Magnetoresistive sensors magnetic field measurement CONTENTS Gene
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Magnetoresistive sensors magnetic field measurement
CONTENTS General field measurement Operating principles Philips magnetoresistive sensors Flipping Effect temperature behaviour Using magnetoresistive sensors Further information advanced users Appendix magnetoresistive effect Appendix Sensor flipping Appendix Sensor layout.
General
Fig.1 Philips magnetoresistive sensors.
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range magnetoresistive sensors characterized high sensitivity detection magnetic fields, wide operating temperature range, stable offset sensitivity mechanical stress. They therefore provide excellent means measuring both linear angular displacement under extreme environmental conditions, because their very high sensitivity means that fairly small movement actuating components example, cars machinery (gear wheels, metal rods, cogs, cams, etc.) create measurable changes magnetic field. Other applications magnetoresistive sensors include rotational speed measurement current measurement. Examples where their properties good effect found automotive applications, such wheel speed sensors motor management systems position sensors chassis position, throttle pedal position measurement. Other examples include instrumentation control equipment, which often require position sensors capable detecting displacements region tenths millimetre even less), electronic ignition systems, which must able determine angular position internal combustion engine with great accuracy. Finally, because their high sensitivity, magnetoresistive sensors measure very weak magnetic fields thus ideal application electronic compasses, earth field correction traffic detection. sensors used maximum advantage, however, important have clear understanding their operating principles characteristics, their behaviour affected external influences their magnetic history. Operating principles Magnetoresistive (MR) sensors make magnetoresistive effect, property current-carrying magnetic material change resistivity presence external magnetic field (the common units used magnetic fields given Table Table Common magnetic units
General
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Permalloy
Current
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Fig.2 magnetoresistive effect permalloy.
Figure shows strip ferromagnetic material, called permalloy (20% Ni). Assume that, when external magnetic field present, permalloy internal magnetization vector parallel current flow (shown flow through permalloy from left right). external magnetic field applied, parallel plane permalloy perpendicular current flow, internal magnetization vector permalloy will rotate around angle result, resistance permalloy will change function rotation angle given
material parameters achieve optimum sensor characteristics Philips Ni19Fe81, which high value magnetostriction. With this material, order more information materials, Appendix obvious from this quadratic equation, that resistance/magnetic field characteristic non-linear addition, each value necessarily associated with unique value (see Fig.3). more details essentials magnetoresistive effect, please refer Section "Further information advanced users" later this chapter Appendix which examines effect detail.
kA/m 1.25 mTesla air) Gauss basic operating principle sensor shown Fig.2.
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General
this basic form, effect used effectively angular measurement some rotational speed measurements, which require linearization sensor characteristic. series sensors, four permalloy strips arranged meander fashion silicon (Fig.4 shows example, pattern KMZ10). They connected Wheatstone bridge configuration, which number advantages: Reduction temperature drift Doubling signal output sensor aligned factory.
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Fig.3
resistance permalloy function external field.
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Fig.4 KMZ10 chip structure.
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further resistors, included, shown Fig.5. These trimming sensor offset down (almost) zero during production process.
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some applications however, effect used best advantage when sensor output characteristic been linearized. These applications include: Weak field measurements, such compass applications traffic detection; Current measurement; Rotational speed measurement. explanation characteristic linearized, please refer Section "Further information advanced users" later this chapter. Philips magnetoresistive sensors
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Based principles described, Philips family basic magnetoresistive sensors. main characteristics sensors given Table
Fig.5
Bridge configuration with offset trimmed zero, resistors
Table
Main characteristics Philips sensors FIELD RANGE (kA/m)(1) -0.5 +0.5 -0.05 +0.05 -2.0 +2.0 -7.5 +7.5 -0.2 +0.2 -0.2 +0.2 SENSITIVITY LINEARIZE Rbridge -EFFECT 16.0 22.0 APPLICATION EXAMPLES compass, navigation, metal detection, traffic control current measurement, angular linear position, reference mark detection, wheel speed compass, navigation, metal detection, traffic control
SENSOR TYPE KMZ10A KMZ10A1(2) KMZ10B KMZ10C
PACKAGE SOT195 SOT195 SOT195 SOT195
KMZ51 KMZ52 Notes
SO16
16.0 16.0
air, kA/m corresponds 1.25 Data given operation with switched auxiliary field.
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Flipping internal magnetization sensor strips stable positions. reason sensor influenced powerful magnetic field opposing internal aligning field, magnetization flip from position other, strips become magnetized opposite direction (from, example, `+x' `-x' direction). demonstrated Fig.6, this lead drastic changes sensor characteristics.
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field (e.g. `-Hx') needed flip sensor magnetization, hence characteristic, depends magnitude transverse field `Hy': greater field `Hy', smaller field `-Hx'. This follows naturally, since greater field `Hy', closer magnetization's rotation approaches 90°, hence easier will flip into corresponding stable position `-x' direction. Looking curve Fig.7 where kA/m, such transverse field sensor characteristic stable positive values reverse field kA/m required before flipping occurs. kA/m however, sensor will flip even smaller values `Hx' approximately kA/m).
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(mV)
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reversal sensor characteristics
Fig.6 Sensor characteristics.
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kA/m (kA/m)
Fig.7 Sensor output `Vo' function auxiliary field `Hx' several values transverse field `Hy'.
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Figure also shows that flipping itself instantaneous, because permalloy strips flip same rate. addition, illustrates hysteresis effect exhibited sensor. more information sensor flipping, Appendix this chapter. Effect temperature behaviour Figure shows that bridge resistance increases linearly with temperature, bridge resistors' temperature dependency (i.e. permalloy) typical KMZ10B sensor. data sheets show also spread this variation manufacturing tolerances this should taken into account when incorporating sensors into practical circuits. addition bridge resistance, sensitivity also varies with temperature. This seen from Fig.9, which plots output voltage against transverse field `Hy' various temperatures. Figure shows that sensitivity falls with increasing temperature (actual values given every sensor datasheets). reason this rather complex related energy-band structure permalloy strips.
General
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bridge
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Fig.8
Bridge resistance KMZ10B sensor function ambient temperature.
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General
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Output voltage `Vo' fraction supply voltage KMZ10B sensor function transverse field `Hy' several temperatures.
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Figure similar Fig.9, with sensor powered constant current supply. Figure shows that, this case, temperature dependency sensitivity significantly reduced. This direct result increase bridge resistance with temperature (see Fig.8), which
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partly compensates fall sensitivity increasing voltage across bridge hence output voltage. Figure demonstrates therefore advantage operating with constant current.
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Fig.10 Output voltage `Vo' KMZ10B sensor function transverse field `Hy' several temperatures.
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Using magnetoresistive sensors excellent properties magnetoresistive sensors, including their high sensitivity, stable offset, wide operating temperature frequency ranges ruggedness, make them highly suitable wide range automotive, industrial other applications. These looked more detail other chapters this book; some general practical points about using sensors briefly described below. ANALOG APPLICATION CIRCUITRY many magnetoresistive sensor applications where analog signals measured measuring angular position, linear position current measurement, example), good application circuit should allow sensor offset sensitivity adjustment. Also, sensitivity many magnetic field sensors drift with temperature, this also needs compensation. basic circuit shown Fig.11. first stage, sensor signal pre-amplified offset adjusted. After temperature effects compensated, final amplification sensitivity adjustment takes place last stage. This basic circuit extended with additional components meet specific requirements modified obtain customized output characteristics (e.g. different output voltage range current output signal). Philips magnetoresistive sensors have linear sensitivity drift with temperature temperature sensor with
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linear characteristics required compensation. Philips series well suited this purpose, their positive Temperature Coefficient (TC) matches well with negative sensor. degree compensation controlled with resistors special op-amps, with very offset temperature drift, should used ensure compensation constant over large temperature ranges. Please refer part this book more information temperature sensors; also Section "Further information advanced users" later this chapter more detailed description temperature compensation using these sensors. USING MAGNETORESISTIVE SENSORS WITH COMPENSATION
COIL
general magnetic field current measurements useful apply `null-field' method, which magnetic field (generated current carrying coil), equal magnitude opposite direction, applied sensor. Using this `feedback' method, current through coil direct measure unknown magnetic field amplitude advantage that sensor being operated zero point, where inaccuracies result tolerances, temperature drift slight non-linearities sensor characteristics insignificant. detailed discussion this method covered Chapter "Weak field measurement".
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offset adjustment KTY82-210 TLC2272
sensitivity adjustment
KMZ10B
op-amp
op-amp (with resistive load greater than
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Fig.11 Basic application circuit with temperature compensation offset adjustment.
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Further information advanced users EFFECT sensors employing effect, resistance sensor under influence magnetic field changes moved through angle given shown that
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Barber pole Magnetization
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Permalloy
where regarded material constant comprising called demagnetizing anisotropic fields. Applying equations equation leads
Fig.12 Linearization magnetoresistive effect.
which clearly shows non-linear nature effect. More detailed information derivation formulae effect found Appendix LINEARIZATION magnetoresistive effect linearized depositing aluminium stripes (Barber poles), permalloy strip angle strip axis (see Fig.12). aluminium much higher conductivity than permalloy, effect Barber poles rotate current direction through (the current flow assumes `saw-tooth' shape), effectively changing rotation angle magnetization relative current from 45°.
Wheatstone bridge configuration also used linearized applications. pair diagonally opposed elements, Barber poles +45° strip axis, while another pair they -45°. resistance increase pair elements external magnetic field thus `matched' decrease resistance equal magnitude other pair. resulting bridge imbalance then linear function amplitude external magnetic field plane permalloy strips, normal strip axis.
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General
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equation linear where H/Ho shown Fig.7. Likewise, sensors using Barber poles arranged angle -45°, equation derives
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This mirror image characteristic Fig.7. Hence using Wheatstone bridge configuration ensures bridge imbalance linear function amplitude external magnetic field. FLIPPING
Fig.13 resistance permalloy function external field after linearization (compare with Fig.6).
sensors using Barber poles arranged angle +45° strip axis, following expression sensor characteristic derived (see Appendix effect):
described body chapter, Fig.7 shows that flipping instantaneous also illustrates hysteresis effect exhibited sensor. This figure Fig.14 also shows that sensitivity sensor falls with increasing `Hx'. Again, this expected since moment imposed magnetization `Hx' directly opposes that imposed `Hy', thereby reducing degree bridge imbalance hence output signal given value `Hy'.
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Fig.14 Sensor output `Vo' function transverse field `Hy' several values auxiliary field `Hx'.
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following general recommendations operating KMZ10 applied: ensure stable operation, avoid operating sensor environment where likely subjected negative external fields (`-Hx'). Preferably, apply positive auxiliary field (`Hx') sufficient magnitude prevent likelihood flipping within intended operating range (i.e. range `Hy'). Before using sensor first time, apply positive auxiliary field least kA/m; this will effectively erase sensor's magnetic `history' will ensure that residual hysteresis remains (refer Fig.6). minimum auxiliary field that will ensure stable operation, because larger auxiliary field, lower sensitivity, actual value will depend value KMZ10B sensor, minimum auxiliary field approximately kA/m recommended; guarantee stable operation values sensor should operated auxiliary field kA/m. These recommendations (particularly first one) define kind Safe Operating ARea (SOAR) sensors. This illustrated Fig.15, which example (for KMZ10B sensor) SOAR graphs found data sheets.
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greater auxiliary field, greater disturbing field that tolerated before flipping occurs. auxiliary fields above kA/m, SOAR graph shows that sensor completely stable, regardless magnitude disturbing field. also seen from this graph that SOAR extended values `Hy'. Fig.15, (for KMZ10B sensor), extension kA/m shown. TEMPERATURE COMPENSATION With magnetoresistive sensors, temperature drift negative. circuits manufactured SMD-technology which include temperature compensation briefly described below. first circuit basic application circuit already given (see Fig.11). provides average (sensor-to-sensor) compensation sensitivity drift with temperature using KTY82-210 silicon temperature sensor. also includes offset adjustment (via R1); gain adjustment performed with second op-amp stage. temperature sensor part amplifier's feedback loop thus increases amplification with increasing temperature. temperature dependant amplification temperature coefficient first op-amp stage approximately: temperature dependent resistance KTY82. values taken certain reference temperature. This usually other applications different reference temperature more suitable. Figure shows example with commonly-used instrumentation amplifier. circuit divided into stages: differential amplifier stage that produces symmetrical output signal derived from magnetoresistive sensor, output stage that also provides reference ground amplification stage. compensate negative sensor drift, with above circuit amplification again given equal positive temperature coefficient, means KTY81-110 silicon temperature sensor feedback loop differential amplifier.
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SOAR
(kA/m)
Fig.15 SOAR KMZ10B sensor function auxiliary field `Hx' disturbing field `Hd' opposing `Hx' (area
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offset
KMZ10B
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Fig.16 KMZ10B application circuit with instrumentation amplifier.
amplification input stage (`OP1' `OP2') given
(11)
where temperature dependent resistance KTY82 sensor bridge resistance magnetoresistive sensor. amplification complete amplifier calculated positive temperature coefficient (TC) amplification
given negative `TC' magnetoresistive sensor required amplification input stage `A1', resistance `RA' `RB' calculated (12)
(13)
(10)
where TCKTY temperature coefficient sensor temperature coefficient amplifier. This circuit also provides adjustment gain offset voltage magnetic-field sensor.
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APPENDIX MAGNETORESISTIVE EFFECT Magnetoresistive sensors make fact that electrical resistance certain ferromagnetic alloys influenced external fields. This solid-state magnetoresistive effect, anisotropic magnetoresistance, easily realized using thin film technology, lends itself sensor applications. Resistance- field relation specific resistance anisotropic ferromagnetic metals depends angle between internal magnetization current according cos2
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Figure shows geometry simple sensor where thickness much smaller than width which turn, less than length (i.e. With current flowing x-direction (i.e. then following equation obtained from equation cos2f(2) with constant current voltage drop x-direction becomes: Besides this voltage, which directly allied resistance variation, there voltage y-direction, given sincos This called planar pseudo Hall effect; resembles normal transverse Hall effect physically different origin. sensor signals determined angle between magnetization `length' axis and, rotates under influence external fields, these external fields thus directly determine sensor signals. assume that sensor manufactured such that e.a. x-direction that without influence external fields, only x-component 180°). energies have introduced when rotated external magnetic fields: anisotropy energy demagnetizing energy. anisotropy energy given crystal anisotropy field which depends material processes used manufacture. demagnetizing energy form anisotropy depends geometry this generally rather complex relationship, apart from ellipsoids where uniform demagnetizing field introduced. this case, sensor set-up Fig.17.
where resistivities perpendicular parallel quotient ||)/ called magnetoresistive effect amount several percent. Sensors always made from ferromagnetic thin films this major advantages over bulk material: resistance high anisotropy made uniaxial. ferromagnetic layer behaves like single domain distinguished direction magnetization plane called easy axis (e.a.), which direction magnetization without external field influence.
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where demagnetizing factor t/w, saturation magnetization induction constant Vs/Am. Fig.17 Geometry simple sensor. field t/w(M0/m0) determines measuring range magnetoresistive sensor, given
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-(6) -cos where |Hy| components external field. simplest case voltages become: (Note: then must replaced Hx/cos Neglecting constant part there main differences between magnetoresistive signal depends square Hy/H0, whereas Hall voltage linear ratio their maximum values L/w; Hall voltage much smaller most cases Magnetization thin layer magnetic field reality slightly more complicated than given equation (6). There solutions angle (with 180° Replacing 180° influence except change sign Hall voltage also that most linearized magnetoresistive sensors. Therefore, avoid ambiguity either short pulse proper field x-axis (|Hx| with correct sign must applied, which will switch magnetization into desired state, stabilizing field x-direction used. With exception advisable stabilizing field this case, values affected non-ideal behaviour layer restricted so-called `blocking curve'. minimum value depends structure sensitive layer order insufficient value will produce open characteristic (hysteresis) sensor. easy axis y-direction leads sensor higher sensitivity, then Linearization shown, basic magnetoresistor square 2000
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resistance-field (R-H) dependence, simple magnetoresistive element cannot used directly linear field measurements. magnetic biasing field used solve this problem, better solution linearization using barber-poles (described later). Nevertheless plain elements useful applications using strong magnetic fields which saturate sensor, where actual value field being measured, such angle measurement. this case, direction magnetization parallel field sensor signal described cos2 function. Sensors with inclined elements Sensors also linearized rotating current path, using resistive elements inclined angle shown Fig.18. actual device uses four inclined resistive elements, pairs each with opposite inclinations, bridge. magnetic behaviour such pattern more complicated determined angle inclination anisotropy, demagnetization bias field present). Linearity maximum 45°, which achieved through proper selection stabilization field (Hst) x-direction necessary some applications, this arrangement only works properly magnetization state.
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Fig.18 Current rotation inclined elements (current magnetization shown quiescent state).
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BARBER-POLE SENSORS number Philips' magnetoresistive sensors `barber-pole' construction linearize relationship, incorporating slanted strips good conductor rotate current. This type sensor widest range linearity, smaller resistance least associated distortion than other form linearization, well suited medium high fields.
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equal widths. characteristic plotted seen that small values relative dependence linear. fact this equation gives same linear dependence planar Hall-effect sensor, magnitude magnetoresistive sensor.
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Permalloy Barber pole
Magnetization
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Fig.19 Linearization magnetoresistive effect with barber-poles (current magnetization shown quiescent state).
Fig.20 Calculated characteristic barber-pole sensor. Barber-pole sensors require certain magnetization state. bias field several hundred generated sensing current alone, this sufficient sensor stabilization, neglected. most applications, external field applied this purpose.
current takes shortest route high-resistivity gaps which, shown perpendicular barber-poles. Barber-poles inclined opposite direction will result opposite sign characteristic, making extremely simple realize Wheatstone bridge set-up. signal voltage Barber-pole sensor calculated from basic equation with 45°):
Sensitivity high demagnetization, most applications field components z-direction (perpendicular layer plane) ignored. Nearly sensors most sensitive fields y-direction, with only having limited even negligible influence.
Definition sensitivity contains signal field variations DH), well operating voltage proportional U0): (10)
where constant arising from partial shorting resistor, amounting 0.25 barber-poles gaps have 2000
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This definition relates unit operating voltage. highest (HG) lowest (Hmin) fields detectable sensor also significance. measuring range restricted non-linearity this assumed approximate value barber-pole sensors given (11) From this equation signal voltage (UBP) barber-pole sensor, following simple relationship obtained: (12) Other sensor types have narrower range linearity therefore smaller useful signal. lowest detectable field Hmin limited offset, drift noise. offset nearly cancelled bridge circuit remaining imbalance minimized symmetrical design offset trimming, with thermal noise negligible most applications (see section sensor layout). Proper film deposition and, necessary, introduction stabilization field will eliminate magnetization switching domain splitting introduction `Barkhausen noise'. Sensitivity essentially determined anisotropy (Hk), demagnetization (Hd) bias (Hx) fields. highest sensitivity achievable with although this case depends purely which less stable than permalloy with thickness greater than equal width excess required which, although possible, drawback producing very resistance unit area. maximum theoretical with this permalloy 2.5%) approximately: (max) -(13) same reasons, sensors with reduced sensitivity should realized with increased which estimated maximum barber-pole sensor kA/m. further reduction sensitivity corresponding growth linearity range attained using biasing field. magnetic shunt parallel magnetoresistor only having small field component sensitive direction also employed with very high field strengths. high signal voltage only produced with sensor that tolerate high supply voltage This 2000
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requires high sensor resistance with large area since there limits power dissipation current density. current density permalloy very high A/cm2 passivation layers), there weak points current reversal meander (see section sensor layout) barber-pole material, with five-fold increased current density. high resistance sensor with maximum results value 10-3 (A/m)-1 converted flux density, 2000 V/T. This value several orders magnitude higher than normal Hall effect sensor, valid only much narrower measuring range. Materials There five major criteria magnetoresistive material: Large magnetoresistive effect Dr/r (resulting high signal operating voltage ratio) Large specific resistance achieve high resistance value over small area) anisotropy Zero magnetostriction avoid influence mechanical stress) Long-term stability. Appropriate materials binary ternary alloys which NiFe (81/19) probably most common. Table gives comparison between some more common materials, although majority figures only approximations exact values depend number variables such thickness, deposition post-processing. Table Comparison magnetoresistive sensor materials (10-8m) 0.07 /(%) k(/m) 2500 2500 2000
Materials NiFe 81:19 NiFe 86:14 NiCo 50:50 NiCo 70:30 CoFeB 72:8:20
nearly independent these factors, itself increases with thickness will decrease during annealing. Permalloys have zero magnetostriction; addition will increase
Magnetoresistive sensors magnetic field measurement
this also considerably enlarges small temperature coefficient required, NiCo alloys preferable. amorphous alloy CoFeB high slightly worse thermal stability absence grain boundaries within amorphous structure, exhibits excellent magnetic behaviour. APPENDIX SENSOR FLIPPING During deposition permalloy strip, strong external magnetic field applied parallel strip axis. This accentuates inherent magnetic anisotropy strip gives them preferred magnetization direction, that even absence external magnetic field, magnetization will always tend align with strips. Providing high level premagnetization within crystal structure permalloy allows stable premagnetization directions. When sensor placed controlled external magnetic field opposing internal aligning field, polarity premagnetization strips switched `flipped' between positive negative magnetization directions, resulting stable output characteristics.
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more magnetization rotates towards therefore becomes easier flip sensor into corresponding stable position `-x' direction. This means that smaller field sufficient cause flipping action seen transverse field strengths (0.5 kA/m) sensor characteristic stable positive values reverse field approximately kA/m required flip sensor. However higher values kA/m), sensor will also flip smaller values kA/m). Also illustrated this figure noticeable hysteresis effect; also shows that permalloy strips flip same rate, flipping action instantaneous.
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Fig.22 Sensor output `Vo' function auxiliary field
reversal sensor characteristics
Fig.21 Sensor characteristics.
sensitivity sensor reduces auxiliary field increases, which seen more clearly This because moment imposed magnetization directly opposes that resulting reduction degree bridge imbalance hence output signal given value
field required flip sensor magnetization (and hence output characteristic) depends magnitude transverse field greater this field, 2000
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Fig.23 Sensor output `Vo' function transverse field
Safe Operating ARea (SOAR) determined magnetoresistive sensors, within which sensor will flip, depending number factors. higher auxiliary field, more tolerant sensor becomes external disturbing fields (Hd) with kA/m greater, sensor stabilized disturbing fields long does irreversibly demagnetize sensor. negative much larger than stabilising field sensor will flip. This effect reversible, with sensor returning normal operating mode again becomes negligible (see 24). However higher greater reduction sensor sensitivity generally recommended have minimum auxiliary field that ensures stable operation, generally around kA/m. SOAR also extended values long transverse field less than kA/m. also recommended apply large positive auxiliary field before first using sensor, which erases residual hysteresis
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Fig.24 SOAR KMZ10B sensor function auxiliary field `Hx' (MLC133).
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APPENDIX SENSOR LAYOUT Philips' magnetoresistive sensors, permalloy strips formed into meander pattern silicon substrate. With KMZ10 (see KMZ51 series, four barber-pole permalloy strips used while KMZ41 series simple elements. patterns used
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different these three families sensors every case, elements linked same fashion form four arms Wheatstone bridge. meander pattern used KMZ51 more sophisticated also includes integrated compensation flipping coils (see chapter weak fields); KMZ41 described more detail chapter angle measurement.
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Fig.25 KMZ10 chip structure.
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pair diagonally opposed elements barber-poles +45° strip axis, with second pair -45°. resistance increase pair elements external magnetic field matched equal decrease resistance second pair. resulting bridge imbalance then linear function amplitude external magnetic field plane permalloy strips normal strip axis. This layout largely eliminates effects ambient variations (e.g. temperature) individual elements also magnifies degree bridge imbalance, increasing sensitivity. indicates further trimming resistors (RT) which allow sensors electrical offset trimmed down zero during production process.
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Fig.26 KMZ10 KMZ11 bridge configuration.
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WEAK FIELD MEASUREMENT Contents Fundamental measurement techniques Application note AN00022: Electronic compass design using KMZ51 KMZ52 Application circuit: signal conditioning unit compass Example Earth geomagnetic field compensation CRT's Example Traffic detection Example Measurement current. Fundamental measurement techniques Measurement weak magnetic fields such earth's geomagnetic field (which typical strength between approximately A/m), fields resulting from very small currents, requires sensor with very high sensitivity. With their inherent high sensitivity, magnetoresistive sensors extremely well suited sensing very small fields. Philips' magnetoresistive sensors nature bi-stable (refer Appendix `Standard' techniques used stabilize such sensors, including application strong field x-direction (Hx) from permanent stabilization magnet, unsuitable they reduce sensor's sensitivity fields measurement, y-direction (Hy). (Refer Appendix Fig. A2.2). avoid this loss sensitivity, magnetoresistive sensors instead stabilized applying brief, strong non-permanent field pulses very short duration µs). This magnetic field, which easily generated simply winding coil around sensor, same stabilizing effect permanent magnet, only present very short duration, after pulse there loss sensitivity. Modern magnetoresistive sensors specifically designed weak field applications incorporate this coil silicon. However, when measuring weak fields, second order effects such sensor offset temperature effects greatly reduce both sensitivity accuracy sensors. Compensation techniques required suppress these effects. OFFSET COMPENSATION `FLIPPING' Despite electrical trimming, sensors have maximum offset voltage ±1.5 mV/V. addition this
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static offset, offset drift temperature variations about (µV/V)K-1 expected assuming ambient temperature resulting offset order mV/V. Taking these factors into account, with external field sensor with typical sensitivity mV/V (kA/m)-1 have offset equivalent field A/m, which itself about four times strength typical weak field such earth's geomagnetic field. Clearly, measures compensate sensor offset value have implemented weak field applications. technique called `flipping' (patented Philips) used control sensor. Comparable `chopping' technique used amplification small electrical signals, only stabilizes sensor also eliminates described offset effects. When bi-stable sensor placed controlled, reversible external magnetic field, polarity premagnetization (Mx) sensor strips switched flipped between output characteristics (see Fig.27).
offset
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Fig.27 Butterfly curve including offset.
This reversible external magnetic field easily achieved with coil wound around sensor, consisting current carrying wires, described above. Depending direction current pulses through this coil, positive negative flipping fields x-direction (+Hx -Hx) generated (see Fig.28).
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Flipping causes change polarity sensor output signal this used separate offset signal from measured signal. Essentially, unknown field `normal' positive direction (plus offset) measured half cycle, while unknown field `inverted' negative direction (plus offset) measured second half. This results different outputs symmetrically positioned around offset value. After high pass filtering rectification single, continuous value free offset output, smoothed pass filtering. Figs Offset compensation using flipping requires additional external circuitry recover measured signal.
,,,, ,,,, ,,,,
coil sensor
current pulses
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Fig.28 Flipping coil.
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FLIPPING SOURCE PREAMPLIFIER OFFSET FILTER
PHASE SENSITIVE DEMODULATOR
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Fig.29 Block diagram flipping circuit.
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time
time
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Fig.30 Timing diagram flipping circuit output voltage; filtered output voltage; output voltage filtered demodulated.
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SENSOR TEMPERATURE DRIFT sensitivity sensors also temperature dependent, with sensitivity decreasing temperature increases (Fig.31).The effect sensor output certainly
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negligible, produce difference factor three within +125 temperature range, fields kA/m. This effect compensated flipping action described last section.
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Fig.31 Output voltage `Vo' fraction supply voltage KMZ10B sensor, function transverse field `Hy', several temperatures.
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simplest form temperature compensation current source supply sensor instead voltage source. this case, resulting reduction sensitivity temperature partially compensated corresponding increase bridge resistance. Thus current source only improves stability
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output voltage `Vo', reduces variation sensitivity factor approximately (compared factor three using voltage source). However, this method requires higher supply voltage, voltage drop current source.
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Fig.32 Output voltage `Vo' KMZ10B sensor function transverse field `Hy' using current source, several temperatures.
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optimal method compensating temperature dependent sensitivity differences measurements weak fields uses electro-magnetic feedback. seen from sensor characteristics Figs sensor output completely independent temperature changes point where external field applied (the null-point). using electro-magnetic feedback set-up, possible ensure sensor always operated this point. achieve this, second compensation coil wrapped around sensor perpendicular flipping coil, that magnetic field produced this coil same plane field being measured. Should measured magnetic field vary, sensor's output voltage will change, change will different different ambient temperatures. This voltage change converted into current integral controller supplied compensation coil, which then itself produces magnetic field proportional output voltage change caused change measured field.
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magnetic field produced compensation coil opposite direction measured field, when added measured field, compensates exactly change output signal, regardless actual, temperature-dependent value. This principle called current compensation because sensor always used `zero' point, compensation current independent actual sensitivity sensor sensitivity drift with temperature. Information measured magnetic signal effectively given current compensating coil. field factor compensation coil known, this simplifies calculation compensating field from compensating current therefore calculation measured magnetic field. this field factor precisely known, then resistor performing current/voltage conversion must trimmed. Figure shows block diagram compensated sensor set-up including flipping circuit.
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flipping field
earth's field
compensation coil compensation field flipping coil sensor KMZ10A1
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Fig.33 Magnetic field directions flipping compensation coils.
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influence other disturbing fields also eliminated provided they well known, adding second current source compensating coil. Such fields might those arising from set-up housing, ferromagnetic components placed close sensor magnetic fields from electrical motors.
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brief summary Table compares types compensation their effects, they assessed their suitability given application. Because these options encompass range costs, individual requirements application should carefully analysed terms performance gains versus relative costs.
handbook, full pagewidth
CLOCK
FLIPPING SOURCE
PRE-AMPLIFIER WITH SUPRESSION OFFSET
PHASESENSITIVE DEMODULATOR
CURRENT REGULATOR VOLTAGE CURRENT OUTPUT
MBH619
Fig.34 Block diagram compensation circuit.
Table
Summery compensation techniques TECHNIQUE EFFECT avoids reduction sensitivity constant stabilization field avoids reduction sensitivity constant stabilization field, well compensating sensor offset offset drift temperature reduction sensitivity drift with temperature factor accurate compensation sensitivity drift with temperature
Setting Flipping Current supply Electro-magnetic feedback
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Magnetoresistive sensors magnetic field measurement
PHILIPS SENSORS WEAK FIELD MEASUREMENT Philips Semiconductors present four different sensors suitable weak field applications, with primary device being KMZ51, extremely sensitive sensor with integrated compensation set/reset coils.(see Fig.35) This sensor ideal many weak field detection applications such compasses, navigation, current
General
measurement, earth magnetic field compensation, traffic detection integrated set/reset coils provide both flipping required weak field sensors also allow setting/resetting orientation sensitivity after proximity large disturbing magnetic fields. Philips also KMZ10A KMZ10A1, similar sensors which have integrated coils therefore require external coils. Table provides summary main single sensors Philips' portfolio weak field measurement.
handbook, full pagewidth
flip conductor
barber-pole
(field measured)
compensation conductor
MBH630
Fig.35 Layout Philips' KMZ51 magnetoresistive sensor.
Table
Properties Philips Semiconductors single sensors weak field applications KMZ10A KMZ10A1 SOT195 ±1.5 ±0.5 KMZ51 ±0.2 KMZ52 SO16 ±1.5 ±0.2 UNIT (mV/V)/ (kA/m) mV/V µV/V/K kA/m
Package Supply voltage Sensitivity Offset voltage Offset voltage temperature drift Applicable field range (y-direction) Set/reset coil on-board Compensation coil on-board Note kA/m. 2000
SOT195 16(1) ±1.5 ±0.5
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APPLICATION CIRCUIT Signal conditioning unit compass Fig.36 shows dimensioned circuit that provides signal conditioning both outputs KMZ52 KMZ51), required compass application. circuit delivers output voltages which proportional magnetic field measured directions respectively. operational principal circuit based that described section
General
circuit following characteristics: supply voltage flip frequency sensitivity bandwidth demonstration purposes, circuit operated with without electromagnetic feedback, depending state switches
handbook, full pagewidth
pre-amplifier offset compensation -VO1 GND1
IC1A
synchronous rectifier amplification IC2A
filter integral controller compensation coil driver IC2B LM324D IC5B COM1 IC3A LM324D IC2D LM324D COM2 IC3B LM324D open loop IC5C feedback open loop feedback
IC5A VCC1
KMZ52 +VO1
LM324D
LM324D IC1B IC4A IC4B
-VO2 VCC2 KMZ52 LM324D +VO2 GND2 IC1C
IC2C LM324D
IC1D LM324D
IC4C
IC4D +10V
CLOCK
BST52 BAS32L BC847
IC4P LM324D IC3C IC5E reference generator IC3D LM324D IC5D IC6P
IC6A IC6B
4011BT CLOCK
4011BT
FLI1
BC857 BAS32L
IC6D
IC6C
FLI2 BST62
4011BT
4011BT
flipping generator
MLD428
Fig.36 Application circuit diagram signal conditioning unit compass.
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Magnetoresistive sensors magnetic field measurement
EXAMPLE Earth geomagnetic field compensation CRT's Earth's geomagnetic field always caused problems monitor manufacturers, influences trajectory electrons tube producing horizontal tilt geometry convergence error shifts. With introduction wide screen picture tubes, this problem become unacceptable, especially with geometric test patterns 16:9 aspect ratios. With continuing goal improving picture quality allowing varying magnetic fields every part world, compensation circuit required reduce this effect. simple one-dimensional solution wrap DC-current carrying coil around neck generate magnetic field opposite Earth's field, cancelling twist electrons path reducing approximately number convergence errors. This coil also additional advantage compensating other extraneous electromagnetic field sources emanating from such loudspeakers. including magnetoresistive sensor detect Earth field, output from sensor used drive compensation field, making adjustment automatic. Although residual picture twist North/South trapezoid errors still seen, simple DC-shift compensation current will eliminate picture twist addition vertical sawtooth (ramp) current, derived from vertical deflection, will remove trapezoid.
handbook, halfpage
General
MBH628
Fig.38 Geometry error North/South trapezoid.
EXAMPLE Traffic detection number vehicles using already congested roads steadily increases, traffic control systems becoming necessary avoid time consuming traffic jams. These systems monitor traffic flow, average speed traffic density, allowing electronic road signs control flow speed traffic known trouble spots. They also have advantage indicating possible incidents, where traffic speeds fall significantly below average certain sections road. Simple modifications these systems allows them used improve safety, also monitor ground traffic airports. Although highly sophisticated computer systems used analyse various inputs traffic systems, currently this input information gained from inductive systems which have number disadvantages. sensitivity offered inductive measuring systems requires large areas road lifted re-surfaced during installation. With their high power consumption, fact they produce very little information regarding type traffic passing over them, makes them both costly inefficient. They also rather unreliable road thermal stress. practically every vehicle manufactured contains high number ferromagnetic components, measurable magnetic field specific individual model from every manufacturer detected, using weak field measurement techniques with magnetoresistive sensors. Even with greater aluminium manufacture vehicle been demagnetized, will still create measurable change geomagnetic field strength flux density.
handbook, halfpage
MBH627
Fig.37 Geometry error horizontal picture tilt.
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Magnetoresistive sensors magnetic field measurement
comparison with inductive methods, with high sensitivity magnetoresistive measuring provide information passing vehicle type. Also, sensor size placement, systems easily quickly installed stretch road, even side road, necessary. Combined with almost negligible power consumption, this makes magnetoresistive control systems inexpensive highly efficient method monitoring traffic levels. MEASUREMENTS ROADS field test with three-dimensional sensor modules set-up, firstly measure signals different vehicles; secondly, relative occurrence signal values three vehicle categories (car, truck). first
General
test, module placed road, under vehicle comparison, second module placed side road. second test, which performed `live' street Hamburg, Germany, module could only positioned side road. local geomagnetic field calibrated zero, that only disturbance field caused passing vehicle would recorded. Figure shows spectra produced Opel Kadett. sensor modules also proved sensitive enough detect distinguish motorbikes (even with engine, frame wheels being made aluminium), which produced following roadside spectra.
handbook, full pagewidth
(A/m)
sensormodules
time
MBH631
Fig.39 Spectra Opel Kadett from ground sensor.
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General
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(A/m)
sensormodules
time
MBH632
Fig.40 Spectra motorbike.
roadside test Hamburg, road chosen random maximum signal value recorded different vehicles, being grouped into cars, vans trucks. relative occurrence signal values shown following diagram.
handbook, full pagewidth percentage
cars
percentage
vans
trucks (est.)
max. signal (A/m)
,,,,
max. signal (A/m)
MBH633
sensormodules
Fig.41 Distribution graphs maximum signal versus occurrence.
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signals each group seem have Gaussian distribution with characteristic maximum (although fact there were only three trucks, values this group estimate). AIRPORT GROUND TRAFFIC CONTROL With constant growth traffic around world, serious consideration improvement safety ability improve handling capacity airports, control traffic around runways. Using traffic control system, possible introduce automatic guidance systems prevent runway incursions even heavily congested airports under visibility conditions, accordance with regulations set-down internationally recognized authorities. Table
General
Although there number possible sensor solutions, traffic systems using magnetoresistive technology have none drawbacks existing radar, microwave, I/R, pressure, acoustic inductive systems (see Table They meet functional environmental restraints, such large temperature ranges, insensitivity climatic changes, power consumption and, most all, cost, high reliability ruggedness. They also perform range signalling functions including detection presence, recognition, classification, estimation speed deviation from path.
Disadvantages various sensors airport ground traffic control units Radar Microwave barriers Cannot installed flush with ground Creates obstacles surveyed area Produce interference Inductive sensors sensitivity short range Poor target information High power consumption Unreliable harsh environments Repairs require traffic stopped diverted signalling Greatly affected weather conditions Complex target identification
High costs Reduced efficiency with large number targets Line sight only Complex target identification resolution Slow response times Pressure sensors Frequent mechanical breakdowns when used harsh environments Associated ageing problems Poor target identification
Acoustic sensors Signal interference when used outdoor weather conditions Trade-off between sensitivity range Large power consumption
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EXAMPLE Measurement current principle measuring current with magnetoresistive sensor straightforward. current, flows through wire, generates magnetic field around which directly proportional current. measuring strength this magnetic field with magnetoresistive sensor, current thus accurately determined.
General
sensitivity magnetoresistive sensors easily adjusted, using different set-ups different electronics (refer selection guide General section), individual sensor optimized specific current measurement application, clear advantage over Hall effect sensors. Set-ups with sensors allow current measurement without breaking conductor interfering with circuit way, providing distinct advantage over resistor based systems. They used example, measuring current headlamp-failure detection system motor vehicles clamp-on (non-contacting) meters, used power industry. accuracy achievable current measurement using magnetoresistive sensors highly dependent specific application set-up. Factors which affect accuracy mechanical tolerances (such distance between sensor wire), temperature drift sensitivity conditioning electronics.
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MGG423
measurement set-up, there always other magnetic fields present besides that generated current, such earth's magnetic field, these interfere with measurement. more accurate measurement set-up uses magnetic field sensors, compensate these external fields (see Fig.43).
Fig.42 Diagram showing field direction current carrying wire.
relationship between magnetic field strength current distance given
handbook, halfpage
(14)
sensor
Some calculated values typical conditions given Table
Hdisturb
Table
Values magnetic field generated current carrying wire various distances currents
sensor
EXAMPLE
CURRENT 1000
DISTANCE
MAGNETIC FIELD 3.18 15.9 kA/m
MGG426
Table clearly indicates that current measurement involve measurement weak strong magnetic fields. 2000
Fig.43 Diagram showing sensors measure current.
Magnetoresistive sensors magnetic field measurement
first sensor detects both interference field current-field positive direction, second sensor detects interference field negative direction current-field positive direction. These signals added, cancelling interference field, leaving signal that representative only current-field. This set-up works with homogeneous interference fields like that from earth. Inhomogeneous fields, which will produce different interference fields inside sensors, will still affect current measurement. This error minimized keeping distance between sensors small. Large magnetic fields which fall outside range sensors also produce errors, size external fields must limited. Table
General
Another advantage using sensors, fixed distance apart, that measurement less sensitive sensor-conductor distance. conductor moved closer first sensor, then distance from second sensor correspondingly increased effect compensated. small differences distance between conductor sensors, sensitivity nearly constant conductor need fixed place. This method lends itself measurement current free cables. Table summarizes various advantages disadvantages one-sensor two-sensor measurement set-ups described above.
Summary advantages disadvantages typical measurement set-ups CURRENT MEASUREMENT WITH MAGNETIC FIELD SENSOR PROS galvanic connection breaking conductor small physical dimensions CONS effects interference from external fields sensitive sensor-conductor distance
CURRENT MEASUREMENT WITH MAGNETIC FIELD SENSORS PROS galvanic connection breaking conductor small physical dimensions reduced sensitivity sensor-conductor distance reduced interference effects from homogeneous fields CONS interference effects from inhomogeneous fields errors generated from large external fields
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proper functionality set-ups shown Fig.42 Fig.43 important limit disturbing fields x-direction (perpendicular sensitive direction) sensor. Thus `flipping' sensor output characteristic (see section) prevented. Applications where `flipping' (respectively disturbing fields x-direction) prevented lead reversed sensor characteristic. This means same current (respectively magnetic field) output voltage sensor bridge positive negative, depending polarity magnetization permalloy. obvious that this phenomenon unwanted, especially current measurements. current measurements, flipping cause frequency doubling output voltage sensor. This happens current measured also produces periodic magnetic field components x-direction therefore sensor flipped periodically. avoid `flipping' transfer characteristic, must ensured that magnetization permalloy reversed disturbing fields x-direction sensor. This achieved applying static magnetic field x-direction, e.g. gluing permanent magnet sensor package. magnitude this stabilizing magnetic field x-direction must chosen properly,
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because hand magnetic fields x-direction avoid unwanted flipping, other hand sensitivity sensor reduced (see section 'Flipping'). sophisticated solution measuring currents, even with presence disturbing magnetic fields x-direction therefore flipping output characteristic, operate sensor zero point. Such arrangement which magnetic field generated current-carrying wire compensated secondary circuit wrapped around ferrite core seen 'null-field' point, detected sensor located between ends core, magnitude current secondary circuit measure current main circuit. Even sensor characteristic reversed disturbing magnetic fields x-direction then output voltage sensor bridge remains zero, because this operating point equivalent point where "normal" "flipped" sensor characteristic intersect. Additionally this arrangement represents more accurate measure currents, because inaccuracies result mechanical tolerances, temperature drift slight non-linearity sensor characteristics reduced.
handbook, full pagewidth
ferrite
sensor
MLC141
Fig.44 Current measurement using compensating coil.
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