7. Anexos
7.4. Documento de recogida de datos
15. Welding Robots 209
Increased quality requirements for products and the trend to automate production processes along with increased profitability result in the use of industrial robots in modern manufactur- ing, Figures 15.1 – 15.2. Since robots have been introduced in industry in the 70s, their most frequently fields of application ranged from installation jobs up to spot welding, and seam welding.
The definition says that an industrial robot for gas welding is an universal movement automaton with more than three axes
which are user-
programmable and may be
sensor-controlled. It is
equipped with a welding torch and carries out weld- ing jobs.
Core of a modern robot welding cell are one or more seam welding robots of swan neck type. Normally, they have six user-programmable axes; so they can access any point within the working range at any orientation of the welding torch. To extend their working range, robots may be installed in over- head position. A further extension of the working range can be achieved by installation of the robot onto a linear carriage with Cartesian axes. Such 'ex- ternal' axes are also user-
programmable, Figure
15.3. Figure 15.1
Inernational Distribution of Installed Welding Robots (1990 -2002) br-er15-01e.cdr 1 9 9 0 1 9 9 2 1 9 9 4 1 9 9 6 1 9 9 8 2 0 0 0 2 0 0 2 0 1000 2000 3000 4000 5000 6000 7000 8000 Europa Amerika Japan Europe America Japan br-er15-02e.cdr research and training 2.656 other workpiece manipulators 8.214 metal cutting machine tools 6022 diecasting and injection moulding 4.681 pressing and forging 2.064 commissioning and palletising 3.234 measurement 1.251 others 1.562 assembly 10.229 machining 1.767 seam welding 8.749 spot welding 12.349 applying bonding
and sealing agents 1.485
surfacing 2.337
15. Welding Robots 210
2005 To turn the workpiece in the welding-favourable downhand position and to ensure accessibil- ity to any joints, workpiece positioners are used as external axes which are steered by the robot control. Multi-station
cycle tables are often used to increase profitability of the complete system instal- lation. The operator feeds and removes the welded workpiece on one side, while the robot is welding on the other side.
The robot control is the centre of an industrial robot system for arc welding,
Figure 15.4. It provides and processes all information for robot mechanics, positioner, weld- ing unit, safety equipment, and external sensors. The robot program transforms information into signals for control of robot- and positioner-mechanics as well as welding power source. Communication with external systems is possible by a host or master computer.
Modern industrial robot controls are build as multi-processor controls due to the multitude of parallel calculations and control functions. Figure 15.5 shows the internal structure of such a control. Individual assem- blies which are designed
for special jobs and
equipped with an own mi- cro-processor are linked with the host computer via the system bus. The host controls and coordinates the actions of the compo- nents based on the operat- ing system and the robot
program. Examples of
Figure 15.3
Examples for Robot Arrangements
br-er15-03e.cdr
Figure 15.4
Industrial Robot System for Arc Welding
industrial robot control robot mechanics power source welding installation welding gun positioner tools sensors host- computer safety device link to SPS offline CAD expert MDR br-er15-04e.cdr
15. Welding Robots 211
such assemblies, which are mostly installed on individual printed boards, are e.g. the axes computers. They are responsible for calculation of movement and for control of power units of the individual axes. To control the drive motors, two interconnected control loops per axis are available which control speed and position of each axis.
Further assemblies control the display screen, the manual programming unit (PHG); these assemblies are responsible for communication with the welding power source, external sen- sors, and peripheral units via digital and analogue in- and outputs and field bus systems. Or they complete the data transmission with external control systems. To reduce downtimes in the case of malfunction,
some robot controls can be connected via internet with telediagnosis sys- tems of the robot manu- facturer to support service personnel during trouble- shooting and commission- ing.
Programming of welding robots can be carried out in different ways which are distinguished in On-Line (programming at the ro- bot) and Off-Line (pro- gramming out of the robot cell), Figure 15.6.
The robot is manually guided along the later
track with decoupled
drives during Play-Back programming. The path of the track is recorded and
Programming Procedures for Welding Robots X Y Z A B C + - - - + + + - -+ + + - -+ + - + - - + + NOT-AUS + X Y Z A B C + - - - + + + - - + + + - - + + - + - - + + NOT-AUS direct programming (online) mixed proced. (online/ offline) indirect programming (off-line) play-back programming teach-in programming sensor supported programming textual programming, teach-in points macropro-gramming, teach-in points chains
textual programming with point coordinates
knowledge-based programming with expert systems graphical programming
with CAD data
movement oriented movement oriented function oriented function oriented br-er15-06e.cdr Figure 15.5
Industrial Robot Control digital I/0 memory assembly master computer analog I/0 compiler computer axses- computer welding unit screen keyboard bulk memory printer PHG sensors welding unit host Computer chaining offline programm expert system MDR brake motor encoder tacho programm interface Internet telediagnosis fieldbus positioner sensors tools welding unit br-er15-05e.cdr p o s it io n c o n tr o l lo o p s p e e d c o n tr o l lo o p
15. Welding Robots 212
2005 sponding robot control pro- gram. This procedure is preferably used for painting jobs.
A common technique to program a robot is the Teach-In procedure. During
Teach-In programming,
with the help of the manual
programming unit, the
welding torch is moved to
notable points of the
groove to be welded which are stored with information about position and orientation. In addition, track parameters must be entered, like e.g. type of movement and speed or welding parameter sets.
During sensor supported Teach-In programming, the path progress through some typical points is only roughly indicated. Then the accurate path is picked-up by sensors and auto- matically calculated in the robot steering control. Afterwards the movement program is sup- plemented by additional information about e.g. welding parameter sets.
Textual programming be- longs to mixed procedures. The sequence program in form of a text file is created on an external computer and is then transmitted to the robot steering control, Figure 15.7. The recording of the position of points is carried out in the same way as with Teach-In program- ming: moving into position and recording. Figure 15.7 TEA 1PTP 2 3 4 5 6 7 8 $ 1 $ 2 $ 1 point file point no. CP/ PTP OV/ SPD AUSG 1 2 3 4 X EXT1 Y EXT2 Z EXT3 BETA EXT5 ALPHA EXT4 GAMMA EXT6 1 2 3 4 5 6 7 8 PTP PTP PTP PTP PTP PTP PTP PTP 100 100 100 100 100 100 100 100 0000 0000 1100 1100 1100 1100 0000 0000 10560 10700 10700 10700 10700 10700 10700 10700 1 1128 1513 2420 3190 3852 4510 4510 1317 1344 1344 1344 1344 1344 1344 1344 17204 15164 14220 14229 13294 14448 15520 15520 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 LIST1=(20,0,0,50,60,75,15,12,0,0) 1 LIST2=(30,0,0,55,70,0,0,0,0,0) 2 MAIN 3 $(1) 4 GP(1-3) 5 GC (4,$2,5,$1,6) 6 GP(7,8,1) 7 END definition of welding parameters selection of welding parameter set1 move to point 1-3 in PTP operation welding with change welding parameters
br-er15-07e.cdr
Figure 15.8
geometric macro welding macro
= TCP = torch angel - welding parameters = TCP - torch angel - welding parameters - welding programme (ignition, welding, crater filling) Length macro
Profile macro
15. Welding Robots 213
Macro-programming is also regarded as a mixed method which shortens programming time at the robot, Figure 15.8. Macros are structured processing sequences which are created online to fulfil working functions and which can be repeated for further similar working func- tions. Geometry macros
contain information about torch guidance to produce certain joints or joint sec- tions. Welding technology parameters for individual
welding situations are
summarised in welding
macros. This applies for torch positioning, torch in- clination, relative position of beads to root and welding parameters.
Using a collection (can be created online or offline) of such macros, the programming time can be shortened for workpieces with often repeated welding jobs, e.g. steel construction when welding stiffeners and head plates
Using offline programming practice, the programming work is shifted out from the producing robot cell. This avoids unproductive stoppages and allows for economic-viable, limited num- ber of pieces to be reduced.
During textual program-
ming, the 3-dimensional
point coordinates and torch orientations are entered into an external computer in a
manufacturer-specific pro-
gram language. To achieve a complete program se- quence, each instruction must be entered individu-
Figure 15.9 Quelle:Cloos, Kuka 2002 Graphical Simulation of Robot Movement br-er15-09e.cdr movement instructions program sequence instructions arithmetical and
logical functions special functions
−
−
−
−
synchronical PTP pro- cedure (point to point) linear interpolation, CP (continious path) cicule and graduated cicule interpolation continuously program- mable tool speed
− − − − − −
sub program techni- que jump instruction conditional instruc- tions repeated loops inquiry of entries programmed stop − − − +, -, *, : boolean operations etc. − − − − −
3D online and offline transformation of pro- gram parts mirroring of program parts processing variables communication with sensors communication with external computers br-er15-10e.cdr
15. Welding Robots 214
2005 The graphical offline programming uses CAD data for modelling the complete robot working cell and parts to be welded. Planning of the path is carried out with CAD functions directly at the workpiece which is dis-
played on a screen. In most cases, the program- ming systems provide a graphical simulation of the movement, e.g. to check for collisions between torch
and workpiece, Figure
15.9. For the following transformation of the pro- gram into the robot control,
a calibration between
model and physical robot working cell is required.
In the case of knowledge- based offline program- ming, the operator is sup-
ported by integrated
expert systems when it comes to creation of robot welding programs, e.g. for
determination of job-
specific welding parame- ters. However, checking and adapting the program must be carried out by the operator.
Modern robot controls provide the programmer with some functions for movement control and for modification of program sequence, Figure 15.10. PTP movement (point to point) serves to move the robot in the space. All axes are controlled in such a way that they reach
Figure 15.11 TEA EDI 1 2 3 4 5 0 LIST1=(20,0,0,50,60,75,15,12,0,0) 1 MAIN 2 $(1) 3 GP(1-3) 4 GC(4) 5 GP(5,1)
definition of welding parameters
selection of welding parameters set 1
move to dots 1-3 in PTP mode
move to dot 4 in CP mode
move to dot 5 and 1 in PTP mode 6 END br-er15-11e.cdr Figure 15.12 TEA EDI 120° 1 0° 1 2 3 4 5 6 7 8 9 10 11 0 LIST1=(20,0,0,50,60,75,15,12,0,0) 1 LIST2=(30,0,0,55,70,0,0,0,0,0) 2 MAIN 3 $(1) 4 GP(1-3) 5 CIRO(1) 6 Cir(3,4,5,50) 7 GP(6-8) 8 $(2) 9 CIRO(0) 10 CIR(8,9,10,0) 11 GP(11,1) 12 END
definition of welding parameters
selection of welding parameter set 1 move to dot 1-3 in PTP mode with rotating the 6 axis circle instruction
selection of welding parameter set 2 lock 6 axis
th
th
15. Welding Robots 215
their set-point at the same time. Thereby the actual path of the torch depends on kinematics of the robot and on current position of the axes.
A linear interpolation (CP procedure, continuous Path), Figure 15.11, is used for accurate movement along a straight line, e.g. movement to weld start point or welding. The active point of the tool 'arc' (Tool-Centre-Point, TCP) is moved along a straight line between two programmed points, adapting torch angle and torch inclination between the two points.
Circles and graduated circles are entered by means of circle interpolation programs, Figure 15.12. Then the orientation of the torch can be adapted through turning the knuckle axis or 6th axis of the robot and the value of spill-weld at the end of the seam can be indicated.
Speed of the torch is user-programmable and, if required, can be superimposed by an oscillation. To control the program run, commands are available for: repeated loops, conditional and unconditional program jumps, waiting periods, waiting for inputs, and working with sub-programs.
The software of modern seam welding robots contains – as special functions – 3-dimansional transfor-mations and mir- roring of programs and partial programs, palletis- ing functions, processing sensor data and com- mands for communication with other robot controls (Master/Slave operation) as well as with external computers, Figure 15.13. Figure 15.13 x x y y z z x y z offset x y z br-er15-13e.cdr
16.
Sensors
16. Sensors 217 The welding process is ex-
posed to disturbances like misalignment of workpiece, inaccurate preparation, ma-
chine and device
tolerances, and proess dis- turbances, Figure 16.1.
The manual welder notices them by eyesight and cor-
rects them manually
according to strategies
learned and gained by ex- perience. To record process
irregularities and path deviations, a fully mechanised welding plant requires sensors provid- ing control signals which are then used in accordance with implemented rules. Using corresponding control elements, the control loop is closed for the welding process.
Scopes of duty of the sensors is finding the weld start point and seam tracking. In addition, with the help of information about joint geometry, process parameters can be adapted online and offline. The ideal sensor for a robot application should measure the welding point (avoid- ance of tracking misalignment), detect in advance (finding the start point of the seam,
recognising corners, avoid- ing collisions) and should be as small as possible (no restriction in accessibility). The ideal sensor which combines all three re- quirements, does not yet exist, therefore one must select a sensor which is suitable for the individual welding job. Figure 16.2 Figure 16.1
© ISF 2002
Adaptive Process Control Manually - Fully Mechanised
br-er16-01e.cdr
process disturbances welding process brain
eye hand control
strategy
sensor
Sensors for Arc Welding Systems Survey
16. Sensors 218
2005 principles used in welding engineering. The most frequently used systems in practice are tac- tile, optical, and arc based
sensor systems with me- chanical arc adjustment.
With tactile scanning sys- tems, the simplest type of scanning is a mechanical sensor. Pins, rollers, balls, or similar devices may be used as sensors.
Such scanning systems show a long distance be- tween sensor and torch, the
application range is limited. Only grooves with large dimensions and relatively straight seam path can be scanned with these systems. Figure 16.3 shows some examples of different
groove geometries.
Tactile sensors can recognise 3-dimensional offsets of the workpiece. Through scanning of three levels the 3-dimensional point of inter- section can be calculated and the robot program for correcting the deviation can be shifted accordingly thus finding the start point of the weld. In this case, the gas nozzle of the torch serves as a sensor, Figure 16.4, which is charged with electrical tension. As soon as the torch touches the workpiece, a current flows, which is then taken by the robot control as a signal for obtaining the level to be scanned.
Inductive sensors are graded as non-contact measurement systems. Due to their function Figure 16.4 © ISF 2002 br-er16-04e.cdr A A' B' C' B Figure 16.3
Scanning Principles With Tactile Sensors
br-er16-03e.cdr
V, X Y seam with ball-probe
I seam with blanc-probe
fillet seam with ball-probe
overlapp seam with ball-probe
multilayer seam with ball-probe
16. Sensors 219 principle, they can be applied for metallic and electrically conductive materials. The simplest type is a ring coil. If alternating current flows though the coil, ,a magnetic field is generated
close to the workpiece. When the coil approaches the workpiece surface, the magnetic field weakens. Figure 16.5 shows the dis- tance-dependent electrical signal. Such simple sen- sors are used to recognise
the workpiece position.
Using several distance
sensors, also a welding groove can be scanned.
With multi-coil arrangements in one sensor, the position of the welding groove, the angle be- tween sensor and workpiece surface and the distance can be recorded. Figure 16.6 shows a principle arrangement. A transmitter coil generates an magnetically alternating field which induces alternating currents in the two receiver coils. In the undisturbed case, these currents are phase-shifted by 180° and neutralise each other. If the sensor is moved crosswise to the groove, magnetical asymmetries will occur in the scanning area, which will show in the pre- sented signal shape. The
output signal will be zero, if the coils are positioned ex- actly above the centre of the groove.
The radar sensor in Fig-
ure 16.6 uses Doppler's
effect to generate a signal. Here the phase difference between transmitter signal and receiving signal is evaluated.
Figure 16.5
© ISF 2002
Principle of an Inductive Sensor (Single Coil and Multicoil Arrangement)
br-er16-05e.cdr
A B
sensor signals groove position
distance coil arrangement for
groove position
coil arrangement for distance measurement transmitter coil reception coil © ISF 2002
Functional Principle of a Radar Sensor
br-er16-06e.cdr radar sensor oscillation workpiece signal path work piece radar sensor transmitting wave receiving wave phase difference functional principle of continuous wave doppler’s radar
16. Sensors 220
2005 transforms such signals into distance values. To record the position and the depth of the groove, the sensor must be continuously moved along the seam. Radar sensors form a so called radar baton, which is focussed onto a measurement spot of about 0,7 mm diameter for this application. Figure 16.6 shows the sensor signal, which represents the relative move- ment along the workpiece. At the moment, the characteristic values of the weld groove can be determined with a resolution in the range of 1/10 mm.
Arc sensors evaluate the continuous change of the welding current with a change of the con- tact tip-to-work distance, Figure 16.7. A signal for side control of the torch is determined by measure- ment and subtraction of the currents on the flanks of a groove. A comparison be-
tween actual welding
current and programmed rated current provides a signal for distance control of the welding torch.
To let this sensor method work, a divergence of the arc or the use of a second arc is required.
To realise this principle, there are numerous possi- bili-ties. Figure 16.8 shows some variants of signal re-
cording. The most
frequently used method is a mechanical oscillation of the welding torch, which is carried out by a rotor Figure 16.7 © ISF 2002 Arc Sensor br-er16-07e.cdr IO l2 l1 ∆l U I I l2 l0 I1 side correction l = 0 ∆ ∆ l1,2 height correction = 2 x I ΣI Soll Figure 16.8 © ISF 2002 Arc Sensor - Signal Detection - br-er16-08e.cdr
mechanical oscillation twin wire welding
16. Sensors 221 movement with an oscillation frequency up to 5 Hz.
The second method is mainly used with submerged arc welding. Both wires are aligned crossways to welding direction and the difference of the two currents is evaluated.
Magnetic fields can diverge only the arc itself. The advantage of this method is a high diver- gence frequency of about 15 Hz. A disadvantage is the size of the electromagnets and the limited accessibility to the workpiece.
The last variant of an arc sensor incorporates a mechanical rotation of the welding wire. In this case, the divergence frequency of the arc can reach up to 30 Hz.
The signal recording is continuous during the movement. In this way, information about orien- tation of the torch and groove width is also provided. The arc sensor principle is limited to groove shapes with clear flanks. Together with the tactile torch gas nozzle sensor, it provides a frequently used combination for seam finding and seam tracking during robot welding.
Optical sensors can be used for a great number of jobs. The easiest method is the recogni- tion of the radiation intensity, which is reflected during welding.
E.g. with laser beam welding, this is carried out through recording the reflected laser radiation with simple sensors for control of penetration depth, Figure 16.9.
The procedure is based on the line-up between the degree of reflection and shaft relation (penetration depth/focus position) of the capillary. The amount of back-reflection of the laser beam power is measured,
which due to multi-
reflection is not absorbed by the workpiece. Changes of penetration depth due to modified laser power or a