Active feedback stabilization of magnetic levitation

Motivation & Problem Description

Most magnetic levitation demos use a traction-type system, which means that the magnetic coil is located above the levitating object. This just doesn't look as cool as levitating an object with repulsion from below. However, repulsive levitation is generally unstable in two directions, which makes it harder to control. Additionally, most commercially available levitation systems have a significant power draw, which results in heating of the magnet & wasting of energy. Finally, the simplest levitation designs use optical sensors to detect the position of the levitated magnet, but there are problems with interference (eg, someone's hand blocking the beam, or the sensor being fooled by sunlight).

Solution

It is possible to create a simple configuration of magnets that is repulsively levitated and yet is unstable along just one axis instead of two. See for instance: levitating motor. A single controller is then able to stabilize the configuration longitudinally. The controller can then be designed to be inexpensive, low-power, and non-optical. Here is an example of an inexpensive controller that uses a Hall effect sensor to detect the magnet position, and an efficient & cheap pulse-width-modulation chip to drive the solenoid Low-cost magnetic levitation system for electronics learning. Additionally, this design uses a permanent magnet which attracts the electromagnet's iron core, so that there is a position where force balance can be achieved without current flowing in the solenoid. This in principle allows the system to draw zero power, on average, to support the magnet, if the stable position is adjusted correctly. The controller would then only drive current if the object was displaced, basically a virtual force that only exists when it needs to!

The next step is to add a negative feedback loop that automatically adjusts the stable levitation position to coincide with the zero-average-power position. This is not a new idea, but it looks as though nobody has implemented this for a small-scale undergrad lab or DIY project. Here is an article on zero-average-power controllers for magnetic levitation: Vibration isolation system combining zero-power magnetic suspension with springs (This also gives you a sense for the bizarre properties of such a system: if you push on the magnet, it moves toward you!)

Further references

Here is a paper on designing a small levitating train, which could be used to replace the air-hockey-style bumper cars in undergrad mechanics labs: maglev train

Thesis project with details of several different configurations

Design, Fabrication, and Control of a Single Actuator Magnetic Levitation System

Magnetic Levitation Experiments for a Control Systems course in an Electrical Engineering Technology Program

PWM Control of a Magnetic Suspension System

"Low-Cost" Magnetic Levitation Project Kits Not actually that inexpensive...

Self-sensing magnetic bearing control system design using the geometric approach Also talks about zero-average power control