1. A position-independent geometric error modeling method of an AC double-turntable five-axis machine tool is characterized by comprising the following steps:

(1) establishing a general forward nominal kinematics model of the five-axis machine tool based on a local exponential product formula;

(2) applying the universal forward nominal kinematics model obtained in the step (1) to forward nominal kinematics modeling of the AC double-turntable five-axis machine tool;

(3) obtaining a full-attitude error model by a positive nominal kinematics model of the AC double-turntable five-axis machine tool, and simplifying the full-attitude error model;

(4) and obtaining a position-independent geometric error model of the AC double-turntable five-axis machine tool based on distance error information according to the distance error measured by the ball rod instrument.

2. The method for modeling the position-independent geometric error of the AC double-turntable five-axis machine tool according to claim 1, wherein in the step (1), the specific steps for establishing the universal forward nominal kinematics model of the five-axis machine tool are as follows:

(1-1) introducing initial pose transformation quantums of two adjacent coordinate systems, and establishing a kinematic model of the series machine tool based on a local exponential product formula;

(1-2) taking a coordinate system of the five-axis machine tool as a base coordinate system, and obtaining a homogeneous coordinate transformation matrix of the workpiece coordinate system of the five-axis machine tool relative to the base coordinate system according to the number of joints of a workpiece branched chain and a main shaft cutter branched chain on a workbench;

(1-3) calculating a conversion matrix of a tool coordinate system of the five-axis machine tool relative to a base coordinate system;

(1-4) calculating homogeneous coordinate transformation of the base coordinate system relative to the workpiece coordinate system;

and (1-5) establishing a positive nominal kinematic model of a tool coordinate system of the five-axis machine tool relative to a workpiece coordinate system.

3. The method for modeling the position-independent geometric error of the AC dual-turret five-axis machine tool according to claim 1, wherein in the step (3), the full pose error model is calculated from the forward nominal kinematics model of the AC dual-turret five-axis machine tool, and the full pose error model is simplified, and the method comprises the following specific steps:

(3-1) obtaining a full pose error model by differentiating the initial pose vector, the joint rotation and the joint displacement;

and (3-2) based on the assumption that kinematic errors are concentrated in an initial pose matrix of a local coordinate system, joint displacement and joint rotation are kept as nominal values in the calibration process, and an error model of full pose information is simplified.

4. The modeling method for the position-independent geometric error of the AC double-turntable five-axis machine tool according to claim 1, wherein in the step (4), the position-independent geometric error model of the AC double-turntable five-axis machine tool based on the distance error information is obtained according to the distance error measured by the ball bar machine, and the method comprises the following specific steps:

(4-1) adopting a linear equation to express a full attitude error model of the AC double-turntable five-axis machine tool;

(4-2) establishing a conversion relation between the position coordinates of the machine tool point relative to the workpiece coordinate system and the tool coordinate system;

(4-3) projecting the position change of the three-dimensional coordinate of the tool point of the machine tool to the axial direction of the ball bar instrument, and keeping the position change consistent with the change of the reading of the ball bar instrument;

and (4-4) calculating a Jacobian matrix based on the change of the measured distance of the ball arm instrument and a position-independent geometric error parameter needing to be identified to obtain a position-independent geometric error model of the AC double-turntable five-axis machine tool based on the distance error information.

5. A position-independent geometric error identification method for an AC double-turret five-axis machine tool, characterized in that the position-independent geometric error identification method is based on the position-independent geometric error modeling method for the AC double-turret five-axis machine tool according to any one of claims 1 to 4, and comprises the following steps:

(I) according to the track points measured by the ball arm instrument, a position-independent geometric error model of the AC double-turntable five-axis machine tool is expanded;

(II) identifying position-independent geometric error parameters in the error model by adopting an iterative least square method with damping factors;

and (III) updating an initial pose transformation matrix of the five-axis machine tool by using the position-independent geometric error parameters, repeating iteration until the modulus of the error vector is less than a set value, and identifying to obtain the position-independent geometric error of the AC double-turntable five-axis machine tool.

6. The method for identifying the position-independent geometric error of the AC double-turntable five-axis machine tool according to claim 5, wherein in the step (I), the specific steps of expanding the position-independent geometric error model of the AC double-turntable five-axis machine tool according to the track points measured by the ball rod instrument are as follows:

(I-I) identifying position-independent geometric error parameters of the AC double-turntable five-axis machine tool, and determining the number of track points measured by the ball bar instrument;

(I-II) expanding a position-independent geometric error model of the AC double-turntable five-axis machine tool based on the distance error according to the number of the track points measured by the ball rod instrument.

7. The method for identifying the position-independent geometric error of the AC double-turntable five-axis machine tool according to claim 5, wherein in the step (II), the position-independent geometric error parameters in the error model are identified by using an iterative least square method with a damping factor.

8. The method for identifying the position-independent geometric errors of the AC double-turntable five-axis machine tool according to claim 5, wherein the method comprises the following specific steps of updating an initial pose transformation matrix of the five-axis machine tool by using the position-independent geometric error parameters, and identifying to obtain the position-independent geometric errors of the AC double-turntable five-axis machine tool:

(III-I) updating the initial pose transformation matrix using the identified position independent geometric error parameters;

(III-II) repeating the iteration of step (III-I) to modulo y of the error vector_bAnd if the | | is smaller than the set micro value epsilon, identifying to obtain the position-independent geometric error of the AC double-turntable five-axis machine tool.

Background

Compared with a three-axis machine tool, the five-axis machine tool is additionally provided with two rotating shafts, so that a cutter of the machine tool can machine the curved surface of a complex part in any posture, and the machining efficiency of the part is remarkably improved. However, under the influence of geometric errors, the machining precision of the five-axis machine tool is greatly reduced, which limits the application of the five-axis machine tool in high-end manufacturing industry to a certain extent.

The calibration technology is generally recognized as a method for effectively improving the machining precision of a machine tool. The four steps of calibration are modeling, measuring, identifying and compensating respectively, wherein error modeling and identifying are the premise of error compensation.

Generally speaking, the geometric errors of the machine tool are divided into Position Independent Geometric Errors (PIGEs) and Position Dependent Geometric Errors (PDGEs). Studies have demonstrated that pdgfs are relatively small compared to PIGEs.

At present, the error modeling and identification method for PIGEs has the following problems: the traditional geometric error modeling method has the theory of a multi-body system, a Homogeneous Transformation Matrix (HTM), a D-H model and the like, the former two methods have the problems that the parameterized modeling is not easy to realize and the error identification process is very complicated, and the D-H model has the singular problem when two motion axes are close to parallel and has poor universality. In the error modeling method based on the global exponential product formula proposed in recent years, the orthogonalization and normalization of each iteration bring calculation errors, and the solving efficiency is reduced. On the other hand, the error identification process mostly uses the gauss-newton method, but the robustness of the method is poor, and the problem that the error can not be iteratively converged under the condition of large noise data may occur.

The specification with the publication number of CN112518422A discloses a five-axis AC swing head gantry machine tool geometric error modeling and separating method, which comprises the steps of firstly establishing a kinematic model of a machine tool from a tool coordinate system to a workpiece coordinate system, then respectively establishing a tool nose point spatial error model only considering a linear axis, only considering a rotating axis and simultaneously considering the linear axis and the rotating axis, then designing a linear axis spatial measurement track, measuring an actual spatial coordinate of a tool nose point under the measurement track, and further realizing the separation calculation of 21 items of geometric errors of the linear axis; finally, A, C-axis independent measuring tracks are designed, the rod length variation of the full stroke in the axial, radial and tangential modes is measured, the correlation between the sensitive direction of the 8-item PIGE errors and the rod length variation is established, and the separation of the 8-item PIGE errors is realized; and then establishing the correlation between the 12 PDGE errors and the variation of the club length of the club instrument, and realizing the separation of the 12 PDGE errors.

The specification with the publication number of CN112558547A discloses a method for quickly optimizing geometric error compensation data of a translational shaft of a five-axis numerical control machine tool, which comprises the following steps: 1) establishing a space positioning error model; 2) establishing an error database; 3) constructing a spatial error compensation table; 4) establishing a compensation data optimization model to form three face diagonal lines and a body diagonal line optimization model in a translation shaft linkage mode; 5) finishing iterative optimization and selection of correction coefficients; 6) completing the compensation of the geometric error of the five-axis numerical control machine tool in the step (5); 7) and (4) iterating the error correction data to a geometric error database, setting linkage track detection, presetting a positioning error threshold, and circulating the steps 2 to 6 to realize a machine tool precision guarantee mode of periodic detection, optimization and compensation.

Disclosure of Invention

The invention aims to provide a position-independent geometric error modeling method of an AC double-turntable five-axis machine tool.

A position-independent geometric error modeling method of an AC double-turntable five-axis machine tool comprises the following steps:

(1) establishing a general forward nominal kinematics model of the five-axis machine tool based on a local exponential product formula;

(2) applying the universal forward nominal kinematics model obtained in the step (1) to forward nominal kinematics modeling of the AC double-turntable five-axis machine tool;

(3) calculating a full pose error model by a forward nominal kinematics model of the AC double-turntable five-axis machine tool, and simplifying the full pose error model;

(4) and obtaining a position-independent geometric error model of the AC double-turntable five-axis machine tool based on distance error information according to the distance error measured by the ball rod instrument.

And determining the initial pose of each local coordinate system. Allowing a local coordinate system to be defined at an arbitrary position of the corresponding link to simplify the initial pose transformation matrix thereof to the utmost. When the five-axis machine tool is in a zero position, firstly defining all local coordinate systems to be in the same direction, then defining a tool nose point as an original point of a machine tool coordinate system, simultaneously setting a workpiece coordinate system to coincide with a tool coordinate system, setting coordinate systems of an X axis, a Y axis and a Z axis to coincide with the machine tool coordinate system when the machine tool coordinate system is in the zero position, measuring relative positions of an A axis coordinate system, a C axis coordinate system and the machine tool coordinate system through a micrometer, and then expressing an initial pose change matrix as a rotation quantity form.

In the step (1), the specific steps of establishing the general forward nominal kinematics model of the five-axis machine tool are as follows:

(1-1) the tandem machine tool is composed of n +1 connecting rods, and introduces the initial pose transformation rotation of two adjacent coordinate systemsT_i-1，i(0) Expressed in the form of an exponential map asKinematic model T of series machine tool based on local exponential product formula_0，n+1Comprises the following steps:

wherein the content of the first and second substances,the initial pose transformation vector of two adjacent coordinate systems;andrespectively representing the joint rotation and the joint displacement at the ith joint in the kinematic model of the serial machine tool; xi₁、ξ₂、…、ξ_nRespectively, the 1 st, 2 nd, … th joint rotation amount n, theta₁、θ₂、…θ_nRespectively, 1 st, 2 nd, … th joint displacement.

(1-2) taking a coordinate system of the five-axis machine tool as a basic coordinate system, wherein m joints are arranged in a workpiece branched chain of a workbench, and n joints are arranged in a main shaft cutter branched chain, and obtaining a homogeneous coordinate transformation matrix T of the workpiece coordinate system of the five-axis machine tool relative to the basic coordinate system_b，wComprises the following steps:

wherein: m + n is 5, and m + n is 5,(i ═ 1, 2, …, m +1) denotes the initial pose transformation of two adjacent coordinate systems in the tandem machine on one side of the workpiece coordinate system,andrespectively representing the joint rotation amount and the ith joint displacement of the ith joint positioned at the end of the workpiece link in the workpiece coordinate system;

(1-2) taking a coordinate system of the five-axis machine tool as a basic coordinate system, wherein m joints are arranged in a workpiece branched chain of a workbench, and n joints are arranged in a main shaft cutter branched chain, and obtaining a homogeneous coordinate transformation matrix T of the workpiece coordinate system of the five-axis machine tool relative to the basic coordinate system_b，wComprises the following steps:

m+n＝5，

wherein the content of the first and second substances,(j ═ 1, 2, …, m +1) denotes the initial pose transformation of two adjacent coordinate systems in the tandem machine on one side of the workpiece coordinate system,andthe joint rotation and joint displacement at the j (j ═ 1, 2, …, m) th joint at the end of the workpiece chain;

(1-3) conversion matrix T of tool coordinate system of five-axis machine tool with respect to base coordinate system_b，tComprises the following steps:

wherein the content of the first and second substances,andrespectively the ith joint rotation and the ith joint displacement of the link end of the workpiece in the tool coordinate system;

(1-4) homogeneous coordinate transformation T of the base coordinate system with respect to the workpiece coordinate system_w，bComprises the following steps:

(1-5) forward nominal kinematic model T of tool coordinate system of five-axis machine tool with respect to workpiece coordinate system_w，tComprises the following steps:

in the step (2), the universal forward name obtained in the step (1) is usedThe kinematics model is applied to forward nominal kinematics modeling of the AC double-turntable five-axis machine tool to obtain a forward nominal kinematics model T of the AC double-turntable five-axis machine tool_w，tComprises the following steps:

wherein the content of the first and second substances,respectively representing nominal initial pose moments, ξ, between the local coordinate systems_a、ξ_c、ξ_x、ξ_y、ξ_zNominal joint rotation, theta, for the axis of rotation A, C and the axis of motion X, Y, Z, respectively_c、θ_a、θ_y、θ_x、θ_zIs a drive command for five motors.

In the step (3), a full pose error model is calculated by a forward nominal kinematics model of the AC double-turntable five-axis machine tool, and the full pose error model is simplified, and the method specifically comprises the following steps:

(3-1) forward kinematics model T_w，tAbout the initial poseThe function of the joint rotation xi and the joint displacement theta is expressed in a mathematical way, namely T_w，tF (p, xi, theta), and differentiating the initial pose vector p, the joint rotation xi and the joint displacement theta to obtain an all-pose error model:

wherein, δ T_w，t·T_w，t ^-1Representing the tool pose error of the five-axis machine tool;

(3-2) based on the assumption that kinematic errors are concentrated in initial pose matrixes among local coordinate systems, joint displacement and joint rotation amount are kept as nominal values in the calibration process, an error model of full pose information is simplified into,

the kinematic errors are all concentrated in the initial pose matrix between the Local coordinate systems, and the assumption that the joint displacement and the joint rotation amount are kept at nominal values during the calibration process is specifically disclosed in Local POE model for robot kinematic calibration [ J ]. Mechanism and Machine Theory, 2001, 36 (11-12): 1215-1239.

In the step (4), a position-independent geometric error model of the AC double-turntable five-axis machine tool based on distance error information is obtained according to the distance error measured by the ball rod instrument, and the method specifically comprises the following steps:

(4-1) the error model of the all-attitude information of the five-axis machine tool is expressed by a linear equation as follows:

y＝J·x

y＝(δT_w，t·T_w，t ^-1)V＝log(T_wt ^aT_wt ^-1)^∨∈R^6×1，

wherein, the V-shaped is a vector which transforms the matrix mapping into a 6-dimensional vector,representing the error parameter to be identified, T_w，t ^aThe pose matrix of the end of the tool is actually measured, y is a pose error vector of the tool, J is a Jacobian matrix, and x is a position-independent geometric error parameter vector;

(4-2)P_wand P_tRespectively, the tool point of the machine tool is seated relative to the workpieceThe conversion relationship between the position coordinates of the coordinate system and the tool coordinate system is as follows:

wherein, P_wAnd P_tRespectively representing the position coordinates of the tool point of the machine tool relative to a workpiece coordinate system and a tool coordinate system;

(4-3) projecting the position change of the three-dimensional coordinate of the tool point of the AC double-turntable five-axis machine tool to the axial direction of the ball bar instrument, and keeping consistent with the change of the reading of the ball bar instrument:

y_b＝[O_bi O_bj O_bk]·δP_w

wherein, y_bAs a change in the reading of the ball bar linear sensor, [ O ]_bi O_bj O_bk]Is a unit direction vector of the cue instrument in a nominal kinematic model;

(4-4) calculating Jacobian matrix J based on changes in the measured distances of the cue meters_bAnd obtaining a position-independent geometric error model of the AC double-turntable five-axis machine tool based on the distance error information according to the position-independent geometric error parameter x to be identified:

y_b＝J_b·x

wherein the content of the first and second substances,

the invention aims to further provide an error identification method for the position-independent geometric error modeling based on the AC double-turntable five-axis machine tool, and the position-independent geometric error parameters are identified by adopting an iterative least square method with damping factors, so that the robustness of the error identification process is improved.

A position-independent geometric error identification method of an AC double-turntable five-axis machine tool is based on position-independent geometric error modeling of the AC double-turntable five-axis machine tool and comprises the following steps:

(I) according to the track points measured by the ball arm instrument, a position-independent geometric error model of the AC double-turntable five-axis machine tool is expanded;

(II) identifying position-independent geometric error parameters in the error model by adopting an iterative least square method with damping factors;

and (III) updating an initial pose transformation matrix of the five-axis machine tool by using the position-independent geometric error parameters, repeating iteration until the modulus of the error vector is less than a set value, and identifying to obtain the position-independent geometric error of the AC double-turntable five-axis machine tool.

The purpose of kinematic error identification is to find suitable error parameters to minimize the least square value of the error model established based on the distance information measured by the cue stick instrument. Considering that the identification process of the error parameters is weak and nonlinear, under the general condition, a better identification effect can be obtained by adopting a Gaussian-Newton method. However, if the noise of the actual measurement error data is too large, which may lead to the situation that the error identification process is not converged, an iterative least square method with a damping factor is proposed to identify the position-independent geometric error parameter x in the error model.

In the step (I), according to the track points measured by the ball bar instrument, the specific steps of expanding the linearization equation based on the distance error are as follows:

(I-I) for the AC double-turntable five-axis machine tool, the kinematic open chain from the machine tool base coordinate system to the workpiece coordinate system has (m +1) connecting rods, the kinematic open chain from the machine tool base coordinate system to the cutter coordinate system has (n +1) connecting rods, and in order to complete the identification of the position-independent geometric error parameter x of the five-axis machine tool, the number k of track points measured by the ball rod instrument is as follows:

k＞6(m+1)+6(n+1)；

(I-II) expanding a position-independent geometric error model based on distance errors according to the number of the track points measured by the cue instrument as follows:

wherein the content of the first and second substances,

distance error data y for k sets of club instruments_bThe vector of the composition is then calculated,is a matrix formed by k sets of Jacobian matrices Jb.

In the step (II), an iterative least square method with a damping factor is adopted to identify a position-independent geometric error parameter x in an error model:

where μ is the damping factor.

Preferably, in an ideal case or a small noise case, the damping factor μ is set to 0 to improve the error identification accuracy, and in a large noise data, a smaller value (0.0001-1) can be set to improve the robustness of the algorithm.

(III) updating an initial pose transformation matrix of the five-axis machine tool by using the position-independent geometric error parameters, and identifying to obtain the position-independent geometric error of the AC double-turntable five-axis machine tool, wherein the method comprises the following specific steps:

(III-I) updating the initial pose transformation matrix using the identified position-independent geometric error parameters:

wherein the content of the first and second substances,in order to obtain an initial pose transformation matrix before updating,for the updated initial pose transformation matrix, s when the driving motor is at the side of the workbench_iWhen the drive motor is on the spindle side, s ═ 1_i＝1，

(III-II) step (III-I) repeat iterations to modulo y of the error vector_bAnd if the | | is smaller than a set small value epsilon, identifying the position-independent geometric error of the AC double-turntable five-axis machine tool.

Compared with the prior art, the invention has the advantages that:

1. the position-independent geometric error modeling method based on the local exponential product formula uniformly expresses the rotating shaft and the moving shaft in the joint rotation quantity, the smooth change of kinematic parameters can avoid matrix singularity, and the universality is good.

2. The position-independent geometric error identification method of the least square method with the damping factors provided by the invention ensures the stability of the algorithm under the condition of large noise data.

3. The method only needs to test the five-axis linkage track once when identifying the position-independent geometric error, has high identification efficiency, and has important significance for quickly finishing the calibration of the five-axis machine tool.

Drawings

FIG. 1 is a schematic structural diagram of an AC double-turntable five-axis machine tool in the embodiment of the invention;

FIG. 2 is a flow chart of error identification of the AC double-turntable five-axis machine tool based on a local exponential product formula in the embodiment of the invention;

FIG. 3 is a schematic diagram of the position distribution of each local coordinate system in the zero position of the AC double-turntable five-axis machine tool in the embodiment of the invention;

FIG. 4 is a schematic diagram of a testing trajectory of a blade tip during error identification according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating an iterative convergence process of average distance error during error identification according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of the distance error predicted by the calibrated kinematic model in the embodiment of the invention.

Detailed Description

Taking the five-axis machine tool shown in fig. 1 as an example, the method for modeling and identifying the position-independent geometric error of the AC dual-turntable five-axis machine tool, as shown in fig. 2, includes:

the modeling of the position-independent geometric error of the AC double-turntable five-axis machine tool comprises the following steps:

(1) based on a local exponential product formula, a general forward nominal kinematics model of the five-axis machine tool is established, and the method specifically comprises the following steps:

(1-1) the tandem machine tool is composed of n +1 connecting rods, and introduces the initial pose transformation rotation of two adjacent coordinate systemsT_i-1，i(0) Expressed in the form of an exponential map asKinematic model T of series machine tool based on local exponential product formula_0，n+1Comprises the following steps:

wherein the content of the first and second substances,the initial pose transformation vector of two adjacent coordinate systems;andrespectively representing the joint rotation and the joint displacement at the ith joint in the kinematic model of the serial machine tool; xi₁、ξ₂、…、ξ_nRespectively, the 1 st, 2 nd, … th joint rotation amount n, theta₁、θ₂、…θ_nRespectively, 1 st, 2 nd, … th joint displacement.

(1-2) taking a coordinate system of the five-axis machine tool as a basic coordinate system, wherein m joints are arranged in a workpiece branched chain of a workbench, and n joints are arranged in a main shaft cutter branched chain, and obtaining a homogeneous coordinate transformation matrix T of the workpiece coordinate system of the five-axis machine tool relative to the basic coordinate system_b，wComprises the following steps:

m+n＝5，

wherein the content of the first and second substances,(j ═ 1, 2, …, m +1) denotes the initial pose transformation of two adjacent coordinate systems in the tandem machine on one side of the workpiece coordinate system,andrespectively, the joint rotation and joint displacement at the j (j ═ 1, 2, …, m) th joint at the end of the workpiece link.

(1-3) conversion matrix T of tool coordinate system of five-axis machine tool with respect to base coordinate system_b，tComprises the following steps:

wherein the content of the first and second substances,andrespectively the ith joint rotation and the ith joint displacement of the link end of the workpiece in the tool coordinate system.

(1-4) homogeneous coordinate transformation T of the base coordinate system of the five-axis machine tool with respect to the workpiece coordinate system_w，bComprises the following steps:

(1-5) Universal Forward Nominal kinematics model T of the tool coordinate System with respect to the workpiece coordinate System of any type of five-axis machine tool_w，tComprises the following steps:

(2) based on the five motor-driven displacements under the machine tool coordinate system, the universal forward nominal kinematics model obtained in the step (1) is applied to the forward nominal kinematics modeling of the AC double-turntable five-axis machine tool, and then the forward nominal kinematics model T of the AC double-turntable five-axis machine tool_w，tComprises the following steps:

wherein the content of the first and second substances,respectively representing nominal initial pose moments, ξ, between the local coordinate systems_a、ξ_c、ξ_x、ξ_y、ξ_zNominal joint rotation, theta, for the axis of rotation A, C and the axis of motion X, Y, Z, respectively_c、θ_a、θ_y、θ_x、θ_zIs a drive command for five motors.

(3) The method comprises the following steps of calculating a full pose error model by a forward nominal kinematics model of an AC double-turntable five-axis machine tool, and simplifying the full pose error model, wherein the method comprises the following specific steps:

(3-1) forward kinematics model T_w，tAbout the initial poseThe function of the joint rotation xi and the joint displacement theta is expressed in a mathematical way, namely T_w，tF (p, ξ, θ). Obtaining a full pose error model by differentiating the initial pose vector p, the joint rotation xi and the joint displacement theta:

wherein, δ T_w，t·T_w，t ^-1And the tool pose error of the AC double-turntable five-axis machine tool is shown.

(3-2) based on the assumption that kinematic errors are concentrated in an initial pose matrix of a local coordinate system, joint displacement and joint rotation amount are kept as nominal values in the calibration process, and an error model of full pose information is simplified as follows:

kinematic errors are concentrated in the initial pose matrix of the Local coordinate system, and the assumption that joint displacements and joint rotations remain at nominal values during calibration is specifically disclosed in Local POE model for robot simulation [ J ]. mechanization and Machine Theory, 2001, 36 (11-12): 1215-1239.

(4) According to the distance error measured by the ball arm instrument, a position-independent geometric error model of the AC double-turntable five-axis machine tool based on distance error information is obtained, and the method specifically comprises the following steps:

(4-1) transform matrix for any oneSum of rotationIs a rotation quantity and can be used as a seatThe symbol is asWherein:

SE (3) is a special Euclidean group, R is a special orthogonal group, p is a position vector, SE (3) is a lie algebra of the special Euclidean group SE (3), and the following results are obtained:

therefore, the error model of the full attitude information of the AC double-turntable five-axis machine tool is represented as follows:

wherein, the V-shaped is a vector which transforms the matrix mapping into a 6-dimensional vector,representing the position independent geometric error parameter to be identified.

The left full pose error model is converted into a matrix logarithm form, and the right full pose error model is converted into a matrix multiplication form, namely:

wherein, T_w，t ^aThe tool end pose matrix is actually measured;

the full attitude error model of the AC double-turntable five-axis machine tool is expressed by a linear equation as follows:

y＝J·x

wherein y is a pose error vector of the cutter, J is a Jacobian matrix, and x is a position-independent geometric error parameter vector.

(4-2) error identification based on 6-dimensional information is theoretically possible, but in practice, the attitude information of the tool of the AC double-turntable five-axis machine tool is difficult to obtain. Numerical control machine tool factories generally adopt a ball bar instrument for measuring the dynamic precision of a five-axis machine tool, and the ball bar instrument can only measure the distance change along the ball bar direction. Therefore, it is necessary to further develop an error model based on the error of the measurement distance of the cue stick machine.

The conversion relation between the position coordinates of the tool nose point of the machine tool relative to the workpiece coordinate system and the tool coordinate system is as follows:

wherein, P_wAnd P_tRespectively representing the position coordinates of the tool point of the machine tool relative to a workpiece coordinate system and a tool coordinate system;

due to the fact that under the tool coordinate system, P_t＝[0 0 0]^T，δP_t＝[0 0 0]^TDifferentiating the above formula to obtain:

due to (delta T)_w，tT_w，t ^-1) E se (3), written in the form of a rotation, i.e.The upper typeConversion to:

the local coordinate system can be defined at any position of the corresponding link to simplify the initial pose thereof to the utmost.

When the AC double-turntable five-axis machine tool is in a zero position, firstly defining that all local coordinate systems have the same direction, as shown in FIG. 3; and then defining the tool nose point as the origin of a machine tool coordinate system, simultaneously setting a workpiece coordinate system to coincide with a tool coordinate system, X, Y and a Z-axis coordinate system to coincide with the machine tool coordinate system at a zero position, and finally measuring the relative positions of the A-axis and C-axis coordinate systems and the machine tool coordinate system by a micrometer, wherein the initial pose momentum can be expressed in a momentum form.

(4-3) projecting the position change of the three-dimensional coordinate of the cutter point of the AC double-turntable five-axis machine tool onto the axial direction of the ball bar instrument, and keeping consistent with the change of the reading of the ball bar instrument:

y_b＝[O_bi O_bj O_bk]·δP_w

wherein, y_bIs the change in the reading of the ball bar gauge linear sensor, [ O ]_bi O_bj O_bk]Is the unit direction vector of the cue stick in a positive nominal kinematics model.

(4-4) the position-independent geometric error model based on the change in the measured distance of the cue stick instrument can be represented linearly as:

y_b＝J_b·x

wherein, y_bDistance variation of cue instruments, J_bThe Jacobian matrix is based on the change of the measurement distance of the cue stick instrument, and x is a position-independent geometric error parameter to be identified.

The purpose of kinematic error identification is to find suitable error parameters to minimize the least square value of the error model established based on the distance information measured by the cue stick instrument. Considering that the identification process of the error parameters is weak and nonlinear, under the general condition, a better identification effect can be obtained by adopting a Gaussian-Newton method. However, if the noise of the actual measurement error data is too large, which may cause the error identification process to be non-convergent, an iterative least square method with a damping factor is required to identify the position-independent geometric error parameter x in the error model.

The embodiment adopts a high-precision QC20-W ball-bar instrument which is used for measuring the distance change of the AC double-turntable five-axis machine tool during five-axis linkage operation.

Under the ideal condition, the AC double-turntable five-axis machine tool has no error, and the distance between a main shaft end ball and a workpiece end ball is kept constant;

because there is geometric error in actual lathe for the distance between the ball at club appearance both ends will take place slight change, and this change information can be caught by the telescopic linear transducer of high accuracy in the club appearance, through bluetooth transmission, on the Ballbar Trace software of real-time recording on the computer.

The error identification method based on the position-independent geometric error modeling of the AC double-turntable five-axis machine tool comprises the following steps of:

(I) according to the track points measured by the ball arm instrument, a position-independent geometric error model of the AC double-turntable five-axis machine tool is expanded, and the method specifically comprises the following steps;

(I-I) for the AC double-turntable five-axis machine tool, a kinematic open chain from a machine base coordinate system to a workpiece coordinate system has (m +1) connecting rods, a kinematic open chain from the machine base coordinate system to a cutter coordinate system has (n +1) connecting rods, and in order to finish the identification of the position-independent geometric error parameter x of the AC double-turntable five-axis machine tool, the number k of track points measured by a ball rod instrument is as follows:

k＞6(m+1)+6(n+1)

and the NC codes of the test tracks refer to a program provided by the Renilsha official network and used for testing the dynamic precision of the five-axis machine tool. The NC code comprises 7204 groups of five-axis motor driving commands, the five-axis motor driving commands are substituted into a nominal forward kinematics model, the position of a tool point in a Cartesian space under a workpiece coordinate system is obtained, and a track formed by the position points is shown in FIG. 4.

And (3) importing the NC codes into the numerical control system, and starting the RTCP function of the numerical control system, so that the ball rod instrument can measure distance change data of the five-axis machine tool in the linkage process due to the existence of geometric errors.

(I-II) according to the number of the track points measured by the ball rod instrument, expanding a position-independent geometric error model of the AC double-turntable five-axis machine tool based on the distance error into:

wherein the content of the first and second substances,

representing k sets of cue meters distance data y_bThe vector of the composition is then calculated,as a k-set Jacobian matrix J_bA matrix of components.

(II) identifying a position-independent geometric error parameter x in the error model using an iterative least squares method with a damping factor:

where μ is the damping factor.

Under the ideal condition or the low-noise condition, the damping factor mu is set to be 0 so as to improve the error identification precision, and under the high-noise data, a smaller value is set to be 0.0001-1 so as to improve the robustness of the algorithm.

(III) updating an initial pose transformation matrix of the five-axis machine tool by using the position-independent geometric error parameters, repeating iteration until the modulus of an error vector is smaller than a set tiny value, and identifying to obtain the position-independent geometric error of the AC double-turntable five-axis machine tool, wherein the method comprises the following specific steps:

(III-I) updating the initial pose transformation matrix using the identified position-independent geometric error parameters:

wherein the content of the first and second substances,in order to obtain an initial pose transformation matrix before updating,for the updated initial pose transformation matrix, s when the driving motor is at the side of the workbench_iWhen the drive motor is on the spindle side, s ═ 1_i＝1，

(III-II) step (III-I) the iteration is repeated, as shown in FIGS. 2 and 5, until the actual measured spatial positionAnd calculating the kinematic model after each iteration to obtain the spatial positionModulo y of the difference_bAnd the | | tends to be stable and is smaller than a set tiny value epsilon, and the position-independent geometric error of the AC double-turntable five-axis machine tool is output.

As shown in fig. 6, the updated initial transformation matrix is substituted into the nominal forward kinematics model of the AC double-turntable five-axis machine tool, so that a calibrated forward kinematics model can be obtained, and compared with the nominal forward kinematics model, and further distance error data can be predicted, the data is compared with the actual distance error measured by the ball rod instrument, and the actual and predicted distance error curves of each sample point are substantially consistent, thereby further verifying the validity of the proposed method.