CN106796097B

CN106796097B - Device and method for temperature-compensated interferometric distance measurement during laser processing of workpieces

Info

Publication number: CN106796097B
Application number: CN201580045781.9A
Authority: CN
Inventors: F·多施; T·哈雷尔; P·奥格; D·普菲茨纳; S·凯斯勒
Original assignee: Trumpf Laser und Systemtechnik GmbH
Current assignee: Trumpf Laser und Systemtechnik GmbH
Priority date: 2014-08-25
Filing date: 2015-08-19
Publication date: 2022-06-03
Anticipated expiration: 2035-08-19
Also published as: DE102014216829B4; WO2016030246A1; DE102014216829A1; CN106796097A

Abstract

In a device (3) for measuring a distance (A) between a reflected workpiece surface (2a) and a reflected reference surface (2b) of a workpiece (2) during laser machining of the workpiece (2), an interferometer (5) has a beam splitter (6) which splits interferometer light (7) onto a measurement arm (9) as a measurement beam (10) and onto a reference arm (11) as a reference beam (12), and a detector (13) which detects the measurement beam (10) reflected at the workpiece surface (2a) and the reference beam (12) reflected at the reference surface (2b), wherein the measurement arm (9) and the reference arm (11) are of equal length within the coherence length of the interferometer light (7). According to the invention, the measuring arm (9) has a measuring fiber (14) and the reference arm (11) has a reference fiber (15), the measuring fiber (14) and the reference fiber (15) extend parallel alongside one another over the entire length or over a partial length thereof, in particular over the entire length of the shorter fiber if the lengths of the fibers (14, 15) differ, and are in thermal contact with one another, the measuring arm (9) has a first surface region (2a) as a reflective workpiece surface and the reference arm (11) has a second surface region (2b) of the workpiece (2) as a reflective reference surface, and a deflecting optical means (16) is arranged between the workpiece (2) and the measuring fiber (14) and/or the end of the reference fiber (15) on the workpiece side, which deflecting optical means moves the measuring beam (10) and/or the reference beam (15) jointly or separately past the measuring surface and/or the reference surface (2a), 2b) .1. the

Description

Device and method for temperature-compensated interferometric distance measurement during laser processing of workpieces

Technical Field

The invention relates to a device for measuring a distance between a reflective workpiece surface and a reflective reference surface of a workpiece during laser machining of the workpiece, comprising an interferometer having: a beam splitter which splits the interferometer light onto the measurement arm as a measurement beam and onto the reference arm as a reference beam; and a detector that detects the measurement beam reflected on the surface of the workpiece and the reference beam reflected on the reference plane, wherein the measurement arm and the reference arm are of equal length within a coherence length of the interferometer light.

Background

Interferometers are used for distance measurements during laser machining processes. For this purpose, the radiation of one of the two interferometer arms (measuring arm) is directed nearly coaxially with the machining laser at the workpiece, which acts as an interferometer mirror. Preferably, the measuring beam is spatially superimposed on the machining beam before focusing, for example by means of a beam splitter in the laser machining head, and focused by the machining optics onto the measurement site. The other interferometer arm (reference arm) is positioned in the measuring appliance (with beam source, beam splitter, detector and analysis processing unit). Typically, the two interferometer arms have approximately equal optical path lengths. It is important for interferometric distance measurements to know the change in optical path length. In reality, the optical path in the measuring arm may change not only due to the distance to be measured, but also due to (unintentional) changes in the optical path of the interferometer arm. For example, in fiber-guided interferometers, temperature differences from the reference fiber and thus temperature-induced interference signal variations, which lead to measurement errors in the pitch measurement, are caused by the heating of the end of the measurement fiber above the sensed weld site. Furthermore, relative measurements are often required in order to determine the distance between two points (or areas) of the workpiece, for example the weld-in depth or capillary depth of the hole relative to the component surface.

Disclosure of Invention

The object of the present invention is to eliminate temperature-induced measurement errors and to sense the topography of the workpiece surface or to perform a spatial averaging in a measuring device of the type mentioned at the outset.

According to the invention, this object is achieved in the following manner: the measuring arm has a measuring fiber and the reference arm has a reference fiber, and the measuring fiber and the reference fiber extend parallel side by side over the entire length or a partial length thereof, in particular over the entire length of the shorter fiber if the lengths of the fibers are different, and are in thermal contact with one another, the measuring arm having a first surface region as a reflective workpiece surface and the reference arm having a second surface region of the workpiece as a reflective reference surface, and a deflecting optical means is arranged between the workpiece and the workpiece-side end of the measuring fiber and/or the reference fiber, which deflecting optical means moves the measuring beam and/or the reference beam jointly over the measuring surface and/or the reference surface or separately over the measuring surface and/or the reference surface.

According to the invention, the measuring fiber and the reference fiber are thermally coupled to one another in such a way that temperature differences of interest do not occur, but rather temperature fluctuations in the transmission path are systematically compensated for. The measuring beam and the reference beam are guided via corresponding optical fibers closely next to each other, but separately, so that the beam paths are slightly offset from each other. As a result, the interference signals of the two beams depend only on the relative distance between the workpiece surface and the reference surface, since the path length fluctuations in the measuring arm and the reference arm are compensated to the greatest extent by the parallel embodiment. The reference beam can thereby be deflected onto the component surface (reference surface) and the measuring beam can be deflected onto the measuring point or measuring surface. The deflecting optics enable the measuring beam and/or the reference beam to be moved jointly or separately past the measuring or reference surface in order to carry out one-dimensional or two-dimensional measurements. Whereby the topography of the workpiece surface can be sensed or a spatial averaging performed.

In an advantageous embodiment, the measuring fiber and the reference fiber are two separate fibers which lie against one another over their entire length or over part of their length, in particular over the entire length of the shorter fiber if the lengths of the fibers differ. The two optical fibers may be guided in thermal contact, for example, in a common light-conducting cable or protective hose. In a further advantageous embodiment, the measuring fiber and the reference fiber are formed by the core and the inner cladding of a double-clad fiber.

For the measurement of the distance between the two surface regions, the measuring arm has one surface region as a reflective workpiece surface and the reference arm has the other surface region of the workpiece as a reflective reference surface, so that the distance measured by the interferometer is measured relative to the workpiece surface and is independent of the optical path length fluctuations along the measuring fiber and the reference fiber.

Furthermore, an imaging light fixture can advantageously be arranged between the workpiece and the workpiece-side end of the measuring fiber and/or the reference fiber, which imaging light fixture images the measuring beam and/or the reference beam in a defined illumination pattern (for example a point, a line or a circle) on the workpiece surface.

Advantageously, the measurement fiber and the reference fiber are spaced apart from one another at their workpiece-side end by a distance of a maximum of a few millimeters, so that the measurement beam and the reference beam are directed at different surface regions of the workpiece. The reference beam may be directed towards the surface of the component and the measuring beam may be directed towards the surface to be measured. Because the fiber ends are spaced apart, the two fibers abut each other only over a partial length.

In particular, it is preferred that the measuring device is arranged in a machining head of the laser machining device in order to measure a distance of the machining head from the workpiece. The measuring beam and, if necessary, the reference beam can also be spatially superimposed on the machining beam by means of a beam splitter of the machining head and focused by the machining optics on the measurement site.

The invention also relates to a method for measuring the distance between a reflective workpiece surface and a reflective reference surface of a workpiece by means of a fiber-guided interferometer during laser machining of the workpiece, wherein a measurement beam and a reference beam of the interferometer are guided in a measurement fiber and a reference fiber, wherein the measurement fiber and the reference fiber extend parallel next to one another over the entire length or a partial length thereof, in particular over the entire length of a shorter fiber if the lengths of the fibers differ, and are in thermal contact with one another, wherein the measurement beam and/or the reference beam are deflected jointly or separately from one another through the measurement surface and/or the reference surface.

Finally, the invention also relates to a method for measuring the distance between two surface regions of a workpiece by means of an interferometer during laser machining of the workpiece, wherein a measuring beam and a reference beam of the interferometer are reflected at the two surface regions, wherein the measuring beam and/or the reference beam are deflected jointly or separately from each other through a measuring surface and/or a reference surface. Preferably, the interferometer light is geometrically divided into a measurement beam and a reference beam at a step of the workpiece present between the two surface regions.

Further advantages of the invention emerge from the claims, the description and the drawings. The features mentioned above and those which will be mentioned below can be used individually as such or in any combination in the case of a plurality of features. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention.

Drawings

The figures show:

FIG. 1 is a measuring device for measuring a distance to a workpiece according to the invention, having a thermally coupled measuring fiber and a reference fiber;

FIG. 2 is another measuring device for measuring the separation between two surface regions of a workpiece according to the invention, having a thermally coupled measuring fiber and a reference fiber;

fig. 3a, 3b show a measuring device according to the invention for measuring the distance between two surface regions of a workpiece by means of a measuring beam and a reference beam, which are guided in the manner of free beam propagation (fig. 3a) and in a common transmission fiber (fig. 3 b); and

fig. 4a, 4b show a measuring device according to the invention for measuring the distance between two surface regions of a workpiece, having a transmission fiber which is arranged before (fig. 4a) or after (fig. 4b) a beam splitter.

In the following description of the figures, identical or functionally identical components are provided with the same reference numerals.

Detailed Description

A processing head 1 of a laser processing machine (not shown) shown in fig. 1 is used for processing a workpiece 2 by means of a processing laser beam (not shown) and has a measuring device 3 for measuring a distance a between a reflected workpiece surface 2a of the workpiece 2 and the processing head 1, more precisely a reference surface 4 of the processing head 1.

The measuring device 3 comprises an interferometer 5 having a beam splitter 6 which splits the interferometer light 7 of an interferometer light source 8 onto a measuring arm 9 as a measuring beam 10 and onto a reference arm 11 as a reference beam 12, and a detector 13 which detects the measuring beam 10 reflected at the workpiece surface 2a and the reference beam 12 reflected at the reference surface 4. The measuring arm 9 has a measuring fiber 14 and the reference arm 11 has a reference fiber 15, both of equal length and extending parallel side by side over their entire length and in thermal contact with each other. The reference surface 4 is formed by the mirrored fiber end of the reference fiber 15 facing the workpiece 2. Alternatively, the reference surface may also be formed by a separate mirror in the reference arm 11. In contrast to that shown in fig. 1, the measuring and

reference fibers

14, 15 may also be unequal in length, wherein in this case the two

fibers

14, 15 extend parallel side by side over the entire length of the shorter fiber, but only over part of the length of the longer fiber and are in thermal contact with each other.

In the measuring arm 9, the measuring beam 10 is guided through the measuring fiber 14, reflected on the workpiece surface 2a, and the reflected measuring beam 10 is coupled back into the measuring fiber 14 and deflected by the beam splitter 6 toward the detector 13. In the reference arm 11, the reference beam 12 is guided through a reference fiber 15, reflected on the reference surface 4 of the reference fiber 15 and the reflected reference beam 12 is diverted by the beam splitter 6 towards the detector 13. The measuring and

reference beams

14, 15, which are again converging, are detected by a detector 13, and the distance a between the workpiece surface 2a and the reference plane 4 can be determined from the interference. Since the two

optical fibers

14, 15 are thermally coupled, no significant temperature differences and thus no temperature-induced measurement errors occur during absolute distance measurement.

The two

optical fibers

14, 15 can be guided in thermal contact, for example as individual optical fibers, in a common light-conducting cable or hose. Alternatively, the measuring fiber 14 may also be formed by the core of a double-clad fiber and the reference fiber 15 by the inner cladding of the double-clad fiber.

In the measuring device 3 shown in fig. 2, in contrast to fig. 1, the measuring arm 9 has the first surface area 2a of the workpiece 2 as a reflective workpiece surface and the reference arm 11 has the second surface area 2b of the workpiece as a reflective reference surface. The measuring and

reference beams

10, 12 are guided through corresponding

optical fibers

14, 15 close to one another, but separately, toward the workpiece 2, so that the beam paths are slightly offset from one another. The reference beam 12 can thus be deflected to a first surface region 2a (for example the workpiece surface as a reference plane) and the measuring beam 10 can be deflected to a measuring point or measuring plane 2 b. The reconverged reflected measuring and

reference beams

14, 15 are detected by the detector 13 and the distance a between the two

workpiece surfaces

2a, 2b is determined from their interference. Due to the thermal coupling of the two

optical fibers

14, 15, no significant temperature differences and thus no temperature-induced errors occur during the relative distance measurement.

A steering or imaging light 16 is arranged between the workpiece 2 and the workpiece-side ends of the measuring and

reference fibers

14, 15. The measuring and/or

reference beams

10, 12 can be moved jointly or separately through the two

surface regions

2a, 2b by means of the deflecting optics 16 in order to carry out one-dimensional or two-dimensional measurements. Thereby, for example, the topography of the workpiece surface may be sensed and spatial averaging performed. The measuring and/or

reference beams

10, 12 can be shaped by means of the imaging optics 16 in order to generate a defined illumination pattern (for example a point, line or circle) on the

surface regions

2a, 2 b.

The reference surface for measuring the distance from the workpiece can be formed, as indicated in fig. 2 by a dashed line, by a mirror 4 which can be pivoted into a reference arm 11. In this way, it is possible to switch between two operating modes, namely the temperature-compensated measurement of the distance to the workpiece and the temperature-compensated measurement of the distance between the two surface regions. The measurement range of the interferometer 5 is determined by the spectral width of the interferometer light source 8 and the difference in optical path length between the measurement and

reference arms

9, 11. In order to be able to adapt the measuring range, the mirror 4 of the reference arm 11 can be moved along the optical axis and thus the path length of the reference arm 11 can be adapted.

In contrast to the illustration in fig. 2, the measuring and

reference fibers

14, 15 can be spaced apart from one another at their workpiece-side end by a spacing of a maximum of a millimeter, so that the measuring and

reference beams

10, 12 impinge on correspondingly spaced-apart

surface regions

2a, 2b of the workpiece 2.

The beam splitting does not have to be forced through the optical elements of the measuring device 3, but can be obtained by different reflection positions on the workpiece 2. In the case of the measuring device 3 shown in fig. 3a, the interferometer light 7, which is incident by the beam splitter 17 in a freely beam-propagating manner, is only geometrically split into the measuring beam 10 and the reference beam 12 at the step 18 of the workpiece 2, which is present between the two

surface regions

2a, 2b, i.e. the interferometer light 7 is reflected as the measuring beam 10 at the surface region 2a and as the reference beam 12 at the other surface region 2 b. When, for example, the spot of the interferometer light 7 on the workpiece surface during laser deep welding is larger than the diameter of the weld capillary, a part of the interferometer light 7 is reflected from the workpiece surface in the surroundings of the weld capillary and another part is reflected in the weld capillary, whereby the interferometer light 7 is "naturally" split into the measurement beam and the

reference beam

10, 12. The reflected measuring and

reference beams

10, 12, which are again converging, are diverted by a beam splitter 17 towards a detector 13. The distance a between the two

workpiece surfaces

2a, 2b can be determined from the interference detected there.

In contrast to fig. 3a, in the measuring device 3 shown in fig. 3b the interferometer light 7 is guided by means of a transmission fiber 19 toward the workpiece 2, and the measuring and

reference beams

10, 12 reflected on both

surface regions

2a, 2b are guided back to the beam splitter 17 by means of the transmission fiber 19.

In the case of the measuring device 3 shown in fig. 4a, 4b, the splitting of the interferometer light 7 into the measuring beam and the

reference beam

10, 12 takes place in a separate beam splitter 6, which is arranged in fig. 4a before the delivery fiber 19 and in fig. 4b after the delivery fiber. In fig. 4b, the beam splitter 5 is also used to direct the measuring and

reference beams

10, 12 at

different surface areas

2a, 2b of the workpiece 2.

Claims

1. A measuring device (3) for measuring a distance (A) between a reflected workpiece surface (2a) and a reflected reference surface (2b) of a workpiece (2) during laser machining of the workpiece (2), having an interferometer (5) with: a beam splitter (6; 18) which splits the interferometer light (7) onto the measuring arm (9) as a measuring beam (10) and onto the reference arm (11) as a reference beam (12); and a detector (13) which detects a measurement beam (10) reflected on the workpiece surface (2a) and a reference beam (12) reflected on the reference surface (2b), wherein the measurement arm (9) and the reference arm (11) are of equal length within a coherence length of the interferometer light (7), wherein,

the measuring arm (9) having a measuring fiber (14) and the reference arm (11) having a reference fiber (15) and the measuring arm (9) having a first surface area of the workpiece (2) as a reflective workpiece surface, characterized in that,

the measuring fiber (14) and the reference fiber (15) are formed by the core and the inner cladding of a double-clad fiber,

the measuring fiber (14) and the reference fiber (15) extend parallel side by side over their entire fiber length with the same length of the measuring fiber (14) and the reference fiber (15) or over the entire length of a shorter fiber with different lengths of the measuring fiber (14) and the reference fiber (15) and are in thermal contact with each other,

the reference arm (11) has a second surface area of the workpiece (2) as a reference surface for reflection, and

a deflection optics (16) is arranged between the workpiece (2) and the workpiece-side end of the measuring fiber (14), said deflection optics moving the measuring beam (10) directly over the first surface region in order to carry out a one-dimensional or two-dimensional measurement of the first surface region.

2. Measuring device according to claim 1, characterized in that the measuring fiber (14) and the reference fiber (15) are two separate fibers abutting each other over their entire length or over part of their length.

3. A measuring device according to claim 1 or 2, characterized in that the measuring fiber (14) and the reference fiber (15) are spaced apart from each other at their workpiece-side ends.

4. Measuring device according to claim 1 or 2, characterized in that the measuring device (3) is arranged in a machining head (1) of a laser machining machine.

5. A measurement method for measuring a distance (A) between a reflective workpiece surface (2a) and a reflective reference surface (2b) of a workpiece (2) by means of a fiber-guided interferometer (5) during laser processing of the workpiece (2), wherein a measurement beam (10) and a reference beam (12) of the interferometer (5) are guided in a measurement fiber (14) and in a reference fiber (15), wherein the measurement fiber (14) and the reference fiber (15) are formed by a core and an inner cladding of a double-clad fiber, wherein the measurement fiber (14) and the reference fiber (15) run parallel and short side by side over the entire fiber length thereof if the length of the measurement fiber (14) and the reference fiber (15) is the same or if the length of the measurement fiber (14) and the reference fiber (15) is different and are in thermal contact with one another, wherein the measuring beam (10) is directly deflected on the workpiece surface (2a) by means of a deflection optics (16) which moves the measuring beam (10) directly across the workpiece surface (2a) in order to carry out one-dimensional or two-dimensional measurements of the workpiece surface.