A virtual target-based calibration method for articulated coordinate measuring machine

Abstract

The application discloses a kind of based on the joint coordinate measuring machine calibration method of virtual calibration member construction.It is as follows: one, the coordinates of 12 sampling points are determined;Two, 12 sampling points are used to construct selected type virtual calibration member.Three, the spindle of precision machine tool drives calibration tool to move to the sampling point position on all virtual calibration members in turn.Each time calibration tool reaches a sampling point, the coordinates of calibration tool are measured using the spherical probe of joint coordinate measuring machine.Four, according to the measurement results, joint coordinate measuring machine is calibrated.The application constructs a large number of different types of virtual calibration members by setting 12 sampling points in precision machine tool, and the size and spatial pose of calibration member can change according to the change of sampling point;Compared with the existing calibration method using entity calibration member that can only change spatial pose, the calibration effect and calibration efficiency of the application are significantly enhanced.

Description

Technical Field

[0001] This invention belongs to the field of coordinate measuring machine calibration technology, specifically relating to a calibration method for an articulated coordinate measuring machine based on a virtual calibration component. Background Technology

[0002] Since its inception, the coordinate measuring machine (CMM) has been widely used in emerging industries such as machinery manufacturing, electronics, automobiles, and aerospace. Its versatility, large 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. These four steps are complementary: 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 specific optimization algorithms to obtain structural parameters; and the fourth step is the process of applying the obtained structural parameters to the calibrated CMM to improve its measurement accuracy. Currently, the reference values ​​used in the calibration of articulated CMMs are mainly divided into the following categories: 1) single-point reference; 2) length values; 3) coordinate values.

[0003] The calibration method using an orthogonal coordinate measuring machine (CMM) as the reference involves placing the articulated CMM within the measurement space of the orthogonal CMM. First, the orthogonal CMM establishes a coordinate system on the base of the articulated CMM, aligning it as closely as possible with the machine coordinate system of the articulated CMM. The articulated CMM is then fixed in position using a fixture, ensuring its probe is within the measurement space of the orthogonal CMM. Next, the orthogonal CMM measures the coordinates of the probe's center of gravity. During this measurement, probe stability must be maintained. While the orthogonal CMM is sampling, the angles of the articulated CMM are simultaneously sampled. After sampling one point, the fixture is adjusted to change the articulated CMM's position, and once the probe stabilizes, the next point is sampled. This process continues until sufficient data has been collected.

[0004] Santolaria used a ball-and-bar standard component to calibrate an articulated coordinate measuring machine (CMM) and conducted an in-depth, systematic study of it. He used seven balls on the sampling rod as sampling points, continuously changing the rod's pose during sampling to obtain sufficient data. Finally, he used the LM method 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.

[0005] Shimojima uses a 3D ball plate to calibrate an 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, and use the articulated coordinate measuring machine to sample and obtain 9 position points, using the coordinates of these points as the reference values.

[0006] The above methods still have shortcomings in calibration, requiring high-precision calibration components, the manufacturing cost of which is considerable. This invention aims to replace physical calibration components with virtual ones. A virtual calibration component is constructed using a higher-precision instrument to complete the calibration process. In scenarios requiring articulated coordinate measuring machines to inspect object dimensions, precision machining equipment is often available. To better integrate with industrial realities, a precision machine tool (preferably a three-axis CNC machine tool) can be used to construct the virtual calibration component. Summary of the Invention

[0007] The purpose of this invention is to propose a calibration method for articulated coordinate measuring machines (CMMs) based on virtual calibration components. During calibration, compared to physical calibration components that can only change their spatial orientation, the virtual calibration components used in this invention are composed of various calibration points. Selecting different calibration points can create virtual calibration components with different geometric shapes, sizes, and spatial orientations. This allows for more complete movement of the articulated coordinate measuring machine and enables targeted calibration strategies to be adopted according to different types of articulated coordinate measuring machines.

[0008] This invention discloses a method for calibrating an articulated coordinate measuring machine by constructing a virtual calibration component. Taking the construction of the virtual calibration component using a precision machine tool as an example, the specific steps are as follows:

[0009] Step 1: Determine N sampling points in the articulated coordinate measuring machine (ACM), where N ≥ 5; select several types of virtual calibration components; these include virtual circle calibration components and virtual sphere calibration components. The calibration parameters for the virtual circle calibration component are the radius of the virtual circle center or the distance between the centers of two virtual circles. The calibration parameters for the virtual sphere calibration component are the radius of the virtual sphere or the distance between the centers of two virtual spheres.

[0010] Step 2: Construct multiple virtual calibration components of the selected type using N sampling points.

[0011] Step 3: Install a calibration tool on the spindle of the precision machine tool to position the probe of the articulated coordinate measuring machine. The spindle of the precision machine tool drives the calibration tool to move sequentially to the sampling point positions on all virtual calibration components. Whenever the calibration tool reaches a sampling point, the ball probe of the articulated coordinate measuring machine is used to measure the coordinates of the calibration tool in various different orientations.

[0012] Step 4: Using the coordinate values ​​of each sampling point obtained in Step 3, acquire the calibration parameter measurement values ​​of all virtual calibration components. Construct an objective function based on the measured and actual calibration parameter values. Use the objective function to calibrate the articulated coordinate measuring machine.

[0013] Preferably, in step two, if the type of virtual calibration element is a virtual circular calibration element, then the virtual circular calibration elements constructed from the sampling points are screened, retaining only those virtual circular calibration elements whose maximum angle of the characteristic triangle is 90°–120° and whose radius is 400mm–500mm. The characteristic triangle refers to the triangle formed by the three sampling points that constitute the virtual circular calibration element. When calibrating using the center of the circle formed by the combination of virtual circular calibration elements, based on the screening results, only combinations of virtual circular calibration elements with a center-to-center distance less than or equal to 100mm are retained.

[0014] Preferably, in step two, if the type of virtual calibration element is a virtual sphere calibration element, then the virtual sphere calibration elements constructed from the sampling points are screened, and only virtual sphere calibration elements with a radius of 600mm to 700mm are retained. When calibrating using the center of a circle formed by combining virtual sphere calibration elements, based on the screening results, only combinations of virtual sphere calibration elements with a center distance less than or equal to 100mm are retained. When calibrating using the center of a circle formed by combining virtual sphere calibration elements, based on the screening results, only combinations of virtual sphere calibration elements with a center distance of 200mm to 300mm are retained.

[0015] Preferably, the calibration tool uses a three-ball conical socket. The three-ball conical socket includes a small shaft section and a large shaft section; the small shaft section is used to connect with the spindle of a precision machine tool; the end face of the large shaft section has a tapered hole, and three spheres are evenly distributed circumferentially along the central axis of the tapered hole.

[0016] Preferably, in step three, the number of samplings for each sampling point is greater than or equal to 30.

[0017] Preferably, the number of sampling points N is 12.

[0018] As a preferred method, the process of setting the positions of N sampling points is as follows:

[0019] First, determine the coordinates (s) of a set of feature points in space. i ,t i ,z i )as follows:

[0020]

[0021] In the formula, i∈{0,...,N-1}, h is the characteristic constant of the Z-axis.

[0022] Then, the coordinates of the N feature points are multiplied by the magnification feature constant to obtain the coordinates of the N sampling points; all N sampling points are within the measurement space of the coordinate measuring machine.

[0023] Preferably, the magnification characteristic constant is greater than or equal to 80% of the measuring radius of the coordinate measuring machine.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. This invention constructs virtual calibration components by providing calibration points, generating virtual calibration components such as virtual spheres and virtual circles. Compared to physical calibration components, virtual calibration components have no structural limitations, and their type, size, and spatial pose can change according to variations in the calibration point position. This provides more options for the calibration of articulated coordinate measuring machines (CMMs), and targeted calibration schemes can be designed for different types of CMMs. Compared to existing calibration methods using physical calibration components that can only change spatial pose, this invention significantly enhances calibration effectiveness and efficiency.

[0026] 2. This invention uses precision machining equipment or precision measuring equipment to calibrate articulated coordinate measuring machines in combination with actual conditions, providing a lower-cost, more efficient, and better calibration method for articulated coordinate measuring machines in some scenarios. Attached Figure Description

[0027] Figure 1 This is a flowchart of the process of the present invention. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings.

[0029] like Figure 1As shown, a calibration method for an articulated coordinate measuring machine based on a virtual calibration component, taking the construction of a virtual calibration component using a precision machine tool as an example, includes the following specific steps:

[0030] Step 1: Power on the precision machine tool (preferably a three-axis CNC milling machine) and return the machine tool spindle to its origin. Install a three-ball conical socket on the machine tool spindle. The small shaft section of the three-ball conical socket is mounted on the machine tool spindle. The three-ball conical socket includes a small shaft section and a large shaft section; the small shaft section is used to connect to the machine tool spindle; the end face of the large shaft section has a tapered hole, and three spheres are evenly distributed circumferentially along the central axis of the tapered hole.

[0031] Step 2: The coordinates of the 12 sampling points are calculated as follows:

[0032] Let 0 ≤ (s) i ,t i ,z i )≤1 (or a certain unit length), (s i ,t i ,z i () is the coordinates of a set of feature points in space, defined as follows:

[0033]

[0034] In the formula, i∈{0,...,N-1}, and N is the number of measurement points; Represents the smallest integer not less than log₂N. Indicates not greater than The largest integer; Mod is the modulo operator; h is the Z-axis characteristic constant, whose value is less than or equal to the maximum Z-axis coordinate value of the articulated coordinate measuring machine.

[0035] Multiply the coordinates of N feature points by the magnification characteristic constant to obtain the coordinates of N sampling points; all N sampling points are within the measurement space. The magnification characteristic constant is greater than or equal to 80% of the measurement radius of the articulated coordinate measuring machine, ensuring that the N sampling points are more fully distributed within the measurement space.

[0036] Step 3: Based on the application scenario and calibration requirements of the articulated coordinate measuring machine, determine the type of virtual calibration component. Virtual calibration component types include virtual circle calibration components and virtual sphere calibration components. The calibration parameters for a virtual circle calibration component are the radius of the virtual circle center and the distance between the centers of the two virtual circles. The calibration parameters for a virtual sphere calibration component are the radius of the virtual sphere and the distance between the centers of the two virtual spheres. Various virtual calibration components are constructed by arranging and combining 12 sampling points using a three-point circle calibration and a four-point sphere calibration method.

[0037] The constructed virtual calibration components are screened using a MATLAB simulation program (in this example, an articulated coordinate measuring machine with a measuring radius of 1250mm is used as an example). During screening, the radii and spacing of both virtual circles and virtual spheres should be prioritized to be smaller. Specific screening conditions are as follows:

[0038] 1) Virtual circle calibration component: Three points are randomly selected from 12 sampling points to form a circle, resulting in a total of... There are various combinations, but some of these virtual circles will result in poor calibration, so it is necessary to select suitable virtual circles.

[0039] ① Using the radius of the virtual circular calibration component and the maximum angle of its characteristic triangle as the selection criteria, only virtual circular calibration components with a maximum angle of 90°–120° and a radius of 400mm–500mm are retained. Virtual circular calibration components with radii less than 400mm, greater than 500mm, or with a maximum angle less than 90° or greater than 120° in their triangles are removed. The characteristic triangle refers to the triangle formed by connecting the three sampling points that constitute the virtual circular calibration component.

[0040] ② Based on the above screening results, the center distance between each pair of the retained virtual circular calibration components is detected to form C. t 2 A virtual circular calibration component combination; t is the number of virtual circular calibration components selected above. Only virtual circular calibration component combinations with a center distance less than or equal to 100mm are retained.

[0041] The radius of the virtual circular calibration component or the center distance of the virtual circular calibration component combination is used as the calibration parameter, and the optimal solution of the structural parameters of the articulated coordinate measuring machine is calculated.

[0042] 2) Virtual sphere calibration component: Four points are randomly selected from 12 sampling points to form a sphere, resulting in a total of... There are various combinations, some of which result in poor calibration performance, so it is also necessary to select suitable virtual balls.

[0043] ① Using the radius of the virtual sphere calibration component as the screening criterion, only virtual sphere calibration components with a radius of 600mm to 700mm are retained. Virtual sphere calibration components with a radius less than 600mm or greater than 700mm are removed.

[0044] ② Based on the above screening results, the center-to-center distance between each pair of the retained virtual sphere calibration components is detected to form... A virtual circular calibration component combination; s is the number of virtual sphere calibration components selected above. Only virtual sphere calibration component combinations with a center-to-center distance of 200mm to 300mm are retained.

[0045] Using the radius of the virtual sphere calibration component or the center distance of the virtual sphere calibration component assembly as calibration parameters, the optimal solution for the structural parameters of the articulated coordinate measuring machine is calculated.

[0046] Step 3: Place the articulated coordinate measuring machine (CMM) so that all sampling points on the retained virtual calibration components are located within the measuring space of the CMM. The spindle of the precision machine tool drives the three ball-and-cone sockets to move sequentially to the sampling point positions on all the retained virtual calibration components.

[0047] Whenever the three-ball conical socket reaches a sampling point, the spherical probe of the articulated coordinate measuring machine (ACM) is inserted into the conical hole of the ACM calibration tool to sample data. After each data sampling, the spherical probe is removed from the conical hole of the ACM calibration tool. For each data sampling, the calibration machine measures the center of the conical hole of the three-ball conical socket 30 times in different postures within the joint's range of motion. These 30 different postures require displacement of all six joints of the ACM. During measurement, it is necessary to ensure full contact between the ACM probe and the three-ball conical socket, while avoiding excessive contact force to prevent deformation of the probe and the three-ball conical socket. During sampling, the joints of the measuring machine should be allowed as much movement as possible.

[0048] Step 4: Data Processing. During data processing, the quantities set for the precision machine tool should be considered the true values, and the measured data should be compared with these true values. Specifically, the measured coordinate values ​​of each sampling point obtained during sampling are used to acquire the calibration parameter measurement values ​​of all virtual calibration components. An objective function is constructed based on the measured and actual values ​​of the calibration parameters. The articulated coordinate measuring machine is then calibrated using the objective function.

[0049] The objective function for calibration using the radius of the virtual circle calibration component is as follows:

[0050] Calculate the total measurement error of the virtual circle radius. Where, r i,j Let δ1 be the measurement radius of the j-th virtual circle in the i-th measurement, r be the actual radius of the j-th virtual circle, m1 be the number of virtual circle calibration parts, and n be the number of calibration point samplings. Using the minimum δ1 as the objective function, find the optimal solution for the structural parameters of the articulated coordinate measuring machine.

[0051] The objective function for calibration using the center distance of the virtual circular calibration component assembly is as follows:

[0052] Calculate the total measurement error of the virtual circle's center distance. Among them, L j Let d be the actual center distance of the j-th virtual circle calibration component assembly. i,j Let δj be the measured center distance of the j-th virtual circular calibration component assembly, m2 be the number of virtual circular calibration component assemblies, and n be the number of calibration point samplings. Using the minimum δ2 as the objective function, find the optimal solution for the structural parameters of the articulated coordinate measuring machine.

[0053] The objective function for calibration using the radius of the virtual sphere calibration component is as follows:

[0054] Calculate the total measurement error of the virtual sphere radius. Where, r j Let r be the actual radius of the j-th virtual sphere. i,j Let δj be the radius measurement value of the j-th virtual sphere, m3 be the number of virtual sphere calibration parts, and n be the number of calibration point samplings. Using the minimum δ3 as the objective function, find the optimal solution for the structural parameters of the articulated coordinate measuring machine.

[0055] The objective function for calibration using the center distance of the virtual sphere calibration components is as follows:

[0056] Calculate the total error in the measurement of the distance between the centers of the virtual spheres. Among them, L j Let d be the actual center distance of the j-th virtual sphere calibration component assembly. i,j Let δ4 be the measured center distance of the j-th virtual sphere calibration component assembly, m4 be the number of virtual sphere calibration component assemblies, and n be the number of calibration point samplings. Using the minimum δ4 as the objective function, find the optimal solution for the structural parameters of the articulated coordinate measuring machine.

Claims

1. A calibration method for an articulated coordinate measuring machine based on constructing a virtual calibration component, characterized in that: Step 1: Determine in the articulated coordinate measuring machine N One sampling point, N ≥5; The process of setting the positions of N sampling points is as follows: First, determine the coordinates of a set of feature points in space. as follows: ； In the formula, ; h The characteristic constant of the Z-axis; After that, N The coordinates of each feature point are multiplied by the magnification feature constant to obtain the coordinates of N sampling points; all N sampling points are within the measurement space of the coordinate measuring machine. Several types of virtual calibration components are selected; the virtual calibration component types include virtual circle calibration components and virtual sphere calibration components; the calibration parameters of the virtual circle calibration component are the radius length of the virtual circle center or the distance between the centers of the two virtual circles; the calibration parameters of the virtual sphere calibration component are the radius length of the virtual sphere or the distance between the centers of the two virtual spheres. Step Two: Utilize N Each sampling point constructs multiple virtual calibration elements of a selected type; If the virtual calibration component is a virtual circular calibration component, then each virtual circular calibration component constructed from the sampling points is screened, and only virtual circular calibration components with a feature triangle having a maximum angle of 90° to 120° and a radius of 400mm to 500mm are retained; the feature triangle refers to the triangle formed by the three sampling points that make up the virtual circular calibration component; when calibrating using the center of the circle of the virtual circular calibration component combination, based on the virtual circular calibration component screening results, only virtual circular calibration component combinations with a center distance less than or equal to 100mm are retained; If the virtual calibration component is a virtual sphere calibration component, then after screening the virtual sphere calibration components constructed from the sampling points, only virtual sphere calibration components with a radius of 600mm to 700mm are retained; when calibrating using the center of a circle formed by a combination of virtual sphere calibration components, based on the screening results, only combinations of virtual sphere calibration components with a center distance less than or equal to 100mm are retained; when calibrating using the center of a circle formed by a combination of virtual sphere calibration components, based on the screening results, only combinations of virtual sphere calibration components with a center distance of 200mm to 300mm are retained. Step 3: Install a calibration tool on the machine tool spindle for positioning the probe of the articulated coordinate measuring machine; the machine tool spindle drives the calibration tool to move sequentially to all sampling point positions; whenever the calibration tool reaches a sampling point, the ball probe of the articulated coordinate measuring machine is used to measure the coordinates of the calibration tool in various different orientations; Step 4: Obtain the calibration parameter measurement values ​​of all virtual calibration components by measuring the coordinate values ​​of each sampling point obtained in Step 3; A target function is constructed based on the measured values ​​and actual values ​​of the calibration parameters; the target function is then used to calibrate the articulated coordinate measuring machine.

2. The calibration method for an articulated coordinate measuring machine based on a constructed virtual calibration component according to claim 1, characterized in that: The calibration tool adopts a three-ball conical socket; the three-ball conical socket includes a small shaft section and a large shaft section; the small shaft section is used to connect with the spindle of the machine tool; the end face of the large shaft section has a conical hole, and three spheres are evenly distributed circumferentially along the central axis of the conical hole.

3. The calibration method for an articulated coordinate measuring machine based on a constructed virtual calibration component according to claim 1, characterized in that: In step three, the number of samples taken at each sampling point is greater than or equal to 30.

4. The calibration method for an articulated coordinate measuring machine based on a constructed virtual calibration component according to claim 1, characterized in that: Number of sampling points N The value is 12.

5. The calibration method for an articulated coordinate measuring machine based on a constructed virtual calibration component according to claim 1, characterized in that: The magnification characteristic constant is greater than or equal to 80% of the measuring radius of the articulated coordinate measuring machine.

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