several years in the development of thickness sensors capable of traversing continuously from edge to edge of the sheet by meeting adequate accuracy and reliability requirements. The degree of accuracy needed nowadays for thickness sensors is in the order of a micrometer (10.sup.-6 m) or less for the thinnest sheets.
The technique generally used for this category of measuring devices is based on two opposite sensing heads comprising a magnetic sensor of relative distance capable of moving on the sheet without friction. It is to be noted that, being a basic requisite of this measuring device the continuous scanning from edge to edge of the sheet, the sensing heads must be mounted on two supports traversing on proper guides longer that the sheet width. It is worth saying that modern lines producing printing paper can be 10 meter wide and have a speed over 1000 meter/min.
In order to make the sensing heads frictionless and to avoid their contact with the sheet surface, the commonly adopted method is the air-cushion technique implemented by means of suitable air jets emitted from the surface of the sensing heads facing the sheet. The jets produce a thin layer of air under pressure which maintains the heads at a distance from the sheet eliminating the friction. In this technique the degree of accuracy of the measurements is mainly dependent on the degree of invariability and repeatability of the thickness of the air cushion.
A device functioning according to the above principle is illustrated in publication WO 84/02772. In this case each of the two opposite sensing heads is attached to a sliding piston moving axially with respect to the plane of the sheet, and it is urged toward the sheet by the force exerted by the feed air pressure in a pressurized rear chamber acting on the piston counteracting the repulsive force of a single frontal air cushion fed by the same pressure air conveyed through the hollow of the piston via its opened far end. Since the equilibrium of the two counteracting forces must be ensured at unchanged cushion gap, the repulsive force has to vary proportionally to the feed air pressure in the same manner as the urging force applied to the piston.
In this described embodiment the effective air cushion is delimited by the inner edge of the annular barrier which creates a restriction to the radial air flow causing a sharp drop of pressure between the internal area and the ambient. The sucking phenomenon occurring in the area immediately outside the edge, due to Bernoulli principle, is incidental and does not play any specific role, its sole effect being that of reducing marginally the repulsive force exerted by the internal positive pressure area.
The own weight of the floating heads should also be considered whenever a rigorous analysis of equilibrium of forces is to be done, but it has been considered negligible by this inventor because of the light floating head-piston assembly compared to the large counteracting pneumatic forces of several newtons.
It is an object of the present invention to provide a device of the above-mentioned type in which the thickness of the air cushion between sensing heads and the sheet to be measured is kept as constant as possible independently from the pressure of air fed.
This is obtained through the features of the measuring device of the within invention for scanni
REFERENCES:
patent: 3528002 (1970-09-01), Dunlavey
patent: 3855524 (1974-12-01), Crawford
patent: 4929895 (1990-05-01), Typpo
patent: 5132619 (1992-07-01), Typpo
patent: 5243849 (1993-09-01), Williams
Aeonic Systems Italia S.r.L.
Strecker Gerard R.
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