METHOD, MEASURING DEVICE, MACHINING SYSTEM AND COMPUTER PROGRAM PRODUCT FOR DETERMINING A CORRECTED HEIGHT SIGNAL FROM MEASUREMENT DATA OBTAINED WITH OPTICAL COHERENCE TOMOGRAPHY

Abstract

A method, measuring device, machining system and computer program product are provided for determining a corrected height signal from measurement data obtained with optical coherence tomography. The measurement data comprises an object signal and a background signal superimposed on the object signal, the object signal and the background signal being subject to different dispersion. A first transformation is performed comprising transforming the measurement data, the first transformation being targeted at the background signal to obtain a height signal, background components in the height signal are determined, the background components in the height signal are compensated to obtain a background-compensated height signal, an inverse transformation is performed comprising back-transforming the background-compensated height signal to obtain background-compensated measurement data, dispersion compensation for the object signal is performed to obtain dispersion-compensated and background-compensated measurement data, and a second transformation is performed comprising transforming the dispersion-compensated and background-compensated measurement data to obtain a dispersion-compensated and background-compensated height signal.

Claims

1. A method for determining a corrected height signal from measurement data obtained with an optical coherence tomograph of a measuring device of a machining system for machining a workpiece using a high-energy machining beam, the method comprising: obtaining measurement data based on interference of sample light guided in a sample arm and reference light guided in a reference arm, the sample arm and the reference arm differing in dispersion, the measurement data comprising an object signal and a background signal superimposed on the object signal, the object signal and the background signal being subject to different dispersion; performing a first transformation on the measurement data using a control unit of the measuring device, the first transformation being targeted at the background signal to obtain a height signal; determining background components in the height signal using the control unit of the measuring device; compensating the background components in the height signal using the control unit of the measuring device to obtain a background-compensated height signal; performing an inverse transformation comprising back-transforming the background-compensated height signal using the control unit of the measuring device to obtain background-compensated measurement data; performing dispersion compensation for the object signal using the control unit of the measuring device to obtain dispersion-compensated and background-compensated measurement data; performing a second transformation comprising transforming the dispersion-compensated and background-compensated measurement data using the control unit of the measuring device to obtain a dispersion-compensated and background-compensated height signal; and controlling the machining of the workpiece by the control unit of the measuring device based, at least in part, on the dispersion-compensated and background-compensated height signal.

2. The method of claim 1, wherein said compensating comprises subtracting a least a portion of the background components from the height signal.

3. The method of claim 1, further comprising the step of performing dispersion compensation for the background signal before the first transformation.

4. The method of claim 1, wherein compensating comprises at least one selected from the group consisting of (i) clipping and (ii) overwriting data points of the height signal for height values not exceeding a predetermined threshold value.

5. The method of claim 4, wherein the threshold value is a maximum of 100 μm.

6. The method of claim 4, wherein the threshold value is a maximum of 50 μm.

7. The method of claim 1, wherein at least one selected from the group consisting of: (i) the first transformation, (ii) the second transformation, and (iii) the inverse transformation, comprise a Fourier transformation.

8. The method of claim 1, wherein at least one selected from the group consisting of: (i) the first transformation, (ii) the second transformation, and (iii) the inverse transformation, comprise a fast Fourier transformation.

9. The method of claim 1, wherein performing dispersion compensation for the object signal comprises multiplying the background-compensated measurement data by a dispersion correction curve.

10. The method of claim 1, wherein at least one method step is based on calculations which are carried out in at least one field programmable gate array.

11. The method of claim 1, wherein all of said method steps are based on calculations which are carried out in at least one field programmable gate array.

12. A measuring device for a machining system for machining a workpiece using a high-energy machining beam, comprising: an optical coherence tomograph configured to generate a sample beam and a reference beam, comprising; a sample arm in which the sample beam is optically guidable; a reference arm in which the reference beam is optically guidable; a sample unit adapted to perform optical coherence tomography measurements by causing the sample beam and the reference beam to interfere to generate measurement data; and a control unit having at least one non-transitory computer readable medium having computer-readable program code portions embodied therein, the control unit having a processing device operatively coupled to the at least one non-transitory computer readable medium, wherein the processing device is configured to execute the computer-readable program code portions to: obtain measurement data based on interference of sample light guided in a sample arm and reference light guided in a reference arm, the sample arm and the reference arm differing in dispersion, the measurement data comprising an object signal and a background signal superimposed on the object signal, the object signal and the background signal being subject to different dispersion; perform a first transformation on the measurement data, the first transformation being targeted at the background signal to obtain a height signal; determine background components in the height signal; compensate the background components in the height signal to obtain a background-compensated height signal; perform an inverse transformation comprising back-transforming the background-compensated height signal to obtain background-compensated measurement data; perform dispersion compensation for the object signal to obtain dispersion-compensated and background-compensated measurement data; perform a second transformation comprising transforming the dispersion-compensated and background-compensated measurement data using the control unit of the measuring device to obtain a dispersion-compensated and background-compensated height signal; and control the machining of the workpiece by the machining system based, at least in part, on the dispersion-compensated and background-compensated height signal.

13. The measuring device of claim 12, wherein the control unit comprises at least one field programmable gate array and wherein the control of the machining of the workpiece by the machining system is based on calculations made in the at least one field programmable gate array.

14. The measuring device of claim 12, wherein compensating comprises subtracting a least a portion of the background components from the height signal.

15. The measuring device of claim 12, wherein the processing device is configured to execute the computer-readable program code portions to perform dispersion compensation for the background signal before the first transformation.

16. The measuring device of claim 12, wherein compensating comprises at least one selected from the group consisting of (i) clipping and (ii) overwriting data points of the height signal for height values not exceeding a predetermined threshold value.

17. The measuring device of claim 16, wherein the threshold value is a maximum of 100 μm

18. A machining system for machining a workpiece using a high-energy machining beam, comprising: a measuring device according to claim 12; and a machining device comprising a machining beam source configured to generate the machining beam and machining beam optics configured to at least one selected from the group consisting of project and focus the machining beam onto the workpiece.

19. A computer program product for determining a corrected height signal from measurement data obtained with an optical coherence tomograph of a measuring device of a machining system for machining a workpiece using a high-energy machining beam, the computer program product comprising at least one non-transitory computer readable medium having computer-readable program code portions embodied therein, the computer-readable program code portions comprising executable portions for: obtaining measurement data based on interference of sample light guided in a sample arm and reference light guided in a reference arm, the sample arm and the reference arm differing in dispersion, the measurement data comprising an object signal and a background signal superimposed on the object signal, the object signal and the background signal being subject to different dispersion; performing a first transformation on the measurement data, the first transformation being targeted at the background signal to obtain a height signal; determining background components in the height signal; compensating the background components in the height signal to obtain a background-compensated height signal; performing an inverse transformation comprising back-transforming the background-compensated height signal to obtain background-compensated measurement data; performing dispersion compensation for the object signal to obtain dispersion-compensated and background-compensated measurement data; performing a second transformation comprising transforming the dispersion-compensated and background-compensated measurement data using the control unit of the measuring device to obtain a dispersion-compensated and background-compensated height signal; and controlling the machining of the workpiece by the machining system based, at least in part, on the dispersion-compensated and background-compensated height signal.

20. The computer program product of claim 19, wherein the computer-readable program code portions comprising executable portions for performing dispersion compensation for the background signal before the first transformation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a schematic representation of a machining system for machining a workpiece using a high-energy machining beam, comprising a measuring device by means of which OCT measurement data can be generated;

[0043] FIG. 2 is a visualization of an example of a transformation of measurement data into a height signal;

[0044] FIG. 3 shows an example of a height signal;

[0045] FIG. 4 shows another example of a height signal;

[0046] FIG. 5 shows a background-compensated height signal;

[0047] FIG. 6 shows an example of background-compensated measurement data;

[0048] FIG. 7 shows an example of a dispersion correction curve;

[0049] FIG. 8 shows an example of a complex signal; and

[0050] FIG. 9 is a sequence diagram of background compensation of measurement data.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0051] FIG. 1 illustrates a machining system 12 comprising a measuring device 10 and a machining device 32. The machining device 32 comprises a machining beam source 50 configured as a machining laser. It generates a machining beam 16 which can be directed onto a workpiece 14 for machining of the latter. This can be, for example, a machining laser beam.

[0052] The machining device 32 comprises a machining scanner 52 that makes the machining beam 16 displaceable. The machining scanner 52 comprises, for example, a mirror arrangement that makes the machining beam 16 automatically displaceable in two spatial directions, e.g. parallel and transverse to a machining direction 54. The machining beam 16 is focused onto the workpiece 14 via a schematically illustrated machining beam optics 56 of the machining device 32.

[0053] In the present case, the machining device 32 includes a machining head 58 that may be attached to an industrial robot, for example, which is not shown.

[0054] The machining system 12 further comprises a measuring device 10. The measuring device 10 comprises an optical coherence tomograph 18. The optical coherence tomograph 18 comprises a sample beam source 60 and a beam splitter 62 coupled thereto. A sample arm 24 and a reference arm 26 extend from the beam splitter 62. A sample beam 20 is optically guided in the sample arm 24. A reference beam 22 is optically guided in the reference arm 26.

[0055] The sample arm 24 and the reference arm 26 are connected to a sample unit 64, within which the sample beam 20 and the reference beam 22 interfere with each other. In the case shown, the sample unit 64 comprises a spectrometer enabling optical coherence measurements on the basis of the interference of the sample beam 20 and the reference beam 22. These measurements allow optical coherence tomography to be carried out, for example to determine a height or depth profile of a portion of the workpiece 14 to be machined and/or already machined and/or currently being machined. It is also possible, for example, to determine a penetration depth of the machining beam 16 into the workpiece 14, in particular into a vapor cavity (also known as “keyhole”) that is formed.

[0056] The sample arm 24 extends from the beam splitter 62 to the workpiece 14. The reference arm 26 extends from the beam splitter 62 to its end at which a reflector 66 is arranged. In the case shown, the reflector 66 is a mirror belonging to a path length adjustment unit 68 that makes an optical path length of the reference arm 26 adjustable. This allows the optical path length of the reference arm 26 to be adjusted to the optical path length of the sample arm 24.

[0057] The sample beam 24 is coupleable into the machining beam 16. In the case shown, the sample beam 20 is guided to a partially transparent mirror 70. It deflects the machining beam 16 and allows the sample beam 24 to be coupled into the machining beam 16.

[0058] The measuring device 10 further comprises a sample scanner 72. The sample scanner 72 comprises, for example, a mirror arrangement that makes the sample beam 20 automatically displaceable in two spatial directions, e.g. parallel and transverse to the machining direction 54. In the present machining system 12, the sample beam 20 is deflectable relative to the machining beam 16 so that impact positions of the two beams can be set independently of one another. As can be seen in FIG. 1, the sample scanner 72 only deflects the sample beam 20 whereas the machining scanner 52 deflects both the machining beam 16 and the sample beam 20. This enables the aforementioned independent displacement of machining the beam 16 and the sample beam 20.

[0059] In addition, a control unit 30 is provided. It may be part of the measuring device 10. The measuring device 10 and the machining device 32 may have separate control units. The control unit 30 shown as an example in FIG. 1 is a common control unit controlling the components of the machining system 12.

[0060] The control unit 30 may be adapted to perform the method described herein. It may have appropriate programming or program code for this purpose. In particular, a computer program product 74 comprising a machine-readable medium such as a flash memory or an EEPROM may be provided. The program code may be stored thereon. It is in particular intended to be executed in a graphics processing unit (GPU) and/or a microcontroller and/or a field programmable gate array (FPGA).

[0061] In some embodiments, the calculations underlying the method described are made entirely in one or more field programmable gate arrays (FPGA). They may be part of the control unit 30.

[0062] The design of the measuring device 10 is to be understood as exemplary. In particular, the method for determining a corrected height signal as described below is in principle appliable to any measuring systems supplying OCT measurement data.

[0063] Below, the determination of a corrected height signal is described. First, measurement data are obtained with optical coherence tomography. In the example shown, this is done with the measuring device 10. The measurement data are available in the form of a spectrum as illustrated in FIG. 2. A height signal can be obtained from the measurement data in a way that is generally known by performing suitable transformation, such as fast Fourier transformation (FFT). Such height signal has a height-dependent intensity. FIG. 2 shows an example of a height signal with a single peak at a specific height value. Here, for example, a certain height value or distance value was measured, e.g. a specific penetration depth.

[0064] FIG. 3 shows an example of a height signal as conventionally obtained upon transformation of measurement data. The measurement data contain both an object signal resulting from the measurement on the workpiece 14 and a background signal resulting from an at least essentially static measurement background due to, for example, protective glasses or the like. Based on such measurement data, software-based dispersion compensation was performed to obtain the height signal shown in FIG. 3. For this purpose, a dispersion compensation parameter was selected which causes dispersion compensation for the object signal. The result is a pronounced peak in the range of medium height values, which is attributable to the object signal. Further, what is apparent in the example are the two broad peaks of low intensity in the range of low height values. They are due to the background signal. While dispersion compensation that was targeted at the object signal was performed, the background peaks were broadened, which made them difficult to distinguish from the background noise.

[0065] FIG. 4 shows the procedure according to a method described herein. Here, measurement data are first transformed using a transformation that is targeted at the background. For this purpose, either no dispersion compensation is performed or dispersion compensation is performed before the transformation that makes the background signal more prominent. The latter case will be discussed below. In the example shown, this results in two narrow large-amplitude background peaks that are easy to recognize and isolate. In the range of the object signal, on the other hand, a broadened peak occurs that is rather difficult to isolate. This is no problem, however, since the method initially deals only with the background signal.

[0066] FIG. 5 illustrates the height signal of FIG. 4 after parts of the background have been subtracted. Generally speaking, background components are compensated. First, the height signal is clipped below a threshold value 46. The values of the height signal are thus set to zero below the threshold value 46. This is 50 μm, for example. In a penetration depth measurement, for example, no such low values are expected for the object signal, i.e. the signal related to the penetration depth. Therefore, clipping the height signal is no problem for the object signal, yet it already removes a significant portion of the background signal easily and reliably. However, it is understood that such threshold-based clipping is purely optional and may be omitted depending on the measurement situation and the desired measurement information.

[0067] In addition, background subtraction is performed by compensating the detected background peaks of the height signal. This can be done by partial or complete subtraction of the background peaks. For this purpose, any suitable method for subtracting individual peaks from spectra may be applied in a generally known manner. For example, the peaks may be fitted and the functions modeled in the process may be subtracted. Also, maxima of the peaks may be searched and peak widths may be determined, and based on this, peaks to be subtracted may be modeled.

[0068] By subtracting at least part of the background signal, the background-compensated height signal shown in FIG. 5 is obtained. In the next step, it may be back-transformed to obtain background-compensated measurement data. In another transformation, a height signal may be obtained again from these data, performing dispersion compensation that is targeted at the object signal is this time.

[0069] FIG. 6 shows an example of background-compensated measurement data 50 obtained by proceeding as described above.

[0070] FIG. 7 shows an example of a dispersion correction curve 52. In the present case, the dispersion correction curve is a complex curve. It comprises a real part 54 and an imaginary part 56. For example, the real part is a cosine function, and the imaginary part is a negative sine function. A second-degree polynomial is used as the argument of each of these functions. The dispersion correction curve defines a phase shift that is adjustable by dispersion compensation parameters. The dispersion compensation parameters may be coefficients of the mentioned polynomial.

[0071] Multiplying the measurement data 50 shown in FIG. 6 by the complex dispersion correction curve 52 shown in FIG. 7 produces a complex signal 58, which is shown by way of example in FIG. 8. It comprises a real part 60 and an imaginary part 62.

[0072] To obtain a dispersion-compensated and background-compensated height signal, the complex signal 58 obtained by the multiplication is subjected to a fast Fourier transformation.

[0073] It is understood that in analogy to this, a dispersion correction curve may be used before the first transformation targeted at the background signal. Such curve may be selected in such a way that it effects dispersion compensation primarily for the background signal. In the example shown, however, no dispersion compensation is performed before the first transformation.

[0074] FIG. 9 illustrates the entire procedure again. First, measurement data 36 are obtained and transformed to a height signal 38 in a first transformation (T1) that is targeted at the background signal. In some embodiments, dispersion correction may be performed before the first transformation (T1). In the height signal 38, background components are compensated as described above, which produces a background-compensated height signal 40 (Corr.). This is back-transformed (T−1) to obtain background-compensated measurement data 42. The background-compensated measurement data 42 are dispersion-corrected, which produces dispersion-corrected and background-compensated measurement data 43. In a second transformation (T2), a dispersion-compensated and background-compensated height signal 44 is obtained from the dispersion-compensated and background-compensated measurement data 43.

[0075] If a dispersion compensation software is used in which the dispersion compensation behavior can be controlled by presetting a certain value of a dispersion compensation parameter, the following procedure is followed. Before performing the first transformation, a first value of the dispersion compensation parameter is selected that causes dispersion compensation for the background signal. In particular, this may include the absence of dispersion compensation, meaning that the selected value causes no dispersion compensation. Before performing the second transformation, however, a second value of the dispersion compensation parameter is selected that differs from the first value and causes dispersion compensation for the object signal.

[0076] By performing two transformations in a row as described, a background signal can first be easily detected and reliably compensated before, following inverse transformation, the measurement data corrected in this way are dispersion-compensated and then transformed in such a way that the object signal emerges.

[0077] It will be understood that any suitable computer-readable medium may be utilized. The computer-readable medium may include, but is not limited to, a non-transitory computer-readable medium, such as a tangible electronic, magnetic, optical, infrared, electromagnetic, and/or semiconductor system, apparatus, and/or device. For example, in some embodiments, the non-transitory computer-readable medium includes a tangible medium such as a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EEPROM or Flash memory), a compact disc read-only memory (CD-ROM), and/or some other tangible optical and/or magnetic storage device. In other embodiments of the present invention, however, the computer-readable medium may be transitory, such as a propagation signal including computer-executable program code portions or executable portions embodied therein.

[0078] It will also be understood that one or more computer-executable program code portions or instruction code for carrying out or performing the specialized operations of the present invention may be required on the specialized computer include object-oriented, scripted, and/or unscripted programming languages, such as, for example, Java, Perl, Smalltalk, C++, SQL, Python, Objective C, and/or the like. In some embodiments, the one or more computer-executable program code portions for carrying out operations of embodiments of the present invention are written in conventional procedural programming languages, such as the “C” programming languages and/or similar programming languages. The computer program code may alternatively or additionally be written in one or more multi-paradigm programming languages, such as, for example, F #.

[0079] Embodiments of the present invention are described above with reference to flowcharts and/or block diagrams. It will be understood that steps of the processes described herein may be performed in orders different than those illustrated in the flowcharts. In other words, the processes represented by the blocks of a flowchart may, in some embodiments, be in performed in an order other that the order illustrated, may be combined or divided, or may be performed simultaneously. It will also be understood that the blocks of the block diagrams illustrated, in some embodiments, merely conceptual delineations between systems and one or more of the systems illustrated by a block in the block diagrams may be combined or share hardware and/or software with another one or more of the systems illustrated by a block in the block diagrams. Likewise, a device, system, apparatus, and/or the like may be made up of one or more devices, systems, apparatuses, and/or the like. For example, where a processor is illustrated or described herein, the processor may be made up of a plurality of microprocessors or other processing devices which may or may not be coupled to one another. Likewise, where a memory is illustrated or described herein, the memory may be made up of a plurality of memory devices which may or may not be coupled to one another.

[0080] It will also be understood that the one or more computer-executable program code portions may be stored in a transitory or non-transitory computer-readable medium (e.g., a memory, and the like) that can direct a computer and/or other programmable data processing apparatus to function in a particular manner, such that the computer-executable program code portions stored in the computer-readable medium produce an article of manufacture, including instruction mechanisms which implement the steps and/or functions specified in the flowchart(s) and/or block diagram block(s).

[0081] The one or more computer-executable program code portions may also be loaded onto a computer and/or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer and/or other programmable apparatus. In some embodiments, this produces a computer-implemented process such that the one or more computer-executable program code portions which execute on the computer and/or other programmable apparatus provide operational steps to implement the steps specified in the flowchart(s) and/or the functions specified in the block diagram block(s). Alternatively, computer-implemented steps may be combined with operator and/or human-implemented steps in order to carry out an embodiment of the present invention.

[0082] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of, and not restrictive on, the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.