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Magnetic Levitation Technology

	Figure 1: Principle of electromagnetic levitation

The principle of Magnetic Levitation
Active electromagnetic levitation is based on the attractive force of a controllable electromagnet on a ferromagnetic body (Figure 1). A control unit adjusts the current in an electromagnet and hence the magnetic force acting on the ferromagnetic body so that the body is held in suspension. A sensor continuously measures the position of the ferromagnetic body. If the ferromagnetic body is above the desired position, the controller reduces the current in the magnet and with it the magnetic force. If the body is below the desired position, the current in the magnet is increased.

Magnetic Bearings
A single electromagnet is incapable of stabilizing all spatial degrees of freedom of a rotor. Two electromagnets arranged in an opposed pair are needed just to orient the position of a rotor in one direction. Two such pairs of electromagnets positioned at right angles to each other form a "radial bearing." Like a ball bearing, this configuration is capable of holding a rotor in one position in a plane (x-y direction). Since the rotor must also be prevented from tilting, it must usually be held at two points with two radial bearings. Another pair of electromagnets is needed as a thrust bearing. Thus, a completely active electromagnetic rotor bearing system requires a total of ten electromagnets. An electronic control unit modulates the current in each electromagnet by using sensor feedback from five separate sensors.

Figure 2: Mechanical and electromagnetic components of a magnetically levitated high-speed spindle

Figure 3: Single-board control unit (sensor system, controller, power-amplifier) for control of a 5-axis magnetic bearing system)

Advantages of Magnetic Bearings
Magnetic bearings have specific properties that differentiate them from mechanical bearings. The principle advantages relate to the absence of physical contact and electronic control of the rotor position.

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