Talking about good vibrations

To give their cars the right acoustic properties on the road, manufacturers assess them on a variety of test rigs, which can prove complex and expensive. A new adaptive car structure test facility combines stress and hydraulic testing stations in a single compact model.

Carmakers test the acoustics of the various models and their tyres at different speeds and on different types of road surfaces before they come to market. The test rigs consist of two to three metre-diameter drums with various road surfacing materials attached round them, on which the cars roll.

Such test rigs are expensive, can be dangerous at high speeds, and require a lot of space. They are also unsuitable for examining the effects of road surface imperfections and similar sources of vibration, which can only be done with hydraulic systems.

Researchers at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt have for the first time developed a test rig that combines all of the various trials in a single system.

"We expanded the hydraulic test rigs by adding a highly dynamic stress interface, so that we can now simulate structural loads in frequency ranges up to 50Hz as well as vibro-acoustic stress in the 50 to 1000Hz," said Michael Matthias, deputy director of the Mechatronic/Adaptronics Competence Centre at LBF.

The researchers begin by using sensors to record vibrations on the tyres and in the car interior that occur when driving on various road surfaces at different speeds and with different tyre profiles. These vibrations are then archived in a database so that they can be quickly fed into the test rig.

"We use an algorithm to modify this vibration signal in the same way that it would change on its way to the ear during real-world driving", said Matthias. "That way, we get a realistic sound." On the test rig, the wheels are replaced by electric motors that work against the car's acceleration and simulate the resistance generated by gradients and road friction.

This "adaptive car structure test facility" gives the scientists an environment in which to develop active systems for preventing vehicle vibrations. But it is also suitable for test operations by auto manufacturers. The researchers have already succeeded in producing realistic vibrations along the Z-axis (vertical in relation to the road). Lateral vibrations, such as those that occur in bends in the road, will require further development time.