CALIBRATION ARTIFACT

Description

AREA OF TECHNOLOGY

[0001] The present invention relates to a calibration artifact for checking the geometric deviations of a machine using an optical position measuring system. STATE OF THE ART

[0002] From WO 01 / 38828 A1, an optical position measuring system is known, designed as a spatial 2D angle measuring system and used to determine position information in three-dimensional space. This system comprises, on the one hand, a transmitting unit, which is arranged, for example, on a measuring object moving in space, whose spatial position and orientation are to be determined; one or more suitable light sources can serve as the transmitting unit. On the other hand, one or more optical receiving units are provided, stationary opposite the moving transmitting unit, each essentially comprising a scanning grating and an optoelectronic detector. Alternatively, the transmitting unit can be stationary and the receiving unit movable in space; the number of transmitting and receiving units can also vary.Using such a system, the position and orientation of the transmitting unit can be measured in space via multiangulation. For this purpose, the direction of the line of sight to the transmitting unit is determined from the respective receiving unit using two angle measurements. Given the known relative positions of two receiving units, the position of the transmitting unit can be determined from the intersection of the defined lines of sight. For further details of this measurement principle, please refer to the aforementioned publication.

[0003] With such an optical position measuring system, it is possible to check any geometric deviations that may be present in a machine during a calibration process. In the case of a machine tool, such a calibration process detects deviations between the actual position and orientation of the workpiece relative to the tool and the target position and orientation. For this purpose, the position measuring system directly determines the pose between several transmitting units or light sources in a defined spatial arrangement on a calibration artifact and at least one receiving unit of the position measuring system within the entire machining area of ​​the machine tool. The light sources are arranged at defined positions on the calibration artifact on the workpiece table in the machine tool, and the receiving unit is located in the tool holder, which is movable relative to it.Such calibration is required repeatedly after machine assembly and later during operation to verify that the required geometric positioning accuracy in the machining area meets the specifications. The data generated during this process can be used to improve the machine's geometric accuracy. Correction values ​​for the machine's axes of movement are determined from the measured deviations, stored in a compensation table, and used during operation for the precise relative positioning of the tool and workpiece. To ensure accurate pose measurement, it is essential that the relative arrangement of the majority of light sources remains unchanged during the calibration process and is not altered by thermal and / or mechanical effects.

[0004] A similar procedure for calibrating machine tools is known from EP 2 786 839 A1. In this case, an interferometer serves as the optical position measuring system, with which length measurements are taken between the interferometer head in the tool holder and several retroreflectors arranged on a carrier plate on the workpiece table of the machine tool. The carrier plate with the four retroreflectors, arranged in the corners of the carrier plate, thus serves as the calibration artifact, which is clamped onto the workpiece table for the calibration process. However, such a calibration artifact is not suitable for the position measuring system described at the beginning, which uses several 2D angle measurements to determine the pose. Furthermore, the relative positions of the four retroreflectors change during calibration, e.g.,The heating of the carrier plate can cause undefined changes, thus preventing the accurate determination of calibration data for the machine tool. Furthermore, the intended calibration artifact requires a specific machine tool geometry and therefore cannot be used universally. SUMMARY OF THE INVENTION

[0005] The present invention is based on the objective of providing a calibration artifact that can be used in a calibration process to check the geometric deviations of a machine. In the calibration process, the pose between the calibration artifact and at least one receiving unit of the position measuring system is to be reliably determined by means of an optical position measuring system via several angle measurements, even if thermal and / or mechanical influences act on the calibration artifact.

[0006] This problem is solved according to the invention by a calibration artifact having the features of claim 1.

[0007] Advantageous embodiments of the calibration artifact according to the invention result from the measures listed in the dependent claims.

[0008] The calibration artifact according to the invention serves to check the geometric deviations of a machine using an optical position measuring system. It comprises a base body on which a first group with at least one selectively activatable light source is arranged. Furthermore, it has a light source carrier arranged on the base body, on which a second group with at least two selectively activatable light sources is arranged, the light source carrier being made of a thermally invariant material.

[0009] Preferably, the first group of light sources and the light source carrier are arranged on the same side of the base body.

[0010] It is possible that the light source support is rod-shaped and that the light sources are arranged on it at fixed intervals along the longitudinal direction of the light source support.

[0011] Furthermore, it may be provided that both the base body and the light source carrier each have one or more flat touch surfaces.

[0012] Preferably, the light source carrier is arranged in a recess of the base body in such a way that both the light source carrier and the base body can expand or contract largely independently of each other in the event of temperature changes.

[0013] It is provided that the recess in the base body is dimensioned in such a way that the touch surfaces of the light source carrier are accessible.

[0014] In this case, the light source carrier can be arranged in the recess of the base body via a fixed bearing and at least one sliding bearing.

[0015] Alternatively, it may be provided that the light source carrier is arranged in the recess of the base body via a web-shaped connecting element, and wherein the contact surfaces between the connecting element and the light source carrier and the recess in the base body are significantly smaller than the surfaces of the light source carrier and the recess of the base body.

[0016] In an advantageous embodiment, the base body is made of steel and the light source support is made of Invar.

[0017] Preferably, the light sources are designed as LEDs.

[0018] It is also possible that at least one electronic control unit is arranged on the base body, via which a plurality of light sources can be selectively activated.

[0019] In this system, several control units can be interconnected via a bus system, and each control unit and each light source can be assigned a defined bus address.

[0020] It is advantageous that the base body has recesses or cutouts for arranging the light sources, the control units, and for arranging cable connections between these components.

[0021] Furthermore, it is possible that the base body has mounting elements for detachable fastening to the workpiece table of a machine tool.

[0022] In one possible embodiment, it is provided that the base body is rotationally symmetric, and a portion of the light sources of the first group are arranged at equal intervals along a first circular line around the rotational symmetry axis of the base body, and another portion of the light sources of the first group are arranged at equal intervals along a second circular line around the rotational symmetry axis of the base body, the radii of the first and second circular lines being different, and the rod-shaped light source support is arranged such that its longitudinal axis intersects the rotational symmetry axis of the base body.

[0023] The calibration artifact according to the invention can be advantageously used in conjunction with an angle-measuring optical position measuring system to check the geometric deviations of machines. For example, it can be used in a machine tool, where the calibration artifact is clamped onto the workpiece table and the receiving unit of the optical position measuring system is located in the tool holder or tool spindle. Alternatively, a coordinate measuring machine can also be calibrated with it. Due to the measures according to the invention, it is ensured that thermal and / or mechanical influences during the calibration process do not distort the measurements. Any temperature-related displacement between the transmitting units or light sources and the receiving unit of the position measuring system can be identified and corrected if necessary.Furthermore, the calibration artifact according to the invention enables a large volume of the machine's working space to be measured without costly modifications, solely by suitable movements of the machine's linear and rotary axes; this is achieved by positioning the receiving unit and / or the calibration artifact in the working space.

[0024] Further details and advantages of the present invention will be explained with reference to the following description of exemplary embodiments of the calibration artifact according to the invention in conjunction with the figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] It shows Figures 1a and 1b each show a highly schematic representation of a machine tool with the calibration artifact according to the invention arranged therein and the position measuring system used for calibration in different views; Figure 2 shows an enlarged top view of the calibration artifact made from the Figures 1a, 1bFigures 3a and 3b each show a schematic sectional view of embodiments of the calibration artifact according to the invention; Figure 4 shows a highly schematic representation of a calibration artifact according to the invention with the interconnected light sources and a control unit. DESCRIPTION THE FORMS OF EXECUTION

[0026] In the Figures 1a and 1b A machine tool 10 is shown in a highly schematic form in a side view and in a top view, in which an optical position measuring system is used to carry out the calibration process described above with the aid of the calibration artifact 20 according to the invention.

[0027] The figures show a 5-axis machine tool 10 which allows positioning of a tool holder 11 along the three linear axes x, y, z as well as pivoting of the workpiece table 12 about the two rotational axes C, A.

[0028] In the present embodiment, the optical position measuring system consists of a spatial 2D angle measuring system, as is known, for example, from the aforementioned publication WO 01 / 38828 A1. The 2D angle measuring system comprises, on the one hand, a receiving unit 30 arranged in the tool holder 11 and, on the other hand, a plurality of selectively activatable light sources 21 on the calibration artifact. Reference is also made here to EP 3 739 287 A1 and EP 3 175 949 A1, which each describe the application of such 2D angle measuring systems; a possible design of the receiving unit is also known from EP 4 145 083 A1. Reference is hereby expressly made to the disclosure content of these publications and the information contained therein regarding details of suitable 2D angle measuring systems.

[0029] During the calibration process, the position measuring system determines the pose between the receiving unit 30, located in the tool holder 11 of the machine tool 10, and the calibration artifact 20, which is located on the workpiece table 12 of the machine tool 10. For this purpose, incremental angle measurements are taken between the receiving unit 30 and the several selectively activatable light sources 21 on the calibration artifact 20, as shown in Figure 1a is indicated schematically. In the Figures 1a and 1bThe reference symbol 21 is assigned to the majority of light sources; a detailed description of the possible arrangement of the light sources 21 on the calibration artifact 20 is given with reference to the following figures. From the angles determined with the aid of the position measuring system between the receiving unit 30 and several light sources 21 at different measuring positions of the machine tool and a rough estimate of the mounting parameters, the position of the calibration artifact 20 on the workpiece table 12, the exact positions of the light sources 21 on the calibration artifact 20, and, via multiangulation, the poses between the receiving unit 30 and the calibration artifact 20, and thus also the poses between the tool holder 11 and the workpiece table 12, can be determined.

[0030] Based on the Figures 2 and 3a The detailed description of a first embodiment of the calibration artifact 20 according to the invention follows.

[0031] The calibration artifact 20 comprises, on the one hand, a base body 22, which in the illustrated embodiment is made of steel and is plate-shaped and rotationally symmetrical about an axis of rotational symmetry S. In this specific example, the base body 22 has the shape of a regular dodecagon or dodecadon in a top view.

[0032] A first group of selectively activatable light sources 21a, e.g., designed as LEDs, is arranged on the base body 22. For arranging the light sources 21a on or in the base body 22, mounting threads, receiving recesses, or cutouts are provided in the base body 22 (not shown in the figures). The first group of light sources comprises at least one such light source 21a; in the present example, a total of 20 light sources 21a are provided in the first group. In the illustrated example, some of the light sources 21a of the first group are arranged uniformly distributed along a first circle K1 around the axis of rotational symmetry S; specifically, twelve of the 20 light sources 21a of the first group are arranged along the first circle K1 at fixed intervals from one another, with an equally spaced arrangement of the light sources being provided in this example.In this embodiment, another portion of the light sources 21a of the first group are arranged uniformly distributed along a second circle K2 around the axis of rotational symmetry S, the radii of the first and second circles K1 and K2 differing, as can be seen. In this case, eight of the 20 light sources 21a of the first group are arranged at equal intervals along the second circle K2, which has a smaller radius than the first circle K1.

[0033] Not shown in the figures are mounting elements in the base body 22, by means of which the base body 22, and thus the calibration artifact 20, can be detachably attached to the workpiece table 12 of the machine tool 10. Suitable mounting elements could, for example, be screw holes in the base body 22, which allow the calibration artifact 20 to be attached to the workpiece table 12 using T-nuts.

[0034] The calibration artifact according to the invention further comprises a light source carrier 23 arranged on the base body 22, which consists of a thermally invariant material. A thermally invariant material is understood here to be a material that does not exhibit any changes in length when exposed to temperature changes within a predetermined temperature range; i.e., the coefficient of thermal expansion of the material is approximately zero. In a preferred embodiment, for example, Invar would be a suitable material for the light source carrier 23; alternatively, Zerodur or a suitable carbon material could also be used.

[0035] A second group of selectively activatable light sources 21b, preferably also designed as LEDs, is arranged on the light source carrier 23. The second group consists of at least two selectively activatable light sources 21b, with four such light sources 21b being provided in the present example, located at precisely defined positions on the light source carrier 23. Due to the choice of material for the light source carrier 23, it is ensured that the positions of the light sources 21b on the light source carrier 23, or their distances from each other, remain unchanged even in the event of temperature changes. While in the illustrated example the first group comprises 20 light sources and the second group four light sources, in principle, a minimum configuration can use only three light sources in total, one of which is assigned to the first group on the base body and two to the second group on the light source carrier.

[0036] As from Figure 2 as well as from the depictions in the Figures 1a and 1b As can be seen, both the first group of light sources 21a and the light source carrier 23 are arranged on the same side of the plate-shaped base body 22. In the example shown, this is the top side of the base body 22, which faces the receiving unit 30 in the tool holder 11.

[0037] The light source support 23, made of the thermally invariant material, is rod-shaped; along the longitudinal direction of the light source support 23, the four light sources 21b of the second group are arranged at fixed distances from one another, with an equally spaced arrangement being provided here. The arrangement of the rod-shaped light source support 23 on the base body 22 is such that its longitudinal axis intersects the rotational symmetry axis S of the base body 22 at mid-length. As further shown from Figure 2As can be seen, in this embodiment the light source support 23 has a length approximately corresponding to the diameter of the second circular line K2, along which eight light sources 21a of the first group are arranged. However, a different arrangement and / or dimensioning of the light source support 23 would also be conceivable.

[0038] Both the base body 22 of the calibration artifact 20 and the light source carrier each have one or more planar probing surfaces 22.1, 23.1. On the base body 22, the planar side surfaces located on the outer edge of the dodecagonal circumference serve as probing surfaces; that is, the base body 22 has twelve usable probing surfaces 22.1. On the rod-shaped light source carrier 23, the two opposing planar end faces function as corresponding probing surfaces 23.1. In the calibration process, these probing surfaces 22.1, 23.1 are used to determine, at least roughly, the position of the base body 22 on the workpiece table or the length of the rod-shaped light source carrier 23 by probing with a measuring probe clamped in the tool holder 11. The position information obtained in this way is further processed within the calibration process.

[0039] As seen in the sectional view in Figure 3aAs can be seen, the base body 22 of the calibration artifact 20 according to the invention has a recess 24 on the arrangement side of the light source carrier 23, in which the light source carrier 23 is arranged. The recess 24 is shown in the top view in Figure 2 how the light source carrier 23 is rectangular and how from Figure 2 or 3a, which is evidently somewhat larger than the light source carrier 23. This dimensioning of the recess 24 ensures, on the one hand, that the light source carrier 23 and the base body 22 can expand and contract largely independently of each other in the event of temperature changes. Furthermore, the dimensioning of the recess 24 in the base body 22 ensures that the end-face contact surfaces 23.1 of the light source carrier 23 are accessible to the measuring probe during calibration.

[0040] To allow for the aforementioned relative expansion and contraction of the base body 22 and the light source support 23, for example, during temperature changes, the illustrated example further provides for the light source support 23 to be arranged in the recess 24 via a fixed bearing 25.1 and a floating bearing 25.2; the floating bearing 25.2 can be, for example, a sliding bearing or a rigid joint. As can be seen, the fixed bearing 25.1 and the floating bearing 25.2 are arranged between the underside 23.2 of the light source support 23 and the bottom 24.1 of the recess 24. In this way, the length-invariant light source support 23 is thermally stably connected to the steel base body 22. If, for example, the base body 22 expands due to a temperature increase, the loose bearing 25.2 compensates for the resulting change in length in the base body 22 and ensures that no forces are exerted on the light source support 23 and that this does not cause any damage.The distances between the light sources 21b of the second group on the light source carrier change in an undefined manner.

[0041] An alternative possibility for arranging the light source carrier 123 in a suitably dimensioned recess 124 of the base body 122 is shown in Figure 3b in a sectional view analogous to Figure 3a shown. The following only highlights the key differences between this variant and the example in Figure 3a received.

[0042] Instead of two bearings for mounting the light source carrier 123 on the floor of the recess 124 in the base body 122, a web-shaped connecting element 125 is provided, which is arranged in the recess 124 between the floor 124.1 of the recess 124 and the underside 123.2 of the light source carrier 123. The contact surfaces between the connecting element 125 and the light source carrier 123 and the recess 124 in the base body 122 are each significantly smaller than the surfaces of the light source carrier 123 and the recess 124 of the base body 122. Therefore, only negligibly small stresses are introduced onto the contact surfaces and thus onto the light source carrier 123 via the connecting element 125 in the event of any temperature changes. The base body 122 can therefore expand or contract freely in the recess 124 iw even when the temperature changes.

[0043] In another variant - not shown in the figures - it would also be possible to arrange the light source carrier on the floor of the recess in the base body via a central fixed bearing and two sliding bearings in the outer areas.

[0044] As mentioned above, both the light sources on the base and those on the light source carrier can be selectively activated. This means that the light sources can be switched on and off as needed, so that only specific light sources are used for angle measurements with the receiver unit during calibration.

[0045] Typically, those light sources are activated or used that are currently within the field of view of the receiving unit, i.e., those light sources whose line of sight to the receiving unit is not interrupted.

[0046] For the selective activation of the various light sources of the calibration artifact according to the invention, at least one electronic control unit is provided on the base body, via which a specific number of light sources can be switched on and off. Figure 4Figure 1 illustrates a specific embodiment of a calibration artifact 220 according to the invention, showing how a total of six such control units 226 are arranged on the base body 222 in suitable receiving recesses or cutouts. Each of the six control units 226 can control four light sources 221a, 221b, which are connected to the respective control unit via a suitable cable connection 227. The six control units 226 are also interconnected via a bus system 228, for example in a so-called daisy-chain arrangement. The cable connections 227 and the bus system 228, like the control units 226 and the light sources 221a, 221b, are arranged on the base body 222 in suitably designed receiving recesses or cutouts.

[0047] A central control unit 30 is also connected to the bus system 228. This unit handles the position determination and the necessary calculations, etc., within the calibration process and synchronizes the activation of the light sources 221a and 221b. Each control unit 226 and each light source 221a and 221b is assigned a defined bus address, which the central control unit 30 uses to activate the appropriate light sources 221a and 221b during the calibration process.

[0048] In addition to the exemplary embodiments of the calibration artifacts according to the invention described above, there are of course further possibilities for their design within the scope of the present invention.

[0049] It is therefore possible to adjust the size of the base body as well as the light source carrier to suit the machine being calibrated; this naturally also entails an adjustment of the number of light sources.

[0050] Furthermore, the shape of the base body can also be suitably adapted; that is, it does not necessarily have to be plate-shaped and / or rotationally symmetrical. To better utilize the emission range of the light sources, the base body could also be designed in a trough, tower, or pyramid shape.

[0051] In principle, a different arrangement of the calibration artifact in a machine tool could also be provided. In this case, the calibration artifact would be located in the tool holder, and one or more receiving units would be on the workpiece table. The base body could then be designed in a tower shape, as mentioned above, with light sources of the first group arranged on the outer sides; the light source carrier would then be located in a recess in the outer wall, etc.

Claims

1. Calibration artifact (20; 120; 220) for checking the geometric deviations of a machine using an optical position measuring system, comprising - a main body (22; 122; 222) on which a first group comprising at least one selectively activatable light source (21a; 121a; 221a) is arranged, and - a light source carrier (23; 123; 223) which is arranged on the main body (22; 122; 222) and on which a second group comprising at least two selectively activatable light sources (21b; 121b; 221b) is arranged, wherein the light source carrier (23; 123; 223) consists of a thermally invariant material.

2. Calibration artifact (20; 120; 220) according to Claim 1, wherein the first group of light sources (21a; 121a; 221a) and the light source carrier (23; 123; 223) are arranged on the same side of the main body (22; 122; 222).

3. Calibration artifact (20; 120; 220) according to Claim 1 or 2, wherein the light source carrier (23; 123; 223) is of rod-like design and the light sources (21b; 121b; 221b) are arranged on it at fixed distances along the direction of longitudinal extent of the light source carrier (23; 123; 223).

4. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein the main body (22; 122; 222) and the light source carrier (23; 123; 223) each have one or more planar probing surfaces (22.1, 23.1; 122.1, 123.1).

5. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein the light source carrier (23; 123; 223) is arranged in a recess (24; 124) of the main body (22; 122; 222) in such a way that the light source carrier (23; 123; 223) and the main body (22; 122; 222) can expand or contract largely independently of each other in the event of changes in temperature.

6. Calibration artifact (20; 120; 220) according to Claims 4 and 5, wherein the recess (24; 124) in the main body (22; 122; 222) is dimensioned in such a way that the probing surfaces (23.1, 123.1) of the light source carrier (23; 123; 223) are accessible.

7. Calibration artifact (20) according to Claim 5, wherein the light source carrier (23) is arranged in the recess (24) of the main body (22) via a fixed bearing (25.1) and at least one floating bearing (25.2).

8. Calibration artifact (120) according to Claim 4, wherein - the light source carrier (123) is arranged in the recess (124) of the main body (122) via a web-like connecting element (125), and - wherein the contact surfaces between the connecting element (125) and the light source carrier (123) and the recess (124) in the main body (122) are considerably smaller than the surfaces of the light source carrier (123) and the recess (124) of the main body (122).

9. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein the main body (22; 122; 222) consists of steel and the light source carrier (23; 123; 223) consists of Invar.

10. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein the light sources (21; 21a, 21b; 121a, 121b; 221a, 221b) are designed as LEDs.

11. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein at least one electronic actuation unit is further arranged on the main body (22; 122; 222), a plurality of light sources (21; 21a, 21b; 121a, 121b; 221a, 221b) being selectively activatable via the electronic actuation unit.

12. Calibration artifact (20; 120; 220) according to Claim 11, wherein a plurality of actuation units (226) are connected to each other via a bus system on the main body (22; 122; 222) and a defined bus address is assigned to each actuation unit (226) and to each light source (21; 21a, 21b; 121a, 121b; 221a, 221b).

13. Calibration artifact (20; 120; 220) according to Claim 11, wherein the main body (22; 122; 222) has receiving cavities or receiving recesses for arrangement of the light sources (21; 21a, 21b; 121a, 121b; 221a, 221b), the actuation units and for arrangement of cable connections between these components.

14. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein the main body (22; 122; 222) has mounting elements for releasable fixing on the workpiece table (12) of a machine tool.

15. Calibration artifact (20; 120; 220) according to at least one of the preceding claims, wherein - the main body (22; 122; 222) is of rotationally symmetrical design, and - a portion of the light sources (21a; 121a; 221a) of the first group are arranged at equal distances along a first circular line (K1) around the axis of rotational symmetry of the main body (22; 122; 222), and - a further portion of the light sources (21a; 121a; 221a) of the first group are arranged at equal distances along a second circular line (K2) around the axis of rotational symmetry of the main body (22; 122; 222), wherein the radii of the first and second circular lines (K1, K2) are different, and - the light source carrier (23; 123; 223) of rod-like design is arranged such that its longitudinal axis crosses the axis of rotational symmetry of the main body (22; 122; 222).