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Sensor-based control systems for arc welding robots
R o b o ~ & Computer-~tegmted Manu~cturing, Vol. 2, No. 2, p& 125-133, 1985
0736-584~85 $3.00 + 0.00 P~gamon P ~ L~.
Palmed ~ ~ e U~.A. A~ ~g~s ~ e d
•
SENSOR-BASED CONTROL SYSTEMS FOR ARC WELDING ROBOTS L. M. SWEET Conu~ The~y and Sy~ems Program, Gen~M E l ~ c Company, Co~orMe R e h a s h and Dev~opmem, P.O. Box 43, Schenec~dy, NY, U.S.A. Arc w a l i n g ~ one ~ the mint impormm areas of appH~tion ~ r i n d u ~ d ~ robots. ~ m o ~ m a n u ~ u d n g ~mafiom, un~nfi~ ~ ~mem~m ~ ~e pa~, ~om~ ~ ~ e j ~ m , and the welding p r ~ e ~ ~ l f make the u ~ ~ ~ m o ~ ~ s t nti~ ~ mainlining weld quali~. ~ th~s paper ~ r ~ ~ p ~ ~ control s y s ~ s for arc welding robes ~e ~dbed: (D ~ n g ~ems ~r ~e~ng ~ e w e ~ pudd~ o v ~ ~ e j ~ , (~ w~d ~em ~n~s ~r m~ntai~ng ~oper ~am ~h and ~netration, and (3) s u p ~ s o ~ c o n ~ o ~ for ~quencing w ~ d ~ g o p ~ w fions. T h e ~ control ~ f i o m ha~ be~ imp~m~d m~s~lry ~ ~ o d ~ f i o n u~ng a ~ m p u ~ r ~ d o n ~ n m r ~ t ~ r a t ~ ~ t o the w ~ n g t o ~ h . E x p e C t a n t ! resu~s are p r e e n e d ~ m o m ~ a f i n g the c a p a b H ~ of the ~em.
I. INTRODUCTION ConventionM robotic arc wdding sy~ems are preprogrammed u~ng a teach pendant to move the welding torch Mong a prescribed path in dose proximity to the joint. Concurrent with th~ motion is a schedule for initiating the ar~ setting the arc cu~ent (or voltage) and wire feed rate to prescribed vMueL and extinguishing the arc at the end of the joint. In addition, the robot controller may be used to activate soMnoids controlling cooling water and shidding gas flows, and to provide interlocks for safe and re~able operation, such as detection of contact of electrode with workpiece or arc failure. Rehance on preprogrammed torch motion and mc parameters o~en resu~s in unsafi~actory welds in many manufacturing applications. The fi~t problem lo be soNed is the deviation of the actuM weld joint from the taught path. In generM practice, the des~ed path is taught for the fi~t pa~ in a production run, with the path data stored in the robot controller for use on succeeding parts. The processes used to produce the pa~s to be joined, in particular those used to manufacture sheet metM parts, are prone to error. Fu~heL heating of the metM pa~s during wdding may produce thermM distortion, or warping, cau~ng the joint to move from its originM position. In pra~ rice, even ff the elec~ode ~ centrM~ positioned over the joint, the weld puddle can move off the centeL
due to therm~ and d e ~ r i c ~ asymmetries in the workp~ce and fixture. The primary function of a sensor-based ~acking sy~em ~ to position the torch co~ectly so that the weld puddle ~ centered over the joint. To assure h ~ h quality wdds ff ~ defirable to monitor and control the wdding piocess in re~ time. In many appl~ation~ variations in loc~ met~ th~kness, joint width, heat ~nking, and grounding geometry may lead to unacceptable errors in bead width, pene~ation and resultant seam geome~y. A fu~her source of error in welding ~ variation in arc ~ngth, due to changes in gap between d e , r o d e and workp~ce. By u~ng sensors that can monitor the wdding process in re~ time, wdd qu~ffy can be maintained through dosed loop process control. Supervisory control functions are performed by the robot controller to coordinate operation of equipment in the wdding cell. Convenfion~ robot con~ol sy~ems based on NC technology rely on the correspondence of time and space to synchron~e wdding operations with torch and part positions. Sensor-based control sy~ems for arc wdding are des~ned to accomodate variable joint ~ngths and wdding vdodties, which require addition~ intelligence in the supervisory control. The con~ol sy~ems discu~ed in this paper are appl~able to many lypes of arc wdding. The ~peofic
Presented at the 9th World Congress of the Internafion~ Federation of Automatic Control, Budape~, Hungary,
2-6 Ju~, 1984. Receded for publication 3 June 1985. 125
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~g. 1. I n ~ e d
~ems described here have been i m ~ e m e n ~ d su~e~Hy for Gas Tung~en Arc Welding (GTAW). H. INTEGRATED WELDING TORCH VISION SENSOR The contr~ sy~ems described in t~s paper may be applied to dosed loop arc w a l i n g robo~ ufing a ~afie~ of sensor. A fundament~ requirement for these sensors is that they be a~e to measure (1) the position of the weld pud~e centred rdafive to the j ~ n t and (2) w~d process param~ers, both in refl time. Fu~her, it is defira~e to integr~e these sensors into the w ~ d ~ g torch itself to permR robot ma~p~ation of the torch into comNex, three~ m e n f i o n ~ pa~ geometries. This paper will focus on control sy~ems ufing a new ~fion sensor developed by Gener~ Electric that meets these requirements. 4 For a number of years, there has been an intenfive ~ r n a f i o n ~ effort to deve~p automatically contrd~d sys~ms for arc waling, for purposes of improved ~ac~ng of the weld joint and weld seam qu~i~. Research and devdopment has focused on ~novations ~ sensor ~chn~ogy, ~gorithms for ~ack~g and weld process c o n t r , . Mecha~c~, opticS, eddy current and magnetic sensors are availa~e commer~ally or are under devdopment. Each has the capability of p r o ~ n g an appropri~e tracking fign~ for accommodating a suffi~enfly broad range of practical welding conditions. The sy~em described here is defigned to meet the following requirements:
welding ~ h
• VNume 2, Numb~ 2, 1985
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• Capab~ ~ r ~acking m d f i ~ e weld joim ~ome~es • Capabfli~ for ~ac~ng j o l t s ~ ~ m ~ d ~ e n f i ~ space • T r a c i n g u~ng s~n~s obt~ned in re~ time • Compa~ hardware, ~ t h ~ m ~ extenfions beyond ~men~ons of convention~ torches • Compafib~ ~ ~gh EMI e n ~ n m e m a~o~a~d ~ am w a l i n g • T r a c i n g during the actu~ w d d ~ g process • No contact between sensor and work~ece The ~sion sensor ~ d into the w d d ~ g torch, shown in Hg. 1, pro~des d~ect images of the weld pud~e and joint as fiewed a ~ y Mong the torch d e . r o d e . ~ e concept of us~g fi~on sensors ~g~d ~ t h the torch was ~ i f i ~ proposed by ~ ~ . 8 The robot moves the torch to a p o ~ t on the j ~ where w e ~ n g ~ to start. A laser pa~ern generator p r ~ e c ~ a hght paaern, such as two p a r ~ l d s ~ s , on the met~ surface a p p r o ~ m ~ d y 7 mm in front of the to~h d e . r o d e . The pa~ern ~ g e n e ~ d by sp~ ti~y m ~ ~ a He-Ne laser beam to produce the des~ed p ~ r n . The beam reaches the end of the t o ~ h ~ter b~ng ~ a ~ m k ~ d ~ough a ~ h ~ e m fib~ optic bun~e. (A cohe~nt bun~e has the ends of the ~d~u~ fibers arranged in ~ e n t ~ m~rices at each bun~e end, reprodu~ng the t w ~ m e n s ~ n ~ ~eO The we~ joint causes breaks or d~tortions ~ the lair s~s, ~ ~ o w n ~hematicM~ in ~g. 2. The d~torfions or breaks can be d ~ e c ~ d , ~ c ~ g the ~ c ~ n ~ me j o ~ , i~ ~ & h , and ~ h e r ~ o m e ~ c
Se~o~b~ed co~r~ ~ems
• L. M. SWEET
127
~ E
Fig. 2. Schematicof features in wdd vision sensor image.
~atures of in~re~. A second coherent fiber optic b u n ~ e h oriented so that it looks ~rectly down the torch. The image shows the laser stripes or other pattern ~ front of the d e , r o d e . The image ~ ~ a n ~ m ~ d to the welding ~fion sy~em, and may be ~s~ a y e d on a video monitor, as shown in Fig. 3. The w d d ~ g ~fion sy~em determines the locations of the breaks or distortions in the ~ser pattern produced by the joint. The joint location data ~ combined with refl-time measurements of pud~e pofition by the control compu~r to determine the direction for the torch to be moved to position the pudd~ directly over the j~nt. The torch position correction data is ~ a n s m ~ d to the robot con~oller, wh~h g u l e s the torch to the proper position. The wdding process produces a pud~e of moRen m ~ e r i ~ under the ~e~rode. The puddle em~s e x ~ e m d y bright fight. Ufing spe~fl optic~ filters R ~ posfib~ to see both the pud~e and the laser pa~ tern in the same image (Fig. 3). The same fiber optic bun~e and camera used to see the laser pattern ~ flso used to sense the w d d pud~e. The pud~e may be observed on the TV mo~tor. The intdligent ~ ion sy~em an~yzes the we~ pudd~ to determ~e one or more of the following geometry parameter: pudd~ width, pud~e length, puddle area, pud~e shape, and pofition of the pud~e rehtive to the joint (Fig. 2).
III. TORCH AND WORKPIECE MOTION CONTROL
The motion control problem may be subdivided into the following steps: • Measurement of location of joint under or immediatdy in ~ont of the torch electrode • Measurement of location of centroid of wdd puddle • Guidance of the torch as the prescribed wdding speed keeping the wdd puddle centered over the joint • Servoing of the torch twist an~e to keep the torch longitudin~ axis tangent to the joint. There are two broad d a ~ e s of Racking sy~ems. The firm is based on d~ect senfing of the joint, using mechanicfl, de~romagnetic or opticfl means. The mechanic~ and opticfl methods detect changes in geome~y of the metfl surface; the electromagnetic methods detect variations in sensor to metfl d~placement and/or variations in metfl thickness. A number of sensor concepts are incompatible with dose proximity to the arc, requiring remote location or two-pass (senfing/wdding) operation. Such sensors have fimRed utility, due to cumbe~ome geome~y and poor utfl~ation of equipment. Sensors that are favored are integrated with the torch, insensitive to EMI, and capable of operating in the presence of arc l~ht.
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Robofi~ and C o m p ~ e r - I n t e ~ M~u~c~fing • V~ume 2, Numb~ 2, 1985
Fig. 3. Gener~ Ele~fic dosed ~op arc waling robot and ~ o n ~ e m , d ~ a ~ n g re~-fime ~deo image of stru~ured laser fight on j~nt and wdd pud~e.
The second class of Packing sy~ems is based on indire~ senfing of the joint p o r t i o n through i n f e r ence ~ o m effec~ on the arc. As the gap between eMctrode and metM change~ the arc voltage varies in a predictable manner. Through use of a weaving p a ~ e r n , variations in joint width and rdafive position can be detected in-proce~ for ce~Mn d a ~ e s of joint geometries. 3 The addition of a Packing sensor to the torch usuM~ requires that the torch be rotated about its twist ax~ to keep the joint within the field of v ~ w of the
sensor. Torches with e x ~ r n ~ wire feed (such as G T A W ) ~ s o require twi~ a ~ s s e r v i n g . The P a c i n g control s y s ~ m shown ~ Fig. 4 performs the functions outlined above. The c o o r ~ n ~ e s y s ~ m used for motion c o n t r ~ ~ oriented ~ o n g the taught p ~ h of the robot (which ~ r e o r i e n ~ d for each taught p ~ h segmen o. T h ~ c o o r ~ n a t e s y ~ e m ~ chosen to c o n ~ r ~ n the torch v d o c i t y to the prescribed w a l i n g speed, as ~ u ~ e d later. Measurements of joint location are first ~ a n s f o r m e d ~ o m sensor c o o r ~ n a t e s to robot (fixed) c o o r ~ n a ~ s , then
Sensor-based control sy~ems • L. M. SWEET PUDDLE CENTROID
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to taught path coordinates to determine the necessary torch motion co~ections. If the joint position measured ~ in ~ont of the torch d e , r o d e , it ~ necessary to delay this data (in a ring buffe 0 to determine the position of the electrode ~ong a fine n o r m ~ to the joint. The length of the delay is a r u n , i o n of preview length and torch v d o d t y . The location of the puddle centroid is ~ansformed in a fimflar manner into taught path coordinates. The control laws used in the sy~em shown in Fig. 4 are determined ~ o m an~yfis of the dynamos of the robot arm and the response of weld puddle location to changes in torch position. The prev~w control is defigned with a c o n f r o n t that the torch velocity t o n g the ~acked joint be equ~ to the nomin~ v d o ~ t y spe~fied for the torch. In many wdding processes ~ is necesary to m~nt ~n the torch in a verfic~ (or down hand) position. To w d d complex three-dimenfion~ pa~s the workpiece must be rotated by a moveab~ table c ~ d a pofitioner. True coordination of the robot arm with the pofitioner requ~es the following c o n f r o n t : the r d a t ~ e vdocity between the torch and workpiece surface must be held invariant. True coordination is difficult, fince the v d o ~ t y of the workpiece surface is a function of the instantaneous radius of the workpiece on the positioning table, as shown in Fig. 5. This c~culation may be performed by the robot controller through coordinate ~ansforms relating robot and pofitioner a~s sy~ems.
The final motion control function ~ regulation of arc length. In conventional system~ arc length ~ maintained by feedback of arc voltage, based on the linear relationship that holds between these two va~ iables. In this system, this concept must be extended, fince the arc current ~ modulated by the weld process control (see next section), as shown in Fig. 4. The performance of this control system ~ discus sed in Section VI. IV. WELD PROCESS CONTROLS Contr~ of wdding processes ~ ~ f f i c ~ t for severfl reasons: • Variables a ~ o o a t e d with the quality of the weN, such as bead pene~ation, are dfffic~t or impos~ • ~ to measure ~rectly in re~ time. • The rdationships among variab~s that may be automaticfl~ controlled, those that may be measured in r e ~ time, and those that describe weld quality are not well known. These rdationships are often ~ r o n ~ y affe~ed by e x ~ r n ~ factors. The theory of w d d ~ g o ~ y expl~ns phenomena qualitat~e~, or quantimt~dy for very fimple geometries and w e n conditions. Re~ance on empiricfl data ~ common. As ~ s c u ~ e d in Richardson, 7 Ashida ~and Dornfeld, ~edback control may be used to compensa~ for the e f ~ s of extern~ variables by comparing measured variables with defired reference conditions. As ~ the case with j ~ n t ~ac~ng, sensor technology is the key
130
Robotics and C o m p m e ~ a ~ d
Manu~ufing
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V~ume 2, Number 2, 1985
FULLY COORDINATED WELDING ROBOT AND POSITIONER CONTROL
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Fig. 5. True coordination of torch and pa~ pofitioner. to weld process control success. A thorough survey of weld process sensors is given in Richardson.7 These sensors may be grouped as follows: • Puddle image sensors (Corby, 4 Richardson, 8 Ashida) • Temperature sensors (Chin, 2 Lukens~ • Puddle oscillation sensors (Richardson, 7 Zackenhouse~. These sensor types have been app~ed to relatively ~ m p ~ laboratory demonstration~ with none fully developed to c o m m e r d a l status. Richardson 8 has d e m o n ~ r a t e d use of current control to regulate puddle width for bead on fiat plate, while mshida 1 varies travel v d o d t y at constant current to ach~ve the same end for welding in a deep rectangular shaped groove. Dornfeld and Tomizuka 5 have d e m o n ~ r a t e d use of back~de thermal measurements for purposes of weld model identification. The video-based sy~em of Ashida ~ notewo~hy because it centers the weld puddle over the joint, thus compensating for asymmetries in grounding. The method used, howeve~ is ~m~ed to the case where the weld is performed in a deep ~shaped joint. The adaptive wdding process contro~er determines the corrections in arc curren~ torch velocity, wire feed rate, or other parameters necessary to maintain the d e , r e d p u d d ~ width. The arc current and wire feed rate corrections are Uansm~ted to the robot, which in turn changes the current and wire feed rate settings on the wdding power supply. The corrected arc curren~ torch velocity, wire feed rate or other parameters m ~ n t ~ n con~ant weld seam width and bead pene~ation.
The weld process control system ~ shown in Fig. 6. The reference inputs to the control system are weld quality descriptors such as bead width and pene~ation. These descriptors are converted to weld puddle g e o m e ~ y parameters based on predetermined functional relationships derived for chosen material properties and metM thickness from anMytical m o d d s and laboratory tests. The g e o m e ~ y parameters are summed with measured puddle g e o m e ~ y to produce error ~gnals for the mulfivariable process con~ol, whose o u t p u ~ are torch velodty, arc current (or voltage) and wire feed rate. Since the joint width ~gnal ~ available for preview action, a feedforward loop ~ provided to enhance sy~em response.
V. SUPERVISORY CONTROL FOR WELDING OPERATION Use of sensory feedback information Mters the operation of the robot controlMr in its roM of coordinating equipment in the welding cell. Under conventionM operation, the operator programs the robot ufing the teach pendant to follow a nominM path Mong the weld joint. The operator also teaches the robot the des~ed nominM w d d i n g conditions, including wdding speed, arc current, wire feed rate, time of arc initiation, etc. The advanced inteH~ence provided by this sy~em ~ illustrated in Fig. 7. Under automatic operation, the role of the human operator changes dramaficM~. Since the sy~em compensates for path errors, less teaching time r e s u l t , fince fewer points win be
Senso~based con~ol sy~ems • L. M. SWEET
MEASUREMENJ T ~ N WI T DTH
131
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Fig. 7. Superv~ory con~ol functions for wdding operation. ~ KEYBOARD~
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132
Robofi~ and Compu~In~gr~ed Manu~ufing • V~ume 2, Number 2, 1985
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~g. 10. Ex~fimental sam~e ~ e s , be~re and after w~ng.
needed to speofy the nomin~ path. The operator is now able to teach weld quality parameters d e , r e d ~ong the path (seam width, peneuafion), in~ead of ~ying to guess the best set of open loop welding conditions. The use of dosed loop controls adds some complications, ~nce the tracked path usually is different in length from the preprogrammed one. Fu~her, variations in welding speed required for process con~ol disturb the taught correspondence between torch position and time. It is necessary to detect the starting and ending points of the weld ufing the vi~on sensor, and to modify the welding operation accordingly, as shown in Fig. 7.
VI. CLOSED LOOP ARC W E L D I N G R O B O T E X P E R I M E N T A L RESULTS
The wel~ng ~fion sensor sy~em has been implemen~d successfully in use with a Gener~ E ~ r i c PS0 robot, arc welding power sup~y, pofitione~ wire feeder and au~fiary equipment shown ~ Fig. 8. The ~ac~ng performance of the sy~em ~ shown ~ Fig. 9, demonstrating coor~nated compensation for errors in twist an~e and position ~ r M to the taught p~h, s u ~ e ~ to the con~ r ~ n t on con~ant torch ve~city. R e s ~ of wel~ng of sample parts are shown ~ Fig. 10, wh~h demon~ trates successful welding of j ~ n ~ with 12 mm ra~us of curv~ure at 12 cm/sec welding speed.
Senso~based con~ol sy~ems • L. M. SWEET
REFERENCES 1. As~da, E. ~ M.: M~hod and apparatus for automaticflly controllhg arc w e ~ g . U.S. Patent No. ~28~13% 1981. 2. Ch~, B.A., Goodling, J.S., Madsen, N.H.: Infrared thermography shows promi~ for sensors ~ robotoc welding. R o b o ~ Today 85-87, 1983. 3. Cook, G.E.: Through-th~arc senfing of ~ c welding. Proc. Conf Produc~on R~earch Techno~gy, SAE PuN~ation P-128. 1983. pp. 141-151. 4. Corby, N.R.: A re~ time mach~e ~fion sysem ~ r robotic TIG w e ~ g . Presented ~ 3rd Intl. Conf. on Robot Vifion and Sensory Con~o~, Cambridge, MA, 1983. 5. Dornfeld, D.A., Tom~hka, M., Langari, G.: Modd~g and adaptive contrd of arc w d d ~ g processe~ In Measurement and Conwol for Ba~h Manufactudng, Hardt, D.E. (Ed 0. American So~e~ ~f Mecha~c~ En~nee~. 1982, pp. 53-64.
133
6. Luken~ W.E., Morris, R.A.: Infrared ~mper~ure senfing of cooling rates for arc wdding contrd. Weld~ g Journd 6: 27-33, 1982. 7. Richardson, R.W.: Renew of the ~ate-oGthe-a~ of adaptive control for the gas tung~en and ~asma are w d d ~ g processes. Center for Welding Research Repo~ 529501-82-5, O ~ o Sta~ U~ve~Ry, 1986. 8. Richardson, R.W., Gutow, D.A., Rao, S.H.: A fifion based sy~em for arc weM p o d fize control. In Measu~ment and Control for Ba~h Manufacturing, Hardt, D.E. (EdO. American Sode~ of Mecha~c~ En~nee~. 1982, pp. 65-77. 9. Zackenhouse, M., Hardt, D..: We~ p o d impedance identification for s~e measurement and contrd. In Measu~ment and Control for Batch Manufacturing, Hardt, D.E. (EdO. American S o o e ~ of Mecha~c~ En~nee~. 1982, pp. 77-88.
0736-584~85 $3.00 + 0.00 P~gamon P ~ L~.
Palmed ~ ~ e U~.A. A~ ~g~s ~ e d
•
SENSOR-BASED CONTROL SYSTEMS FOR ARC WELDING ROBOTS L. M. SWEET Conu~ The~y and Sy~ems Program, Gen~M E l ~ c Company, Co~orMe R e h a s h and Dev~opmem, P.O. Box 43, Schenec~dy, NY, U.S.A. Arc w a l i n g ~ one ~ the mint impormm areas of appH~tion ~ r i n d u ~ d ~ robots. ~ m o ~ m a n u ~ u d n g ~mafiom, un~nfi~ ~ ~mem~m ~ ~e pa~, ~om~ ~ ~ e j ~ m , and the welding p r ~ e ~ ~ l f make the u ~ ~ ~ m o ~ ~ s t nti~ ~ mainlining weld quali~. ~ th~s paper ~ r ~ ~ p ~ ~ control s y s ~ s for arc welding robes ~e ~dbed: (D ~ n g ~ems ~r ~e~ng ~ e w e ~ pudd~ o v ~ ~ e j ~ , (~ w~d ~em ~n~s ~r m~ntai~ng ~oper ~am ~h and ~netration, and (3) s u p ~ s o ~ c o n ~ o ~ for ~quencing w ~ d ~ g o p ~ w fions. T h e ~ control ~ f i o m ha~ be~ imp~m~d m~s~lry ~ ~ o d ~ f i o n u~ng a ~ m p u ~ r ~ d o n ~ n m r ~ t ~ r a t ~ ~ t o the w ~ n g t o ~ h . E x p e C t a n t ! resu~s are p r e e n e d ~ m o m ~ a f i n g the c a p a b H ~ of the ~em.
I. INTRODUCTION ConventionM robotic arc wdding sy~ems are preprogrammed u~ng a teach pendant to move the welding torch Mong a prescribed path in dose proximity to the joint. Concurrent with th~ motion is a schedule for initiating the ar~ setting the arc cu~ent (or voltage) and wire feed rate to prescribed vMueL and extinguishing the arc at the end of the joint. In addition, the robot controller may be used to activate soMnoids controlling cooling water and shidding gas flows, and to provide interlocks for safe and re~able operation, such as detection of contact of electrode with workpiece or arc failure. Rehance on preprogrammed torch motion and mc parameters o~en resu~s in unsafi~actory welds in many manufacturing applications. The fi~t problem lo be soNed is the deviation of the actuM weld joint from the taught path. In generM practice, the des~ed path is taught for the fi~t pa~ in a production run, with the path data stored in the robot controller for use on succeeding parts. The processes used to produce the pa~s to be joined, in particular those used to manufacture sheet metM parts, are prone to error. Fu~heL heating of the metM pa~s during wdding may produce thermM distortion, or warping, cau~ng the joint to move from its originM position. In pra~ rice, even ff the elec~ode ~ centrM~ positioned over the joint, the weld puddle can move off the centeL
due to therm~ and d e ~ r i c ~ asymmetries in the workp~ce and fixture. The primary function of a sensor-based ~acking sy~em ~ to position the torch co~ectly so that the weld puddle ~ centered over the joint. To assure h ~ h quality wdds ff ~ defirable to monitor and control the wdding piocess in re~ time. In many appl~ation~ variations in loc~ met~ th~kness, joint width, heat ~nking, and grounding geometry may lead to unacceptable errors in bead width, pene~ation and resultant seam geome~y. A fu~her source of error in welding ~ variation in arc ~ngth, due to changes in gap between d e , r o d e and workp~ce. By u~ng sensors that can monitor the wdding process in re~ time, wdd qu~ffy can be maintained through dosed loop process control. Supervisory control functions are performed by the robot controller to coordinate operation of equipment in the wdding cell. Convenfion~ robot con~ol sy~ems based on NC technology rely on the correspondence of time and space to synchron~e wdding operations with torch and part positions. Sensor-based control sy~ems for arc wdding are des~ned to accomodate variable joint ~ngths and wdding vdodties, which require addition~ intelligence in the supervisory control. The con~ol sy~ems discu~ed in this paper are appl~able to many lypes of arc wdding. The ~peofic
Presented at the 9th World Congress of the Internafion~ Federation of Automatic Control, Budape~, Hungary,
2-6 Ju~, 1984. Receded for publication 3 June 1985. 125
126
Robofi~ and C o m p u ~ I m e ~ a ~ d
M~a~ufi~
~g. 1. I n ~ e d
~ems described here have been i m ~ e m e n ~ d su~e~Hy for Gas Tung~en Arc Welding (GTAW). H. INTEGRATED WELDING TORCH VISION SENSOR The contr~ sy~ems described in t~s paper may be applied to dosed loop arc w a l i n g robo~ ufing a ~afie~ of sensor. A fundament~ requirement for these sensors is that they be a~e to measure (1) the position of the weld pud~e centred rdafive to the j ~ n t and (2) w~d process param~ers, both in refl time. Fu~her, it is defira~e to integr~e these sensors into the w ~ d ~ g torch itself to permR robot ma~p~ation of the torch into comNex, three~ m e n f i o n ~ pa~ geometries. This paper will focus on control sy~ems ufing a new ~fion sensor developed by Gener~ Electric that meets these requirements. 4 For a number of years, there has been an intenfive ~ r n a f i o n ~ effort to deve~p automatically contrd~d sys~ms for arc waling, for purposes of improved ~ac~ng of the weld joint and weld seam qu~i~. Research and devdopment has focused on ~novations ~ sensor ~chn~ogy, ~gorithms for ~ack~g and weld process c o n t r , . Mecha~c~, opticS, eddy current and magnetic sensors are availa~e commer~ally or are under devdopment. Each has the capability of p r o ~ n g an appropri~e tracking fign~ for accommodating a suffi~enfly broad range of practical welding conditions. The sy~em described here is defigned to meet the following requirements:
welding ~ h
• VNume 2, Numb~ 2, 1985
~ .
• Capab~ ~ r ~acking m d f i ~ e weld joim ~ome~es • Capabfli~ for ~ac~ng j o l t s ~ ~ m ~ d ~ e n f i ~ space • T r a c i n g u~ng s~n~s obt~ned in re~ time • Compa~ hardware, ~ t h ~ m ~ extenfions beyond ~men~ons of convention~ torches • Compafib~ ~ ~gh EMI e n ~ n m e m a~o~a~d ~ am w a l i n g • T r a c i n g during the actu~ w d d ~ g process • No contact between sensor and work~ece The ~sion sensor ~ d into the w d d ~ g torch, shown in Hg. 1, pro~des d~ect images of the weld pud~e and joint as fiewed a ~ y Mong the torch d e . r o d e . ~ e concept of us~g fi~on sensors ~g~d ~ t h the torch was ~ i f i ~ proposed by ~ ~ . 8 The robot moves the torch to a p o ~ t on the j ~ where w e ~ n g ~ to start. A laser pa~ern generator p r ~ e c ~ a hght paaern, such as two p a r ~ l d s ~ s , on the met~ surface a p p r o ~ m ~ d y 7 mm in front of the to~h d e . r o d e . The pa~ern ~ g e n e ~ d by sp~ ti~y m ~ ~ a He-Ne laser beam to produce the des~ed p ~ r n . The beam reaches the end of the t o ~ h ~ter b~ng ~ a ~ m k ~ d ~ough a ~ h ~ e m fib~ optic bun~e. (A cohe~nt bun~e has the ends of the ~d~u~ fibers arranged in ~ e n t ~ m~rices at each bun~e end, reprodu~ng the t w ~ m e n s ~ n ~ ~eO The we~ joint causes breaks or d~tortions ~ the lair s~s, ~ ~ o w n ~hematicM~ in ~g. 2. The d~torfions or breaks can be d ~ e c ~ d , ~ c ~ g the ~ c ~ n ~ me j o ~ , i~ ~ & h , and ~ h e r ~ o m e ~ c
Se~o~b~ed co~r~ ~ems
• L. M. SWEET
127
~ E
Fig. 2. Schematicof features in wdd vision sensor image.
~atures of in~re~. A second coherent fiber optic b u n ~ e h oriented so that it looks ~rectly down the torch. The image shows the laser stripes or other pattern ~ front of the d e , r o d e . The image ~ ~ a n ~ m ~ d to the welding ~fion sy~em, and may be ~s~ a y e d on a video monitor, as shown in Fig. 3. The w d d ~ g ~fion sy~em determines the locations of the breaks or distortions in the ~ser pattern produced by the joint. The joint location data ~ combined with refl-time measurements of pud~e pofition by the control compu~r to determine the direction for the torch to be moved to position the pudd~ directly over the j~nt. The torch position correction data is ~ a n s m ~ d to the robot con~oller, wh~h g u l e s the torch to the proper position. The wdding process produces a pud~e of moRen m ~ e r i ~ under the ~e~rode. The puddle em~s e x ~ e m d y bright fight. Ufing spe~fl optic~ filters R ~ posfib~ to see both the pud~e and the laser pa~ tern in the same image (Fig. 3). The same fiber optic bun~e and camera used to see the laser pattern ~ flso used to sense the w d d pud~e. The pud~e may be observed on the TV mo~tor. The intdligent ~ ion sy~em an~yzes the we~ pudd~ to determ~e one or more of the following geometry parameter: pudd~ width, pud~e length, puddle area, pud~e shape, and pofition of the pud~e rehtive to the joint (Fig. 2).
III. TORCH AND WORKPIECE MOTION CONTROL
The motion control problem may be subdivided into the following steps: • Measurement of location of joint under or immediatdy in ~ont of the torch electrode • Measurement of location of centroid of wdd puddle • Guidance of the torch as the prescribed wdding speed keeping the wdd puddle centered over the joint • Servoing of the torch twist an~e to keep the torch longitudin~ axis tangent to the joint. There are two broad d a ~ e s of Racking sy~ems. The firm is based on d~ect senfing of the joint, using mechanicfl, de~romagnetic or opticfl means. The mechanic~ and opticfl methods detect changes in geome~y of the metfl surface; the electromagnetic methods detect variations in sensor to metfl d~placement and/or variations in metfl thickness. A number of sensor concepts are incompatible with dose proximity to the arc, requiring remote location or two-pass (senfing/wdding) operation. Such sensors have fimRed utility, due to cumbe~ome geome~y and poor utfl~ation of equipment. Sensors that are favored are integrated with the torch, insensitive to EMI, and capable of operating in the presence of arc l~ht.
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Robofi~ and C o m p ~ e r - I n t e ~ M~u~c~fing • V~ume 2, Numb~ 2, 1985
Fig. 3. Gener~ Ele~fic dosed ~op arc waling robot and ~ o n ~ e m , d ~ a ~ n g re~-fime ~deo image of stru~ured laser fight on j~nt and wdd pud~e.
The second class of Packing sy~ems is based on indire~ senfing of the joint p o r t i o n through i n f e r ence ~ o m effec~ on the arc. As the gap between eMctrode and metM change~ the arc voltage varies in a predictable manner. Through use of a weaving p a ~ e r n , variations in joint width and rdafive position can be detected in-proce~ for ce~Mn d a ~ e s of joint geometries. 3 The addition of a Packing sensor to the torch usuM~ requires that the torch be rotated about its twist ax~ to keep the joint within the field of v ~ w of the
sensor. Torches with e x ~ r n ~ wire feed (such as G T A W ) ~ s o require twi~ a ~ s s e r v i n g . The P a c i n g control s y s ~ m shown ~ Fig. 4 performs the functions outlined above. The c o o r ~ n ~ e s y s ~ m used for motion c o n t r ~ ~ oriented ~ o n g the taught p ~ h of the robot (which ~ r e o r i e n ~ d for each taught p ~ h segmen o. T h ~ c o o r ~ n a t e s y ~ e m ~ chosen to c o n ~ r ~ n the torch v d o c i t y to the prescribed w a l i n g speed, as ~ u ~ e d later. Measurements of joint location are first ~ a n s f o r m e d ~ o m sensor c o o r ~ n a t e s to robot (fixed) c o o r ~ n a ~ s , then
Sensor-based control sy~ems • L. M. SWEET PUDDLE CENTROID
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to taught path coordinates to determine the necessary torch motion co~ections. If the joint position measured ~ in ~ont of the torch d e , r o d e , it ~ necessary to delay this data (in a ring buffe 0 to determine the position of the electrode ~ong a fine n o r m ~ to the joint. The length of the delay is a r u n , i o n of preview length and torch v d o d t y . The location of the puddle centroid is ~ansformed in a fimflar manner into taught path coordinates. The control laws used in the sy~em shown in Fig. 4 are determined ~ o m an~yfis of the dynamos of the robot arm and the response of weld puddle location to changes in torch position. The prev~w control is defigned with a c o n f r o n t that the torch velocity t o n g the ~acked joint be equ~ to the nomin~ v d o ~ t y spe~fied for the torch. In many wdding processes ~ is necesary to m~nt ~n the torch in a verfic~ (or down hand) position. To w d d complex three-dimenfion~ pa~s the workpiece must be rotated by a moveab~ table c ~ d a pofitioner. True coordination of the robot arm with the pofitioner requ~es the following c o n f r o n t : the r d a t ~ e vdocity between the torch and workpiece surface must be held invariant. True coordination is difficult, fince the v d o ~ t y of the workpiece surface is a function of the instantaneous radius of the workpiece on the positioning table, as shown in Fig. 5. This c~culation may be performed by the robot controller through coordinate ~ansforms relating robot and pofitioner a~s sy~ems.
The final motion control function ~ regulation of arc length. In conventional system~ arc length ~ maintained by feedback of arc voltage, based on the linear relationship that holds between these two va~ iables. In this system, this concept must be extended, fince the arc current ~ modulated by the weld process control (see next section), as shown in Fig. 4. The performance of this control system ~ discus sed in Section VI. IV. WELD PROCESS CONTROLS Contr~ of wdding processes ~ ~ f f i c ~ t for severfl reasons: • Variables a ~ o o a t e d with the quality of the weN, such as bead pene~ation, are dfffic~t or impos~ • ~ to measure ~rectly in re~ time. • The rdationships among variab~s that may be automaticfl~ controlled, those that may be measured in r e ~ time, and those that describe weld quality are not well known. These rdationships are often ~ r o n ~ y affe~ed by e x ~ r n ~ factors. The theory of w d d ~ g o ~ y expl~ns phenomena qualitat~e~, or quantimt~dy for very fimple geometries and w e n conditions. Re~ance on empiricfl data ~ common. As ~ s c u ~ e d in Richardson, 7 Ashida ~and Dornfeld, ~edback control may be used to compensa~ for the e f ~ s of extern~ variables by comparing measured variables with defired reference conditions. As ~ the case with j ~ n t ~ac~ng, sensor technology is the key
130
Robotics and C o m p m e ~ a ~ d
Manu~ufing
•
V~ume 2, Number 2, 1985
FULLY COORDINATED WELDING ROBOT AND POSITIONER CONTROL
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Fig. 5. True coordination of torch and pa~ pofitioner. to weld process control success. A thorough survey of weld process sensors is given in Richardson.7 These sensors may be grouped as follows: • Puddle image sensors (Corby, 4 Richardson, 8 Ashida) • Temperature sensors (Chin, 2 Lukens~ • Puddle oscillation sensors (Richardson, 7 Zackenhouse~. These sensor types have been app~ed to relatively ~ m p ~ laboratory demonstration~ with none fully developed to c o m m e r d a l status. Richardson 8 has d e m o n ~ r a t e d use of current control to regulate puddle width for bead on fiat plate, while mshida 1 varies travel v d o d t y at constant current to ach~ve the same end for welding in a deep rectangular shaped groove. Dornfeld and Tomizuka 5 have d e m o n ~ r a t e d use of back~de thermal measurements for purposes of weld model identification. The video-based sy~em of Ashida ~ notewo~hy because it centers the weld puddle over the joint, thus compensating for asymmetries in grounding. The method used, howeve~ is ~m~ed to the case where the weld is performed in a deep ~shaped joint. The adaptive wdding process contro~er determines the corrections in arc curren~ torch velocity, wire feed rate, or other parameters necessary to maintain the d e , r e d p u d d ~ width. The arc current and wire feed rate corrections are Uansm~ted to the robot, which in turn changes the current and wire feed rate settings on the wdding power supply. The corrected arc curren~ torch velocity, wire feed rate or other parameters m ~ n t ~ n con~ant weld seam width and bead pene~ation.
The weld process control system ~ shown in Fig. 6. The reference inputs to the control system are weld quality descriptors such as bead width and pene~ation. These descriptors are converted to weld puddle g e o m e ~ y parameters based on predetermined functional relationships derived for chosen material properties and metM thickness from anMytical m o d d s and laboratory tests. The g e o m e ~ y parameters are summed with measured puddle g e o m e ~ y to produce error ~gnals for the mulfivariable process con~ol, whose o u t p u ~ are torch velodty, arc current (or voltage) and wire feed rate. Since the joint width ~gnal ~ available for preview action, a feedforward loop ~ provided to enhance sy~em response.
V. SUPERVISORY CONTROL FOR WELDING OPERATION Use of sensory feedback information Mters the operation of the robot controlMr in its roM of coordinating equipment in the welding cell. Under conventionM operation, the operator programs the robot ufing the teach pendant to follow a nominM path Mong the weld joint. The operator also teaches the robot the des~ed nominM w d d i n g conditions, including wdding speed, arc current, wire feed rate, time of arc initiation, etc. The advanced inteH~ence provided by this sy~em ~ illustrated in Fig. 7. Under automatic operation, the role of the human operator changes dramaficM~. Since the sy~em compensates for path errors, less teaching time r e s u l t , fince fewer points win be
Senso~based con~ol sy~ems • L. M. SWEET
MEASUREMENJ T ~ N WI T DTH
131
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Fig. 7. Superv~ory con~ol functions for wdding operation. ~ KEYBOARD~
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132
Robofi~ and Compu~In~gr~ed Manu~ufing • V~ume 2, Number 2, 1985
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~g. 10. Ex~fimental sam~e ~ e s , be~re and after w~ng.
needed to speofy the nomin~ path. The operator is now able to teach weld quality parameters d e , r e d ~ong the path (seam width, peneuafion), in~ead of ~ying to guess the best set of open loop welding conditions. The use of dosed loop controls adds some complications, ~nce the tracked path usually is different in length from the preprogrammed one. Fu~her, variations in welding speed required for process con~ol disturb the taught correspondence between torch position and time. It is necessary to detect the starting and ending points of the weld ufing the vi~on sensor, and to modify the welding operation accordingly, as shown in Fig. 7.
VI. CLOSED LOOP ARC W E L D I N G R O B O T E X P E R I M E N T A L RESULTS
The wel~ng ~fion sensor sy~em has been implemen~d successfully in use with a Gener~ E ~ r i c PS0 robot, arc welding power sup~y, pofitione~ wire feeder and au~fiary equipment shown ~ Fig. 8. The ~ac~ng performance of the sy~em ~ shown ~ Fig. 9, demonstrating coor~nated compensation for errors in twist an~e and position ~ r M to the taught p~h, s u ~ e ~ to the con~ r ~ n t on con~ant torch ve~city. R e s ~ of wel~ng of sample parts are shown ~ Fig. 10, wh~h demon~ trates successful welding of j ~ n ~ with 12 mm ra~us of curv~ure at 12 cm/sec welding speed.
Senso~based con~ol sy~ems • L. M. SWEET
REFERENCES 1. As~da, E. ~ M.: M~hod and apparatus for automaticflly controllhg arc w e ~ g . U.S. Patent No. ~28~13% 1981. 2. Ch~, B.A., Goodling, J.S., Madsen, N.H.: Infrared thermography shows promi~ for sensors ~ robotoc welding. R o b o ~ Today 85-87, 1983. 3. Cook, G.E.: Through-th~arc senfing of ~ c welding. Proc. Conf Produc~on R~earch Techno~gy, SAE PuN~ation P-128. 1983. pp. 141-151. 4. Corby, N.R.: A re~ time mach~e ~fion sysem ~ r robotic TIG w e ~ g . Presented ~ 3rd Intl. Conf. on Robot Vifion and Sensory Con~o~, Cambridge, MA, 1983. 5. Dornfeld, D.A., Tom~hka, M., Langari, G.: Modd~g and adaptive contrd of arc w d d ~ g processe~ In Measurement and Conwol for Ba~h Manufactudng, Hardt, D.E. (Ed 0. American So~e~ ~f Mecha~c~ En~nee~. 1982, pp. 53-64.
133
6. Luken~ W.E., Morris, R.A.: Infrared ~mper~ure senfing of cooling rates for arc wdding contrd. Weld~ g Journd 6: 27-33, 1982. 7. Richardson, R.W.: Renew of the ~ate-oGthe-a~ of adaptive control for the gas tung~en and ~asma are w d d ~ g processes. Center for Welding Research Repo~ 529501-82-5, O ~ o Sta~ U~ve~Ry, 1986. 8. Richardson, R.W., Gutow, D.A., Rao, S.H.: A fifion based sy~em for arc weM p o d fize control. In Measu~ment and Control for Ba~h Manufacturing, Hardt, D.E. (EdO. American Sode~ of Mecha~c~ En~nee~. 1982, pp. 65-77. 9. Zackenhouse, M., Hardt, D..: We~ p o d impedance identification for s~e measurement and contrd. In Measu~ment and Control for Batch Manufacturing, Hardt, D.E. (EdO. American S o o e ~ of Mecha~c~ En~nee~. 1982, pp. 77-88.