When everything shakes, precision is usually impossible. Anyone who has ever tried to take a photo with shaky hands or jot down handwritten notes on a bumpy bus ride knows this. In technical precision measurements, even the smallest vibrations are a major problem, for example with high-performance microscopes or perfectly aligned telescope mirrors. Even the smallest vibrations, which are not even perceptible to humans, can render the measurement result unusable.
The Technical University of Vienna has invented a new vibration damping technology that solves this problem in an unusual way: electropermanent magnets are used. These are magnets that, like ordinary permanent magnets, retain their magnetism permanently without a power supply, but are additionally equipped with a coil so that their magnetization can be changed extremely quickly by an electrical pulse. This makes it possible, for example, to actively suppress the vibrations of the mirrors of large telescopes and thus significantly increase their performance.
A floating platform with nanometric precision
The vibration damping system at TU Wien consists of a fixed base and a floating platform above it. The platform is suspended in the air and 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 of several kilograms is mounted on this platform.
“In sensitive applications, such as the positioning of mirror segments, the position of this platform must be kept stable to within a few tens of nanometers,” explains Professor Ernst Csencsics from the Institute of Automation and Control Technology at TU Wien. “This is only possible if even minimal floor vibrations, such as those that occur when someone walks past the laboratory or those caused by normal building vibrations, can be compensated for.”
The position of the platform must therefore be measured with extreme precision and any movement must be immediately neutralized. This allows vibrations to be very effectively suppressed, especially low-frequency vibrations, which are usually a problem in such applications.
Electromagnets need constant power
“To actively dampen vibrations, electromagnets are usually used,” explains Professor Georg Schitter, director of the institute. “A current flows through coils in a magnetic field and, depending on the intensity of this current, different forces can be generated. This works very quickly and precisely.”
The major disadvantage of this technology is that the current must flow continuously, otherwise the magnetic forces disappear instantly. A permanent magnet, on the other hand, can retain its magnetic properties for an indefinite period of time without any external energy input – once it has been magnetized by a very strong magnetic field.
Everyday permanent magnets, such as those known from magnetic boards or refrigerator magnets, are also manufactured in this way: a suitable magnetizable material is required and it is exposed once to a strong magnetic field. This creates a magnetic order in the material, which allows it to remain permanently magnetic.
Targeted remagnetization of permanent magnets
The researchers succeeded in combining the advantages of electromagnets and permanent magnets to dampen vibrations with an electropermanent magnet. “This is a permanent magnet that is also equipped with a coil,” explains Csencsics. As long as the power of the permanent magnet is in the appropriate range, it does not require any power and the floating platform is held in place. Only small corrective measures of the actuators are necessary to compensate for the vibrations.
If the power of the permanent magnet is no longer suitable, for example because the weight of the floating platform has changed or because it has to be tilted, then more radical methods are used: a short, strong current pulse is sent through the coil, which creates a very strong magnetic field for a moment and also changes the magnetization of the permanent magnet. By selecting the appropriate magnetic pulse power, the permanent magnet can be set to a new operating point, at which it then remains constant without the need for an energy supply.
Working prototype, patent pending
This control can be automated: the system automatically recognizes whether the desired operating point is still close 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 Csencsics. The invention has already been patented with the support of the research and transfer team at the Technical University of Vienna.
“With our prototype, we have demonstrated that it is possible to suppress vibrations extremely precisely and energy-efficiently,” explains Georg Schitter. “This technology would be ideal for large telescopes, for example, which consist of several mirror segments. The telescope must be able to be oriented towards different areas of the sky, and the mirrors must then be aligned with great precision and held stable in each position. This is exactly what our technology would be ideal for.”
In principle, the electro-permanent magnet vibration damping technology could of course also be applied to other areas, such as the precision production of semiconductor chips and high-quality large optics, adaptive actuators or precision measurement technology in the laboratory. “Wherever the highest possible precision is required that could be disturbed by vibrations, our technology is an interesting solution,” the researchers are convinced.
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