WORKPIECE PROCESSING MACHINE AND METHOD FOR PROCESSING A WORKPIECE, IN PARTICULAR BY WELDING

Abstract

A workpiece processing machine that includes: a beam emission head for providing a beam for processing the workpiece, an optical interferometer for splitting, redirecting, and detecting the beam, an adjustment element for changing a second portion of a power of the beam redirected from a retroreflector to a detector, and a control unit for actuating the adjustment element to control a ratio between a first power portion of the beam redirected from the workpiece to the detector and the second power portion of the beam redirected from the retroreflector to the detector to a target ratio.

Claims

1. A workpiece processing machine comprising: a beam emission head configured to provide a beam having a source power, the beam being configured to process a workpiece; an optical interferometer comprising: a beam splitter unit configured to split the source power of the beam between a reference path and a measurement path, wherein the workpiece is positioned within the measurement path, a retroreflector positioned within the reference path, and a detector configured to detect a total power of the beam redirected to the detector from the workpiece along the measurement path and from the retroreflector along the reference path, an adjustment element configured to change a second power portion of the total power of the beam redirected from the retroreflector to the detector; and a control unit configured to act on the adjustment element to control a ratio between a first power portion of the beam redirected from the workpiece to the detector and the second power portion of the beam redirected from the retroreflector to the detector.

2. The workpiece processing machine of claim 1, wherein the adjustment element is configured to change the splitting of the source power of the beam between the reference path and the measurement path.

3. The workpiece processing machine of claim 1, wherein the adjustment element is configured to change the power portion of the beam in the reference path or the power portion in the measurement path.

4. The workpiece processing machine of claim 3, wherein the adjustment element is configured to change a reflectivity of the retroreflector positioned within the reference path.

5. The workpiece processing machine of claim 3, wherein the adjustment element is configured to act on a focusing unit to change a focus position of the beam in the reference path.

6. The workpiece processing machine of claim 5, wherein the focusing unit comprises a movable lens displaceable along a beam axis of the reference path.

7. The workpiece processing machine of claim 3, wherein the adjustment element comprises an optical filter unit.

8. The workpiece processing machine of claim 3, wherein the adjustment element is a first adjustment element and the workpiece processing machine further comprises: a second adjustment element configured to change the source power of the beam source, wherein the control unit is configured to actuate the second adjustment element configured to control the total power redirected to the detector below a total power threshold value.

9. The workpiece processing machine of claim 1, wherein the optical interferometer comprises an optical coherence tomography unit.

10. The workpiece processing machine of claim 9, wherein the optical coherence tomography unit is configured to spectrally resolve detection of the total power reflected back to the detector.

11. The workpiece processing machine of claim 10, further comprising: an evaluation unit configured to determine at least one parameter of the spectrally resolved total power, wherein the control unit is configured to adapt at least one of the target ratio and the total power based on the at least one parameter.

12. The workpiece processing machine of claim 1, further comprising: a deflection unit configured to deflect the beam in the measurement path to different positions on the workpiece.

13. The workpiece processing machine of claim 12, wherein the deflection unit is configured to deflect the beam in the measurement path to a first position leading in relation to a position of the machining beam on the workpiece, to the position of the machining beam on the workpiece, and to a second position trailing in relation to the position of the machining beam on the workpiece.

14. The workpiece processing machine of claim 1, wherein the beam comprises a laser beam.

15. A method of processing a workpiece, the method comprising: generating a beam having a source power, splitting the source power of the beam between a reference path, comprising a retroreflector and a measurement path, detecting a total power of the beam, wherein the beam comprises a first beam that is redirected by the workpiece along the measurement path and a second beam that is redirected by the retroreflector along the reference path, wherein the first beam and the second beam interfere with one another, and controlling a ratio between a power portion of the detected beam redirected from the workpiece and a power portion of the detected beam redirected from the retroreflector to a target ratio by adjusting the power portion of the beam redirected from the retroreflector to the detector of the total power, during processing of the workpiece.

16. The method of claim 15, wherein adjusting the power portion of the total power of the beam redirected from the retroreflector comprises a change of the splitting of the source power of the beam between the reference path and the measurement path.

17. The method of claim 15, wherein adjusting the power portion of the total power of the beam redirected from the retroreflector comprises at least one of an attenuation, of the power portion of the beam in the reference path, and an attenuation, of the power portion in the measurement path.

18. The method of claim 17, wherein to attenuate the power portion of the beam in the reference path, a focus position of the beam in the reference path is changed, by acting on a on a focusing unit in the reference path.

19. The method of claim 15, further comprising: changing the source power to control the detected total power below a total power threshold value.

20. The method of claim 15, wherein the total power of the beam redirected back is detected in a spectrally resolved manner and the method further comprises: determining at least one parameter of the spectrally resolved total power, and adapting at least one of the target ratio and the total power in dependence on the at least one determined parameter.

Description

DESCRIPTION OF DRAWINGS

[0041] Further advantages of the invention result from the description and the drawing. The above-mentioned features and the further features set forth hereafter can also be used alone or in multiples in arbitrary combinations. The embodiments shown and described are not to be understood as an exhaustive list, but rather have exemplary character for the description of the invention.

[0042] In the figures:

[0043] FIG. 1 shows a schematic illustration of an exemplary embodiment of a workpiece processing machine for welding machining of a workpiece having an optical coherence tomography unit for scanning the workpiece during the machining using a laser beam,

[0044] FIG. 2 shows a simplified illustration of a beam path in the optical coherence tomography unit of FIG. 1 having a measurement path and having a reference path,

[0045] FIGS. 3A and 3B show illustrations of the spectrally resolved detected total power of the radiation of a beam source of the optical coherence tomography unit, which is incident on the workpiece and reflected thereby at the position of a machining beam and at two positions leading and trailing the position of the machining beam,

[0046] FIGS. 4A and 4B show schematic illustrations of the spectrally resolved detected total power similar to FIGS. 3A and 3B during the determination of parameters to control a power ratio between a measurement path and a reference path, and

[0047] FIGS. 5A and 5B show schematic illustrations of the spectrally resolved detected total power during a successful control of the power ratio.

DETAILED DESCRIPTION

[0048] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0049] FIG. 1 shows an exemplary structure of a workpiece processing machine 1 for machining a workpiece 2, which is welding machining in the example shown. The workpiece processing machine 1 comprises a machining head 3, to which a machining beam, in the example shown a laser beam 4, is supplied by a laser source (not shown). The laser beam 4 is focused on the workpiece 2 at a focusing lens 5 arranged in the machining head 3 and forms a capillary or a keyhole 6 thereon, in which the material of the workpiece 2 is melted and/or partially vaporized. The machining head 3 is moved by means of a movement unit (not shown in greater detail) during the welding process along a feed direction V over the surface 2a of the workpiece 2. It will be understood that alternatively or additionally the workpiece 2 can also be moved by means of a suitable movement unit in relation to the machining head 2.

[0050] The workpiece processing machine 1 comprises an optical interferometer in the form of an optical coherence tomography unit 7, which comprises a beam source 8 for generating (measurement) radiation 9. In the example shown, the beam source 8 is a superluminescent diode, which in the example shown generates radiation 9 having a wavelength or having wavelengths of greater than 800 nm. It will be understood that the beam source 8 can also be designed to generate radiation 9 at other wavelengths . The radiation 9 originating from the beam source 8 initially propagates freely and is coupled into an optical fiber 11 at a coupling optical unit 10a and is coupled out of the optical fiber 11 at an output-side end via an output coupling optical unit 10b before the radiation 9 is incident in free beam propagation on a beam splitter unit in the form of a polarization beam splitter 12.

[0051] At the beam splitter 12, the radiation 9, which is generated having a source power P.sub.Q by the beam source 8, is split between a reference path 13 and a measurement path 14. A further optical fiber 15 is arranged in the reference path 13, at the output-side end of which the radiation 9 exits divergently and is collimated by a first lens 16a. A second lens 16b is used for focusing the collimated radiation 9 on a retroreflector in the form of a planar end mirror 17. The radiation 9, more precisely the radiation portion which is coupled into the measurement path 14 by the polarization beam splitter 12, is guided via a deflection unit in the form of a scanner unit 19, which comprises two scanner mirrors (not shown in greater detail) in the example shown, to deflect the radiation 9 in the measurement path 14, before it is coupled into the beam path of the laser beam 4.

[0052] With the aid of the scanner unit 19, the radiation 9 in the measurement path 14 can be deflected or aligned at different positions on the workpiece 2. A position P, at which the laser beam 4 is incident on the workpiece 2, a first position P1 leading the position P at which the laser beam 4 is incident on the workpiece 2, and a second position P2 trailing the position P at which the laser beam 4 is incident on the workpiece 2, are shown by way of example in FIG. 1. With the aid of the scanner unit 19, the radiation 9 in the measurement path 14 can be deflected or aligned at all three positions P1, P, P2.

[0053] The radiation 9 reflected and/or scattered from the workpiece 2 in the measurement path 14 and the radiation 9 reflected and/or scattered from the end mirror 17 in the reference path 13 is combined at the polarization beam splitter 12 and passes through the optical fiber 11 in the reverse direction. At a further beam splitter 20 arranged in front of the beam source 8, which is designed as a partially-transmissive mirror, the reflected radiation 9 is deflected and supplied to a spectrometer 21, which comprises a detector 22 in the form of a detector line and a diffraction grating 23, at which the reflected and/or scattered radiation 9 is decomposed into its spectral components, which are incident on different detector surfaces of the detector 22 designed as a detector line.

[0054] The total power P.sub.D of the radiation 9 which is incident on the detector 22 or on each individual detector surface 22a-k (cf. FIG. 2), is the total of a power portion P.sub.R of the radiation 9, which has passed through the reference path 13 and is reflected and/or scattered at the end mirror 17 back to the detector 22, and a power portion P.sub.M of the radiation 9, which has passed through the measurement path 14 and is reflected and/or scattered at the workpiece 2 back to the detector 22 (P.sub.D=P.sub.M+P.sub.R).

[0055] For the case in which the splitting of the source power P.sub.Q is carried out at the polarization beam splitter 12 between the reference path 13 having a reference portion S.sub.R and the measurement path 14 having an identical measurement portion S.sub.M, i.e., S.sub.R=S.sub.M=0.5 (wherein S.sub.R+S.sub.M=1), the following applies for the total power P.sub.D, which is incident on the detector 22, more precisely on each individual detector surface 22a-k:

I.sub.D(k)=P.sub.D(k)=[P.sub.R(k)+P.sub.M(k)],

[0056] wherein k=2/ denotes the wave number (in [m.sup.1]), denotes the wavelength of the radiation 9 or of the respective radiation portion, I.sub.D(k) denotes the measurement current at the respective detector surface 22a-k, and denotes the proportionality constant between incident power P.sub.D(k) and the measurement current I.sub.D(k) proportional thereto (, for example, in ampere/watt).

[0057] The measurement current I.sub.D(k) comprises three components (a)-(c), which are indicated hereafter:

[00001]$I_D\left(k\right)=.Math.\begin{array}{cc}\frac{}{4}\left[P_Q\left(k\right).Math.\left(R_R+R_{S.Math..Math.1}+R_{S.Math..Math.2}+\mathrm{.Math.}\right)\right]& \left(a\right)\\ +\frac{}{4}\left[P_Q\left(k\right).Math.\underset{n=1}{\overset{N}{.Math.}}.Math..Math.\sqrt{R_R.Math.R_{\mathrm{Sn}}}.Math.\left(\cos \left[2.Math.k\left(z_R-z_{\mathrm{Sn}}\right)\right]\right)\right]& \left(b\right)\\ +\frac{}{4}\left[P_Q\left(k\right).Math.\underset{nm=1}{\overset{N}{.Math.}}.Math..Math.\sqrt{R_{\mathrm{Mn}}.Math.R_{\mathrm{Sm}}}.Math.\left(\cos \left[2.Math.k\left(z_{\mathrm{Sn}}-z_{\mathrm{Sm}}\right)\right]\right)\right]& \left(c\right)\end{array}$

[0058] For the explanation of the meaning of the variables used in the components (a)-(c), reference is made to FIG. 2, which shows a simplified illustration of the optical coherence tomography unit 7 having the reference path 13 and the measurement path 14. As can be inferred from FIG. 2, R.sub.R denotes the reflectivity of the end mirror 17 and R.sub.S1, R.sub.S2, . . . , R.sub.SN denote the reflectivity of the workpiece 2 in different measurement planes, which each correspond to different depths in the workpiece 2. The optical path length z.sub.R, which the radiation 9 covers in the reference path 13, and the optical path length z.sub.S or z.sub.Sn, which the radiation 9 covers in the measurement path 14 to the respective measurement plane 1, n, N, are not shown in FIG. 2.

[0059] The first component (a) is a so-called DC component, i.e., a constant component which is determined, for example, by means of a black adjustment before the beginning of the detection or measurement and which is filtered out during the detection from the total power P.sub.R(k)+P.sub.M(k) detected by the detector 22 or the measurement current I.sub.D(k) proportional thereto. The DC component (a) reduces the contrast range of the detector 22 and should therefore turn out to be as small as possible.

[0060] The second component (b) is a so-called cross-correlation component, which contains the actual depth information, i.e., the actual desired measurement signal, as an interferometric signal component. The auto-correlation component (c) contains, in the case of the reflection at different measurement planes 1, N at the workpiece 2, interference components between different measurement planes 1, . . . , N. The auto-correlation component (c) involves artifacts which corrupt the actual measurement result, i.e., the auto-correlation component (c) should also turn out to be as small as possible.

[0061] FIG. 1 also shows an evaluation unit 24, which records and analyzes the total power P.sub.R(k)+P.sub.M(k), which is incident on the detector 22, more precisely the measurement signal I.sub.D(k) proportional thereto, spectrally resolved in dependence on the wave number k. On the basis of the spectrally resolved total power P.sub.R+P.sub.M, information about the welding process can be determined, for example, about the welding penetration depth Z, which can be determined in the spectrum 25 shown in FIG. 1 on the basis of a spectral distance between two peaks in the recorded spectrum 25, as indicated by way of example in FIG. 1.

[0062] As can be seen in FIGS. 3A and 3B, the spectrally resolved total power P.sub.R+P.sub.M and thus the associated spectrum 25 are strongly dependent on the power portion R.sub.S reflected at the workpiece 2 (and/or on the power portion R.sub.S1, . . . , R.sub.SN reflected at the workpiece 2 in a respective measurement plane), which refers in FIGS. 3A and 3B to the total source power P.sub.Q and is in turn dependent on the position P, P1, P2, at which the radiation 9 in the measurement path 14 is incident on the workpiece 2. In the example shown in FIG. 3A, in which the radiation 9 is incident at the position P of the laser beam 4 on the workpiece 2, the power portion R.sub.S of the source power P.sub.Q reflected at the workpiece 2 is comparatively small and is, for example, R.sub.S=0.02, while the power portion R.sub.R reflected at the end mirror 17 of the source power P.sub.Q is approximately 0.5, i.e., R.sub.R<<1R.sub.S, which results in a comparatively large DC component (a), as is recognizable in FIG. 3A on the basis of the comparatively tall or large spectrum 25.

[0063] In the example shown in FIG. 3B, the radiation 9 in the measurement path 14 is reflected from the first position P1 leading the position P of the laser beam 4 and/or from the second position P2 trailing the position P of the laser beam 4. At these two positions P1, P2, the power portion R.sub.S reflected at the workpiece 2 or at the surface 2a of the workpiece 2 of the source power P.sub.Q is comparatively large, for example, R.sub.S=0.98, while for the radiation portion R.sub.R reflected at the end mirror 17: R.sub.R=0.02, so that: R.sub.S1R.sub.R. In this case, the auto-correlation component (c) of the spectrum 25 is excessively large and it is comparatively irregular.

[0064] To obtain the highest possible signal quality and/or a good signal-to-noise ratio as independently as possible of the possibly strongly differing reflectivity R.sub.S of the workpiece 2, the workpiece processing machine 1 shown in FIG. 1 comprises a control unit 26, to control the ratio between the power portion P.sub.R of the radiation 9 in the measurement path 14 reflected back from the workpiece 2 to the detector 22 to the power portion P.sub.M of the radiation 9 in the reference path 13 reflected back from the end mirror 17 to a target ratio P.sub.MS/P.sub.RS. The target ratio P.sub.MS/P.sub.RS can in particular be a constant value, which is preferably between approximately 0.01 and approximately 100, more preferably between 0.1 and 10 and is ideally P.sub.MS/P.sub.RS=1.

[0065] To carry out the control, the control unit 26 can first determine by computation the (actual) power portion P.sub.R of the radiation 9 from the reference path 14 which is reflected to the detector 22, for example in the following manner:

P.sub.R=S.sub.R.sup.2P.sub.QR.sub.R,

[0066] wherein S.sub.R denotes the reference portion of the source power P.sub.Q of the radiation 9 which is coupled into the reference path 14 (see above) and R.sub.R denotes the reflectivity of the end mirror 17. The squaring of the reference component S.sub.R of the source power P.sub.Q in the above equation is to be attributed to the fact that there are two passes through the beam splitter 12. On the basis of the total power P.sub.D, which is incident on the detector 22, the (actual) power portion P.sub.M of the measurement path can be determined in the evaluation unit 24 as P.sub.M=P.sub.DP.sub.R.

[0067] With the aid of the actual ratio P.sub.M/P.sub.R determined in thisor possibly in another mannerthe control to the desired target ratio P.sub.MS/P.sub.RS can be performed in the control unit 26. For this purpose, the control unit 26 can act on an adjustment element 27, which enables it to change the power portion P.sub.R in the reference path 13. In the example shown in FIG. 1, the adjustment element 27 is designed as a movement unit, which is used for moving, more precisely for displacing the first lens 16a in the reference path 13 along the beam axis 28 of the reference path 13.

[0068] The focus position F1 of the first lens 16a is changed by the displacement of the first lens 16a, so that it is no longer in the plane of the output-side end of the optical fiber 11, i.e., the distance between the output-side end of the optical fiber 11 and the first lens 16a no longer corresponds to the focal length f of the first lens 16a, whereby the radiation 9 in the reference path 13 becomes defocused, which results in an attenuation of the radiation 9 reflected back to the detector 22. This attenuation A can be considered to be a (for example, percentage) reduction of the reflectance R.sub.R in the reference path 13, i.e., the reduced reflectance R.sub.R*A is no longer exclusively determined by the reflectivity R.sub.R of the end mirror 17.

[0069] Additionally or alternatively, the adjustment element 27 can also be designed to displace the second lens 16b in the reference path 13 in the direction of the beam axis 28, whereby the further above-described defocusing or attenuation occurs with respect to the distance between the second lens 16b and the end mirror 17. It will be understood that in the workpiece processing machine 1 shown in FIG. 1, the second lens 16b is not absolutely necessary, i.e., it is not absolutely necessary to focus the collimated radiation 9 in the reference path 13 on the end mirror 17.

[0070] The adjustment element 27 can also act directly on the end mirror 17 to change its reflectivity R.sub.R, without having to act on the lenses 16a, b for this purpose, for example, by rotating the end mirror 17 around its axis. It is also possible that the adjustment element 27 moves an optical filter unit 29 shown in FIG. 1 into the beam path of the reference path 13 and back out of it, as also indicated in FIG. 1 by a double arrow, whereby an attenuation A or a change of the reflectance A*R.sub.R in the reference path 13 also occurs. Alternatively or additionally, the or an optical filter unit 29 can also be arranged permanently in the beam path of the reference path 13, wherein the filter action or the attenuation A of the optical filter unit can be changed with the aid of the control unit 26, i.e., it is a controllable optical filter unit 29.

[0071] The adjustment element 27a for changing the power portion PR, which is reflected from the reference path 13 back to the detector 22, can also be used to change the splitting of the source power P.sub.R of the beam source 8 between the reference path 13 and the measurement path 14, i.e., the ratio S.sub.R to S.sub.M. For this purpose, the or an adjustment element 27a can act, for example, in a suitable manner on the (polarization) beam splitter 12, for example, by rotating it around the beam axis of the radiation 9 incident thereon from the beam source 8, as indicated in FIG. 1, whereby the ratio S.sub.R/S.sub.M and thus also the ratio of the power portions P.sub.R/P.sub.M typically changes.

[0072] In the example shown, the control unit 26, in addition to controlling the ratio of the power portions P.sub.R/P.sub.M, is also used to control the total power P.sub.R+P.sub.M, which is incident on the detector 22, specifically to a value which is less than a total power threshold value P.sub.RS+P.sub.MS, which is predefined by the sensitivity of the detector 22 and is not supposed to be exceeded in order to avoid the detector 22 going into saturation or exceeding its contrast range. To achieve this, the control unit 26 acts on a further adjustment element 30, which in the example shown is a controllable current source for supplying a current signal to the beam source 8 in the form of the superluminescent diode. With the aid of the further adjustment element 30, the source power P.sub.Q of the beam source 8 can thus be changed, whereby the total power P.sub.R+P.sub.M incident on the detector 22 can be controlled so that it is always less than the total power threshold value P.sub.RS+P.sub.MS.

[0073] FIGS. 4A and 4B show by way of example the spectrally resolved total power P.sub.R+P.sub.M incident on the detector 22 or a spectrum 25 resulting therefrom for the first measurement plane (n=1) or for a measurement plane (n>1) located deeper in the workpiece 2, respectively, in dependence on the wave number k. With the aid of the evaluation unit 24, characteristic parameters in the respective spectrum 25 can be determined, which can be used by the control unit 26 for the control. For example, at the wave number at which the maximum of the current or the total power P.sub.R+P.sub.M is measured, the mean value of the reflectivity R.sub.R of the reference path 13 and the reflectivity R.sub.S of the workpiece 2 may be read, i.e., it may be determined as a parameter (R.sub.R+R.sub.S1)/2. The proportionality factor {square root over (R.sub.RR.sub.S1)} of the cross-correlation component (b) can also be determined on the basis of the spectrum 25 and in this manner, for example, the reflectivity R.sub.S1 of the workpiece 2 at the first measurement plane (n=1) may be determined.

[0074] On the basis of the at least one parameter, for example, the proportionality factor {square root over (R.sub.RR.sub.S1)} of the cross-correlation component (b), the target ratio P.sub.MS/P.sub.RS and/or the total power P.sub.R+P.sub.M can be adapted suitably to optimize the signal-to-noise ratio. However, it will be understood that both the target ratio P.sub.MS/P.sub.RS and also the total power P.sub.R+P.sub.M can be controlled to a predetermined, constant value. In this case, an adjustment of the detector 22 for adaptation to different types of material, etc., can be disposed with.

[0075] FIGS. 5A and 5B show two spectra 25, which were optimized with the aid of the control unit 26 so that both the DC component (a) and also the auto-correlation component (c) are comparatively small, so that essentially the cross-correlation component (b) containing the depth information remains. The underlying light conditions during the recording of the two spectra 25 are very different and correspond to the light conditions or reflectances R.sub.S of the workpiece 2 shown in FIGS. 3A and 3B. As the comparison of FIG. 5A and FIG. 5B shows, the two spectra 25 only differ insignificantly in spite of the very different light conditions on the workpiece 2, i.e., the signal quality is high both at the two positions P1, P2 leading and trailing the position P of the laser beam 4 and also at the position P of the laser beam 4 itself.

[0076] Although welding machining of the workpiece 2 was described above, the optimization of the signal quality can also advantageously be performed in other machining processes which are carried out by means of the workpiece processing machine 1 or a suitably modified workpiece processing machine 1. For example, the machining can be a (laser) cutting process or a (laser) drilling process. The above-described method can also be used in optical interferometers other than an optical coherence tomography unit, since the problem also occurs therein that the signal quality or the signal-to-noise ratio cannot be optimized by the change of the exposure time of the detector 22 and/or by the change of the source power P.sub.Q without an adaptation of the power ratio P.sub.R/P.sub.M. The workpiece 2 does not necessarily have to be a metallic workpiece 2 (a plate), rather other objects can also be machined with the aid of the workpiece processing machine 1, which are denoted as the workpiece 2 in the present application for simplification.