All car manufacturers need to maintain and strengthen their positions
long term by achieving higher quality, safety and sustainability, plus
user-friendliness, performance and design. In this case study, involving
the body welding cells at car manufacturer BMW AG's Dingolfing plant,
Jörg Lochmüller and Andreas Mühlthaler look at the use of 14,500
Profinet IO nodes to bring benefits to car production.
MODERN PRODUCTION systems can only be efficiently
controlled, monitored, optimised and
diagnosed with standardised and integrated
communication. Correspondingly uniform
standards in networking technology are also
essential. This is now becoming a reality for
BMW using the standardisation that derives
from industrial Ethernet.
Since June 2008, production in the body-inwhite
section of the company's Dingolfing plant
has used Profinet IO (PN IO) throughout the
entire welding cell involved in the manufacture
of BMW 7 series cars.
The BMW 7 Series that is produced in Dingolfing.
The requirements
Car production comprises five core steps: press
shop, body-in-white, paint shop, engine and
transmission production, and final assembly.
In body-in-white, precisely shaped pressed
sheet metal parts are assembled to form a
finished car body shell. Body production at the
Dingolfing plant is distributed across individual
welding robot cells. In being processed, the
parts are machined by industrial robots and
combined again to form larger units, and are
then further processed.
Almost all parts must pass through several
welding cells before a nearly complete vehicle
is obtained. The different machining stations
are interconnected via conveyor systems, such
as suspended monorails, roller conveyors,
elevators, decoupling buffer belts and accumulating
roller conveyors (Fig. 1).
Fig. 1: View of a welding robot cell. The welding cell for
the floor panel. The parts are moved to the next
machining step by elevator via connecting conveyors
The essential need was to use a single bus
system (network) for all applications. This
should handle all communication tasks, such
as transfer of individual job data and the safety
and control signals. Interfaces needed to be
avoided as far as possible, and specialist
knowledge and the number of commissioning
and engineering tools needed to be reduced. A
further demand was to achieve tool changing
within 500ms using real-time communication;
all the while, the latest safety requirements
had to be complied with.
In addition, it was important to provide
production system analysis via a single system
to make diagnostics of the entire network as
transparent as possible.
Cyclically precise in-feed
Every car rolling off the Dingolfing plant's
production line is a customised product. Using
ISO-on-TCP communication1, individual
customer orders are sent by higher-level control
to an order management system that controls
complete production. By connecting each
processing machine's controller via Ethernet,
production status is reported back to the
control system for the job in hand. The machine
operator sees the overview via the operator
console, so knows at any time when a particular
menu unit is to be used.
Smaller parts to be machined are fed into the
welding cell workstations partly manually by
the machine operator, who has to supply
several stations - so distances and timings are
optimised for maximum efficiency. To guarantee
safety for personnel and machinery, operator
workstations can only be entered with the
robots automatically stopped and interlocked
against restart (Fig. 2). Following sheet metal
insertion, the manufacturing process is
restarted. Larger metal sheets are inserted
using robots. The correct positioning of metal
sheets is checked automatically.
Fig. 2: Workpiece infeed. The workpiece is fed in partly by the machine operator who obtains an overview for the
remaining production sequence via the operator console seen to the right in this photo.
Although the plant is disconnected from the
operator cycle to an extent because of the robot
placement, it still needs many special mechanical
solutions, so commissioning is costly.
The plant system
About 1200 robots - mainly for spot welding
- are involved in body construction for the
BMW 7 Series and 5 Series cars. Other robots are
used for bonding, riveting, etc.
A total of around 14,500 Ethernet nodes must
communicate smoothly with each other in
exchanging data. These include the following
products:
• 3600 x Simatic ET200S distributed I/O
stations with standard and safety I/O
• 1300 x CP1616 communications processors
• 700 x Scalance X208 and X216 switches
• 150 x PN/PN couplers2
• 200 x Simatic S7-416F-3PN/DP controllers
• 4 x Simatic S7-317-2PN/DP controllers
• 4 x Simatic S7-319-3PN/DP controllers.
Previously, several bus systems were used,
but the use of Profinet IO allows only a single
bus system to meet all system requirements.
With Siemens as the automation provider for
the Dingolfing plant, the Simatic S7-416F
safety-related controller is the core of every
production cell. Between 10 and 15 controllers
are used in the plant area. The floor panel, for
example, consists of its own plant area
controlled by four Simatic S7-416F-3PN/DP
controllers. Each of these controls typically 10
robots that weld small metal sheets and fix
various fasteners. Around 60 to 100 controllers
are required to make each car body. Manufacture
of an external side frame alone, comprising 94
parts, needs a cycle time of just 2.1 minutes,
during which 46 robots with 60 welding tongs
and 52 'handlings' (gripper operations) apply
631 spot welds and bond 2.3m2 of material
together.
One bus suits all
I/O connection has changed with every model
in line with technological developments - from
the parallel input/output modules (I/Os),
through Interbus, up to PN IO. Previously, a
robot control cabinet had to be configured or
programmed with three different installation
technologies and with different commissioning
tools. There were point-to-point connections
(Interbus), a bus for the safety-related I/O
(Safetybus), and Ethernet controlling robot
PCs. Fault analysis was hampered by the varying
bus systems.
With real-time-enabled Profinet IO, all the
communication services, PCs, controllers, I/O
modules, robots, etc. can now be operated via
a single, standardised bus system (Industrial
Ethernet/Profinet). This results in just one
Ethernet-based installation technology and
correspondingly fewer configuring tools, such
as the Step7 programming environment3. The
robot's control centre is a PC-based solution,
where the CP1616 Ethernet card with integral
switch simultaneously functions as the Profinet
IO controller and IO device. Therefore, the robot
is addressed as a PN IO device by the higherlevel
PN IO controller, but is itself the PN IO
controller for its own I/O.
It is also possible to reduce the number of
engineering tools with Profinet - for example,
using Step7 to configure, program and diagnose
safety modules together with standard modules
means that users no longer have to deal with
different hardware systems (Fig. 3). The pointto-
point connections of the physical Ethernet
bus also make troubleshooting easier.
Fig. 3: View of the control cabinet. Showing ET200S distributed I/O with standard and safety inputs/outputs
Overcoming EMC problems
The original plan was to use polymer optical
fibre (POF) cables to control areas subject to
electromagnetic interference, such as welding
robot arms. However, extensive EMC measurements
and transmission characteristic tests
confirmed that Ethernet copper cables
(industrial Ethernet/Profinet) work fault-free
even in difficult electromagnetic environments
and with significant mechanical load. Cabling
of the entire welding cell is, therefore, possible
using this type of cable.
There are very significant time and cost
benefits from simply replacing industrial
Ethernet cables with IE FC RJ45 Plugs4 using
the FastConnect system. This needs no
specialist knowledge.
Maximum safety
The high safety demanded by today's standards
also presents a challenge. Safety category 3
(SIL 3) is achieved system-wide in the welding
cells. Even Category 4 would be possible on the
basis of the electrical systems used, but not for
the pneumatic, mechanical and robot systems.
The IEC 61508 certified Profisafe meets the
highest safety requirements with SIL 3 or
Category 4 in accordance with EN 954-1. It
enables flexibility through the use of stop
functions 'safe shutdown' (EMERGENCY OFF)
and 'safe stop' (EMERGENCY STOP), which puts
the plant into a safe state.
Simatic S7-416F controllers (Fig. 4) - which
have a long service life - with their integral
safety functions are responsible for control of
conventional plant sequences, as well as
production cell safety. This represents real
progress compared with the previous safety
technology, which was very wiring intensive
and costly. Previously, the production orders
were sent to the controller, not with ISO-on-
TCP, but using the Sinec-H1 protocol5.
Fig. 4: Control cabinet showing CPU with I/O. This is
fitted with a safety-related Simatic S7-416F controller
with I/O
For local operation, panel PCs are used in
most control cabinets. Remote operator
terminal solutions now work with (Microbox)
PCs, but these do not normally have the
necessary software packages and instead use
a terminal-server architecture to access the
main terminal where the full functionality is
available (Fig. 5).
Fig. 5: An operator panel. This cell contains a Simatic PC627B industrial PC for user-friendly local operation in the
welding cell control cabinets. This uses a terminal server architecture to access the main terminal
The plant structure, designed for long-term
availability, provides short transport routes for
car body parts, and there are subnets separated
by routers and switches for secure communication
between the welding cells and with the
higher-level IT via Profinet.
The enterprise connection
Each control cabinet is office network
connected for data exchange between
commercial/higher-level IT and automation.
For safety, IT assigns the plant its own subnet
with which IP addresses can be managed
autonomously. Layer 3 functionality is achieved
using separating routers. This strict network
segregation means that significantly less office
access is needed, which saves money.
Responsibilities are clearly defined if a fault
occurs, so that all plant-internal faults can be
corrected by the maintenance personnel
without needing external specialists. Again,
this saves time and money.
Connecting all welding cells to the higher-level
control system, plus the communication between
these systems and the higher-level backbone
necessitate using firewalls and routers for
security. Without these, all of BMW's Ethernet
nodes would be unprotected against unauthorised
access from anywhere in the world.
Connection between cells
Within the production cells, switches (Scalance
X-200) are fully integrated into Profinet, so
special infrastructure programs/programming
aids are unnecessary. If a fault occurs (e.g.
cable break), a message is automatically sent
to the control system, or via standard channels
(SNMP traps6) to the higher level office
network.
If a cable does break, its location can be
roughly determined using the Topological View
in Step7 or the Web interface of the CPU, and
its precise location can be found using runtime
measurement integrated into the switch.
To enable real-time data exchange between
automation cells, they are interconnected using
PN/PN couplers so that, for example,
emergency-off functions also take effect in
neighbouring cells. Figure 6 shows a view of
the distribution cabinet containing Scalance
switch, Microbox PC and ET200S distributed I/O.
Fig. 6: A view of the distribution cabinet. This contains
Scalance switch,Microbox PC and ET200S distributed I/O
With a new body construction plant such as
this, every single module used is subjected to a
feasibility study. Along with technical aspects,
commercial factors such as procurement costs
and maintenance costs also play an important
role. Components, such as Ethernet switches,
must be very strongly made with high ingress
protection and high resistance to EMC effects.
Simplicity and, therefore, faster commissioning
of the new communication technology
compared with that previously used, resulted
in significant economic benefits - even at the
planning stage. In addition, higher plant availability
has been achieved because faults can
be located and corrected faster.
As a result, this BMW Dingolfing site solution
using Profinet in the welding cell has now also
already been implemented in the company's
Regensburg and Munich plants.
Notes
1. ISO-on-TCP: Communication service commonly used in
automation
2. The PN/PN coupler enables cross-plant, fast and deterministic
I/O data coupling between two Profinet networks.
3. Step7: Configuring or programming software for Simatic S7
controllers
4. Rugged, industry-standard Ethernet connectors
5. The Sinec-H1 protocol is an Ethernet-based (communication)
service
6. SNMP: Simple Network Management Protocol (Trap:Message
frame for a network management system).
Jörg Lochmüller is Presales Consultant at the Industry
Automation Business Unit (Sensors and Communication),
Siemens AG, Nuremberg.
Andreas Mühlthaler is Promoter Automobile Industry at
Industry Sector Division - Industry Automation, Siemens AG, Munich.
Back
