Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Transformer wikipedia , lookup
Stray voltage wikipedia , lookup
Skin effect wikipedia , lookup
Mains electricity wikipedia , lookup
Induction motor wikipedia , lookup
Alternating current wikipedia , lookup
Electric machine wikipedia , lookup
Geophysical MASINT wikipedia , lookup
Galvanometer wikipedia , lookup
11. Magnetic sensors Introduction Principle of work: • coupling between electric and magnetic circuits is changed • influence of magnetic field on material parameters of a sensor. Sensor materials: hard and soft magnetics semiconductors (structures) sensitive to mag. field. Electronic compass (Honeywell) Vehicle detector 1 Classification of sensors Inductive sensors inductivity (change of L) transformer – type (change of M – mutual inductance) electromagnetic - induced EMF - induced eddy currents Magnetogalvanic sensors Hall sensors magnetoresistors magnetotransistors Magnetoelastic sensors Magnetic field sensors SQUIDs 2 Inductivity sensors Coil inductance L is proportional to magnetic flux B generated by current I : N B = L I It equals ca: L o r N 2 A 2 N R l R where: R - reluctance, N- number of turns l - length A – cross section r – relative magnet. permeability of core l A o r L is changed through N or more often through R. Magnetic coil as an element of a magnetic circuit: B R 1 NI R 2 NI l l 1 2 A A o o r 3 Inductivity sensors With changing air slit Impedance module Z RL2 2L2 RL RCu R Z RL effective resist . of losses Z L For small losses N2 N2 N2 L 1 2l p RFe R p R p 0 A Z 1 lp R reluc . 4 Inductivity sensors With changing cross section of a slit 5 Inductivity sensors Differential sensor Two coils with impedances Z1 i Z2 with common keeper Slits: l1 l0 l2 l0 Uout R I R Î1 Î 2 R( I1 I 2 ) I1,2 U kA 2 2 ( L0 ) RS2 l0 Dla 0 < δ < 0.4l0 ΔI ~ δ Weak dependence of Uout on frequency and supply voltage 6 Inductivity sensors With changing core position 7 Inductivity sensors Two bridge-connected coils Shift of a core from the medium position gives the output voltage Uout U out 1 L U in 2 L 0 For a small displacement Δx L dL x dx Hence U out 1 dL x U in 2L 0 dx 8 Transformer type sensor Linear variable differential transformer (LVDT) Separation of the power supply and output circuits, large number of turns of the output circuit. Compensation of thermal noise (work range from criogenic tempwrature till 1500C). Z1 and Z2 connected in a push-pull mode. In the middle core position Uout = 0 High sensitivity in the range of displacements from 10-7m to 1 m, with nonlinearity error <3%. Special construction of coils is adopted. 9 Electromagnetic sensors with induced EMF EMF is induced by changing magnetic flux (Faraday’s law): df/dt In sensoric solutions one uses generally permanent magnets and changing flux is obtained by moving electric circuits in a magnetic field B or on the contrary – by moving source of B or changing magnetic reluctance for resting electric circuit. Moving electric circuit in a constant B Linear velocity sensor N` BA d dx v dt dt N xB A l 10 Moving electric circuit in a constant B Angular velocity sensor N B A cos N B A cos t d N B A sin t dt Some minimal angular velocity is necessary. For high ω the amplifier is not necessary. 11 Change of magnetic reluctance Electromagnetic tachometer 12 Motion of B source Tachogenerator Frequency of induced voltage: f~n·p p – number of poles n – number of revolutions Typical working range: 150 – 3000 rpm For small angular velocity the number of magnetic poles is increased. Generally sensors with induced EMF are used for: • measurements of angular velociy • investigation of vibrations: x v dt k dt a dv d k dt dt 13 Signal conditioniong for a tachometer with gear 14 Electromagnetic sensors based on eddy currents Induced currents and not EMFs are important B rot E t Changing magnetic field induces rotational electric field Eddy current proximity sensors Magnetic coil is a part of resonsnce circuit of LC generator Dynamics 1 – 50 mm Resolution 0.1 mm 15 Magnetogalvanic sensors Hall sensors Lorentz force: F=qvxB UH = (RH/d) I B = γ I B for elongated sample In general it is necessary to introduce the geometrical factor G and an offset voltage: UH = γ G I B + Ur Ur –offset voltage, constant or varying slowly for B=0. 16 Hall sensors Hall constant for sample with carriers of one type is equal: RH = ± r/ nq r – coeff. dependent on the mechanism of carriers scattering n – carriers concentration q – elementary charge Large UH signal is obtained for samples of high carriers mobility µ ( InSb, GaAs ): EH/Ex = µBz EH – Hall field Ex – field generating current 17 Hall sensors shapes Hall sensor labelling CC/HC –interchangeable contacts CC – curent contacts HC – Hall contacts IC technology (vertical), B field - tangent to the sample surface 18 Hall sensors technology - samples from bulk material - thin film structures - integrated structures: - MOS structures - epitaxial GaAs structures - superlattices in MBE (Molecular Beam Epitaxy) technology - bipolar IC structures Most of recently produced commercial Hall sensors are manufactured using bipolar integrated circuits technology (IC). 19 Hall sensors technology Isolation of the Hall structure is accomplished by the reverse polarization of p/n junctions Active part of the structure is epitaxial n layer with diffused isolation p areas and n+ contacts. 20 Hall sensor parameters • absolute sensitivity: SA = ∂ UH / ∂ B dla I = const • sensitivity vs. supply current: SI = SA / I • sensitivity vs. supply voltage: SU = SA / U • offset: equivalent field Bo giving offfset voltage Uo : Bo = Uo / S.A. 21 Applications of hall sensors Contactless measurement of position 22 Contactless measurement of position differential connection of Hall sensors 23 Contactless measurement of current Around the current lead (linear) concentric magnetic field is generated B = μoI/(2πr), then B ~ I for r = const Simple construction, good linearity High sensitivity is obtained applying the magnetic core with a slit δ ~ 1 mm, in which a Hall sensor is placed. 24 Contacless measurement of current Flux of field B: B R NI R R l Fe 0 r A R 0 A N – number of turns Rµ, Rδ – reluctances of a core and a slit B field in a slit: B ΦB μ0 N I μ0 N I UH I A l Fe δ δ μr Current measurement range: 10A – several kA 25 Contactless measurement of power Multiplication feature of the Hall sensor is utilized Load current iL produces field B which is measured as described before: B ~ iL Voltage uL is transformed giving current iin supplying the Hall sensor: iin ~ uL Voltage indicated by the sensor: u H iin B ur ( t ) k u L iL ur ( t ) k pL ur ( t ) u L ( t ) U 0 L cos t For the resistance load: iL ( t ) I 0 L cos t ur ( t ) U 0 r cos t offset voltage 1 u H ( t ) k U 0 L I 0 L ( 1 cos 2 t ) U 0 r cos t 2 Using low-pass filter one obtains signal prop. to the average power 1 u H k U 0 L I 0 L 2 26 Brushless DC motor Rotor has a built-in permanent magnet. Driving coils, being a part of a stator, are controlled by two Hall sensors. Hall sensors register relative positions of a rotor and with the help of transistors control the coils currents. Currents in a stator change very smooth. Advantages of the motor: • prolonged lifetime (only bearings are weared) • low noise • lack of sparking 27 Magnetoresistors Demonstrate the dependence of resistance on magnetic field. Early years of magnetoresistors development were based on utilization of semiconductors, eg. InSb for B > 2kGs. Recently magnetoresistors are manufactured using: • ferromagnetic metals (Thomson effect) also known as AMR (anisotropic magnetoresistance) effect, • layer magnetic structures (GMR – giant magnetoresistance) • magnetic tunnel junctions (MTJ) AMR elements come into use with the development of thin film technologies. The following alloys are used: Ni Fe Ni Co Ni Fe Co Change of resistance as a function of magnetic field depends on the angle between the current and the axis of magnetic anisotropy (axis of easy magnetization). 28 AMR magnetoresistor For ε = ± 45° the dependence is quasilinear Practical solution Metallic stripes force the direction of current flow BARBER-POLE sensor manufactured by PHILIPS. Characteristics nearly linear. 29 Giant magnetoresistance (GMR) Sudden drop of resistance in the presence of a magnetic field for the multilayer structure, where magnetic layers ( Fe, Co ) are separated by nonmagnetic layers ( Cu, Ag ) ( Baibich 1988). Scattering of electrons depending on the direction of spin vs. magnetization vector M a – spin up, b – spin down 30 GMR In practise the superlattices are manufactured Chracteristics of GMR superlattice [Co (1.1nm) Cu(0.9nm)] · 100 Sensitivities for small magnetic fields are obtained when the structure has a form of so called magnetic valve. 31 GMR IBM Almaden Res. Center 32 Magnetic Tunnel Junction (MTJ) Two ferromagnetic electrodes CoFeB are separated by the tunnel layer of MgO insulator. The current flows perpendicularly to the junction. For antiparallel orientations of a free layer (upper) and fixed (lower) one gets high resistance (IrMn – the layer causing fixation). Change of magnetization of upper layer for parallel orientation gives drop of the resistance. CR Magnetics Inc. 33 Magnetic Tunnel Junction (MTJ) The spin-up electrons are those with spin orientation parallel to the external magnetic field, whereas the spin-down electrons have anti-parallel alignment with the external field. If no voltage is applied to the junction, electrons tunnel in both directions with equal rates. With a bias voltage U, electrons tunnel preferentially to the positive electrode. With the assumption that spin is conserved during tunneling, the current can be described in a two-current model. The total current is split in two partial currents, one for the spin-up electrons and another for the spin-down electrons. These vary depending on the magnetic state of the junctions. 34 MTJ sensor Micro Magnetics The STJ-001 low-field magnetic microsensor in die form. Active areas as small as 1x2 microns The die is 1.9 mm square and 300 microns thick. It has four gold wirebonding pads which allow four-point measurement of the device resistance. The field sensitivity of the STJ-001 is 5 nT, which is ten thousand times smaller than the magnetic field of the Earth. 35 Applications of MR sensors Read head in a disc drive Principle of operation of magnetoresistive read head. Important are only changes of flux in the direction perpendicular to the magnetic medium. 36 Disc drive IBM Almaden Res. Center 37 Applications of MR sensors Read head in a disc drive MR read head in comparison to induction heads: • signal is independent of the tape speed • higher sensitivity, then higher writing density These heads cannot be however at the same time the write heads (inductive) • First head with MR sensor – 1970r. • Heads for reading tapes, IBM –1985r. • Recently all hard disc drives utilize MR elements for reading. 38 HD Tunnel Reader Industry first 120 GB 2.5-in Seagate Momentus II high capacity mobile drive with MTJ reading element. 39 Applications of MR sensors • Contactless measurements of DC and AC currents, transformation of DC curents • Registration of position and revolution of magnetic materials • Digital compass • Credit card readers • Checking of failed wirebonding and defects in semiconductor structures • MRAM memories 40 Galvanic isolation DC current transformer Summation of two currents 41 Identification of coins When the coin is moved in the field of a coil, eddy currents are induced. Phase shift between the coil signal and magnetoresistor signal are measured, what is characteristic for a given coin, independently of the speed of coin moving.. 42 ABS system with MR sensors At sliding tendency electronic and hydraulic systems control respective breaks. 43 Application of MTJ sensors Defects in microstructure connections 44 Application of MTJ sensors Failed wirebonding 45 Magnetoelastic sensors Change of magnetic properties under the influence of mechanical interactions Stress σ rotates magnetization MS by an angle θ in respect to magnetic field H. From the condition of minimum energy (for θ<< θ0) one obtains: sin 2 0 MSH Optimal case: θ0 = 450, H - small A measure of torque FR for a rod fixed at one side is a torsion angle θ 46 Measurement of torque with a magnetoelastic sensor of cylindrical shape With no stress (torque t = 0), there is no coupling between coils After application of torque t the output voltage is generated U out = kt 47 SQUID Magnetometer SQUID (Superconducting Quantum Interference Device) is a very sensitive magnetometer used to measure extremely subtle magnetic fields, based on superconducting loops containing Josephson junctions. SQUIDs are sensitive enough to measure fields as low as 5 aT (5×10−18 T). Animals produce very small magnetic fields in the range 10-9T to 10-6T. Two Josephson junctions are connected in parallel in a superconducting loop. In the absence of any external magnetic field, the input current I splits into the two branches equally. If a small external magnetic field is applied to the superconducting loop, a screening current IS, begins circulating in the loop that generates a magnetic field canceling the applied external flux. The induced current is in the same direction as I in one of the branches of the superconducting loop, and is opposite to I in the other branch; the total current becomes I/2 + IS in one branch and I/2 - IS in the other. As soon as the current in either branch exceeds the critical current IC, of the Josephson junction, a voltage appears across the junction. 48 If the input current is more than Ic, then the SQUID always operates in the resistive mode. The voltage in this case is thus a function of the applied magnetic field and the period equal to Φ0 . The screening current is the applied flux divided by the selfinductance of the ring. Thus ∆Φ can be estimated as the function of ∆V (flux to voltage converter) as follows: ∆V = R ∆I 2I = 2 ∆Φ/L, where L is the self inductance of the superconducting ring ∆V = (R/L) ∆Φ Left: Plot of current vs. voltage for a SQUID. Upper and lower curves correspond to nΦ0 and (n+1/2)Φ0 respectively. Right: Periodic voltage response due to flux through a SQUID. The periodicity is equal to one flux quantum, Φ0. 49