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Industrial Ethernet Book Issue 69 / 37
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Ethernet powers fast return for car maker

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.

www.automation.siemens.com


Source: Industrial Ethernet Book Issue 69 / 37
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