| CPC B23K 9/0284 (2013.01) | 2 Claims |
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1. A method for conformal processing of a cylindrical shell inner weld seam by a special mobile robot, wherein in the method, a weld seam contour is scanned and measured to obtain point cloud data of a contour of a weld seam area first; then weld seam feature identification is carried out to each generatrix, and misidentified generatrices are filtered out to obtain weld seam left and right boundaries; an ideal weld seam processing contour is generated conformally according to the appearance of the weld seam area, and weld seam coarse grinding is carried out after correction and compensation; weld seam contour information after coarse grinding is obtained by scanning again after the coarse grinding is completed; process parameters of the grinding are controlled according to an actual weld seam contour, weld seam fine grinding is carried out, and shape of an inner weld seam contour is trimmed to complete the conformal processing; and the method comprises the following specific steps:
step 1: first weld seam contour measurement and feature identification
putting a robot in a cylindrical shell, stopping the robot after the robot is judged to reach a weld seam according to a real-time image collected by a wide-angle camera, and using a laser scanner to measure an annular weld seam area in the cylindrical shell; matching measurement data of the laser scanner with position information of each kinematic axis of a three-axis drive mechanism to obtain point cloud data of a contour of a cylindrical shell inner weld seam area; representing the obtained point cloud data as N generatrices (K1, K2, . . . KN) in an X-R plane in a cylindrical coordinate system (X-R-A coordinate system); then carrying out weld seam feature identification to each generatrix in the X-R plane; taking the Mth generatrix KM as an example, selecting the leftmost m points [(x1, r1), (x2, r2) . . . (xm, rm)] within the range of the generatrix as initial points, and fitting a left inner wall contour line according to formula (1); starting from the (u=m+1)th leftmost point, calculating a distance dul from each point to the left inner wall contour line in sequence from left to right according to formula (2);
 where, (xi, ri) is a coordinate of a point involved in fitting, and n is the number of points involved in fitting; x is an x average value of the points involved in fitting; r is an r average value of the points involved in fitting; and r=ax+b is the fitted inner wall contour line;
 where, (xi, ri) is a point coordinate; and r=ax+b is a straight-line formula;
setting a threshold d0l for judgement: if dul≤d0l, it is judged that a point u is still an inner wall contour point, re-fitting the left inner wall contour line by the leftmost u points, and letting u=u+1 to continue the judgement of a next data point; if dul>d0l, and considering an error in identification, an actual weld seam feature point should be on the left side of point u; setting a compensation amount v (which is an integer of 5-12) according to the actual condition of the weld seam to carry out compensation, then the (u-v)th point is judged as a weld seam left feature point PML, the judgement cycle is ended, and a final left inner wall contour line is lML; similarly, fitting a right inner wall contour line by the rightmost points, and judging each point from right to left to obtain a weld seam right feature point PMR and a final right inner wall contour line lMR; taking a middle point PMC of the left and right feature points as a weld seam center feature point of the generatrices KM, thus to obtain left feature points (P1L, P2L, . . . PNL), right feature points (P1R, P2R, . . . PNR), center feature points (P1C, P2C, . . . PNC), left inner wall contour lines (l1L, l2L, . . . lNL) and right inner wall contour lines (l1R, l2R, . . . lNR) of the generatrices;
step 2: filtering out of weld seam feature misidentified generatrices caused by debris and oil stains
completing weld seam feature point identification of each generatrix in sequence, and filtering out weld seam feature misidentified generatrices; taking the Mth generatrix KM as an example, and obtaining weld seam width by calculation from left and right weld seam feature points PML (xPML, rPML) and PMR (xPMR, rPMR) of the generatrix according to formula (3);
wM=xPMR−xPML (3)
setting the minimum weld width wmin and the maximum weld width wmax according to actual inner weld seam features, and judging generatrices with wM>wmax and wM<wmin as weld seam width identification abnormal generatrices;
in a rectangular coordinate system, fitting a plane LT by the weld seam center feature points (P1C, P2C, . . . PNC) of various generatrices according to formula (4), and calculating a distance dMf from a weld seam center feature point PMC of the generatrix KM to the plane LT according to formula (5);
 where, Ax+By+Cz+D=0 is a plane generated by fitting; n is the quantity of the points involved in fitting; and (xi, yi, zi) is a three-dimensional coordinate of a point involved in fitting;
 where, df is a distance from a point to a plane; (xi, yi, zi) is a point coordinate; and Ax+By+Cz+D=0 is a plane formula;
setting a threshold for a distance d0f from a weld seam center point to a fitted plane, and judging a generatrix with dMf>d0f as a weld seam position identification abnormal generatrix; filtering out weld seam feature misidentified generatrices by the above two judgement methods, with the remaining Z qualified generatrices being (K1′, K2′, . . . KZ′);
step 3: obtaining of maximum boundary of weld seam
fitting a weld seam center plane Lc by inner weld seam center feature points (P1C′, P2C′, . . . PZC′) of the remaining Z qualified generatrices (K1′, K2′, . . . KZ′), and calculating distances (d1L′, d2L′, . . . dZL′) from left feature points (P1L′, P2L′, . . . PZL′) of the qualified generatrices to the weld seam center plane Lc according to formula (5); taking the maximum distance dL′=max(d1L′, d2L′, . . . dZL′), and generating a parallel surface Ll at a distance dL′ from the left side of the weld seam center plane Lc, namely a weld seam left boundary plane; obtaining a weld seam right boundary plane Lr in a similar way; finally, in a three-dimensional coordinate system, calculating intersection points of the left inner wall contour lines (l1L, l2L, . . . lNL) and the weld seam left boundary plane Ll in all of the Ngeneratrices (K1, K2, . . . KN), namely weld seam left boundary points (T1L, T2L, . . . TNL) of the generatrices; obtaining weld seam right boundary points (T1R, T2R, . . . TNR) of the generatrices in a similar way; thus, obtaining of maximum boundary of inner weld seam is completed;
step 4: ideal weld seam contour conformal generation and coarse grinding
after obtaining of maximum boundary of weld seam is completed, conformally generating an ideal weld seam contour, and carrying out coarse processing; taking the Mth generatrix KM as an example, and calculating a middle position xTMC=(xTML+xTMR)/2 of a left boundary point TML (xTML, rTML) and a right boundary point TMR (xTMR, rTMR); calculating a point (xTMC, r1) of the left inner wall contour line lML at the middle position and a point (xTMC, r2) of the right inner wall contour line lMR at the middle position; considering the influence of seam misalignment during cylindrical shell welding, and comparing the point heights r1 and r2 the left and right inner wall contour lines lML and lMR at the middle position; if r1>r2, it is judged that the left inner wall is higher than the right inner wall in the generatrix, and the coordinate of a middle point TMC is (xTMC, r1); if r1≤r2, it is judged that the right inner wall is higher than the left inner wall in the generatrix, and the coordinate of the middle point TMC is (xTMC, r2); connecting the three points TML, TMC and TMR, wherein lM1 is a connecting line between the point TML and the point TMC, lM2 is a connecting line between the point TMC and the point TMR, and lM1 and lM2 form the ideal weld seam contour of the generatrix, which is represented by formula (6);
 where, r=aM1x+bM1 and rM2=aM2x+bM2 are respectively expressions of lM1 and lM2; and xTMC is the middle position of the left and right boundaries;
due to errors in robot processing, and in order to avoid damage to the inner wall, generating a processing contour parallel to the ideal weld seam contour at a certain safe distance δ upward; discretizing the processing contour into processing points, generating a G code, and carrying out coarse grinding to a cylindrical shell inner weld seam;
step 5: second weld seam contour measurement and fine grinding parameter control
after the coarse grinding is completed, using the laser scanner (3) to scan again, and changing the dwell time of a flap wheel to realize grinding amount control of each part of the weld seam and realize weld seam fine grinding; first, obtaining contours after coarse grinding of generatrices (K1, K2, . . . KN) by laser scanner scanning; generating a grinding track of the flap wheel 7 according to ideal weld seam middle points (T1C, T2C, . . . TNC) of various generatrices in a three-dimensional space;
taking the Mth generatrix KM as an example, setting a variable contour difference ξM to represent the difference between the weld seam contour after coarse grinding and the ideal weld seam contour of the generatrix KM, and calculating a distance dMi from each point in the weld seam area to an ideal weld seam according to formula (7);
 where, ξM represents a weld seam contour difference of the generatrix KM; n represents the quantity of points in the weld seam area of the generatrix; and dMi represents a distance from an actual weld seam contour point to the ideal weld seam contour;
as the flap wheel is flexible, when used for processing the generatrix KM, the flap wheel will also have a grinding influence on the area around the generatrix KM; therefore, a grinding weight μi is introduced to represent the grinding influence of the flap wheel on the weld beam of various generatrices when used for grinding at the generatrix KM; when the flap wheel is used for grinding at the generatrix KM, the generatrix KM is most influenced by cutting, the value of corresponding μ0 is the largest, and the weight μi of each nearby generatrix is determined according to the degree of influence; obtaining a weld seam grinding dwell time coefficient ψM at KM by weighted calculation of the weld seam contour difference ξi of various generatrices according to formula (8):
 where, ψM represents the grinding dwell time coefficient at the generatrix KM; μi represents the grinding weight; and ξi represents the weld seam contour difference of the generatrix KM;
finally, obtaining the dwell time tM of flap wheel grinding at the generatrix KM according to formula (9);
tM=ψMt0 (9)
where, t0 represents unit dwell time; ψM represents the dwell time coefficient; and tM represents the dwell time at the generatrix KM;
similarly, calculating the dwell time (t1, t2, . . . tN) at the generatrices (K1, K2, . . . KN), generating a G code, and carrying out fine grinding to the weld seam area to complete the conformal processing of the cylindrical shell inner weld seam.
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