Standard device and calibration method for calibrating multi-source comprehensive error of on-machine measurement system

Abstract

The present invention provides a standard device and a calibration method for calibrating the multi-source comprehensive error of an in-machine measurement system, aiming to solve the technical problems of high cost, complex operation, and low accuracy in the existing comprehensive error calibration methods for in-machine measurement systems. The present invention designs a standard device with a special structure. By using high-precision measuring instruments such as a coordinate measuring machine with high precision, the calibration values of the positions of each standard ball on the standard device can be obtained accurately. By comparing the measurement results of the positions of each standard ball in the in-machine measurement with the calibration values, the multi-source comprehensive error affected by multiple machine tool geometries and probe errors can be directly obtained. Furthermore, the comprehensive error distribution in the measurement area of the in-machine measurement can be obtained. There is no need for a complex geometric error decoupling and accumulation process, nor multiple setups and adjustments, which improves the calibration efficiency and the calibration result is relatively accurate. Since there is no need to use professional precision instruments such as laser interferometers and ballbar testers, the cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of in-machine measurement, and particularly relates to a standard device and a calibration method for calibrating multi-source comprehensive errors in an in-machine measurement system. Background Art

[0002] In recent years, with the development of intelligent manufacturing technology, in order to change the dilemma of off-line measurement of workpieces during the machining process and improve the machining inspection efficiency, an in-machine measurement system and method based on a numerically controlled machine tool have emerged as the times require. The carrier of the in-machine measurement system is a numerically controlled machine tool. During the machining of parts, it is not necessary to remove the parts to be measured from the numerically controlled machine tool, and the geometric dimensions of the machining features of the workpiece can be measured on the numerically controlled machine tool, thus integrating the machining and measurement of the parts and improving the production efficiency of the parts.

[0003] Due to the non-uniform spatial errors of the numerically controlled machine tool, the comprehensive errors introduced into the in-machine measurement system will reduce the measurement accuracy of the in-machine measurement system. Therefore, the measurement accuracy of the in-machine measurement system is restricted by the accuracy of the numerically controlled machine tool, resulting in doubts about the credibility of the measurement results in its application.

[0004] In order to evaluate the measurement accuracy of the in-machine measurement system, a method for obtaining its comprehensive errors has emerged as the times require:

[0005] The comprehensive errors of the in-machine measurement system with a three-axis numerically controlled machine tool as the carrier include the errors introduced by the machine tool probe and the spatial errors of the machine tool itself. Among them, the spatial errors of the machine tool include 3 items of positioning errors, 6 items of straightness errors, 9 items of angular motion errors, and 3 items of perpendicularity errors, a total of 21 items of geometric errors.

[0006] An in-machine measurement error prediction method considering the positioning errors of a five-axis machine tool disclosed in the patent document with the application number 202211449731.5 calibrates 3 items of positioning errors using a laser interferometer. The in-machine measurement and correction method for the machining error of the face gear tooth surface considering geometric errors disclosed in the patent document with the application number 202311172268.9 calibrates 3 items of positioning errors using a laser interferometer, and uses the "9-line method" to calibrate the errors coupled by angular motion errors, perpendicularity errors, and straightness errors, and decouples and calculates them to obtain each single error. However, the disadvantages of these solutions are that the laser interferometer is expensive, requires multiple installations, adjustment of the mirror group and the refracting mirror group, takes a long time, and requires a complex geometric error decoupling and accumulation process.

[0007] A geometric / thermal error online measurement and compensation system for a numerically controlled machine tool is disclosed in the patent document with the application number 201910100940.0. The positioning errors of each axis of the numerically controlled machine tool are calibrated by arranging acceleration sensors in the machine tool. However, the disadvantage of this solution is that only the positioning errors of the numerically controlled machine tool are calibrated, and the straightness error and angular motion error in the spatial error are not calibrated, and it is impossible to accumulate into a relatively accurate comprehensive error. Summary of the Invention

[0008] The present invention provides a standard device and a calibration method for calibrating the multi-source comprehensive error of an in-machine measurement system, aiming to solve the technical problems of high cost, complex operation and low accuracy of the existing comprehensive error calibration methods for in-machine measurement systems.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] A standard device for calibrating the multi-source comprehensive error of an in-machine measurement system, characterized in that:

[0011] It includes a base, a support assembly, an upper plate, a standard ball set and a pressing plate;

[0012] The base is a flat plate for installing the standard device on the workbench surface of the numerically controlled machine tool;

[0013] The support assembly is vertically arranged on the base and includes an inner support rod and an outer support rod sleeved coaxially. The upper part of the inner support rod extends out of the outer support rod and is connected to the middle part of the lower end surface of the upper plate; the inner support rod is detachably connected to the outer support rod, or the inner support rod is detachably connected to the upper plate;

[0014] The upper plate is a flat plate, and the standard ball set is arranged on its upper end surface; one end of the upper plate is fixed on the base through the pressing plate, and the other end is suspended;

[0015] The standard ball set includes at least five standard balls, and the number and distribution of the standard balls are determined according to the required calibration accuracy; one of the standard balls is located at the geometric center of the upper plate as the central ball, and the remaining standard balls are evenly distributed around the central ball.

[0016] Further, a plurality of pin holes are arranged on the side wall of the outer support rod along its height direction, and the outer support rod and the inner support rod are connected by pins.

[0017] Further, the standard ball is installed on the upper plate through a connecting rod; a threaded hole is arranged at the bottom of the connecting plate, and a screw passes through the upper plate and is screwed into the threaded hole to fasten the connecting rod and the upper plate; a spring washer is arranged between the screw and the upper plate.

[0018] Further, the upper plate is a rectangular plate, and one standard ball is located at the intersection of the diagonals of the upper plate as the central ball, and the remaining standard balls are evenly distributed on each diagonal of the upper plate, and the number of standard balls on each diagonal is equal.

[0019] Further, two sets of weight-reducing holes that are symmetric about the geometric center are provided on the upper plate; each set of weight-reducing holes includes two through holes with a right-angled triangle cross-section that are axially symmetrically arranged, and one of the right-angled sides of these two through holes is arranged face to face.

[0020] Further, the diameter accuracy of the standard ball is 0.0005 mm, and the roundness is not less than 0.8 μm.

[0021] The present invention also provides a method for calibrating the multi-source comprehensive error of an in-machine measurement system by using the above-mentioned standard device, which is characterized in that it includes the following steps:

[0022] Step 1: Taking the center of the central ball on the standard device as the origin, taking the plane normal vector formed by the three plane normal theoretical measurement points on the upper surface of the upper plate of the standard device as the positive direction of the Z axis, taking the vector direction formed by the two roll angle theoretical measurement points on the side surface of the upper plate of the standard device as the positive direction of the X axis, and determining the positive direction of the Y axis according to the right-hand rule, establish a calibration coordinate system, and measure the center positions of the standard balls on the standard device in the calibration coordinate system;

[0023] Step 2: Taking the center of the central ball on the standard device as the origin, establish a workpiece coordinate system with the coordinate axis directions consistent with those of the numerical control machine tool, and obtain the pre-travel error of the in-machine probe at any calibration position in the workpiece coordinate system;

[0024] Step 3: By aligning the standard device in the numerical control machine tool, obtain the attitude relationship between the calibration coordinate system and the workpiece coordinate system, and then according to this attitude relationship and the center positions of the standard balls on the standard device in the calibration coordinate system, obtain the center positions of the standard balls on the standard device in the workpiece coordinate system;

[0025] Step 4: Calibration of the spatial position error of the in-machine measurement system

[0026] Define the two opposite ends on the upper plate of the standard device as end A and end B respectively. When end A is in contact with the base and end B is suspended, the standard device is in attitude one, and when end B is in contact with the base and end A is suspended, the standard device is in attitude two;

[0027] Step 4.1 When the standard device is in attitude one, calibrate the spatial position error of the in-machine measurement system

[0028] Install the standard device on the workbench of the CNC machine tool in posture one, call out the probe of the on-machine measurement system, and respectively detect each standard ball along the positive X, negative X, positive Y, negative Y, and negative Z directions in the workpiece coordinate system according to the center positions of the standard balls on the standard device in the workpiece coordinate system, measure the measurement results corresponding to each axis, and then obtain the measurement values of the center positions of each standard ball;

[0029] Subtract the measurement values of the center positions of each standard ball from the center positions of the standard balls on the standard device in the workpiece coordinate system obtained in step 3, and the obtained result is the spatial position error distribution one of the on-machine measurement system;

[0030] Step 4.2 Spatial position error calibration of the on-machine measurement system when the standard device is in posture two

[0031] Switch the standard device to posture two, and use the same method as in step 4.1 to obtain the spatial position error distribution two of the on-machine measurement system;

[0032] Step 4.3 Obtain the spatial position error set of the on-machine measurement system within a spatial region

[0033] Merge the spatial position error distribution one of the on-machine measurement system obtained in step 4.1 with the spatial position error distribution two of the on-machine measurement system obtained in step 4.2 to obtain the spatial position error set of the on-machine measurement system within a spatial region containing 2N - 1 calibration points;

[0034] Step 5: Obtain the spatial position error of the on-machine measurement system at any calibration position

[0035] According to the spatial position error set of the on-machine measurement system, use interpolation or estimation algorithms to fit the spatial position error of the on-machine measurement system at any calibration position;

[0036] Step 6: Calculate the multi-source comprehensive error of the on-machine measurement system

[0037] Accumulate the pre-travel error of the on-machine probe at any calibration position obtained in step 2 and the spatial position error of the on-machine measurement system obtained in step 5 to obtain the multi-source comprehensive error of the on-machine measurement system.

[0038] Furthermore, between steps 4 and 5, there is also a step:

[0039] A1: Move or raise the standard device on the workbench surface of the CNC machine tool so that the calibration spatial region of the standard device at this time partially overlaps with the previous calibration spatial region, and fix the standard device;

[0040] A2: Use the same method as in steps 3 - 4 to obtain the spatial position error set of the on-machine measurement system within the current calibration spatial region;

[0041] A3: Obtain the calibration difference between any two standard spheres with approximately the same positions within the overlapping region of the current calibration space region and the previous calibration space region, and subtract the said calibration difference from the set of spatial position errors of the in-machine measurement system within the current calibration space region to obtain the set of spatial position errors of the in-machine measurement system within the current calibration space region after registration;

[0042] A4: Using the same method as in steps A1 - A3, obtain the sets of spatial position errors within different calibration space regions after registration respectively until the entire measurement region of the in-machine measurement system is covered;

[0043] A5: Combine the sets of spatial errors within all spatial regions to obtain the set of spatial errors within the entire measurement region of the in-machine measurement system.

[0044] Further, the specific steps of step 3 are as follows:

[0045] Step 3.1 Coarse alignment

[0046] Obtain the pitch angle θ′ and yaw angle φ′ of the upper plate in the standard device in the workpiece coordinate system;

[0047] Rotate the workpiece coordinate system counterclockwise around its own X-axis by θ′, and then rotate it counterclockwise around its own Y-axis by φ′ to establish the alignment coordinate system WCS′;

[0048] Obtain the roll angle between the alignment coordinate system WCS′ and the workpiece coordinate system

[0049] Rotate the alignment coordinate system WCS′ counterclockwise around its own Z-axis by to obtain the updated alignment coordinate system WCS″;

[0050] Calculate the rotation matrix R′ along the pitch angle θ′, yaw angle φ′ and roll angle θ , R φ ′ and

[0051] Step 3.2 Fine alignment

[0052] Obtain the pitch angle θ″, yaw angle φ″ and roll angle between the alignment coordinate system WCS″ and WCS′

[0053] Calculate the rotation matrix R along the pitch angle θ″, yaw angle φ″ and roll angle θ ″, R φ ″,

[0054] Step 3.3 According to the rotation matrix R′ θ , R φ ′ and Rotation matrix R θ ″, R φ ″ and the positions of the centers of the standard balls on the standard device in the calibration coordinate system, calculate the coordinates of the centers of the standard balls on the standard device in the workpiece coordinate system WCS.

[0055] The present invention also provides a method for compensating the measurement error of an on-machine measurement system, which is characterized in that it includes the following steps:

[0056] Calibrate the multi-source comprehensive error of the on-machine measurement system by using the above method;

[0057] During the measurement of the on-machine measurement system, use the multi-source comprehensive error to compensate the measurement results of the on-machine measurement system.

[0058] Compared with the prior art, the beneficial effects of the present invention are:

[0059] 1. The present invention designs a standard device, with which the pre-travel error and spatial position error of the on-machine measurement system can be calibrated, providing a basis for obtaining the comprehensive error of the on-machine measurement system.

[0060] 2. The support component in the standard device of the present invention includes an inner support rod and an outer support rod. By opening a plurality of pin holes on the side wall of the outer support rod and connecting the inner support rod and the outer support rod with a cylindrical pin, on the one hand, it is convenient for the disassembly and assembly of the two to switch the attitude of the upper plate, and on the other hand, it is also convenient to adjust the installation height of the inner support rod in the outer support rod, thereby adjusting the height of the entire support component, and further adjusting the angle between the upper plate located above the inner support rod and the base, avoiding interference between the standard device and the components on the numerical control machine during use, and enabling the standard device to adapt to various types of numerical control machines.

[0061] 3. The calibration method of the present invention is simple to operate and has high accuracy. Only by using the standard device of the present invention and existing high-precision measuring instruments such as a coordinate measuring machine can the on-machine measurement system be calibrated, without using professional precision instruments such as a laser interferometer and a ball bar, and the cost is low.

[0062] 4. The present invention can obtain accurate calibration values of the positions of the standard balls on the standard device by using a high-precision coordinate measuring machine. By comparing the measurement results of the positions of the standard balls measured on the machine with the calibration values, the multi-source comprehensive error affected by multiple machine tool geometries and probe errors can be directly obtained. Furthermore, the comprehensive error distribution in the measurement area of the on-machine measurement can be obtained, without the need for a complex geometric error decoupling and accumulation process, nor multiple installations and adjustments, improving the calibration efficiency.

[0063] 5. The comprehensive error calibrated by the present invention includes the pre-travel error introduced by the probe and the spatial error of the measurement system. The spatial error of the measurement system includes 21 geometric errors, and the calibration result is relatively accurate. Description of the Drawings

[0064] Figure 1 It is a perspective view of the standard device of the present invention.

[0065] Figure 2 It is a front view of the standard device of the present invention.

[0066] Figure 3 It is a right view of the standard device of the present invention.

[0067] Figure 4 It is a left view of the standard device of the present invention.

[0068] Figure 5 It is a flowchart of the method for calibrating the multi-source comprehensive error of the in-machine measurement system of the present invention.

[0069] Figure 6 It is a schematic diagram of the calibration coordinate system established when aligning the standard device of the present invention.

[0070] Figure 7 It is a schematic diagram of calibrating the probe pre-travel error according to the detection direction in the calibration method of the present invention.

[0071] Figure 8 It is a schematic diagram of calibrating the comprehensive spatial error of the in-machine measurement system when the standard device of the present invention is in posture one.

[0072] Figure 9 It is a schematic diagram of calibrating the comprehensive spatial error of the in-machine measurement system when the standard device of the present invention is in posture two.

[0073] Figure 10 It is a schematic diagram of a single calibration area.

[0074] Figure 11 It is a splicing schematic diagram of two calibration areas.

[0075] Figure 12 It is the measurement process of the turbine casing process 500.

[0076] Figure 13 It is the measurement result of the position degree of each hole.

[0077] Description of the reference numerals:

[0078] 1 - Base; 2 - Outer support rod; 3 - Connector; 4 - Upper plate; 5 - Connecting rod; 6 - Inner support rod; 7 - Standard ball group; 8 - Pressure plate; 9, 11, 12 - Screws; 10 - Nut. Detailed Description of the Invention

[0079] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0080] Referring Figure 1-4 , the standard device provided by the present invention for calibrating the multi-source comprehensive error of the in-machine measurement system includes a base 1, a support assembly, a connecting member 3, an upper plate 4, a connecting rod 5, a standard ball set 7, and a pressing plate 8.

[0081] The base 1 is a flat plate, and its shape and size are not limited as long as it can provide an installation foundation for other components of the standard device and install the standard device on the workbench of the numerical control machine tool.

[0082] The support assembly includes an inner support rod 6 and an outer support rod 2 coaxially sleeved. The inner support rod 6 and the outer support rod 2 are detachably connected to facilitate the switching of two postures of the upper plate 4 in the standard device. Preferably, the inner support rod 6 and the outer support rod 2 are detachably connected by a cylindrical pin 13. The outer support rod 6 is vertically arranged on the upper end surface of the base 1 and is fixed by screws 11 or welded. An installation hole is provided axially in the outer support rod 6, and a plurality of pin holes are provided on the side wall along its height direction. The inner support rod 6 is located inside the outer support rod 2 and the two are coaxial. The upper part of the inner support rod 6 extends out of the outer support rod 2 and is connected to the connecting member 3; the inner support rod 6 can be fixed inside the outer support rod 2 by inserting the cylindrical pin 13 into the pin hole on the side wall of the outer support rod 2. The installation height of the inner support rod 6 inside the outer support rod 2 and the fixing position of the cylindrical pin 13 are adjustable, and thus the overall height of the support assembly can be adjusted.

[0083] The upper plate 4 is a flat plate and serves as the installation and bearing foundation for the standard ball set. The shape and size of the upper plate 7 are not limited as long as it can meet the installation requirements of the standard ball set. Preferably, the upper plate 4 is a rectangular flat plate. A connecting member 3 is provided in the middle of the lower end surface of the upper plate 4. The middle of the lower end surface of the upper plate 4 is connected to the upper part of the inner support rod 6 in the support assembly through the connecting member 3. One end of the upper plate 4 is fixed on the base 1 by the pressing plate 8 and fasteners (screws 9, nuts 10, and gaskets) and is in line contact with the base 1, and the other end of the upper plate 4 is suspended, so that it is inclined on the base 1. When the axial position of the inner support rod 6 inside the outer support rod 2 is adjusted to change the height of the support assembly, the included angle between the upper plate 4 and the base 1 will also change accordingly. This design enables the standard device to adapt to various types of numerical control machine tools and avoids interference during use. In order to reduce the weight of the standard device and reduce gravity deformation, the present invention also provides a weight reduction hole on the upper plate 4. Preferably, with the dual optimization goals of minimum weight and minimum gravity deformation, the weight reduction holes are designed in two groups, symmetric about the geometric center of the upper plate 4, and can also provide an installation space for the pressing plate 8 at the same time; each group of weight reduction holes includes two axially symmetrically arranged through holes with a right-angled triangle cross-section, and one of the right-angled sides of these two through holes is arranged face to face.

[0084] The standard ball set 7 is fixedly arranged on the upper end surface of the upper plate 4 through the connecting rod 5. The standard ball set 7 includes at least 5 standard balls, and one of the standard balls is arranged at the geometric center of the upper plate 4 as the central ball, which is used to establish a coordinate system and calibrate the pre-travel error, and the remaining standard balls are evenly distributed around the central ball. Preferably, the upper plate 4 is a rectangular plate, and one of the standard balls is arranged at the intersection of the diagonals of the upper plate 4 as the central ball, and the remaining standard balls are evenly distributed on each diagonal of the upper plate 4, and the number of standard balls on each diagonal is equal; such a design can better show the influence of 21 geometric errors. The number and distribution of the standard balls are determined according to the actual calibration point density. When the stroke of the CNC machine tool is large, in order to ensure the calibration accuracy, the number of standard balls can be set more to increase the density of the calibration points. In order to minimize the influence of the ambient temperature on the calibration result and improve the calibration accuracy, the standard balls should be made of materials with a small coefficient of thermal expansion, such as ceramic materials. In addition, the present invention does not make special requirements on whether the radii of the standard balls are the same, but it is necessary to ensure that the diameter accuracy of the standard balls is 0.0005 mm and the roundness is not less than 0.8 μm, so as to ensure that there is no accuracy loss in the process of transferring the calibration accuracy of high-precision measuring instruments such as coordinate measuring machines to the calibration accuracy of the in-machine measuring system, that is, to minimize the manufacturing error of the standard device.

[0085] The bottom of the connecting rod 5 is machined with a threaded hole. A spring washer is sleeved on the screw, and the screw is passed through the upper plate 4 and then screwed into the threaded hole at the bottom of the connecting rod 5 to fasten the connecting rod 5 and the upper plate 4 to reduce the connection looseness caused by the time-varying effect. Refer to Figure 5 , a method for calibrating the multi-source comprehensive error of the in-machine measuring system by using the standard device provided by the present invention includes the following steps:

[0086] Step 1: Measure the positions of the centers of the standard balls on the standard device

[0087] Step 1.1 Establish a calibration coordinate system

[0088] First, as Figure 6 shown, taking a corner of the upper surface of the upper plate 4, such as the lower right corner, as the origin, and the direction of the outer normal vector of the upper surface as the positive direction of the Z R axis, and the direction along the side to the right as the positive direction of the X R axis, a rough calibration coordinate system is established.

[0089] Then, in the roughly established calibration coordinate system, measure three theoretical measurement points p 1 ~p 3 of the plane normal vectors on the upper surface of the upper plate 4, and it is required that p 1 ~p 3 are not on the same straight line and the mutual distance is not less than 150 mm; from p 1 ~p 3 calculate the plane normal vector of the upper plate 4

[0090]

[0091] Next, measure the two theoretical rolling angle measurement points p on the side of the upper plate 4. 4 and p 5 , requiring p 4 and p 5 The vector composed Not consistent with the Z coordinate system of the rough calibration R The axes are parallel and the length is not less than 150mm.

[0092] Finally, the plane normal vector Set to the positive Z direction of the calibration coordinate system, with p 4 and p 5 The vector composed The direction is the positive direction of the X-axis of the calibration coordinate system. The center position of the central ball is set as the origin of the calibration coordinate system. The calibration coordinate system is established according to the right-hand rule of the Cartesian coordinate system.

[0093] Since it is difficult to ensure the flatness and verticality of the entire outer surface when processing the plane of the upper plate 4, the plane normal vectors measured at different positions are With vector are not completely consistent, so in order to avoid the existence of benchmark differences between multiple calibration processes, it is required to ensure p 1 ~p 5 The position is consistent.

[0094] Step 1.2 Obtain the relative position of the center of each standard ball relative to the center of the central standard ball after calibration

[0095] Place the assembled standard in a high-precision measuring instrument such as a three-coordinate measuring machine, and measure the position set C = {B i (x i ,y i ,z i )|i=1,2,…,N}, where N is the number of standard balls, B i (x i ,y i ,z i ) are the coordinates of the center of the i-th standard sphere.

[0096] Step 2: Calibrate the pre-travel error of the on-machine probe

[0097] Before calibrating the multi-source comprehensive error, it is necessary to eliminate the errors introduced by the probe of the in-machine measurement system, mainly including eccentricity error and pre-travel error. The eccentricity error is caused by the non-coincidence of the probe axis and the spindle axis of the CNC machine tool during the probe installation process. This error can be eliminated in advance by using existing mature methods with the assistance of a dial indicator by adjusting the adjusting screw of the probe. Therefore, this invention only considers the calibration of the pre-travel error.

[0098] Due to the internal structure of the probe, the actual contact time of the probe is inconsistent with the trigger time of the probe and the reception time of the receiver. This time lag causes a position error between the probe detection point recorded by the CNC system and the actual detection point. This error is called the pre-travel error. During the measurement process, after the probe contacts the workpiece, it continues to move and receives a continuously increasing contact force from the workpiece. When the contact force reaches the threshold, the probe is triggered. Due to the anisotropy of the internal structure of the probe, the threshold of the contact force when the probe triggers the workpiece in different directions is different. It is manifested that the pre-travel error detected in different directions is anisotropic.

[0099] Refer to Figure 7 , and the pre-travel error of the probe can be calibrated using the standard device of this invention. The specific method is as follows:

[0100] Step 2.1 Install the standard device on the workbench of the CNC machine tool so that the long side of the upper plate 4 is parallel to the X direction of the machine coordinate system, and adjust the support assembly to an appropriate height to ensure that there is no interference when the CNC machine tool measures each standard ball on the upper plate 4.

[0101] Step 2.2 Iteratively measure the center coordinates of the center ball at any standard ball on the standard device, and establish a workpiece coordinate system WCS at the center of the center ball. The directions of the coordinate axes of the workpiece coordinate system WCS are the same as those of the coordinate axes of the machine coordinate system.

[0102] Step 2.3 Calibrate the pre-travel error related to the detection direction in the workpiece coordinate system WCS, as Figure 4 shown. During the calibration process, since the pre-travel error is related to the actual detection direction, we use the detection direction of the probe at the detection point on the spherical surface of any standard ball on the standard device to represent the pre-travel error δ pre (i,j,k):

[0103]

[0104] In the formula, x m , y m , z m are the actual coordinate values of the surface points of the standard ball measured along the detection direction in the workpiece coordinate system WCS, R is the radius of the standard ball, and (i,j,k) is the unit normal vector of the detection direction in the workpiece coordinate system WCS.

[0105] Step 2.4 Without interference during the movement, calibrate the pre-travel errors in several detection directions to obtain the pre-travel error set Δ pre :

[0106] Δ pre ={δ pre (i,j,k)}.

[0107] Step 2.5 According to the pre-travel error set Δ pre , interpolation algorithm or estimation algorithm can be used to fit the pre-travel error of the probe at any calibrated position The distribution and quantity of the calibrated normal vectors are determined according to the fitting accuracy requirements.

[0108] Step Three: Align the standard device to establish the attitude relationship between the calibration coordinate system and the workpiece coordinate system WCS, so as to convert the center positions of the standard balls on the standard device in the calibration coordinate system into coordinates in the workpiece coordinate system

[0109] Before calibrating the spatial position error of the in-machine measurement system using the standard device of the present invention, it is necessary to clarify the pose of the standard device in the CNC machine tool after installation. The standard device has two poses during calibration: Pose one is as Figure 8 shown. In the workpiece coordinate system WCS, with the positive direction of the X-axis to the right, the left side of the upper plate 4 is in contact with the base 1, and the right side is suspended. Pose two is as Figure 9 shown, the right side of the upper plate 4 is in contact with the base 1, and the left side is suspended.

[0110] The specific process of aligning the standard device is as follows:

[0111] Step 3.1 Coarse alignment of the standard device

[0112] First, in the workpiece coordinate system WCS, use the probe of the in-machine measurement system to measure the coordinates p 1 ′~p 3 ′ of any three non-collinear points on the upper surface of the upper plate 4, and calculate the unit plane normal vector

[0113]

[0114] Then, calculate the pitch angle θ′ and yaw angle φ′ of the upper plate 4 in the standard device:

[0115]

[0116] φ′ = arcsin a′

[0117] Again, construct the alignment coordinate system WCS′. The alignment coordinate system WCS′ is established by rotating the workpiece coordinate system WCS counterclockwise around its own X-axis by θ′ and then counterclockwise around its own Y-axis by φ′;

[0118] Next, in the aligned coordinate system WCS′, use the probe of the on-machine measurement system to measure any two points p along the X-axis direction of the aligned coordinate system 4 ′(x 4 ,y 4 ), p 5 ′(x 5 ,y 5 ), and calculate the roll angle between the aligned coordinate system WCS′ and the workpiece coordinate system WCS

[0119]

[0120] Then, rotate the aligned coordinate system WCS′ counterclockwise around its own Z-axis by to obtain the updated aligned coordinate system WCS″.

[0121] Finally, calculate the rotation matrices R′ along the three attitude angles, namely the pitch angle θ′, the yaw angle φ′, and the roll angle θ , R φ ′, respectively as follows:

[0122]

[0123]

[0124]

[0125] Step 3.2 Fine alignment of the standard device

[0126] In the updated aligned coordinate system WCS″, use the probe of the on-machine measurement system to measure the same 5 theoretical measurement points p as in the calibration process of Step 1 1 ~p 5 , including three theoretical measurement points p of the plane normal vector 1 ~p 3 and two theoretical measurement points p of the roll angle 4 ~p 5 . Calculate the normal vector of the upper plane of the upper plate 4 in the aligned coordinate system WCS″ 1 ~p 3 from the measurement results of p and the pitch angle θ″ and yaw angle φ″ between the aligned coordinate system WCS″ and WCS′:

[0127]

[0128]

[0129] From p 4 , p 5Calculating the roll angle between the alignment coordinate system WCS″ and WCS′ based on the measurement results

[0130] φ″ = arcsina″

[0131] Along the three attitude angles, namely the pitch angle θ″, the yaw angle φ″ and the roll angle of the rotation matrix R θ ″, R φ ″, are respectively:

[0132]

[0133]

[0134]

[0135] The coordinates of the centers of the standard balls on the standard device in the workpiece coordinate system WCS are:

[0136]

[0137] where i is the number of the standard ball, taking 1, 2,..., N respectively.

[0138] Step 4: Calibrating the spatial position error of the on-machine measurement system

[0139] Step 4.1 Calibrating the spatial position error of the on-machine measurement system when the standard device is in attitude one

[0140] Install the standard device on the workbench of the CNC machine tool in attitude one, call out the probe of the on-machine measurement system, and according to the positions of the centers of the standard balls of the standard device in the workpiece coordinate system WCS Probe the standard balls of the standard device along the positive X direction, negative X direction, positive Y direction, negative Y direction, and negative Z direction respectively in the workpiece coordinate system WCS, and the measurement results of the corresponding axes are Then the measurement results of the centers of the standard balls can be expressed as:

[0141]

[0142] Furthermore, the spatial position error of the on-machine measurement system at the positions of the centers of the standard balls can be calculated as:

[0143]

[0144] Step 4.2 Calibrating the spatial position error of the on-machine measurement system when the standard device is in attitude two

[0145] Step 4.2.1 Pull out the cylindrical pin 13 between the inner support rod 6 and the outer support rod 2 in the standard device. After taking out the upper plate 4 and the inner support rod 6 as a whole and rotating them 180° around the axis of the inner support rod 6, reinstall them and insert the cylindrical pin 13 back for fixation to achieve Attitude 2.

[0146] Step 4.2.2 Install the standard device on the workbench of the CNC machine tool in the current attitude. Using the same method as in Step 4.1, the spatial position error of the in-machine measurement system at the center positions of each standard ball can be calibrated when the standard device is in the current Attitude 2.

[0147] Step 4.3 Obtain the set of spatial position errors of the in-machine measurement system within a spatial region

[0148] Merge the spatial position errors obtained in Step 4.1 and Step 4.2, and a spatial region Ω containing 2N - 1 calibration points can be obtained 1 (a cuboid region in this embodiment), where N represents the number of standard balls on the standard device, and the set of spatial position errors of the in-machine measurement system (as shown in the example Figure 10 )

[0149]

[0150] Step Five: By obtaining, registering, and merging the sets of spatial position errors of the in-machine measurement system within different spatial regions, the set of spatial position errors of the in-machine measurement system within the entire measurement space is obtained

[0151] Since the size of the upper plate in the standard device is limited, the calibration range of the standard device can only cover part of the measurement space of the in-machine measurement system with a small stroke. For the in-machine measurement system for measuring large-sized workpieces such as aircraft panels and engine casings, the calibration range of the standard device is difficult to cover the entire measurement space of the in-machine measurement system, and it is impossible to obtain the error distribution of the entire measurement space through one-time calibration in Step Four. Therefore, it is necessary to calibrate multiple spatial regions separately, obtain the sets of spatial position errors of the in-machine measurement system within multiple spatial regions, and unify their references and merge them into a single set of spatial position errors of the in-machine measurement system, as Figure 11 shown. Specifically as follows:

[0152] Step 5.1 Move or raise the standard device on the workbench of the CNC machine tool so that the calibration region of the standard device at this time partially overlaps with the calibrated region Ω 1 and fix the standard device.

[0153] Step 5.2 Using the same method as in Steps Three - Four above, obtain the set of spatial position errors of the in-machine measurement system within the second spatial region Ω 2 :

[0154]

[0155] Step 5.3 Obtain the spatial region Ω 1 and Ω 2 the calibration difference between any two standard spheres with approximately the same position in the overlapping region is δ Ω1,Ω2 , then the spatial position error set of the in-machine measurement system within the registered measurement space Ω 2 is:

[0156]

[0157] Step 5.4 Using the same method as in Steps 5.1 - 5.3, respectively obtain the spatial position error sets 3 of the registered measurement spaces Ω 4 ..., Ω n until the entire measurement area of the in-machine measurement system is covered.

[0158] Step 5.5 Combine the spatial position error sets of the in-machine measurement system in all spatial regions, that is, obtain the spatial error set of the in-machine measurement system within the entire measurement area Ω of the in-machine measurement system:

[0159]

[0160] Step Six: Obtain the spatial position error of the in-machine measurement system at any calibration position

[0161] According to the spatial position error set Δ spa of the in-machine measurement system, interpolation or estimation algorithms can be used to fit the spatial position error of the in-machine measurement system at any calibration position as If it is necessary to improve the fitting accuracy, the number of standard spheres in the standard device can be increased.

[0162] Step Seven: Calculate the multi-source comprehensive error of the in-machine measurement system

[0163] From the pre-travel error of the probe at any calibration position obtained in Step Two and the spatial position error of the in-machine measurement system at any calibration position obtained in Step Six, the multi-source comprehensive error of the in-machine measurement system at any calibration position and any measurement normal vector can be calculated as

[0164]

[0165] Using the method of the present invention to calibrate the multi-source comprehensive error of the in-machine measurement system at any calibration position and any measurement normal vector After that, during the process of using the in-machine measurement system to measure a workpiece, the measurement result p mea (x, y, z) of each measurement point can be compensated according to the theoretical position and detection direction of the measurement point, and the compensated measurement result p​com (x, y, z) is:

[0166]

[0167] After compensating the measurement results of the in - machine measurement system with the multi - source comprehensive error calibrated by the present invention, the accuracy of the measurement results of the in - machine measurement system can be effectively improved.

[0168] Exemplarily, in an in - machine measurement system composed of a DMU50 CNC machining center and an OMP400 probe, for the array of holes on the outer edge of the turbine casing process 500 as Figure 12 shown, compare the in - machine measurement results, the measurement results after compensating the in - machine measurement comprehensive error, and the measurement results of a coordinate measuring machine. The comparison results are as Figure 13 shown. It can be seen that after calibrating and compensating the multi - source comprehensive error obtained by the present invention, the in - machine measurement accuracy can be significantly improved.

[0169] The above - shown are only specific examples. Those skilled in the art can make adaptive changes based on the actual scenario requirements. For example, if the standard device of the present invention is only used on a certain fixed - type CNC machine tool, there is no need to design the support component as a height - adjustable type. At this time, the inner support rod and the outer support rod can be fixedly connected, and the inner support rod and the upper plate can be designed as a detachable connection, and still can realize the switching of the two postures of the standard device. For example, the inner support rod and the upper plate can also be connected by direct welding without being transferred through a connecting piece. Another example is that in some other embodiments, if the measurement space of the in - machine measurement system can be covered by using the above step four, then the above step five can be omitted.

Claims

1. A method for calibrating the multi-source comprehensive error of an on-machine measurement system using a standard device, characterized in that: The standard device comprises a base, a support assembly, an upper plate, a standard ball set and a pressure plate; The base is a flat plate, which is used to install the standard on the working table of the CNC machine tool; The support assembly is vertically arranged on the base, and includes an inner support rod and an outer support rod which are coaxially sleeved, wherein the upper portion of the inner support rod extends out of the outer support rod and is connected to the middle portion of the lower end surface of the upper plate; the inner support rod is detachably connected to the outer support rod, or the inner support rod is detachably connected to the upper plate; The upper plate is a flat plate, and the standard ball set is arranged on its upper end surface; one end of the upper plate is fixed to the base through the pressing plate, and the other end is suspended; The standard ball group includes at least five standard balls, and the number and distribution of the standard balls are determined according to the required calibration accuracy; one of the standard balls is located at the geometric center of the upper plate as the central ball, and the remaining standard balls are evenly distributed around the central ball; The method comprises the following steps: Step 1: Take the center of the central ball on the standard as the origin, the plane normal vector formed by the three plane normal vector theoretical measurement points on the upper surface of the upper plate of the standard as the positive direction of the Z axis, the vector direction formed by the two roll angle theoretical measurement points on the side of the upper plate of the standard as the positive direction of the X axis, and the positive direction of the Y axis is determined according to the right-hand rule. Establish a calibration coordinate system and measure the center position of each standard ball on the standard in the calibration coordinate system; Step 2: With the center of the central ball on the standard as the origin, establish a workpiece coordinate system whose coordinate axis directions are consistent with the coordinate axes of the CNC machine tool, and obtain the pre-travel error of the on-machine probe at any calibration position in the workpiece coordinate system; Step 3: by aligning the standard in the CNC machine tool, the posture relationship between the calibration coordinate system and the workpiece coordinate system is obtained, and then the center position of each standard ball on the standard in the workpiece coordinate system is obtained according to the posture relationship and the center position of each standard ball on the standard in the calibration coordinate system; Step 4: Calibration of spatial position error of the on-machine measurement system It is defined that the two opposite ends on the upper plate of the standard are end A and end B respectively, when end A contacts the base and end B is suspended in the air, the standard is in posture one, and when end B contacts the base and end A is suspended in the air, the standard is in posture two; Step 4.1 When the standard is in attitude 1, calibrate the spatial position error of the on-machine measurement system The standard device is installed on the workbench of the CNC machine tool in attitude 1, and the probe of the on-machine measurement system is called out. According to the center position of each standard ball on the standard device in the workpiece coordinate system, each standard ball is detected along the positive X, negative X, positive Y, negative Y and negative Z directions in the workpiece coordinate system, and the corresponding measurement results of each axis are measured, and then the measurement value of the center position of each standard ball is obtained; The center position of each standard ball on the standard device in the workpiece coordinate system obtained in step 3 is subtracted from the measured value of the center position of each standard ball, and the result is the spatial position error distribution 1 of the on-machine measurement system; Step 4.2: Calibrate the spatial position error of the on-board measurement system when the standard is in attitude 2 Switch the standard device to posture 2, and use the same method as step 4.1 to obtain the spatial position error distribution 2 of the on-machine measurement system; Step 4.3 Obtain the spatial position error set of the on-machine measurement system in a spatial region Combine the spatial position error distribution 1 of the on-machine measurement system obtained in step 4.1 with the spatial position error distribution 2 of the on-machine measurement system obtained in step 4.2 to obtain the spatial position error set of the on-machine measurement system in the spatial region containing 2N-1 calibration points; Step 5: Obtain the spatial position error of the on-machine measurement system at any calibration position According to the spatial position error set of the on-machine measurement system, an interpolation or estimation algorithm is used to fit the spatial position error of the on-machine measurement system at any calibration position; Step 6: Calculate the multi-source comprehensive error of the on-machine measurement system The pre-travel error of the on-machine probe at any calibration position obtained in step 2 and the spatial position error of the on-machine measurement system obtained in step 5 are accumulated to obtain the multi-source comprehensive error of the on-machine measurement system.

2. The method according to claim 1, characterized in that: Between steps 4 and 5, there are also steps: A1: Move or raise the standard on the work surface of the CNC machine tool so that the calibration space area of ​​the standard now partially overlaps with the previous calibration space area, and fix the standard; A2: Using the same method as steps 3-4, obtain the spatial position error set of the on-machine measurement system in the current calibration space area; A3: Obtain the calibration difference of any two standard balls with roughly the same position in the overlapping area of ​​the current calibration space area and the previous calibration space area, and subtract the calibration difference from the spatial position error set of the on-machine measurement system in the current calibration space area to obtain the spatial position error set of the on-machine measurement system in the current calibration space area after registration; A4: Using the same method as steps A1-A3, obtain the spatial position error sets in different calibration space areas after registration, until the entire measurement area of ​​the on-machine measurement system is covered; A5: Combine the spatial error sets in all spatial regions to obtain the spatial error set in the entire measurement area of ​​the on-machine measurement system.

3. The method according to claim 2, characterized in that: Step 3 is as follows: Step 3.1 Rough alignment Obtain the pitch angle θ′ and the yaw angle φ′ of the upper plate in the standard device in the workpiece coordinate system; Rotate the workpiece coordinate system counterclockwise around its own X-axis by θ′, and then rotate it counterclockwise around its own Y-axis by φ′ to establish the alignment coordinate system WCS′; Get the roll angle between the alignment coordinate system WCS′ and the workpiece coordinate system Rotate the positive coordinate system WCS′ counterclockwise around its own Z axis Get the updated alignment coordinate system WCS″; Calculate the pitch angle θ′, yaw angle φ′ and roll angle The rotation matrix R θ ′,R φ 'and Step 3.2 Precision Alignment Get the pitch angle θ″, yaw angle φ″ and roll angle between the alignment coordinate systems WCS″ and WCS′ Calculate the pitch angle θ″, yaw angle φ″ and roll angle The rotation matrix R θ ″,R φ ″, Step 3.3 According to the rotation matrix R θ ′,R φ 'and Rotation matrix R θ ″,R φ ″, As well as the position of the center of each standard ball on the standard device in the calibration coordinate system, calculate the coordinates of the center of each standard ball on the standard device in the workpiece coordinate system WCS.

4. The method according to any one of claims 1 to 3, characterized in that: The side wall of the outer support rod is provided with a plurality of pin holes distributed along the height direction thereof, and the outer support rod is connected with the inner support rod through pins.

5. The method according to claim 4, characterized in that: The standard ball is mounted on the upper plate via a connecting rod; a threaded hole is provided at the bottom of the connecting plate, and a screw is passed through the upper plate and screwed into the threaded hole to fasten the connecting rod and the upper plate; a spring gasket is provided between the screw and the upper plate.

6. The method according to claim 5, characterized in that: The upper plate is a rectangular plate, wherein one standard ball is located at the intersection of the diagonals of the upper plate as a central ball, and the remaining standard balls are evenly distributed on the diagonals of the upper plate, and the number of standard balls on each diagonal is equal.

7. The method according to claim 6, characterized in that: The upper plate is provided with two groups of weight-reducing holes symmetrical about its geometric center; each group of weight-reducing holes includes two through holes with right-angled triangular cross-sections arranged axially symmetrically, and one of the right-angled sides of the two through holes is arranged face to face.

8. The method according to claim 7, characterized in that: The diameter accuracy of the standard ball is 0.0005mm and the roundness is not less than 0.8μm.

9. A method for compensating measurement errors of an on-machine measurement system, characterized in that: The following steps are involved: Calibrate the multi-source comprehensive error of the on-machine measurement system using the method described in any one of claims 1 to 8; During the measurement of the on-machine measurement system, the multi-source comprehensive error is used to compensate the measurement result of the on-machine measurement system.

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