Regulating surface treatment by laser beam

#The present invention relates to a method for regulating the position of the focusing plane in a treatment of parts by means of a focused laser beam.

#Numerous operations are known, in particular in mechanical engineering, in which a tool may be replaced by a focused laser beam. As an example there may be cited various machining operations, such as stamping or boring, assembly operations, such as welding, or surface heat treatment. In all these operations the laser beam is focused by a suitable optical system, such as a lens or a curved mirror; it thus forms a focusing cone whose apex, located in the focal plane of the optical system used, has the maximum light power density of the beam. For reasons of energy saving and operational precision, this maximum power point has to be positioned as accurately as possible with respect to the treated part; for example, in the case of surface treatment the focal point is usually brought into the plane tangential to the surface of the part at the point where the intended operation is to be performed.

#Although it is important that this point is positioned correctly with respect to the part to be treated, it is equally important that this relative position remains as constant as possible.

#The single FIGURE is a diagrammatic illustration of the manner in which the method of the present invention may be performed.

#A laser beam with a diameter of 20 mm and a wavelength of 10.6 .mu.m is focused by a focusing element comprising a lens whose focal length is 60 mm. Accordingly, the reference distance between the surface of a part to be treated by the focused beam and the point at which the axis of the beam passes through the median plane of the lens is 60 mm, when the apex of the focusing cone is on the said surface.

#Referring specifically to the drawing, consider a surface 1 of a workpiece to be treated moving in a direction of arrow 2. For example, the workpiece surface 1 may be heated by a laser beam 3. This laser beam passes through a lens 4 held in a frame 6. The lens focuses the laser beam 3 into a conical shaped side having an apex which must be located at the treatment point of surface 1. The lens 6 carries a sensor 7 for measuring the distance between the surface 1 and an arbitrary point of the lens assembly 4, 6. The measurement system is not part of the invention; therefore, any known system may be employed. The device illustrated in the drawing comprises a sensor which measures the distance between a point O, fixed with respect to the lens assembly, and the surface 1, in a direction parallel to the axis of the laser beam 2.

#It is well known in the art that the surface of a workpiece is never perfectly plane so that the representation of surface 1 in the drawing is a somewhat wavy profile. The method of the present invention involves regulating the location (i.e., distance) of the lens with respect to irregularities of the surface 1.