Magnetic Levitation

FPAA Projects

Magnetic Levitation

By Subramanian Ramachandran and Lav Thyagarajan Under the guidance of Dr. Stephen Grodzinsky

INTRODUCTION

Magnetic Levitation!!! The name itself brings about an expression of awe on every individual’s face. The experiment involves suspending i.e. LEVITATING an object in a magnetic field. This wonder of magnetism has inspired many and has become an important field of study in today’s world. The concept has been very successful in the industrial and transportation sectors. Due to this magical phenomenon, MAGNETICALLY LEVITATED TRAINS have set ground transportation speeds to as high 430 Km/Hr with high efficiencies in power consumption!!! In this experiment, we have taken the initiative of levitating an object through the use of a new programmable analog device called as an FPAA – Field Programmable Analog Arrays.

COMPONENTS NEEDED 1. 2. 3. 4. 5. 6. 7. 8.

AN221K04 Development board from ANADIGM. MCT2E Optocoupler 2N2222 Transistor IRF540 Power Mosfet Air Core Bobbin. Neodymium magnet. Object for Levitation Hall Sensor – Ratiometric and Linear - A1321EUA from Allegro Microelectronics (www.allegromicro.com - one can order samples after online registration) OR SS490 from Honeywell.

9. Bread Board. 10. DC Power supply: +/- 10 V for the FPAA Board. 11. DC Power supply: 5V (for Hall sensor) and 20 V for the Solenoid. 12. DC Power supply: 0 – 5V (variable) for the Set point voltage.

BLOCK DIAGRAM

DC POWER SOURCE

GND

+10V

-10V

FPAA IP1 + IN1 IP2 IN2 ON1(-) + – OP1(+)

+V GND

DC +20 Source GND

Variable DC Power Source 0 – 5 V.

+5

CV

GND +5 V

D S

POWER SWITCHING CKT.

 Figure 1 D: Drain lead of Power MOSFET S: Source lead of Power MOSFET L: Levitating distance between object and solenoid. CV: This is the connection to the base of the 2N2222 transistor. A detailed circuit is shown in Figure 2. Vcc: The Vcc shown in the diagram above is for the Hall sensor.  Note: Dark circles indicate connections.

PROJECT DESCRIPTION Our basic idea was to levitate the neodymium + object at a certain close distance from the solenoid. Given below is the detailed description of how to go about doing the experiment.

SOLENOID WINDING Any small steel bobbin can be used for this purpose. Even a high strength plastic or wooden bobbin can be used. The next step is to wind insulated copper wire around the bobbin so that the resistance is 21 ohms or higher. Make sure that the winding is uniform – start slowly from one edge of the bobbin and proceed towards the other end. Also make sure that the thickness of the winding around the bobbin is more or less uniform throughout the length of the bobbin. This helps to create a more uniform magnetic field strength along the air core center point above/below the solenoid. Once wound, apply electrical insulation tape around the w inding to hold it in place.  HALL SENSOR Mount and fix the Hall sensor on the bottom surface of the solenoid. Make sure the sensor is centered on the surface – this helps to measure the magnetic field better for better control. The Hall sensor has three leads – Vcc, GND and Vout. Connect Vcc lead to +5V and the GND lead to the common ground of the system. The Vout lead is connected to the input terminal labeled IP1 of the FPAA. The terminal IN1 of the FPAA is connected to the common ground of the system.

 POWER SWITCHING CIRCUIT  The circuit diagram below shows the connections for rigging up the power switching device. A simple 2N2222 transistor is in series with an optocoupler to transmit the switching pulses to the gate of the power MOSFET. The optocoupler’s role is to provide a safe isolation between the low voltage signals and the high voltage/current part of the circuit.

DETAILED CIRCUIT DIAGRAM OF THE POWER SWITCHING CIRCUIT

MCT2E

 Figure 2  Pins 3 and 6 of the Optocoupler MCT2E are to be left unconnected.

FPAA PROGRAMMING

The software used: ‘AnadigmDesigner’ can be downloaded from the website: www.anadigm.com . One has to register online to obtain the authentication code and License key for the same. Programming is done through the serial port of the PC. If  unavailable, a USB to COM Port cable has to be used. The software contains basic analog device/circuits for easy analog system design. Our goal was to use this programmable board to demonstrate one of the many such capabilities and applications possible as a part of new course in our university. Hence its use and implementation in the magnetic levitation project. Circuits can be easily constructed through a drag and drop option from the CAM list. (CAM : Configurable Analog Modules). We used a simple comparator circuit module with a dual input voltage option. The input voltage from the Hall sensor and the set point voltage (from an outside DC voltage source) are the two inputs to the comparator module. Since the hall sensor voltage varies between 0 and 5 volts DC, the set point should also be set within this range. Note: It is not advisable to set low set point voltages as the capability of the solenoid to hold or levitate the object magnet decreases. This again depends on the strength of  the magnetic field the solenoid is able to generate at 20 volts. One can also increase the voltage to 24 volts to generate a higher magnetic field but again, this is at the expense of an increase in solenoid heating due to the high current drawn.

POLARITIES

The orientation of the hall sensor is important for proper operation. The voltage output of the hall sensor should increase as the object is brought closer to the solenoid. If the hall sensor output voltage decreases as the object nears the solenoid,  flip the orientation of the hall sensor.  LOGIC 

The logic used is simple: When the hall sensor voltage crosses the set point, the  output of the comparator goes low and when it is below the set point, the output of   the comparator is high. The levitation in this experiment is achieved through ATTRACTION of the object magnet. When the hall sensor voltage is below the set point voltage, the comparator output voltage is high and consequently the transistor is switched on. A gate voltage (= 4.8 V) exists at the gate of the MOSFET. The IRF540 is an N-channel MOSFET and needs a positive gate voltage to turn ON – hence it starts to conduct and current flows in the solenoid to attract the object magnet closer to the solenoid. When the Hall sensor voltage goes higher than the set point voltage, the output of the comparator goes low. This switches off the transistor and hence the power MOSFET. The current in the solenoid goes to zero and there is no attraction of the object magnet. The object magnet drops to a distance when the hall sensor voltage goes below the set point and attraction starts again. This repeated on-off control accounts for the switching which develops a set frequency about the set point. Hence the levitation about the set point. PROCEDURE 

Give a set point of about 4.0 V. This brings the object magnet levitation point closer to the solenoid. Due to the interaction between the solenoid and the object magnet, current fluctuations will be visible on the power supply current display. At the levitation point, a grab will be felt due to the attraction and the weight of the object will be taken off the person’s fingers. At this point, the person should slowly leave the object without causing any horizontal movements to it. The output comparator voltage from the FPAA (from Pin OP1 – which connected to the base of  the 2N2222 transistor) will start to fluctuate about the set point. A set PWM frequency will develop as a result of keeping the object in stable levitation. It also observed that the current taken by the solenoid at the levitation point is minimum. Please also note that the weight of the object is also very critical for levitation. A heavy object will not be held by the magnetic field. A light object will be very unstable as a lower mass leads to lower damping – oscillations will increase to cause unstability and the object will not levitate. The correct object mass which provides the required damping for stable levitation can only be chosen through a trail and error basis – PATIENCE IS ALWAYS REWARDED!!!