Semi-Active Control Devices To Cut Rail Noise

Test-bridge during field tests

Trains that shake houses do not encourage public support for the switch of freight from road to rail. CASCO promotes social acceptance for trains by making them quieter using small, low-energy damping devices.

The environmental, social and economic costs of freight transport by road are key reasons to encourage the switch from lorries to the railways. Trains are cleaner, often quicker, use less energy and are safer for pedestrians – but railway companies that want to build new tracks or bring old lines back into use meet strong resistance. Concerns about infrastructure noise and vibration exposure raise strong objections from citizens and their representatives.

Noise and vibration can arise from several sources: the rough contact between wheels and track is one, but others are less obvious, such as the interaction between wheel, track and bridge structures that can travel through the ground and induce vibrations in neighbouring buildings. The CASCO project brings a new approach to reducing noise and vibration on the railways using innovative semi-active control devices that make a more effective use of materials and minimise resource consumption.

“To reduce noise and vibration in steel structures you need to dissipate energy by damping – this is traditionally done by passive control,” explains project coordinator Dr Helmut Wenzel, managing director of Vienna Consulting Engineers (VCE). “Passive controls use the structure’s system response to achieve damping. Active controls are another current technology, these rely on big electrically driven actuators, but for large structures they consume a lot of energy and need regular maintenance.

“The semi-active devices CASCO develops are stand-alone, miniature devices with low power requirements, based on magneto-rheological fluid dampers. These contain fluids that change viscosity dramatically in a magnetic field and can be set to dissipate vibrations from the structures they support. Such actuators are installed at critical locations throughout structures and underneath railway tracks to eliminate vibration. They prevent the vibrational kinetic energy from passing into the soil by dissipating the energy locally. A key advantage is that because of their size and robustness they can be retro-fitted in many cases.”

Bridging theory and experiment

Railway tunnels and bridges are strong sources of noise, both airborne and by vibrations through the soil. The CASCO consortium selected the Rohrbach railway bridge in Austria for trials; this steel bridge was built in 1903 and is no longer in use.

Initially extensive measurements were carried out using sensors attached to the sleepers and girders on the bridge. Emitted noise was measured both with and without a train passing – and the noise of the train itself was carefully eliminated to isolate structural noise. By matching the observed vibrational frequencies to structural elements of the bridge, it was possible to isolate the main contributions to noise.

The bridge was then dismantled, transported and reassembled at the European Laboratory for Structural Assessment (ELSA) at the European Commission’s Joint Research Centre DG in Ispra, Italy. ELSA is a partner in the project with state-of-the-art equipment for active structural testing. “Once we rebuilt the bridge at Ispra, we repeated our measurements to ensure the system response of the structure was the same in the new location,” says Dr Wenzel. “This was important for calibration and to allow a true comparison.”

Another partner, the University of Applied Sciences in Vienna, developed a unique approach to modelling rheological fluids that was used in the design of the CASCO prototypes. It also has potential for more widespread application.

The outcome of the project is a suite of innovative semi-active devices shown to have excellent results in damping vibrational energy. In particular a new ‘cup-spring’ isolator was developed, the subject of a patent and capable of using both hydraulic and magneto-rheological mechanisms. Partners continue to develop the cup-spring isolator and are moving to commercial development for selected prototypes. Further, full-scale devices are being made for in-situ testing by Austrian Railways (ÖBB), which has dedicated an operational steel bridge for this purpose.

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