A method for measuring the linkage accuracy of machine tool linear axes using a dual ballbar system

Abstract

The present invention belongs to the field of machine tool linear axis precision measurement and relates to a method for measuring the linkage accuracy of machine tool linear axes using a dual ballbar system. Two ballbars are used to measure the linkage accuracy of the two moving axes of a machine tool. Specifically, the two ballbars are simultaneously installed between the moving and fixed components of the machine tool using a fixture. A single center point of the moving component is replaced by a straight line passing through the two moving sphere centers as the analysis and measurement element. A closed-loop measurement vector model is established based on the principles of kinematic geometry, and error motion parameters are decoupled by solving equations. Using the dual ballbar system method to measure the linkage accuracy of machine tool linear axes is simple to operate, can decouple multiple error motion parameters, and is more conducive to improving the accuracy and efficiency of precision testing during machine tool assembly and commissioning.

Description

Technical Field

[0001] The invention belongs to the field of machine tool linear axis precision measurement, and relates to a method for measuring the linkage precision of a machine tool linear axis by using a double-set ballbar system. Background Art

[0002] During the assembly and commissioning of multi-axis machine tools or machining centers, it is often necessary to test the motion accuracy of their moving axes. Specific displacement data is acquired through various precision sensors, and the motion errors of the worktable (or spindle box) under the linkage drive of the moving axes are fitted and analyzed to ensure that the accuracy of the machine tool's moving axes meets design specifications and processing requirements. Currently, when measuring the linkage accuracy of two moving axes on a machine tool, a single ballbar is generally used for circular trajectory testing. A magnetic ball mount is used to precisely mount the ballbar between the fixed end of the machine tool and the linkage end of the moving axis. The ballbar's telescopic displacement data is then acquired by running the two-axis linkage along a circular trajectory. The data is then fitted and analyzed using the data for the two-axis linkage accuracy parameters. This type of testing method relies on a single, independent parameter as the data is based on the unidirectional displacement characteristics of a single point on the moving component. The deflection error motion information of the measured moving component cannot be decoupled, and the installation position of the ballbar on the fixed and moving components affects the measurement results. This, to a certain extent, affects the accuracy of the two-axis linkage accuracy assessment.

[0003] Patent number CN113211185A, inventors Chen Guangsheng et al., claim a method for detecting linear errors in CNC machine tool linear axes based on a ballbar. This method uses a single ballbar to perform measurements along five predefined trajectories. The linear error is pre-fitted using a collection of sine functions, and the coefficients are solved using a particle swarm algorithm to determine the linear axis error. This method effectively measures linear errors in moving axes. However, because the measurement process uses five trajectories, the cumulative measurement time is long, and only a single independent measurement parameter can be obtained, making it difficult to analyze the runout error of the measured linear axis.

[0004] The dual ballbar system consists of two ballbars and a measurement fixture. Using a single installation of two ballbars for simultaneous measurement, the required fixture is simple. Compared to the commonly used single ballbar circular trajectory measurement, this system does not increase the complexity of installation and measurement trajectory planning. While retaining the ballbar's advantages of easy installation and efficient measurement, it also enhances the dimensionality of independent measurement parameters obtained with a single installation, enabling the simultaneous acquisition of the motion trajectory of two relatively fixed markers on the measured moving component. Combining the advantages of the dual ballbar system, it is possible to establish a kinematic geometry model encompassing the sensor measurement parameters, the measured geometric elements, and the error motion parameters of the measured moving component by studying the relationship between sensor measurement parameters and the motion trajectory of the measured moving component's geometric elements. Solving the model equations yields the error motion parameters and allows the evaluation of linear axis linkage accuracy. Therefore, leveraging the advantages of the dual ballbar system allows for efficient measurement of machine tool linear axis linkage accuracy, overcoming the limitations of existing single ballbar measurement methods, which are unable to detect parameters such as the swing error of the measured component. Summary of the Invention

[0005] To overcome the existing problems of single ballbars in measuring the accuracy of machine tool linear axis linkages, such as insufficient measurement parameters, inability to decouple some error motion parameters of the measured component, and insufficient error motion parameters that can be analyzed and obtained from a single measurement, this invention proposes a method for measuring the accuracy of machine tool linear axis linkages using a dual ballbar system. This method utilizes a fixture to simultaneously install two ballbars. After installation, two moving ball points are identified on the measured component as measurement markers. Simultaneous measurements are performed along the same machine tool linkage motion trajectory, and trajectory information for the two moving ball points on the measured component is obtained within a single measurement run. By studying the mapping relationship between the displacement vectors of the two ballbar sensors and the error motion trajectory of the line determined by the two moving ball points, the error motion parameters of the measured moving component can be decoupled. This dual ballbar system method is simple to operate for measuring the accuracy of machine tool linear axis linkage motion, can decouple multiple error motion parameters, and is therefore more conducive to improving the accuracy and efficiency of precision testing during machine tool assembly and commissioning.

[0006] The technical solution adopted in the present invention is:

[0007] A method for measuring the linkage accuracy of machine tool linear axes using a dual ballbar system uses two ballbars to measure the linkage accuracy of the machine tool's two moving axes. Specifically, the two ballbars are simultaneously mounted between the machine tool's spindle box and mounting base using a fixture. A straight line passing through the two moving sphere centers on the moving component replaces a single sphere center point as the analysis and measurement element. A closed-loop measurement vector model is established based on the principles of kinematic geometry, and error motion parameters are decoupled by solving equations. The measurement method includes the following steps:

[0008] Step 1: Install the double-ball bar mounting fixture and adjust the relative position between the two moving ball bases: A slide that can move up and down is installed on the side of the machine column, and the spindle box is fixed on the slide. Install the double-ball bar mounting fixture on the output shaft of the spindle box; the moving ball base is adsorbed on the lower surface of the double-ball bar mounting fixture. Adjust and record the distance L between the two bases;

[0009] Step 2: Install the dual ballbar: Use the magnetism of the fixed ball base to adsorb it on the mounting base. The mounting base is fixed relative to the machine tool bed during the measurement process. Manually adjust the distance and relative direction of the two fixed ball bases to make the distance between the fixed ball bases approximately L and the relative direction approximately the direction of the two moving ball bases. Adsorb the two base fine-tuning standard balls (essentially standard spheres with the same diameter as the ballbar head) that come with the dual ballbar on the adaptive magnetic bowls of the two fixed ball bases respectively. The adaptive magnetic bowls can achieve a small range of adjustment on the fixed ball bases. Displacement and swing: By moving the X, Y, and Z axes of the machine tool, the spindle box drives the two moving ball bases on the installation fixture to approach their corresponding fixed ball bases. When the position is close to the magnetic suction range of the moving ball base bowl, the fine-tuning standard ball moves with the fixed ball base adaptive magnetic suction bowl under the action of magnetic attraction, so that the fine-tuning standard ball is adsorbed on the fixed ball base bowl and the moving ball base bowl at the same time. Rotate the locking handle on the fixed ball base to lock the adaptive bowl. At this time, the fixed ball point installation position on the fixed ball base is the precise position, and the precise adjustment of the fixed ball base is completed. Remove the auxiliary installation fine-tuning standard ball, translate the machine tool moving axis to the reference length distance of the ballbar used, and install ballbar No. 1 and ballbar No. 2 between the two sets of moving and fixed bases respectively; the fixed ball points of ballbar No. 1 and ballbar No. 2 are S1 and S2 respectively, and the moving ball points are S3 and S4 respectively.

[0010] Step 3: Establish a fixed coordinate system and a moving coordinate system: The origin of the fixed coordinate system is located at the fixed point S1, and the X coordinate axis of the fixed coordinate system is X f , the direction is from S1 to S2; the origin of the motion coordinate system is located at the moving ball point S3, and the X coordinate axis of the motion coordinate system is X m , the direction is from S3 to S4, and the measurement movement is

[0011] ||R f ||=||R m || (1) where R f is the relative position vector of the two fixed points, from fixed point S1 to fixed point S2, R m is the relative position vector of the two moving ball points, pointing from moving ball point S3 to moving ball point S4.

[0012] Homogeneous transformation matrix M between fixed coordinate system and moving coordinate system mf for

[0013]

[0014] Where α, β, and γ are the deflection angle, pitch angle, and roll angle of the moving coordinate system relative to the fixed coordinate system, respectively; c represents cosine, s represents sin, and X, Y, and Z are the displacement parameters of the moving coordinate system relative to the fixed coordinate system in three directions;

[0015] Step 4: Set the two-axis motion circle trajectory and measure the data: The data λ1 and λ2 obtained by ballbar No. 1 and ballbar No. 2 correspond to the sensor measurement vector R b1 and R b2 ;

[0016] Step 5. Establish the measurement motion geometry model: The closed-loop vector equation of the double ballbar system measurement process is:

[0017] R f +R b2 =R b1 +R m (3)

[0018] Step 6: Solve the motion parameters of the moving axis linkage error:

[0019] For any motion position i of the machine tool during the measurement process, the sensor measurement vector R of the two ballbars is expressed in the form of a circular vector b1i and R b2i :

[0020]

[0021] in is the rotation angle corresponding to ballbar No. 1 at position i, is the rotation angle of ballbar No. 2 at position i, λ 1i and λ 2i are the data values ​​measured by ballbar No. 1 and ballbar No. 2 at position i, respectively, and e I1 and e I2 are the unit direction vectors of the moving ball points of ballbar No. 1 and ballbar No. 2 relative to the fixed ball point respectively;

[0022] For the measurement of two sets of ballbars, the measurement data is insensitive to the normal vector direction of the plane formed by the two linked linear axes. After projecting the vector equation to the error-sensitive direction, the homogeneous transformation matrix between the moving and fixed coordinate systems degenerates into

[0023]

[0024] Combining equations (1), (3), and (4) we have

[0025]

[0026] Substituting the measurement data and attitude transformation matrix from ballbars 1 and 2 into Equation (6), and simultaneously projecting the vector equation in Equation (3) onto the two coordinate axes of the fixed coordinate system, we can solve for the X, Y, and deflection angle α of the spindle box (the component being measured) at any position i in the linked motion. When measuring machine tools with different kinematic chain configurations, the components being measured are different. These components are those that actually move during the measurement process, such as the spindle box or worktable.

[0027] The beneficial effects of the present invention are as follows: the measurement method utilizes a dual-ballbar measurement system, with the instruments installed synchronously for simultaneous measurement. Two independent sets of measurement parameters are obtained from a single measurement motion trajectory. Kinematic geometry methods are used to establish error parameter solution equations, effectively solving the motion error parameters of the machine tool's linear axis linkage. The measurement fixture and instrument are simple to install, the measurement trajectory is simple to set, and measurement efficiency is high. The method can detect the runout error of a moving component in a machine tool's two linear axes linkage using a single circular trajectory motion, improving the efficiency and accuracy of linear axis motion precision measurement during machine tool assembly and adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This diagram shows the installation of a dual ballbar system for measuring the accuracy of a machine tool's linear axis linkage. Schematic diagram: 1 - machine column, 2 - ballbar No. 1, 3 - machine bed, 4 - mounting base, 5 - ballbar No. 2, 6 - dual ballbar mounting fixture, 7 - spindle box, 8 - slide plate.

[0029] Figure 2 The closed-loop vector equation diagram for the two-axis linkage accuracy measurement of a machine tool with two sets of ballbars. f -X f Y f Z f is a fixed coordinate system, O m -X m Y m Z m is the motion coordinate system, S1 is the fixed ball point No. 1 of the ballbar, and its position coincides with the origin of the fixed coordinate system, S2 is the fixed ball point No. 2 of the ballbar, S3 is the moving ball point No. 1 of the ballbar, and its position coincides with the origin of the motion coordinate system, S4 is the moving ball point No. 2 of the ballbar, Tr1 is the motion trajectory of the moving ball point No. 1 of the ballbar, Tr2 is the motion trajectory of the moving ball point No. 2 of the ballbar, S3' is the schematic position of the moving ball point No. 1 of the ballbar at a certain moment in the measurement process, S4' is the schematic position of the moving ball point No. 2 of the ballbar at a certain moment in the measurement process, R f is the sensor position element vector, pointing from point S1 to point S2, R m is the vector of the geometric element being measured, pointing from the moving ball point S3 to the ball point S4, R b1 is the measurement parameter vector of ballbar No. 1, which points from the fixed ball point S1 to the moving ball point S3, R b2It is the measurement parameter vector of ballbar No. 2, pointing from the fixed ball point S2 to the moving ball point S4.

[0030] Figure 3 Flowchart of the method for measuring the two-axis linkage accuracy of machine tools using dual ballbars. DETAILED DESCRIPTION

[0031] The specific implementation of the present invention is described in detail below with reference to the accompanying drawings and technical solutions.

[0032] The present invention is a method for measuring the motion accuracy of a linear axis of a machine tool, which uses a double set of ballbars for measurement. Figure 1 As shown. A slide 8 that can move up and down is installed on the side of the machine tool column 1. A spindle box 7 is fixed to the slide 8. A double-ballbar mounting fixture 6 is mounted on the output shaft of the spindle box 7. A moving ball base is attached to the lower surface of the double-ballbar mounting fixture. Two moving ball bases are mounted on the machine tool's spindle box 7 via the double-ballbar mounting fixture 6. Two fixed ball bases are mounted on the machine tool's mounting base 4. Two ballbars (ballbar 1 No. 2 and ballbar 2 No. 5) are mounted between a pair of moving ball bases and fixed ball bases. When using this method to measure machine tools or machining centers with different kinematic chain configurations, the fixture structure can be appropriately adjusted to effectively connect the moving ball base to the machine tool's moving components.

[0033] Figure 3 This is a flow chart of the measurement method of this embodiment. The specific steps of the method for measuring the linkage motion accuracy of the linear axis of a machine tool using a dual ballbar system are as follows:

[0034] 1. Install the measuring fixture and adjust the relative position between the two moving ball bases: Install the measuring fixture through the spindle end tool holder of the machine tool. Attach the moving ball points S3 and S4 to the fixture. Adjust the position of the two bases to be larger than the selected standard length of the ballbar to prevent interference between the two ballbars. Record the adjusted distance.

[0035] 2. Install the fixed ball base and ballbar: Install two magnetic bases at appropriate positions on the machine tool mounting base (or bed). Drive the machine tool linear axis so that the moving ball base is close to the fixed ball base. Use the fine adjustment standard ball to adjust the fixed ball base to the correct position. Move the moving end of the machine tool horizontally to the standard length of the selected ballbar. Install two ballbars respectively.

[0036] 3. Establish a fixed coordinate system O f -X f Y f Z f With the motion coordinate system O m -X m Y m Z m , see Figure 2, the origins of the fixed coordinate system and the moving coordinate system are located at the fixed spherical point S1 and the moving S3 respectively, and a closed-loop vector model of the measurement process including the three measurement elements is established;

[0037] 4. Set up the machine tool motion program to make the machine tool move in a circular trajectory with two linear linkages. The trajectory radius corresponds to the selected ballbar standard length. Execute the measurement trajectory to obtain two ballbar measurement data. To protect the measuring instrument, the program can be run dry without the ballbar installed during measurement and debugging to check whether the trajectory setting is correct.

[0038] 5. Establish the kinematic geometry model equations of the measurement process (3);

[0039] 6. Solve the model equations according to formulas (1)-(6) to obtain the error motion parameters of the spindle box when the two linear axes are linked, including two position parameters X, Y and a deflection angle parameter α.

[0040] This invention rationally combines a dual-ballbar measurement system and applies the principles of kinematic geometry to establish a mapping relationship between dual-ballbar measurement data and the error motion of the measured moving component. This method then derives a solution for the measured error motion parameters. This simple method for measuring the precision of the linked motion of machine tool linear axes allows for accurate measurement of the measured error, including the runout of the measured component. It effectively distinguishes between spindle box motion errors caused by positional errors and deflection angle errors, achieving high accuracy. This method significantly improves the efficiency and accuracy of linear axis precision measurement and enhances the efficiency of assembly and commissioning of machine tool linear axes.

Claims

1. A method for measuring the linkage accuracy of machine tool linear axes using a dual ballbar system, characterized in that: Two ballbars are used to measure the accuracy of the linked motion of two moving axes of a machine tool. Specifically, the two ballbars are simultaneously mounted between the machine tool's spindle box and the mounting base using a fixture. A straight line passing through the two moving sphere centers on the moving component replaces a single sphere center point as the analysis and measurement element. A closed-loop measurement vector model is established based on the principles of kinematic geometry, and the error motion parameters are decoupled by solving equations. The measurement method includes the following steps: Step 1: Install the double-ball bar mounting fixture and adjust the relative position between the two moving ball bases: A slide that can move up and down is installed on the side of the machine column, and the spindle box is fixed on the slide. Install the double-ball bar mounting fixture on the output shaft of the spindle box; the moving ball base is adsorbed on the lower surface of the double-ball bar mounting fixture. Adjust and record the distance L between the two bases; Step 2: Install the double ballbar: Use the magnetism of the fixed ball base to adsorb it on the mounting base. The mounting base is fixed relative to the machine bed during the measurement process. Manually adjust the distance and relative direction of the two fixed ball bases to make the distance between the fixed ball bases L and the relative direction similar to the mutual direction of the two moving ball bases. Adsorb the two base fine-tuning standard balls of the double ballbar on the adaptive magnetic ball bowls of the two fixed ball bases respectively. The adaptive magnetic ball bowl can achieve a small range of displacement and swing on the fixed ball base. By moving the X, Y, and Z axes of the machine tool, the spindle box drives the two moving ball bases on the mounting fixture to approach their corresponding fixed ball bases. When the position is close to the ball bowl of the moving ball base, When the ball is within the magnetic range, the fine-tuning standard ball moves with the adaptive magnetic bowl of the fixed ball base under the action of magnetic attraction, so that the fine-tuning standard ball is simultaneously adsorbed on the bowl of the fixed ball base and the bowl of the dynamic ball base. The locking handle on the fixed ball base is rotated to lock the adaptive bowl. The fixed ball point installation position on the fixed ball base is now the precise position, and the precise adjustment of the fixed ball base is completed. Remove the auxiliary fine-tuning standard ball, translate the machine tool's moving axis to the reference length of the ballbar being used, and install ballbars No. 1 and No. 2 between the two sets of dynamic and fixed bases respectively. The fixed ball points of ballbars No. 1 and No. 2 are S1 and S2 respectively, and the dynamic ball points are S3 and S4 respectively. Step 3: Establish a fixed coordinate system and a moving coordinate system: The origin of the fixed coordinate system is located at the fixed point S1, and the X coordinate axis of the fixed coordinate system is X f , the direction is from S1 to S2; the origin of the motion coordinate system is located at the moving ball point S3, and the X coordinate axis of the motion coordinate system is X m , the direction is from S3 to S4, and the measurement movement is ||R f ||=||R m || (1) where R f is the relative position vector of the two fixed points, from fixed point S1 to fixed point S2, R m is the relative position vector of the two moving ball points, pointing from moving ball point S3 to moving ball point S4; Homogeneous transformation matrix M between fixed coordinate system and moving coordinate system mf for Where α, β, and γ are the deflection angle, pitch angle, and roll angle of the moving coordinate system relative to the fixed coordinate system, respectively; c represents cosine, s represents sin, and X, Y, and Z are the displacement parameters of the moving coordinate system relative to the fixed coordinate system in three directions; Step 4: Set the two-axis motion circle trajectory and measure the data: The data λ1 and λ2 obtained by ballbar No. 1 and ballbar No. 2 correspond to the sensor measurement vector R b1 and R b2 ; Step 5. Establish the measurement motion geometry model: The closed-loop vector equation of the double ballbar system measurement process is: R f +R b2 =R b1 +R m (3) Step 6: Solve the motion parameters of the moving axis linkage error: For any motion position i of the machine tool during the measurement process, the sensor measurement vector R of the two ballbars is expressed in the form of a circular vector b1i and R b2i : in is the rotation angle corresponding to ballbar No. 1 at position i, is the rotation angle of ballbar No. 2 at position i, λ 1i and λ 2i are the data values ​​measured by ballbar No. 1 and ballbar No. 2 at position i, respectively, and e I1 and e I2 are the unit direction vectors of the moving ball points of ballbar No. 1 and ballbar No. 2 relative to the fixed ball point respectively; For the measurement of two sets of ballbars, the measurement data is insensitive to the normal vector direction of the plane formed by the two linked linear axes. After projecting the vector equation to the error-sensitive direction, the homogeneous transformation matrix between the moving and fixed coordinate systems degenerates into Combining equations (1), (3), and (4) we have Substituting the measurement data and attitude transformation matrix of ballbar No. 1 and ballbar No. 2 into equation (6), and projecting the vector equation of equation (3) onto the two coordinate axes of the fixed coordinate system in parallel, the two scalar equations obtained can be solved to obtain the motion position X, Y and deflection angle α of the spindle box of the measured component at any position i of the linkage motion. When measuring machine tools with different kinematic chain configurations, the measured components are different, and the measured components are the components that actually move during the measurement process.

Images