Introduction - Electronics Now, Feb 1996

How Does It Work?

Magnetism and a closed-loop control system are the secrets to the stunning presentation produced by this electromagnetic levitator.

An electromagnet creates a magnetic field that attracts a hollow steel globe or similar object upward. The globe doesn't crash against the magnet, however. Instead, as it draws near, the magnet's intensity weakens, letting the globle drop slightly. As it drops, the matnet's intensity again increases, pulling the globe up again. The process is so smooth, however, that the globe appears to float, held in space by invisible forces.

An infrared emitter and detector mounted across from each other create an invisible beam that passes slightly below the coil. As the object rises towards the electromagnet, it begins to block the beam. When the beam becomes blocked, theoutput of the detector is reduced which in turn reduces the current in the electromagnet's coil. The reduced current weakesn the magnetic field, the object begins to drop and the detector once again sees more of the beam. This causes the circuit to increase the magnetic field and the cycle repeats as the object is attracted upwards again.

The circuit is designed so that eventually an equilibrium is reached where the magnetic attraction exactly balances the force of gravity pulling on the object. The object then remains perfectly suspended in the infrared beam's path with no visible means of support!

Basics of Closed-Loop Control

Open- and closed-loop control systems surround us. An open-loop system is one in which the signal that controls the output is independent of the output itself. By contrast, the signal controlling the output of a closed-loop system is dependent on the system's output.

A space heater or table-top fan are examples of open loop control systems. they are controlled by a switch, and once they are turned on, their output remains constant regardless of how hot or cold it gets.

A home furnace and air conditioner are examples of closed-loop control systems. Both monitor their outputs — the hot or cold air — with a thermostat, and regulate their outputs accordingly. The thermostat closes the loop by feed back an error signal to the furnace or air conditioner.

This article is reprinted from Electronics Now, February 1996.