The use of Hall effect sensors in magnetic levitation systems SIBILSKA-MROZIEWICZ Anna1, a, CZUBAJ Sławomir1,b, ŁADYŻYŃSKAKOZDRAŚ Edyta1,c and SIBILSKI Krzysztof2,d 1
Warsaw University of Technology, Faculty of Mechatronics, 8 A.Boboli str., 02-525 Warsaw Poland 2
Air Force Institute of Technology, 6 Księcia Bolesława str., 01-494 Warsaw Poland
a
[email protected], bs,
[email protected],
[email protected], d
[email protected]
Keywords: Hall effect sensors, Meissner effect, magnetic levitation, magnetic field strength.
Abstract. This paper presents a new method of non-contact measurement and control of the magnetic field strength. The article discusses at first magnetic levitation phenomena and commercial Mag-Lev suspensions systems. Then it explains the Hall effect physics and example use of Hall effect sensor in educational magnetic levitation device. Next it lists some example constructions of Hall effect sensors. Finally it reveals potential new use of Hall-sensor in control system of unmanned aircraft catapult using Meissner effect. Introduction Even today, in the era of mass media and advanced technology, people are amazed seeing material objects suspended without visible support in space. Overcoming the gravitational interaction fascinated countless thinkers ranging from Far Eastern mystics, by scientists such as Leonardo da Vinci, Benjamin Franklin and Robert Goddard, to the Hollywood film producers. In the movie "Avatar" of James Cameron human race wanted to master the Pandora, due to the levitating at room temperature superconductor, while levitating vehicles appeared in films such as "Star Wars" or "Back to The Future". The levitation term comes from the Latin Levitas i.e. lightness and means physical phenomenon in which the body is at rest without contact with the material basis. Mechanical systems using phenomenon of levitation would move without friction between the body and the substrate. That approach would significantly improve the energy efficiency of the system [1]. Lifted by the buoyant force balloon is the most common example of levitation. With the current technology development phenomenon of levitation can take place due to aero and hydrodynamic, acoustic, optical, and mainly magnetic and electromagnetic interactions. At the heart of magnetic levitation systems lies one of the fundamental physical laws, that the opposite pool magnets generates attractive force, while the same pools repels each other. If the magnetic force balances the gravitation, then the object is levitating. However, according to Earnshaw law, levitation resulting from static magnetic repulsion force is unstable equilibrium state [2]. Even slight change of magnetization vector direction of one of the magnets changes the nature of the impact to attraction force and the magnets are moving closer to each other with high acceleration. The system stabilization can be achieved by blocking part of the degrees of freedom, giving to one of the magnets gyroscopic moment, or by adding to the levitating system feedback loop, allowing to control acting on the system forces. In this type of control systems it is crucial to monitor the position of the levitated object. Position information could by collected by optical sensors or Hall effect sensors. This paper presents examples of measuring and controlling the intensity of magnetic field using Hall effect sensors in magnetic levitation systems. Mag-Lev suspensions systems To date, magnetic levitation had the strongest impact on the industry of railway transport systems (Mag-Lev) and magnetic bearings.
Nowadays there are two kinds of commercial solutions in Mag-Lev systems [3]. The first one, called electromagnetic suspensions (EMS) was developed by the German Transrapid system. Strong electromagnets mounted on the underside of the train and oriented toward the rail are attracted to metal rails. Distance between train and rails is controlled by feedback loop, achieved by changing the strength of a magnetic field produced by electromagnets. Another solution is electrodynamics suspension that uses very strong permanent magnets or superconducting electromagnets placed along the train path. Train is lift up by repulsive force between opposite polarization of magnets. Main EDS system flaw, comparing to EMS systems, is that it needs minimum speed of 30km/h. Central Japan Railway Company (JR Central) is building commercial line Tokyo-Nagoya based on EDS technology. Passive suspensions using diamagnetic materials, in particular high temperature superconductors are very attractive alternative to commercial solutions presented above. This paper is part of innovative project of using Magnetic levitation phenomenon in UAV catapult.
Fig. 1. Commercial Mag-Lev systems: Transrapid (electromagnetic suspension) and Japan Railways Company (electrodynamics suspension) photos from www.wikipedia.org
Fig. 2. Mag-Lev suspensions schema: EMS left from: http://www.angelfire.com/de/ScienceFB/ LSLIM.html and EDS (right) from: http://archplanbaltimore.blogspot.com/2014/10/the-fascinationwith-magnetic-levitation.html Levitator Popular educational device levitator shown in fig 3 uses the magnetic levitation phenomena [4]. The structure of levitator consists of electromagnet connected to the fixed base, levitating object made of magnetic material, and feedback control loop. Hall effect sensor is mounted below the electromagnet. Sensor reads the magnetic field strength. If generating magnetic field levitating object is higher, then sensors readings are stronger. Levitation height is thereby interpolated by magnetic field strength, measured by Hall effect sensor. Read by Hall sensor signal must be filtered
in order to isolate the magnetic field generated by the levitating object of magnetic field generated by the electromagnet.
Fig. 3. Educational levitator devise and schema [4] Recently some interesting gadgets using magnetic levitation phenomena began to enter to commercial market. Hall effect Hall effect is manifested by the appearance of a small voltage difference at the ends of the semiconductor [5,6]. That voltage difference is called Hall voltage. At the core of the phenomenon lies the Lorentz force. The effect occurs only in the presence of an external magnetic field with nonzero component perpendicular to the direction of movement of electric charge. When the semiconductor is placed in a magnetic field, then on electrons flowing through him acts Lorentz force, which deflects the trajectory of the particle. According to right hand rule Hall voltage is perpendicular to an electric current in the semiconductor and an external magnetic field Fig.5.
Fig.5. Hall effect [6] The curvature of the electric charge trajectory leads to accumulation of the equal and opposite sign charges on opposite ends of the) semiconductor. The charge separation sets up electric field that prevents further charge migration. Force equilibrium is maintained as long as charge is flowing through semiconductor eq. 1.
(1) F q( E v B) 0 The Hall voltage can be computed using eq. 2, where I is the current across the plate length, B is the external magnetic field, t is the thickness of the plate, e is the elementary charge, and n is the charge carrier density of the carrier electrons.
VH
IB nte
(2)
The Hall coefficient is defined as the ratio of the induced electric field to the product of the current density and the applied magnetic field. The Hall coefficient can be computed using eq. 3, where j is the density of carrier electrons, and Ey is the induced electric field. The Hall coefficient is usually expressed as m3/C or Ωcm/G. RH
Ey jx B
(3)
Hall sensor is a device whose operating principle is based on the classic Hall effect. Hall generators are made on the basis of semiconductor materials with a high mobility of charge carriers. Hall sensors process the magnetic energy into electrical energy. A Hall effect sensor has many technical applications [7]. They are used as current sensors, magnetic field detectors in passive components to measure and control the intensity of the magnetic field. Construction of the Hall effect sensor The Hall sensor is composed of a semiconductor wafer. During the measurements the plate is placed in a magnetic field. Applied to sensor voltage forces the flow of electrons in the perpendicular to the field direction. By measuring the voltage on the electrodes placed perpendicular to the field and the direction of flow of electrons, the density of the magnetic field is determined. In other words - a magnetic field and the flow of electrons through a semiconductor cause relevant to the field strength voltage variations on the respective electrodes.
Fig.6. Hall sensor scheme [4] Hall voltage is very low, about of 30 μv for the field of 1 Gauss. Therefore the measuring circle must consist differential amplifier bipolar transistor, with a low noise level, high impedance output and a moderate gain. A regulator is needed to keep input current at a constant level. As a result, the output changes respond only to a magnetic field.
Fig.7. An example detector of magnetic field Examples of commercial Hall effect sensors Series A3141EU, A3144L, A3144E, SS441A, SS449A are unipolar magnetic field sensors using Hall effect (in occurs when there is a magnetic field, off occurs when the magnetic field disappears) [10]. They have open collector output that changes state after the disappearance of the magnetic field. Versions differ in the value of the magnetic field, for which the switching output status. The output can be loaded long-term current to 25 mA. They have a wide operating temperature range and small dimensions. They have internal temperature compensation.
Fig.8. Hall effect Sensors photos from www.ebay.com A3503 is an example of a linear magnetic field sensor based on the Hall effect [8]. The system provides an output voltage proportional to the intensity of the magnetic field. Versions vary, sensitivity and value of the magnetic field strength for which the output switches. For the A3515, A3516 and A3518 output can be loaded long-term current to 10 mA. For the remaining 25 mA. Programmable sensor MLX91208 proposed by Melexis company [9] seems to be very good option. It’s parameters such as sensitivity; gain and offset are programmable by the user and stored in the internal EEPROM. Calibration is done by a special protocol, which involves the administration on one pin modulated voltage. The system is designed to measure the intensity of AC and DC currents in the temperature range from -40 to + 150 ° C and a frequency range up to 200 kHz. The sensor comes in two versions differing programmed magnetic susceptibility. The circuit using Hall effect sensor in feedback loop Fig 9. shows example circuit used in Levitator [10]. The device is controlled by the detection of the magnetic field. Presented scheme consists of three parts: a position sensor, modulator and actuator. A Hall-effect sensor measures the system state – the levitating object height. The levitating
object is a source of magnetic field, for example neodymium magnet. The output voltage of the sensor increases in proportion to the intensity of the magnetic field, wherein the sensor is located. The field produced by an electromagnet alternating current is adjustable with adjustable duty cycle. With increasing distance between the magnet and the solenoid the magnetic field strength is reduced.
Fig. 9. Diagram of the magnetic field control system based on the levitator schema Hall-effect sensor SS 495A is a sensor of magnetic field. The Hall voltage generated in the sensor depends on the distance between the levitating object and a solenoid. The sensor is supplied to the PWM controller 502 MIC BN, which then controls the solenoid through the system LMD 18201. As we approach the permanent magnet to electromagnet see that at some point he begins to tremble and is attracted, but only to a certain point. The magnet levitates above electromagnet, until the solenoid is energized. Levitating magnet can be made to rotate around a vertical axis. There is no energy dissipation, cause there is no friction. Magnetic UAV catapult Fig. 10 shows a prototype of magnetic UAV (Unmanned Air Vehicles) catapult based on the phenomenon of magnetic levitation [11]. Magnetic launch system consists of two components. First, the fixed runway made of strong neodymium magnets. The runway consists of two rails with glued three rows of rectangular magnets that generate a magnetic field. The polarity of magnets varies across the tracks, so that the magnetic field lines are in the shape “the gutter”. The shape of the magnetic field affects the stability of levitation system. The second element of catapult is levitating cart. It rises above the tracks, supported on four levitating supports. These supports are boxes made of good temperature insulator. Inside the boxes are four high-temperature superconductors flooded with liquid nitrogen. The impact between the supports and tracks depends on the intensity of the magnetic field, which varies depending on the position of the boxes, relative to the tracks. The magnetic field is rapidly losing strength with the change of levitating cart height. Magnets configuration generates a gradient magnetic field across the track, while the magnetic field along the track should have a constant value. Thereby the movement of the levitating cart along the track is lossless. However, this is only theoretical
assumption, because in reality the field will be affected by magnets geometry, and imperfect of their arrangement.
Fig.10. Magnetic UAV catapult The position and orientation of levitating cart would be interpolated based on the Hall effect sensors readings. Information from a single sensor is insufficient to determine the location of the model. The system of sensors evenly spaced along the surface of the object should provide much more information; so all six levitating cart degrees of freedom could be calculated. The response of the measuring system is the family of characteristics of the magnetic induction as a function of displacement. The Hall effect sensors output would be changing depending on levitation height and levitating cart orientation across the track. A microprocessor would read each sensor state and compared it with data stored in the memory. Voltage signals from sensors magnetic field will be measured using data acquisition card using a computer (PCI or USB). To interpolate the position a special sophisticated algorithm would be developed implemented. Summary The proposed in article measuring system is the beginning of a broader study on the use of Hall sensors to measure and control the magnetic field. The control loop of magnetic UAV catapult using Hall effect sensor seems to be very interesting and worthwhile solution. The accurate position of levitating cart, relevant to the track during taking-off and landing procedures need to be known, in order to avoid cart-magnetic rails collisions. The proposed bench would bee also very good place and develop and implement appropriate algorithms for processing Hall effect sensors readings. Those studies can help in the construction of various control systems in automation and mechanics. Application of non-contact measurement transducer provides high durability. Hall-effect sensor is resistant to dust, dirt, water, which clearly makes it better than the methods of optical and electromechanical if we take into account the position sensors. References [1] F.C. Moon.: Superconducting Levitation: Application to bearings and magnetic transportation. ISBN 0-471-55925-3. Willey-Vch, August 1994 [2] S. Earnshaw.: On the nature of the molecular forces which regulate the constitution of the luminiferous ether. Trans. Camb. Soc. 7:97-112, 1842
[3] K. Bonsor: How Maglev trains work[online], 13.10.2000 HowStuffWorks.com. [acces: 5.06.2015 at:http://science.howstuffworks.com/transport/engines-equipment/maglev-train.htm]. [4] K.A. Lilienkamp: “Low-cost magnetic levitation project kits for teaching feedback system design," American Control Conference, 2004. Proceedings of the 2004 , vol.2, no., pp.1308,1313 vol.2, June 30 2004-July 2 2004 [5] A. Kobus, J. Tuszyński, Z. Lech: Techniques of Hall-effect, WNT, Warsaw 1980 (in Polish) [6] W. Giriat, J. Raułuszkiewicz: Hallotrons. The use of the Hall effect in practice, PWN, Warsaw 1961 (in Polish) [7] T. Figielski: In the two-dimensional world of electrons, Wiedza i Życie nr 4/1999 (in Polish) [8] Magnetic field sensors [online], 2009 [acces: 15.05.2015], at: http://www.cyfronika.com.pl/ semi/cz_magn.htm [9] Hall effect current sensor aims at electric vehicles[online], 06.12.2013 [acces: 15.05.2015], at: http://www.electronicsweekly.com/news/products/sensors/hall-effect-current-sensor-aims-atelectric-vehicles-2013-12 [10] Guy Marsden.: Levitation! Nuts and Volts Magazine, Vol. 24, no. P, pp 58-61, September 2003 [11] E. Ładyżyńska-Kozdraś, K. Falkowski, A. Sibilska-Mroziewicz: Physical model of cart of UAV catapult using Meissner effect, Mechanika w Lotnictwie ML-XVI T.I i II / Sibilski Krzysztof (red.), 2014, Polskie Towarzystwo Mechaniki Teoretycznej i Stosowanej, ISBN 97883-932107-3-2, pp. 231-242 (in Polish)
