Abstract:
A method for performing multi-layer welding using a welding robot is disclosed, in which welding on the second and subsequent layers is carried out by backwardly displacing the position of a torch tip by the amount of a preset value. This displacement is made possible through making a constant determined by a torch shape increase by the preset value.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and device for tracer correction in a welding robot. 
     When welding large structures by a welding robot of the simple play-back system in which the robot merely repeats the line of actions as taught and welding is performed at a low tacking accuracy, it is frequently the case that there is around 5 to 10 mm error between the taught weld line and the weld line of the next work, hence it is difficult to make satisfactory welding. 
     To compensate for this drawback, it is practiced to cause the robot to follow in on-line tracing the actual weld line by detecting torch tip error against a weld line through the monitoring of welding current or by detecting the torch tip error against the weld line through the detection of arc produced by welding by means of a TV camera installed to the welding torch. 
     The coordinate system for detecting the error differs according to the type of error detector (hereinafter referred to as a sensor) being employed. For example, as shown in FIG. 1(a), a sensor for monitoring welding current detects the error in the A--A&#39; and B--B&#39; directions which are respectively parallel and orthogonal with respect to a torch 3, while a sensor employing a TV camera detects the torch tip error in the C--C&#39; and D--D&#39; directions which are inclined by 45 degrees with respect to the torch 3 as shown in FIG. 2, thus the coordinate systems for detecting the torch tip error differ one from the other. The coordinate system of FIG. 1 and that of FIG. 2 are hereafter referred to as the U coordinate system and the V coordinate system for the purpose of explanation. In prior art, it has been common to make the robot control axis correspond to the detection coordinate system of a sensor being used to cope with the difference in the detection coordinate systems for each type of the sensor. 
     However, with robots having such control axis, weldable works are unaviodably limited, and welding narrow parts such as the inside of a box is difficult. A device has been proposed to enable welding of narrow parts in which an arm rotatable in the horizontal and vertical planes as shown in FIG. 3(a) and (b) is provided between a robot body 5 and the torch 3. 
     It should be noted here that the detection coordinate system of the sensor installed to the welding torch is based on the torch while the torch posture is based on the coordinate system of the robot control axis. As a result, the two coordinate systems differ from each other. Particularly when a robot performs welding while changing the posture of a wrist 4 as shown in FIG. 4, a correcting operation, that is, converting the torch tip error detected by the senser into the robot displacement has become necessary. 
     In addition, when groove forms vary as shown in FIG. 5(a) through FIG. 5(d), welding conditions such as torch movement vary accordingly, and in many cases a sensor that can be used varies. In such cases, it is preferable to select and install to the torch 3 a sensor optimum to individual weldment. Prior art robot controllers have failed to make the above correcting operation in response to each type of sensor. 
     SUMMARY OF THE INVENTION 
     Accordingly, this invention is directed to solve the above problems. 
     An object of this invention is to provide a tracer correction device for welding robots, in which specified coefficient values are sequentially read out from a memory storing coefficient values corresponding to the coordinate system of the sensor used according to the current posture of the arm, the coefficient value thus read out is multiplied by the torch tip error detected by the sensor, the resultant value is converted to the displacement values of basic three axes, X,Y, and Z of the robot, and the converted values (hereinafter referred to as correction values) are applied to a robot controller for correcting the displacement values of the above three axes, this invention will be described in detail based on an embodyment of the invention with reference to attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIGS. 1 and 2 are explanatory illustration of welding torch movements, 
     FIG. 3(a), and FIG. 3(b) are illustrations showing the wrist used in the welding robot, 
     FIG. 4 is an illustration of arm movement during the tracer control, 
     FIG. 5(a) through FIG. 5(d) are drawings showing various groove forms, and 
     FIG. 6 is a block diagram of an embodiment of tracer correction device according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 3(a) and FIG. 3(b), a robot associated with a correction device according to this invention is one that includes a wrist 4 having a certain degree of freedom. An arm 5a of a robot body 5 is controlled so as to move in the axial directions X, Y, and Z, and at the front end of the arm 5a, the wrist 4 is installed. This wrist 4 is controlled so as to rotate by predetermined angles θ and φ within the horizontal and vertical planes respectively, to the front end 4a of the wrist 4 being installed a welding torch 3. Accordingly the movements of the torch 3 consist of movements in the direction of X, Y, and Z axes (basic three axes of the robot) and rotations θ and φ of the wrist 4 on the horizontal and vertical planes. A robot controller 6 is for controlling movement in X-, Y- and Z-axis directions of the arm 5a of the robot body 5 and controlling rotation of the wrist 4 on the horizontal and vertical planes. The robot controller 6 memorizes a plural number of teaching points, i.e., posture data Dx, Dy, and Dz of the arm 5a and posture data Dθ and Dφ of the wrist 4, through teaching, and sequentially outputs control signals ex, ey, ex, eθ, and eφ (FIG. 6) for controlling axial movements of of the robot and rotation angle θ and φ of the wrist 4 based on the posture data Dx, Dy, Dz, Dθ, and Dφ corresponding to the teaching points during tracer control, i.e., during actual welding. The arm 5a is controlled in the X-, Y- and Z-axis directions according to control signals ex, ey, and ez, while the wrist 4 is controlled for proper rotation according to control signals eθ and eφ. 
     Referring to FIG. 6, a correction device according to this invention is connected to the robot controller 6, and generates signals corresponding to the current posture of the wrist 4, signal S indicating the type of sensor used, and correction signals Δx, Δy and Δz corresponding to control signals ex, ey and ex from the robot controller 6 outputted based on signal Ds corresponding to the torch tip error against a weld line L as detected by a sensor. FIG. 6 shows the arrangement which can deal with two types of sensors. A tracer correction device 10 comprises memories 11 and 12 for storing correction coefficient values for individual coordinate system, a selector 13 for selecting coefficient values corresponding to the sensor used out of the coefficient values read out from memories 11 and 12 and outputting the selected values, and a arithmetic circuit 14 for converting the coefficient values thus selected to the basic three axial displacements of the robot body 5 by multiplying these values by the value of the torch tip error sent from the sensor. The memory 11 stores, for example, correction coefficient values Ku (Kux, Kuy, Kuz) for converting the value of the torch tip error as detected by the above mentioned U coordinate system (FIG. 1(a)) into a value corresponding to each of X, Y, and Z axes of the robot. These coefficient values are sequentially stored for every predetermined rotation angle, for example, for every degree while taking posture θ and φ of the wrist 4 as parameters. In tracer control, in response to the current posture of the wrist 4 θ and φ these memories 11 and 12 sequentially output Ku (Kux, Kuy, Kuz) and Kv (Kvx, Kvy, Kvz). 
     In addition, the robot controller 6 outputs control signal ex, ey, ez eθ and eφ corresponding to data Dx, Dy, Dz, Dθ and Dφ memorized in the teaching stage during the tracer control, drives the arm 5a and the wrist 4 of the robot body 5, and controls the torch 3. The sensor outputs signal S indicating the type of sensor and signal Ds corresponding to the value of torch tip error against the weld line L, and applies the signal S to the selector 13, the signal Ds to the arithmetic circuit 14. To the memories 11 and 12 are sequentially applied signals corresponding to the current posture θ and φ of the wrist 4 and the corresponding coefficients Ku (Kux, Kuy, Kuz) and Kv (Kvx, Kvy, Kvz) are sequentially outputted and applied to the selector 13. The selector 13 selects either of the coefficient values Ku and Kv, and applies the coefficient value thus selected to the arithmetic circuit 14. The arithmetic circuit 14 outputs the correction value Δ (Δx, Δy, Δz) on the basis of the selected coefficient value and current error value Ds of the torch 3 from the weld line. The correction value Δ(Δx, Δy, Δz) is a correction value resulted from the conversion of Ds of the torch 3 into the displacement of the robot body 5 in X-, Y- and Z-axis directions. This correction value Δ(Δx, Δy, Δz) is applied to the robot controller 6. The robot controller corrects control signals ex, ey, and ez according to the correction value C, outputting the signals thus corrected as signals ex&#39;, ey&#39; and ez&#39;. Then, the robot controlled 6 controls the posture of arm 5a based on these corrected control signals ex&#39;, ex&#39;, and ex&#39;, and also controls the posture of the torch 3. 
     To make the above description clearer and more concrete, description will be made on the operations of the memory 12 and the arithmetic circuit 14 in the case that sensor associated with the V coordinate system is employed. The V coordinate system, as described above, is a coordinate system which is fixed to the torch 3 and in this system, detection of the torch tip error is made in the directions inclined by 45 degrees with respect to the torch 3, that is, directions C--C&#39; and D--D&#39; in FIG. 2. If c and d represent components of the detected values in C--C&#39; and D--D&#39; directions, and Δx, Δy and Δz represent X-, Y- and Z-axis components of the correction value Δ, then 
     
         Δx=c sin (45°-φ) sin θ+d cos (45°-φ) cos θ                                                   (1) 
    
     
         Δy=c sin (45°-φ) sin θ+d cos (45°-φ) sin θ                                                   (2) 
    
     
         Δz=c cos (45°-φ)+d sin (45°-φ) (3) 
    
     In the memory 12, which corresponds to the V coordinate system, there are being stored sets of six values; these values are obtained, for each φ and θ, from sin (45°-φ) sin θ and cos (45°-φ) cos θ for the coefficient KVx, from sin (45°-φ) sin θ and cos (45°-φ) sin θ for the coefficient KVy, and from cos (45°-φ) and sin (45°-φ) for the coefficient KVz. In response to input signals corresponding to φ and θ as address signals, the memory 12 outputs the above six values corresponding to φ and θ. The signal Ds includes the value c and d detected in the directions of C--C&#39; and D--D&#39; in FIG. 2. The arithmetic circuit 14 performs arithmetic operation on Equations (1) through (3) to output the correction values Δx, Δy, and Δz in response to the input of c, d and the above six values. 
     In this way, the basic three axes of the robot body 5 are corrected according to the current posture of the wrist 4 and the type of the sensor, and the torch 3 is controlled so as to follow the weld line L. 
     Memories 11 and 12 need not necessarily be provided according to the type of the sensor. For example, an arrangement may be made in which a single memory data is provided to store coefficient values corresponding to the sensor used with the memory content being rewritten each time the sensor type is changed. 
     As described above, according to this invention, the tracer control to cause the torch to precisely follow the weld time becomes possible by applying correction value resulted from the conversion of the value of the torch tip error into the basic three axes displacement of the robot body based on the inputs of the current posture of the wrist, the coordinate systems of the sensor detection, and the value of the torch tip error.