principles to align an assembly line (assembly cell)...
TRANSCRIPT
The 6th
edition of the
Interdisciplinarity in Engineering International Conference
“Petru Maior” University of Tîrgu Mureş, Romania, 2012
PRINCIPLES TO ALIGN AN ASSEMBLY LINE (ASSEMBLY
CELL) FOR SPOT WELDING OR RIVETING OF CAR BODY
Lucian MUDURA, Macedon GANEA
University of Oradea, Oradea
str. Universitatii nr. 1, Oradea, Romania
[email protected], [email protected]
ABSTRACT The research problem is to measure and align a cell assembly from an assembly line for
spot welding or riveting for car body where this is done by one robot or more robots. The
fixtures can have several working positions, which mean to find the fixture position
according with the robot position. For one fixture can work several robots, which can be
6-axis or 7-axis.
Keywords: fixtures, bodyside welding, measurements, robots, laser tracker
1. Introduction
To meet the requirements on quality, cost and
performance automotive experts concerns were
directed towards practical exploitation of new
technologies arising in their implementation in
manufacturing and assembly, design and
implementation of systems design, testing, repair and
control computer assisted production.
Thus, modern manufacturing faces two main
challenges: high quality at lower prices and improved
productivity. These are the requirements to keep the
major manufacturers in developed countries which
are in direct competition with countries with low-
wage. Other important features of production systems
are flexible and agile manufacturing process because
companies must answer a very dynamic market with
products that have short life cycles due to fashion
trends and global competition. [5]
Therefore, robotic welding is essential for
automatic welding in many industries. The fact is that
25% of all industrial robots are used in welding
processes. Robot welding is used to have a high
quality level of welds in short cycle’s times [5].
Market demands are indicators which
determine what kind of automotive types and variants
has to be made, thus requires the development and
implementation of production systems capable of
processing small batches of parts in terms of quality,
efficiency and high productivity. To fulfill these
requirements is necessary to move from flexible
manufacturing units to their integration into a flow of
materials and information lead by computer.
Flexible manufacturing system can be defined
as a cyber system whose elements are coordinated by
computer for self-regulation and optimization of me-
chanical processing. A flexible manufacturing system
has two or more flexible manufacturing cells connect-
ed by an automated transport (conveyor).
A flexible cell is used in body in white to
assembly of a frame and panels, here the detachable
components are fitted to the body [3].
During the last decades the layout of car
manufacturing plants are changing very often, always
adapted to the new demands and present conditions.
Due to the fact that the terms lean production gains
more importance and the minimization of non-value
activities are taken in consideration, new plant have
to be developed for the future as well [11].
The measurement system has a critical role in
any dimensional evaluation process. In automotive
industries, the measurement system is very important
and his role is particularly influential. The fixture
(tool) suppliers measure most part fixtures in absolute
space (X, Y, and Z coordinates) rather than as relative
distances between points. Absolute space measure-
ments are more complex, particularly for angled
surfaces [1].
The body coordinate system has been widely
used in the automotive industry for drawing of body
parts, product and process design. The origin of the
body coordinate system (OX) is defined at the front
center of a vehicle, its indicates a length of car and
the coordinate system (OZ) is below its underbody
indicates height of car and the coordinate system
67
(OY), its starting point is the center of car body
indicates width of car. [1]
In figure 1 and figure 2 we have one assembly
line from Jaguar plant in Castle Bromwich, UK.
Fig.1 - Production line for Jaguar
Fig.2 - Production line for Jaguar
2. Materials and Methods
A flexible manufacturing cell made in GMAB
Consulting is shown in the figure below (fig. 3) [11].
The flexible manufacturing cell is formed by:
� electrical cabinet
� robots and robots controllers (in our
cell are 11 robots)
� Man/machine interface with operator
panel
� spot welding guns fixed on robot wrists
or spot welding guns fixed on the
platform
� grippers for the finishing robots
� tip dressers
� geometrical tools (fixture), turntables
� conveyor for part extraction
� safety device for operation in front of
the turntable
� safety fences
Fig.3 - A flexible manufacturing cell
In a flexible manufacturing cell or line we can
have one or more 7 axis robot. For the robots of the
type IRB 7600, the track motion acts as an integrated
seventh axis, as shown in figure 4.
Fig.4 - Robot on track motion (7 axis robot)
We used a track motion from ABB (fig. 5)
model type IRBT 7003S. The illustration shows the
principle layout of the track motion in the Compact
design [12].
Fig.5 - IRBT 7003S compact.
The component parts of the showing compact
track motion IRBT 7003S are [14]: 1. Gear rack; 2.
Linear guides; 3 & 8. End plates; 4. Side cover; 5.
Motor; 6. Serial Measurement Box / Brake release
box; 7. Carriage; 9. Cable chain; 10. Cable tray; 11.
Gearbox.
68
The table below contains technical data
performance for track motion IRBT 7003S (table 1).
Table 1 - Technical specifications
The illustration (fig. 6) shows the length mea-
surements of IRBT 7003S from the front.
Fig. 6 - IRBT 7003S, length measurement [14]
The length of track motion depends on travel
length, in next table are showing how many stands
(quantity N) we need for different travel length. One
stand is 1000mm long.
Table 2 - value of N for different travel lengths
When a 7-axis robot is mounted, this can be
mounted straight on the floor or on frame, if we need
that 7-axis to reach highest points. First is necessary
to fix plates under each stand or frame in the floor, on
these plates the track/frame is welded or bolted.
It is very important that before mounting the
robot on track, this has to be level. It is recommended
to use a laser level in the track motion’s direction of
travel and a spirit level across this to obtain precise
adjustment. Always the measurements are done on a
machined surface such as the linear guide or gear
rack. The accuracy must at least be ± 0,5 mm along
the track length and ± 0,1 mm in height between side
to side.
Geometric leveling of track motion may be
performed according to three different methods:
Method 1: Leveling the track motion by using
a spirit leveler for leveling the carriage horizontally
along the complete travel length.
Method 2: Leveling the track motion by using
position measurement equipment for leveling the
carriage horizontally along the complete travel length.
Method 3: Leveling the track motion by using
a laser leveling instrument based on available
geometric system layout.
We used method 3, using a FARO Laser
Tracker ION as leveling instrument and Metrolog
XG13 as measurement software. Also we place in
position using de datum from layout a several roller
beds, lifters and underbodies.
Usual the origin of the assembly line is same
with origin of the first underbody in line. First we
place the underbody in line, then lifters and roller
beds.
3. Results and Discussion
In the picture below (fig. 7) is represented one
report after leveling the 7-axis. The length of the
track was 6m.
Below I will present the results from zone with
framers where we had to align 3 robots 7-axis, 2
robots 6-axis, and one underbody, one roller bed with
lifter, 2 framers, one fixture for roof (figure 8 and 9)
and one conveyor on track motion (figure 10).
I used the same procedure which I presented in
work paper called “The measurements and
alignment of fixture in a cell assembly for car body” to align the fixture with robots.
Fig. 7 - Leveling report for 7-axis
Fig. 8 - Top view of alignment
Fig. 9 - Left side view of alignment
69
Fig. 10 - Conveyor report for leveling
I developed a procedure to align new fixtures
in assembly line which was aligned before and now it
running. In this case I couldn’t measure the flange of
robot because it has mounted tools for rivet. The new
fixtures were for different type of car, which was
shorter than the first car. Because I had the same
origin of the car I could use the old alignment to
position the laser tracker with the old tool then create
the reference for this tool. The robot already has the
position for the old tool, so I could create relationship
between 4 points from the robot in car line and the
old reference. After this I created a new alignment in
the reference of the new tool and generate de 4 points
for the robot for the new fixture, with the new
positions.
4. Conclusions
Is important to have a layout where is present
the position of fixture in the assembly cell or line.
For robots 7-axis is important to know where
is located the origin of the robot coordinate system.
The origin for 7-axis robot is located in manipulator
base frame center (fig. 11).
Fig. 11 - System coordinate for 7-axis robot
When we align an assembly line is important
to use same reference points to place the underbody,
roller beds, lifters and framers. When I create the line
for the middle of the car it has to be longer as
possible, to not have errors put all line together.
In case of having the fixture on the turntable is
important to know the angles for working position
and the turntable is able to rich every time this
position.
Acknowledgment
This work was partially supported by the
strategic grant POSDRU/88/1.5/S/50783, Project
ID50783 (2009), co financed by the European Social
Fund – Investing in People, within the Sectoral
Operational Programme Human Resources
Development 2007-2013.
References
[1] Auto/Steel Partnership Program - Body Systems
Analysis Task Force: Automotive Body
Measurement System Capability.
[2] Richard S. Figliola, Donald E. Beasley (2011):
Theory and Design for Mechanical
Measurements, John Wiley & Sons, Inc U.S.A.
[3] Lorenzo Morello, Lorenzo Rosti Rossini,
Giuseppe Pia, Andrea Tonoli (2011): The
Automotive Body, Volume I – Components,
Design Springer Science + Business Media B.V.
2011, ISBN 978-94-007-0512-8.
[4] Alan S. Morris. (2001), Measurement and
Instrumentation Principles, Butterworth-
Heinemann, A division of Reed Educational and
Professional Publishing Ltd, ISBN 0750650818.
[5] J. Norberto Pires, Altino Loureiro and Gunnar
Bölmsjo (2006): Welding Robots - technology,
systems issues and applications, London
Limited UK: Springer-Verlag.
[6] The ABB website (2012). [Online]. Available:
http://www.abb.com/
[7] W. Schreiber (2004): Planning of a car factory -
Automotive Product Development and
Production.
[8] The Metrologic Group website (2012). [Online].
Available: http://www.metrologic.fr/
[9] GMAB Consulting database with designed
fixtures.
[10] ABB product manual (2006): Product Manual
Track Motion - IRBT 7003S ABB Automation
Technologies AB, Sweden.
70