Electricity: measuring and testing – Magnetic – Combined
Patent
1984-09-11
1987-03-03
Karlsen, Ernest F.
Electricity: measuring and testing
Magnetic
Combined
324229, 73 377, G01B 710, G01B 1304, G01N 2772
Patent
active
046478558
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to apparatus for measuring the thickness of a moving web, there being two axially opposite measuring heads, with the web insertable between them, the measuring head being axially movable, supplied with a pressurized fluid and provided with ejection means for urging the pressurized fluid in a direction towards the web for maintaining a constant distance between the respective measuring head and the web, electrical transducer means being disposed in the heads for generating signals in response to the distance between the heads and thereby to the thickness of the web.
BACKGROUND OF THE INVENTION
Apparatus functioning according to such a principle is illustrated in U.S. Pat. No. 4,107,606. This publication only illustrates the said principle very schematically, and is more specifically directed towards a construction for electrical distance measurement between two measuring heads. How these are to be designed and implemented is not described in detail particularly as to the maintenance of distance provided pneumatically.
Another apparatus of the same kind is disclosed in U.S. Pat. No. 3,528,002, although what is described in detail really consists of a fixed measuring head, from which the web is caused to maintain a constant distance while only the opposing head is movable. The distance maintenance is described as taking place by compressed air being caused to emanate through a porous plug, for generating an air cushion and a lifting force counteracted by a constant counterforce, which is illustrated as being generated by a relatively heavy mass. It is obvious that this is a great disadvantage if a rapidly-acting thickness meter is desired, since large mass results in large inertia.
Another prior art method for measuring thickness utilizes a single movable transducer, the position of the web being maintained fixed against an underlying plate, e.g., a suction plate, while the movable transducer is kept by means of a pneumatic distance maintenance at a constant distance from the other side of the web. Such a device is known from U.S. Pat. No. 3,818,327, the distance between the suction plate and transducer being measured electromagnetically. A similar device is known from U.S. Pat. No. 3,617,872. With regard to the movable transducer, it is disclosed in both cases that an air cushion generated by compressed air is used together with a relatively heavy structure, the gravitational force of which counteracts the air pressure.
A substantially lighter structure for a transducer having pneumatic distance maintenance is known from applicant's own Swedish Published Specification No. 7900795-1. In this structure there is a light, movable and hollow measuring means, which is carried in air bearings, is axially movable in a holder, has at one end an air-blowing nozzle and at the other end a disc arranged with annular clearance in an outwardly open cylindrical chamber, the portion of the cylindrical chamber partially closed off by the disc being supplied with compressed air such that a counterforce acts on the measuring means for urging it in a direction away from the measured surface. This structure functions relatively well, but it would be desirable to make it faster in assuming its state of equilibrium, and to reduce the distance between nozzle and measuring surface.
OBJECT OF THE INVENTION
One object of the present invention is to provide an apparatus for measuring the thickness of a moving web of the kind described above, the apparatus having the ability of reacting very quickly, both measuring heads being able to follow the web, one on either side thereof, even if the web itself oscillates at right angles to its plane. It is then necessary to have a large directional force for keeping the constant distance between the measuring head and web on either side of the web, i.e., for an incident change in the distance between the web surface and the measuring head, there must occur a large corrective force between the web surface and the measuring head, and the measuring
REFERENCES:
patent: 4325249 (1982-04-01), Berglund
patent: 4450404 (1984-05-01), Williams et al.
Karlsen Ernest F.
Snow Walter E.
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