Adaptive Control of Industrial Robots for Arc Welding

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ADAPTIVE CONTROL OF INDUSTRIAL ROBOTS FOR ARC WELDING D. N. Karastojanov and G. N. Nachev 111.\/;11111' 0/ ' ,,(/11.\(1"/(/1 (.\-/)('nll,tin (filii R o/wlin (fICR J. Hulg(ll"j(w :1.("(1(11'111.'" vf" Snf'1IC1's .

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ABSTRACT, THE PAPER EXAMINES THE PROBLEM OF ADAPTATION TO INEXACT PREPARED OR INACCURATE POSITIONED PARTS WHEN WELDING BY INDUSTRIAL ROBOTS, AN IDEA FOR ADAPTIVE CONTROL IS PRESENTED, BASED ON THE PARTS DEVIATION FROM THEIR PRELIMINARY TAUGHT MODEL, AN EXAMPLE OF THE MODEL IS DESCRIBED FOR THE CASE OF STRIGHTFORWARD JOINT, TAKING INTO ACCOUNT THE WELDING TORCH POSITION AND THE SENSOR PARAMETERS, RESULTS FROM EXPERIMENTS MADE WITH THE ARC WELDING ROBOT RB 251 ARE PRESENTED, SOME NOICE AND ACCURACY PROBLEMS ARE DISCUSSED, AND SUGGESTIONS FOR FURTHER DEVELOPMENTS ARE GIVEN, KEYWORDS , ADAPTIVE CONTROL; ROBOTS; WELDING; SENSORS; MODELS,

INTRODUCTIOr~

VELY IT IS NECESSARY TO INTRODUCE AN ADAPTIVE CONTROL, WHICH CAN REACT TO PREVIOUSLY UNKNOWN DEVIATIONS, (KUROPATKIN, 1980),

THE TEACHING OF INDU STRIAL ROBOTS INCLUDES A DESCRIPTION OF THE DESIRED PATH AS A SEQUENCE OF POINTS AND INDICATION OF THE TYPE OF MOVEMENT BETWEEN THEM (STRAIGHT LINE, CIRCLE ARC), AS WELL AS A DESCRIPTION OF THE TECHNOLOGICAL PROCESS BY MEANS OF ITS PARAMETERS (IN THE CASE OF ARC WELDING THEY ARE VO LTAGE AND CURRENT), AN OPTIMAL CONTROL WHICH REALISES THE DESIGNED IN THIS WAY TAS K, IS BASED ON SEVERAL CRITERIA (E, G, ACCURACY, SPEED, EXPENSES),

REALISATION PRINCIPES OF ADAPTIVE BEHAVIOR THE STRUCTURE AND THE TYPE OF FUNCTIONING OF A COMPLEX SYSTEM SUCH AS AN INDUSTRIAL ROBOT, DEPENDS ON A NUMBER OF VARIABLES WHICH ARE THE RESULT OF DECISION MADE AT DIFFERENT TIMES BY PEOPLE WITH VARIOUS PROFESSIONS, IDEAS AND GOALS, DURING THE ACTUAL EXECUTION OF CONCRETE MOVING TASKS, THE QUALITY OF THE INDUSTRIAL ROBOT PERFORMANCE IS ALSO A SUBJECT TO VARIATY OF GEOMETRICAL, TECHNOLOGICAL AND OTHER FACTORS, SOME OF THE PHASES IN THE ROBOT DESIGN WHICH HAVE AN INFLUENCE ON THE SYSTEM AS A WHOLE, ARE SHOWN AT FIG, 1 IN THE FORM OF A GRAPH, IN TABLE 1 THEIR MEANINGS ARE INDICATED, AND THE FACTORS WITH MAJOR INFLUENCE ON THE QUALITY OF THE SEAM (THE LOWEST LEVEL IN THE GRAPH AT FIG, 1) ARE GIVEN AS WELL.

IT IS WELL KNOWN HOWEVER, THAT DUE TO MANUFACTURING TOLERANCES, INADEQUATE FIXTURING, HEAT DEFORMATIONS AND PROGRAMMING DEVIATIONS FROM THE TAUGHT-IN MODEL MAY OCCUR, IN THIS CASE THE ACCURATE POSITIONING OF THE TORCH ACCORDING TO THE TAUGHTIN PATH OR WELDING IN THE PROGRAMMED MODE ARE NOT SUFFICIENT ENOUGH, IF THE DEVIATIONS ARE WITHIN A SMALL RANGE FOR THE DEFINITE SYSTEM (E, G, COMPATIBLE WIT H THE DIAMETER OF THE WELDING WIRE) THE CONTROL SYS TEM IS PASSIVELY ADA PTING AND THE QUALIT Y OF WELDING IS GOOD ENOUGH, ALTERNATI-

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D. N. Karastojanov and G. N. Nachev

TABLE 1 DECISION, MADE DURING DEVELOPMENT AND PRODUCTION OF WELDED PRODUCTS

I. 2. 3.

4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14.

15. 16. 17. 18.

19. 20. 21.

BASIC MATERIALS, USED IN THE PRODUCTS THICKNESS OF THE PARTS DIMENSIONS AND WEIGHT OF THE PARTS AND ASSEMBLIES SHAPE (CONFIGURATION) OF THE ASSEMBLIES SPECIFICATIONS OF THE WELDS: STRENGHT, APPEARANCE ETC WELDING PROCESS TO BE APPLIED COMPOSITION OF ELECTRODE WIRE COMPOSITION OF PROTECTIVE GAS WELDING CURRENT-AC, DC AND POLARITY STATIC AND DYNAMIC CHARACTERISTICS OF THE ARC POWER SUPPLY WIRE DIAMETER EDGE PREPARATION TOLERANCES OF THE PARTS STATIC AND DYNAMIC REPEATABILITY OF THE ROBOT'S MANIPULATING SYSTEMS FIXTURING TOLERANCES ORIENTATION OF THE JOINT DURING WELDING (RELATIVE TO GRAVITY) ANGLE BETWEEN THE TORCH AXIS AND THE JOINT WEAVING PARAMETERS: AMPLITUDE, FREQUENCE, SHAPE ARC VOLTAGE ARC CURRENT VELOCITY OF THE TORCH MOVEMENT ALONG THE JOINT

FACTORS, WHICH INTERVENCE DURING WELDING

22.

23.

24. 25. 26.

27. 28.

DEVIATION OF THE JOINT LINE FROM IT'S PREPROGRAMMED PLACE, RESULTING FROM PART'S DISTORTOINS, INEXACT FIXTURING, DEVIATIONS OF THE ROBOT'S MANIPULATING SYSTEMS ETC DISTORTIONS OF THE JOINT, RESULTING FROM THE TERMAL INFLUENCE LATTERAL COMPONENT OF THE DEVIATION CHANGE OF THE GAP BETWEEN THE TWO PARTS CHANGE OF THE DISTANCE BETWEEN THE TORCH AND THE PARTS (LONGITUDINAL COMPONENT OF THE DEVIATION) CHANGE OF THE ARC VOLTAGE WITH RESPECT TO IT'S PREPROGRAMMED VALUE CHANGE OF THE ARC CURRENT WITH RESPECT TO IT'S PREPROGRAMMED VALUE

THE MOVEMENT OF THE TORCH ALONG A PATH IS REALISED BY SIMULTANEOUS CONTROL OF ALL JOINTS WITH PREDETERMINED SPEED AND DIRECTION. ANY CHANGE IN THE SPEED AND DIRECTION OF JOINT MOVEMENT LEADS TO DEVIATION FROM THE PREPROGRAMMED TRAJECTORY. THUS THE ADAPTATION TO THE GEOMETRICAL DEVIATIONS IS REDUCED TO A SEQUENCE OF DECISION ABOUT THE TYPE, SIZE AND DIRECTION OF THE JOINT-MOVEMENT CORRECTIONS, MADE ON THE BASIS OF INFORMATION FROM THE ENVIRONMENT AND AIMED AT COMPENSATING INACCURACIES OF A KNOWN CHARACTER, (KARASTOJANOV AND BoNEV, 1981). ON THE BASIS OF SEQUENTIAL AND LOGICAL CONNECTIONS, THE GENERAL TASK FOR ADAPTIVE BEHAVIOR OF AN INDUSTRIAL ROBOT CAN BE DECOMPOSED INTO THE FOLLOWING SUCCESSIVELY SOLVED SUBTASKS, (SARIDIS, 1977): - INFORMATION GATHERING FROM THE ENVIRONMENT - INFORMATION PROCESSING AND DECISION-MAKING - REALISATION OF THE CORRECTIVE ACTION. THE SENSOR INFORMATION GATHERING FROM THE ENVIRONMENT IS BASED ON MEASUREMENT OF AN OBSERVABLE PHYSICAL PHENOMENON (CF TABLE 2). THE ARC ITSELF CAN SERVE DIRECTLY AS A SIGNAL SOURCE (D) OR SOME INDIRECT (I) SOURCES CAN BE USED. FOR CONVENIENCE THE RESULT OF THE MEASUREMENT IS CATEGORIZED AS: - RATIO BETWEEN SIGNALS - DISTANCE LESS THAN 10 MM - DISTANCE MORE THAN 10 MM. IT IS OBVIOUS THAT NOT ALL COMBINATIONS FROM THESE THREE CHARACTERISTICS CAN BE PRACTICALLY REALISED OR APPLIED IN ARC WELDING. THE IDEAL SENSOR SHOULD BE ABLE TO: - MEASURE GEOMETRICAL VALUES (ANGLES, DIRECTIONS, DISTANCES ETS) CHARACTERISTIC FOR THE WELDED PARTS AND THE TORCH - BE COMPATIBLE WITH THE ARC - BE COMPATIBLE WITH WEAVING - NOT INCREASE CONSIDERABLY THE TORCH DIMENSIONS - FUNCTION WITHOUT ADDITIONAL DEGREES OF

Control of Industrial Robots for Arc We ldin g

FREEDOM - FUNCTION WITH DIFFERENT, POSSIBLY ALL, CONFIGURATIONS OF THE JOINT AREA - BE COMPATIBLE WITH OTHER METHODS FOR INFORMATION GATHERING (ALLOWS TO GET MORE RELIABLE RESULTS) - BE RELIABLE, IICR, SOFIA DEVELOPED A SENSOR FOR INFORMATION GATHERING DURING WELDING BASED ON A LASER RANGE FINDER, SCANNING THE WELDED PART'S SURFACE NEAR THE WELDING TORCH, THE DEVICE IS COMPACT AND COMPARATIVELY CHEAP, (FIG, 2A), IT CAN BE MOUNTED REMOTELY ON THE TORCH AND THE EMITTING RANGE IS CHOSEN NOT TO BE INFLUENCED BY THE ARC DISTURBANCES, THUS USING A LOW POWER LASER EMITTER DISTANCES WITHIN 50-300 MM CAN BE MEASURED ACCURATELY ENOUGH, (FIG, 2B), THE NEW SENSOR ALLOWS TO CALCULATE: - LATTERAL DEVIATION OF THE TORCH AXIS FROM THE JOINT LINE - DISTANCE BETWEEN THE CONTACT TIP AND THE JOINT LINE - DISTANCE BETWEEN THE TWO WELDED PARTS - MISALIGNEMENT OF THE EDGES OF THE TWO PARTS - VOLUME OF THE EDGE PREPARATION (IF ANY),

ADAPTIVE CONTROL

FOR THE PROPER PROCESSING OF THE SENSOR DATA INFORMATION ABOUT THE ACTUAL POSITION AND ORIENTATION OF THE TORCH TIP IS NEEDED, THE PROCESSING OF INFORMATION RECEIVED DURING THE PROGRAM EXECUTION DEPENDS ON THE CHARACTERISTICS OF THE OF THE ENVIRONMENT AND THE SENSOR, (FIG, 2c), WE ASSUME, THAT THE CHARACTERISTICS PARAMETERS, DESCRIBING THE SENSOR REALISATION ARE PREVIOUSLY KNOWN, AND IT WILL BE NECESSARY TO BUILD AN ENVIRONMENTAL MODEL DURING THE TEACH-IN, IN PARTICULAR BY USING THE SAME OBSERVATION MECHANISM AS DURING PROGRAMM EXECUTION, THE TASK FOR DETERMINING THE MODEL PARAMETERS REPRESENTS AN OBJECT IDENTIFICATION PROCEDURE BASED ON A KNOWN OBJECT CLASS TYPE,

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BASED ON THE MODEL, IT IS POSSIBLE TO GENERATE THE EXPECTED VALUE OF THE SIGNAL FOR EACH MEASUREMENT POINT IN EXECUTION MODE, DUE TO CHANGES IN THE ENVIRONMENTAL PARAMETERS FOR THE SPECIFIC PART, THE ACTUAL VALUE OF THE SIGNAL MAY VARY WITHIN A SERTAIN RANGE, AFTER SOME PRIMARY FILTRATION AND PROCESSING OF THE SENSOR DATA, THE FOLLOWING COMPONENTS IN PART DEVIATION CAN BE POINTED OUT, DEPENDING ON THE MODIFIED PARAMETER: - ANGLE BETWEEN THE TORCH AND THE LEFTHAND PLANE - ANGLE BETWEEN THE TORCH AND THE RIGHTHAND PLANE - ANGLE BETWEEN THE TORCH AXIS AND THE JOINT LINE - LATTERAL DISPLACEMENT OF THE TORCH AXIS WITH RESPECT TO THE JOINT LINE - DISTANCE BETWEEN CONTACT TIP AND JOINT LINE - DISTANCE (GAP) BETWEEN THE TWO PARTS TO BE WELDED, AND MISALIGNEMENT OF THE EDGES - VOLUME OF THE EDGE PREPARATION, THIS DATA IS THEN SENT TO THE UPPERMOST LEVEL OF THE CONTROL SYSTEM'S HIERARCHY, WHICH CHANGES THE PROGRAMME ACCORDINGLY, FIG 3 SHOWS A FLOW-CHART OF AN ADAPTIVE SUBSYSTEM FOR SEAM-TRACKING IN WELDING ROBOTS, THE DECISION MAKING ABOUT THE TYPE OF THE PART DEVIATION FROM ITS TAUGHT-IN POSITION IS REALISED ACCORDING TO THE COMBINATION OF DEVIATION COMPONENTS FOR EACH OF THE TWO PARTS IN THE BASIC COORDINATE SYSTEM OF THE ROBOT, THE ROBOr THE FIRST ROBOT TO MAKE USE OF THE UNIQUE FEATURES OF THE NEW SENSOR IS ISOTHRON RB 251 - A CARTESIAN COORDINATES ROBOT, HAVING A 5 AXIS ON THE TORCH MANIPULATOR AND UP TO 4 SERVO-CONTROLLED AXIS FOR PART ORIENTATION (FIG, 4), ITS WORKING SPACE IS 2500 / 1250 / 630, AND NC-LANGUAGE IS USED FOR PROGRAMMING, TEACHING THE ADAPTIVE WELDER IS VERY EASY, WHEN SHWOWING TO IT THE SUCCESSIVE POINTS, CHARACTERIZING THE PATH (I, E, THE BEGINNING AND THE END OF A STRAIGHT LINE) THE SENSOR MEASURES THE GE-

D. N. Karastojanov and C. N. Nachev

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OMETRY OF THE PARTS IN THE NEIGHBOURHOOD OF THE CORRESPONDING POINT, DETERMINES THE ABOVE-MENTIONED PARAMETERS, AND STORES OF THEM TOGETHER WITH THE COORDINATES OF THAT POINT. DURING PROGRAMME EXECUTION THE ROBOT BRINGS THE TORCH AT SOME DISTANCE BACK FROM THE JOINT LINE (TIPICALLY 15 MM), MEASURES AND DETERMINES THE ACTUAL PARAMETERS OF THE JOINT, COMPARES THEM WITH THE STORED ONES, AND FIRMLY APPROACHES THE BEGINNING OF THE SEAM. WHEN WELDING ALL MEASUREMENTS ARE REPEATED AT A RATE ABOUT 3 TIMES PER SECOND, WHICH ALLOWS CONTINUOUS UPDATING OF THE PROGRAMME, AND HENCE SEAM FOLLOWI NG.

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CONCEPTUAL --\

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DESIGN

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COIKLUSION

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PROD. I 13 ORGAN I - I SA T! ON I

THE ADAPTATION TO INACCURATELLY PRODUCED OR POSITIONED PARTS IN ARC WELDING ROBOTS INCREASES CONSIDERABLY THEIR CAPABILITIES AND THE QUALITY OF WELDS. THE DESCRIBED STRATEGY ALLOWS OUR ROBOT TO ADAPT TO ANY KIND OF JOINTS: CORNER, LAP, T END EDGE (EVE N WITHOUT EDGE PREPARATION), DEVIATING FROM THE PROGRAMMED POSITION UP TO 10 MM IN ANY DIRECTION.

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REFEREIKES

KARASTOJANOV, D. N., AND N. K. BONEV (1981). GEOMETRIC ADAPTATION IN THE CONTROL OF ARC WELDING ROBOT. AUTOMATIC AND COMPUTER DESIGN, VOL. 4, SOFI~ PP 49-53

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KUROPATKIN, P. (1980). OPTIMALE AND ADAPTI VE CONTROL, VIYSHA SCO LA, MOSKOW.

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SARIDIS, G. (1977). SELF-ORGANIZING CONTROL OF STOCHASTIC SYSTEMS, MARSEL DEKKER, NEW YORK.

FEEDBACK FIG. 1

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