Calibration standard for geometry calibration of a measurement system operating by tactile and/or optical means, method for calibration, and coordinate measuring machine

Abstract

A calibration standard for geometry measurement calibration of a measurement system operating by tactile and/or optical means is provided which includes a flat surface having a structure that is capturable by a measurement system operating by optical and/or tactile means. The structure has a changeable periodicity that is capturable by a sensor in a first direction and/or in a second direction and for a change in the periodicity to code position information and/or direction information. In addition, a method for calibrating a coordinate measuring machine operating by tactile and/or optical means and to a coordinate measuring machine for such a method or having such a calibration standard is provided.

Claims

1. A calibration standard for geometry measurement calibration of a measurement system operating by tactile and/or optical means, the calibration standard comprising: a flat surface having a structure capturable by the measurement system operating by the tactile and/or optical means, wherein the structure has in at least one of a first direction and a second direction a periodicity that is capturable by a sensor and that changes, and wherein a change in the periodicity codes at least one of position information and direction information.

2. The calibration standard according to claim 1, wherein the first direction and the second direction are perpendicular to each other.

3. The calibration standard according to claim 1, wherein the structure is embodied in at least one of the first direction and the second direction in accordance with a sine function.

4. The calibration standard according to claim 3, wherein at least one of a frequency and an amplitude of the sine function are modulated with a modulation function.

5. The calibration standard according to claim 4, wherein the modulation function is a linear function or the sine function.

6. The calibration standard according to claim 5, wherein the modulation function is the sine function with a linearly changing amplitude.

7. The calibration standard according to claim 3, wherein for the sine function of the structure a frequency is modulated in the first direction and for the sine function an amplitude is modulated in the second direction.

8. The calibration standard according to claim 1, wherein a frequency of the periodicity lies at least partially above the frequency that typically occurs when measuring surfaces.

9. A method for calibrating a coordinate measuring machine operating by the tactile and/or optical means on a calibration standard according to claim 1, the method comprising: performing the measurement on the structure in one direction; and evaluating the measurement for decoding at least one of the position information and the direction information.

10. The method according to claim 9, wherein the calibration standard has a frequency-modulated sine structure in the first direction and an amplitude-modulated sine structure in the second direction, which is perpendicular to the first direction, and wherein the method further comprises determining an envelope of measured structure information.

11. The method according to claim 9, further comprising comparing at least one of the position information and the direction information with at least one of the position information and the direction information of the coordinate measuring machine.

12. A coordinate measuring system configured to carry out the method according to claim 9.

13. A coordinate measuring system comprising the calibration standard according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure will now be described with reference to the drawings wherein:

(2) FIG. 1 shows a perspective view of a coordinate measuring machine;

(3) FIG. 2 shows a perspective view of a calibration standard that is mounted to a holder according to an exemplary embodiment of the disclosure;

(4) FIGS. 3 and 4 show schematic cross-sectional views of a surface structuring of the calibration standard in different directions;

(5) FIG. 5 shows a schematic perspective view of the surface structure of the calibration standard according to an exemplary embodiment of the disclosure; and

(6) FIG. 6 shows a schematic illustration of a measurement signal for evaluating the highest possible spatial frequency.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(7) FIG. 1 shows a coordinate measuring machine 10 in a perspective illustration. The coordinate measuring machine 10 includes a table 12 including a base 14 and a plate 16 made, for example, of hard rock. The plate 16 serves to receive a workpiece 18, the surface of which is intended to be measured. In the illustrated exemplary embodiment, the measurement is a spatially resolved roughness measurement.

(8) The table 12 carries a positioning device 20, with which a measurement apparatus 22 can be positioned relative to the table 12 with high accuracy. In the exemplary embodiment illustrated, the positioning device 20 has a gantry-type configuration and includes a gantry 24, which is mounted with two feet 26, 28 at the peripheries of the table 12 and is displaceable along the table 12 in the horizontally extending X-direction in a motor-driven manner. A cantilever 32 is mounted on a gantry crossbeam 30, which interconnects the two feet 26, 28, in such a way that said cantilever can be displaced in a motor-driven manner along the longitudinal direction of the gantry crossbeam 30, i.e., in the likewise horizontally extending Y-direction, as is indicated by a double-headed arrow. A measurement carrier 36 is received in a vertically aligned receptacle 34 of the cantilever 32 and is displaceable in a motor-driven manner along the vertically extending Z-direction.

(9) A control and evaluation device 38, which can exchange control and measurement data with the measurement apparatus 22, is provided at a distance from the table. Said exchange can be effected—as is illustrated in FIG. 1—via appropriate lines or via a radio interface.

(10) The range that can be reached by the measurement carrier 36 as a result of displacement movements along the X-, Y-, and Z-axes is of the order of approximately 2 m.sup.3 in the illustrated exemplary embodiment, and thus even significantly larger workpieces 18 can be measured than what is illustrated in FIG. 1. This is merely exemplary, however. Other measurement systems with a larger or smaller measurement volume can of course also be used in connection with the disclosure.

(11) For each of the three displacement directions X, Y, and Z, the positioning device 20 has at least one transducer, which returns information relating to the travels covered to the evaluation and control device 38. The evaluation and control device 38 controls the movements of the positioning device 20 and evaluates the measurement values transferred from the measurement apparatus 22. The evaluation also includes the computational correction of the measurement values supplied by the measurement apparatus 22. As a result, it is possible to take account of static and dynamic influences of the positioning device 20, thermal deformations of the table 12, and also the bend of tactile probes caused by contact forces.

(12) FIG. 2 illustrates a calibration standard 100 placed on a holder 40. The calibration standard 100 has a structured surface 102, which can be used for calibration measurements. Additionally provided on the calibration standard 100 are measurement calibration aids 104 and a half shell 106 for self-centering probing contact.

(13) The holder 40 can be mounted at different sites of the coordinate measuring machine 10, for example to the plate 16, to a retainer for a measurement calibration sphere, or to a probe interchanging magazine that may be present. The calibration standard 100 itself can be interchangeably mounted to the holder 40 with a magnetic retainer and be interchangeable.

(14) FIG. 3 shows in a schematic illustration in a diagram 200 the function with which the surface 102 of the calibration standard 100 is structured in a first direction, specifically in the Y-direction. The illustration thus also depicts as it were a cross section of the structured surface 102 of the calibration standard 100 along the plane defined by the directions Y and Z. The abscissa 202 of the diagram 200 represents a length, for example 1 mm overall, along the Y-direction. The ordinate shows the height of the structure in the Z-direction, overall 2 μm, for example. The exemplary embodiment illustrated is a sine function with a specific base frequency. This base frequency of the sine wave is frequency-modulated with a frequency that clearly differs from the base frequency. The modulation function in the present case is again a sine function with a linearly changing amplitude. In the function 206 shown in FIG. 3, the amplitude increases from the left to the right. The result is the following function term:

(15) The following holds true:
sin(a.Math.x+b.Math.x.Math.sin(c.Math.x)),
wherein a>>b.

(16) In the case of a measurement with the coordinate measuring machine 10 on the calibration standard 100 in the Y-direction, the modulation function gives the spatial frequency of the calibration standard 100 with respect to the respective measured Y-position. A regional Fourier analysis of the measurement profile can be calculated for an evaluation for ascertaining the Y-position.

(17) In principle, a calibration standard 100 having a surface 102 that is shown only in the Y-direction in FIG. 3 would already be advantageous. It would be possible to ascertain in the Y-direction absolute spatial coordinates in the Y-direction and to thus ascertain for example an advance accuracy.

(18) The surface structure 102 of the exemplary embodiment shown in FIG. 2 is at the same time likewise modulated in the X-direction and thereby codes the respective X-position. A sine wave with a specific frequency is likewise provided as a base function in the X-direction. In contrast to the Y-direction, the steady component, that is to say the carrier function, is amplitude-modulated. The modulation function for the amplitude is likewise a sine function with a linearly changing amplitude. The resulting function is illustrated—analogously to FIG. 3—in the diagram 300 shown in FIG. 4. The abscissa 302 of the diagram 300 again represents the length along the X-direction, and the ordinate 304 represents the height of the structure in the Z-direction with overall lengths or heights similar to the diagram 200. One exemplary function 306 is:
m.Math.x.Math.sin(n.Math.x)

(19) It is advantageous in the case of the simultaneous modulation in the Y- and X-directions if the modulation frequency of the amplitude modulation is clearly higher, as shown in FIG. 4, than that of the frequency-modulated surface signal in order to thus ensure good separation between the X- and Y-position data. In the actual measurement, the amplitude height in the X-direction represents the position within the structure with respect to the X-direction.

(20) FIG. 5 depicts the combined surface structure in a schematic illustration. It shows how the base frequency in the Y-direction is clearly higher than in the Y-direction. At the same time, the frequency of the modulation function is clearly higher in the X-direction than that of the frequency modulation in the Y-direction. However, this is not easily evident from FIG. 5.

(21) The division of amplitude modulation and frequency modulation in the X- and Y-direction in the present case is random and can also be the other way round. It is likewise possible for example for the amplitude modulation to be realized with a simple linear scaling of the amplitude.

(22) In order to then perform a measurement on the calibration standard 100, the position of the calibration standard 100 can be determined in a first step by a measurement calibration with the coordinate measuring system 10, for example on the measurement calibration aids 104, 106. Next, a measurement with a surface measurement system on the calibration standard 100, in particular on the structure 102, can be performed. This measurement can be performed from a desired first point on the surface 102 to a desired second point on the surface 102. The resulting probed section thus includes in the normal case components in the X-direction and components in the Y-direction.

(23) The resulting measurement signal can be subjected to a Fourier analysis. The latter initially provides statements about the Y-position that will have to be corrected later.

(24) At the same time, the measurement signal can be analyzed with respect to the amplitude. For this purpose, for example the envelope of the measurement signal can be determined. The start and end amplitude of the envelope at the start and at the end of the measurement process represent the components in the X-direction of the start position and the end position. With the incorporation of the overall measurement length L of the probed section, it is possible with simple triangulation to determine the angle tilted by which with respect to the Y-direction the measurement process was performed. In this way, it is possible in a simple manner already from the analysis of the envelope to ascertain both the X-position and the Y-position for the start and end point of the measurement.

(25) In addition or alternatively, the envelope of the Fourier transform can be considered. Its amplitudes at the beginning and at the end of the measurement—ascertained for example from the envelope—allow the determination of the start position and the end position in the Y-direction. This can likewise be corrected with the correction factor obtained from the angle enclosed by the measurement section with the Y-direction.

(26) It is possible to ascertain, using the above-described measurement method, the following properties via the measurement system:

(27) TABLE-US-00001 Parameter of the measurement of the Limiting property geometry Information about of the geometry measurement the measurement measurement # calibration standard system calibration standard 1 Start point Absolute position Tolerance position XYZ standard to measurement calibration aid 2 Scanning direction Alignment of the Resolution sensor in plane of orthogonal to the the standard main direction 3 Scanning length Accuracy advance Resolution in main direction 4 Inclination of the Alignment of the Planarity of the scan sensor orthogonal standard to the plane of the standard 5 Height of the profile Linearity of the Low deviation of measurement the target profile, system high reproducibility 6 Standard deviation Adjustment of s.a. of the profile the measurement system 7 Frequencies of the Dynamic High spatial profile behavior, frequencies up to MTF 25 μm wavelength

(28) Reference data about periodicity and amplitude can be assigned to the standard from nominal data or a prior calibration measurement. For this purpose, a data carrier can also be supplied. The standard can allow machine-readable identification (RFID, barcode, etc.) for assignment purposes.

(29) The deviation of the measurement data from the calibration measurement of said reference data can be used to calculate the information according to the above table.

(30) The information obtained can be stored for the purpose of later correction as CAA table, FFT transformed, as spline or family of polynomials. For example, it is possible to describe the swing with linearization correction parameters.

(31) Measurement errors that are not correctable—for example from a repetition measurement or from residues in the spline fitting—can additionally be assigned to the corresponding states (speed of signal change, swing, inclination of the scan, etc.) and be correspondingly stored.

(32) The correction data and the residual error information can be stored on a data carrier, in a database, or in the sensor and be loaded into the controller or correction computation unit during the measurement. The correction data are used for correcting the measurement signal during the measurement. It is possible to continuously determine from the residual errors a state-dependent contribution to the measurement uncertainty and to transmit it to the evaluation software.

(33) In addition to the pure ascertainment of spatial and directional information, a plurality of geometric variables can be ascertained not only with a single measurement. If the highest spatial frequencies of the standard exceed the typical region of surfaces, a frequency transmission of the measurement system can be determined at the same time. This allows statements to be made about the performance and the installation conditions of the measurement system. Such an exemplary measurement is shown in FIG. 6. FIG. 6 shows a diagram 600, which illustrates on the abscissa 602 the position during a measurement. The entire measurement can extend for example over 3 mm. The ordinate 604 presents the measurement signal obtained, which corresponds to a profile depth. The entire profile depth measured can be, for example, 2 μm. As is shown in FIG. 6, the measurement signal 606 initially follows the profile depth of the calibration standard. In a specific region, which in FIG. 6 begins at the location 608, the measurement system can no longer completely follow the surface structuring. The measured amplitude of the measurement signal 606 decreases, even though this does not correspond to the actual structure. From a specific point, which occurs in the measurement signal 606 at the location 610, the frequency of the structured surface is so high that the measurement signal no longer perceives it and then only outputs a constant signal 612. In this way, the highest evaluable spatial frequency can be easily ascertained. The quality of the sensor advance for stylus systems can be determined with the aid of high-frequency grooves on the calibration standard. In this case, the advance can be moved counter to the movement of the machine, and the relative movement can be determined on the basis of the deflection of the stylus instrument.

(34) The position of the structured surface 102 relative to the measurement calibration aids for the location determination can be determined with tactile or optical measuring instruments in order to calibrate the location for each individual.

(35) The system can also be used for optically scanning sensors. The beam angle of the sensor can here be determined on the basis of a known measurement position on the standard of unknown coordinate measuring machine coordinates.

(36) In principle, all possible functions that allow a separation of the X- and Y-information are permissible in the construction of the surface.

(37) It is understood that the foregoing description is that of the exemplary embodiments of the disclosure and that various changes and modifications may be made thereto without departing from the spirit and scope of the disclosure as defined in the appended claims.