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Magnetic Levitation

Magnetic Levitation


This is a simple magnetic levitator which suspends objects a set distance below an electromagnet. The physics behind it is to simply provide a magnetic force which equal and opposite to the gravitational force on the object. The two forces cancel and the object remains suspended. Practically this is done by a circuit which reduces electromagnet force when an object gets to close, and increases it when the object is out of range.

Infrared Magnetic Levitator

Magnetic Levitator Schematic

This circuit works by comparing the signals from the sensors with the first op-amp and sending out a voltage proportional to the difference or "error". The error signal is then sent through a compensation network which acts a high-pass filter, allowing quick changes in error to pass easier than slow changes. This is required to stabilize the control loop, and without it objects would just flutter close to the electromagnet due to the system being unstable. The signal is then amplified to it's original amplitude, since the compensation network attenuated it, and finally drives the TIP122 Darlington transistor, which controls the electromagnet current. The extra diodes around the transistor are to prevent damage to the transistor. The signal diode on the base prevents reverse biasing the base, which is damaging, while the two 1N4001's give a path for the magnetizing current to flow when the electromagnet turns off. The optical components used aren't too critical, as long as their wavelengths match up ok, and the detection/emission angles aren't too narrow. The IR LEDs are TIL38, which are 940nm peak, have 15 degree spread, 35mW and 100mA max. The detectors are PT204-6B, which are IR phototransistors.

Levitator setup

Construction Details
The top detector is a reference detector and the bottom one senses when an object is in levitating position. The object detector must be level with the IR LED. The reference detector must see the IR diode at all times, even when levitating an object. The electromagnet should have maximum 15 ohms of resistance, any more and it will not be able to lift anything. Too little resistance and the transistor will have problems regulating electromagnet current and will also dissipate more heat. When constructing an electromagnet there are two things to remember. Magnetic force is proportional to the number of turns and current. So when using copper wire the magnetic force is roughly proportional to the square of the power dissipated in it, for all practical use. In my coil I used 70 meters of 0.45mm magnet wire. The coil can be wound on almost any ferrous metal rod if you're not concerned with efficiency. Remember that keeping the surface area of the face small will keep the object centered better. Constructing the circuit correctly is easiest if built in two parts; part 1 with the detectors and first op-amp to make sure the output swings when an object is put in the beam, and then part 2 with the rest. This way it will be much easier to troubleshoot.

Troubleshooting
If the levitator fails to suspend anything, but objects “flutter”, you will need to adjust the compensation network. Try changing resistor values and capacitor values. The compensation network is the 150K, 1K resistor and 100nF cap between op-amp1 and 2. Decreasing the value of the 150K and/or increasing the value of the 1K will often solve the fluttering problems. Sometimes fluttering problems are caused by the reference detector not being setup properly. If the coil doesn’t turn off even when an object is put in the beam, check the reference and object detectors.

Levitated Objects

Floating bolt  Floating Lego mini figure  Floating hexnut

I scavenged a small neodymium magnet from a CD-ROM drive, which is quite powerful for it's size. Using a permanent magnet will increase the force the electromagnet exerts on an object, and allow for greater distances or heavier objects to be levitated using the same amount of power.

Pliers levitated with magnet

Youtube Video



Hall-Effect Magnetic Levitator


Another method for detecting the levitated object's position is using hall-effect sensors. These are sensors which detect the intensity of the magnetic field they are currently in, and provide an output voltage which is proportional to it. Hall effect sensors are often used in brushless DC motors to detect the position of the rotor, and as such can be found in floppy and disk drives. After experimenting with some hall-effect sensors scavenged from floppy and CD-ROM drives I found they are usually linear, symmetrical 4-pin devices. Which means they have no defined polarity and diagonal pins correspond to input/output and anode/cathode pairs. The sensors I used in the end were taken from a CD-ROM spindle motor, because the one I had from a floppy drive had been crushed. Remember to shield the sensor from objects suddenly being drawn to the electromagnet. I epoxied a 2mm thick plastic cover over mine to keep the impact force spread over a larger area. Initially I experimented with a feedback design, but found I couldn't stabilize the system. Unequal amounts of flux through the two sensors from the electromagnet and reduced sensitivity when in a strong magnetic field caused strong hysteresis in the system. After a throwing the board out the window a few times I finally accepted the KISS principle.

Schematic

The voltage between the + and - output of the hall effect sensor can swing positive and negative, but when referenced to ground one will go up and the other down (or the opposite, depending on the magnetic field's polarity). In order to get a useful sensor output the two voltages are compared and amplified by a differential amplifier. You'll need to decide which magnetic pole you want as positive, this can be changed by reversing the sensor leads. Whichever you choose, make sure the output of the LM358 rises positively to it's approach, or the levitator will repel the object!  The sensor output is compared to the reference, which is set to some defined error level. The TL494 will adjust the duty cycle of the electromagnet in order to keep the sensor output at the correct level. I've added a bonus feature to use up the other op-amp in the LM358, a temperature sensor. It's glued to the electromagnet, and will reduce the magnet current once it gets too warm. Almost unnecessary in this model, as it operates very efficiently. You will find maximum power cannot be achieved with your electromagnet using this circuit. I could levitate objects at twice the distance using an earlier prototype, see the first picture below, but I was unable to stabilize it. I'm unsure as to why it limits power so much, but it's sufficient for light loads. Some details on the electromagnet: The core is an M8 (8mm in diameter) flat-head carriage bolt, taken from my junk box. The wire was taken from a PFC inductor, wire gauge is estimated to be 1mm in diameter. I used all of the wire available, which might have been 50 meters. The DC resistance of the electromagnet is 2,4 ohms. The physical dimensions of the electromagnet are 2,5cm long by 4cm in diameter.

 

A lot of people have trouble finding these little symmetrical hall-effect sensors, but a standard three-pin linear hall effect sensor such as the SS495 can be used instead. Due to the variations in hall effect sensor design and sensitivity certain component values have to be found through experimentation. Here's an alternate schematic featuring a three-pin HE sensor.

Hall Effect Levitator Demonstration


Other Magnetic Levitators:

Barry's Magnetic Levitation
Home-Built Magnetic Levitator






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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.