Electric heating – Metal heating – By arc
Reexamination Certificate
1999-01-22
2001-01-23
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830, C219S124340
Reexamination Certificate
active
06177649
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention generally relates to welding, brazing, metal fusion and the like (hereinafter “welding”), and in particular to a control system for welding that operates by means of infrared thermography.
In the welding of two metal plates disposed end-to-end with the use of a welding head which generates a high energy-density welding beam, e.g. a laser beam or electron beam, there are essentially three phases of the operation:
temperature rise in the material;
fusion of a certain amount of the metal; and
cooling of the welded joint and environs.
The time, spatial location, and temperature are the three variables which define the thermal aspects of the process; i.e., for fixed conditions of welding, at a given instant a point disposed in the zone being welded has a given “thermal level” giving rise to IR emissions, and representing enthalpy or the like.
Accordingly, analysis of the thermal image of the weld during the welding process enables one to monitor the quality of the welded joint and to control the quality of the welding process, in real time, amid changing variables.
It is known to achieve such monitoring and control using an infrared-sensitive camera (IR camera) which indicates the thermal profile prior to, during, and/or after the fusion.
Depending on the type of information sought, the IR camera may observe the zone immediately ahead of the advancing zone of fusion; this enables one to have, e.g., a thermal image which can be used for guiding the welding head. The trough in the thermal profile transverse to the plane of the joint represents the gap separating the two metal plates to be welded together. The IR camera may also observe the thermal image at the fused mass of metal in order to indicate the temperature at the surface of the metal, and the width of the molten mass. Or the IR camera may observe the zone behind the fusion zone so as to determine the depth of penetration and variations in the welding process.
Three forms of visual display may be provided for the thermal images of the zone observed by the IR camera along the length of the weld:
a longitudinal thermal plot;
a transverse profile; and
a three-dimensional thermal surface.
The IR cameras used in this type of application may be of a scanning type wherein the elementary field is moved by optomechanical means or electronic means. Optomechanical means include complete rotation of a system, oscillation of a plane mirror, spinning rotation of a drum bearing a polygonal mirror (i.e. a mirror in the shape of a polygonal parallelepiped), rotation (e.g. spinning rotation) of a refractive polygonal prism, rotation of a drum bearing lenses, use of a diaporameter, and use of a linear bank of detectors. The latter arrangement using a linear bank of detectors has the advantage of not requiring mechanical movements. One may also employ a “matrix camera”.
The aforesaid known method of monitoring welds and controlling the welding process with the use of IR thermography is described in the following publication articles:
La Thermographie Infrarouge,
pub. Technique et Documentation Lavoisier, 3rd Ed. 1989;
Capteurs Infrarouges: Le Soudures Analys{acute over (e)}es En Temps Reel—Infrared Detectors—Real-Time Analysis Of Welding,
in the journal
Mesures
, Jan. 19, 1987.
In order to be able to make practical use of the images produced by the IR camera for monitoring welds and controlling the welding process in view of the high temperature gradients present in welding (which are typically several hundred degrees Centigrade per millimeter), one must have full and accurate control over the position of the field of view of the camera with respect to the point of impact of the welding beam, in terms of:
longitudinal position along the axis of the weld (along the plane of the joint);
position transverse to said axis; and
focusing of the optics of the camera.
If one does not perfectly control the positioning of the field of view of the camera with respect to the point of impact of the welding beam, it will be difficult to interpret the thermal plots obtained because they will not represent the field supposed but rather a neighboring zone.
Thus, e.g., if the field of view of the camera is theoretically 5 mm behind the point of impact of the welding beam along the axis of welding but the actual field of view is only 3 mm behind said point of impact, the interpretation of the thermal image in terms of the maximum temperature may result in one unwarrantedly reducing the power of the beam, thereby leading to the delivery of welded plates in which the weld is of poor quality. If the actual field of view of the camera is shifted transversely to the welding axis compared to the supposed field of view, the thermal profile will be shifted and will be interpreted as a shifting of the welding axis with respect to the plane of the joint of the plates being welded. Corrective shifting of said welding axis will not restore the welding axis as supposed but rather will shift the actual welding axis to a laterally incorrect locus even though the thermal profiles signaled by the camera will now be interpreted as perfectly centered.
Thus, it is important to have full and accurate control of the position of the field of view of the camera with respect to the point of impact of the welding beam. This can be achieved by achieving accurately reproducible regulation of the position of the thermographic IR camera relative to the welding head; i.e. such that said relationship is preserved with each regulative action.
One may consider regulating both the position of the welding head and the position of the camera with respect to a reference point fixed in space, e.g. a point on the housing of the welding machine. In practice, such a reference point and frame of reference are indeed reliable in providing a means of regulating the relative position of the welding head and the camera; however, they cannot guarantee constancy of the exact position of the zone of the metal plates observed by the camera relative to the point of impact of the welding beam on said plates, because of variabilities due to tolerances in the dimensions of various components which are components of the camera and of the welding head, and because of possible time wise variations in the optical paths of these apparatuses.
SUMMARY OF THE INVENTION
The object of the invention is a method of regulating the geometric position of a camera in a thermal or thermographic control and/or monitoring arrangement associated with a system for welding together of at least two metal plates, which method guarantees accurate positioning of the zone of the plates observed by the camera, particularly with respect to the point of impact of the welding beam on said plates, wherewith the above-mentioned problems of tolerances and of variations of optical paths are avoided.
The inventive method which accomplishes the stated object of the invention is a method of regulating the position of a camera in a thermal or thermographic control and/or monitoring arrangement associated with a system for welding together of at least two metal plates along a joint plane describing the joint of the plates being welded, wherein the position of said camera is regulated with respect to a welding head, and wherewith the welding apparatus comprises:
a welding head which generates a high energy-density welding beam, e.g. a laser beam or electron beam, and
a camera for thermographically monitoring the welding being carried out by means of said beam, which welding head and camera are both supported by a frame member; wherewith, in the subject method,
in a first step, the welding beam is moved into a reference position with respect to a point source of light disposed in a zone of the welding apparatus or installation, which zone is accessible by the welding beam, which reference position is in a plane P close to or coincident with the “median” plane between the two planes containing the respective principal faces of the metal plates which faces are directed generally toward the welding head and the camer
Juret Thierry
Philippe Jean
Cole Thomas W.
Evans Geoffrey S.
Nixon & Peabody LLP
Sollac
LandOfFree
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