Presentation on theme: "Biomedical Electronics Engineering, School of Mechatronics"— Presentation transcript:
1Biomedical Electronics Engineering, School of Mechatronics ENT 215Biomedical Electromagnetic Theory
23.5 Faraday’s Law In 1831, Michael Faraday had done an experiment to prove that magnetic field could produce current.He wound two separate windings on an iron toroidand placed a galvanometer in one circuit and abattery in the other. Upon closing the batterycircuit, there is deflection on galvanomater andsimilar deflection in opposite direction whenbattery diconnected.
3Faraday’s Law (cont’d) A time-varying magnetic field produced anelectromotive force (emf) which then could producecurrent in a closed circuit. An emf is merely avoltage arises from conductors moving in amagnetic field or from changing magnetic fields
4Faraday’s Law (cont’d) A non-zero value of dФ/dt may result from :A time-changing flux linking a stationary closed path.Relative motion between a steady flux and a closed path.Combination of the two.Minus sign tells that the emf is in such a directionas to produce a current whose flux, if added to theoriginal flux would reduce the magnitude of the emf. This is known as Lenz’s Law.
5Faraday’s Law (cont’d) We can also define emf as :This is a voltage about a specific closed path.
6Faraday’s Law (cont’d) Givenor,Force per unit charge on the left hand of theequation is called motional electric field intensityEm,
7Faraday’s Law (cont’d) Finally we obtain,This is the motional emf produced by movingconductor.
83.6 Magnetic Properties of Materials Magnetic field can be enhanced by wrapping aconductor wire around an iron core. The iron coreis considered as a magnetic material since it caninfluence magnetic field. Different materialsactually have different degree of influence tomagnetic field.
9Magnetic Properties of Materials (Cont’d) We define magnetic susceptibility χm as the degreeof magnetization of material in response to anapplied magnetic field. While magnetization is theproperty of materials that describes to what extentthey are affected by magnetic fields.So, magnetization is
10Magnetic Properties of Materials (Cont’d) Magnetization can also be defined as vector sum ofmagnetic dipole moment per unit volume. In termof magnetic flux densityWe’ve also learned that B depends on relativepermeability of material, µr
11Magnetic Properties of Materials (Cont’d) TYPES OF MAGNETIC MATERIALSDiamagnetic - materials with µr slightly less thanone. Have very little influence on the magneticfield. e.g bismuthParamagnetic - exhibit no magnetic behavior whenmagnetic field absence. When magnetic fieldpresent, magnetic dipole experiences a torque. e.galuminium
12Magnetic Properties of Materials (Cont’d) Ferromagnetic - strongly magnetic even in absenceof external applied field. Used for permanentmagnets e.g iron, nickel, cobaltSuperparamagnetic – ferromagnetic particlessuspended in a dielectric. Magnetization willsaturates at very large fields. e.g magnetic audioand video tapes.
133.7 Electromagnetic Flowmeter The electromagnetic flowmeter measuresinstantaneous pulsatile blood flow. It is based onthe principal that electric magnetic force will beinduced if a conductor is cutting through amagnetic field.WhereB: Magnetic flux density, in the unit of TL: length of conductor, in the unit of mU: instant velocity of the blood, in the unit of m/s
14Electromagnetic Flowmeter (cont’d) When the directions of the velocity, magnetic fluxdensity and the conductor are perpendicular toeach other, the electric magnetic force is easilydetermined as indicated in the following equation.e = BLv
15Electromagnetic Flowmeter (cont’d) PROBE DESIGNThe toroidal-type cuff probe has twooppositely wound windings on eachhalf of the core. The magnetic fluxthus leaves the top of both sides,flows down in the center of the cuff,enters the base of the toroid and flowsup through both sides. The electrodesare mounted at the center of the bothsides of the toriod. This probe can beeasily fit snuggly to the vessel. Chooseproper size of the probe based on thesize of the blood vessel.
16Electromagnetic Flowmeter (cont’d) Unfortunately there is error in measurementcaused by :1) Velocity profile is asymmetric especially around the curve of aorta.2) Shunting effects of the wall of blood vessel.3) Circulating currents flow in the axial direction.4) Magnetic flux density is not uniform in the transverse plane.Callibration is recommended before measuring toovercome these errors.EM flowmeter actually can be used to measure theflow of all types of counducting liquid.
17PRACTICAL APPLICATION TransformerA current passing through the primary coil creates a magnetic field. Changing magnetic field within a coil of wire induces a voltage across the ends of the coil. By changing the current in the primary coil, it changes the strength of its magnetic field; since the changing magnetic field extends into the secondary coil, a voltage is induced across the secondary. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron
18LOUDSPEAKERSPaper or plastic cone affixed to a voice coil (electromagnet) suspended in a magnetic field.AC Signals to the voice coil moves back and forth, resulting vibration of the cone and producing sound waves of the same frequency as the AC signal
20MAGLEV (Cont’d)Interaction between electromagnets in the train and the current carrying coils in the guide rail provide levitation.By sending waves along the guide rail coils, the train magnet pushed/pulled in the direction of travel. The train is guided by magnet on the side of guide rail.Computer algorithms maintain the separation distance.