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[Quarterly Journal of Japan Welding Society , Vol. 13, No. 1, pp. 28-38 (1995)]
CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication By Yuji SUGITANI, Yoshihiro KANJO and Masatoshi MURAYAMA
Abstract
Recently, the serious problems facing heavy industries such as the increase in the average age of employees, shortage of skilled workers and environmental problems in the workplace are being addressed. The bridge panel fabrication system we have developed is such an approach to solve these problems. It consists of two parallel box girder panel sub-assembly lines, one exclusively for web panels and the other for flange . Each line is made up of stages for fitting, welding , re-forming, drilling and finishing. All of the line equipment is controlled by the line computer in the central control room.
The box girder panel is reinforced by the stiffeners and the fillet welding is the objective of each welding stage. In total, 14 articulated-type arc welding robots are applied in the welding stages. In the web panel welding stage, 8 robots are hung from stationaly transverse girders with 2 m span in longitudinal direction and can be used for operations up to 4 m wide by 16 m long. In the flange panel welding stage, 2 welding robots are hung from each of the 3 transverse sliding units that are arranged under the mobile gantry . In total, 6 welding robots cover the 5 m wide by 16 m long work area .
By adopting the High Speed Rotating Arc welding process, we have doubled welding efficiency as compared with conventional processes. Furthermore, this process is more effective to reduce panel deformation because of the lower welding heat input. The welding robot system is stabilized with precise accuracy by newly developed joint end and bead end sensing techniques as well as a seam tracking arc sensor system based on the High Speed Rotating Arc.
All of the welding robot is linked with LAN to the newly developed, teachingless CAD/CAM system that is based on the computerized design fabrication system for bridge
fabrication. This system is especially effective for bridge panel fabrication where almost all the panels are of different shapes.
Thus, a step towards computer-integrated manufacturing (CIM) in steel structure
fabrication has been achieved.
Key Words : CIM, FMS, CAD/CAM , Robot, Off-line Programming, Arc Sensor, High Speed Rotating Arc welding , Bridge, Fabrication
1. Introduction
Recently, serious problems are being addressed
that face the heavy industries . These problems include the increase in the average age of
employees, shortage of skilled workers and envi -
ronmental problems in the workplace . The crea-tion of an innovative fabrication system geared to
the 21st century has been sought with this in
mind.
The more important technologies for achieving
the necessary innovation are : robotization,
systemization, highly efficient production and
stabilization of the fabrication process . The
bridge panel sub-assembly lines installed at the Tsu works are an approach to solving these
problems.Fourteen articulated-type arc welding robots
are hung from gantries in the welding stage . The use of the High Speed Rotating Arc welding method developed by NKK's R & D section has
enhanced production efficiency. Furthermore, economies have been achieved in the total num-ber and locations of the robots .
The use of arc sensing through the High Speed
Rotating Arc system permits highly accurate seam tracking and joint end and bead end sensing . This system has improved the tolerance for work
Recived : 22 March, 1994 .
Member, Engineering Research Center NKK
溶 接 学 会 論 文 集 第13巻(1995)第1号29
setting errors and reduced panel waving distor-tion and panel deformation from welding heat input. The sensing system has also contributed to system stability and product quality.
The fully integrated welding robots are controlled by a newly developed, teaching-less direct CAD/CAM system that is based on NKK's own computerized design-fabrication system for bridge fabrication. This system is especially effective for bridge panel fabrication where the
panels are different shapes.The work stages consist of fitting, welding,
reformation, drilling and finishing. The instru-mentation of all of these stages are connected by LAN. All of the CAD/CAM and production control data are wholly controlled by line com-
puters in the central control room.This report introduces the functions of the
welding robot system and presents the results of its application.
2. Outline of box girder panel fabrication lines
2.1 Construction of the lineFig. 1 is a schematic diagram of a box girder
bridge section. Box girders consist of vertical web panels and horizontal flange panels. The conventional fabrication method is as follows:
First, the stiffeners are fitted to the web and
flange plates. Each panel is then fitted to the box.
Finally, welding operations inside of the box are
completed. The space inside is too narrow and
complex for the required operations, so that this
approach is not effective or suitable for automa-
tion.
In this "panel fabrication approach," all opera-
tions such as fitting, welding, reforming and dril-
ling are then completed at the sub-assembly line,
followed by assembly to the box.
Fig. 2 shows the configuration of the box girder
panel fabrication lines based on the new concept.
There are two parallel lines, one exclusively for
web fabrication and the other for flanges. Each
line is made up of stages for fitting, welding,
reforming, drilling and finishing and is fully
controlled from the central control room.
Fig. 1 Cross-section of box girder type bridge.
Fig. 2 General view of panel assemble lines.
30 研究論文 Yuji SUGITANI et al: CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication
2.2 Integrated line control system
All of the line equipment is controlled by the
line computer in the central control room. An
Ethernet LAN is used to link the line computers
to each other and to the host computer, which
runs the CAD/CAM system. In addition, all of the individual pieces of equipment are linked to
each other by a programmable controller net-work. Sequential communications are controlled by the master programmable controller in the
Fig. 3 Configuration of integrated control system.
Fig. 4 Typical figure of panels.
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central control room. The network information consists of CAD/CAM data (e.g., NC equipment data) and production control data (e.g., monitor-ing information, production completion data, etc.). The configuration of the integrated control
system is shown in Fig. 3. 2.3 Configuration of the panel
Fig. 4 is a schematic diagram of the box girder
panel. The web panel is reinforced by vertical and horizontal stiffeners, while the flange panel is reinforced by logitudinal stiffeners. The longitu-dinal stiffener has some discontinuous sections of welding because of drain holes and diaphragm sections. Fillet welding, including the edge treat-ment of these stiffeners, is the objective of each welding stage. Table 1 is a specifications of panels.
Table 1 Specifications of panels.
3. Welding robot system
3.1 Configuration of welding robot system(1) Web panel welding robot system
Fig. 5 shows the configuration of the web panel welding robot system. An articulated robot is hung from a stationary transverse girder with a 4 m wide stroke (additional axis). The robot has a total of seven axes. Eight welding robots are arranged with a 2 m span in the longitudinal direction. Therefore, the welding robot system
Table 2 Specifications of web panel welding robot system.
Fig. 5 Configuration of web panel welding robot system.
32 研究論文 Yuji SUGITANI et al: CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication
Fig. 6 Configuration of flange panel welding robot system.
can be used for operations up to 4 m wide by 16 m long.
Generally, each robot moves independently, and the movement regions of adjacent robots overlap. To avoid interference, the motion
sequence is optimized, and a mutual interlock system installed. Table 2 shows specifications for the web panel welding robot system.
(2) Flange panel welding robot system Fig. 6 shows the configuration of the flange
panel welding robot system. Two robots are hung from each of three transverse sliding units
that are arranged under the mobile gantry. This
gantry travels in the longitudinal direction. In total, six welding robots cover the 5 m wide by 16
m long work area. Welding is performed by pairs of robots that face each other at either side of the longitudinal
stiffener. Up to three . stiffeners can be welded simultaneously.
By using articulated welding robots, continuous welding is possible, even if some of the stiffeners have discontinuous sections with individual arc on and off operations. At the same time, the
robots do not interfere with each other, and the
gantry motion is not interrupted. Table 3 shows specifications for the flange panel welding robot
system. 3.2 High Speed Rotating Arc welding robot
In this system, a total of fourteen sets of the
High Speed Rotating Arc welding robot are used. The High Speed Rotating Arc welding torch is
Table 3 Specifications of flange panel welding robot system.
mounted at the tip of the robot. Specifications for
the High Speed Retating Arc welding robot are shown in Table 4. Newly-developed arc sensor techniques are used with both joint end and bead
end sensors, in addition to the conventional seam tracking sensor. These sensing techniques help
the wire touch sensor and improve the sensing
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Table 4 Specifications of arc welding robot.
accuracy.The system is also equipped with many error
sensors to detect arc miss, wire stic, torch contact and other problems. For peripheral devices, wire nippers, nozzle cleaners and fume collectors are installed to complete the perfect unmanned weld-
ing system. 3.3 Control systems 3.3.1 Line control system The line computer generates the line schedules,
transfers NC data, monitors the line conditions and collects processed data from each stage. The flow of these functions is as follows : First, the
line schedule is generated. NC data is then auto-matically transferred to the machines based on the scheduled order. Some of the NC data, such
as for welding or drilling, are translated to object code by an intermediate FA (Factory Automa-tion) computer before transmission to the end
controller. Thirdly, the work operations are carried out, and all of the machine information is monitored. Finally, several pieces of operating information, such as the actual running and arc
time ratios, are collected and reported after the operations are completed. 3.3.2 Teaching-less CAD/CAM system The use of welding robots for making bridge
panels presents problems typically associated with the industrial production of complex units made in small quantities. The main problem is how to effectively generate motion data for the
work. The off-line teaching system has been
generally used in place of teaching for this pur-
pose. However, this approach permits only the
Fig. 7 Integrated teaching-less CAD/CAM sys-tem.
working style to be changed, i.e., from the practi-cal teaching operation using actual robots to the
definition of the motion using a computer screen with software. Furthermore, almost all off-line teaching systems can only be used for one robot
at any one time. Thus, programming applications with multiple welding robot systems for diversified small-quantity production remains a
problem. For this purpose, a teaching-less direct CAD/
CAM system was developed based on NKK's computerized design-fabrication system for
bridge fabrication. In this system, motion data for every welding robot is generated automati-cally from member geometric data, material codes and welding design data (i.e., fillet welding
sizes), which are supplied from the computerized design-fabrication system database. Fig. 7 shows a flow diagram for the teaching-less direct CAD/CAM system. The system work
flow is as follows : First, geometric, material and welding design data for the member are derived. Secondly, joint locations such as arc on and off
points and motion change points are recognized in the robot motion data generation system. Third, welding design data is added for each
joint. Data for the robot is stored in the robot
34 研究論文 Yuji SUGITANI et al : CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication
database, downloaded to the robot control system
and finally converted to motion pattern codes,
locational data and welding design codes. These
tasks are all carried out in the host computer.
The FA computer in the robot system processes
the actual motion pattern data from the motion
macro database. Each pattern is then applied to
Fig. 8 Robot motion generation system .
the joint location data. Next, the welding condi-tions are chosen from the welding database according to the welding design code and added to the motion pattern data. Finally, motion pattern data is compiled into binary data and transferred to the robot controller. Fig. 8 shows a flow diagram for the web panel
robot motion data generation system. The proc-ess consists of work generation, welding line definition, interference avoidance, robot area division and motion pattern selection. 3.3.3 Robot motion sequence for unmanned sys-
temThe sequence for the operator-free welding
robot is shown in Fig. 9. Robot motion data is transferred before the panel is installed. The
panel is carried by the automatic roller conveyor, and welding is started after adjustment of the work.
The robot motions for one welding joint is as
Fig. 9 Flow of unmanned control for arc weld-ing robot.
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follows: Nozzle cleaning and wire cutting are first carried out, followed by the approach motion
to the arc start point. Next, wire touch sensing at the arc start point compensates for panel setting
errors. This is followed by cornering treatment of welding at the stiffener edge. The high speed rotating arc sensor seam tracking system then
corrects for deviations in the welding line. At the
arc end point, the joint end sensor or bead end sensor detect the actual end position, and another weld cornering treatment follows. Finally,
retract motion from the arc end point takes place,
and the process continues recursively.After completion of all of the weld joints, the
panel is automatically loaded onto the next stage. Thus, an unmanned welding system has been
achieved by these sequential processes.
4. Function of the High Speed Rotating Arc
welding process
4.1 The principle of High Speed Rotating Arc
welding processThe principle of the High Speed Rotating Arc
welding process is shown in Fig. 10. The rotating
arc is maintained over the weld pool by a mechan-ically rotated electrode nozzle. This process
easily achieves a high speed arc rotation of over 100 Hz, which can be compared to conventional mechanical weaving at 4 or 5 Hz.
A 1.2 mm¢ solid wire is used for welding, along with a shielding gas composition of Ar-20%CO2.
These welding parameters are used to enhance bead appearance and minimize spatter for the
high welding currents of over 400 A. 4.2 High speed welding with high welding cur-
rents
The operating characteristics of the high speed rotating arc are due to physical factors such as
the arc pressure and heat distribution over the
molten pool. Fig. 11 shows the relationship
between bead flatness and the rotation speed of the arc at 400A welding current and 1.0 m/min
welding speed. In general, the bead appearance is
assumed to be convex for high current, high speed welding. However, in the rotating process, the
arc pressure is distributed and decreased over the molten pool, and bead flatness is controlled, even
under the high current, high welding speed condi-tions.
Some examples of the welding conditions and bead appearance are shown in Fig. 12 for
different leg sizes. It can be seen that this process allows the welding current and speed to be in-
creased, so that it is possible to significantly increase the welding efficiency compared to con-ventional GMAW. Fig. 13 shows the relation
between welding current and deposition rate/ wire feed rate. In our experience, the deposition
rate depends mainly on the welding current, and explicit differences in the deposition rate at the
same welding current have not been observed
between multiple, mixed shielding gas processes and this process.
Furthermore, high current, high speed welding
Fig. 10 Principle of High Speed Rotating Arc welding process.
Fig. 11 Pelation between rotating speed and bead flatness.
Fig. 12 Examples of bead shape cross sections.
36 研究論文 Yuji SUGITANI et al: CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication
Fig. 13 Relation between welding current and deposition rate/wire feed rate.
Fig. 14 Relation between welding current and blow hole formation.
using a small wire size reduces distortion compar-
ed to conventional SAW methods. In addition,
higher welding currents are favorable for welding
steel plate coated with inorganic zinc rich
primers. Fig. 14 shows the relation between
welding current and porosity formation in mild
steel with inorganic zinc rich primer (primer
thickness of 15 to 20ƒÊ). The peak porosity range
is from 300 A to 350 A, and blow hole generation
is reduced at higher and lower currents.
4.3 Seam tracking with High Speed Rotating Arc
sensor
It is common to use arc sensors for sensing the
groove in oscillating or rotating welding torch
systems. The torch position can be controlled in
real-time by detecting deviations between the
groove center and the oscillation center through
Fig. 15 Principle of seam tracking control by
arc sensor.
Fig. 16 Principle of joint end sensor.
Fig. 17 Principle of bead end sensor.
analysis of the welding current or arc voltage waveforms. The advantages of this method are that special detectors are not necessary because
the arc itself is used as a sensor. This simplifies the design of the torch. Moreover, information
from the area just below the arc can be detected in real-time during welding. In the panel welding
system, weld line tracking requires high accuracy
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to accommodate errors in setting the work, bends in the skin plate and welding distortion.
The principle of the arc sensor in the High
Speed Rotating Arc system is shown in Fig. 15.
The oscillation frequency is limited to several Hz for conventional repetitive oscillating arc sen-
sors, whereas high speed rotation at over 100 Hz can be easily obtained in this method. According-
ly, the sampling rate is higher with the higher rotation speeds, so that automatic seam tracking
with higher precision and better response is achieved by this method.
4.4 Joint end and bead end detection by High Speed Rotating Arc sensor
Joint ends and bead ends can be detected during welding by using the high speed rotating arc
sensor. Fig. 16 shows the principle of the joint end sensor, while Fig. 17 shows the principle of
the bead end sensor. The joint end can be detect-
ed by comparing the arc voltage waveforms of the right and left sides of arc rotation. The bead
end can be detected by comparing the front and rear waveforms. Sensing accuracy is maintained within 1 mm. Stable cornering beads and bead
connections are possible by using these sensors.
5. The result of application
The new welding line has been in operation
since April 1992. As an example, Photo. I shows the web panel welding robot system in operation,
and Photo. 2 shows the flange panel welding robot system. To illustrate seam tracking, Photo. 3
shows the bead appearance of a flange panel weld where the plate thickness changes with a 1/5
taper. The welding robot was not given any instruction at the transition ; nevertheless, the figure shows that seam tracking control can be
achieved with good response using the arc sensor. This seam tracking ability is clearly effective for the curved panel shown in Photo. 2.
Photo. 4 shows an example of a cornering bead appearance made by using the arc sensor joint
Photo. I Web panel welding robot system.
end detection, and Photo. 5 shows an example of
a bead connection using arc sensor head end
detection.
Table 5 shows an example of time simulation
Photo. 2 Flange panel welding robot system.
Photo. 3 An example of bead appearance at
plate thickness transition.
Photo. 4 Cornering bead appearance by arc sensor joint end detection.
Photo. 5 Finished head appearance by arc sen-sor head end detection.
38 研究論文 Yuji SUGITANI et al: CAD/CAM Welding Robot System in Steel Bridge Panel Fabrication
Table 5 Time estimation in a standard web
panel.
for robotic welding operations in a typical web
panel.
6. Conclusions
A welding robot system has been developed and applied for bridge panel fabrication. The fea-tures of the system can be summarized as fol-lows:(1) Fourteen sets of articulated welding robots are controlled by the teaching-less, direct CAD/ CAM system. Robot motion data and welding conditions are automatically generated without teaching.(2) Welding efficiencies that are twice that possible with conventional processes can be obtained by the application of the High Speed Rotating Arc welding process. This process improves weldability and the resistance to poros-
ity formation of primer-coated plate. The low heat input welding process decreases welding
distortion.
(3) The use of arc sensors with the High Speed Rotating Arc process enables highly accurate
seam tracking, joint end sensing and bead end sensing. Thus, the welding robot system is fur-ther stabilized with precise accuracy.
(4) The panel fabrication lines are connected by LAN. Thus, a successful step toward computer-integrated manufacturing (CIM) in bridge fabri-cation has been achieved.
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