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

lmudura@yahoo.com, macedonganea@yahoo.com

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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