METHOD AND APPARATUS FOR MULTI-LAYER BUILDUP WELDING

Abstract: Out of the welding operations carried out by a welding robot, a multi-layer buildup welding operation is a very important operation since it enables the strength of a weld zone to be secured but it has many technical difficulties. In order to carry out the first-layer welding according to the present invention, an approximate straight line or an approximate curve between taught points is determined on the basis of coordinates values through which a welding torch actually passes between the taught points, and points on the approximate straight line or approximate curve are computed on the basis of the distances of the taught points, each actual welding pass being then defined by these points. Regarding the loci in the welding from the second-layer welding onward, the direction and quantity of shift are determined by carrying out computation on the basis of the two points taught as the welding starting and ending points during the teaching for the first-layer welding and the two reference points designated for the purpose of defining the direction of shift. The multi-layer buildup welding is carried out after computing the loci of operations for the welding from the second-layer welding onward in this manner. In order to control the posture of the welding torch during the welding from the second-layer welding onward, the loci of operations therein are determined on the basis of the locus of operation of the welding torch during the first-layer welding and the angle of rotation given from the outside of the welding torch, and the welding torch is then turned by a given angle around a vector of direction of advance determined at a taught point, the posture data being computed. The welding from the second-layer welding onward is carried out on the basis of the posture data thus determined. Thus the first-layer welding and the subsequent welding runs, i.e. a multi-layer buildup welding can be carried out with a high accuracy owing to these operations.

Description: [TECHNICAL FIELD] The present invention relates to a method for multi-layer welding according to the preamble of claim 1 (US-A-4508953). [TECHNICAL BACKGROUND] Welding robots now come into wide use since they fulfill objectives such as labor saving, quality-enhancement or improvement in environmental conditions of welding work or the like, in place of conventional welding work relying upon experiences or intuition of an operator. Among welding operations performed by the welding robot, multi-layer welding is a very important operation for the reason such as securing a strength of a welded portion but has many technical difficulties. As one prior art is introduced in Japanese Patent Application Laid-Open (Kokai) No. 58(1983)-188572 (corresponding to U.S. Patent No. 4,508,953), in the multi-layer welding operation, generally, the first layer is welded according to the tracking function, and therefore, a deviation occurs between a teaching path and an actual welding path. Even if a teaching path is used as a reference path for determining paths for the second and succeeding layers, the tracking function is necessary for the second and succeeding layers similar to the first layer. However, in the second and succeeding layers, the first layer is already filled with bead, and therefore, a current variation cannot be obtained. Accordingly, groove position information is not known. For this reason, as a method which uses no tracking function for the second and succeeding layers, there is a method for storing an actual welding path when the first layer is welded and determining paths for the second and succeeding layers on the basis of the stored path. As this method, there is a method for corresponding the locus when the first layer is welded to instructed points of the teaching path to store it as a point having passed during the actual welding and generating an actual welding path at the stored point, as proposed in the aforesaid Japanese Patent Application Laid-Open (Kokai) No. 58(1983)-188572. However, in the case where generally, the tracking function is used, a welding torch follows an actual welding line with unevenness under the tracking conditions, and therefore, in the aforesaid method, in the case where the actual welding path is defined merely on the basis of the point having passed during actual welding, that is, in the case where the instructed points are defined by two passed points, said unevenness becomes included in data of the passed points, and as a result, an error occurs between original welding lines. More specifically, as shown in FIG. 1, an error occurs in the obtained actual welding path (indicated at the broken line between x and x) irrespective of presence or absence of a positional deviation between the instructed path and the actual welding line (a curve indicated by the solid line). Accordingly, if the actual welding path is defined by the conventional method to determine paths of the second and succeeding layers using the actual welding path as a reference, an error in the first layer becomes included in the final layer. [SUMMARY OF THE INVENTION] It is an object of the present invention to provide a procedure for performing multi-layer welding with high accuracy. More specifically, the object of the present invention is to use a calculation for preparing a locus of the first layer to minimize the aforementioned error thus performing multi-layer welding with high accuracy. For achieving the aforesaid object, according to the invention, multi-layer welding according to a path of welding defined by interpolating a plurality of instructed points by a suitable line is performed by storing N coordinates actually passed by a welding torch in the welding operation of the first layer at the instructed points, calculating and storing approximate straight lines and approximate curves using a method of least square at the instructed points on the basis of said stored N coordinates at the instructed points, and calculating points on each of the approximate straight lines or approximate curves on the basis of the distance of the instructed points to define each actual welding path by said points. In the present invention, the actual welding line is calculated in the following procedure. (1) N coordinates actually passed by a welding torch in the welding operation of the first layer at the instructed points are stored. (2) Approximate straight lines are calculated and stored using a method of least square on the basis of the N coordinates stored in (1) above. (3) Point on each approximate straight lines are calculated on the basis of the distance of the instructed points, these points being points used to define each actual welding path. In this manner, the welding path of the first layer as a reference of the actual welding path of the second and succeeding layers can be defined with high accuracy, and therefore, the lowering of the welding accuracy caused by accumulation of errors can be prevented. [BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is an explanatory view showing a deviation between an actual welding path and an instructed path in a conventional method and an actual welding path obtained by a method according to the invention. FIG. 2 is an explanatory view of a multi-layer welding. FIG. 3 is a block diagram showing an embodiment of the invention. FIG. 4 is a flowchart showing processing by a method of least square of the invention. FIG. 5 is a flowchart showing processing in a joint between steps. FIG. 6 is an explanatory view of a joint point. [BEST MODE FOR EMBODYING THE INVENTION] The present invention will be described in detail in conjunction with the embodiments. FIG. 3 is a control block diagram showing an example of configuration of a multi-layer welding apparatus according to the invention. The whole apparatus is divided into a human interface block 1 and a trajectory control block 2, both of which is coupled by a 2-port memory. The human interface block 1 has a CRT 12, an operating panel 13 and a teach box 14 connected thereto using a peripheral control section 11 as a control section. The trajectory control block 12 has a calculating section 22, a sensor control section 23, a servocontrol section 24 and an I/O control section 25 connected thereto using a motion control section 21 as a control section. Jobs registered from the teach box 14 or the operating panel 13 are stored in the 2-port memory 3, and when a start instruction is given to the robot, the designated job is read and executed by the motion control section 21. The sensor control section 23 is processed in synthronism with the execution. N coordinates are stored therein (first means). The calculation section 22 performs processing for calculating the coordinates of the robot and determining a welding line (second and third means). FIG. 4 shows a flowchart of a method of least square used in the present invention. Processing between steps is performed every control clock. Step 100: Initial Here, judgment is made of if it is a start of initial movement at the instructed points. Step 110: Preprocessing of calculation of interpolation Here, the divided number N is obtained as the value obtained by dividing the distance between two points by speed, and the count number k of a counter is reset to 0. Step 120: Calculation of sampling intervals for a method of least square Here, initialization of relevant data shown below is performed. Σt = 0, Σt² = 0 Σx = 0, Σy = 0, Σz = 0 Σxt = 0, Σyt = 0, Σzt = 0 Sampling count number sk = 0 Step 130: Here, "1" is added to the count number k to execute the thereafter processing. Step 140: Here, interpolation calculation processing is performed by use of a known procedure. Step 150: Here, fine corrected amounts ΔX, ΔY and ΔZ on a rectangular coordinate system are transmitted from a sensor, and control points are corrected thereby. At that time, as present values, x, y and z are used. Step 160: Sampling position Here, two points are subjected to sampling at equi-intervals according to the number of samplings. Step 170: Here, calculation of preprocessing is performed prior to calculation (Step 190) by a method of least square. Sampling points may be successively stored in a memory and finally the calculation by a method of least square for all may be performed. However, this step is used to perform calculation in order to save the memory and calculation time. That is, possible calculation is performed during sampling in order to prevent the calculation from concentration on a certain timing. "1" is added to the count value sk of a sampling counter. Σt = Σt + k/N, Σt² = Σt² + (k/N)² Σx = Σx + x, Σy = Σy + y, Σz = Σz + z, Σxt = Σxt + x × (k/N), Σyt = Σyt + y × (k/N), Σzt = Σzt + z × (k/N) Step 180: If the divided number N is smaller than the sample number, processing of step 190 is performed. Step 190: Here, a straight line (start point: x o , y o , z o and direction cosines: ℓ, m, n) is obtained by a method of least square. w k = sk × Σt² - (Σt)² a x = {(sk × Σx - Σx × Σt)/wk] *(k/N) a y = {(sk × Σy - Σy × Σt)/wk} *(k/N) a z = {(sk × Σz - Σz × Σt)/wk} *(k/N) D = √ a x ² + a y ² + a z ² ℓ = a x /D, m = a y /D, n = a z /D x₀ = x - a y , y₀ = y - a y , z₀ = z - a z From the above, the start point of the straight line and the direction cosines can be obtained. FIG. 5 shows a joint flow between steps. As for the joint flow, as shown in FIG. 6, when points P₀, P₁, P₂ are instructed and straight lines a and b are subjected to sensing operation, actual loci obtained by a method of least square is a' and b', in which case, it is necessary to obtain an actual point P₁' corresponding to the point P₁. This point P₁ is called the joint point between steps. A processing for obtaining the joint point is called the joint flow. Middle points of a common perpendicular line of two straight lines on the space are obtained. Middle points (p x , p y , p z ) of a straight line L₁ (x₁, y₁, z₁, ℓ₁, m₁, n₁) and a straight line L₂ (x₂, y₂, z₂, ℓ₂, m₂, n₂) are obtained by cosϑ = ℓ₁ℓ₂ + m₁m₂ + n₁n₂ sin²ϑ = 1 - cos²ϑ wherein ϑ is an angle formed between L₁ and L₂. D x = x₂ - x₁, D y = y₂ - y₁, D z = Z₂ - Z₁ R₁ = ℓ₁D x + m₁D y + n₁Dz R₂ = ℓ₂D x + m₂D y + n₂D z S = (R₁ - R₂cosϑ)/sin²ϑ T = (R₁cosϑ - R₂)/sinϑ P₁: a point of intersection between the straight line L₁ and the common perpendicular line P 1x = x₁ + S × ℓ₁ P 1y = y₁ + S × m₁ P 1z = z₁ + S × n₁ P₂: a point of intersection between the straight line L₂ and the common perpendicular line P 2x = x₂ + T × ℓ₂ P 2y = y₂ + T × m₂ P 2z = z₂ + T × n₂ Point P P x = (P 1x + P 2x )/2 P y = (P 1y + P 2y )/2 P z = (P 1z + P 2z )/2 From the above, a passage point corresponding to each instructed point actually moved is obtained, and the obtained point is shifted and operated to thereby render multi-layer welding possible. FIG. 1 shows comparison between the actual welding path obtained by the above-described method and a conventional method. As mentioned above, according to the multi-layer welding apparatus of the invention, in the multi-layer welding having the tracking function in the welding of the first layer, the coordinates in the number of N actually passed by a welding torch at the instructed points in the welding operation of the first layer are stored, approximate straight lines and approximate arcuate lines are calculated by a method of least square at the instructed points on the basis of the N coordinates, and the instructed points are interpolated by a suitable line with respect to coordinates obtained by adding a predetermined shift amount to said approximate coordinate value to determine a path of welding. Thereby, the actual welding path is obtained even if unstable factors in the tracking function occur, and therefore, calculation can be made on the basis of the actual welding path of the second and succeeding layers to determined a path of welding, thus obtaining excellent welding quality. It is to be noted that the tracking function of the second and succeeding layers is not necessary. [INDUSTRIAL APPLICABILITY] The present invention can be utilised in the field in which a thick plate or the like is subjected to multi-layer welding using a welding robot.

Claims: A method of executing multi-layer welding according to a path of welding defined by interpolating a plurality of instructed points (P₀, P₁, P₂) by a line (a, b), which method comprises: a first step of storing a number N of coordinates actually followed by a welding torch in the welding operation of a first layer at the instructed points (P₀, P₁, P₂,), a second step of calculating and storing coordinates of straight lines and curves (a', b') on the basis of said stored N coordinates, and a third step of determining a welding path of a second and succeeding layers by interpolation by a suitable line with respect to coordinates obtained by adding predetermined shift amounts to said coordinates stored in the second step at the instructed points (P₀, P₁, P₂) at the time of welding the second and succeeding layers, characterised in that said straight lines and curves (a', b') are approximated by using the method of least squares.

Inventors: NAKAZATO TATSUMI KOKURA PLANT, OKUMURA SHINJI KOKURA PLANT KA, TAKAOKA KEIICHI KOKURA PLANT K, NAKAZATO, TATSUMI KOKURA PLANT, KABUSHIKI K.Y.D.S., OKUMURA, SHINJI KOKURA PLANT, KABUSHIKI K.Y.D.S., TAKAOKA, KEIICHI KOKURA PLANT, KABUSHIKI K.Y.D.S.
Applicants: YASKAWA DENKI SEISAKUSHO KK, KABUSHIKI KAISHA YASKAWA DENKI SEISAKUSHO
Application year: 1990
Main classification: B23K9
IPC codes: G05B19, B23K9