Calibration artifact

Abstract

To provide calibration artifacts that can be used in calibration process to inspect geometric misalignment of a machine.SOLUTION: A calibration artifact includes a substrate 22 on which a first group having at least one selectively activatable light source 21a is disposed, and a light source carrier 23 on which a second group comprising at least two selectively activatable light sources 21b is disposed on the substrate 22. The light source carrier 23 comprises a thermally invariant material.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a calibration artifact for checking geometric misalignment of a machine using an optical position measuring system. [Background technology]

[0002] In the patent document 1, an optical position measuring system is known, which is formed as a spatial 2D angle measuring system and can be used to determine position information in three-dimensional space. It comprises, on the one hand, a transmitting unit, which is arranged, for example, on a measuring object movable in space, whose spatial location and orientation in space is to be determined, for example one or more suitable light sources can act as transmitting units. On the other hand, one or more optical receiving units are arranged fixedly opposite the movable transmitting unit, which optical receiving units essentially each comprise a scanning grating and an optoelectronic detector. Therefore, alternatively, it can also be arranged such that the transmitting unit is fixed and the receiving units are arranged movably in space, and it is also possible to change the number of the transmitting units and the receiving units. With such a system, the orientation of the transmitting unit in space, i.e. its spatial location and orientation, can be measured by multiangulation. For this, the direction of the line of sight to the transmitting unit from each receiving unit is determined by two angle measurements. If the relative positions of the two receiving units are known, the orientation of the transmitting unit can be determined from the intersection of certain lines of sight. For further details of this measurement principle, the reader is referred to the aforementioned publication.

[0003] With such optical position measuring systems, it is possible to check the geometrical deviations present in the machine in a calibration process, if necessary. In the case of a machine tool, deviations from the actual position and orientation of the workpiece relative to the tool compared to the target position and orientation are ascertained in such a calibration process. For this reason, in a calibration process with a position measuring system, the orientation between several transmitting units or light sources in a defined spatial arrangement on the calibration artifact and at least one receiving unit of the position measuring system in the entire machining space of the machine tool is directly determined. For this reason, the light sources are arranged in a predefined position on the calibration artifact on the workpiece table in the machine tool, and the receiving units are arranged in a tool receptacle that is movable relative to it. Such a calibration is always necessary after the machine is assembled and during operation to check whether the required geometrical positioning accuracy in the machining space meets the requirements. In this case, the generated data can be used to improve the geometrical accuracy of the machine. For this reason, correction values ​​of the machine's motion axes are determined from the determined deviations, stored in a correction table and used during operation for precise relative positioning of the tool and the workpiece. To ensure accurate pose measurements, one must ensure that the relative placement of the multiple light sources remains unchanged during the calibration process and, if possible, is not altered by thermal and / or mechanical effects.

[0004] A similar procedure for calibrating a machine tool is known from DE 10 200 43 511 A1. Here, an interferometer acts as an optical position measuring system, with which length measurements are performed between an interferometer head in a tool receptacle and a number of retroreflectors arranged on a carrier plate on the workpiece table of the machine tool. In this solution, a carrier plate with four retroreflectors arranged at the corners of the carrier plate is therefore used as a calibration artifact that is spread over the workpiece table for the calibration process. However, such a calibration artifact is not suitable for the above-mentioned position measuring system, which performs several 2D angular measurements to determine the attitude. In addition, the relative positions of the four retroreflectors can change in an unspecified way during the calibration, for example due to heating of the carrier plate, and therefore an accurate determination of the calibration data of the machine tool cannot be guaranteed. Furthermore, the provided calibration artifact requires a specific geometry of the machine tool and therefore cannot be used globally. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO 01 / 38828 [Patent Document 2] European Patent Application Publication No. 2786839 [Patent Document 3] European Patent Application Publication No. 3739287 [Patent Document 4] European Patent Application Publication No. 3175949 [Patent Document 5] European Patent Application Publication No. 4145083 Summary of the Invention [Problem to be solved by the invention]

[0006] The problem of the invention is to identify a calibration artefact which can be used in a calibration process to check the geometric deviations of a machine, in which the attitude between the calibration artefact and at least one receiving unit of a position measuring system should be reliably determined by means of an optical position measuring system via several angle measurements, even in the case of thermal and / or mechanical influences on the calibration artefact. [Means for solving the problem]

[0007] This problem is solved by a calibration artefact having the features of claim 1.

[0008] Advantageous embodiments of the calibration artefact according to the invention result from the measures recited in the dependent claims.

[0009] The calibration artifact according to the invention is used for checking the geometrical deviation of a machine by means of an optical position measuring system. It comprises a substrate on which a first group with at least one selectively activatable light source is arranged. It further comprises a light source carrier arranged on the substrate, on which a second group with at least two selectively activatable light sources is arranged. The light source carrier is made of a thermally invariant material.

[0010] Preferably, the first group of light sources and the light source carrier are arranged on the same side of the substrate.

[0011] The light source carrier is formed in a rod shape, and the light sources can be arranged at regular intervals along the longitudinal extension of the light source carrier.

[0012] It may further be contemplated that the substrate and light source carrier each have one or more flat contact surfaces.

[0013] Preferably, the light source carrier is disposed within a recess in the substrate such that the light source carrier and the substrate can expand or contract largely independently of each other upon temperature changes.

[0014] In this case, the recess in the base is intended to be dimensioned to allow access to the contact surface of the light source carrier.

[0015] In this case, the light source carrier can be arranged in the recess of the base via a fixed bearing and at least one floating bearing.

[0016] Alternatively, the light source carrier is arranged in a recess of the base body via a web-shaped connecting element, and the contact surface between the connecting element and the light source carrier and between the connecting element and the recess in the base body are significantly smaller than the surface of the light source carrier and the recess in the base body; It may be contemplated that.

[0017] In one advantageous embodiment, the substrate is made of steel and the light source carrier is made of Invar.

[0018] Preferably, the light source is formed as an LED.

[0019] It is further possible that at least one electronic control unit is disposed on the substrate, and that the plurality of light sources are selectively operable via the electronic control unit.

[0020] In this case, several control units can be connected to one another via a bus system and each control unit and each light source can be assigned a defined bus address.

[0021] Advantageously, it is provided that the base has receiving recesses or recesses for arranging the light source, the control unit and for arranging the cable connections between these components.

[0022] Additionally, the base may have mounting elements for removably fastening to a workpiece table of a machine tool.

[0023] In one possible embodiment, - the base body is rotationally symmetrical, - a portion of the first group of light sources are equally spaced along a first circumference about an axis of rotational symmetry of the base; - another portion of the light sources of the first group are equally spaced along a second circumference about the axis of rotational symmetry of the base body, wherein the first circumference and the second circumference have different radii; - the rod-shaped light source carrier is arranged such that the longitudinal axis of the light source carrier intersects the axis of rotational symmetry of the base body; It is intended to be so.

[0024] The calibration artifact according to the invention can be advantageously used together with an optical position measuring system for measuring angles in order to check the geometric deviations of the machine. In this case, for example, the calibration artifact is spread on the workpiece table and the use in a machine tool is possible, where the receiving unit of the optical position measuring system is installed in the tool receptacle or in the tool spindle. Alternatively, a coordinate measuring machine can also be calibrated. By the measures according to the invention, it is ensured that the measured values ​​are not corrupted by thermal and / or mechanical influences during the calibration process. Any temperature-related deviations between the transmitting unit or the light source and the receiving unit of the position measuring system can be identified and, if necessary, corrected. Furthermore, the calibration artifact according to the invention makes it possible to measure large volumes of the machine's working space without complex modifications, simply by suitable movements of the linear and rotational axes of the machine, by positioning the receiving unit and / or the calibration artifact in the working space thereon.

[0025] Further details and advantages of the invention are explained in the following description of an embodiment of a calibration artifact according to the invention, together with the drawings. [Brief description of the drawings]

[0026] [Figure 1a] 1 shows a highly schematic view of a machine tool with a calibration artefact according to the invention arranged therein and a position measuring system used for the calibration; [Figure 1b] 3 shows another highly schematic view of a machine tool with a calibration artefact according to the invention arranged therein and a position measuring system used for the calibration; [Diagram 2] FIG. 1 shows an enlarged top view of the calibration artifact from FIGS. 1a and 1b. [Figure 3a] 1 shows a schematic cross-sectional view of an embodiment of a calibration artifact according to the invention; [Figure 3b] 1 shows a schematic cross-sectional view of an embodiment of a calibration artifact according to the invention; [Figure 4] 1 shows in a highly schematic diagram a calibration artefact according to the invention with a light source and a control unit connected to each other; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] 1a and 1b show in very schematic form a machine tool 10 in side and top view, in which an optical position measuring system is used to carry out the initially described calibration process using a calibration artefact 20 according to the invention.

[0028] These figures show a five-axis machine tool 10 allowing positioning of a tool holder 11 along three linear axes x, y, z, and rotation of a workpiece table 12 about two rotational axes C, A.

[0029] In the present embodiment, the optical position measuring system consists of a spatial 2D angle measuring system, as is basically known, for example from the patent document 1 cited at the outset. In this respect, the 2D angle measuring system comprises, on the one hand, a receiving unit 30 arranged in the tool receptacle 11 and, on the other hand, a number of selectively activatable light sources 21 on the calibration artefact. Furthermore, reference should be made to patent documents 3 and 4, which each show the application of such a 2D angle measuring system, and furthermore a possible construction of the receiving unit is known from patent document 5. Here, explicit reference is made to the disclosure content of these publications and the information contained therein for details of suitable 2D angle measuring systems.

[0030] Via the position measuring system, in the calibration process, the pose between the receiving unit 30 arranged in the tool receptacle 11 of the machine tool 10 and the calibration artefact 20 arranged on the workpiece table 12 of the machine tool 10 is determined. For this purpose, incremental angle measurements are performed between the receiving unit 30 and a number of selectively activatable light sources 21 on the calibration artefact 20, as shown diagrammatically in FIG. 1a. In this case, in FIGS. 1a and 1b, the light sources are assigned the reference number 21, and a detailed description of the possible arrangement of the light sources 21 on the calibration artefact 20 is given with reference to the following figures. From the rough estimation of the angles and mounting parameters determined by the position measuring system between the receiving unit 30 and the several light sources 21 at various measuring positions of the machine tool and the position of the calibration artefact 20 relative to the workpiece table 12, the exact position of the light sources 21 relative to the calibration artefact 20 and the pose between the receiving unit 30 and the calibration artefact 20 via polycentric surveying, in particular also between the tool receptacle 11 and the workpiece table 12, can be determined.

[0031] Based on Fig. 2 and Fig. 3a, a detailed description of a first embodiment of a calibration artefact 20 according to the invention is given below.

[0032] On the one hand, the calibration artefact 20 comprises a base body 22 which, in the illustrated embodiment, is made from steel and is plate-like and rotationally symmetrical about an axis of rotational symmetry S. In a specific example, the base body 22 has a regular dodecagon or a dodecagon in top view when viewed from above.

[0033] A first group of selectively activatable light sources 21a, for example formed as LEDs, is arranged on the base 22. For arranging the light sources 21a on or in the base 22, the base 22 is provided with, for example, mounting screws (not shown), receiving recesses or receiving cavities. Here, the first light source group comprises at least one such light source 21.a, in the present embodiment, 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 in an evenly distributed manner along a first circumference K1 around the axis of rotational symmetry S, in particular 12 of the 20 light sources 21a of the first group are arranged at regular intervals from one another along the first circumference K1, whereby, in the present invention, the light sources are arranged in an evenly spaced arrangement. In this embodiment, another part of the light sources 21a of the first group is arranged uniformly distributed along a second circumference K2 around the axis of rotational symmetry S, with the radii of the first and second circumferences K1, K2 being different as shown in the figure. In this case, 8 of the 20 light sources 21a of the first group are arranged equally spaced along the second circumference K2, which has a radius smaller than the first circumference K1.

[0034] Although not shown in the figures, there are mounting elements in the base 22, via which the base 22, and thus the calibration artefact 20, can be removably fixed to the workpiece table 12 of the machine tool 10. A suitable mounting element may, for example, be a threaded hole in the base 22, which can attach the calibration artefact 20 to the workpiece table 12 by means of a sliding block.

[0035] The calibration artefact according to the invention further comprises a light source carrier 23 arranged on the substrate 22, the light source carrier 23 being made of a thermally invariant material. In this respect a thermally invariant material is understood to be a material which does not show a change in length when the temperature changes within a given temperature range, i.e. the material has a thermal expansion coefficient of approximately zero. In a preferred embodiment, for example, Invar is a suitable material for the light source carrier 23, alternatively Zerodur or a suitable carbon material can also be used.

[0036] A second group of selectively activatable light sources 21b is arranged on the light source carrier 23, which light sources 21b are preferably also formed as LEDs. The second group consists of at least two selectively activatable light sources 21b, where in an embodiment of the invention four such light sources 21b are provided, which are present at precisely defined positions on the light source carrier 23. Depending on the choice of material of the light source carrier 23, the positions of the light sources 21b on the light source carrier 23 or the spacing of the light sources 21b from one another remain unchanged even in the case of temperature changes.

[0037] In the illustrated example, the first group includes 20 light sources and the second group includes 4 light sources, but essentially, in a minimum configuration, only three light sources in total can be used, one of which is assigned to the first group on the base and two of which are assigned to the second group on the light source carrier.

[0038] As can be seen from Fig. 2 and the schematic diagrams of Figs. 1a and 1b, 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 illustrated example, this is in this case the upper side of the base body 22, which upper side faces the receiving unit 30 in the tool cradle 11.

[0039] The light source carrier 23 made of a thermally invariant material is rod-shaped, and along the longitudinal extension of the light source carrier 23 the four light sources 21b of the second group are arranged at regular intervals from one another, with an even arrangement being envisaged in the present invention. The arrangement of the rod-shaped light source carrier 23 on the base body 22 is envisaged in such a way that the longitudinal axis of the light source carrier 23 intersects the axis of rotational symmetry S of the base body 22 at half its length. As can further be seen from FIG. 2, the light source carrier 23 in this embodiment has a length that corresponds approximately to the diameter of the second circumference K2 along which the eight light sources 21a of the first group are arranged. In principle, however, different arrangements and / or dimensionings of the light source carrier 23 are also conceivable.

[0040] The base body 22 and the light source carrier of the calibration artefact 20 each comprise one or more flat contact surfaces 22.1, 23.1. At the sides of the base body 22, the flat side faces located outside the perimeter of the dodecagon serve as contact surfaces, i.e. the base body 22 has twelve usable contact surfaces 22.1. At the rod-shaped light source carrier 23, two opposing flat rod end faces serve as corresponding contact surfaces 23.1. In the calibration process, these contact surfaces 22.1, 23.1 are used to at least roughly determine the position of the base body 22 on the workpiece table or the length of the rod-shaped light source carrier 23 by contacting them with a measuring probe that is extended in the tool receptacle 11. The position information thus obtained is further processed during the calibration.

[0041] As can be seen from the cross-sectional view in Fig. 3a, the base 22 of the calibration artefact 20 according to the invention has a recess 24 on the side where the light source carrier 23 is arranged, in which the light source carrier 23 is arranged. According to the plan view in Fig. 2, the recess 24 is rectangular like the light source carrier 23 and, as can be seen from Fig. 2 and also from Fig. 3a, is slightly larger than the light source carrier 23. On the one hand, such a dimensioning of the recess 24 ensures that the light source carrier 23 and the base 22 can expand and contract almost independently of each other during temperature changes. Furthermore, the dimensioning of the recess 24 in the base 22 ensures that the front contact surface 23.1 of the light source carrier 23 is accessible by a measuring probe during calibration.

[0042] In the illustrated example, in order to allow the above-mentioned relative expansion or contraction of the base 22 and the light source carrier 23 in the case of a change in temperature, it is further provided for the light source carrier 23 to be arranged in the recess 24 via a fixed bearing 25.1 and a floating bearing 25.2, in which case, for example, a sliding bearing or a fixed joint can be used as the floating bearing 25.2. 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 carrier 23 and the bottom 24.1 of the recess 24. In this way, the light source carrier 23, which has a fixed length, is thermally stably connected to the steel base 22. For example, if the base 22 expands due to an increase in temperature, the floating bearing 25.2 compensates for the resulting change in length in the base 22 and ensures that no forces are applied to the light source carrier 23 and that, therefore, the distance between the light sources 21b of the second group on the light source carrier does not change in an undefined way.

[0043] An alternative possibility for arranging the light source carrier 123 in a suitably dimensioned recess 124 in the base 122 is shown in Fig. 3b in a cross-sectional view similar to Fig. 3a. In the following, only the significant differences of this variant compared to the embodiment of Fig. 3a will be discussed.

[0044] Thus, two supports for arranging the light source carrier 123 on the bottom of the recess 124 in the base body 122 are no longer provided, but rather one connecting element 125 in the form of a web, which is arranged in the recess 124 between the bottom 124.1 of the recess 124 and the underside 123.2 of the light source carrier 123. In this case, the contact surface between the connecting element 125 and the light source carrier 123 and the contact surface of the recess 124 in the base body 122 are significantly smaller than the surfaces of the light source carrier 123 and the recess 124 of the base body 122, respectively. In the event of a temperature change, therefore, only very small stresses are introduced via the connecting element 125 onto the contact surface and thus onto the light source carrier 123. The base body 122 can thus freely expand or contract in the recess 124 in the event of a temperature change.

[0045] In another variant (not shown), the light source carrier can also be arranged via a central fixed bearing and two floating bearings in the outer area on the bottom of the recess in the base.

[0046] As already mentioned above, the light source on the substrate and the light source on the light source carrier are selectively activatable. This means that the light source can be switched on and off as required, so that during calibration, only a specific light source is used for angle measurement with the receiving unit. In this case, typically, the light source that is currently within the field of view of the receiving unit, i.e. the light source that has an unobstructed line of sight to the receiving unit, is activated or used.

[0047] In order to selectively activate the various light sources of the calibration artefact according to the invention, at least one electronic control unit is provided on the base, via which a certain number of light sources can be switched on and off. In Fig. 4 it is shown how a total of six such control units 226 are arranged in suitable receiving cavities or recesses on the base 222 for a particular embodiment of the calibration artefact 220 according to the invention. In the present invention, four light sources 221a, 221b can be controlled via each of the six control units 226, which are each connected to the assigned control unit via a suitable cable connection 227. Furthermore, the six control units 226 are connected to one another via a bus system 228, for example in a so-called daisy chain arrangement. The cable connection 227 and the bus system 228 are arranged on the base 222 in suitably formed receiving cavities or recesses, like the control units 226 and the light sources 221a, 221b.

[0048] Furthermore, the central control unit 30 is connected to a bus system 228, which performs position determination and necessary calculations etc. within the scope of the calibration process and synchronizes the control of the light sources 221a, 221b to be activated, whereby each control unit 226 as well as each light source 221a, 221b is assigned a defined bus address via which the central control unit 30 activates the appropriate light source 221a, 221b in the calibration process.

[0049] In addition to the described embodiment of a calibration artefact according to the invention, there are of course further design options within the scope of the invention.

[0050] It is therefore of course possible to adapt the size of the substrate and light source carrier to suit the machine to be calibrated, which therefore naturally includes adapting the number of light sources.

[0051] Furthermore, the shape of the substrate can also be adapted appropriately, i.e. it does not necessarily have to be formed in a plate-like and / or rotationally symmetrical manner: in order to better utilize the emission range of the light source, the substrate can also be formed essentially in a trough-, tower- or pyramidal shape.

[0052] Basically, different arrangements of the calibration artifact on the machine tool can also be envisaged, where the calibration artifact is arranged in the tool receptacle and one or more receiving units are arranged on the workpiece table, where the base body can be formed as a tower as already mentioned above and the first group of light sources can be arranged on the outside, where the light source carrier is arranged in a recess in an outer wall or the like.

Claims

1. A calibration artifact (20; 120; 220) for checking the geometric deviation of a machine using an optical position measuring system, a substrate (22; 122; 222) on which a first group is arranged with at least one selectively activatable light source (21a; 121a; 221a), a light source carrier (23; 123; 223) arranged on a base (22; 122; 222) on which a second group having at least two selectively activatable light sources (21b; 121b; 221b) is arranged, said light source carrier (23; 123; 223) being made of a thermally invariable material, Calibration artifacts (20; 120; 220).

2. 2. The 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 substrate (22; 122; 222).

3. 3. A calibration artifact (20; 120; 220) as claimed in claim 1 or 2, wherein the light source carrier (23; 123; 223) is formed in a rod shape and the light sources (21b; 121b; 221b) are arranged thereon at regular intervals along the longitudinal extension direction of the light source carrier (23; 123; 223).

4. A calibration artefact (20; 120; 220) according to at least one of claims 1 to 3, wherein the substrate (22; 122; 222) and the light source carrier (23; 123; 223) each have one or more flat contact surfaces (22.1, 23.1; 122.1, 123.1).

5. A calibration artifact (20; 120; 220) according to at least one of claims 1 to 4, wherein the light source carrier (23; 123; 223) is arranged in a recess (24; 124) in the base (22; 122; 222) such that the light source carrier (23; 123; 223) and the base (22; 122; 222) can expand or contract largely independently of each other upon temperature changes.

6. A calibration artifact (20; 120; 220) according to claims 4 and 5, wherein the recess (24; 124) in the base (22; 122; 222) is dimensioned such that the contact surface (23.1, 123.1) of the light source carrier (23; 123; 223) is accessible.

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

8. the light source carrier (123) is arranged in a recess (124) of the base (12) via a web-shaped connecting element (125), and the contact surface between the connecting element (125) and the light source carrier (123) and between the connecting element (125) and the recess (124) in the base (122) is significantly smaller than the surface of the light source carrier (123) and the recess (124) of the base (122); The calibration artifact (20) of claim 4.

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

10. Calibration artefact (20; 120; 220) according to at least one of claims 1 to 9, wherein the light sources (21; 21a, 21b; 121a, 121b; 221a, 221b) are formed as LEDs.

11. A calibration artifact (20; 120; 220) according to at least one of claims 1 to 10, further comprising at least one electronic control unit arranged on the base (22; 122; 222) and via which the plurality of light sources (21; 21a, 21b; 121a, 121b; 221a, 221b) can be selectively activated.

12. A calibration artifact (20; 120; 220) according to claim 11, in which on the substrate (22; 122; 222) several control units (226) are connected to each other via a bus system and each control unit (226) and each light source (21; 21a, 21b; 121a, 121b; 221a, 221b) is assigned a defined bus address.

13. A calibration artifact (20; 120; 220) according to claim 11, wherein the base (22; 122; 222) has accommodation recesses or recesses for arranging the light sources (21; 21a, 21b; 121a, 121b; 221a, 221b), the control unit and for arranging cable connections between these components.

14. Calibration artefact (20; 120; 220) according to at least one of claims 1 to 13, wherein the base (22; 122; 222) has mounting elements for removably fastening to a workpiece table (12) of a machine tool.

15. the base body (22; 122; 222) is of rotationally symmetrical design, a portion of the first group of light sources (21a; 121a; 221a) are equally spaced along a first circumference (K1) around the axis of rotational symmetry of the base (22; 122; 222), another portion of the first group of light sources (21a; 121a; 221a) is equally spaced along a second circumference (K2) around the axis of rotational symmetry of the body (22; 122; 222), with the radii of the first circumference (K1) and the second circumference (K2) being different; the light source carrier (23; 123; 223) formed in rod-like form is arranged such that its longitudinal axis intersects the axis of rotational symmetry of the base body (22; 122; 222); A calibration artefact (20; 120; 220) according to at least one of the claims 1 to 14.

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