Tram drivers have to bring their passengers safely to their chosen destination through frequently chaotic traffic, negotiating jams and road works. They are not only exposed to noise from surrounding traffic and machinery, but also to the noise made by their own vehicles – this often significantly affects their concentration and raises their stress level. The monotonous hum from the piston compressor, for example, is always clearly audible. It is located in an air-conditioning unit above the driver’s seat and ensures that the temperature in the cab remains at a comfortable level. While it is running, it produces vibrations that travel along the roof of the tram and reverberate inside the vehicle. As part of the InMAR project (Intelligent Materials for Noise Reduction), engineers at the Fraunhofer Institute for Structural Durability and System Reliability (LBF) in Darmstadt, Germany, recently developed an active interface that isolates the vibrations from the compressor and prevents them from being propagated to the tram roof. For the driver, this means less noise – so less stress. “Three active mountings are used to reduce vibrations from the compressor up to 15 decibels, both over a wide frequency spectrum and at individually selectable frequencies,” says Christoph Axt, a member of LBF’s Mechatronics/Adaptronics team. The original prototype which was created for InMAR has since been further developed in cooperation with ISYS Adaptive Solutions, and will be shown at the Hannover Messe (Adaptronics joint booth in Hall 2, Stand D34). The manufacturer of the air conditioning unit had identified the compressor as the source of noise. Following experimental studies showed that in order to reduce vibrations, the range of travel needed had to be bigger than the one achieved with conventional piezoelectric stack actuators such as those used in injection systems in the automotive industry, which only permit travel of around a thousandth of their length. Axt says: “Since we had only limited space available to accommodate the mounting, we implemented a transmission for the actuators.” In the new anti-vibration mount, four actuators exert pressure on large rams, which in turn act on a small ram through an elastomer transfer medium, in a similar manner to a hydraulic transmission. The horizontal arrangement of the actuators combined with vertical force transfer enable the required shallow design to be achieved. The modular construction, with the elastomer area in the middle and the actuators to the left and right, has the additional advantage that defective parts can be easily replaced. Moreover, the component has been enclosed in a robust casing to protect the sensitive internal components from environmental influences. The piezoelectric actuator unit is supplied with power using a low-cost piezoelectric amplifier – its particularity is to operate on 12 V, the standard on-board voltage for today’s vehicles. “Even when it came to the electromechanical components, we were careful to choose a cost-effective, compact and modular design”, says Dr Frerk Haase, who was also involved in the development. “If we need more electrical power, we can add more modules.”

With an eye to volume production, the scientists are now attempting to simplify the design of these active structural solutions and are using standard components wherever possible. This reduces costs, even for small quantities. The piezoelectric actuators in the compressor mounting, normally used in vehicle fuel injection systems, are just one example. “Apart from the impressive technical performance demonstrated by such systems, the sales aspect is crucial. Costs and technical reliability are at the forefront of all our discussions with customers,” says head of department Dr Tobias Melz. The researchers are currently testing the active mounting in the laboratory, after which it will undergo life testing.