Workpiece and cutter pose calibration method based on robotic edge milling error tracking

Patent metadata

Field	Value
Publication number	US-12122009-B2
Application number	US-202418733869-A
Country	US
Kind code	B2
Filing date	Jun 5, 2024
Priority date	Jul 5, 2022
Publication date	Oct 22, 2024
Grant date	Oct 22, 2024

Abstract

A workpiece and cutter pose calibration method based on robotic edge milling error tracking, including: 1. generating an edge milling trajectory point cloud; 2. obtaining an actual edge milling three-dimensional point cloud; 3. generating an updated edge milling three-dimensional point cloud; 4. calculating an edge milling allowance error and a posture inclination error; 5. solving position errors of the workpiece and cutter; 6. solving posture errors of the workpiece and cutter; 7. updating pose parameters of the workpiece and cutter; 8. repeating steps 4 to 7 until pose error vectors of the workpiece and cutter are both not greater than corresponding preset thresholds. The disclosure performs error comparison, error modeling, and error tracking on the three-dimensional point cloud and edge milling trajectory point cloud, even if the cutter has system errors such as axis deviation, the disclosure can accurately identify pose errors of the workpiece and cutter during edge milling.

First claim

What is claimed is:

1. A workpiece and cutter pose calibration method based on robotic edge milling error tracking, comprising steps as follows:
Step S 10 , constructing an edge milling path of a cutter to the workpiece to generate a robotic edge milling trajectory point cloud Q;
Step S 20 , obtaining an actual edge milling three-dimensional point cloud P of the workpiece;
Step S 30 , matching an edge milling trajectory point cloud Q and the actual edge milling three-dimensional point cloud P into same coordinate system to generate a pose updated edge milling three-dimensional point cloud P′;
Step S 40 , calculating an edge milling allowance error e i and a posture inclination error α i according to the edge milling trajectory point cloud Q and the edge milling three-dimensional point cloud P′;
Step S 50 , establishing respectively an influence model of a position error of the workpiece on the edge milling allowance error and an influence model of a position error of the cutter on the edge milling allowance error when milling an i-th point of the workpiece according to the edge milling allowance error e i , and solving a workpiece position error W d and a cutter position error U d z ;
Step S 60 , establishing respectively an influence model of a posture error of the workpiece on the edge milling error and an influence model of a posture error of the cutter on the edge milling error when milling the i-th point of the workpiece according to the posture inclination error α i , and solving a workpiece posture error U δ x and a cutter posture error W δ;
Step S 70 , updating a workpiece pose parameter and a cutter pose parameter according to the workpiece position error W d, the cutter position error U d z , the workpiece posture error U δ x , and the cutter posture error W δ;
Step S 80 , repeating the Step S 40 to the Step S 70 until a workpiece pose error vector is W V and a cutter pose error vector is U V and both of vectors are less than or equal to corresponding preset thresholds.

2. The workpiece and cutter pose calibration method according to claim 1 , wherein the Step S 10 comprises:
discretizing uniformly a boundary cross-section of a design model of the workpiece to generate the uniform and orderly edge milling trajectory point cloud Q={q 1 , q 2 , . . . , q i , . . . , q n }, wherein any point q i in the edge milling trajectory point cloud Q is a vector of 3×1, a unit normal vector w i of q i is perpendicular to a boundary lateral cross-section, and a positive direction is toward outside of the cross-section and is same as an edge milling depth direction, wherein a first unit tangent vector τ i1 of the point q i is parallel to a cross-section boundary direction and is same as a motion direction of an edge milling trajectory, wherein a second unit tangent vector τ i2 of the point q i is parallel to a curved surface thickness direction; constituting a coordinate system of the point q i together by three vectors (τi 1 , τ i2 , w i ), wherein the three vectors respectively correspond to directions of an x-axis, a y-axis, and a z-axis.

3. The workpiece and cutter pose calibration method according to claim 1 , wherein the Step S 20 comprises steps as follows:
Step S 21 , defining respectively an initial workpiece pose parameter and an initial cutter pose parameter as W B T and U B T, wherein W B T represents a pose of a workpiece coordinate system {W} relative to a base coordinate system {B}, U B T represents a pose of a cutter coordinate system {U} relative to the base coordinate system {B}, and both of the workpiece pose parameter W B T and the cutter pose parameter U B T are homogeneous transformation matrices of 4×4;
Step S 22 , positioning an edge milling pose of a robot through the workpiece pose parameter W B T and the cutter pose parameter U B T;
Step S 23 , performing edge milling on a workpiece blank according to the edge milling path, which is constructed, and after completion, performing three-dimensional scanning on a surface to be milling processed of the workpiece by a three-dimensional scanning device to obtain the actual edge milling three-dimensional point cloud P of the workpiece, wherein P={p 1 , p 2 , . . . , p a , . . . , p m }, and each point on the actual edge milling three-dimensional point cloud P is a vector of 3×1.

4. The workpiece and cutter pose calibration method according to claim 3 , wherein the Step S 30 comprises steps as follows:
Step S 31 , searching for a closest point to the point q i in the actual edge milling three-dimensional point cloud P and denoting as p a , wherein the search is for any point q i in the edge milling trajectory point cloud Q, and i=1, 2, . . . , n;
Step S 32 , constructing a matching objective function ƒ(R,t) based on uniform allowance using point pair (q i , p a ), wherein R, t respectively represents a rotation posture matrix of 3×3 of the actual edge milling three-dimensional point cloud P and a translation position matrix of 3×1 of the edge milling trajectory point cloud Q; then solving pose parameters R and t by minimizing an objective function;
Step S 33 , updating a position of any one of the points p a on the actual edge milling three-dimensional point cloud P as p′ a =Rp a +t, and assigning p a =p a ′ to obtain the pose updated edge milling three-dimensional point cloud P′, wherein P′={p 1 ′, p 2 ′, . . . , p a ′, . . . , p m ′}.

5. The workpiece and cutter pose calibration method according to claim 3 , wherein an objective function to be minimized in the Step S 32 is min

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wherein d i =∥Rp a +t−q i ∥, d i represents a distance from point p a ′ to point q i , and n represents the number of points in the edge milling trajectory point cloud Q.

6. The workpiece and cutter pose calibration method according to claim 3 , wherein the Step S 40 comprises steps as follows:
Step S 41 , extracting a closest point p a ′ to the point q i from the edge milling three-dimensional point cloud P′ according to the edge milling trajectory point cloud Q, wherein the edge milling allowance error of the point q i is e i =(q i −p a ′) T w

Assignees

• Univ Hunan

Inventors

• Xie He
• Wang Yaonan
• Zhang Hui
• Mao Jianxu

Classifications

• B23Q2717/006

  in milling machines · CPC title

• B23C2235/21

  Calibration by electronic detection of position of probes and cutting wheels · CPC title

• B23C3/12 Primary

  Trimming or finishing edges, e.g. deburring welded corners · CPC title

• G05B2219/45145

  Milling · CPC title

• G05B2219/37404

  Orientation of workpiece or tool, surface sensor · CPC title