Application Example: Quality Control

Quality assurance on BMW convertibles

Measuring System: TRITOPCMM

Keywords: Process optimization in production line, Quality assurance in
production, Validation of fitting dimensions, Calibration stability of assembly
jigs, MCS, PCS

Mobile optical 3D coordinate measuring technology is used for quality

assurance on the production line in the BMW Regensburg plant. For the
assembly of roof modules for convertibles, optical measuring equipment is used
in process planning for the machine capability study (MCS) even before the start
of serial production. On the assembly line the photogrammetric system then
enables flexible inspection to be carried out during manufacture. Thus trends
and deviations in production can be detected at an early stage. As result of
using optical metrology rework time is reduced and production costs are cut
down significantly. Also Quality Management can be specifically improved by
means of process capability studies.

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Application Example: Quality Control

Quality assurance on BMW convertibles

Measuring System: TRITOPCMM

Keywords: Process optimization in production line, Quality assurance in
production, Validation of fitting dimensions, Calibration stability of assembly
jigs, MCS, PCS

In the BMW Regensburg plant, all the necessary production steps for
automobile manufacture, from the press and body shops to final assembly, are
located on one site. In assembly, as the last section at the heart of the
manufacturing process, finished BMWs are produced from approximately 20,000
components in about 100 working steps. Here, in a unique range of variants,
the BMW 1-Series (5-door), the sedan, the coupe and the convertible from the
BMW 3-Series as well as the corresponding M3 models, and also the BMW Z4
Roadster are produced in a mixed stream on a variant-neutral main conveyor.
This type of individualized automobile production requires classical
manufacturing care and the most up-to-date logistical control in assembly. The
individual parts not only have to be supplied "just in time" but also "just in
sequence", in other words in the order corresponding to the vehicles on the
assembly line. In the case of the 3-Series convertible and the Z4 Roadster, roof
module assembly is a particularly sensitive task. It places high demands on the
operatives, the process and the auxiliary equipment used. To secure this process
the mobile optical TRITOPCMM system is used to monitor the distortion-free
installation of the convertible hardtop modules, the quality of the seals and
uniform gaps.

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This measuring system enables independent and flexible inspection to be
carried out directly on the assembly line. It allows the calibration stability of
assembly jigs to be indirectly checked, and sensitive work steps and critical
vendor parts to be analyzed. By detecting unacceptable deviations at an early
stage and as a result of the short response times, the TRITOPCMM system
contributes significantly to saving rework time, optimizing processes and
thereby reducing production costs.

Measuring task: Inspection of assembly brackets

In order to fit the hardtop modules without distortion, the assembly brackets
(connecting pieces between the body and the roof module to be fitted) must be
precisely positioned. This is the only way to ensure the correct engagement of
the roof shell with the windshield panel and the exact fit of the seals to remain
functional in the long term. Accurate assembly also prevents compression of the
hard top, and troublesome noises when opening and closing. Uniform gap and
flush of the seals relative to one another must also be achieved. For this reason,
two operatives connect the six brackets (three on the right and three on the
left) to the body using a roof jig (Fig. 1). A maximum tolerance limit of 0.7 mm
is allowed. Continuous position control of the pins, threads and holes in the
brackets is necessary to guarantee consistent quality in the respective assembly
processes. Of particular importance here is the ability to intervene in the
assembly process quickly and selectively as soon as unacceptable deviations are
detected.

Fig. 1: Assembly brackets for the 3-part foldaway hard top roof module and fitting
positions in the body

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TRITOPCMM: Optical 3D Coordinate Measurement System
TRITOPCMM is a transportable optical measuring system which precisely
determines the 3D coordinates of individual points on an object (Fig. 2). The
areas to be checked are identified with self-adhesive or magnetic markers
before the measuring process (Fig. 3). Adapters, which in this case have been
modified to suit the specific task, can be used for measuring features such as
cylinders, holes, threads, balls and border lines (Fig. 3).

Fig. 2: TRITOPCMM measuring system: Fig. 3: TRITOPCMM consumables: Self-adhesive and

Photogrammetric camera with magnetic markers, control adapter, special adapter
accessory for bracket measurement

The measuring object is photographed from different viewing angles with the
TRITOPCMM photogrammetric camera. From the digital images, the 3D coordi-
nates for the markers and adapters are automatically calculated on-line by
means of bundle adjustment computation. Two certified scale bars guarantee
the accuracy and process security of the measurement.

Fig. 4: Display of bundle adjustment Fig. 5: 3D coordinates measured from a series of

of three camera positions 2D images in the TRITOP software

Camera case, laptop and scale bar case can easily be carried by one person.
Likewise, only one person is needed to carry out the measuring process. No
external power supply is required neither for the measurement nor for the
subsequent evaluation.

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Requirements of the measuring system: Precise, mobile and fast
Before being put into operation the optical TRITOPCMM measuring system was
subjected by the operatives to the typical BMW incoming inspection and
probationary tests. Here, the MCS (machine capability study) with different
users consistently gave outstanding results with regard to precision, repeatabi-
lity and reproducibility. "It soon became clear to us that measurement from
images is a precise and controllable process, as, thanks to the pixel deviation
and double scale bars, the system makes the measuring accuracy of a project
verifiable in many respects", explains Manfred Grimm, responsible user on site.
"The system is also easy to use and can be quickly calibrated by the user before
each project to suit the environmental conditions", adds Alexander Besenreiter,
another TRITOPCMM operator at BMW. "It is also a major advantage that we now
have a measuring system which enables us to take measurements directly on the
assembly line so that we are no longer dependent on inflexible tactile measu-
ring equipment. In the past, we had to book a measuring station, take the
vehicle out of the assembly shop into the measuring room. Because of this
time-consuming planning and logistical procedure, we could sometimes only
check one vehicle per day", explains Manfred Grimm. A control measurement is
currently carried out on every 15 vehicles on average, so that up to 5 vehicles
can now be checked in one shift. The mobile TRITOPCMM system can be comfor-
tably carried to the place of use by one person (Fig. 6, 7) and the measurement
can also be carried out by a single person.

Fig. 6, 7: TRITOPCMM portable optical measuring system, operation without external power
supply

Approximately 45 minutes per measuring process are required including to

channel the vehicle out of the production line and back in again. Any number
of features can be measured without significantly increasing the measuring
time.

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Measuring procedure: TRITOPCMM in the BMW Regensburg plant
The pins, external threads and internal threads of the assembled brackets for
the BMW 3-Series convertible and the BMW Z4 are fitted with appropriately
marked adapters for carrying out the measurement (Fig. 8). The pattern of
reference points on the adapters is calibrated to the respective fixing point of
the bracket surface. This enables CMM points with their respective deviations
relative to the desired position to be output after the measurement (Fig. 10).

Fig. 8: Fitting the pins, external threads and internal threads of the assembled roof
brackets with the appropriate TRITOPCMM adapters for carrying out the measurement

Fig. 9: Measuring process

directly adjacent to the
assembly belt

Coded orientation crosses and two scale bars are also placed in the measuring
field, before the area of the measurement is photographed with several shots
from different viewing angles (Fig. 9). The images are automatically transferred
to the laptop as the measurement is carried out. The 3D coordinates of all
bracket assembly points are calculated less than a minute after the images have
been transferred. Measuring reports corresponding with the manufacturing
requirements are automatically produced with the help of the adapters (Fig. 10).

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Deviations are detected immediately thanks to the rapid evaluation directly
adjacent to the assembly line. This enables problems in the assembly chain to be
analyzed fast and to introduce necessary corrective measures quickly.

Fig. 10: Measuring reports produced immediately after the measurement with the CMM
points of the adapter brackets on the vehicle. The corresponding deviations from the
desired position can easily be seen from the clear visualization.

Measuring results: Targeted analyses with TRITOPCMM

"On the one hand, the TRITOPCMM system enables us to routinely check the
specified tolerance limits close to the assembly line when fitting the brackets,
and on the other to react quickly and objectively if these are exceeded",
explains Siegfried Gebhard, the project manager responsible for this section of
the assembly. "The measurements allow us to continuously monitor the jigs and
their calibration stability. This enables out-of-calibration jigs to be corrected at
an early stage before they are used to fit many more brackets to the vehicles. A
replacement jig can thus be used while the assembly jig to be corrected is
recalibrated", continues Siegfried Gebhard. "At the same time, the mobile
measuring equipment gives us the opportunity to analyze fitting problems. This
enables us to optimize assembly processes and to provide efficient retraining
for our fitters." For example, the TRITOPCMM system has enabled us to establish
that the sequence of tightening the screws when assembling the brackets is
important. It is also crucial that the fitters work in synchronism, as otherwise a
large part of the tolerance buffer would be used up. Furthermore, typical
fitting problems, such as the incomplete locking of the assembly jig when
fitting the screws, can be detected as such and remedial action taken appropria-
tely quickly.

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The TRITOPCMM software not only enables features to be inspected directly by
easy to understand and well visualised measuring reports. Long-term trends can
also be detected and analyzed by exporting the measurement data in table
form (Fig. 11).

Fig. 11: User-defined measuring report with CMM measuring points and output of
measurement data in table format for analyzing long-term trends (cp, cpk values)

Additional benefit: Verification of vendor parts

The easy-to-use TRITOPCMM system requires no Measuring Technician as
measurements and evaluations can be carried out by assembly operatives, as is
the case here in Regensburg. As these operatives are highly familiar with the
individual processes in the assembly line, they can very quickly put the signifi-
cant additional benefits of mobile measuring technology into practice. While
initially only the bracket adapters were measured, the position and dimensions
of the rubber seal of the A-pillar node are now also verified simply by attaching
measuring markers (Fig. 12). This susceptible fit of the seal between roof shell
and windshield panel can therefore also be inspected as part of the standard
measuring process. The possible existence of a batch with problematic vendor
parts, such as inaccurately manufactured brackets or roof modules which are
inconsistent in themselves, can also be checked using the optical measuring
equipment. Unlike with tactile measuring techniques, this does not increase the
measuring time, as the additional measuring points are visibly located within
the measuring volume. The operatives in Regensburg have therefore also made
a new adapter for determining the peak of the roll-over bar. This guarantees
reliable setting and positioning of the position adapter on the assembly jig -
and this is certain to be far from the last idea from the BMW operatives.

Fig. 12: Verification measurement of the rubber seal on the A-pillar by attaching measuring
markers also enables vendor parts to be analyzed within the standard measuring process

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Well-established: Optical Metrology in the BMW Regensburg plant
"After thorough investigation, we have very quickly learned to appreciate the
advantages of optical surface measurement", concludes Manfred Grimm, one of
the operatives responsible for measurement. "The mobile measuring system
enables us to carry out measurements close to production, immediately on the
assembly belt. Thanks to the lack of dependence on the tactile measuring room
and the fast measuring time of less than 45 minutes including channeling the
vehicle in and out of the line, we can inspect many more automobiles per shift.
The short measurement and response time enables us to detect inconsistencies in
the assembly line and to respond very quickly. At the same time, we can also
analyze whether a jig is out of calibration, whether there is a fitting problem or
perhaps that there are vendor parts which do not comply with the agreed
specifications."

Fig. 13: The TRITOPCMM system helps to comply with the high optical and functional
requirements of the automobile industry. By saving retrospective adjustment, the system
helps to reduce production costs and optimize working procedures.

The sooner inconsistencies in the assembly line are detected, the quicker
deviations can be prevented and rectified which avoids unnecessarily long
rework. This is because small adjustments can be made efficiently with
reconditioning aids. And the early measurement and routine inspection of the
assembly brackets can also prevent a possible collision of roof modules and
windshield panel during testing later in the assembly line.
"The TRITOPCMM system helps us to comply with the high optical and functional
requirements of the automobile industry, such as compatible seals and uniform
gaps. The mobile optical measuring system helps us by saving rework, reducing
production costs and optimizing working procedures", says Siegfried Gebhard
as the responsible project manager, summarizing the experience with the
TRITOPCMM system.

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