Magnetic field sensing

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Magnetic Field Sensors!

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Magnetic field sensing

  1. 1. Magnetic Field Sensing Course: Nanomagnetic Materials and DevicesNAST 736 Submitted to Dr.A.Kasi.Vishwanath Associate Professor Center for Nanoscience & Technology Submitted by Zaahir Salam
  2. 2. Contents • • • • • • • • What are Sensors? Detectable Phenomenon Physical Principles – How Do Sensors Work? Need for Sensors Choosing a Sensor Market analysis and World wide Revenue General Applications Types of Sensors
  3. 3. What are Sensors? • American National Standards Institute (ANSI) Definition – A device which provides a usable output in response to a specified measurand. Input Signal Output Signal Sensor • A sensor acquires a physical parameter and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical)
  4. 4. Detectable Phenomenon Stimulus Acoustic Biological & Chemical Electric Magnetic Quantity Wave (amplitude, phase, polarization), Spectrum, Wave Velocity Fluid Concentrations (Gas or Liquid) Charge, Voltage, Current, Electric Field (amplitude, phase, polarization), Conductivity, Permittivity Magnetic Field (amplitude, phase, polarization), Flux, Permeability Optical Refractive Index, Reflectivity, Absorption Thermal Temperature, Flux, Specific Heat, Thermal Conductivity Mechanical Position, Velocity, Acceleration, Force, Strain, Stress, Pressure, Torque
  5. 5. Physical Principles • Amperes’s Law – A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer) • Curie-Weiss Law – There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behavior • Faraday’s Law of Induction – A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer) • Photoconductive Effect – When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor)
  6. 6. Need for Sensors • Sensors are omnipresent. They embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications. • Without the use of sensors, there would be no automation !! – Imagine having to manually fill water bottles.
  7. 7. Choosing a Sensor
  8. 8. Market analysis - magnetic sensors • 2005 Revenue Worldwide - $947M • Growth rate 9.4% Application Medical $24M Aerospace Defense $37M Industrial, $156M Auto $338M 8 Other $11M Type Research, $0.8M HT SQUID, $0.38M LT SQUID, $5.3M NDE $0.1M Computer, $380M Magnetometer $5.5M Compass, $4.8M Position sensor, $3.4M GMR, $40.2M AMR $121.6M Hall element, $94.7M “World Magnetic Sensor Components and Modules/Sub-systems Markets” Frost & Sullivan, (2005) Hall IC, $671.2M
  9. 9. Worldwide Revenue Forecast for Magnetic Sensors in Industrial and Medical Applications
  10. 10. Applications • • • • • Health Care Geophysical Astronomical Archeology Non-destructive evaluation (NDE) • Data storage Bio-magnetic tag detection Frietas, ferreira, Cardoso, Cardoso J. Phys.: Condens. Mater 19, 165221 (2007)
  11. 11. Magneto-encephalography MagnetoCardiography Mars Global Explorer (1998) “Biomagnetism using SQUIDs: Status and Perspectives” Sternickel, Braginski, Supercond. Sci. Technol. 19 S160–S171 (2006). Magnetic RAM North Caroline Department of Cultural Resources “Queen Anne’s Revenge” shipwreck site Beufort, NC
  12. 12. Introduction • Magnetic sensors can be classified according to whether they measure the total magnetic field or the vector components of the magnetic field. • The techniques used to produce both types of magnetic sensors encompass many aspects of physics and electronics. • There are many ways to sense magnetic fields, most of them based on the intimate connection between magnetic and electric phenomena. Fig. 1. Estimate of sensitivity of different magnetic sensors. The symbols and GMN are used to indicate the strength of the Earth’s magnetic field and geomagnetic noise, respectively. The symbols E and GMN are used to indicate the strength of the Earth’s magnetic field and geomagnetic noise, respectively.
  13. 13. Types of Magnetic Sensors • Vector Magnetometers. • Total Field Magnetometers. – insensitivity to rotational vibrations. – splitting between some electron or nuclear spin energy levels is proportional to the magnitude of the magnetic field over a field range sufficient for magnetometry.
  14. 14. Vector Magnetometers  Measures both the magnitude and the direction.  First, nearly all vector magnetometers suffer from noise, especially 1/f noise (Geomagnetic Noise). Solution- MEMS flux concentrator  which will shift the operating frequency above the range where noise dominates.  Another major problem with vector magnetometers is that they are affected by rotational vibrations.
  15. 15. Search-Coil Magnetometer • The principle of working Faraday’s law of induction. • The search coil (also known as Inductive Sensor) is a sensor which measures the variation of the magnetic flux. • It is just coils wound around a core of high magnetic permeability. • They measure alternating magnetic field and so can resolve changes in magnetic fields quickly, many times per second. Photograph of the search coil magnetometers used on the THEMIS and Cluster/Staff mission
  16. 16. • The signal detected by a search-coil magnetometer depends on the permeability of the core material, the area of the coil, the number of turns, and the rate of change of the magnetic flux through the coil. • The frequency response of the sensor may be limited by the ratio of the coil’s inductance to its resistance, which determines the time it takes the induced current to dissipate when the external magnetic field is removed. The higher the inductance, the more slowly the current dissipates, and the lower the resistance, the more quickly it dissipates. • Detect fields as weak as 20 fT , and there is no upper limit to their sensitivity range. • Their useful frequency range is typically from 1 Hz to 1 MHz, the upper limit being that set by the ratio of the coil’s inductance to its resistance. • They require between 1 and 10 mW of power.
  17. 17. In addition to this passive use, one can also operate a search coil in an active mode to construct a proximity sensor. A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. Magnetic proximity fuze It is a type of proximity fuze that initiates a detonator in a piece of ordnance such as a land mine, naval mine, depth charge, or shell when the fuse's magnetic equilibrium is upset by a magnetic object such as a tank or a submarine.
  18. 18.  Fig 2(a), a balanced inductive bridge where an inductance change in one leg of the bridge produces an out-ofbalance voltage in the circuit.  Fig. 2(b), incorporates a resonant circuit where a change in inductance results in a change in the circuit’s resonant frequency.  Called eddy-killed oscillator, since conductive materials near the active coil will have eddy currents induced, which will produce a mutual inductance change in the circuit. Ferrite cores are often used in this approach because they can be designed with the coil to offer a temperature insensitive impedance.  Fig. 2(c) uses a single coil in the sensor and the remainder of the electronics is connected remotely.
  19. 19. Fluxgate Magnetometer • The fluxgate magnetometer consists of a ferromagnetic material wound with two coils, a drive and a sense coil. It exploits magnetic induction together with the fact that all ferromagnetic material becomes saturated at high fields. This saturation can be seen in the hysteresis loops shown on the right side of Fig. 4.
  20. 20. • When a sufficiently large sinusoidal current is applied to the drive coil, the core reaches its saturation magnetization once each half-cycle. • As the core is driven into saturation, the reluctance of the core to the external magnetic field being measured increases, thus making it less attractive for any additional magnetic field to pass through the core. • This change is detected by the sense coil. When the core comes out of saturation by reducing the current in the drive coil, the external magnetic field is again attracted to the core, which is again detected by the sense second coil. • Thus, alternate attraction and lack of attraction causes the magnetic lines of flux to cut the sense coil. The voltage output from the sense coil consists of even-numbered harmonics of the excitation frequency.
  21. 21. • The sensitivity of this sensor depends on the shape of the hysteresis curve. For maximum sensitivity, the magnetic field magnetic induction (B-H) curve should be square, because this produces the highest induced electromotive force (emf) for a given value of the magnetic field. For minimum power consumption, the core material should have low coercivity and saturation values.
  22. 22.  But they consume roughly five times more power than proximity sensors.  Most of these achieve lower power consumption by operating the sensor on a minor hysteresis loop, thus not driving the core from saturation to saturation.
  23. 23. Superconductor Magnetometers • SQUID sensors The most sensitive of all instruments for measuring a magnetic field at low frequencies ( 1 Hz) is the superconducting quantum interference device (SQUID) illustrated in Fig. 6.  It is based on the remarkable interactions of electric currents and magnetic fields observed when certain materials are cooled below a superconducting transition temperature. At this temperature, the materials become superconductors and they lose all resistance to the flow of electricity.
  24. 24. • For a large number of applications extremely small magnetic signals have to be detected and accurately measured. – Sensitivities of magnetic sensors: Hall probes ~ mT Flux gate sensors ~ nT SQUIDs ~ fT • SQUIDs allow to detect and characterize the magnetic signals which are so small as to be virtually immeasurable by any other sensors. • How sensitive? Allows to measure magnetic fields produced by the nerve currents associated with the physiological activity of the human heart (magneto cardiogram – MCG) or the human brain (magnetoencephalogram – MEG); these signals have a typical strength ~ pT. • Best of the SQUID sensors have energy sensitivity approaching Planck’s constant. • SQUIDs are the most sensitive detectors of extremely small changes in magnetic flux. Fluxes can be created by currents – therefore the most sensitive current sensors as well
  25. 25. SQUIDs - basic facts • • • • SQUIDs combine the physical phenomena of flux quantization in superconducting loops and Josephson tunneling. The Josephson effect refers to the ability of two weakly coupled superconductors to sustain at zero voltage a supercurrent associated with transport of Cooper pairs, whose magnitude depends on the phase difference between the two superconductors. The maximum current which a Josephson weak link can support without developing any voltage across it is known as its critical current Ic. When the current passed through a Josephson weak link exceeds Ic, a voltage appears across it If a closed loop made of superconductor magnetic field cannot enter the loop (“ideal diamagnetism”). But if there is a weak link flux enters the loop in quanta! Flux quantum h 0   2.07 1015T  m 2 2e
  26. 26. Applications of SQUIDS Magnetoencephalograph Magnetocardiography imaging currents in semiconductor packages Rock magnetometry Biosensors
  27. 27. Hall Sensor • Utilizes the Lorentz force on charge carriers • predominantly use n-type silicon when cost is of primary importance and GaAs for higher temperature capability due to its larger band gap.
  28. 28. Temperature Dependence of Hall Resistance and Hall Voltage Different materials and different doping levels result in tradeoffs between sensitivity and temperature dependence.
  29. 29. Applications of Hall Field Sensors Response to South or North Polarity Motor-Tachometer application where each rotation of the motor shaft is to be detected When ring magnet rotates w/ motor, South Pole passes the sensing face of the Hall sensor after each revolution. Sensor Actuated when the South Pole approaches sensor Deactuated when South Pole moves away from sensor Single digital pulse produced for each revolution.
  30. 30. application continued….. Gear Tooth Sensing • Sense movement of ferrous metal targets (magnetically biased) • Sensor detects change in flux level • Translates it into a change in the sensor output (high to low)
  31. 31. AMR – anisotropic magnetoresistance
  32. 32. Operation of AMR
  33. 33. Typical application of AMR Sensors • • • • Cylinder position sensing in pneumatic cylinders Elevator sensor Lid sensor for laptop computers Position sensor for materials handling equipment (lift trucks) • Blood analyzer • Magnetic encoders
  34. 34. GMR is achieved by using a four layer structure that consists of two thin ferromagnets separated by a conductor. The fourth layer is an antiferromagnet that is used to pin (inhibit the rotation) the magnetization of one of the ferromagnetic layers. The ferromagnet layer that is being pinned is between the conductor and the antiferromagnet. The pinned ferromagnet is called the hard ferromagnet and the unpinned ferromagnet is called the soft ferromagnet. This structure is called a spin valve.
  35. 35. • The difference in resistivity between the case when the magnetizations are parallel to when they are antiparallel can be as large as 12.8% at room temperature. • To optimize the effect the layers must be a very thin, i.e. about a nanometer thick. • For the low field response of the sensor to be a linear function of the field, it is necessary that the soft ferromagnetic have its easy axis of magnetization in zero field perpendicular to the magnetization of the pinned ferromagnet. • The zero field orientation of the two magnetizations is depicted in Fig. 11(a). The resistance is measured either in the plane of the ferromagnetics or perpendicular to this plane.
  36. 36. • This multilayer geometry increases the percentage resistance change because it increases the probability of spin flip scattering by increasing the number of interfaces where spin flip scattering occurs.
  37. 37. Total Field Magnetometers • Total field magnetometers have the important advantage of insensitivity to rotational vibrations. Total field magnetometers use the fact that the splitting between some electron or nuclear spin energy levels is proportional to the magnitude of the magnetic field over a field range sufficient for magnetometry. Optically Pumped Magnetometer: Based on the Zeeman effect. In 1896, the Dutch physicist Zeeman showed that some of the characteristic spectral lines of atoms are split when the atoms are placed in a magnetic field; one spectral line becomes a group of lines with slightly different wavelengths. The splitting is particularly pronounced in alkali elements such as cesium and rubidium.
  38. 38. Nuclear-Precession Magnetometer

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