Electromagnetic suspension is magnetic levitation achieved using servo-controlled electromagnets.
Suspension
The attraction from a fixed strength magnet decreases with increased distance, and increases at closer distances. This is termed 'unstable'. For a stable system, the opposite is needed, variations from a stable position should push it back to the target position.
Stable magnetic levitation can be achieved by measuring the position and speed of the object being levitated, and using a feedback loop which continuously adjusts one or more electromagnets to correct the object's motion, thus forming a servomechanism.
Many systems use magnetic attraction pulling upwards against gravity for these kinds of systems as this gives some inherent lateral stability, but some use a combination of magnetic attraction and magnetic repulsion to push upwards.
This is termed Electromagnetic suspension (EMS).
For a very simple example, some tabletop levitation demonstrations use this principle, and the object cuts a beam of light to measure the position of the object. The electromagnet is above the object being levitated; the electromagnet is turned off whenever the object gets too close, and turned back on when it falls further away. Such a simple system is not very robust; far more effective control systems exist, but this illustrates the basic idea.
EMS magnetic levitation trains are based on this kind of levitation: The train wraps around the track, and is pulled upwards from below. The servo controls keep it safely at a constant distance from the track.
Dynamic stability
Although adjusting the magnetic field can give static stability it does little to deal with dynamic stability; the object will wobble, and it turns out that if there is a slight time delay in the magnetic field adjustment with distance (as there typically is) this wobble will increase in amplitude until suspension is lost.
For dynamic stability to occur, if, for example, the levitated object starts to move in a sinusoidal fashion, then a magnetic field at 90 degrees to the position (negative feedback on the speed) must be provided.
See also
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