A pose space based articulated coordinate measuring machine measurement error compensation method

Abstract

The application discloses a kind of joint coordinate measuring machine measurement error compensation methods based on pose space;The calibration method is as follows: one, the initial structure parameter is obtained by static calibration to joint coordinate measuring machine.Two, the probe of joint coordinate measuring machine is moved along the preset trajectory, and the sampling point data of probe is continuously detected and recorded;Sampling point data includes probe coordinates and the angle of each joint.Three, data processing, the pose space of multiple joint coordinate measuring machines is calibrated respectively.The application divides the pose space to the first three joints of joint coordinate measuring machine which has greater influence on measurement accuracy, and calibrates the structure parameter for each pose space respectively, overcomes the problem that the additional error caused by joint rotation of joint coordinate measuring machine leads to unstable measurement accuracy at different positions, and improves the reliability of joint coordinate measuring machine in subsequent detection.

Description

Technical Field

[0001] This invention belongs to the field of coordinate measuring machine calibration technology, specifically relating to a method for compensating measurement errors in an articulated coordinate measuring machine based on attitude space. Background Technology

[0002] Since its inception, the coordinate measuring machine (CMM) has been used in emerging industries such as machinery manufacturing, electronics, automotive, and aerospace. Its versatility, wide measurement range, high precision, high efficiency, and good performance have led to its widespread application and significant development. The calibration of an articulated CMM can be divided into four steps: 1. Establishing a measurement model; 2. Acquiring calibration data; 3. Obtaining structural parameters from the calibration data; 4. Experimentally verifying the effectiveness of the structural parameters. The measurement model established in the first step forms the mathematical foundation of the CMM; the second step mainly involves using the articulated CMM to probe specific standard parts to obtain a large amount of angular information based on the designed calibration scheme; the third step generally involves processing the obtained angular information using a specific optimization algorithm to obtain the structural parameters; the fourth step is the process of applying the obtained structural parameters to the calibrated CMM to improve its measurement accuracy.

[0003] Building upon this foundation, Santolaria used a ball-and-bar standard component to calibrate an articulated coordinate measuring machine (CMM) and conducted an in-depth, systematic study. Seven balls on the sampling rod were used as sampling points, and the rod's pose was continuously changed during sampling to obtain sufficient data. Finally, the LM method was used to process the obtained data, achieving good results. Santolaria's model for the six-DOF articulated CMM is a redundant parameter model containing 27 structural parameters. However, Santolaria's method requires changing the probe type to match the sampling standard balls. The probe used cannot be used in actual measurements, so probe parameter calibration is necessary. Among all calibration methods based on length, the self-made rod with tapered holes at both ends is the simplest and most effective. Because the standard component has tapered holes at both ends, a spherical probe can be directly used for sampling without the need for a specially made probe. The calibrated parameters can then be directly applied to measurements, facilitating the verification of measurement results.

[0004] In addition, Shimojima uses a three-dimensional ball plate to calibrate the articulated coordinate measuring machine. The method is as follows: use a high-precision orthogonal coordinate measuring machine to measure the coordinates of the center of the ball on the ball plate, then place the ball plate at 5 different positions around the measuring machine, use the articulated coordinate measuring machine to sample and obtain 9 position points, and use the coordinates of these points as reference values ​​to calibrate the articulated coordinate measuring machine.

[0005] However, the articulated coordinate measuring machine (CMM) calibrated using the above-mentioned calibration method suffers from inconsistent measurement accuracy across different regions. This problem arises from joint rotation, specifically from errors in the manufacturing and assembly process of the CMM, namely, the difference between the design and actual values. Joint rotation causes changes in some actual values; for example, the actual position of the joint's rotation axis differs from the design position, leading to changes in structural parameters. Therefore, even after calibration, the CMM still exhibits inconsistent measurement values ​​and accuracy across different measurement spaces due to varying orientations. This invention addresses this issue by dividing the rotation angle range of some joints into intervals, combining these intervals to correspond to different spatial poses of the end effector, and terming this combination "attitude space." Error compensation is then applied to each attitude space using calibration techniques. Summary of the Invention

[0006] The purpose of this invention is to propose a calibration method for multiple measurement models of an articulated coordinate measuring machine based on joint space.

[0007] A method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space includes the following steps:

[0008] Step 1: Perform static calibration on the articulated coordinate measuring machine to obtain the initial structural parameters.

[0009] Step 2: Move the probe of the articulated coordinate measuring machine along the preset trajectory, and continuously detect and record the sampling point data of the probe; the sampling point data includes the probe coordinates and the angles of each joint.

[0010] Step 3: Data processing. Calibrate the multiple attitude spaces of the articulated coordinate measuring machine.

[0011] Step 3-1. Divide the rotation range of the s joints of the articulated coordinate measuring machine near the base into multiple angle intervals; 1≤s≤4; arrange and combine the different angle intervals of the s joints to form multiple posture spaces.

[0012] Step 3-2. Perform calibration on each attitude space in sequence. The calibration process is as follows: for the first attitude space, the initial structure parameters obtained in Step 1 are used as the initial values ​​of the nonlinear least squares method during calibration; for the remaining attitude spaces, the final structure parameters of the previous attitude space are used as the initial values ​​of the nonlinear least squares method during calibration; the sampling point data corresponding to the currently calibrated attitude space are used as the measured values ​​of the objective function, and the nonlinear least squares method is executed to obtain the final structure parameters of the currently calibrated attitude space.

[0013] Preferably, the number of joints 's' used to divide the angle interval in step 3-1 is 3. The conditions for sorting each attitude space are as follows: the attitude spaces are sorted according to the first principle of ascending first joint angle, the second principle of ascending second joint angle, and the third principle of ascending third joint angle.

[0014] Preferably, in step two, the minimum distance between the preset path of the probe and the center point of the measurement space of the articulated coordinate measuring machine (ACM) is less than 20% of the measurement radius; the maximum distance between the preset path of the probe and the center point of the measurement range is greater than 80% of the measurement radius; there is at least one point on the preset path such that the second and third joints of the ACM simultaneously reach their maximum rotation angle. There is also at least one point on the preset path such that the second and third joints of the ACM simultaneously reach their minimum rotation angle. During the complete passage of the probe through the preset path, the first joint of the ACM rotates at least one revolution.

[0015] Preferably, in step 3-1, each joint that is divided into angle intervals has 3 to 12 angle intervals.

[0016] Preferably, the objective function described in step 3-2 is a multi-point calibration objective function, the expression of which is shown in equation (1):

[0017]

[0018] Where x, y, and z are the average values ​​of the X-axis, Y-axis, and Z-axis coordinates of all sampled points in the currently calibrated attitude space, respectively. j y j , z j , where are the X-axis, Y-axis, and Z-axis coordinates of the j-th sampling point in the currently calibrated attitude space, respectively; s is the number of sampling points in the currently calibrated attitude space.

[0019] Preferably, the objective function described in step 3-2 is a length calibration objective function, the expression of which is shown in equation (2):

[0020]

[0021] Among them, L j d represents the true length of the j-th line segment in the currently calibrated pose space; j t represents the measured length of the j-th line segment in the currently calibrated attitude space; t represents the number of sampling points in the currently calibrated attitude space.

[0022] As a preferred method, during the coordinate measurement process using a calibrated articulated coordinate measuring machine, the rotation angles of the s joints of the articulated coordinate measuring machine near the base determine the corresponding attitude space, the final structural parameters corresponding to this attitude space are selected, and the coordinate measurement values ​​are calculated.

[0023] In step two, the probe of the articulated coordinate measuring machine is moved by the spindle of the orthogonal coordinate measuring machine. The probe coordinates in the sampling point data are obtained by detection using the orthogonal coordinate measuring machine.

[0024] Preferably, the probe of the articulated coordinate measuring machine (ACM) is fixed to the spindle of the orthogonal coordinate measuring machine (CMM) via a connecting assembly. The connecting assembly includes a first three-ball conical socket and a second three-ball conical socket. The first three-ball conical socket is coaxially and detachably fixed to the CMM spindle via a stepped shaft. The second three-ball conical socket has a central hole; the central hole of the second three-ball conical socket is fitted onto the rod portion of the probe of the calibrated ACM. The three positioning balls of the second three-ball conical socket contact the tip ball portion of the probe. Both the first and second three-ball conical sockets have multiple connecting holes; the first three-ball conical socket and the second three-ball conical socket are fixed together via connecting holes and bolts, thus fixing the probe of the articulated coordinate measuring machine to the spindle of the orthogonal coordinate measuring machine.

[0025] Preferably, the static calibration process in step one is as follows: three calibration components are set at different positions in the measurement space of the articulated coordinate measuring machine. The articulated coordinate measuring machine is used to collect coordinates of the three calibration components multiple times; a set of structural parameters is identified through a differential evolution algorithm and used as the initial structural parameters.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. This invention divides the attitude space of the three joints of the articulated coordinate measuring machine that have a significant impact on the measurement accuracy, and calibrates the structural parameters for each attitude space. This overcomes the problem of unstable measurement accuracy at different positions caused by the additional error brought about by the joint rotation of the articulated coordinate measuring machine, and improves the reliability of the articulated coordinate measuring machine in subsequent testing.

[0028] 2. This invention moves the probe of the articulated coordinate measuring machine along a preset path and uses an orthogonal coordinate measuring machine to continuously sample the probe of the articulated coordinate measuring machine. While eliminating the tedious operation of arranging calibration parts in various poses in space, it can quickly acquire a large amount of calibration data, meeting the large data volume requirements of structural parameter calibration in multi-pose space. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the connection components used in this invention;

[0030] Figure 2 This is a schematic diagram of the first three-ball conical socket in the connecting assembly used in this invention;

[0031] Figure 3 This is a schematic diagram of the second and third ball-and-cone sockets in the connecting assembly used in this invention;

[0032] Figure 4 This is a flowchart of the calibration of the articulated coordinate measuring machine in step four of the present invention.

[0033] Reference numerals in the attached diagram: 1. First three-ball cone socket; 2. Second three-ball cone socket; 3. Probe; 4. Stepped shaft. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings.

[0035] Example 1

[0036] A method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space, the specific steps of which are as follows:

[0037] Step 1: Static calibration. Three calibration components are set at different locations in the measurement space using an articulated coordinate measuring machine (ACM). All three calibration components are conical recesses. The ACM performs 30 coordinate acquisitions on each of the three calibration components; a set of structural parameters is identified using a differential evolution algorithm, which serves as the initial structural parameters for subsequent solutions.

[0038] Step 2: Power on the orthogonal coordinate measuring machine (OCM) and return the spindle of the OCM to its origin. Use the connecting assembly to fix the spindle of the OCM to the probe of the articulated OCM.

[0039] like Figure 1 , 2 As shown in Figure 3, the connecting assembly includes a first three-ball conical socket 1 and a second three-ball conical socket 2. The first three-ball conical socket 1 is coaxially and detachably fixed to the spindle of the orthogonal coordinate measuring machine via a stepped shaft 4. The second three-ball conical socket 2 has a central hole; the central hole of the second three-ball conical socket 2 is fitted onto the rod portion of the probe 3 of the calibrated articulated coordinate measuring machine. The three positioning balls of the second three-ball conical socket 2 are in contact with the tip ball portion of the probe 3. Both the first three-ball conical socket 1 and the second three-ball conical socket 2 have multiple connecting holes; the first three-ball conical socket 1 and the second three-ball conical socket 2 are fixed through connecting holes and bolts, thereby fixing the probe of the articulated coordinate measuring machine to the spindle of the orthogonal coordinate measuring machine.

[0040] Step 3: Input the curve program corresponding to the preset path into the orthogonal coordinate measuring machine (CMM). The CMM then drives the probe of the articulated CMM to move along the preset path. During the movement, continuously collect the joint angles of the articulated CMM and the spindle coordinates of the orthogonal CMM.

[0041] The measuring range of an articulated coordinate measuring machine (ACM) is a sphere with its measuring radius as its radius. The minimum distance between the preset path of the ACM's probe and the center point of the measuring range is less than 20% of the measuring radius; the maximum distance between the preset path of the ACM's probe and the center point of the measuring range is greater than 80% of the measuring radius; there must be at least one point on the preset path such that the second and third joints of the ACM simultaneously reach their maximum rotation angle. There must also be at least one point on the preset path such that the second and third joints of the ACM simultaneously reach their minimum rotation angle. During the complete passage of the probe through the preset path, the first joint of the ACM rotates at least one revolution.

[0042] Step 4, as follows Figure 4 As shown, data processing is performed to complete the calibration of the articulated coordinate measuring machine.

[0043] 1) Divide the rotation ranges of the first, second, and third joints of the articulated coordinate measuring machine (CMM) into m angle intervals; n angle intervals; p angle intervals; 3≤m≤12; 3≤n≤12; 3≤p≤12; forming m×n×p attitude spaces. Sort the m×n×p attitude spaces according to the following principle: first, the rotation angles of the first joint are arranged from smallest to largest; second, the rotation angles of the second joint are arranged from smallest to largest; and third, the rotation angles of the third joint are arranged from smallest to largest. All sampling points measured in step two are assigned to the corresponding attitude spaces based on the rotation angles of the first, second, and third joints at the time of sampling.

[0044] 2) Set the initial value of i to 1.

[0045] 3) Identify the structural parameters for the i-th attitude space. Using the final structural parameters of the (i-1)-th attitude space as the initial values ​​for the nonlinear least squares method, and the sampled point data corresponding to the i-th attitude space as the measured values ​​of the objective function, perform the nonlinear least squares method to obtain the structural parameters corresponding to the i-th attitude space. Use the initial structural parameters obtained in step two as the final structural parameters for the 0th attitude space.

[0046] The objective function adopts a multi-point calibration objective function, and its expression is shown in equation (1):

[0047]

[0048] Where x, y, and z are the average values ​​of the X-axis, Y-axis, and Z-axis coordinates of all sampling points in the currently calibrated i-th attitude space, respectively. j y j , z jδ1 represents the X-axis, Y-axis, and Z-axis coordinates of the j-th sampling point in the currently calibrated i-th attitude space, respectively; s represents the number of sampling points in the currently calibrated i-th attitude space. The objective function is to minimize δ1.

[0049] 4) Increase i by 1 and repeat step 3) until i is greater than m×n×p. The corresponding structural parameters are generated in m×n×p attitude spaces, and the calibration of the articulated coordinate measuring machine is completed.

[0050] Step 5: Use the calibrated articulated coordinate measuring machine to perform coordinate measurements. During the coordinate measurement process, the articulated coordinate measuring machine selects the final structural parameters corresponding to the attitude space where the first, second, and third joint rotation angles are located, and calculates the coordinate measurement values.

[0051] Example 2

[0052] An error compensation method for articulated coordinate measuring machines based on attitude space is proposed. The only difference between this embodiment and Embodiment 1 is that the objective function used in step four is different.

[0053] The objective function used in this embodiment is a length calibration objective function, the expression of which is shown in equation (2):

[0054]

[0055] Among them, L j d represents the true length of the j-th line segment in the currently calibrated pose space; j δj represents the measured length of the j-th line segment in the currently calibrated i-th attitude space; t represents the number of sampling points in the currently calibrated i-th attitude space. The objective function is to minimize δ2. A line segment is formed by connecting any two sampling points in the same attitude space.

Claims

1. A method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space, characterized in that: Includes the following steps: Step 1: Perform static calibration on the articulated coordinate measuring machine to obtain initial structural parameters; Step 2: Move the probe of the articulated coordinate measuring machine along a preset trajectory, and continuously detect and record the sampling point data of the probe; the sampling point data includes the probe coordinates and the angles of each joint; Step 3: Data processing, calibrating the multiple attitude spaces of the articulated coordinate measuring machine; Step 3-1. Divide the rotation range of the s joints of the articulated coordinate measuring machine near the base into multiple angle intervals; 1≤s≤4; arrange and combine the different angle intervals of the s joints to form multiple posture spaces; Step 3-2. The attitude spaces are calibrated sequentially. The calibration process is as follows: for the first attitude space, the initial structure parameters obtained in Step 1 are used as the initial values ​​for the nonlinear least squares method during calibration; for the remaining attitude spaces, the final structure parameters of the previous attitude space are used as the initial values ​​for the nonlinear least squares method during calibration. Using the sampling point data corresponding to the currently calibrated attitude space as the measured value of the objective function, the nonlinear least squares method is executed to obtain the final structure parameters of the currently calibrated attitude space.

2. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: In step 3-1, the number of joints s used to divide the angle interval is 3. The conditions for sorting each attitude space are as follows: the attitude spaces are sorted according to the first principle of ascending first joint angle, the second principle of ascending second joint angle, and the third principle of ascending third joint angle.

3. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: In step two, the minimum distance between the preset path of the probe and the center point of the measurement space of the articulated coordinate measuring machine is less than 20% of the measurement radius; the maximum distance between the preset path of the probe of the articulated coordinate measuring machine and the center point of the measurement range is greater than 80% of the measurement radius; there is at least one point on the preset path that allows the second and third joints of the articulated coordinate measuring machine to reach their maximum rotation angle simultaneously; there is at least one point on the preset path that allows the second and third joints of the articulated coordinate measuring machine to reach their minimum rotation angle simultaneously. During the process of the probe completely traversing the preset path, the first joint of the articulated coordinate measuring machine rotates at least once.

4. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: In step 3-1, each joint that is divided into angle intervals has 3 to 12 angle intervals.

5. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: The objective function described in step 3-2 is a multi-point calibration objective function, and its expression is shown in equation (1): Where x, y, and z are the average values ​​of the X-axis, Y-axis, and Z-axis coordinates of all sampling points in the currently calibrated attitude space, respectively; x j y j , z j These are the X, Y, and Z coordinates of the j-th sampling point in the currently calibrated attitude space, respectively; n is the number of sampling points in the currently calibrated attitude space. The objective function is calibrated at multiple points.

6. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: The objective function described in step 3-2 is a length-calibrated objective function, the expression of which is shown in equation (2): in, This represents the true length of the j-th line segment in the currently calibrated pose space; t represents the measured length of the j-th line segment in the currently calibrated attitude space; t represents the number of sampling points in the currently calibrated attitude space. The objective function for length calibration.

7. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: During coordinate measurement using a calibrated articulated coordinate measuring machine; the rotation angles of the s joints of the articulated coordinate measuring machine near the base determine the corresponding attitude space, the final structural parameters corresponding to the attitude space are selected, and the coordinate measurement values ​​are calculated. In step two, the probe of the articulated coordinate measuring machine is moved by the spindle of the orthogonal coordinate measuring machine; the probe coordinates in the sampling point data are obtained by detection by the orthogonal coordinate measuring machine.

8. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: The probe of the articulated coordinate measuring machine is fixed together with the spindle of the orthogonal coordinate measuring machine by a connecting assembly; the connecting assembly includes a first three-ball conical socket (1) and a second three-ball conical socket (2); the first three-ball conical socket (1) and the spindle of the orthogonal coordinate measuring machine are coaxially and detachably fixed by a stepped shaft (4); the second three-ball conical socket (2) is provided with a central hole; the central hole of the second three-ball conical socket (2) is fitted onto the rod part of the probe (3) of the calibrated articulated coordinate measuring machine; the three positioning balls of the second three-ball conical socket (2) are in contact with the tip ball part of the probe (3); both the first three-ball conical socket (1) and the second three-ball conical socket (2) are provided with multiple connecting holes; the first three-ball conical socket (1) and the second three-ball conical socket (2) are fixed by connecting holes and bolts, so that the probe of the articulated coordinate measuring machine is fixed to the spindle of the orthogonal coordinate measuring machine.

9. The method for compensating measurement errors of an articulated coordinate measuring machine based on attitude space according to claim 1, characterized in that: The static calibration process in step one is as follows: three calibration components are set at different positions in the measurement space of the articulated coordinate measuring machine; the coordinates of the three calibration components are acquired multiple times using the articulated coordinate measuring machine; a set of structural parameters is identified through the differential evolution algorithm and used as the initial structural parameters.

Images