Double-Tube Tunnel Opens to Traffic with Redundant PROFIBUS

By 22 September 2014

The Pfänder tunnel is not only one of the most important motorway connections between Germany and Austria, but is also used by many commuters to bypass Bregenz, the capital of the Austrian province of Vorarlberg. The construction of the tunnel with a length of approx. 6.7 km in the 1970s proved very difficult due to the geological conditions of the site, with increased swelling and water seepage potential. For cost reasons, only a single tunnel tube with one traffic lane in each direction was built at that time. Today about 30,000 vehicles, including approx. 5,000 trucks, use the Pfänder tunnel eSofting_Tunnelueberwachung_F6B1109_01very day, often causing long traffic jams on both sides of the tunnel. To ensure a smooth traffic flow and to reduce the risk of accidents in the tunnel, the construction of a second tunnel tube was begun in 2006. Following its completion and the termination of the refurbishment works on the existing tube in July 2013, the Pfänder tunnel now offers two lanes in both directions and traffic jams are largely a thing of the past.

The re-opened Pfänder tunnel features cutting edge supervisory control system based on PROFIBUS DP.
The construction of a second tube for the Pfänder motorway tunnel in Austria involved a modernization of the supervisory control system for the entire tunnel and the integration with the tunnel control center. This upgrade to state-of-the-art technology was performed by evon GmbH. The requirements included the integration of all field devices into the communication system and the implementation of a redundancy solution. In view of the long distances to be covered in the overall facility and the huge data volumes to be transmitted, evon decided to employ the PROFIBUS DP Master Stack from Softing Industrial Automation.

Redundant system for highest availability
The connection of the various field devices in the Pfänder tunnel is based on PROFIBUS DP technology with baud rates of 1.5 MBaud and 93.87 kBaud in the fieldbus, respectively. All in all, the various PROFIBUS configurations with their copper or optical fiber cables use 129 PROFIBUS DP slaves whose cycle times are < 20 milliseconds. The requirement for highest availability and stability of the entire supervisory control system – which is key to ensuring reliable 24/7 tunnel control – had an impact on the design of the PROFIBUS DP network. It was intended to implement a redundancy solution with two PROFIBUS DP Masters, which is able to switch over seamlessly from the active master to a backup master, if needed. This means, in particular, that after the redundancy switchover, each PROFIBUS DP device continues to work and transfer its data without interruption and without requiring re-parameterization of the field devices. As master redundancy has not been specified in the PROFIBUS DP standard,

evon had to find a way to implement the required functionality.
evon found the ideal solution at Softing Industrial Automation who offered the desired redundancy functionality for Softing’s own PROFIBUS DP Master. This way, all that was left for evon to do was to integrate the new functionality into the XAMControl supervisory control system.

Continuous data exchange instead of re-parameterization
Using two PROFIBUS DP Masters and switching between the active and the backup master alone does not provide the required redundancy functionality yet. It is also necessary to include all field devices in such a way that switching over to the other PROFIBUS-DP-Grafik_ENPROFIBUS DP Master will not result in a default restart of the PROFIBUS DP slaves. This behavior can only be achieved if the backup master adopts the bus address of the previously active master, and thus seamlessly continues the cyclic data exchange after the switchover. The PROFIBUS DP Master from Softing, and the Softing PCI interface card employed in the XAMControl supervisory control system support the required redundancy functionality.

To ensure the redundancy functionality, it is also necessary to analyze the state of the backup master to find out whether its bus interface is still operational. For this purpose, the backup master initially uses a different bus address than the active PROFIBUS DP Master so that the two masters can exchange special test frames based on PROFIBUS FDL. This process not only checks the local bus connection, but also the PROFIBUS segment between the two masters. The backup master only adopts the bus address from the active master when a redundancy switchover occurs. A special function then allows the backup master to take over the preset master parameterization and to immediately start the data exchange based on the current process image held by the backup master. This ensures that the field devices connected to the PROFIBUS DP network remain unaware of the switchover.

As a result, the PROFIBUS DP interface card from Softing was successfully integrated into XAMControl and the requirements placed on the Pfänder tunnel supervisory control system and the integration with the tunnel control center were completely fulfilled. For Andreas Leitner, Managing Director at evon GmbH, the possibility to also apply this solution to other projects is equally important: “We’ve been able to seamlessly implement the interface from our Pfänder tunnel application into other projects without a need for modifications. For example, these projects can access PROFIBUS via small compact controllers, PC interface cards, or also via PROFIBUS remote interfaces in the Ethernet network. This way, we are well prepared for the future.”