For some motorists, the hearty rattling of an engine is part of the fun of driving. But most people prefer a motor with a somewhat more unobtrusive sound. For this reason, car makers thoroughly test the acoustics of the various models and their tires at different speeds and on different types of road surfaces. In special test rigs, cars roll on drums that measure two to three meters in diameter, with various road surfacing materials attached round them in adjacent strips. Such test rigs are expensive, can be dangerous at high speeds, and require a good deal of space. What is more, they are not suitable for examining the effects of road surface imperfections and similar sources of vibration – this 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 50 Hz as well as vibro-acoustic stress in the 50 to 1000 Hertz range", explains Michael Matthias, deputy director of the Mechatronic / Adaptronics Competence Center at LBF. The researchers begin by using sensors to record vibrations on the tires and in the car interior that occur when driving on various road surfaces at different speeds and with different tire profiles. These vibrations are then archived in a database, so that they can be quickly be 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", explains 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) – they still need a bit more time on the lateral vibrations, such as those that occur in curves.
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