Magnetic Field Sensing
Submitted toDr.A.Kasi.VishwanathAssociate ProfessorCenter for Nanoscience & Technology Zaahir Salam
Course: Nanomagnetic Materials and Devices-NAST 736
What are Sensors?
Physical Principles How Do Sensors Work?
Need for Sensors
Choosing a Sensor
Market analysis and World wide Revenue
Types of Sensors
What are Sensors?
American National Standards Institute (ANSI) Definition
A device which provides a usable output in response to aspecified measurand.
A sensor acquires a physical parameter and converts it into asignal suitable for processing (e.g. optical, electrical,mechanical)
Input Signal Output Signal
Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave
Biological & Chemical Fluid Concentrations (Gas or Liquid)
Electric Charge, Voltage, Current, Electric Field (amplitude,
polarization), Conductivity, Permittivity
Magnetic Magnetic Field (amplitude, phase, polarization), Flux,
Optical Refractive Index, Reflectivity, Absorption
Thermal Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical Position, Velocity, Acceleration, Force, Strain, Stress,
Physical Principles Amperess 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
Faradays 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)
Need for Sensors
Sensors are omnipresent. They embedded in our bodies,automobiles, airplanes, cellular telephones, radios, chemicalplants, industrial plants and countless other applications.
Without the use of sensors, there would be no automation !!
Imagine having to manually fill water bottles.
Choosing a Sensor
8Market analysis - magnetic sensors 2005 Revenue Worldwide - $947M
Growth rate 9.4%
World Magnetic Sensor Components and Modules/Sub-systems MarketsFrost & Sullivan, (2005)
Worldwide Revenue Forecast for Magnetic Sensors in Industrial and Medical Applications
Non-destructive evaluation (NDE)
Bio-magnetic tag detection
Frietas, ferreira, Cardoso, CardosoJ. Phys.: Condens. Mater 19, 165221 (2007)
Mars Global Explorer (1998)
Biomagnetism using SQUIDs: Status andPerspectives Sternickel, Braginski, Supercond.Sci. Technol. 19 S160S171 (2006).
North Caroline Department of Cultural Resources Queen Annes Revenge
shipwreck site Beufort, NC
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 sensorsencompass many aspects of physics and electronics.
There are many ways to sense magnetic fields, most of them based on theintimate connection between magnetic and electric phenomena.
Fig. 1. Estimate of sensitivity of different magnetic sensors. The symbols and GMN are used toindicate the strength of the Earths magnetic field and geomagnetic noise, respectively.
The symbols E and GMN are used to indicate the strength of the Earths magnetic fieldand geomagnetic noise, respectively.
Types of Magnetic Sensors
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.
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 thatthey are affected by rotational vibrations.
The principle of working Faradays law of induction.
The search coil (also known as Inductive Sensor) is a sensor whichmeasures 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 inmagnetic fields quickly, many times per second.
Photograph of the search coil magnetometers used on the THEMIS and Cluster/Staff mission
The signal detected by a search-coil magnetometer dependson the permeability of the core material, the area of the coil,the number of turns, and the rate of change of the magneticflux through the coil.
The frequency response of the sensor may be limited by theratio of the coils inductance to its resistance, whichdetermines the time it takes the induced current to dissipatewhen the external magnetic field is removed. The higher theinductance, the more slowly the current dissipates, and thelower the resistance, the more quickly it dissipates.
Detect fields as weak as 20 fT , and there is no upper limit totheir 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 coilsinductance to its resistance.
They require between 1 and 10 mW of power.
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 pieceof ordnance such as a land mine, naval mine, depth charge, orshell when the fuse's magnetic equilibrium is upset by amagnetic object such as a tank or a submarine.
Fig 2(a), a balanced inductive bridgewhere an inductance change in oneleg of the bridge produces an out-of-balance voltage in the circuit.
Fig. 2(b), incorporates a resonantcircuit where a change in inductanceresults in a change in the circuitsresonant frequency.
Called eddy-killed oscillator, sinceconductive materials near the activecoil will have eddy currents induced,which will produce a mutualinductance change in the circuit.Ferrite cores are often used in thisapproach because they can bedesigned with the coil to offer atemperature insensitive impedance.
Fig. 2(c) uses a single coil in the sensorand the remainder of the electronicsis connected remotely.
The fluxgate magnetometer consists of a ferromagnetic material woundwith two coils, a drive and a sense coil.
It exploits magnetic induction together with the fact that all ferromagneticmaterial becomes saturated at high fields. This saturation can be seen in thehysteresis loops shown on the right side of Fig. 4.
When a sufficiently large sinusoidal current is applied to thedrive coil, the core reaches its saturation magnetization onceeach half-cycle.
As the core is driven into saturation, the reluctance of thecore to the external magnetic field being measured increases,thus making it less attractive for any additional magnetic fieldto pass through the core.
This change is detected by the sense coil. When the corecomes out of saturation by reducing the current in the drivecoil, 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 themagnetic lines of flux to cut the sense coil. The voltage outputfrom the sense coil consists of even-numbered harmonics ofthe excitation frequency.
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 inducedelectromotive force (emf) for a given value of the magnetic field. Forminimum power consumption, the core material should have lowcoercivity and saturation values.
But they consume roughly five times morepower than proximity sensors.
Most of these achieve lower powerconsumption by operating the sensor on aminor hysteresis loop, thus not driving thecore from satu