Error self-calibration method of parallel coordinate measuring machine based on characteristic component

Abstract

The invention discloses an error self-calibration method for a parallel coordinate measuring machine based on a characteristic component, and the method comprises the steps: constructing a geometric error model of a parallel mechanism, measuring the characteristic component through the coordinate measuring machine, installing the characteristic component in a working space of the parallel coordinate measuring machine, and carrying out the self-calibration of the error of the parallel coordinate measuring machine. Carrying out data acquisition based on a sphere center distance constraint and a sphere diameter constraint, and correspondingly constructing an error equation based on the sphere center distance constraint and an error equation based on the sphere diameter constraint; combining the error equations to obtain an error calibration model based on multiple feature constraints, using an LM method to identify, correct and substitute kinematic error parameters of the error calibration model, repeating the steps until the kinematic error parameters do not change any more, and stopping iteration to obtain corresponding geometric error parameters of the parallel coordinate measuring machine; and compensating the tail end attitude error caused by the geometric error in the coordinate measuring machine according to the geometric error parameter. According to the invention, the motion precision of the parallel coordinate measuring machine is greatly improved.

Description

Technical Field

[0001] The invention belongs to the field of error calibration, and in particular relates to an error self-calibration method of a parallel coordinate measuring machine based on a characteristic component. Background Art

[0002] Parallel robots have been widely used in precision manufacturing, aerospace and other fields due to their high precision and high rigidity. However, due to the influence of errors in machining and assembly, there will be deviations between the geometric parameters of the parallel coordinate measuring machine and their design values. On this basis, the motion solution of the parallel coordinate measuring machine will inevitably lead to the position deviation of the end measuring head. Therefore, it is necessary to calibrate the geometric parameters and joint zero position of the parallel coordinate measuring machine to improve the positioning accuracy of the end and lay the foundation for the stable control of the robot.

[0003] Calibration methods are mainly divided into external calibration method and internal calibration method. The internal calibration method is to install sensors on the active and driven joints that exceed the robot's degrees of freedom and measure redundant motion information in real time to identify error parameters. The internal calibration method has the advantages of no need for additional measuring instruments, strong environmental adaptability, fast detection speed, easy to realize automatic calibration and online error compensation. However, the internal calibration method requires measuring the error information at all joints, and the installation of sensors will increase the difficulty of parallel mechanism design and increase the processing and manufacturing costs.

[0004] The external calibration method for calibrating the geometric parameters of parallel robots currently determines the parameters by directly measuring the position and posture of the robot's end effector through coordinates, but it often requires high-precision measurement equipment and is costly. For example, Chinese patent application publication number CN114894086A discloses an error calibration method based on a laser tracker, which has high calibration costs and complex measurements. Chinese patent application publication number CN113183137A discloses a parameter calibration device based on a displacement sensor. The device includes three mutually orthogonal planes, on which 1, 2, and 3 displacement sensors are fixed respectively, and the axis of the measuring rod of the displacement sensor is perpendicular to the corresponding plane. The device is fixed on a fixed platform of a six-degree-of-freedom parallel mechanism through a bracket, and the change in the end position is obtained by the telescopic amount of the displacement sensor. Its disadvantage is that it requires the use of more sensors and cannot calibrate the entire workspace of the parallel robot. Summary of the invention

[0005] In order to solve the problems of high calibration cost, long time consumption and poor environmental adaptability in the existing methods, the present invention provides an error self-calibration method of a parallel coordinate measuring machine based on a characteristic component, comprising:

[0006] Building a parallel mechanism and constructing a geometric error model of the parallel mechanism to determine error parameters;

[0007] The characteristic component is measured by a coordinate measuring machine to obtain the radius value of each standard sphere and the spherical center distance between every two standard spheres; after the measurement is completed, the characteristic component is installed in the workspace of the parallel coordinate measuring machine, and data is collected based on the spherical center distance constraint and the spherical diameter constraint, and an error equation based on the spherical center distance constraint and an error equation based on the spherical radius constraint are correspondingly constructed;

[0008] The error equation based on the ball center distance constraint and the error equation based on the ball radius constraint are combined to obtain an error calibration model based on multiple feature constraints, and the kinematic error parameters of the error calibration model are identified, corrected and substituted using the LM method, and the iteration is repeated until the kinematic error parameters no longer change, thereby obtaining the corresponding geometric error parameters of the parallel coordinate measuring machine;

[0009] The end posture error caused by the geometric error in the coordinate measuring machine is compensated according to the geometric error parameter.

[0010] Preferably, the parallel mechanism comprises a static platform, a branch chain and a dynamic platform;

[0011] The parallel mechanism is driven by a moving pair input consisting of a servo motor, a ball screw and a guide rail;

[0012] The branch chain includes a moving pair and a Hooke's hinge;

[0013] A trigger probe is installed at the end of the moving platform, which is used to output a contact trigger signal between the coordinate measuring machine and the measured part. When the parallel mechanism controller receives the motion command, it decomposes the target motion into the displacement of four translation axes, and the servo motor of the corresponding axis drives the ball screw to rotate to realize the movement of the slider on the translation axis. Through the coordinated movement of the four-axis slider, the translational motion of the end moving platform in the X, Y, and Z axes and the rotational motion around the Z axis are realized.

[0014] Preferably, the process of constructing the geometric error model of the parallel mechanism includes establishing kinematic equations;

[0015] The process of establishing the kinematic equations includes:

[0016] A base coordinate system O-xyz of the parallel mechanism is established at the center of the static platform, and a dynamic coordinate system O'-xyz of the parallel mechanism is established at the center of the dynamic platform;

[0017] Solving the kinematic inverse solution of the parallel mechanism in the kinematic inverse solution model of the parallel mechanism, wherein the input variable is the posture of the moving coordinate system relative to the base coordinate system, and the output is the displacement of the four moving pairs;

[0018] To analyze the branch chain, take the vector from the center of the base coordinate system O to point A on the base as The unit vector of the moving pair's motion direction is The distance from point P on the moving sub-slider to point A on the base is That is, the driving variable of the parallel mechanism; the vector from point P on the moving pair to point D, the center of Hooke's joint 1, is The length of the rod from Hooke's hinge 1 to Hooke's hinge 2 is The direction vector is The vector from the center of the moving coordinate system O' to the center of Hooke's hinge 2, point B, is The vector from the center O of the base coordinate system to the center O' of the moving coordinate system is Let the rotation matrix from the base coordinate system to the moving coordinate system be R, and then the vector equation obtained by the closed-loop vector method is:

[0019] a i +p i e i +d i +L i u i =q+Rb i (1)

[0020] Based on the connecting rod length L i The invariant constraints are obtained:

[0021]

[0022] Where E i =q+Rb i -a i -d i ;

[0023] Use Newton's method to iteratively solve the kinematic solution of the parallel structure, and set the iteration precision. When the second norm of the difference between the current and the next two iteration results is less than the iteration precision, the iteration ends.

[0024] Preferably, the process of constructing the geometric error model of the parallel mechanism includes establishing an error mapping model;

[0025] The process of establishing the error mapping model includes:

[0026] Considering the error sources of workpiece machining error and assembly error, the branch chain is analyzed, a kinematic error mapping model is established, and the linearly related vectors are removed to obtain:

[0027] [u x ,u y ,u x p,-u y p,-u T R,-1]·Δε=[u x ,u y ,u z ,u x(-b x sθ-b y cθ)+u y (-b x cθ-b y sθ)]·ΔY (3)

[0028] where ΔY=[Δx,Δy,Δz,Δθ z ]=[ΔP,Δθ z ];

[0029]

[0030] ΔY is the difference between the actual end position and the theoretical position, ΔP is the position error of the end, and Δθ z represents the z-axis attitude error of the end, Δε represents the geometric error parameter, represents the direction vector of the connecting rod;

[0031] The error model of the branch chain is further expressed as:

[0032] A i ΔY=B i ·Δε i ,i=1,2,3,4(4)

[0033] The geometric error model of the parallel mechanism can be obtained by combining the error models of the branch chains:

[0034] ΔY=J q -1 J ε Δε=J p ·Δε (5)

[0035] Among them J q =[A1,A2,A3,A4],J ε =diag(B1,B2,B3,B4);

[0036] The influence relationship between the geometric error of the mechanism and the terminal posture error is calculated based on the geometric error model.

[0037] Preferably, the workpiece machining error includes: the position error of the three Hooke joints of the moving platform Connecting rod length Position of the Hooke's hinge adjacent to the slider Error of column position relative to the center of static platform

[0038] The assembly error includes: the axis direction of the moving pair and the position on the static platform The initial position positioning error affects the initial position Δp of the ball screw slider.

[0039] Preferably, the process of collecting data based on the sphere center distance constraint and the sphere diameter constraint includes:

[0040] The end of the coordinate measuring machine is controlled to contact the standard ball in the feature component. At each position, the end of the coordinate measuring machine is controlled to use a contact probe to evenly measure the surface of the standard ball on the feature component for multiple times. The measurement readings of each measurement and the position readings of the four translation axes are read and recorded; the errors at different workspace positions are obtained, the positions of the feature components in the workspace of the parallel coordinate measuring machine are changed multiple times, and multiple sets of data are collected.

[0041] Preferably, the process of constructing the error equation based on the sphere center distance constraint includes:

[0042] The recorded translation axis data and measurement readings are divided into several groups according to the position of the characteristic components in the measurement. The spherical center distance after fitting is subtracted from the theoretical spherical center distance measured by the coordinate measuring machine to obtain the spherical center distance error. The error equation based on the spherical center distance constraint is constructed, and the expression is:

[0043]

[0044] Among them, J s,i =[J p,i1 ,J p,i2 ,J p,i3 ,J p,i4 ] T , J s,i It represents the influence of error parameters on the pose error of the four sampling points of each set of standard balls.

[0045] Preferably, the process of constructing the error equation based on the spherical radius constraint includes:

[0046] The measured data of each ball are grouped, and the radius of each group of balls obtained by fitting is subtracted from the theoretical ball radius measured by the coordinate measuring machine to construct an error equation based on the ball radius constraint.

[0047] Preferably, the formula expression of the error calibration model based on multiple feature constraints is:

[0048]

[0049] Among them, the error model of the i-th group of spherical sampling points is:

[0050]

[0051] n is the number of groups of sampling standard balls, Indicates the probe radius error.

[0052] Compared with the prior art, the present invention has the following advantages and technical effects:

[0053] Compared with the traditional parallel robot external calibration method, the method of the present invention has the advantages of low cost and good portability, does not require additional sensors, and is easy to operate; the use of multiple feature constraints increases the number of constraint equations, thereby reducing the number of required calibration positions and shortening the calibration cycle; at the same time, the robustness and noise resistance of the algorithm are improved, effectively solving the problem of kinematic error parameter identification of the parallel coordinate measuring machine, and greatly improving the motion accuracy of the parallel coordinate measuring machine. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0055] Figure 1 The figure is a schematic diagram of a method flow of an embodiment of the present invention. DETAILED DESCRIPTION

[0056] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0057] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0058] like Figure 1 As shown, in this embodiment, a method for self-calibration of an error of a parallel coordinate measuring machine based on a characteristic component is provided, comprising:

[0059] Build a parallel mechanism and construct a geometric error model of the parallel mechanism to determine the error parameters;

[0060] The characteristic component is measured by a coordinate measuring machine to obtain the radius value of each standard sphere and the spherical center distance between every two standard spheres; after the measurement is completed, the characteristic component is installed in the workspace of the parallel coordinate measuring machine, and data is collected based on the spherical center distance constraint and the spherical diameter constraint, and the error equation based on the spherical center distance constraint and the error equation based on the spherical radius constraint are constructed accordingly;

[0061] The error equation based on the ball center distance constraint and the error equation based on the ball radius constraint are combined to obtain an error calibration model based on multiple feature constraints. The LM method is used to identify, correct and substitute the kinematic error parameters of the error calibration model. This process is repeated until the kinematic error parameters no longer change, and the corresponding geometric error parameters of the parallel coordinate measuring machine are obtained.

[0062] The end attitude error caused by the geometric error in the coordinate measuring machine is compensated according to the geometric error parameters.

[0063] Furthermore, in the first step, a 4PUU parallel mechanism is built;

[0064] The 4PUU parallel mechanism consists of a static platform, four branches and a dynamic platform. Each branch consists of a moving pair (P pair) and two Hooke hinges (U pairs). The parallel mechanism is driven by four moving pair inputs consisting of servo motors, ball screws and guide rails. A trigger probe is installed at the end of the dynamic platform to output the contact trigger signal between the coordinate measuring machine and the measured part. When the parallel mechanism controller receives the motion command, it decomposes the target motion into the displacement of the four translation axes, and the servo motors of the corresponding axes drive the ball screws to rotate to realize the movement of the slider on the translation axis. Through the coordinated movement of the four-axis slider, the translational motion of the end dynamic platform in the X, Y, and Z axes and the rotational motion around the Z axis can be realized.

[0065] Furthermore, in the second step, a geometric error model of the 4PUU parallel mechanism is established to determine the error parameters;

[0066] Further optimize the scheme and establish the kinematic equations;

[0067] The kinematic model of the 4PUU parallel mechanism is established by establishing the base coordinate system O-xyz of the parallel mechanism at the center of the static platform and the dynamic coordinate system O'-xyz of the parallel mechanism at the center of the dynamic platform.

[0068] First, solve the kinematic inverse solution of the parallel mechanism: In the kinematic inverse solution model of the parallel mechanism, the input variable is the posture of the moving coordinate system relative to the base coordinate system, and the output is the displacement of the four moving pairs.

[0069] Analyze one of the branches and take the vector from the center of the base coordinate system O to point A on the base as The unit vector of the moving pair's motion direction is The distance from point P on the moving sub-slider to point A on the base is That is, the driving variable of the parallel mechanism; the vector from point P on the moving pair to point D, the center of Hooke's joint 1, is The length of the rod from Hooke's hinge 1 to Hooke's hinge 2 is The direction vector is The vector from the center of the moving coordinate system O' to the center of Hooke's hinge 2, point B, is The vector from the center O of the base coordinate system to the center O' of the moving coordinate system is The rotation matrix from the base coordinate system to the moving coordinate system is denoted by R.

[0070] Then the vector equation obtained by using the closed-loop vector method is:

[0071] a i +p i e i +d i +L i u i =q+Rb i (1)

[0072] Based on the connecting rod length L i The invariant constraints are obtained:

[0073]

[0074] Where E i =q+Rb i -a i -d i ;

[0075] Since Equation 1 cannot solve the explicit analytical expression of the position q of the dispatching platform, the Newton method is used to iteratively solve the kinematic solution of the 4PUU structure. Since the display unit of the parallel coordinate measuring machine is μm, the iteration accuracy is set to 1e-7 to meet the required accuracy. When the second norm of the difference between the current and the last two iteration results is less than the iteration accuracy, the iteration ends.

[0076] Further optimize the scheme and establish the error mapping model;

[0077] The workpiece machining error includes: the position error of the three Hooke joints of the moving platform Connecting rod length Position of the Hooke's hinge adjacent to the slider Error of column position relative to the center of static platform

[0078] Assembly errors include: axis direction of the moving pair and the position on the static platform The initial position positioning error affects the initial position Δp of the ball screw slider.

[0079] Considering the above error sources, one of the branches is analyzed, a kinematic error mapping model is established, and the linearly related vectors are removed to obtain:

[0080] [u x ,u y ,u x p,-u y p,-u T R,-1]·Δε=[u x ,u y ,u z ,u x (-b x sθ-b ycθ)+u y (-b x cθ-b y sθ)]·ΔY (3)

[0081] where ΔY=[Δx,Δy,Δz,Δθ z ]=[ΔP,Δθ z ];

[0082]

[0083] ΔY is the difference between the actual end position and the theoretical position, ΔP is the position error of the end, and Δθ z represents the z-axis posture error of the end, and Δε represents the geometric error parameter. Represents the direction vector of the connecting rod.

[0084] The error model of the branch chain is further expressed as:

[0085] A i ΔY=B i ·Δε i ,i=1,2,3,4 (4)

[0086] The error model of the parallel mechanism can be obtained by combining the error models of the four branches:

[0087] ΔY=J q -1 J ε Δε=J p ·Δε (5)

[0088] Among them J q =[A1,A2,A3,A4],J ε =diag(B1,B2,B3,B4);

[0089] According to Formula 5, the influence of the geometric error of the parallel mechanism on the terminal posture error can be calculated.

[0090] Further, the third step is measurement and installation;

[0091] The standard ball plate is composed of four standard balls. The centers of the two balls are 200mm apart. The standard balls are solid steel balls with a diameter of 38.1mm and a spherical roundness of 1μm. The ball plate can adjust its posture through the ball joint. A high-precision coordinate measuring machine (CMM) is used to measure the characteristic components to obtain the radius value of each standard ball and the distance between the centers of each two standard balls; after the measurement is completed, the characteristic components are installed in the workspace of the parallel coordinate measuring machine.

[0092] Further, the fourth step is data collection;

[0093] The data collection process based on the ball center distance constraint and ball diameter constraint is as follows: Use the handheld operator to control the end of the coordinate measuring machine to contact the standard ball in the feature component, and control the end of the coordinate measuring machine to use the contact probe to evenly measure the surface of the standard ball on the feature component multiple times at each position, and read and record the measurement readings of each measurement and the position readings of the four translation axes. In order to reduce the measurement error and obtain the error at different workspace positions, the position of the feature component in the parallel coordinate measuring machine workspace is changed multiple times to collect multiple sets of data.

[0094] Because the sampling points all satisfy the spherical constraint, if 4 points are evenly collected on the sphere, the following equation is satisfied:

[0095] (P c -P i ) T (P c -P i )=(R i +r) 2 ,i=1,2,3,4 (6)

[0096] Where R i is the radius of the ball, r is the radius of the probe, and after the radius error calibration, it is a known quantity. Differentiating the above formula yields:

[0097]

[0098] That is, the mapping relationship between the sampling point error and the sphere center error and sphere radius error.

[0099] Further, in the fifth step, parameter identification;

[0100] The recorded translation axis data and measurement readings are divided into several groups according to the position of the characteristic components in the measurement. The spherical center distance after fitting is subtracted from the theoretical spherical center distance measured by CMM to obtain the spherical center distance error, and an error equation based on the spherical center distance constraint is constructed.

[0101] Among them, the sphere center distance constraint equation is:

[0102]

[0103] Spherical center distance error mapping equation:

[0104]

[0105] Substituting into formula 5 and formula 8, the error equation based on the sphere center distance constraint can be obtained as:

[0106]

[0107] J s,i =[J p,i1 ,Jp,i2 ,J p,i3 ,J p,i4 ] T , J s,i It represents the influence of error parameters on the pose error of the four sampling points of each set of standard balls.

[0108] The measurement data of each ball are grouped, and the radius of each group of balls obtained by fitting is subtracted from the theoretical ball radius measured by CMM to construct an error equation based on ball radius constraints.

[0109] By combining the error equations obtained by the two constraints, we can obtain the error calibration model based on multiple feature constraints.

[0110]

[0111] The error model of the i-th group of spherical sampling points:

[0112]

[0113] Where n is the number of groups of sampling standard balls, Indicates the probe radius error.

[0114] The LM method is used to identify the kinematic error parameters. The kinematic error model is corrected using the model parameters calculated in each step, and then the corrected parameters are substituted into the kinematic error model. This process is repeated until the error parameters no longer change, and the corresponding geometric error parameters of the parallel coordinate measuring machine are obtained.

[0115] Further, in the sixth step, error compensation;

[0116] Substitute the geometric error parameters identified in the fifth step into the kinematic model containing the error parameters, and directly modify the kinematic error parameters in the controller to obtain the accurate mapping relationship between the active axis position and the end attitude. In actual control, only the kinematic model containing the error parameters is needed to compensate for the end attitude error caused by the geometric error in the coordinate measuring machine.

[0117] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An error self-calibration method for a parallel coordinate measuring machine based on a characteristic component, characterized in that: include: Building a parallel mechanism and constructing a geometric error model of the parallel mechanism to determine error parameters; The characteristic components are measured by a coordinate measuring machine to obtain the radius value of each standard sphere and the distance between the sphere centers of every two standard spheres; After the measurement is completed, the characteristic component is installed in the workspace of the parallel coordinate measuring machine, data is collected based on the ball center distance constraint and the ball diameter constraint, and an error equation based on the ball center distance constraint and an error equation based on the ball radius constraint are correspondingly constructed; The error equation based on the ball center distance constraint and the error equation based on the ball radius constraint are combined to obtain an error calibration model based on multiple feature constraints, and the kinematic error parameters of the error calibration model are identified, corrected and substituted using the LM method, and the iteration is repeated until the kinematic error parameters no longer change, thereby obtaining the corresponding geometric error parameters of the parallel coordinate measuring machine; The end posture error caused by the geometric error in the coordinate measuring machine is compensated according to the geometric error parameter.

2. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1 is characterized in that: The parallel mechanism comprises a static platform, a branch chain and a dynamic platform; The parallel mechanism is driven by a moving pair input consisting of a servo motor, a ball screw and a guide rail; The branch chain includes a moving pair and a Hooke's hinge; A trigger probe is installed at the end of the moving platform, which is used to output a contact trigger signal between the coordinate measuring machine and the measured part. When the parallel mechanism controller receives the motion command, it decomposes the target motion into the displacement of four translation axes, and the servo motor of the corresponding axis drives the ball screw to rotate to realize the movement of the slider on the translation axis. Through the coordinated movement of the four-axis slider, the translational motion of the end moving platform in the X, Y, and Z axes and the rotational motion around the Z axis are realized.

3. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The process of constructing the geometric error model of the parallel mechanism includes establishing kinematic equations; The process of establishing the kinematic equations includes: A base coordinate system O-xyz of the parallel mechanism is established at the center of the static platform, and a dynamic coordinate system O'-xyz of the parallel mechanism is established at the center of the dynamic platform; Solving the kinematic inverse solution of the parallel mechanism in the kinematic inverse solution model of the parallel mechanism, wherein the input variable is the posture of the moving coordinate system relative to the base coordinate system, and the output is the displacement of the four moving pairs; To analyze the branch chain, take the vector from the center of the base coordinate system O to point A on the base as The unit vector of the moving pair's motion direction is The distance from point P on the moving sub-slider to point A on the base is That is, the driving variable of the parallel mechanism; the vector from point P on the moving pair to point D, the center of Hooke's joint 1, is The length of the rod from Hooke's hinge 1 to Hooke's hinge 2 is The direction vector is The vector from the center of the moving coordinate system O' to the center of Hooke's hinge 2, point B, is The vector from the center O of the base coordinate system to the center O' of the moving coordinate system is Let the rotation matrix from the base coordinate system to the moving coordinate system be R, and then the vector equation obtained by the closed-loop vector method is: a i +p i e i +d i +L i u i =q+Rb i (1) Based on the connecting rod length L i The invariant constraints are obtained: Where E i =q+Rb i -a i -d i ; Use Newton's method to iteratively solve the kinematic solution of the parallel structure, and set the iteration precision. When the second norm of the difference between the current and the next two iteration results is less than the iteration precision, the iteration ends.

4. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The process of constructing the geometric error model of the parallel mechanism includes establishing an error mapping model; The process of establishing the error mapping model includes: Considering the error sources of workpiece machining error and assembly error, the branch chain is analyzed, a kinematic error mapping model is established, and the linearly related vectors are removed to obtain: [u x ,u y ,u x p,-u y p,-u T R,-1]·Δε=[u x ,u y ,u z ,u x (-b x sθ-b y cθ)+u y (-b x cθ-b y sθ)]·ΔY (3) where ΔY = [Δx, Δy, Δz, Δθ z = [ΔP, Δθ z ; ΔY is the difference between the actual end position and the theoretical position, ΔP is the position error of the end, and Δθ z represents the z-axis attitude error of the end, Δε represents the geometric error parameter, represents the direction vector of the connecting rod; The error model of the branch chain is further expressed as: A i ·ΔY=B i ·No i ,i=1,2,3,4 (4) The geometric error model of the parallel mechanism can be obtained by combining the error models of the branch chains: ΔY=J q -1 J ε ·No=J p ·No (5) Among them J q =[A1,A2,A3,A4],J ε =diag(B1,B2,B3,B4); The influence relationship between the geometric error of the mechanism and the terminal posture error is calculated based on the geometric error model.

5. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 4 is characterized in that: The workpiece machining error includes: the position error of the three Hooke joints of the moving platform Connecting rod length Position of the Hooke's hinge adjacent to the slider Error of column position relative to the center of static platform The assembly error includes: the axis direction of the moving pair and the position on the static platform The initial position positioning error affects the initial position Δp of the ball screw slider.

6. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The process of data collection based on sphere center distance constraint and sphere diameter constraint includes: The end of the coordinate measuring machine is controlled to contact the standard ball in the feature component. At each position, the end of the coordinate measuring machine is controlled to use a contact probe to evenly measure the surface of the standard ball on the feature component for multiple times. The measurement readings of each measurement and the position readings of the four translation axes are read and recorded; the errors at different workspace positions are obtained, the positions of the feature components in the workspace of the parallel coordinate measuring machine are changed multiple times, and multiple sets of data are collected.

7. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The process of constructing the error equation based on the sphere center distance constraint includes: The recorded translation axis data and measurement readings are divided into several groups according to the position of the characteristic components in the measurement. The spherical center distance after fitting is subtracted from the theoretical spherical center distance measured by the coordinate measuring machine to obtain the spherical center distance error. The error equation based on the spherical center distance constraint is constructed, and the expression is: Among them, J s,i =[J p,i1 ,J p,i2 ,J p,i3 ,J p,i4 ] T , J s,i It represents the influence of error parameters on the pose error of the four sampling points of each set of standard balls.

8. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The process of constructing the error equation based on the spherical radius constraint includes: The measured data of each ball are grouped, and the radius of each group of balls obtained by fitting is subtracted from the theoretical ball radius measured by the coordinate measuring machine to construct an error equation based on the ball radius constraint.

9. The error self-calibration method of a parallel coordinate measuring machine based on characteristic components according to claim 1, characterized in that: The formula expression of the error calibration model based on multiple feature constraints is: Among them, the error model of the i-th group of spherical sampling points is: n is the number of groups of sampling standard balls, Indicates the probe radius error.

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