THE MEASUREMENTS AND ALIGNMENT OF FIXTURE IN A CELL...
Transcript of THE MEASUREMENTS AND ALIGNMENT OF FIXTURE IN A CELL...
The 6th
edition of the
Interdisciplinarity in Engineering International Conference
“Petru Maior” University of Tîrgu Mureş, Romania, 2012
THE MEASUREMENTS AND ALIGNMENT OF FIXTURE IN
A CELL ASSEMBLY FOR CAR BODY
Lucian MUDURA, Macedon GANEA
University of Oradea, Oradea
str. Universitatii nr. 1, Oradea, Romania
[email protected], [email protected]
ABSTRACT The research problem for this paper is to measure and align one fixture for a spot
welding or riveting in a cell assembly for car body where this is done by one robot or
more robots. The fixture can have several working positions, which means to find the
fixture position according with the robot position.
Keywords: fixtures, bodyside welding, measurements, robots, laser tracker
1. Introduction
The automotive industries are one of the most
important industries in the world and are the largest
companies. In recent years the automotive industry
has increasingly improved the assembly line of the
car body. Is desired to make an automatic process for
spot welding / riveting of car body as much possible,
but all along with this line was to be flexible, to be
able to assemble different car models on it. [6]
The robot is an automatic mechanism that can
replace human to perform some operations, being
able to change one execution cycle (by photoelectric
sensing, actuators).
Industrials robots are basic components of
today's modern industry and especially of the future
industry. Request for using industrial robots is
particular for their potential for flexibility, as
intelligent machines that can perform operations in
repetitive mode, at low price of cost and high quality.
Robotic welding is one of the most important
and successful applications are used industrial robots.
Many welding processes require assembling each of
the components.
Industry using the most industrial robots for
welding is automotive industry using these robots for
spot welding operations (riveted), loading / unloading
parts etc.
In next figure is a 6-axis robot with the
notation of the axis (the example is ABB robot
IRB6400 [5]).
The ABB IRB6400 robot is a large six-degree
of freedom open loop robot. The IRB6400 is used for
a wide variety of applications, it is predominantly
used for automotive applications such as body
painting and welding. The robot is capable of
carrying up to a 200 kg load at the end of its 3-meter
maximum reach. Typically, the motors are
constrained to have the tool center point (TCP) move
at a velocity around 2 meters per second, although the
robot can travel at greater speeds [5].
Fig. 1 - Axis notation for ABB robot IRB6400
Each robot has a coordinate system which has
the origin in center of the base of the robot. The robot
can work in more than one coordinate system, but
only one is active and the system can be changed
during the work by program. The coordinate systems
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for robot are shown in figure below.
Fig. 2 – Coordinate systems for robots.
In next figure (fig. 3) is presented a fixture
used to assemble car parts trough spot welding or
riveting. This fixture is on turntable and has more
than one working position.
Fig. 3 - Fixture used for spot welding/riveting [10].
2. Materials and Methods
The paper studies the specific measurements
made to align one fixture with one robot in an
assembly cell (fig. 4). I used one fixture which has
more working position. The fixture (STN540) is
mounted on turntable and the zero position for
turntable is as show in figure 4. This fixture is
working with on robot for riveting (R540) and is
using another robot for loading (R520). The turntable
is turn by a drive with high precision and accuracy
which gives high repeatability to rich the working
position.
The working positions for this station (STN
540) are:
• POS 1 = 10 deg (rivet b stg1)
• POS 2 = 75 deg (rivet c stg1)
• POS 3 = 45 deg (rivet f Stg1)
• POS 4 = -30 deg (load with R520)
Fig. 4 - Fixture mounted on turntable [10].
By ”cell alignment” we understand the position
of fixture origin related to the origin of the base of the
robot, in other words how far is the zero car system
coordinate from the zero robot. This is important to
know then we can know if robot can reach the points
where robot has to weld or rivet.
When I measure the ”cell alignment”, I
measure the reference of the fixture and then I
measure 4 points for the robot R540 on the tool
flange according with car system coordinates. From
RobCAD, software used to simulate the cell assembly
and generate a theoretical work program, I have the
distance from de origin of the robot (center of the
robot base) to the origin of my coordinate system (car
coordinate system).
That 4 points measured are giving the actual
position of robot R540 with the working position of
fixture. For each working position is necessary to re-
measure the reference points to have the new distance
between the robot R540 and fixture STN540.
The same procedure we used to measure the
loading position (POS 4 at -30deg) with robot R520,
using the same reference point for the fixture and
measure 4 points for the robot R520.
In this case I had three measurements for
riveting with robot R540 and one measurement with
robot R520 for loading parts.
The theoretical values (riveting data) for
measuring from RobCAD were (table 1):
Table 1 - Data riveting from RobCAD
For measuring I used a laser tracker FARO
ION (fig. 5) with Metrolog XG13. The program
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Metrolog XG13 is made by Metrologic Group.
Metrologic Group specializes in the design and
manufacture of industry reputed 3D inspection
software and electronics.
Metrologic has developed more than 60 direct
machine interfaces to connect its software to any
controller including CNC CMMs, articulated arms,
laser trackers and 3D optical Scanners [9].
Fig. 5 - FARO
® Laser Tracker ION™
The system specifications are[8]:
Dimensions Head size: 311mm (W) x 556mm (H)
Head weight: 17.7kg (19.5kg w/IFM option)
Controller size: 282mm (L) x 158mm (D) x
214mm (H)
Controller weight: 5.2kgHorizontal Scale Bar
Measurement (2.3 m)
Range Horizontal envelope: ± 270°
Vertical envelope: +75° to -50°
Minimum working range: 0 meters
Maximum working range: 55m with select targets
40m with standard 1.5” & 7/8” SMRs
30m with standard 1/2” SMR
Distance Measurement Performance (Agile ADM) Resolution: 0.5µm
Sample rate: 10,000/sec
Accuracy: 8µm + 0.4µm/m
R0 Parameter: 8µm
The measurements of the fixture are done with
the 3D model of the car body. The measurements are
done according with the car coordinate system. The
touching points are measure as surface points and the
locators are measure as cylinders (8 points are
measure for a cylinder) [2]. For the locators is created
intersection between cylinder and the plane of the
hole in the panel. The tolerances for touching points
are 0.13 mm and for locators is 0.1 mm.
3. Results and Discussion
In the picture below are represented the
references (fig. 6) and values (table 2) used to align
the fixture with 3D model used for inspection which
is in car system coordinate [1]. The measurements
units are millimeters.
Fig. 6 - Fixture references in car coordinate
system.
Table 2- Value of fixture references
In next figure is represented a surface point
measured with the panel data.
Fig. 7 - Touching points between tool and car part
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Fig. 8 - Locator position in car part
The theoretical model cannot be implemented
in reality as in theory. Because of this it is important
to find out the error position for fixture or turntable.
For this is important to measure the robot position
according with position of the fixture in car line.
In table 3 we have the deviation for fixture
alignment with the robot. This means that we have to
compensate this deviation through program than the
offline program match with what we have.
Table 3 - Alignment deviation
4. Conclusions
Is important to have a layout where is present
the position o fixture in the cell.
It is important to use a laser tracker to put the
fixtures and robots on floor in the cell.
It is important to have a start point (zero point)
for the assembly line and a start point (zero point) for
assembly cell in relationship with start point of the
assembly line.
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] User manual – Metrolog XG13.003.
[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] Patrick Willoughby, Course 2.05 December 5,
2000: Position Kinematics of the ABB IRB6400.
[6] J. Norberto Pires, Altino Loureiro and Gunnar
Bölmsjo (2006): Welding Robots - technology,
systems issues and applications, London
Limited UK: Springer-Verlag.
[7] The ABB website (2012). [Online]. Available:
http://www.abb.com/
[8] The FARO website (2012). [Online]. Available:
http://www.faro.com/
[9] The Metrologic Group website (2012). [Online].
Available: http://www.metrologic.fr/
[10] GMAB Consulting database with designed
fixtures.
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