When everything is shaking, precision is often impossible—and anyone who has ever tried to take a picture with shaky hands or take handwritten notes on an uneven bus ride knows this. And with technically precise measurements, even the smallest vibrations are a major problem, for example with high-performance microscopes or precisely aligned telescope mirrors. Even the smallest vibrations, too small for humans to detect, can render a measurement unusable.
A new type of vibration damping technology has been invented at the University of Vienna that solves such problems in an unusual way: permanent electromagnets are used. These magnets, like ordinary permanent magnets, maintain their magnetism permanently without the need for a power source, but they are also equipped with a coil so that their magnetism can be changed very quickly with an electrical pulse. This makes it possible, for example, to actively suppress vibrations in the mirrors of large telescopes and thus significantly increase their performance.
Floating platform with nanometer precision
The vibration damping system at Vienna University of Technology consists of a permanently fixed base and a free-floating platform above it. The platform is suspended in the air, held in place by strong magnetic forces. Several electromagnetic actuators can then adjust the position of the platform with high precision in fractions of a second – even when a load weighing several kilograms is attached to the platform.
“In sensitive applications, such as positioning mirror parts, the position of this platform must be kept stable to within a few tens of nanometres,” says Professor Ernst Sensics from the Institute of Automation and Control Technology at the University of Vienna. “This is only possible if you can compensate for even small ground vibrations, such as those caused when someone walks outside the laboratory, or those caused by normal building vibrations.”
Therefore, the position of the platform must be measured very precisely and any movement must be resisted immediately. This makes it possible to suppress vibrations very efficiently, especially low-frequency vibrations, which are usually a problem in such applications.
Electromagnets require constant power.
“Electromagnets are usually used for such active vibration damping. Current flows through coils in a magnetic field, and depending on how strong this current is, different forces can be generated. This works very quickly and precisely,” explains the institute’s director, Professor Georg Schetter.
But one of the main drawbacks of this technology is that the current must flow continuously, otherwise the magnetic forces disappear immediately. On the other hand, a permanent magnet can maintain its magnetic properties for any period of time without any external energy source – once it is magnetized by a very strong magnetic field.
The permanent magnets we use in our daily lives, as we know them from magnetic plates or refrigerator magnets, are made this way too: you need a suitable, magnetic material and expose it to a strong magnetic field once. This creates a magnetic system in the material, making it remain magnetic permanently.
Targeted remagnetization of permanent magnets
The researchers have now succeeded in combining the advantages of electromagnets and permanent magnets in absorbing vibrations using a so-called permanent electromagnet. “This is a permanent magnet that also has a coil,” says Sensics. As long as the strength of the permanent magnet is in the right range, it requires no power and the floating platform is held in place. Only small corrective measures are needed by the motors to compensate for the vibrations.
However, if the strength of the permanent magnet is no longer suitable, for example because the weight supporting the floating platform has changed or because it needs to be tilted, more drastic methods are used: A short, strong current pulse is sent through the coil, which creates a very strong magnetic field for a moment and thus also changes the magnetism of the permanent magnet. By selecting the correct magnetic pulse strength, the permanent magnet can be set to a new operating point, where it remains stable again without the need for a power source.
Working prototype, patent pending
This control can be automated: the system automatically recognizes whether it is still close to the desired operating point or whether remagnetization is necessary. “We have developed the necessary control technology over the past two years and it already works very well,” says Ernst Sensics. The invention has already been patented with the support of the research and transport support team at the Vienna University of Technology.
“With our prototype we have demonstrated that it is possible to suppress vibrations with great precision and save energy,” says Georg Schetter. “This technology would be ideal for large telescopes, for example, which consist of several mirror segments. It would be possible to align the telescope with different areas of the sky, and the mirrors would then have to be aligned with great precision and kept stable in each position. This is exactly what our technology would be ideal for.”
But in principle, the vibration damping technology using electromagnets could of course be applied in other areas, such as the precision production of semiconductor chips and large, high-quality optics, adaptive actuators or precision measurement technology in laboratories. “Our technology is an interesting solution wherever you need the highest possible precision that can be affected by vibrations,” the researchers say.
