CN113532275B - Non-contact R-test sphere center coordinate calibration method adopting laser displacement sensor - Google Patents
Non-contact R-test sphere center coordinate calibration method adopting laser displacement sensor Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
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- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
Abstract
A non-contact R-test sphere center coordinate calibration method adopting a laser displacement sensor firstly sets measuring points and collects experimental data; calibrating the conversion matrix, and preliminarily solving the coordinates of the point sphere center; then based on Gaussian regression, establishing a primary GPR error model which takes displacement data of three laser displacement sensors as input variables and takes errors of each position point in the directions of an X axis, a Y axis and a Z axis as dependent variables, and further solving error residual quantity; removing a plurality of points with space errors larger than a critical value, selecting a Matern2/5 kernel function to carry out secondary Gaussian process regression, establishing a secondary GPR position error calculation model of the data of the laser displacement sensor and the position point coordinate errors, and further obtaining the coordinate errors of all the position points; then, calculating the sphere center coordinate of the non-contact R-test; finally, the calculated sphere center coordinates are used in the calculation process of the machine tool key errors of the RTCP parameters and the geometric error thermal errors of the five-axis machine tool; the invention realizes accurate and efficient calculation of the sphere center coordinate.
Description
Technical Field
The invention belongs to the technical field of numerical control machines, and particularly relates to a non-contact R-test sphere center coordinate calibration method adopting a laser displacement sensor.
Technical Field
With the increase of the requirement of the machine tool for machining precision, the measurement of main key errors of the machine tool, such as geometric errors, dynamic errors and thermal errors in the machining process, is increasingly important, and the compensation of the main key errors by using the measurement result is an effective method for improving the machining precision of parts. An R-test tester is developed by S.Weikert of the Federal rational in Switzerland in 2004, can measure the position of a tool nose in three directions in a workpiece coordinate system, and has rich and practical application. In recent years, along with research and development of scholars, the R-test tester has been well applied to machine tool error measurement such as geometric errors and thermal errors.
The rapid and accurate calculation of the coordinates of the sphere center is a key problem of R-test measurement, and the R-test is divided into a contact type R-test and a non-contact type R-test according to different sensor types of the R-test. Compared with a contact type R-test, the non-contact type R-test avoids abrasion caused by contact friction and can realize measurement under a high-speed rotation condition; meanwhile, the non-contact R-test has higher measurement precision and higher sampling frequency, can realize the acquisition of the high-frequency displacement signals of the cutter point, and has higher sensitivity. However, for the calculation of the sphere center of the non-contact R-test, due to the influence of the curvature of the spherical surface of the detection sphere, the data change of the sensor and the displacement of the detection sphere are in a nonlinear relationship, so that a coordinate conversion method of a calibration conversion matrix of the contact R-test cannot be used, and the calculation method is complex and not perfect.
In the prior art, chinese patent (CN 109032069 a) discloses a non-contact R-test measuring instrument using an eddy current displacement sensor, which calculates the center coordinates by using a differential evolution algorithm according to an induced voltage characteristic curve equation of the eddy current displacement sensor and a sensor induced plane equation, and solves the coordinate result of an accurate center point in a measurement coordinate system.
Hong and Ibaraki compare four laser displacement sensors with different measurement principles in an article Non-contact R-test with laser displacement sensors for error calibration of five-axis machine tools, and provide a coordinate calculation method based on target sphere curvature compensation.
In conclusion, the prior art has the defect of low calculation efficiency, and the calculation accuracy needs to be further improved. At present, a non-contact R-test sphere center coordinate calculation method which adopts a laser displacement sensor and is efficient, accurate, simple and convenient is urgently needed.
Disclosure of Invention
In order to solve the defects of the prior art, the invention aims to provide a non-contact R-test sphere center coordinate calibration method adopting a laser displacement sensor, so that the sphere center coordinate can be accurately and efficiently calculated.
In order to achieve the purpose, the invention adopts the technical scheme that:
a non-contact R-test sphere center coordinate calibration method adopting a laser displacement sensor comprises the following steps:
1) setting measuring points and acquiring experimental data: setting calibration measuring points at certain intervals, and acquiring displacement data of each laser displacement sensor to obtain displacement values of three non-contact R-test laser displacement sensors;
2) calibrating a conversion matrix, and preliminarily solving the coordinates of the point sphere center: solving a conversion matrix T by referring to a contact type R-test sphere center calculation method based on the R-test sphere center theoretical position obtained in the step 1) and the displacement values of the three laser displacement sensorsLaserPreliminarily solving the sphere center coordinates of the measuring points by using the conversion matrix, and reducing the error of the sphere center coordinates to a micron level as shown in a formula (1);
OW=PSen·TLaser (1)
in the formula OWFor the coordinate of the test point in the workpiece coordinate system, PSenReading by a laser displacement sensor;
3) based on Gaussian regression, establishing a primary GPR error model which takes displacement data of three laser displacement sensors as input variables and takes errors of each position point in the directions of an X axis, a Y axis and a Z axis as dependent variables, and further solving error residual quantity;
adopting a Gaussian process regression method in machine learning, selecting a kernel function as a rational quadratic kernel function, defining the Gaussian process as the formula (2),
f(x)~GP(m(x),k(x,x′)) (2)
where m (x) is a mean function and k (x, x') is a covariance function;
for the multiple nonlinear regression problem, the model is set as formula (3),
y=f(x)+ε (3)
in the formula: x is the input vector; y is the output value; ε -noise;
assuming that the noise satisfies a random distribution, ε -N (0, σ)n 2) Obtaining prior distribution of output value y as formula (4), observed value y*And a predicted value f*The joint prior distribution of (a), as in equation (5),
in the formula: k (X, X) -an nxn order symmetric positive definite covariance matrix, K (X, X)*) -test point x*Covariance matrix of order n X1, K (X) with input X*,x*) -test point x*(ii) its own covariance; i isn-an identity matrix of order n,. sigman-standard deviation of the random distribution of noise;
calculate the predicted value f*The posterior distribution of (2) is shown in the formula (6), and the predicted value f*The mean and variance of (A) are respectively expressed by the formula (7) and the formula (8),
in the formula:test point x*Corresponding predicted value f*The mean value of (a); cov (f)*)——f*The variance of (a);
based on the Gaussian Process Regression (GPR) method, data of three laser displacement sensors are used as input, error components of predicted point coordinates in the X direction, the Y direction and the Z direction are respectively used as output, and a GPR error model is generated;
selecting a rational quadratic kernel function as the kernel function of the Gaussian process regression, wherein the kernel function is in the form of formula (9),
4) removing a plurality of points with the spatial error larger than a critical value in the step 3), selecting a Matern2/5 kernel function to carry out secondary Gaussian process regression, establishing a secondary GPR position error calculation model of three laser displacement sensor data and position point coordinate errors, and further obtaining the coordinate error of each position point;
the quadratic GPR selects the Matern5/2 kernel function, which is shown in equation (10),
in the formula: ζ -smoothing coefficient; Gamma-Gamma function; h is a Bassel function, and when the smoothing coefficient is 0.5, the function is an exponential kernel; when the smoothing coefficient is infinite, the smoothing coefficient is a Gaussian kernel;
5) calculation of the center of sphere coordinates of the non-contact R-test: performing coordinate calculation by the transformation matrix in the step 2) to preliminarily obtain the center coordinates of each position point under the workpiece coordinate system, and compensating the preliminarily obtained center coordinates of each position point by calculating the center coordinates of each position point by the Gaussian regression model in the steps 3) and 4) twice to obtain accurate center coordinates of the non-contact R-test;
6) and (4) applying the sphere center coordinates calculated in the step 5) to the calculation process of the machine tool key errors of the RTCP parameters and the geometric error thermal errors of the five-axis machine tool.
When the dots are discarded in the step 4, the dots with large spatial errors are removed, the original data are kept as much as possible, and the critical value is selected to be 0.7 μm.
The invention has the following beneficial effects:
according to the non-contact R-test sphere center coordinate calibration method adopting the laser displacement sensor, the non-contact R-test sphere center coordinate can be accurately and efficiently calculated, and through verification, under a large sample (169416 x 3), compared with a neural network method, the method reduces the average space error from 0.303 mu m to 0.237 mu m, and improves the coordinate calculation precision by 21.8%; the calculation time is reduced from 6 minutes and 52 seconds to 4 seconds, and the calculation efficiency is improved by 99 percent; the method has important significance for development and use of the R-test and further detection and compensation of various errors in the whole machine tool industry.
Drawings
FIG. 1 is a calibration flow chart of the present invention.
FIG. 2 is a displacement diagram of three laser displacement sensors of a non-contact R-test according to an embodiment of the present invention.
FIG. 3 is a diagram of 729 coordinate prediction points and theoretical points in space according to an embodiment of the present invention.
FIG. 4 is a graph of error difference of 729 predicted points in the X-axis, Y-axis and Z-axis directions according to an embodiment of the present invention.
FIG. 5 is a diagram of residuals in three coordinate axes of GPR at a time in accordance with an embodiment of the present invention.
FIG. 6 is a diagram of a spatial error of a GPR predicted coordinate according to an embodiment of the present invention.
FIG. 7 is a diagram of residual errors in three coordinate axes of secondary GPR in accordance with an embodiment of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and examples.
Referring to fig. 1, a non-contact R-test sphere center coordinate calibration method using a laser displacement sensor includes the following steps:
1) setting measuring points and acquiring experimental data: the measuring range of the adopted non-contact R-test laser displacement sensor is +/-1 mm, and the offset of the sphere center of the detection sphere in the direction of a single coordinate axis cannot reach +/-1 mm in a workpiece coordinate system; in order to ensure the safety of a machine tool, a detection ball and a laser displacement sensor, the offset of each direction of a workpiece coordinate system is set within the range of +/-0.4 mm; the calibrated measuring points are arranged at intervals of 0.1mm, and the total is 93The measuring points (namely 729) collect the displacement data of each laser displacement sensor to obtain three laser displacement transmissions of the non-contact R-testDisplacement values of the sensor, as shown in FIG. 2;
2) calibrating a conversion matrix, and preliminarily solving the coordinates of the center of a point sphere: based on the theoretical position of the center of the sphere of the R-test obtained in the step 1) and the displacement values of the three laser displacement sensors, obtaining the predicted coordinate values of 729 measuring points by referring to a coordinate calibration method of a contact type R-test and through a conversion matrix; firstly, extracting 27 measurement points from 729 space points, and obtaining a coordinate conversion matrix of a non-contact R-test by using R-test displacement and theoretical coordinates of the corresponding points, wherein the coordinate conversion matrix is shown in a formula (1);
using the transformation matrix T obtained by solvingLaserAccording to the formula (2), calculating the prediction coordinates of 729 measured points obtained by measurement;
OW=PSen·TLaser (2)
in the formula OWFor the coordinates of the test point in the workpiece coordinate system, PSenReading by a laser displacement sensor;
the coordinates of 729 predicted points and the coordinates of the theoretical points are shown in FIG. 3, in which the coordinates of the predicted points and the theoretical points are approximately overlapped, but there is a certain degree of error; separating the position error of each coordinate point according to three directions of an X axis, a Y axis and a Z axis to obtain error components of each measuring point in three coordinate axis directions, as shown in FIG. 4;
3) based on Gaussian regression, establishing a primary GPR error model (error model for short) which takes displacement data of three laser displacement sensors as input variables and takes errors of each position point in the directions of an X axis, a Y axis and a Z axis as dependent variables, and further solving error residual quantity;
adopting a Gaussian process regression method in machine learning, selecting a kernel function as a rational quadratic kernel function, defining the Gaussian process as the formula (3),
f(x)~GP(m(x),k(x,x′)) (3)
where m (x) is a mean function and k (x, x') is a covariance function;
for the multiple nonlinear regression problem, the model is set as formula (4),
y=f(x)+ε (4)
in the formula: x is the input vector; y is the output value; ε -noise;
assuming that the noise satisfies a random distribution, ε -N (0, σ)n 2) Obtaining the prior distribution of the output value y as a formula (5), and the joint prior distribution of the observed value y and the predicted value f as a formula (6),
in the formula: k (X, X) -an nxn order symmetric positive definite covariance matrix, K (X, X)*) -test point x*Covariance matrix of order n X1, K (X) with input X*,x*) -test point x*(ii) its own covariance; i isn-an identity matrix of order n,. sigmanStandard deviation of the random distribution of noise, K (X, X)*) And K (x)*X) have the same meaning;
calculate f*Has a posterior distribution of the formula (7), f*The mean and variance of (A) are respectively expressed by the formula (8) and the formula (9),
in the formula:-test point x*Corresponding predicted value f*The mean value of (a); cov (f)*)——f*The variance of (a);
based on the Gaussian Process Regression (GPR) method, data of three laser displacement sensors are used as input, error components of predicted point coordinates in the X direction, the Y direction and the Z direction are respectively used as output, and a GPR error model is generated;
selecting a rational quadratic kernel function as a kernel function of Gaussian process regression, wherein the kernel function is in the form of a formula (10),
the displacement of three laser displacement sensors is input into a GPR error model to obtain error residuals, as shown in FIG. 5, the maximum value of the residuals exceeds 2 μm in the X direction, the residual of more than 10 points exceeds 1 μm, the maximum value of the residuals is close to 3 μm in the Y direction, the maximum value of the residuals exceeds 2 μm in the Z direction,
table 1 shows the information of the primary GPR error model of the rational quadratic kernel function, RMSE is the root mean square error, and the smaller the value, the closer the predicted result is to the true value; the R-square is the goodness of fit, the ideal value is 1, and the total time of one-time GPR error model training is about 15 s.
TABLE 1 GPR error model training information
4) Discarding a plurality of points with larger space errors after the first Gaussian regression, considering to remove the points with large errors and reserving original data as much as possible when discarding the points, selecting a critical value of 0.7 mu m, selecting a Matern5/2 kernel function to perform the second Gaussian regression, establishing a second GPR position error calculation model (coordinate error calculation model for short) between three displacement sensor data and position point coordinate errors, and further obtaining the position point coordinate errors;
the linear axis positioning error compensation is to realize accurate calibration of R-test, and needs to compensate positioning errors of three linear axes in advance, 729 points need to be positioned in the non-contact R-test calibration process, and the positioning accuracy of each linear axis is difficult to ensure, so that the positioning of the linear axes at some points is inaccurate, and the error of a few micrometers (the maximum is about 3 micrometers) exists; due to the influence of points with inaccurate positioning, the accuracy of a secondary GPR position error calculation model is reduced, so that a predicted point has a larger error with a theoretical point at some positions, and the accuracy and the effectiveness of a predicted result are difficult to ensure;
based on the reasons, on the basis of a primary GPR error model, the method uses GPR again for error calculation, and obtains the spatial errors corresponding to 729 measuring points and each point according to the primary GPR error model, as shown in FIG. 6;
selecting a point corresponding to data with an error value larger than a certain specific value (0.7 mu m in the text) as a point with a larger spatial error, and discarding the data of the point from the original laser displacement sensor displacement data to obtain a group of new laser displacement sensor displacement data, wherein compared with the original data, the data amount of the new data is reduced to 617 points, when the points are discarded, the points with the large error are removed, the original data is kept as much as possible, otherwise, the GPR accuracy of a secondary position error calculation model is influenced;
selecting a Matern5/2 kernel function to carry out secondary Gaussian regression, establishing a secondary GPR position error calculation model of three displacement sensor data and position point coordinate errors, selecting a Matern5/2 kernel function as the secondary GPR, wherein the kernel function is shown in a formula,
in the formula: ζ -smoothing coefficient; Gamma-Gamma function; h is a Bassel function, and when the smoothing coefficient is 0.5, the function is an exponential kernel; when the smoothing coefficient is infinite, the smoothing coefficient is a Gaussian kernel;
after regression (GPR) of a quadratic Gaussian process, residual errors in three coordinate axis directions are shown in FIG. 7, and the residual values are all smaller than 0.7 μm;
the information of the secondary GPR position error calculation model is shown in Table 2, compared with the information of the primary GPR, the RMSE values of the secondary GPR position error calculation model are all reduced, the R sides are all increased by 0.01, and the training time is basically kept unchanged and is about 5 s;
TABLE 2 quadratic GPR position error calculation model information
5) Calculating the sphere center coordinates of the non-contact R-test;
performing coordinate calculation by the transformation matrix in the step 2) to preliminarily obtain the center coordinates of each position point under the workpiece coordinate system, and compensating the preliminarily obtained center coordinates of each position point by calculating the center coordinates of each position point by the Gaussian regression model in the steps 3) and 4) twice to obtain accurate center coordinates of the non-contact R-test;
6) the sphere center coordinate calculated in the step 5) can be used in the calculation process of machine tool key errors such as RTCP parameters, geometric error thermal errors and the like of the five-axis machine tool, and has important significance for detection and compensation of various errors in the whole machine tool industry.
Claims (2)
1. A non-contact R-test sphere center coordinate calibration method adopting a laser displacement sensor is characterized by comprising the following steps:
1) setting measuring points and acquiring experimental data: setting calibration measuring points at certain intervals, and acquiring displacement data of each laser displacement sensor to obtain displacement values of three non-contact R-test laser displacement sensors;
2) calibrating a conversion matrix, and preliminarily solving the coordinates of the center of a point sphere: solving a conversion matrix T by referring to a contact type R-test sphere center calculation method based on the theoretical position of the R-test sphere center obtained in the step 1) and the displacement values of the three laser displacement sensorsLaserPreliminary solution using transformation matrixMeasuring the coordinates of the center of sphere of the point, and reducing the error of the coordinates of the center of sphere to micron level as shown in formula (1);
OW=PSen·TLaser (1)
in the formula OWFor the coordinate of the test point in the workpiece coordinate system, PSenReading by a laser displacement sensor;
3) based on Gaussian regression, establishing a primary GPR error model which takes displacement data of three laser displacement sensors as input variables and takes errors of each position point in the directions of an X axis, a Y axis and a Z axis as dependent variables, and further solving error residual quantity;
adopting a Gaussian process regression method in machine learning, selecting a kernel function as a rational quadratic kernel function, defining the Gaussian process as the formula (2),
f(x)~GP(m(x),k(x,x′)) (2)
where m (x) is a mean function and k (x, x') is a covariance function;
for the multiple nonlinear regression problem, a model is set as formula (3),
y=f(x)+ε (3)
in the formula: x is the input vector; y is the output value; ε -noise;
assuming that the noise satisfies a random distribution, ε -N (0, σ)n 2) Obtaining prior distribution of output value y as formula (4), observed value y*And a predicted value f*The joint prior distribution of (a), as in equation (5),
in the formula: k (X, X) -an nxn order symmetric positive definite covariance matrix, K (X, X)*) -test point x*Covariance matrix of order n X1, K (X) with input X*,x*) -test point x*(ii) its own covariance; i isn-an identity matrix of order n,. sigman-standard deviation of the random distribution of noise;
calculate the predicted value f*The posterior distribution of (2) is shown in the formula (6), and the predicted value f*The mean and variance of (A) are respectively expressed by the formula (7) and the formula (8),
in the formula:-test point x*Corresponding predicted value f*The mean value of (a); cov (f)*)——f*The variance of (a);
based on the Gaussian Process Regression (GPR) method, data of three laser displacement sensors are used as input, error components of predicted point coordinates in the X direction, the Y direction and the Z direction are respectively used as output, and a GPR error model is generated;
selecting a rational quadratic kernel function as the kernel function of the Gaussian process regression, wherein the kernel function is in the form of formula (9),
4) removing a plurality of points with the spatial error larger than a critical value in the step 3), selecting a Matern2/5 kernel function to carry out secondary Gaussian process regression, establishing a secondary GPR position error calculation model of three laser displacement sensor data and position point coordinate errors, and further obtaining the coordinate error of each position point;
the quadratic GPR selects the Matern5/2 kernel function, which is shown in equation (10),
in the formula: ζ -smoothing coefficient; Gamma-Gamma function; h is a Bassel function, and when the smoothing coefficient is 0.5, the function is an exponential kernel; when the smooth coefficient is infinite, the coefficient is a Gaussian kernel;
5) calculation of the center of sphere coordinates of the non-contact R-test: preliminarily obtaining the spherical center coordinates of each position point under the workpiece coordinate system by performing coordinate calculation through the conversion matrix in the step 2), and compensating the preliminarily obtained spherical center coordinates through the spherical center coordinate errors of each position point obtained by calculating the Gaussian regression model twice in the step 3) and the step 4) to obtain the accurate spherical center coordinates of the non-contact R-test;
6) and (3) using the sphere center coordinates calculated in the step 5) in the calculation process of the machine tool key errors of the RTCP parameters and the geometric error thermal errors of the five-axis machine tool.
2. The non-contact R-test sphere center coordinate calibration method adopting the laser displacement sensor as claimed in claim 1, wherein: when the dots are discarded in the step 4, the dots with large spatial errors are removed, the original data are kept as much as possible, and the critical value is selected to be 0.7 μm.
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