Laser processing head and method for processing a workpiece by means of a laser beam

1. A laser processing head for processing a workpiece by means of a laser beam, comprising:

a housing through which a beam path for the laser beam is guided, and which has a focusing optics for focusing the laser beam onto a joint of the workpiece to be processed,

a light cutting device fitted on the housing having a light source for generating on the workpiece a light line which cuts a location to be joined at a predetermined distance (d) from the joint,

a camera with an optical bandpass filter arranged in an observation beam path in front of the camera, the light source having an at least local emission maximum in the wavelength passband of the bandpass filter, and the optical bandpass filter being configured so that the laser beam is not transmitted, and the camera imaging the light line at the location to be joined as well as the portion transmitted by the bandpass filter of the process light of a melt pool, produced by the laser beam impinging on the workpiece to be processed, at the joint at regular time intervals, and

a processing unit with a memory for receiving the recorded image data from the camera, which is designed

to determine, by image processing of the received image data, the intersection point of the light line with the location to be joined, as well as the midpoint of the melt pool at the joint,

to store the intersection points between light line and location to be joined as target trajectory in the memory, and

to determine a minimum distance between the stored target trajectory and the current melt pool midpoint.

2. The laser processing head as claimed in claim 1 , wherein the optical bandpass filter is an interference filter.

3. The laser processing head as claimed in claim 2 , wherein the interference filter is a Fabry-Perot filter.

4. The laser processing head as claimed in claim 1 , wherein the optical bandpass filter has a wavelength passband whose full width at half maximum is preferably less than 50 nm.

5. The laser processing head as claimed in claim 4 , wherein the optical bandpass filter has a wavelength passband whose full width at half maximum is less than 20 nm.

6. The laser processing head as claimed in claim 4 , wherein the optical bandpass filter has a wavelength passband whose full width at half maximum is less than 10 nm.

7. The laser processing head as claimed in claim 1 , further comprising:

a comparator which is designed to output an error report upon overshooting of the minimum distance between target trajectory and current welding midpoint beyond a prescribed distance value.

8. The laser processing head as claimed in claim 1 , further comprising:

an actuator which is designed to control the minimum distance between target trajectory and current welding midpoint to zero by moving the laser beam transverse to a movement direction of the laser processing head by a controlling distance (a).

9. The laser processing head as claimed in claim 8 , wherein the actuator is designed to move the laser beam perpendicular to the movement direction by the controlling distance (a).

10. The laser processing head as claimed in claim 1 , characterized in that the camera comprises an image recording device that is designed to process the image data by means of an HDR method.

11. The laser processing head as claimed in claim 1 , further comprising a beam splitter that couples an observation beam path of the camera coaxially into the laser beam path.

12. The laser processing head as claimed in claim 1 , wherein the light cutting device is designed to generate by means of the light source a light fan that strikes the workpiece to be processed obliquely relative to the optical axis (L) of the laser beam so that a light line is generated on the workpiece, and determine a distance between the focusing optics and the workpiece by means of triangulation.

13. The laser processing head as claimed in claim 12 , wherein the light line projected onto the workpiece is straight and runs perpendicular to the movement direction of the laser processing head.

14. The laser processing head as claimed in claim 12 , wherein the light line projected onto the workpiece runs circularly around the point of impingement of the laser beam on the workpiece.

15. The laser processing head as claimed in claim 1 , characterized in that the light source is a laser, in particular a semiconductor laser.

16. A robot device with a laser processing head as claimed in claim 1 , which is designed to move the laser processing head so that the weld midpoint is guided along the target trajectory, the current movement direction of the laser beam striking the workpiece being set so that said movement direction points away from the current weld midpoint tangentially relative to the target trajectory.

17. A method for processing a workpiece by means of a laser processing head as claimed in claim 1 , by using a laser beam, having the steps of:

generating on the workpiece a light line that cuts a location to be joined at a predetermined distance (d) in front of a joint,

imaging the light line at the location to be joined as well as a melt pool ( 46 ), produced by the laser beam impinging on the workpiece, at the joint at regular time intervals by means of the camera, and

processing the image data produced by the camera, having the steps of

determining an intersection point of the light line with the location to be joined, as well as the midpoint of the melt pool,

storing the intersection points determined at different instants as target trajectory in a memory, and

determining a minimum distance between target trajectory and current melt pool midpoint.

18. The method as claimed in claim 17 , further having the step of controlling the minimum distance between target trajectory and current melt pool midpoint by moving the laser beam transverse to the movement direction of the laser beam to zero by a controlling distance (a).