Magnetic Levitation

Magnetic Levitation

Author:- M.Sundeep Goud (sundeep.2488@gmail.com)

               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.

               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.

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.