How to calibrate a shape measuring machine

Abstract

To calibrate a rotary table which is mounted to a shape measuring machine in fewer steps while maintaining accuracy, even when at least one axis of the direct-acting mechanism of the shape measuring machine is restored to origin.SOLUTION: A calibration method of a shape measuring machine comprises: a first step of an initial setting that sets an origin position by returning a mobile body to the origin point; a second step of determining an initial value of a center position of a rotary table corresponding to an initial value of the origin position of the mobile body; a third step of returning a linear motion shaft of the mobile body to the origin after driving the mobile body; and a fourth step of updating the center position of the mobile body in response to return to the origin of the mobile body. The fourth step comprises: determining one or more center positions of a reference sphere having a known diameter, that is disposed on the rotary table at one or two circumferential positions; determining the center position of the rotary table from one or two center positions regarding the reference sphere; and updating the center position of the rotary table having been determined in the second step.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a calibration method for a shape measuring machine that measures the shape and dimensions of a workpiece, and more particularly to a calibration method suitable for a shape measuring machine having a rotary table. [Background technology]

[0002] To measure the outer diameter, inner diameter, etc. of a workpiece with high precision, a shape measuring machine capable of three-dimensional measurement is often used. This machine measures the workpiece by contacting a spherical probe with the workpiece or by approaching an optical probe or other device for non-contact measurement. In such a shape measuring machine, a movable body is provided on a surface plate that can move in the X-axis direction in a horizontal plane, the Y-axis direction perpendicular to the X-axis direction, and the Z-axis direction perpendicular to both the X-axis and the Y-axis. The probe is connected to these movable bodies and can move in the X-axis, Y-axis, and Z-axis directions. The workpiece is placed on the surface plate. If the workpiece has rotational symmetry, a rotary table or a θ-table that can align the workpiece's axis with its own Z-axis can be attached to the surface plate to reduce the measurement time.

[0003] When the X-, Y-, and Z-axis moving bodies and the rotary table are installed, each axis is adjusted and the origin is set. The origins of these tables can shift due to environmental changes after long-term operation or when the power is turned off. When restarting a form measuring instrument, if the X-, Y-, and Z-axis moving bodies are returned to their origins, the origin of the rotary table can shift by several tens of microns. Therefore, when returning to the origin after long-term use or restarting after a power outage, calibration similar to that performed when the form measuring instrument was installed was required.

[0004] Calibration, similar to that performed when installing a shape measuring machine, requires a great deal of time and effort, and there is a need to simplify or shorten the calibration process while maintaining high accuracy. Various methods have been proposed to resolve the above-mentioned problems with rotary tables. For example, in the rotary table for a coordinate measuring machine described in Patent Document 1, calibration of the rotary table is performed without removing the jig for mounting the workpiece from the rotary table.

[0005] Specifically, a first reference sphere is placed near the center of rotation of the rotary table, and a second reference sphere is placed at a different height from the first reference sphere. The centers of rotation are then determined, and the center of rotation of one of the reference spheres is provisionally set as the center of rotation of the rotary table. The tilt of the line passing through the centers of rotation of the two reference spheres is then determined as the tilt of the rotary table. A third reference sphere is then placed in a position that avoids the workpiece jig, and its center of rotation is determined. When a new calibration is required due to environmental changes, for example, the center of rotation of the third reference sphere is determined each time, and the provisionally set center of rotation of the rotary table is corrected based on the difference from the previously determined center of rotation of the third reference sphere.

[0006] Another method for calibrating a rotary table is disclosed in Patent Document 2. In the CMM described in this publication, the coordinate positions of a reference point on the surface of the rotary table are measured at at least three angular positions in order to register the rotary table's coordinate system. The calibration jig used for measurement has three spheres of the same diameter as contact parts, and a support column and base as support parts, and is capable of simultaneously contacting the probe's spherical tip at three points. During measurement, the calibration jig is placed on the surface of the rotary table, and the probe is brought close to the calibration jig. The coordinate position is read with the probe's spherical tip in contact with the three spheres simultaneously.

[0007] An example of an origin gauge that performs highly accurate origin calibration of a shape measuring instrument is disclosed in Patent Document 3. The origin gauge described in this publication includes a base, and a first reference sphere and a second reference sphere installed above the base. The base is disk-shaped, and the center of the first reference sphere is located on the central axis of the base. The center of the second reference sphere is offset from the central axis of the base. The relative positions of the first and second reference spheres are known. The measurement probe is placed in the outer diameter measurement position to measure the first reference sphere, and the origin for the outer diameter measurement position is calibrated based on the measurement results. Furthermore, the probe is placed in the inner diameter measurement position to measure the second reference sphere, and the origin for the inner diameter measurement position is calibrated based on the measurement results. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 6-249641 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-83192 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-15437 Summary of the Invention [Problem to be solved by the invention]

[0009] The conventional rotary table described in Patent Document 1 allows calibration of the rotary table without removing the jig that secures the object to be measured (workpiece) to the rotary table. Therefore, even if the center position of the rotary table or its indicated value changes due to temperature changes or the like, the center position of the rotary table can be accurately calibrated.

[0010] However, the rotary table described in this publication requires the reference sphere to be measured at three or more positions for recalibration, which increases the labor required for calibration. Furthermore, the rotary table calibration described in this publication requires the center of rotation of the third reference sphere on the rotary table to be determined in advance. This calibration requires measurements at three or more positions to determine the center of rotation, which increases the labor required for calibration. Furthermore, the rotary table calibration described in this publication requires the third reference sphere to be fixed and held on the rotary table even while the workpiece is being measured. This method cannot be applied to measuring general workpieces, where it is highly unlikely that such a location can be obtained on the rotary table.

[0011] Patent Document 2 describes measuring the reference position of the rotary table's coordinate system at three or more angular positions to register the rotary table's coordinate system. This publication only describes calibration, which is typically performed on the rotary table in its initial state. It does not consider calibrating the rotary table mounted on a profile measuring machine to check the rotary table's coordinate system after a series of workpiece measurements or for measuring a new workpiece, thereby improving measurement accuracy. Specifically, when a rotary table is mounted on a profile measuring machine, the origin of the linear motion mechanism of the profile measuring machine changes due to temperature changes, etc. This changes the origin position of the rotary table mounted on the profile measuring machine, but no consideration is given to how to deal with this situation. Furthermore, the publication does not take into account interference between the jig for holding the workpiece and the reference sphere. If the jig weighs, for example, 60 kg or more, removing and reinstalling the jig for calibration is difficult, resulting in a significant increase in labor costs. Reinstallation, in particular, typically requires a significant amount of time to align the screw holes in the rotary table with the screw positions on the jig. Furthermore, this publication requires that three reference spheres be placed on the rotary table, which is also a factor in the occurrence of the interference mentioned above. Furthermore, the method described in the publication does not measure both near and far from the rotary table surface, and when the rotary table undergoes precession (precessing motion), the rotation axis vibrates significantly, resulting in large errors. In other words, this is a calibration method for high-precision, expensive rotary tables.

[0012] Patent Document 3 discloses that calibration is performed with the arm attached to the measurement probe not extended, to prevent excessive extension of the arm during calibration and arm deflection from affecting the measurement. However, the origin gauge described in this publication does not take into consideration that the origin position of the linear motion part of the shape measuring device may change due to temperature changes or repeated start / stop cycles after the shape measuring device measures the workpiece, causing the origin position of the disk-shaped pedestal mounted on the shape measuring device to change. Furthermore, it does not consider calibrating the changed origin position in fewer steps and in less time while maintaining accuracy.

[0013] The present invention has been made in consideration of the above-mentioned drawbacks of the conventional technology, and its object is to calibrate a rotary table in a form measuring machine equipped with a rotary table in fewer steps or in less time while maintaining calibration accuracy, even when at least one axis of a linear motion mechanism of the form measuring machine, such as a coordinate measuring machine, is returned to the origin. Another object of the present invention is to enable calibration of the rotary table while a workpiece holding jig remains mounted on the rotary table. A further object of the present invention is to determine the rotation center axis with high accuracy even with a low-precision, inexpensive rotary table. The present invention aims to achieve at least one of the above-mentioned objects. [Means for solving the problem]

[0014] The feature of the present invention for achieving the above-mentioned object is that a calibration method for a shape measuring machine including a base, a movable body having at least one axis movable in linear motion relative to the base, and a rotary table rotatable relative to the base, comprises a first initialization step of setting an initial value of the origin by returning the movable body to its origin, a second initialization step of determining and storing an initial value of the center position of the rotary table and the rotation axis of the rotary table corresponding to the initial value of the origin position of the movable body, a third initialization step of returning at least one axis of the movable body to the origin after driving the movable body, and a fourth step of updating the center position of the rotary table in accordance with the return of the movable body to its origin, wherein the fourth step includes a step of rotating the rotary table to determine the center position of a reference sphere of known diameter arranged near the surface of the rotary table at at least two circumferential positions of the rotary table, and a step of determining the center position of the rotary table from the center positions of at least two points on the reference sphere, and updating the initial value of the center position of the rotary table stored in the second step.

[0015] In this feature, it is preferable that the movable body is an (X, Y, Z) movable body that is movable in three mutually perpendicular axis directions, and the rotary table is rotatable around one of the three axes of the (X, Y, Z) movable body or around an axis different from any of these three axes.

[0016] Furthermore, after executing the second step and before executing the third step, it is desirable to place the reference sphere of known diameter near the surface of the turntable and determine and store an initial value for its center position, and when executing the fourth step, to drive the turntable to determine the center position of the reference sphere at two rotationally symmetrical circumferential positions, to position a measurement probe close to the reference sphere based on the stored initial value for the center position of the reference sphere, to determine the center position of the reference sphere, to determine a new center position of the turntable based on the two determined center positions, and to update the center position of the turntable determined in the second step with the new center position and store it.

[0017] Furthermore, after the second step is executed and before the third step is executed, the reference sphere with a known diameter is placed near the surface of the rotary table and an initial value for its center position is determined and stored; when the fourth step is executed, the rotary table is driven to determine the center position of the reference sphere at three or more different circumferential positions, and a measurement probe is placed close to the reference sphere to determine the center position of the reference sphere based on the initial value for the center position of the reference sphere that has been stored; a new center position of the rotary table is determined based on the three or more determined center positions of the reference sphere; and the center position of the rotary table determined in the second step is updated with this new center position.

[0018] Furthermore, after executing the third step, the reference sphere with a known diameter may be placed near the surface of the turntable, and when executing the fourth step, the turntable may be driven to determine the center position of the reference sphere at two rotationally symmetric circumferential positions, a new center position of the turntable may be determined from the two determined center positions, and the center position of the turntable determined in the second step may be updated with the new center position, or the reference sphere may be removed from the turntable after executing the fourth step.

[0019] In the above feature, the second step can include placing the reference sphere on the rotary table, measuring and storing an initial value of the center position of the reference sphere at a reference position around the circumference of the rotary table, and the fourth step can include newly determining the center position of the reference sphere at a reference position (one location) around the circumference of the rotary table instead of at least two different points around the circumference of the rotary table, and updating the center position of the rotary table determined in the second step with this new center position. [Effects of the Invention]

[0020] According to the present invention, in a method for calibrating a rotary table attached to a profile measuring machine, the rotation axis is determined with high accuracy in advance, and after the linear motion mechanism, which is the moving body of the profile measuring machine, is returned to its origin, calibration of the center position of the rotary table can be achieved with a small number of measurements. Furthermore, calibration of the rotary table can be completed in one measurement if a reference sphere is placed near the rotation center, and in two measurements if the reference sphere is placed around the rotary table. In other words, while accurate calibration of the center position of the rotary table was previously impossible without measuring the center position of the reference sphere three or more times, it can now be achieved with one or two measurements. Because the time required for each calibration is reduced, frequent calibration is possible, and highly accurate measurements can always be obtained. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a perspective view of an embodiment of a shape measuring machine equipped with a rotary table according to the present invention; [Figure 2] 3 is a diagram for explaining steps 1 and 2 in one embodiment of the method for calibrating a rotary table according to the present invention. FIG. [Figure 3] 10A to 10C are diagrams illustrating steps 3 and 4 in the rotary table calibration method according to one embodiment and another embodiment of the present invention. [Figure 4] 10 is a diagram illustrating steps 3 and 4 in still another embodiment of the method for calibrating a rotary table according to the present invention. FIG. [Figure 5] 3 is a flowchart of a method for calibrating a rotary table according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] Several embodiments of a shape measuring machine according to the present invention will be described below with reference to the drawings. Fig. 1 is a perspective view of a three-dimensional shape measuring machine (also referred to as a coordinate measuring machine or a shape measuring machine) 100 equipped with a rotary table 180 according to the present invention. In the three-dimensional measuring machine 100 of this embodiment, a rotary table (also referred to as a rotary table) 180 is provided on a robustly constructed surface plate 150 so as to be rotatable in the θ direction. The θ direction will be described in detail later, but is generally a direction around the Z axis.

[0023] Y-axis guides 140, 170, consisting of guide rails or the like extending in the front-to-rear direction, are provided on the upper surface edge of surface plate 150, and Y-carriages 130, 160 extending in the vertical direction are engaged so as to be able to move linearly. The two Y-carriages 130, 160 are connected at their upper ends by X-axis guide 110, allowing Y-carriages 160, 130 to move synchronously.

[0024] X-axis guide 110 is disposed substantially horizontally, and Z-carriage 120, which has probe holder 190 attached to its tip, is attached so as to be movable in the X-axis direction. Here, the X-axis direction is the width direction of coordinate measuring machine 100, and Z-carriage 120 is configured to be movable in the Z-axis direction, which is perpendicular to the X-axis direction.

[0025] A probe 192 corresponding to the measurement target and measurement content of the coordinate measuring machine 100 is attached to the tip of probe holder 190 provided on Z carriage 120, and can be moved up and down by Z carriage 120, i.e., in directions perpendicular to the X and Y axes. In Fig. 1, probe 192 is a probe with a spherical contact means at its tip, arranged in a cross shape in a horizontal plane.

[0026] In the coordinate measuring machine 100 configured as described above, the Y carriages 130, 160, Z carriage 120, X-axis guide 110, Y-axis guides 140, 170, etc. constitute a linearly movable body of the coordinate measuring machine 100. The movable body includes an X movable body, a Y movable body, and a Z movable body. In this embodiment, a coordinate measuring machine is used as an example for explanation, and therefore the configuration is movable in three axial directions, but the following explanation can also be applied to a measuring machine with a movable body that can only move in one axial direction.

[0027] A workpiece W can be mounted on a rotary table 180 attached to the coordinate measuring machine 100. The workpiece W is fixed to the rotary table using a jig or the like (not shown). Generally, it is convenient to use the rotary table 180 when the workpiece W has a rotationally symmetrical configuration, and in FIG. 1 , the workpiece W shown by the dashed line is, for example, a bevel gear. In the case of a bevel gear, the same state appears for each angle (pitch) obtained by dividing the circumference by the number of teeth, so the rotary table 180 is rotated by the pitch to measure and confirm the shape. The measurement results are sent to the control and arithmetic unit 210, stored in a memory means provided in the control and arithmetic unit 210, and used for evaluating the quality of the workpiece W.

[0028] Incidentally, in order to perform precise measurements using the coordinate measuring machine 100, the machine is calibrated at various stages to ensure its accuracy. In general, the coordinate measuring machine 100 is calibrated when it is installed, during periodic calibration once a year or once every few months, at the end or start of a batch in the case of batch processing, and when the coordinate measuring machine 100 is restarted after its power has been turned off upon completion of measurement.

[0029] In the past, when calibrating a coordinate measuring machine (CMM) 100, the same calibration procedures as those performed at the time of installation were performed, not only when the machine was installed but also at other calibration times. Because the calibration at the time of installation is necessary to start up the machine, it is unavoidable that it takes a significant amount of time. However, it is desirable to perform calibration as quickly and accurately as possible after the machine is installed and operational. Therefore, we attempted to shorten the calibration time by utilizing the results of the calibration performed at the time of installation. As a result, by performing the calibration shown in the third and fourth steps below, we were able to maintain calibration accuracy while shortening the calibration time and simplifying the calibration process. The content of each step is explained in detail below. While the following describes a contact-type probe, the same applies to non-contact probes. In other words, a contact-type probe requires proximity and contact with a reference sphere, whereas a non-contact probe, such as an optical probe, performs measurement by bringing the probe close to the surface of a reference sphere and irradiating it with laser light, etc., but there is no fundamental difference in the calibration of a rotary table.

[0030] <The first step in proofreading> The first and second steps of the calibration are performed as an initial calibration when the equipment is introduced, as a regular calibration once a year, or when the operating environment has changed significantly. These steps will be explained using FIG. 2. FIG. 2(a) is a perspective view of only the rotary table 180 mounted on the coordinate measuring machine 100, and FIG. 2(b) is a perspective view of the rotary table 180 mounted on the coordinate measuring machine 100, and FIG. 2(c) is a perspective view of the rotary table 180 mounted on the coordinate measuring machine 100, and FIG. 2(d) is a perspective view of the rotary table 180 mounted on the coordinate measuring machine 100, and FIG. 2(e) is a perspective view of the rotary table 180 mounted on the coordinate measuring machine 100, and FIG. 2(f ... R1 , S R2 2(c) is a schematic perspective view showing how the center positions Ω1 and Ω2 of the respective loci are determined from the loci 222 and 224 of the centers of the first and second reference spheres S. R1 , S R2 FIG.

[0031] The first step of calibration involves returning the three-dimensional linear motion mechanism (X, Y, Z) excluding the rotary table 180 to its original position, thereby establishing the original position. As shown in FIG. 1, the coordinate measuring machine 100 determines its X-direction position by moving the Z carriage 120, which also serves as the X carriage, left and right along the X-axis guide 110, and its Y-direction position by moving the Y carriages 130 and 160 back and forth along the Y-axis guides 140 and 170. The Z-direction position is determined by moving the probe holder 190 up and down within the Z carriage 120. The amount of movement of each carriage 120, 130, and 160 and the probe holder 190 is measured using incremental measuring means (not shown), such as a linear scale. During the original position return, the original position is determined by moving each axis to the position of an original position sensor (not shown). Setting this original position is conventional, and therefore a detailed description is omitted.

[0032] <Second step of proofreading> After completing the origin return of the three orthogonal axes (X, Y, Z axes) of the coordinate measuring machine 100, initial calibration of the rotary table 180 mounted on the coordinate measuring machine 100 is performed. In the initial calibration of the rotary table 180, first and second reference spheres S, each of which has a known diameter based on a previous measurement or a manufacturing design value, are placed at two positions on the rotary table 180. R1 , S R2 The calibration jigs 12 and 14 are removably fixed and attached. The calibration jigs 12 are provided with a diameter φD so that the height from the top surface of the rotary table 180 is Z1. R1 Standard ball S R1 Similarly, the calibration jig 14 is provided with a diameter φD so that the height from the top surface of the rotary table 180 is Z2. R2 Standard ball S R2 can be attached.

[0033] DiameterφD R1 and φD R2 may be approximately the same size or may be different. On the other hand, the height Z1 is R1 is located as close as possible to the rotary table 180, and the height Z2 isR2 In order to keep the second reference sphere S as far away from the rotary table 180 as possible, it is desirable that the second reference sphere S be equal to or greater than the height of the workpiece W to be measured. However, if the workpiece W to be measured is huge, the second reference sphere S R2 The heights Z1 and Z2 are set to a height that ensures rigidity such that the measurement probe 22 does not bend when it comes into contact with the surface. This does not apply if the measurement probe 22 is not in contact with the surface. The heights Z1 and Z2 are set to a height that ensures rigidity such that the measurement probe 22 does not bend when it comes into contact with the surface. R Also, the first and second reference spheres S R1 , S R2 The greater the distance from the rotation center O of the rotary table 180, the greater the distance between the center position O of the rotary table 180 and the rotation axis A of the rotary table. R In other words, by measuring two reference spheres with different heights, the present invention is expected to enable calibration with reduced influence even on the rotation axis of an inexpensive rotary table that has precession.

[0034] Rotary table 180 has rotation axis A R Since the rotary table 180 is rotatable around the reference sphere S, the vicinity of the position on the top surface of the base 150 of the coordinate measuring machine 100 where the rotary table 180 is placed is set as the reference position 0 (°) for the rotation of the rotary table 180. The amount of rotation (angle) of the rotary table from the reference position is set as θ. The first reference sphere S at angle θ R1 and the second reference ball S R2 The positions of the first and second reference spheres S in the coordinate system fixed to the coordinate measuring machine 100 are (X1, Y1, Z1) and (X2, Y2, Z2), respectively. R1 , S R2 The position of each reference sphere S R1 , S R2 This means the central positions ω1 and ω2.

[0035] As shown in FIG. 2(b), the rotary table 180 is rotated to measure the first and second reference spheres S at a plurality of positions. R1 , S R2The central positions ω1 and ω2 of the first reference sphere S are determined. The number of measurement positions is at least three, and preferably about six to twelve. The measurement error can be reduced by setting the intervals of the circumferential angle θ of the measurement positions to be approximately equal. R1 The center position ω1 of 1i (i=1, ...n: n is the number of measurement points), and the second reference sphere S R2 The center position ω2 of 2i (i=1, ...m: m is the number of measurement points). Measurement point group π 1i From the first reference ball S R1 The locus 222 of the center position ω1 of the measurement point group π 2i From the second reference ball S R2 The locus 224 of the center position ω2 of the first reference sphere S is obtained by approximating the locus 222 of the discrete points with an arc. R1 The center position Ω1 of the circle formed by the trajectory 224 of the discrete points is calculated, and the second reference sphere S R2 Here, if Z1 is small enough, Ω1 can be used as the center of rotation O. Also, if Z1 is known, we can subtract Z1 from the Z-direction component of Ω1 (i.e., Ωz) to obtain Ω1'(Ω X ,Ω Y ,Ω Z -Z1) becomes the rotation center. More generally, if a line is drawn between the center positions Ω1 and Ω2 of the two circles, the point where this line intersects with the top surface of the turntable 180 becomes the rotation center O of the turntable 180. The rotation center O thus found is the reference rotation center O(X R0 , Y R0 , Z R0 ) and stored in the control and arithmetic device 210.

[0036] In the calibration process, the first and second reference spheres S R1 , S R2 When using a contact-type measurement probe 22, the center positions ω1 and ω2 of the reference sphere S are determined as shown in FIG. R1 , S R2 The measurement probe 22 is placed in contact with the periphery of the reference sphere S R1 , S R2The coordinates of the measurement probe 22 are measured at three or more points around the reference sphere S. R1 , S R2 The center positions ω1 and ω2 of the reference sphere S may be determined using a known diameter value (diameter fixed fit). However, since it is assumed that the diameter of the probe 22 may contain an error, R1 , S R2 The surface of the reference sphere S is measured at four or more points or by scanning. R1 , S R2 The surface of the sphere is measured evenly at multiple points, and the diameter is free-fit to measure the reference sphere S. R1 , S R2 It is preferable to find the center coordinates of The diameter of the measuring probe 22 and the reference sphere S R1 , S R2 Diameter φD R1 , φD R2 By making the distance θ known, automatic measurement without manual intervention, which will be described later, becomes possible, and measurement efficiency is improved. The first and second steps of the calibration can be programmed in advance to enable automatic measurement. After the second step of the calibration is completed, the first and second reference spheres S R1 , S R2 is removed from the rotary table 180. This makes it possible to carry out three-dimensional measurement of the workpiece W without any problems.

[0037] <Third step of proofreading> In the subsequent calibration steps, the third reference sphere S R3 Use the third reference ball S R3 The installation of the third reference sphere S R3 Both automatic and manual measurement methods are possible for position measurement using the third reference sphere S. R3 is the first reference sphere S R1 may be the same as the diameter φd R3 (=φD) is known by prior measurement or manufacturing design value. First, the third reference sphere S R3 The initial position measurement will be described.

[0038] Figure 3 shows the third reference sphere SR3 10A is a diagram illustrating the case where automatic calibration is performed using the third reference sphere S. R3 180. The same figure (b) is a schematic perspective view showing the state in which the third reference sphere S is installed from the center O of the rotary table 180. R3 10(c) shows the distance to the third reference sphere S. R3 FIG.

[0039] A third reference sphere S is located at a position eccentric to the rotation center O of the rotary table 180 and as close to the top surface of the rotary table 180 as possible. R3 The third calibration jig 16 is set up with a reference sphere S R3 Since the radial position and height of the third calibration jig 16 on the rotary table are almost the same as those of the first calibration jig 12, the third calibration jig 16 may also be used as the first calibration jig 12. Now that the third calibration jig 16 has been set, the reference position θ0 for rotation of the rotary table 180 is set to the surface plate 150 of the coordinate measuring machine 100. This reference position is set as θ0 = 0°. The reference position can be set to any position.

[0040] Next, the rotary table 180 is rotated to set it at the reference position 0°, and the third reference sphere S R3 The center position P(x, y, z) of the obtained third reference sphere S R3 The center position of the image is stored in the control and calculation device 210 as P0 (X1, Y1, Z1).

[0041] Next, after measurements of the workpiece W have been repeated a predetermined number of times, after the measurement environment has changed, etc., a home return operation is performed on the coordinate measuring machine 100 in order to confirm and correct the origin of the incremental measuring devices of each axis provided in the coordinate measuring machine 100. This is a routine calibration (hereinafter also referred to as daily calibration) that is usually performed before and after the power is turned off, or after measurements of one batch of workpieces W have been completed and a lull has occurred. Note that home return does not necessarily have to be performed for all three axes of the coordinate measuring machine 100; it is also possible to return the origin to only one axis that moves frequently.

[0042] <The fourth step of proofreading> When at least one axis of the coordinate measuring machine 100 is returned to the origin, the component of the axis of the rotary table 180 that has been returned to the origin becomes unclear. For example, the central position O(a1,Y R0 ,Z R0 ) is returned to the X-axis origin, the X coordinate of the center position of the rotary table 180 will be a value different from a1. However, the change in the center coordinate due to the origin return or environmental changes will not cause the probe to deviate so far that it becomes impossible to approach or contact it during automatic measurement.

[0043] Third reference ball S R3 is attached near the top surface of the rotary table 180 at a position eccentric from the center. R3 It is most preferable to place the reference sphere S on the rotary table 180 so that it does not affect the measurement of the workpiece W, since this is the same as the initial calibration state. In other words, it is expected that error factors such as precession due to the rotation of the rotary table are small near the upper surface of the rotary table, and therefore it is expected that the rotation center position O of the rotary table can be accurately determined. However, R3 If it is difficult to place the third reference sphere S with high reproducibility, R3 It is practical to set it in a position where the third reference sphere S can be placed. R3 The first reference sphere S R1 In this case, the first reference sphere S R1 A part of the measurement results is taken from the third reference sphere S R3 If the initial value is set to be , the number of steps can be reduced.

[0044] The following shows the case of automatic calibration. The third reference sphere S R3 In order to measure the center coordinates of the third reference sphere S stored in the coordinate measuring machine 100, R3 The measuring probe 22 is automatically moved based on the assumed position of the third reference sphere S. R3 Before starting calibration using the third reference sphere S R3The center position P of the third reference sphere S and the center of rotation O of the rotary table 180 obtained during the initial calibration can be used as the center position P of the third reference sphere S. R3 Distance L from the rotation center position ω3 and reference sphere S R3 This is the height direction position of the third reference sphere S R3 The assumed center position P(X,Y,Z) is X=X R0 +LCOS(θ-θ0), Y=Y R0 +LSIN(θ-θ0), Z=Z1 It can be expressed as:

[0045] Here, L and θ are the distances from the third reference sphere S as shown in FIG. 3(b). R3 are the xy plane components of the distance from the rotation center O of the rotary table 180 and the rotation angle of the rotary table 180. θ0 is the value of the third reference sphere S R3 θ is the angular position at which the center position P of the workpiece W is measured, and if the measurement was performed at the reference position (0°) during initial calibration, θ0 is 0°. Also, it is assumed that there is no change in the height direction of the rotary table 180 before and after measurement of the workpiece W. The control and calculation device 210 of the coordinate measuring machine 100 changes the rotation angle θ of the rotary table 180 to measure the third reference sphere S R3 The assumed position of the vertex is calculated and stored, and the following measurements are performed:

[0046] The measurement probe 22 is brought close to and in contact with two points on the rotary table 180 where the rotation angle θ is rotationally symmetric, and the third reference sphere S R3 The center position P of the rotary table 180 is measured. The coordinates of the two obtained points are P1 (X2, Y2, Z2) and P2 (X3, Y3, Z3). These coordinate values ​​of each point are the measurement values ​​after return to the origin. Note that if the measurement points on the rotary table 180 are three or more different points, the measurement positions in the rotation direction of the rotary table 180 (when θ is changed) do not need to include two rotationally symmetric points. Using two rotationally symmetric points is for minimizing the number of measurements, so if there is a high demand for reducing the number of measurements or the measurement time, measurement at two rotationally symmetric points, which requires only two measurements, should be used. However, if setting the rotationally symmetric positions requires a lot of work, it is better to measure at any three or more different points.

[0047] Third reference sphere S at two rotationally symmetric points R3 Since the center positions P1 and P2 of the rotary table 180 have been determined, the center O of the rotary table 180 can be determined from the geometric relationship. R1 , Y R1 , Z R1 ) each axis component is X R1 =(X2+X3) / 2, Y R1 =(Y2+Y3) / 2, Z R1 =(Z2+Z3) / 2 These values ​​are stored in the control / arithmetic device 210 and are used for subsequent measurements of the workpiece W. A reference sphere S is placed at a position eccentric to the center of the rotary table 180. R3 If you install the reference sphere S at the rotationally symmetric position of the rotary table 180, R3 In other words, measurements are only required at two circumferential positions. R3 The reference sphere S is measured by tracing it. R3 This is desirable because it reduces the variation when finding the center of the

[0048] In three-dimensional measurement, the rotary table 180 moves (linearly) in the X-axis, Y-axis, and Z-axis directions, but does not move with a change in inclination relative to each axis as a whole. R does not change its vector direction, which is not affected by the origin return operation. Since the calibration of the center coordinate O of the rotary table 180 is completed, the third reference sphere S R3 The workpiece W is then placed on the rotary table 180, and measurement by the coordinate measuring machine 100 is started. R3 It is necessary to remove it when it interferes with the workpiece W, etc.

[0049] The above is the case of automatic calibration, but another embodiment in which manual calibration is performed will be described below. In either automatic calibration or manual calibration, the third reference sphere S R3The procedure for mounting the third reference sphere S on the rotary table 180 and removing it from the rotary table 180 after daily calibration is the same. R3 The timing for attaching the third reference sphere S may be after the origin return operation of the coordinate measuring machine 100. The main difference between manual calibration and automatic calibration is that the third reference sphere S R3 This is the action of bringing the measurement probe 22 close to the object. In manual calibration, the measuring probe 22 is aligned with the third reference sphere S R3 Therefore, if the measurement probe 22 is a contact probe, the operator can operate the probe 22 using a joystick to move the third reference sphere S R3 If the measurement probe 22 is a non-contact type probe such as an optical type, the probe 22 is visually aligned with the third reference sphere S R3 be close to the surface of the Furthermore, when measuring three or more points, there is no need to accurately determine the rotationally symmetric positions, so an experienced person can perform the measurement in approximately the same amount of time as automatic calibration. R3 Measurement after contact with the rotary table 180 and calculation of the center position O of the rotary table 180 are carried out in the same manner as in the above embodiment.

[0050] Third reference ball S R3 Another embodiment in which daily calibration is automatically performed using the third reference sphere S will be described below with reference to FIG. 4. Here, FIG. 4(a) shows the state before daily calibration, and FIG. 4(b) shows the state during daily calibration. This embodiment differs from the first embodiment in that the third reference sphere S R3 This is intended to be applied when the workpiece W can be measured without being removed after the initial calibration. Specifically, the third reference sphere S R3 is placed at or near the center O of the rotary table 180. By placing it as close as possible, this reference sphere S R3 The change in the center coordinate of can be considered as a change in the center of rotation due to a return to the origin or an environmental change. R3 When the reference sphere S is installed, the reference sphere S may be displaced due to error factors such as thermal expansion of the rotary table 180 other than the change in the center of rotation. R3Since the position of the third reference sphere S changes, it cannot be measured accurately. In this measurement, scanning measurement takes in multiple measurement points, so it is extremely effective in obtaining accurate center coordinates with as little variation as possible. In addition, it does not interfere with the workpiece W or the workpiece fixing jig, etc., and the third reference sphere S can be measured at any time. R3 It goes without saying that scanning measurements are desirable for this measurement as well, in order to reduce the effects of measurement variability.

[0051] In the daily calibration after returning at least one axis of the coordinate measuring machine 100 to the origin due to temperature drift or the like, the third reference sphere S is R3 The center position O of the rotary table 180 can be obtained by simply measuring the center position P of the third reference sphere S. R3 The assumed position of the third reference sphere S R3 The central position P0 (X1, Y1, Z1) measured in the initial measurement is used as the assumed position (one reference position) of the third reference sphere S. R3 At this rotation angle θ0, the measurement probe is used to set the third reference sphere S R3 The control and calculation device 210 measures the center position P0 (X1, Y1, Z1) at the time of initial calibration, the center position P1 (X2, Y2, Z2) after the origin return operation, and the center position O (X R0 , Y R0 , Z R0 ), the rotation center position O(X R1 , Y R1 , Z R1 ) is calculated using the following formula: X R1 =X R0 +(X2-X1), Y R1 =Y R0 +(Y2-Y1), Z R1 =Z R0 +(Z2-Z1) Rotation axis A of rotary table 180 Ris the same vector as that at the time of initial calibration for the same reason as above. These measured and calculated values ​​are stored in the control and calculation device 210, and three-dimensional measurement of the workpiece W is then carried out.

[0052] A flowchart of a calibration method according to the present invention, based on the above-described embodiments, is shown in Figure 5. This flowchart illustrates an example in which at least one of the (X, Y, Z) axes of the coordinate measuring machine 100 is returned to the origin, but the same applies to cases in which calibration is performed when the origin of an incremental measuring machine has changed due to temperature drift or the like. Note that, although step S544 is also provided after step S540 in this flowchart, it may also be provided after step S522.

[0053] When the coordinate measuring machine 100 is installed, initial settings are first performed. Note that although the initial settings are shown here for the case when the machine is first installed, the initial settings also include calibrations that are performed when the machine is significantly modified or when regular annual inspections are performed. In step S502, first and second reference spheres S, each with a known diameter, are placed on the rotary table 180. R1 ,S R2 The first reference sphere S R1 is located close to the top surface of the rotary table 180, and the second reference sphere S R2 is installed at a position above and away from the rotary table 180.

[0054] Next, the rotary table 180 is rotated, and the first and second reference spheres S are aligned at a plurality of positions, preferably 6 to 12 positions, at a rotation angle θ. R1 , S R2 The center positions ω1 and ω2 of the first and second reference spheres S are measured (step S504). R1 , S R2 The measured value of the center position ω1, ω2 is π 1i , π 2i From the first and second reference balls S R1 , S R2 The locus circles 222 and 224 are approximated, and the control and calculation device 210 calculates and finds the center positions Ω1 and Ω2 of the locus circles (step S506). The point where the line passing through the two points Ω1 and Ω2 intersects with the upper surface of the rotary table 180 is defined as the rotation center O(X R0 ,YR0 ,Z R0 ) and store it (step S508). R1 is close enough to the rotary table 180, the central coordinates of Ω1 are set to the rotation center O(X R0 ,Y R0 ,Z R0 ) In this case, the vector of the line passing through the two points Ω1 and Ω2 is the rotation axis vector A of the rotary table 180. R In subsequent calibrations, the rotation axis vector A R Since the initial calibration of the rotary table 180 is completed, the first and second reference spheres S R1 , S R2 The device is then removed from the rotary table 180 (step S510). The above steps S502 to S510 are the initial calibration described in the present invention.

[0055] Next, in step S512, a third reference sphere S with a known diameter is R3 Prepare the third reference ball S. R3 is the first reference sphere S R1 The third reference sphere S may be the same as the third reference sphere S. R3 is used for daily calibration, and this third reference sphere S R3 There are three main methods based on the difference in the calibration method using the third reference sphere S. R3 The question is whether the third reference sphere S can be permanently installed (step S514). R3 If it can be permanently installed on the rotary table 180, the process proceeds to step S516. Since the workpiece W is placed on the upper surface of the rotary table 180, the third reference sphere S R3 In many cases, it is not possible to permanently install the above-mentioned on the rotary table 180. In such cases, the process proceeds to step S530.

[0056] The second division is whether the daily calibration is performed simply or accurately over time (step S516). When accurate calibration is desired for automation, this corresponds to the first embodiment of the present invention, and data is acquired before the daily calibration in order to prepare for automation. Specifically, proceeding to step S520, a third reference sphere S is placed on the rotary table 180 at a position close to the rotary table 180 but away from the center of rotation O. R3 The third reference sphere S R3 is measured at a position of an arbitrary rotation angle θ0 of the rotary table 180 and stored in the control and arithmetic unit 210 (step S522). Step S520 constitutes a preparation stage for daily calibration.

[0057] Step S530 is the measurement of workpiece W, which is the intended use of the coordinate measuring machine 100. Strictly speaking, step S530 is not included in the calibration of the rotary table 180. As described above, daily calibration is performed in conjunction with returning the (X, Y, Z) axes of the coordinate measuring machine 100 to their origins, such as after one or several batches of workpieces are completed, at the start of work for the day, etc. The following steps S540 to S550 constitute the daily calibration in this embodiment.

[0058] If the origin is to be returned, the process proceeds to step S540, and at least one of the (X, Y, Z) axes is returned to the origin. For automatic calibration, the measurement probe 22 is moved to the third reference sphere S R3 It is necessary to approach as close as possible to the third reference sphere S R3 Since the position of is not an accurate position due to the return to the origin, the data stored in step S522 is used to calculate the assumed center position P (step S544).

[0059] In the case of the contact type measurement probe 22, the measurement probe is placed on the third reference sphere S R3 The third reference sphere S R3 The center position P of the non-contact type is measured using the third reference sphere S. R3 From the output in the vicinity of the third reference sphere S R3 The center position of the third reference sphere S is detected at two rotationally symmetric positions with respect to the rotation angle θ of the rotary table. R3The rotation center positions P1 and P2 are measured (step S546).

[0060] The third reference sphere S measured at a rotationally symmetric rotation angle θ R3 From the rotation center positions P1 and P2 of the workpiece W, the center position O of the rotary table 180 is calculated as (P1+P2) / 2 and stored in the control / calculation device 210 (step S548). R1 , Y R1 ,Z R1 ) is used as a reference for measurements.

[0061] In step S514, the third reference sphere S R3 If the non-permanent installation is selected, the second embodiment is performed, and the process proceeds to step S530. In step S530, three-dimensional measurement is performed on the workpiece W. When the measurement of the workpiece W is completed or when the measurement operation is started, the movable bodies of at least one of the X movable body, Y movable body, and Z movable body are returned to the origin (step S570), and the process proceeds to step S572 to perform daily calibration. As in the case of automatic calibration, the third reference sphere S R3 is placed on the rotary table 180 (step S572), and the third reference sphere S R3 The center positions P1 and P2 of the rotary table 180 are measured at two rotation angles θ that are rotationally symmetric (step S574). The center position O of the rotary table 180 is calculated as (P1+P2) / 2 and stored in the control and calculation device 210 (step S576). At this time, the rotation center axis vector A R is not changed. After that, the third reference sphere S is R3 (Step S578) Steps S572 to S578 constitute the daily calibration in this embodiment.

[0062] In step S516, the third reference sphere S R3 If simple calibration using one measurement is selected, this corresponds to the last example above, and the process proceeds to step S526. Unlike the first two examples, R3is placed at or near the rotation center 0 of the rotary table 180. Next, the third reference sphere S is placed at an arbitrary rotation angle θ0 of the rotary table 180. R3 After the measurement of the workpiece W (step S530) is completed, at least one axis of the (X, Y, Z) table is returned to the origin (step S560), and the third reference sphere S R3 The third reference sphere S is rotated at the same rotation angle θ0 as when the center position P1 of the R3 The center position P2 (X2, Y2, Z2) of the rotary table is measured and stored in the control and calculation device 210 (step S562). Next, using these data, the control and calculation device 210 calculates the center O (X R1 ,Y R1 ,Z R1 ) to X R1 =X R0 The calculation is performed using an equation such as +(X2-X1) and stored (step S564). This completes the daily calibration. In this example, steps S560 to S564 constitute the daily calibration.

[0063] As described above, according to each embodiment of the present invention, daily calibration can be easily performed with a small amount of time or labor, enabling high-quality three-dimensional measurements at all times. Furthermore, since the same first and third reference spheres can be used, new jigs or other components are not required, simplifying the calibration device. Furthermore, it is possible to calibrate a CMM according to the type of workpiece being measured, and conversely, it is also possible to calibrate a CMM using only one type of calibration jig for all types of workpieces. [Explanation of symbols]

[0064] 12...(first) calibration jig, 14...(second) calibration jig, 16...(third) calibration jig, 22...measuring probe, 100...three-dimensional measuring machine, 110...X-axis guide, 120...Z carriage, 130...Y carriage, 140...Y-axis guide, 150...surface plate, 160...Y carriage, 170...Y-axis guide, 180...rotary table, 190...probe holder, 192...probe, 210...control and calculation device, 222...(first reference sphere) center locus, 224...(second reference sphere) center locus, A R …(rotary table) rotation axis (vector), d R1 , d R2 ...Diameter (of the first and second reference spheres), D...Diameter (of the third reference sphere), O...Center of rotation (position), L...Distance (from center O) (xy plane components), P, P1, P2...Center position (of the third reference sphere), S R1 , S R2 , S R3 …reference ball, W…workpiece, π 1i , π 2i ...(measured center of the first and second reference spheres), ω1, ω2...(center position of the first and second) reference spheres, Ω1, Ω2...(center position of the first and second reference spheres)

Claims

1. A method for calibrating a shape measuring machine including a surface plate, a movable body having at least one axis capable of linear movement relative to the surface plate, and a rotary table rotatable relative to the surface plate, comprising: a step of returning at least one axis of the moving body to an origin after driving the moving body; updating the pre-stored center position of the rotary table in response to the return of the movable body to the origin; the step of updating the center position includes a step of rotating the rotary table to determine, at at least two circumferential positions of the rotary table, the center positions of a reference sphere disposed near the surface of the rotary table and having a diameter previously measured or stored; A method for calibrating a shape measuring machine, comprising the steps of: determining a center position of the rotary table from the center positions of at least two points on the reference sphere; and updating the pre-stored center position.

2. 2. The method for calibrating a shape measuring machine according to claim 1, wherein the step of determining the center position of the reference sphere is a step of determining the center position of the reference sphere by driving the rotary table to bring a measurement probe close to the reference sphere at two rotationally symmetric circumferential positions based on a center position of the reference sphere that was stored in advance before the reference sphere was returned to the origin.

3. 2. The method for calibrating a shape measuring machine according to claim 1, wherein the step of determining the center position of the reference sphere is a step of determining the center position of the reference sphere by driving the rotary table to bring a measurement probe closer to the reference sphere at three or more different circumferential positions based on a center position of the reference sphere that has been stored in advance before the reference sphere is returned to the origin.

4. and a step of disposing the reference sphere near the surface of the rotary table after the step of returning to the origin and before the step of updating the center position.

2. The method for calibrating a shape measuring machine according to claim 1, wherein the step of determining the center position of the reference sphere is a step of driving the rotary table to determine the center positions of the reference sphere at two rotationally symmetrical circumferential positions.

5. A method for calibrating a shape measuring machine including a surface plate, a movable body having at least one axis capable of linear movement relative to the surface plate, and a rotary table rotatable relative to the surface plate, comprising: a step of returning at least one axis of the moving body to an origin after driving the moving body; updating the pre-stored center position of the rotary table in response to the return of the movable body to the origin; the step of updating the center position includes a step of rotating the rotary table to determine, at one reference position in the circumferential direction of the rotary table, a center position of a reference sphere disposed near the surface of the rotary table and having a diameter previously measured or stored; a step of determining a center position of the rotary table from the determined center position and the center position of the reference sphere measured at the reference position before returning to the origin, and updating the pre-stored center position.

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