Weird Science

Defying Gravity: How Photoelectric Stabilization Keeps Magnetic Levitation Steady

Polish ver­sion is here

Magne­tic Levi­ta­tion: Impos­si­ble?

The English word "levi­ta­tion" comes from the Latin term "levi­tas," mea­ning light­ness. Levi­ta­tion is the phe­no­me­non of an object fre­ely suspen­ding without phy­si­cal con­tact with any sur­face.

Histo­ri­cally, levi­ta­tion was con­si­de­red a para­nor­mal phe­no­me­non or an illu­sio­nist’s trick invo­lving peo­ple or objects flo­a­ting in the air. Howe­ver, today, there are nume­rous tech­ni­cal methods to achieve prac­ti­cal levi­ta­tion, even for large objects. Active magne­tic levi­ta­tion is one of the sim­pler and more relia­ble methods, which is why this article focu­ses on it.

Ope­ra­ting Prin­ci­ples

An object in a state of levi­ta­tion is sub­jec­ted to a force that holds it suspen­ded without direct con­tact with its sur­ro­un­dings. In the case of magne­tic levi­ta­tion, this force results from magne­tic inte­rac­tion. Levi­ta­tion occurs when the object rema­ins suspen­ded in equ­i­li­brium without fal­ling. The for­ces acting on the object are illu­stra­ted in the dia­gram below:

Ilustracja

Here, Fg repre­sents the gra­vi­ta­tio­nal force, while Fm repre­sents the oppo­sing force gene­ra­ted by the elec­tro­ma­gnet. Accor­ding to New­ton’s First Law of Motion, an object rema­ins at rest when no for­ces act on it or when the for­ces acting on it are balan­ced. In this case, the lat­ter applies: for the object to levi­tate, the con­di­tion Fg = Fm must be met, where gra­vi­ta­tio­nal force equ­als magne­tic force.

A signi­fi­cant prac­ti­cal chal­lenge is that no sta­tic and sta­ble con­fi­gu­ra­tion of magne­tic or elec­tro­sta­tic for­ces can main­tain levi­ta­tion! Earn­shaw's the­o­rem sta­tes that for­ces inver­sely pro­por­tio­nal to the squ­are of the distance cre­ate inhe­ren­tly unsta­ble sys­tems.

None­the­less, there are methods to achieve sta­ble levi­ta­tion. One of these is active sta­bi­li­za­tion. In this appro­ach, the magne­tic attrac­tion force is regu­la­ted thro­ugh nega­tive feed­back based on the object's posi­tion. The object's distance from the elec­tro­ma­gnet is con­ti­nu­o­u­sly mea­su­red (in this case, using a pho­to­e­lec­tric method), and the magne­tic force is adju­sted accor­din­gly. This ensu­res sta­ble levi­ta­tion. The sim­pli­fied dia­gram below illu­stra­tes the prin­ci­ple of ope­ra­tion:

Ilustracja

The sys­tem inc­lu­des two pho­to­sen­sors: one mea­su­res the object's posi­tion, while the other moni­tors ambient light levels. This setup eli­mi­na­tes inter­fe­rence from exter­nal light sour­ces other than the laser. The signals from the pho­to­sen­sors are pro­ces­sed by a con­trol cir­cuit, which ampli­fies and dif­fe­ren­tia­tes the signals. The out­put signal dri­ves a tran­s­i­stor that regu­la­tes the elec­tro­ma­gnet.

Con­struc­tion

To build a magne­tic levi­ta­tion device, visit the nea­rest elec­tro­nics store and obtain the com­po­nents listed below:

The semi­con­duc­tor laser is a stan­dard laser poin­ter. Care­fully disas­sem­ble the poin­ter, sol­der wires to its con­tacts, and insu­late the con­nec­tions using a heat-shrink tube for pro­tec­tion.

Ilustracja:

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Next, build the elec­tro­ma­gnet. I wound mine using ena­me­led cop­per wire with a dia­me­ter of 0.3mm (appro­xi­ma­tely 0.012 inches) on a card­bo­ard bob­bin. The core is made of trans­for­mer lami­na­tion she­ets, but an ordi­nary steel bolt can also be used.

Ilustracja:

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Assem­ble all com­po­nents accor­ding to the sche­ma­tic dia­gram:

Ilustracja:

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In this sys­tem, T1 acts as the posi­tion sen­sor, while T2 mea­su­res the back­gro­und illu­mi­na­tion. U2A func­tions as a dif­fe­ren­tial ampli­fier with a gain of appro­xi­ma­tely 2 (deter­mi­ned by the ratio of R5 to R4). Its out­put is fed into a dif­fe­ren­tia­tor cir­cuit con­si­sting of C4 and R7. The dif­fe­ren­tia­tor (or deri­va­tive) is a mathe­ma­ti­cal func­tion that indi­ca­tes the rate of change of the input signal. The dif­fe­ren­tia­ted signal is then ampli­fied by a second-stage ampli­fier with a gain of about 47x (the ratio of R9 to R8). This ampli­fied signal dri­ves the BUZ11 MOS­FET tran­s­i­stor, which con­trols the elec­tro­ma­gnet coil. Diode D1 acts as a fly­back diode, sup­pres­sing vol­tage spi­kes indu­ced by chan­ges in coil cur­rent. The vol­tage regu­la­tor U1 pro­vi­des a sta­ble sup­ply vol­tage for the ope­ra­tio­nal ampli­fiers and pho­to­tran­si­stors. Capa­ci­tor C1 smo­o­ths the sup­ply vol­tage, while resi­stor R1 adju­sts the laser’s bri­ght­ness. If neces­sary, adjust resi­stors R2 and R3 to fine-tune the sys­tem.

All com­po­nents sho­uld be secu­rely moun­ted on a PCB, except for the laser, elec­tro­ma­gnet, and pho­to­tran­si­stors, which sho­uld be posi­tio­ned as shown in the dia­gram above. The assem­bled setup sho­uld resem­ble the fol­lo­wing exam­ple:

Ilustracja

Care­fully place a small fer­ro­ma­gne­tic object or magnet within the laser beam’s path. Depen­ding on the object’s wei­ght, you may need to adjust its distance from the elec­tro­ma­gnet. As shown in the video below, the levi­ta­ting objects are highly sta­ble­—they remain suspen­ded even when distur­bed or spun. Sur­pri­sin­gly heavy objects, such as half of a fer­rite core from an old black-and-white TV fly­back trans­for­mer, can also be levi­ta­ted due to the strong magne­tic field gene­ra­ted by the elec­tro­ma­gnet.

It is worth noting that a simi­lar feed­back and auto­ma­tion sys­tem is used in magne­tic levi­ta­tion tra­ins, such as those in the MAGLEV sys­tem. The MLX01 model (shown below) ope­ra­tes using this prin­ci­ple and can reach spe­eds exce­e­ding 500 km/h (310 mph).

Ilustracja

Source: http://dai­ly­ge­ek­show.com/wp-con­tent/uplo­ads/2014/11/maglev-shin­kan­sen-sur-rails.jpg, acces­sed: 11/25/2014

There are also other prac­ti­cal methods of sta­bi­li­zing levi­ta­tion, inc­lu­ding some that do not requ­ire addi­tio­nal energy input.

We hope you enjoy explo­ring this fasci­na­ting and edu­ca­tio­nal pro­ject. Happy expe­ri­men­ting!

Fur­ther rea­dings:

Marek Ples

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