Electric heating – Metal heating – By arc
Reexamination Certificate
2000-12-22
2003-04-08
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121690, C219S121720
Reexamination Certificate
active
06545250
ABSTRACT:
The invention relates to a method and an apparatus for, the laser machining of workpieces, a laser beam being two-dimensionally positionable with the aid of a laser machining head with respect to a surface of a workpiece and it being intended for the machining of the workpiece to take place with predeterminable contours. The corresponding contours should as far as possible be in the form of closed lines, it being possible for a wide variety of geometrical shapes to be concerned, such as circles, triangles, squares, rectangles and others.
For a laser machining operation, usually one positioning of the laser machining head, with the aid of which a laser beam can be deflected and shaped, takes place by a corresponding relative movement with respect to the workpiece. The positioning.may in this case take place by two-dimensional movement of the laser machining head or of the workpiece alone or by simultaneous corresponding movement of these two elements.
If the laser beam is consequently to be guided or deflected over the surface of a workpiece in a way corresponding to the respective predetermined contour, in order for example to carry out a welded connection or some other targeted local heat treatment, or to produce a correspondingly contoured opening in the workpiece by a cutting operation, problems arise, in particular in the case of small-format and filigree structures, and the required positioning accuracy cannot in every case be ensured by the conventional drive mechanisms for the laser machining head or the workpiece, in particular at high machining speeds.
Furthermore, distortion which has occurred as a consequence of previous machining steps on the workpiece may also lead to corresponding dimensional and positional errors, so that the required positioning accuracy cannot be achieved in every case.
If, for example, hollow profiles, in particular thin-walled hollow profiles, are to be butt-welded into openings of a metal sheet by means of a laser beam, as far as possible without filler material, extremely high positional accuracy is required in the alignment and movement of the laser beam, so that the latter must be guided very accurately along the contour which is predetermined as the gap between [lacuna] corresponding to the outer contour and dimensions of the hollow profile and the opening formed in a corresponding way in the metal sheet, in order to be able to produce an adequately solid and sealed connection between the metal sheet and the respective hollow profile. The distortion mentioned may in this case already lead to corresponding positioning and alignment errors, so that production errors may occur with a set of hollow profiles to be welded if no correction of possibly occurring dimensional and positional errors is carried out.
Known positioning aids are, for example, separate markings which can be sensed by suitable sensors which are arranged externally. With the aid of such elements, a corresponding relative movement is generally initiated in order to correct the positioning error which has occurred, which leads to increased expenditure of time since this usually has to be carried out in several stages. With these means, however, it is only possible with difficulty, if at all, to compensate for positioning errors as a consequence of distortion.
If known image processing systems are used for monitoring the positional accuracy, the cost for a corresponding installation increases and an image sensing system required for this purpose must be arranged in such a way that it can monitor the respective machining area under all conditions, so that it is essential to evaluate a perspective image with correspondingly increased computing power.
It is therefore the object of the invention to propose possible ways in which high positional accuracy can be ensured in the laser machining of workpieces in predetermined contours at low cost and with little effort.
This object is achieved according to the invention by the features of claim
1
for a corresponding method and the features of claim
9
for a corresponding apparatus. Advantageous embodiments and developments of the invention are obtained with the features specified in the subordinate claims.
The solution according to the invention falls back on a laser machining head known per se, which can be designed for beam guidance and shaping, at least one scanner mirror allowing a deflection of the laser beam in two dimensions being used in each case. This may, for example, be a cardanically suspended scanner mirror, allowing a deflection of the laser beam in two axes aligned orthogonally with respect to one another over the surface of a workpiece.
For cost reasons, and possibly to increase the deflecting speed of a laser beam, it is, however, also quite possible to use two separate scanner mirrors which are arranged one after the other in the path of rays of the laser beam.
In this case, each of the scanner mirrors has a pivoting drive and an angle encoder, which are connected to an electronic evaluation and control unit, so that a defined pivoting of the scanner mirror or mirrors is possible by an electronic control, with simultaneous feedback, i.e. transmission of the respective angular positions of the scanner mirror(s).
Optionally, a focusing element may be arranged in the laser machining head, but also outside it, in the path of rays of the laser beam, it being possible, likewise optionally, for this to be a simple lens or a zoom lens.
Added to this, an additional light source, for example a laser diode, is used, the light beam of which can likewise be directed onto the scanner mirror or mirrors in a focused form.
With the aid of this light beam, an actual-set value comparison is carried out before the actual laser machining with respect to the positioning of the workpiece to be machined with predeterminable machining contours with respect to the laser machining head. In this case, the laser beam is deflected with the aid of the scanner mirror or mirrors over the surface of the workpiece, light from the surface of the workpiece being reflected back during the deflection and directed via the scanner mirror or mirrors onto an optical detector, by which the intensity of the reflected light can be measured. Since the corresponding measured light intensity values of the angular position or positions of the one or more scanner mirror(s) are sensed in an assigned form, predetermined contours on the surface of the workpiece can be detected and locally assigned with their contour edges or contour limits by the fall and/or rise of the measured intensity of the reflected light.
Since not only the shape but also the size and position of the respective machining contour are stored in the evaluation and control unit, a set-actual value comparison can be carried out with the measured position and the stored position, and the measured deviation of the shape, size or position of the contour from the set values can be compensated by intervention in the control program for the deflection of the laser beam, without a relative movement of the laser machining head and/or workpiece having to be carried out.
In the simplest and most favorable case, it may be adequate for the positional determination of the respective contour on the surface of the workpiece to carry out a deflection of the light beam along one axis, it then being possible with such a light beam deflection for at least two contour edges or contour limits to be sensed. This fact can be utilized in particular in the case of symmetrically designed contours.
Generally, however, a deflection of the light beam over the, surface of the workpiece along at least two axes will be required to determine the actual position of the laser machining head with respect to the contour formed on the surface of the workpiece or arranged there.
The axes in which the light beam is deflected may in this case be aligned parallel to or at a known angle with respect to one another, it undoubtedly being particularly favorable in many cases to carry out a deflection along axes aligned orthogonall
Hartmann Andreas
Klotzbach Annett
Kretzschmar Frank
Morgenthal Lothar
Pollack Dieter
Elve M. Alexandra
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
Leydig , Voit & Mayer, Ltd.
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