KINEMATIC CALIBRATION

Abstract

A calibration method for numerical controlled machine tools (1), which uses a kinematic model to generate a compensation model for the positioning error occurring with the movement of the linear (X, Y, Z) and rotation axes (B, C, A) of a machine tool (1). The calibration method measures the positions of a calibration ball (6) with a measurement sequence which includes the combined movement of the calibration ball (6) around two rotation axes (C, B or C, A), wherein the measurements around a first rotation axis (C) include at least two rotational position movements around a second axis (B or A).

Claims

1. A calibration method for a numerical controlled machine tool (1), wherein the numerical controlled machine tool (1) comprises a working table (2) and a machine head (3) with a main spindle (4), the numerical controlled machine tool (1) further comprises at least 3 linear axes (X, Y, Z) and at least 2 rotation axes (A, B, C) by which the main spindle (4) and the working table (2) can be moved relatively to each other, the numerical controlled machine tool (1) having a jig (5) with a calibration ball (6) mounted on the working table (2) and the main spindle (4) having mounted a touch probe (7), wherein the calibration method includes the step to measure the position of the calibration ball (6) by conducting with the touch probe (7) at least 4, preferably 5, contact measurements with the mounted touch probe (7), wherein the position of the calibration ball (6) is changed at least three times by rotating the working table (2) or the machine head (3) around the rotation axes (A, B, C) and the step of measuring the position of the calibration ball (6) is repeated for every position change of the calibration ball (6), and wherein the calibration method uses a kinematic model for generating from the at least three measured positions of the calibration ball (6) a compensation model to compensate the kinematic errors occurring with a relative movement between the working table (2) and the main spindle (4), wherein the calibration method measures the positions of the calibration ball (6) with a measurement sequence which includes the combined movement of the calibration ball (6) around two, preferably three, rotation axes (A, C, B), wherein the measurements around a first rotation axis (C, B, A) include at least two, preferably m, rotational position movements around a second axis (B, A, C).

2. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein the measurement sequence for the rotational positions (C.sub.i; B.sub.j) of the calibration ball (6) is $\left(C_1;B_1\right),\left(C_2;B_1\right),\left(C_3;B_1\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_1\right)$ $\left(C_1;B_2\right),\left(C_2;B_2\right),\left(C_3;B_2\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_2\right)$ $\left(C_1;B_3\right),\left(C_2;B_3\right),\left(C_3;B_3\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_3\right)$ $\mathrm{.Math.}.Math.\left(C_1;B_m\right),\left(C_2;B_m\right),\left(C_3;B_m\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_m\right)$ $\mathrm{or}.Math.\left(C_{1,1};B_1\right),\left(C_{2,1};B_1\right),\left(C_{3,1};B_1\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,1};B_1\right)$ $\left(C_{1,2};B_2\right),\left(C_{2,2};B_2\right),\left(C_{3,2};B_2\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,2};B_2\right)$ $\left(C_{1,3};B_3\right),\left(C_{2,3};B_3\right),\left(C_{3,3};B_3\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,3};B_3\right)$ $\mathrm{.Math.}.Math.\left(C_{1,m};B_m\right),\left(C_{2,m};B_m\right),\left(C_{3,m};B_m\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,m};B_m\right).$

3. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein to describe the relative movements between the working table (2) and the main spindle (4) the kinematic model uses homogeneous transformation matrices for considering the kinematic errors of the rotation axes (A, B, C).

4. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein the position of the calibration ball (6) is measured in the coordinates of the linear axes X, Y and Z, wherein each measured calibration ball position (X.sub.k, Y.sub.k, Z.sub.k) is memorized in a position table.

5. The calibration method for a numerical controlled machine tool (1) according to claim 4, wherein the touch probe (7) is mounted in the main spindle (4) and is a trigger-based contact measurement device transmitting its trigger signal to the numerical controlled machine tool (1) through wireless transmission, preferably the touch probe (7) is also used to determine the position of a part to be machined.

6. The calibration method for a numerical controlled machine tool (1) according to claim 5, wherein the kinematic model of the calibration method calculates the kinematic error occurring by the movement of the rotation axes (A, B, C) based on the linear axis positions (X.sub.k, Y.sub.k, Z.sub.k) memorized in the position table.

7. The calibration method for a numerical controlled machine tool (1) according to claim 6, wherein the compensation model generated by the kinematic model includes an optimization algorithm for compensating the kinematic errors occurring with a relative movement between the working table (2) and the main spindle (4).

8. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein when the measurement sequence includes the combined movement of the calibration ball (6) around two rotation axes (C, B, A), the kinematic model considers 8 compensation parameters characterizing the kinematic errors of the rotation axes (A0C, B0C, X0C, Y0C, A0B, C0B, X0B, Z0B or A0C, B0C, X0C, Y0C, B0A, C0A, Y0A, Z0A, respectively).

9. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein the kinematic model uses the position errors of the calibration ball (6) in respect of the calibration ball's theoretically correct position to generate a compensation model for the numerical controlled machine tool (1) to compensate the positioning error of the working table movements in respect of the main spindle.

10. The calibration method for a numerical controlled machine tool (1) according to claim 1, wherein the kinematic model generating a compensation model is calculated by a processing unit located outside the numerical controlled machine tool (1) and wherein the compensation model is transmitted by communication means, preferably via telecommunication means, to the numerical control system of the numerical controlled machine tool (1).

11. A numerical controlled machine tool (1) using a calibration method according to claim 1, wherein the calibration method is applied by the machine control unit of the numerical controlled machine tool (1).

12. The numerical controlled machine tool (1) according to claim 11, wherein the kinematic model generating a compensation model is calculated by a processing unit located outside the numerical controlled machine tool (1), preferably the calculated compensation model is transmitted from the processing unit via telecommunication means, most preferably via internet or telephone connection, to the numerical controlled machine tool (1).

Description

DRAWINGS

[0021] The invention and its application will be described in the following with reference to attached figures. The figures enclosed to this specification show:

[0022] FIG. 1 Shows a typical multi-axis machine tool with 3 linear and two rotational degrees of freedom

[0023] FIG. 2 Shows the working table of FIG. 1 with clamped jig and calibration ball

[0024] FIGS. 3 to 6 Show the steps of the inventive calibration method will be described based on an example.

DETAILED DESCRIPTION

[0025] FIG. 1 shows a typical multi-axis machine tool 1 with three linear axes X, Y and Z and two rotational axes B and C. The working table 2 can be moved linearly along the axis X. The linear movements along the Y and Z axis are on the other hand conducted by the machining head 3. The working table 2 can be rotated around the rotation axis C and B. A further rotational axis A could be implemented in the machine tool 1, for instance by the foreseeing possibility to tilt the machine head 3 around the X axis (see arrow A).

[0026] FIG. 2 shows the working table 2 already displayed in FIG. 1 on which a jig 5 is mounted with a calibration ball 6 on its top. The position of the calibration ball 6 respectively of its center is measured by a touch probe 7 mounted on the machine head 3 of the machine tool 1.

[0027] In the following, the inventive calibration method will be described based on an example. FIG. 3 presents the whole proposed calibration procedure. Before starting the kinematic calibration of the rotation axes C and B of the machine tool 1 (see foregoing FIGS. 1 and 2), the linear axes X, Y and Z as well as the touch probe 7 of the machine tool 1 must be properly calibrated. That calibration is conducted according to the state of the art. Once the jig 5 with the calibration ball 6 is mounted on the working table 2, the positions of the rotation axes to investigate are predefined (for example as illustrated in the coming FIG. 6, the positions of the C and of the B axes, C.sub.i and B.sub.j with i=1,j, 2,j, 3,j . . . n,j and j=1, 2 . . . m). Subsequently, the measurement of the calibration ball 6 in the different predefined rotational axis positions is executed and the new kinematic model is computed based on the measured positions of the calibration ball. The machine controller of the machine tool is subsequently updated with the determined kinematic model to compensate the actual positioning errors occurring with movement of the machine axes. More explanations regarding the measurement of the calibration ball in the predefined rotational axis positions are given in consideration of the next FIG. 4.

[0028] FIG. 4 shows that after setting and measuring the reference position of the jig 5 and the calibration ball 6 with the rotation axes in zero position, i.e., corresponding to the condition where the linear axes of the machine have been initially calibrated, the position of the calibration ball 6 around a first rotation axis, e.g., the rotation axis C, is increased by a (partial) turn of the calibration ball 6 around that first rotation axis (for example, from C.sub.1,1 to C.sub.2,1 position). After that turn, the position of the calibration ball 6 respectively of its center is measured. The center position of the calibration ball 6 is determined through typically 5 contact measurements by the touch probe 7 at different locations around the periphery of the calibration ball 6. The measurement of the calibration ball's center is repeated for all the predefined positions around the first rotation axis, e.g., the rotation axis C. Then the calibration ball 6 is (partially) turned around a second rotation axis, e.g., the rotation axis B (for example, from B.sub.1 to B.sub.2 position) and the center of the calibration ball is measured again for all or a subset of the first rotation axis positions. For each measurement, the position of the calibration ball's 6 center X.sub.k, Y.sub.k, Z.sub.k, (with k=1, 2 . . . N, wherein N is the total number of different calibration ball position measurements) according to the three linear axes X, Y and Z are reportedfor exampleinto a table (see table in FIG. 6 including the measured positions 1, 2, 3 . . . . N of the calibration ball's center 6 at the predefined rotation and linear axes positions).

[0029] In contrast to all known calibration methods, the measurement of the positions of the calibration ball's center is done with the inventive calibration method by an incremental and combined turning of the calibration ball around two rotation axes (e.g., B and C). The measurements around a first axis will consequently be repeated after an incremental change of the position of the calibration ball around the second axis. Subsequently, the position of the calibration ball around the second axis is changed again and the measurements around the first axis will be repeated, etc. The sequence of the measured positions is consequently different to the known measurement methods. Example: The measurement sequence according to the inventive method is: [0030] Position (C.sub.1,1; B.sub.1).fwdarw.Position (C.sub.2,1; B.sub.1).fwdarw.Position (C.sub.3,1; B.sub.1).fwdarw. . . . .fwdarw.Position (C.sub.n,1; B.sub.1).fwdarw. [0031] Position (C.sub.1,2; B.sub.2).fwdarw.Position (C.sub.2,2; B.sub.2).fwdarw.Position (C.sub.3,2; B.sub.2).fwdarw. . . . .fwdarw.Position (C.sub.n,2; B.sub.2).fwdarw. [0032] Position (C.sub.1,3; B.sub.3).fwdarw.Position (C.sub.2,3; B.sub.3).fwdarw.Position (C.sub.3,3; B.sub.3).fwdarw. . . . .fwdarw.Position (C.sub.n,3; B.sub.3).fwdarw. [0033] . . . [0034] Position (C.sub.1,m; B.sub.m).fwdarw.Position (C.sub.2,m; B.sub.m).fwdarw.Position (C.sub.3,m; B.sub.m).fwdarw. . . . .fwdarw.Position (C.sub.n,m; B.sub.m).

[0035] This example constitutes the general case where for all different positions of the second axis position B.sub.1, B.sub.2 . . . B.sub.m, a different number and even different angles are used for the first rotational axis.

[0036] In comparison the measurement sequence according to the state of the art: [0037] Position (C.sub.1; B.sub.1).fwdarw.Position (C.sub.2; B.sub.1).fwdarw.Position (C.sub.3; B.sub.1).fwdarw. . . . .fwdarw. Position (C.sub.n; B.sub.1).fwdarw. [0038] Position (C.sub.1; B.sub.1).fwdarw.Position (C.sub.1; B.sub.2).fwdarw.Position (C.sub.1; B.sub.3).fwdarw. . . . .fwdarw.Position (C.sub.1; B.sub.m).

[0039] The measurement sequences of the state of the art do not combine the rotation around two axes. With those methods the kinematic error of each axis (e.g., axis C is measured independently of the other axis (e.g., axis B).

[0040] In contrast to the state of the art, the measurement sequences of the inventive method combine the movement of twoor eventually even moreaxes (e.g., C and B). By this, the kinematic error of combined axial movements will be measured and evaluated. The resulting set of measured positions of the calibration ball (e.g., represented by a table according to FIG. 6) is used to compute the kinematic model. Using the inventive measurement method sequence, results in a substantially more accurate kinematic model.

[0041] Based on the exact location of the calibration ball 6 measured for all selected positions around the first and second rotation axes (e.g., C and B), the occurred kinematic errors of the rotation axes of the machine tool are retrieved using a physical model of the machine kinematics to compute the kinematic model. This kinematic model considers the whole kinematic chain between the machining tool clamped in the machine head 3 and the part clamped on the working table 2, as the example in FIG. 5 illustrates. The kinematic model is mathematically represented using homogeneous transformation matrices (see literature [3]). A standard optimization method, for instance nonlinear least-squares curve fitting (read literature [4]), is used to calculate the different kinematic errors related to the rotation axes (for example the values A0C, B0C, X0C, Y0C, A0B, C0B, X0B, Z0B in FIG. 5) best fitting the effectively measured positions X, Y and Z coordinates of the calibration ball center listed in table of FIG. 6. Finally, to compensate the positioning errors of the machine tool, the parameters of the numerical controller are updated with the obtained kinematic errors calculated with the kinematic model.

[0042] As described above, the inventive calibration method is intended for a numerical controlled machine tool, wherein the numerical controlled machine tool comprises a working table and a machine head with a main spindle. The numerical controlled machine tool further comprises at least 3 linear axes (directions X, Y, Z) and at least 2 rotation axes, eventually 3 rotation axes, by which the main spindle and the working table can be moved relatively to each other. To execute the new calibration method, it is necessary that the numerical controlled machine tool is equipped with a jig, having a calibration ball. The jig with the calibration ball is mounted on the working table, while the main spindle of the machine head needs to be equipped with a touch probe. The inventive calibration method includes the step of measuring the position of the calibration ball by conducting with the touch probe at least 4, preferably 5, contact measurements with the mounted touch probe, wherein the position of the calibration ball is changed at least three times by rotating the working table or the machine head around the rotation axes (A, B, C) and the step of measuring the position of the calibration ball is repeated for every position change of the calibration ball.

[0043] The calibration method uses a kinematic model which calculates from the at least three measured positions of the calibration ball the kinematic errors of the numerical controlled machine tool to compensate the positioning error of the working table movements in respect of the main spindle.

[0044] The kinematic model uses the position errors of the calibration ball in respect of the calibration ball's theoretically correct position to generate a compensation model for the numerical controlled machine tool to compensate the positioning error of the working table movements in respect of the main spindle.

[0045] The inventive calibration method measures the positions of the calibration ball with a measurement sequence which includes the combined movement of the calibration ball around two, preferably three, rotation axes, wherein the measurements around a first rotation axis include at least two, preferably m, rotational position movements around a second axis.

[0046] According to the inventive calibration method, the measurement sequence for the rotational positions (C.sub.i; B.sub.j) of the calibration ball (6) can be as follows:

[00002]$\left(C_1;B_1\right),\left(C_2;B_1\right),\left(C_3;B_1\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_1\right)$$\left(C_1;B_2\right),\left(C_2;B_2\right),\left(C_3;B_2\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_2\right)$$\left(C_1;B_3\right),\left(C_2;B_3\right),\left(C_3;B_3\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_3\right)$$\mathrm{.Math.}.Math.\left(C_1;B_m\right),\left(C_2;B_m\right),\left(C_3;B_m\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_n;B_m\right)$$\mathrm{or}.Math.\left(C_{1,1};B_1\right),\left(C_{2,1};B_1\right),\left(C_{3,1};B_1\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,1};B_1\right)$$\left(C_{1,2};B_2\right),\left(C_{2,2};B_2\right),\left(C_{3,2};B_2\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,2};B_2\right)$$\left(C_{1,3};B_3\right),\left(C_{2,3};B_3\right),\left(C_{3,3};B_3\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,3};B_3\right)$$\mathrm{.Math.}.Math.\left(C_{1,m};B_m\right),\left(C_{2,m};B_m\right),\left(C_{3,m};B_m\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(C_{n,m};B_m\right).$

[0047] Preferably, the inventive calibration method for a numerical controlled machine tool describes the relative movements between the working table and the main spindle with a kinematic model using homogeneous transformation matrices for considering the kinematic errors of the rotation axes (e.g., A, B, C).

[0048] The position of the calibration ball can be measured in the coordinates of the linear axes X, Y and Z, wherein each measured calibration ball position (e.g., Xk, Yk, Zk) is memorized in a position table.

[0049] Further, the touch probe can be mounted in the main spindle as a trigger-based contact measurement device, transmitting its trigger signal to the numerical controlled machine tool through wireless transmission. Preferably, the touch probe can also be used to determine the reference position of a part to be machined.

[0050] The kinematic model of the calibration method can for instance calculate the kinematic error occurring by the movement of the rotation axes (e.g., A, B, C) based on the linear axis positions (e.g., Xk, Yk, Zk) memorized in the position table.

[0051] The compensation model generated by the kinematic model includes an optimization algorithm for compensating the kinematic errors occurring with a relative movement between the working table and the main spindle.

When the measurement sequence includes the combined movement of the calibration ball around two rotation axes (e.g., C, B or C, A), the kinematic model considers 8 compensation parameters characterizing the kinematic errors of the rotation axes (e.g., A0C, B0C, X0C, Y0C, A0B, C0B, X0B, Z0B or A0C, B0C, X0C, Y0C, B0A, C0A, Y0A, Z0A, respectively).

[0052] The kinematic model can use the position errors of the calibration ball in respect of the calibration ball's theoretically correct position to generate a compensation model for the numerical controlled machine tool to compensate the positioning error of the working table movements in respect of the main spindle.

[0053] The kinematic model generates preferably a compensation model calculated by a processing unit located outside the numerical controlled machine tool. The compensation model is transmitted for instance by communication means, e.g., via telecommunication means, to the numerical control system of the numerical controlled machine tool.

[0054] The calibration method is applied for instance by the machine control unit of the numerical controlled machine tool.

[0055] The invention includes the possibility that the kinematic model generating a compensation model is calculated by a processing unit located outside the numerical controlled machine tool. In such a case, the calculated compensation model can be transmitted from the processing unit via telecommunication meansmost preferably via internet or telephone connectionto the numerical controlled machine tool.

[0056] The present invention is not limited to the explicitly explained examples and embodiments. The illustrated alternatives are rather to be considered suggestions intended to motivate the person skilled in the art to implement the invention in a most favorable way.

LITERATURE

[0057] [1] S. Weikert, R-Test, a New Device for Accuracy Measurements on Five Axis Machine Tools, CIRP AnnalsManufacturing Technology, 53/1, 2004, pages 429-432 [0058] [2] Touch probe cycle 451 in Heidenhain Benutzer-Handbuch Zyklenprogrammierung iTNC530, 2010 [0059] [3] T. Bajd et al., Chapter 2 in Robotics, Springer, 2010 [0060] [4] D. M. Bates and D. G. Watts, Nonlinear Regression and its Applications, New York, Wiley, 1988

REFERENCES

[0061] 1 numerical controlled machine tool [0062] 2 working table [0063] 3 machine head [0064] 4 main spindle [0065] 5 jig [0066] 6 calibration ball [0067] 7 touch probe [0068] A, B, C rotation axes of the working table or machine head [0069] X, Y, Z linear axes of the working table or machine head