A Novel Displacement Sensor for Magnetic Levitation

Magnetic levitation is a technique to suspend an object without any support other than that of a magnetic field. The technique finds application in various fields where friction or abrasion between metallic surfaces (like bearings) or products of friction are detrimental to the environment or to the rubbing surfaces themselves. In a simple configuration, an electromagnet is used to lift up a ferromagnetic object and the gravity pulls down the object. A stable position is expected when these two forces are equal. This is basically an unstable system. To make the system stable, a feedback signal proportional to separation between the electromagnet and the levitated object and its time derivative is required to control the current of the electromagnet. Most of the workers have used an optical sensor to obtain a signal dependent upon separation between the object and the electromagnet. An optical sensor requires additional mountings and circuitry that make the system clumsy and unreliable. To avoid the problems caused by optical sensor, an alternative method to obtain a signal depending upon separation between the magnet and the object has been presented in this paper. An additional winding is put on the electromagnet and change in its inductance due to displacement of the object is determined by employing synchronous demodulation technique.