Understanding Levitation

Electromagnetism and Levitation

In basic levitation (Fig 1) an object appears to float due to the invisible forces of magnets. A magnet creates a field that forms two opposing poles: North and South (Fig 2). Opposite poles attract each other while similar poles repel. For magnetic levitation, there is a fixed magnet and a smaller free moving permanent magnet, which is the object that will levitate. This object has two forces exerted on it: downward force from gravity and upward force from the fixed magnet.

Figure 1
Levitation using electricity to create a magnetic field.
http://www.dimi.uniud.it/~franco/levita_0.jpg

In levitation, these two forces must be equal. The gravitational force is F = ma = mg, where m is the mass of the object and g is the gravitational constant 9.81 m/s^2. The force resulting from the magnetic field interaction is very difficult to calculate. For our experiment we will be using more of a guess and test method.

Figure 2
Fields caused by magnets can be “shown” using a magnet and iron filings
www.lhup.edu/~dsimanek/ scenario/analogy.htm

The problem with levitation using permanent magnets is that there is exactly one equilibrium position in which the force from the magnet counteracts the force of gravity. If the object becomes closer, it is pulled with an even stronger force towards the magnet and if it becomes further away, the force from the field is too weak and the object simply falls to the ground. This is what makes levitation unstable.
One way to counteract this problem is to replace the permanent magnet with a coil of wire that creates a magnetic field called a solenoid. (Fig 3) When a current is run through a solenoid it produces a magnetic field. The magnitude of this field is given by

where B is the magnetic field, is a constant, and n is the number of turns per length in the coil. The magnetic field of the solenoid is controlled by the amount of current going though it. Solenoids are used to make magnetic levitation more stable. The strength of the solenoid is controlled through circuitry, which alters the amount of current going through the solenoid depending on the position of the object. There are a number of ways to measure the position of the object which will be discussed in the circuit section.

Figure 3
A solenoid and a bar magnet and their magnetic fields.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/barsol.gif

Summary of Patent 4,910,633

On March 20, 1990 Louis Quinn was granted a patent for a levitation control circuit using two sensors on either side of a ferromagnetic core. More specifically, the circuit contains two linear Hall effect sensors, an element to sum their two outputs, a reference voltage, an element which compares the reference voltage and the summed sensor outputs, and an element which then adjusts the electromagnet voltage accordingly. In team Ukiyo’s circuit, the summing elements are TL084 Operational Amplifiers and the adjustment element is the IRF840 field effect transistor. Also, the reference level voltage is set by the second potentiometer (marked POT2). The patent improves upon and simplifies the tradition levitation control systems which the patent claims are “complicated, generally expensive” and “operate unsatisfactorily” (5). Older circuits relied on producing exactly the right force to counteract gravity. This meant if the field was off by even a small amount, the levitated object would either be attracted to the electromagnet or lose levitation completely.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic fields and levitation and more particularly to a method and apparatus for levitating objects using a magnet attached to the object to principally lift an object toward a core element of an electromagnet while driving the electromagnet with a variable duty cycle and frequency power source to fine tune the levitation forces. The invention further relates to a method of detecting magnetic field strength and variations therein between the core of an electromagnet and an adjacently suspended permanent magnet.

2. Background of the Related Art

The ability to levitate objects in a magnetic field is considered to be useful for many applications. One obvious application is in the area of model displays and toys. Levitation is very useful for adding a sense of realism and accuracy as part of suspending many models, such as those of satellites, aircraft, spacecraft, and the like, in mid-air. It is also desirable to be able to suspend some objects that comprise an artistic formation or work in mid-air. At the same time, however, levitation has beneficial applications for scientific work such as where isolation of a chemical or material both electrically and physically from its surroundings is desired. It may also be desirable to use magnetic levitation to suspend some materials during processing or storage to counterbalance some of the forces of gravity or to better control material positioning in low gravity environments.

In the past, several attempts have been made to provide methods or apparatus for levitating objects. Generally, such apparatus comprises one or more electromagnets, although permanent magnets have been used in some configurations, powered by an adjustable strength current source. The electromagnets are suspended above, or below, an object to be levitated and generate magnetic fields which are used to attract metal in the object, or repel permanent magnets mounted on the object. The electrical current supplied to the electromagnets is adjusted to vary the strength of the magnetic field established by the electromagnet so as to just counter the force exerted by gravity on a suspended object.

One major problem in previous levitation apparatus was to sufficiently or properly balance magnetic attraction, or repulsion, against gravitational forces on a levitated object to achieve levitation. That is, the object must be levitated with sufficient force to prevent releasing it to fall and, at the same time, without attracting it so strongly as to cause it to contact the magnet or surrounding structure. This is accomplished using a combination of sensors to detect the magnetic field strength, and feedback control over the electromagnets based on the sensor data. However, previous attempts at such controls have produced complicated, generally expensive, control circuits which operate unsatisfactorily in many applications. The sensors require very precise or critical alignment which precludes many commercial applications. Transient lateral motion or wobble of the object also causes severe problems for the feedback controls typically resulting in loss of levitation.

What is needed is a method and apparatus for levitating objects that has less stringent construction.

For more information visit: http://patft.uspto.gov