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A New Perspective on Magnetic Field Sensing

Michael J. Caruso Dr. Carl H. SmithTamara Bratland Robert SchneiderHoneywell, SSEC Nonvolatile Electronics, Inc.

12001 State Highway 55 11409 Valley View RoadPlymouth, MN 55441 Eden Prairie, MN 55344

<www.ssec.honeywell.com> <www.nve.com>

ABSTRACT

The earliest magnetic field detectors allowed navigationover trackless oceans by sensing the Earth's magneticpoles. Magnetic field sensing has vastly expanded asindustry has adapted a variety of magnetic sensors todetect the presence, strength, or direction of magneticfields not only from the Earth, but also from permanentmagnets, magnetized soft magnets, vehicle distur-bances, brain wave activity, and fields generated fromelectric currents. Magnetic sensors can measure theseproperties without physical contact and have becomethe eyes of many industrial and navigation control sys-tems. This paper will describe the current state of sev-eral methods of magnetic sensing and how the sensorsare used—many with integrated functions. Finally, sev-eral applications will be presented for magnetic sensingin systems.

INTRODUCTION

Magnetic sensors have been in use for well over 2,000years. Early applications were for direction finding, ornavigation. Today, magnetic sensors are still a primarymeans of navigation but many more uses haveevolved. The technology for sensing magnetic fieldshas also evolved driven by the need for improved sen-sitivity, smaller size, and compatibility with electronicsystems. This paper will overview various types ofmagnetic sensors and their applications. It is not in-tended as a how-to description of building sensor sys-tems but more of a what is this sensor and how does itdetect magnetic fields. The newest types of siliconbased magnetic sensors will be emphasized—aniso-tropic magnetoresistive (AMR) and giant magnetore-sistive (GMR) sensors. Applications for AMR and GMRmagnetic sensors are presented.

A unique aspect of using magnetic sensors is thatmeasuring magnetic fields is usually not the primaryintent. Another parameter is usually desired such aswheel speed, presence of a magnetic ink, vehicle de-tection, or heading determination. These parameterscannot be measured directly, but can be extracted from

temperature

pressure

strain

light

output

direction

presence

rotation

current

angle

sensorV/I

variationof a

magneticfield

signalprocessing

output

sensorV/I

Figure 1. Conventional vs. Magnetic Sensing

changes, or disturbances, in magnetic fields. Figure 1shows other sensors, such as temperature, pressure,strain, or light that can be detected using an appropri-ate sensor. The output of these sensors will directlyreport the desired parameter. On the other hand, usingmagnetic sensors to detect direction, presence, rota-tion, angle, or electrical currents only indirectly detectthese parameters. First, the enacting input has to cre-ate, or modify, a magnetic field. A current in a wire, apermanent magnet, or sensing the Earth's magneticfield can create this field. Once the sensor detects thatfield, or change to a field, the output signal requiressome signal processing to translate the sensor outputinto the desired parameter value. This makes magneticsensing a little more difficult to apply in most applica-tions, but it also allows for reliable and accurate sens-ing of parameters that are difficult to sense otherwise.

One way to classify the various magnetic sensors is bythe field sensing range. These sensors can be arbitrar-ily divided into three categories—low field, mediumfield, and high field sensing. Sensors that detect mag-netic fields less than 1 microgauss will be classed lowfield sensors. Sensors with a range of 1 microgauss to10 gauss will be considered Earth’s field sensors andsensors that detect fields above 10 gauss will be con-sidered bias magnet field sensors for this paper. Table1 lists the various sensor technologies and illustratesthe magnetic field sensing ranges [1].

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Magnetic SensorTechnology

Squid

Fiber-Optic

Optically Pumped

Nuclear Procession

Search-Coil

Anisotropic Magnetoresistive

Flux-Gate

Magnetotransistor

Magnetodiode

Magneto-Optical Sensor

Giant Magnetoresistive

Hall-Effect Sensor

Detectable Field Range (gauss)*

10 -8 10 -4 10 0 10 4 10 8

* Note: 1gauss = 10 -4Tesla = 10 5gamma

Earth’s Field

Table 1. Magnetic Sensor Technology Field Ranges

In the following sections, several types of magnetic fieldsensors are described—both the physical principleswhich cause them to work and the embodiment ofthese principles into sensors. The magnetic field is avector quantity that has both magnitude and direction.Magnet sensors measure this quantity in various ways.Some magnetometers measure total magnitude but notdirection of the field (scalar sensors). Others measurethe magnitude of the component of magnetizationwhich is along their sensitive axis (omni-directionalsensors). This measurement may also include direction(bi-directional sensors). Vector magnetic sensors have2 or 3 bi-directional sensors. Some magnetic sensorshave a built in threshold and produce an output onlywhen that threshold is passed. The types of magneticsensors which will be described will include older tech-niques including Reed Switches, Variable ReluctanceSensors, Flux-gate Magnetometers, Magneto-InductorSensors, and Hall Devices as well as the relatively newsolid state sensors including Anisotropic Magnetoresis-tive (AMR) Sensors and Giant Magnetostrictive (GMR)Sensors.

LOW FIELD SENSORS (less than 1 microgauss)

The low field sensors are used for medical applicationsand military surveillance. They generally tend to bebulky and costly compared to other magnetic field sen-sors. Care must be taken to account for the effects ofthe Earth’s field since daily variations in the Earth’s fieldmay exceed the measurement range of a low field sensor.

SQUID

The most sensitive low field sensor is the Supercon-ducting QUantum Interference Device (SQUID). Devel-oped around 1962 with the help of Brian J. Josephson'swork that developed the point-contact junction tomeasure extremely low current [1]. The SQUID mag-netometer has the capability to sense field in the rangeof several fempto-tesla (fT) up to 9 tesla. That is arange of over 15 orders of magnitude! This is key formedical use since the neuromagnetic field of the hu-man brain is only a few tenths of a fempto-tesla [2].That is 10-8 times weaker than the Earth's magneticfield. The present designs require cooling to liquid he-lium temperature (4 K) but higher temperature tech-niques are being developed. SQUID devices, like theHS07, are available from F.I.T. in Germany and re-search is being done at Shimadzu Corporation in Ja-pan. SQUID magnetometers can be found at manyresearch institutes and universities that are used for thecharacterization of magnetic materials.

Search-Coil

Another common low field sensor is the basic search-coil magnetometer based on Faraday’s law of induc-tion—which states that the voltage induced in a coil isproportional to the changing magnetic field in the coil.This induced voltage creates a current that is propor-tional to the rate of change of the field. The sensitivityof the search-coil is dependent on the permeability ofthe core, and the area and number of turns of the coil.In order for the search-coil to work, the coil must eitherbe in a varying magnetic field or moving through amagnetic field. This restricts the search-coil from de-tecting static, or slowly changing, fields. These sensorsare commonly found in the road at traffic control sig-nals. They are low cost and easily manufactured.

Other Low Field Sensors

Other low field sensor technologies include nuclearprecession, optically pumped, and fiber-optic magne-tometers. These are precision level instruments usedfor laboratory research and medical applications. Forinstance, the long-term stability of the nuclear proces-sion magnetometer can be as low as 50pT/year. Thesemagnetometers will not be discussed in this paper.

EARTH’S FIELD SENSORS (1 microgauss to 10 gauss)

The magnetic range for the medium field sensors lendsitself well to using the Earth’s magnetic field. Severalways to use the Earth’s field are to determine compassheadings for navigation, detect anomalies in it for vehi-cle detection, and measure the derivative of the changein field to determine yaw rate.

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Fluxgate

Fluxgate magnetometers are the most widely usedsensor for compass navigation systems. They weredeveloped around 1928 and later refined by the militaryfor detecting submarines. Fluxgate sensors have alsobeen used for geophysical prospecting and airbornemagnetic field mapping. The most common type offluxgate magnetometer is called the second harmonicdevice [3-5]. This device involves two coils, a primaryand a secondary, wrapped around a common high-permeability ferromagnetic core. The magnetic induc-tion of this core changes in the presence of an externalmagnetic field. A drive signal is applied to the primarywinding at frequency f (e.g. 10 kHz) that causes thecore to oscillate between saturation points. The secon-dary winding outputs a signal that is coupled throughthe core from the primary winding—see Figure 2. Thissignal is affected by any change in the core permeabil-ity (slope of B-H curve) and appears as an amplitudevariation in the sense coil output. By using a phasesensitive detector, the sense signal can be demodu-lated and low pass filtered to retrieve the magnetic fieldvalue. Another way of looking at the fluxgate operatingprinciple is to sense the ease, or resistance, of satu-rating the core caused by the change in its magneticflux. The difference is due to the external magnetic field.

VDRIVE

VSENSE

core

10KHz

Vaverage = f(Hmeasure)

Hm

easu

re

Figure 2. Fluxgate Magnetometer Operation

Fluxgate magnetometers can sense signal in the tensof microgauss range with careful design consideration.Fluxgates can measure both magnitude and directionof static magnetic fields and have an upper frequencyband limit of around 1 kHz—due to the drive frequencylimit of around 10 kHz. They also tend to be bulky andnot as rugged as smaller, more integrated, sensortechnologies. Fluxgates are available from AppliedPhysics Systems (APS520), Zemco, Inc. (DE-710),Bartington Instruments (Mag-03), Walker Scientific Inc.(WS-43), and Haltek Electronics (Sunnyvale, CA).

Magnetoinductive

Magnetoinductive magnetometers are relatively newwith the first patent issued in 1989. The sensor is sim-ply a single winding coil on a ferromagnetic core thatchanges permeability within the Earth's field. Thesense coil is the inductance element in a L/R relaxationoscillator. The frequency of the oscillator is proportionalto the field being measured. A static dc current is usedto bias the coil in a linear region of operation (see Fig-ure 3). The observed frequency shift can be as muchas 100% as the sensor is rotated 90 degrees from theapplied magnetic field. The oscillator frequency can bemonitored by a microprocessor's capture/compare portto determine field values. These magnetometers aresimple in design, low cost, and low power. They areavailable from Precision Navigation, Inc. and used incompass applications. They have a limited temperaturerange of -20 to 70 degree C, and are repeatable towithin 4 milligauss. The small size and shape makes itdifficult to for automatic assembly and axis alignment.

+-

V+ V+

MI

outputfrequencyvaries withHmeasure

Hmeasure

V+

Figure 3. Magnetoinductive (MI) Sensor Circuit

Anisotropic Magnetoresistive (AMR)

William Thompson, later Lord Kelvin [6], first observedthe magnetoresistive effect in ferromagnetic metals in1856. This discovery had to wait over 100 years beforethin film technology could make a practical sensor forapplication use. Magnetoresistive (MR) sensors comein a variety of shapes and form. The newest marketgrowth for MR sensors is high density read heads fortape and disk drives. Other common applications in-clude automotive wheel speed and crankshaft sensing,compass navigation, vehicle detection, current sensing,and many others.

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The anisotropic magnetoresistive (AMR) sensor is onetype that lends itself well to the Earth’s field sensingrange. AMR sensors can sense dc static fields as wellas the strength and direction of the field This sensor ismade of a nickel-iron (Permalloy) thin film deposited ona silicon wafer and is patterned as a resistive strip. Theproperties of the AMR thin film cause it to change re-sistance by 2-3% in the presence of a magnetic field.Typically, four of these resistors are connected in aWheatstone bridge configuration (see Figure 4) so thatboth magnitude and direction of a field along a singleaxis can be measured. For typical AMR sensors, thebandwidth is in the 1-5 MHz range. The reaction of themagnetoresistive effect is very fast and not limited bycoils or oscillating frequencies. The key benefit of AMRsensors is that they can be bulk manufactured on sili-con wafers and mounted in commercial integrated cir-cuit packages. This allows magnetic sensors to beauto-assembled with other circuit and systems compo-nents. AMR sensors are available from Philips, HL Pla-nar, and Honeywell.

AMRSensor

-

+V+

Vout

V+

Figure 4. AMR Sensor Circuit

AMR Sensor Characteristics

AMR sensors provide an excellent means of measuringboth linear and angular position and displacement inthe Earth’s magnetic field. Permalloy thin films depos-ited on a silicon substrate in various resistor bridgeconfigurations provide highly predictable outputs whensubjected to magnetic fields [6-8]. Low cost, high sen-sitivity, small size, noise immunity, and reliability areadvantages over mechanical or other electrical alterna-tives. Highly adaptable and easy to assemble, thesesensors solve a variety of problems in custom applications.

Most AMR sensors are made of Permalloy (NiFe) thinfilm deposited onto a silicon substrate and patterned toform a Wheatstone resistor bridge. A common bridgeresistance is 1 kohm.

-60

-40

-20

0

20

40

60

-20 -15 -10 -5 0 5 10 15 20

Applied Field (Oe)

Brid

ge O

utpu

t (m

V)

2 sweeps

Figure 5. AMR Output Transfer Curve

The AMR film properties are well behaved only whenthe film's magnetic domains are aligned in the samedirection. This assures high sensitivity and good re-peatability with minimal hysteresis. During fabrication,the film is deposited in a strong magnetic field. Thisfield sets the preferred orientation, or easy axis, of themagnetization vector (M) in the Permalloy resistors(see Figure 6). The M vector is set parallel to the lengthof the resistor and can be set to point in either direc-tion, left or right, in the film. Assume for a moment thatthere is a current in the film flowing at a 45-degree an-gle to the length of the film. This creates an angle theta(θ) between the current flow and M vector. The electri-cal properties of the Permalloy film has a relationshipbetween the M vector in the film and the current flowingthrough the film. Figure 6 illustrates this property. Thefilm resistance is the greatest when the current flowsparallel to the M vector.

If an external magnetic field is applied normal to theside of the film, the Magnetization vector will rotate andchange the angle θ. This will cause the resistancevalue to vary (∆R/R) and produce a voltage outputchange in the Wheatstone bridge. This change in thePermalloy is termed the magnetoresistive effect and isdirectly related to the angle of the current flow and themagnetization vector.

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Current IMagnetization Mθ

Easy Axis

Permalloy (NiFe) Resistor

Current I M

Happlied

no applied field

Figure 6. Magnetoresistive Effect

Note in Figure 7 that the ∆R/R change in resistance issymmetric about the angle θ axis and that there is alinear region about the 45-degree angle. The methodused to cause the current to flow at a 45-degree anglein the film is called barber pole biasing. This is accom-plished through a layout technique by placing low re-sistance shorting bars across the film width. The cur-rent prefers to take the shortest path through the film,thus causing it to flow from one bar to the next at a 45-degree angle. Figure 8 illustrated this effect for all fourresistors in a simple Wheatstone bridge.

∆RR

Angle ( θ) ofMagnetization Field

to Current FlowBarber Pole

Bias

Magneto-Resistance

LinearOperating

Region

90°45°0°-90°

Figure 7. Magnetoresistive Variation with Angle Theta

The magnetoresistive characteristic of the Permalloycauses a resistance change (∆R) in the bridge inducedby the presence of an applied magnetic field. Thiscauses a corresponding change in voltage output asshown in Figure 5. The sensitivity of the bridge is oftenexpressed as mV/V/Oe. The middle term (V) of this unitrefers to the bridge voltage, Vb. When the bridge volt-age (Vb) is set to 5 volts, and the sensitivity (S) is3mV/V/Oe, then the output gain will be 15mV/Oe.Through careful selection of a bridge amplifier, outputlevels of 1 microvolt can be achieved. This results in amagnetic resolution of 67 microoersted, or 1 part in15,000 per oersted. If the bridge output is amplified by

a gain of 67, then the total output sensitivity would be1V/gauss (=67 x 15 mV/gauss). If a full-scale range of±2 gauss is desired, this implies a 4 volt output swingcentered on the 2.5V bridge center value—or a span of0.5 to 4.5V. This signal level is suitable for most A/Dconverters. Using an AMR sensor and amplifier, pre-cise magnetic field information can be derived that pro-vide field magnitude as well as directional information.

Gnd

BiasCurrent

Permalloy Shorting Bars

SensitiveAxis

Vb

Out-

Out+Easy Axis

Figure 8. AMR Barber Pole Bias

A concern for any magnetic sensor made of ferromag-netic material is the exposure to a disturbing magneticfield. For AMR sensors, this disturbing field actuallybreaks down the magnetization alignment in the Per-malloy film that is critical to the sensor operation. Thedirection and magnitude of vector M is essential to re-peatable, low noise, and low hysteresis output signals.The top film in Figure 9 illustrates the AMR film whenexposed to a disturbing magnetic field. The Permalloystrip is broken up into random oriented magnetic do-mains that degrades the sensor operation shown inFigure 6.

Easy Axis

Permalloy (NiFe) Resistor

RandomDomain

Orientations

After a Setor Reset Pulse

Magnetization

Figure 9. Magnetic Domain Orientation in AMR Thin Films

To recover the magnetic state, a strong magnetic fieldmust be applied along the length of the Permalloy film.Within tens of nanoseconds the random domains willline up along the easy axis as shown in the lower film ofFigure 9. Now the M vector is restored and the predict-able magnetoresistive effect will occur. The M vectorwill stay in this state for years as long as there is nomagnetic disturbing field present.

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A common method used to realign these domains is touse a coil around the Wheatstone bridge resistors.Switching a high current pulse through the coil (Figure10) will create a large magnetic field of 60-100 gaussand restore the M vector [9]. This process is referred toas flipping the magnetic domains with a set pulse. Thisflipping action will also take place for a pulse in the op-posite direction through this external coil. In this case,the reset pulse, the domains will all point in the oppo-site direction along the easy axis. The KMZ-10A AMRsensor from Philips requires an external coil around thepackage to create the set and reset fields.

Vb

Iset

Applied Field

-+

Vb

Ireset

Vb

Applied Field

Vb

-+Vset Vreset

Figure 10. Set and Reset Flipping Circuits

Honeywell’s family of AMR sensor has a patented on-chip strap that replaces the external coil to create theset and reset field effects.

Offset Reduction in AMR sensors

Before addressing specific applications it is useful tounderstand how to operate the AMR sensor. Specifi-cally, undesirable effects are inherent in the sensor thatmay interfere with magnetic field sensing such asbridge offset voltages and temperature effects. Thissection addresses these concerns and describes tech-niques to perform automatic gain adjustment and real-time offset cancellation.

Additional benefits to using a set/reset pulse besidesrestoring the sensor properties after exposure to a highmagnetic field. Figure 11 shows the transfer curves fora sensor after it has been set, and then reset, showsan inversion of the gain slope and a common crossoverpoint on the bridge output axis. This crossover point isthe zero field bridge offset voltage. For the sensor inFigure 11, the bridge offset is around –3 mV. This isdue to the resistor mismatch during the manufactureprocess. This offset voltage is usually not desirable andcan be reduced, or eliminated, using one of four tech-niques described below.

MANUAL OFFSET TRIM—The most straightforwardtechnique for offset reduction is to add a parallel trimresistor across one leg of the bridge to force both out-

puts to the same voltage. This must be done in a zeromagnetic field environment, usually in a zero gausschamber. It is labor intensive since each sensor mayrequire a different value trim resistor.

-15

-10

-5

0

5

10

15

-2 -1 0 1 2

Reset

Set

Applied Field (gauss)

Brid

ge O

utpu

t (m

V)

-20

Figure 11. Set and Reset Output Transfer Curves

OFFSET STRAP—Another method of removing theoffset voltage is by using a coil to create a field in thesensitive axis direction. Static current through this coilcan be set to null the bridge offset by adding or sub-tracting a field equal to the offset voltage. Honeywell’sfamily of AMR sensors has a patented on-chip offsetstrap to accomplish offset adjustment. Again, the offsetcurrent must be determined in a zero gauss environ-ment and requires a constant dc source. In this paper,further references to the offset strap will imply eitherthe on-chip strap or an external coil.

SET/RESET WITH MICROPROCESSOR—A thirdmethod to cancel the bridge offset (Vos) is by usingnumerical subtraction. To measure a field Happlied,first activate a set pulse, see Figure 12. Then, after ithas settled, take a reading and store it as Vset. Repeatthese steps for a reset pulse and store the reading asVreset. The expressions for these two readings, andtheir difference, are:

Vset = S * Happlied + Vos (1)Vreset = -S * Happlied + Vos (2)Vset - Vreset = 2 * S * Happlied (3)

Note that in equation (3) there is no Vos term and thedesired field, Happlied, is doubled. The benefit of offsetcancellation using this method is that any temperaturedrift of the bridge offset, including the amplifier, is

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eliminated! This is a powerful technique and easy toimplement if the readings are controlled by a microprocessor.

Set/ResetPulse

VosHapplied

Happlied

Vset

Vreset

time

Vout

Set

Rst

Set

Figure 12. Set/Reset Effect On Bridge Output

A variation of this third method is to add Vset and Vre-set instead of subtracting them; the result is 2*Vos.This approach can be used to periodically check theoffset voltage, say during power-on cycle or once every10 minutes. The Vos can then be subtracted from allsubsequent readings. This will allow increased inputsignal bandwidth and help reduce power consumption.

ELECTRONIC FEEDBACK—A fourth method to elimi-nate the bridge offset is to do it electronically using afeedback amplifier (see Figure 13). The basis of opera-tion is to modulate the sensor input signal to a higherfrequency, remove the offset, and then demodulate itback to a dc voltage. This can be accomplished by us-ing the set/reset switching property shown in Figure 10.By using a square wave of frequency 200 Hz to alter-nately create set and reset pulses, the bridge outputvoltage will switch between Vset and Vreset as de-scribed in equations (1) and (2). This switching ofVout1 helps to reduce the signal noise by modulatingthe low frequency signals of interest to a higher band,away from the 1/f noise, and where the flatband noiseis minimal. Before the output of amplifier #1 is con-nected, the intermediate signal, Vout1, is in the form ofa square wave with amplitude related to 2*Happliedand an offset level of Vos as shown in Figure 12.

Amplifier #1 is designed with a low pass frequency re-sponse so that its output will not follow the 200 Hzsquare wave from the bridge. Instead, it will output anegative dc level corresponding to the Vos of thebridge and any offset of amplifier #2. When this signalis connected to the (+) input of amplifier #2, it cancelsthese offsets. Now, the intermediate signal Vout1 is inthe form of a square wave with amplitude related to2*Happlied and centered around Vref. By using a se-lectable +/-1 gain block controlled by Vset/reset, theoutput signal, Vdemod, will be demodulated. This pro-

duces a dc level that is directly proportional to Hap-plied. An additional low pass filter (~10 Hz) should filterthe Vdemod signal to eliminate any residual switchingnoise at frequency 400 Hz out of the demodulator. Thiscircuit has very low temperature drift since the bridgeoffset and temperature variations are continuously be-ing cancelled as well as the offset and temperatureeffects of the bridge amplifier. The magnetic signalbandwidth is somewhat limited to 10 Hz for this example.

MagneticSensor

+-

HMC1021

+5V

S/Rstrap

-+

Vref

Vref

~1 Hz response

-Vos

Vdemod

Av = ±1

Vout1Happlied

Vset/reset @ 200 Hz

1

2 3

Figure 13. Electronic Feedback for Offset Reduction

Compensating for hard iron effects

Any external magnetic field can be canceled by drivinga defined current through the offset strap. This is usefulfor eliminating the effects of stray hard iron distortion ofthe Earth’s magnetic field. For example, reducing theeffects of a car body on the Earth’s magnetic field in anautomotive compass application. If the MR sensor is ina fixed position within the automobile, the effect of thecar on the Earth’s magnetic field can be approximatedas a shift, or offset, field. If this shift in the Earth's fieldcan be determined, then it can be compensated for byapplying an equal and opposite field using the offset strap.

In-circuit gain calibration

The offset strap can also be used to auto-calibrate theAMR bridge while in the application during normal op-eration. This is useful for occasionally checking thebridge gain for that axis or to make adjustments over atemperature drift. This can be done during power-up oranytime during normal operation. The concept is sim-ple; take two points along a line and determine theslope of that line—the gain. When the bridge is meas-uring a steady applied magnetic field the output willremain constant. Record the reading for the steadyfield and call it H1. Now apply a known current throughthe offset strap and record that reading as H2. This can

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be as simple as switching a 1 kohm resistor in serieswith the offset strap using a microprocessor output.The current through the offset strap will cause achange in field the MR sensor measures—call that thedelta applied field (∆Ha). The AMR sensor gain is thencomputed as:

AMRgain = (H2-H1) / ∆Ha

Closed-loop circuit for precision measurements

The offset strap can be used as a feedback element ina closed loop circuit—Figure 14. Using the offset strapin a current feedback loop can produce desirable re-sults for measuring magnetic fields. To do this, connectthe output of the bridge amplifier to a low pass filterdriver connected to the offset strap. Using high gainand negative feedback in the loop, this will create acanceling, or offsetting, magnetic field that will drive theAMR bridge output to zero. The resultant currentthrough the offset strap indicates how strong the field isbeing cancelled. This current is measured using a re-sistor, Rsense, which generates an output voltage,Vsense. This method gives extremely good linearityand temperature characteristics. The idea in this circuitis to always operate the AMR bridge in the balancedresistance mode. That is, no matter what magnetic fieldis being measured, the current through the offset strapwill cancel it out. The bridge always “sees” a zero fieldcondition. The resultant offset current required to can-cel the applied field is a direct measure of that fieldstrength and can be translated into the field value.

There are many other uses for the offset strap thanthose described here. The key point is that the ambientfield and the offset field simply add to one another andare measured by the AMR sensor as a single field.

MagneticSensor

HMC1021

+5V

S/Rstrap

VdemodHapplied

Vset/reset @ 200 Hz

Vref

VsenseOffsetstrap

LPFDriver

Rsense

BridgeAmplifier

(see Figure 13)

Figure 14. Closed-Loop Circuit with AMR Sensor

BIAS MAGNET FIELD SENSORS (above 10 gauss)

Most industrial sensors use permanent magnets as asource of the detected magnetic field. These perma-nent magnets magnetize, or bias, ferromagnetic ob-jects close to the sensor. The sensor then detects thechange in the total field at the sensor. Bias field sen-sors not only must detect fields which are typicallylarger than the Earth’s field, but they also must not bepermanently affected or temporarily upset by a largefield. Sensors in this category include reed switches,InSb magnetoresistors, Hall devices, and GMR sen-sors. Although some of these sensors, such as mag-netoresistors, are capable of measuring fields up toseveral teslas, others such as GMR sensors can detectfields down to the milligauss region with research ex-tending their capabilities to the microgauss region.

Reed Switches

Possibly the simplest magnetic sensor which producesa useable output for industrial control is the reedswitch. It consists of a pair of flexible, ferromagneticcontacts hermetically sealed in an inert gas filled con-tainer, often glass. The magnetic field along the longaxis of the contacts magnetizes the contacts causingthem to attract one another closing the circuit. There isusually considerable hysteresis between the closingand releasing fields so they are quite immune to smallfluctuations in the field.

Reed switches are maintenance free and have a highimmunity to dirt and contamination. Rhodium platedcontacts insure long contact life. Typical capabilities are0.1 to 0.2 A switching current and 100 to 200 V switch-ing voltage. Contact life is measured at 106 to 107 op-erations at 10 mA. Reed switches are available withnormally open (NO), normally closed (NC), and class C(SPDT) contacts. Latching reed switches are alsoavailable. Mercury wetted reed switches can switchcurrents as high as 1 A and have no contact bounce.

Low cost, simplicity, reliability, and zero power con-sumption make reed switches popular in many applica-tions. A reed switch together with a separate smallpermanent magnet make a simple proximity switchoften used in security systems to monitor the openingof doors or windows. The magnet, affixed to the move-able part, activates the reed switch when it comesclose enough. The desire to sense almost everything incars is increasing number of reed switch sensing appli-cations in the automotive industry.

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Lorentz Force Devices

There are several sensors that utilize the Lorentz force,or Hall effect, on charge carriers in a semiconductor.The Lorentz Force equation describes the force FLexperienced by a charged particle with charge q mov-ing with velocity v in a magnetic field B.

FL = q (v x B).

Since the quantities FL, v, and B are vector quantities,they have both magnitude and direction. The Lorentzforce is proportional to the cross product between thevectors representing velocity and magnetic field so it isperpendicular to both of them and, for a positivelycharge carrier, has the direction of advance of a right-handed screw rotated from the direction of v towardsthe direction of B. The acceleration caused by the Lor-entz force is always perpendicular to the velocity of thecharged particle; therefore, in the absence of any otherforces, a charge carrier follows a curved path in amagnetic field.

The Hall Effect is a consequence of the Lorentz force insemiconductor materials. When a voltage is appliedfrom one end of a slab of semiconductor material to theother end, charge carriers start to flow. If at the sametime a magnetic field is applied perpendicular to theslab, the current carriers are deflected to the side bythe Lorentz force. Charge builds up along the side untilthe resulting electrical field produces a force on thecharged particle sufficient to counteract the Lorentzforce. This voltage across the slab perpendicular to theapplied voltage is called the Hall Voltage. Figure 15 is aschematic of the geometry involved in the Hall effect.

Magnetic Field

Applied Voltage

force from

electric fie

ld+ hole

-electron

force from magnetic field

path

path

Figure 15. A semiconductor slab showing magneticfield, applied voltage, forces on electrons and holes,and paths of electrons and holes.

Magnetoresistors

The simplest of Lorentz force devices are magnetore-sistors using semiconductors such as InSb and InAswith high room-temperature carrier mobility. If a voltageis applied along the length of a thin slab of semicon-ductor material, a current will flow and a resistance canbe measured. When a magnetic field is applied per-pendicular to the slab, the Lorentz force will deflect thecharge carriers. If the width of the slab is greater thanthe length, the charge carriers will cross the slab with-out a significant number of them collecting along thesides. The effect of the magnetic field is to increase thelength of their path and, therefore, the resistance. Anincrease in resistance of several hundred percent ispossible in large fields. In order to produce sensorswith hundreds to thousands of ohms of resistance,long, narrow semi-conductor stripes a few µm wide areproduced using photolithography. The required lengthto width ratio is accomplished by forming periodic lowresistance metal shorting bars across the traces. Eachshorting bar produces an equipotential across thesemiconductor stripe. The result is, in effect, a numberof small semiconductor elements with the proper lengthto width ratio connected in series. A second method isused in commercial devices manufactured by Siemensuses lapped wafers cut from boules which have needleshaped low resistance precipitates of NiSb in a matrixof InSb. These precipitates serve as the shorting bars[10]. Figure 16 shows the effect of these shorting barson the current path. Notice that the higher the magneticfield the longer the current path and the higher the re-sistance.

M agnetic F ie ldM agnetic F ie ld

E lectric F ie ldCurre

nt path

InSb slab

N iSb precip itatesN iSb precip itates

Figure 16. Schematic diagram of the current path in aslab of InSb with NiSb precipitates.

10

Magnetoresistors formed from InSb are relatively in-sensitive at low fields but exhibit large changes in re-sistance at high fields. The resistance changes ap-proximately as the square of the field. They are sensi-tive only to the component of the magnetic fieldperpendicular to the slab and are not sensitive towhether the field is positive or negative. The large tem-perature coefficients of resistivity are caused by thechange in mobility of the charge carriers with tempera-ture. Sensors are made with either single resistors orpairs of spaced resistors. The second type is used tomeasure field gradients and is usually combined withexternal resistors to form a Wheatstone bridge. A per-manent magnet is often incorporated in the field gradi-ent sensor to bias the magnetoresistors up to a moresensitive part of their characteristic curve Figure 17shows the change of resistance of a typical InSb sen-sor with field and temperature. Different doping levelsof the semiconductor material account for the differ-ences in characteristics as well as differences in con-ductivity—200 (Ωcm)-1 for D material and 550 (Ωcm)-1for L material.

Siemens D material

0

5

10

15

20

25

30

35

40

45

0 0.5 1

magnetic field (T)

norm

aliz

ed r

esis

tanc

e

Siemens L material

0

1

2

3

4

5

6

7

8

9

0 0.5 1

magnetic field (T)

norm

aliz

ed r

esis

tanc

e

Figure 17. Resistance vs. field for an InSb magnetore-sistors at different temperatures. Resistance is nor-malized to the resistance at zero field. Temperaturesfrom top to bottom are: -20, 0, 25, 60, 90, and 120 ºC.

Hall Sensors

The second type of sensor, which utilizes the Lorentzforce on charge carriers, is a Hall sensor. These de-vices predominantly use n-type silicon when cost is ofprimary importance and GaAs for higher temperaturecapability due to its larger band gap. In addition, InAs,InSb, and other semiconductor materials are gainingpopularity due to their high carrier mobilities which re-

sult in greater sensitivity and in frequency responsecapabilities above the 10 to 20 kHz typical of Si Hallsensors. Compatibility of the Hall sensor material withsemiconductor substrates is important since Hall sen-sors are often used in integrated devices which includeother semiconductor structures.

A Hall sensor uses a geometry similar to that shown inFigure 15; however, in this case the length in the direc-tion of the applied voltage is long compared to thewidth. Charge carriers are deflected to the side andbuild up until they create a Hall voltage across the slabwhose force equals the Lorentz force on the chargecarriers. At this point the charge carriers travel thelength in approximately straight lines, and additionalcharge does not build up. Since the final charge carrierpath is essentially along the applied electric field, theend-to-end resistance changes little with magnetic field.The Hall voltage is measured between electrodesplaced at the middle of each side. This differential volt-age is proportional to the magnetic field perpendicularto the slab. It also changes sign when the sign of themagnetic field changes. The ratio of the Hall voltage tothe input current is called the Hall resistance, and theratio of the applied voltage to the input current is calledthe input resistance.

The Hall resistance and Hall voltage increase linearlywith applied field to several teslas (10s of kilogauss).The temperature dependence of the Hall voltage andthe input resistance of Hall sensors are governed bythe temperature dependence of the carrier mobility andthat of the Hall coefficient. Different materials and dif-ferent doping levels result in tradeoffs between sensi-tivity and temperature dependence. Figures 18 and 19illustrate the temperature dependence of the input re-sistance and Hall voltage for several materials [11].

0

200

400

600

800

1000

1200

1400

1600

-100 -50 0 50 100 150 200

Temperature (C)

Inpu

t Res

ista

nce

(ohm

s)

InSbGaAsInAs

Figure 18. Input resistance of several Hall sensors withdifferent semiconductor materials vs. temperature.

11

0

50

100

150

200

250

300

-100 -50 0 50 100 150 200Temperature (C)

Hal

l vol

tage

at 0

.05

T (

mV

)

InSb @1VGaAs@6VInAs @6V

Figure 19. Hall voltage at 0.05 T (500 Oe) of severalHall sensors with different semiconductor materials vs.temperature. The input voltage is given for each material.

Integrated Hall Sensors

Hall devices are often combined with semiconductorelements to make integrated sensors. By adding com-parators and output devices to a Hall element manu-factures provide unipolar and bipolar digital switches.Adding an amplifier increase the relatively low-voltagesignals from a Hall device to produce ratiometric linearHall sensors with an output centered on one half thesupply voltage. Power usage can even be reduced toextremely low levels by using a low duty cycle [12].

Giant Magnetoresistive (GMR) Devices

Large magnetic field dependent changes in resistanceare possible in thin-film ferromagnet/non-magneticmetallic multilayers. This phenomenon was first ob-served in France in 1988 [13]. Changes in resistancewith magnetic field of up to 70% were observed. Com-pared to the few percent change in resistance ob-served in anisotropic magnetoresistance (AMR), thisphenomenon was truly giant magnetoresistance (GMR)The resistance of two thin ferromagnetic layers sepa-rated by a thin non-magnetic conducting layer can bealtered by changing whether the moments of the ferro-magnetic layers are parallel or antiparallel. Layers withparallel magnetic moments will have less scattering atthe interfaces, longer mean free paths, and lower re-sistance. Layers with antiparallel magnetic momentswill have more scattering at the interfaces, shortermean free paths, and higher resistance. These differ-ences are shown schematically in Figure 20. In orderfor spin dependent scattering to be a significant part ofthe total resistance, the layers must be thinner than themean free path of electrons in the bulk material. For

many ferromagnets the mean free path is tens of na-nometers, so the layers themselves must each be typi-cally less than 10 nm (100 Å). It is not surprising, then,that GMR was only recently observed with the devel-opment of thin-film deposition systems.

Magnetic LayerNon-magnetic ConductorMagnetic Layer

Antiparallel MomentsHigh Interface ScatteringHigh Resistance

Parallel MomentsLow Interface ScatteringLow Resistance

Figure 20. Scattering from two different alignment ofmagnetic moments in a GMR “sandwich” with twomagnetic layers separated by a conducting non-magnetic layer.Various methods of obtaining antiparallel magneticalignment in thin ferromagnet-conductor multilayers asshown in Figure 20 have been discussed elsewhere[14-16]. The structures currently being used in GMRsensors are unpinned sandwiches and antiferromag-netic multilayers although spin valves are of consider-able interest especially for magnetic read heads.

Unpinned sandwich GMR materials consist of twosoft magnetic layers of iron, nickel and cobalt alloysseparated by a layer of a non-magnetic conductor suchas copper. With magnetic layers 4 to 6 nm (40 to 60 Å)thick separated by a conductor layer typically 3 to 5 nmthick there is relatively little magnetic coupling betweenthe layers. For use in sensors, sandwich material isusually patterned into narrow stripes. The magneticfield caused by a current of a few mA per µm of stripewidth flowing along the stripe is sufficient to rotate themagnetic layers into antiparallel or high resistancealignment. An external magnetic field of 3 to 4 kA/m(35 to 50 Oe) applied along the length of the stripe issufficient to overcome the field from the current androtate the magnetic moments of both layers parallel tothe external field. A positive or negative external fieldparallel to the stripe will both produce the same changein resistance. An external field applied perpendicular tothe stripe will have little effect due to the demagnetizingfields associated with the extremely narrow dimensionsof these magnetic objects. The value usually associ-ated with the GMR effect is the percent change in re-sistance normalized by the saturated or minimum re-sistance. Sandwich materials have values of GMR typi-cally 4 to 9 % and saturate with 2.4 to 5 kA/m (30 to 60Oe) applied field. Figure 21 shows a typical resistancevs. field plot for sandwich GMR material.

12

0 .7

0 .7 0 5

0 .7 1

0 .7 1 5

0 .7 2

0 .7 2 5

0 .7 3

0 .7 3 5

0 .7 4

-5 0 -4 0 -3 0 -2 0 -1 0 0 1 0 2 0 3 0 4 0 5 0a p p lie d f ie ld ( O e )

volta

ge (

V)

Figure 21. Voltage vs. applied field for a 2 µm widestripe of unpinned sandwich GMR material with 1.5 mAcurrent. GMR = 5 %.

Antiferromagnetic multilayers consist of multiplerepetitions of alternating conducting magnetic layersand non-magnetic layers. Since multilayers have moreinterfaces than do sandwiches, the size of the GMReffect is larger. The thickness of the non-magnetic lay-ers is less than that for sandwich material (typically 1.5to 2.0 nm) and the thickness is critical. Only for certainthicknesses, the polarized conduction electrons causeantiferromagnetic coupling between the magnetic lay-ers. Each magnetic layer has its magnetic moment an-tiparallel to the moments of the magnetic layers oneach side—exactly the condition needed for maximumspin dependent scattering. A large external field canovercome the coupling which causes this alignmentand can align the moments so that all the layers areparallel—the low resistance state. If the conductinglayer is not the proper thickness, the same couplingmechanism can cause ferromagnetic coupling betweenthe magnetic layers resulting in no GMR effect.

A plot of resistance vs. applied field for a multilayerGMR material is shown in Figure 22. Note the higherGMR value, typically 12 to 16 %, and the much higherexternal field required to saturate the effect, typically 20kA/m (250 Oe). Multilayer GMR materials have betterlinearity and lower hysteresis than typical sandwichGMR material.

Spin valves , or antiferromagnetically pinned spinvalves, are similar to the unpinned spin valves orsandwich materials described earlier. An additionallayer of an antiferromagnetic material is provided onthe top or the bottom. The antiferromagnetic materialsuch as FeMn or NiO couples to the adjacent magneticlayer and pins it in a fixed direction. The other magneticlayer is free to rotate. These materials do not requirethe field from a current to achieve antiparallel alignment

3.6

3.7

3.8

3.9

4

4.1

4.2

-400 -200 0 200 400

applied field (Oe)

resi

stan

ce (

kohm

)

Figure 22. Resistance vs. applied field for a 2 µm widestripe of antiferromagnetically coupled multilayer GMRmaterial. GMR = 14 %.

or a strong antiferromagnetic exchange coupling toadjacent layers. The direction of the pinning layer isusually fixed by elevating the temperature of the GMRstructure above the blocking temperature. Above thistemperature, the antiferromagnet is no longer coupledto the adjacent magnetic layer. The structure is thencooled in a strong magnetic field which fixes the direc-tion of the moment of the pinned layer. If the spin valvematerial is heated above its blocking temperature, itcan loose its orientation. The operating temperature ofa spin valve sensor is limited to below its blocking tem-perature Since the change in magnetization in the freelayer is due to rotation rather than domain wall motion,hysteresis is reduced. Values for GMR are 4 to 20 %and saturation fields are 0.8 to 6 kA/m (10 to 80 Oe).

Spin valves are receiving a high level of research inter-est due to their potential for use in magnetic readheads for high density data storage applications [17].IBM has announced the introduction of a 16.8 gigabytehard drive with a spin valve read head. Bridge sensordesigns using spin valve materials have also been de-scribed in the literature [18] and rotational position sen-sors in a product bulletin [19].

Spin dependent tunneling (SDT ) structures are verysimilar to those shown in Figure 20 except that an ex-tremely thin insulating layer is substituted for the con-ductive interlayer separating the two magnetic layers.The conduction is due to quantum tunneling throughthe insulator. The size of the tunneling current betweenthe two magnetic layers is modulated by the directionbetween the magnetization vectors in the two layers[20]. The conduction path must be perpendicular to theplane of the GMR material since there is such a largedifference between the conductivity of the tunneling

13

path and that of any path in the plane. Extremely smallSDT devices several µm on a side with high resistancecan be fabricated using photolithography allowing verydense packing of magnetic sensors in small areas. Al-though these recent materials are very much a topic ofcurrent research, values of GMR of 10 to 25 % havebeen observed. The saturation fields depend upon thecomposition of the magnetic layers and the method ofachieving parallel and antiparallel alignment. Values ofsaturation field range from 0.1 to 10 kA/m (1 to 100 Oe)offering the possibility of extremely sensitive magneticsensors with very high resistance suitable for batteryoperation.

Colossal Magnetoresistance Scientists, to surpassthe term giant, have proceeded on to collosal magne-toresistance materials (CMR). Under certain conditionsthese mixed oxides undergo a semiconductor to metal-lic transition with the application of a magnetic field of afew tesla (10s of kilogauss). The size of the resistanceratios, measured at 103 to 108 %, have generated con-siderable excitement even though they required highfields and liquid nitrogen temperatures. Recently aca-demic groups have developed CMR materials whichwork at room temperature and fabricated Wheatstonebridge topology sensors out of these materials [21].These CMR materials are still a long ways from commercialapplications but are a new development to watch.

GMR Circuit Techniques

To date the best utilization of GMR materials for mag-netic field sensors has been in Wheatstone bridge con-figurations, although simple GMR resistors and GMRhalf bridges can also be fabricated. A sensitive bridgecan be fabricated from four photolithographically pat-terned GMR resistors, two of which are active ele-ments. These resistors can be as narrow as 2 µm al-lowing a serpentine 10 kΩ resistor to be patterned in anarea as small as 100µm by 100µm. The vary narrowwidth also makes the resistors sensitive only to thecomponent of magnetic field along their long dimen-sion. Small magnetic shields are plated over two of thefour equal resistors in a Wheatstone bridge protectingthese resistors from the applied field and allowing themto act as reference resistors. Since they are fabricatedfrom the same material, they have the same tempera-ture coefficient as the active resistors. The two re-maining GMR resistors are both exposed to the exter-nal field. The bridge output is therefore twice the outputfrom a bridge with only one active resistor. The bridgeoutput for a 10 % change in these resistors is approxi-mately 5 % of the voltage applied to the bridge.

Additional permalloy structures are plated onto thesubstrate to act as flux concentrators. The active re-

sistors are placed in the gap between two flux concen-trators as is shown in Figure 23. These resistors expe-rience a field which is larger than the applied field byapproximately the ratio of the gap between the fluxconcentrators, D1, to the length of one of the flux con-centrators, D2. In some sensors the flux concentratorsare also used as shields by placing two resistors be-neath them as is shown for R3 and R4. The sensitivityof a GMR bridge sensor can be adjusted in design bychanging the lengths of the flux concentrators and thegap between them. In this way, a GMR material whichsaturates at approximately 300 Oe can be used to builddifferent sensors which saturate at 15, 50, and 100 Oe.To produce sensors with even more sensitivity, exter-nal coils and feedback can be used to produce sensorswith resolution in the 100 mA/m or milli-gauss range.

External field

R1R4

B

A

R3R2

D2 D1

R2R1

R3 R4

A B

Figure 23. Configuration of GMR resistors in a Wheat-stone bridge sensor. Flux concentrators are shown: D1is the lengths of the gap between the flux concentra-tors, and D2 is the length of one flux concentrator.

Smart sensors with both sensing elements and associ-ated electronics such as amplification and signal condi-tioning on the same die are the latest trend in modernsensors. GMR materials are deposited on wafers withsputtering systems and can, therefore, be directly inte-grated with semiconductor processes. The small sizesensing elements fit well with the other semiconductorstructures and are applied after most of the semicon-ductor fabrication operations are complete. Due to thetopography introduced by the many layers of polysili-con, metal, and oxides over the transistors, areas mustbe reserved with no underlying transistors or connec-tions. These areas will have the GMR resistors. TheGMR materials are actually deposited over the entirewafer, but the etched sensor elements remain only onthese reserved, smooth areas on the wafers [22].

Functions included in an integrated sensor include regulatedvoltage or current supplies to the sensor elements; thresholddetection to provide a switched output when a preset field isreached, amplifiers, logic functions including divide by 2 cir-cuits; and various options for outputs. Using such elements,a two-wire sensor can be designed which has two currentlevels—a low current level when the field is below a thresh-old and a high current level when the field is above thethreshold.

14

On-board sensor electronics can increase signal levelsto significant voltages with the least pickup of interfer-ence. It is always best to amplify low-level signals closeto where they are generated. Converting analog signalsto digital (switched) outputs within the sensor is anothermethod of minimizing electronic noise. The use ofcomparators and digital outputs makes the non-linearityin the output of sandwich GMR materials of less con-cern. Even the hysteresis in such materials can beuseful, since some hysteresis is usually built into com-parators to avoid multiple triggering of the output due tonoise. Figure 24 shows the circuit diagram and outputcharacteristics for a commercial digital GMR sensor.

Differential

Field

Operate pointRelease point

Logical “1”

Logical “0”

Operate point

Release point

Figure 24. The schematic diagram and logic outputcharacteristic of an integrated digital GMR sensor.

GMR materials have been successfully integrated withboth BiCMOS semiconductor underlayers and bipolarsemiconductor underlayers. The wafers are processedwith all but the final layer of connections made. GMRmaterial is deposited on the surface and patterned fol-lowed by a passivation layer. Windows are cut throughthe passivation layer to allow contact to both the uppermetal layer in the semiconductor wafer and to the GMRresistors. The final layer of metal is deposited and pat-terned to interconnect the GMR sensor elements andto connect them to the semiconductor underlayers. Thefinal layer of metal also forms the pads to which wireswill be bonded during packaging. A final passivation

layer is deposited, magnetic shields and flux concen-trators are plated and patterned, and windows areetched through to the pads.

GMR SENSOR APPLICATIONS

Proximity Detection

A magnetic field sensor can directly sense a magnetic fieldfrom a permanent magnetic, an electromagnet, or a current.In sensing the presence of a ferrous object, a biasing mag-net is often used. The biasing magnet magnetizes the fer-romagnetic object such as a gear tooth, and the sensor de-tects the combined magnetic fields from the magnetizedobject and the biasing magnet. A biasing magnet is affixedto the sensor in a position such that its direct influence onthe sensor is minimal. Usually the biasing magnet ismounted on the top of the sensor with its magnetic axis per-pendicular to the sensitive axis of the sensor. The biasingmagnet can be centered such that there is little or no field inthe sensitive direction of the sensor. In this way a reason-able large biasing magnet can be used. Occasionally aspacer is used between the sensor and the magnet to re-duce the field at the sensor and, therefore, reduce how criti-cally the magnet must be positioned. Figure 25 shows therelative positions of a sensor and a biasing magnet. Themagnetic field lines are shown both in the absence andpresence of a ferromagnetic object. Note the induced mag-netic moment in the ferromagnetic object.

MagnetMagnet

SpacerSpacer

SensorSensor

NSNS

F e r r o u sF e r r o u sO b je c tO b je c t

SSNN

N SN S

Figure 25. Side view of biasing magnet and sensor in 8 pinpackage shown with and without a ferrous object present.

15

The technique of using a biasing magnet is customarilyused only if the ferrous object is in close proximity. It isdifficult to magnetize an object several meters awayusing the field from a sensor-sized permanent magnet.The field from a dipole magnet falls off at the reciprocalof the distance cubed. In some applications such asdetection of motor vehicles, the Earth field acts as abiasing magnet resulting in a magnetic signature fromvarious parts of the car which are magnetized by theEarth’s field. One such application is the counting andclassification of motor vehicles passing over portable orpermanent sensors in the road. Small, low-poweredGMR sensors allow the sensors, electronics, memory,and battery to be packaged in a low-profile, protective,aluminum housing the size of your hand [23]. A secondapplication in which the biasing magnet is not mountedon the sensor is currency detection. The particles in theink on many countries’ currency have ferromagneticproperties. Bills are passed over a permanent magnetarray and magnetized along their direction of travel. Amagnetic sensor located several inches away with itssensitive axis parallel to the direction of travel can de-tect the remnant field of the ink particles. The purposeof the biasing magnet in this case is to achieve a con-trolled orientation of the magnetic moments of the inkparticles resulting in a maximum and recognizablemagnetic signature. Reversing the magnetizing fieldcan actually invert the signature.

Displacement Sensing

GMR bridge sensors can be effectively employed toprovide position information from small displacementsassociated with actuating components in machinery,proximity detectors, and linear position transducers.Due to the nonlinear characteristic of dipole magneticfields produced by permanent magnets, the range oflinear output may be limited. Figure 26 shows the posi-tion and motion of two sensors with differing sensitiveaxis directions relative to a cylindrical permanent mag-net. The sensitive axis of the sensor is indicated by thedouble headed arrow on each sensor. The rate ofchange of the component of the magnetic field alongthe sensitive axis for each sensor is shown superim-posed on the line of motion. Note that the field for thelower sensor changes direction and is negative in thecenter and positive at both ends.

Rotational Reference Detection

GMR sensors offer a rugged, low cost solution to rota-tional reference detection. High sensitivity and dc op-eration afford the GMR bridge sensor an advantageover inductive sensors which have very low outputs atlow frequencies and can generate large noise signalswhen subjected to high frequency vibrations. GMR

sensors are field sensors and do not measure the in-duced signal from the time rate of change of fields, asdo variable reluctance sensors. The output from aGMR bridge sensor will have a minimum when thesensor is centered over a tooth

S NS N

Figure 26. Sensors positioned to measure displace-ment relative to a permanent dipole magnet. Sensitiveaxes are indicated and the component of field along thesensitive axes for the two sensor are graphed.

or a gap and a maximum when a tooth approaches orrecedes. Figure 25 illustrates a bridge sensor in posi-tion for angular position sensing.

Current Sensing

Currents in wires create magnetic fields surroundingthe wires or traces on printed wiring boards. The fielddecreases as the reciprocal of the distance from thewire. GMR bridge sensors can be effectively employedto sense this magnetic field. Both dc and ac currentscan be detected in this manner. Bipolar ac current willbe rectified by the sensors omnipolar sensitivity unlessa method is used to bias the sensor away from zero.Unipolar and pulsed currents can be measured withgood reproduction of fast rise time components due tothe excellent high frequency response of the sensors.Since the films are extremely thin, response to fre-quencies up to 100 megahertz is possible. Figure 27shows the relative position of a GMR bridge sensor anda current carrying wire to detect the current in the wire.A wire placed immediately over or under the sensor willproduce a field of approximately 0.080 A/m (one mOe)

16

per mA of current. The sensor can also be mountedimmediately over a current carrying trace on a circuitboard. High currents may require more separation be-tween the sensor and the wire to keep the field withinthe sensor’s range. Low currents may be best detectedwith the current being carried by a trace on the chipimmediately over the GMR resistors.

Axis ofsensitivity

Direction ofcurrent flow

Figure 27. Proper orientation of a GMR bridge sensorto detect the magnetic field created by a current carry-ing wire.

AMR SENSOR APPLICATIONS

AMR sensors available today do an excellent job ofsensing magnetic fields within the Earth’s field—below1 gauss. These sensors are used in applications fordetecting ferrous objects such as planes, train, andautomobiles that disturb the Earth’s field. Other appli-cations include magnetic compassing, rotational sens-ing, current sensing, underground drilling navigation,linear position sensing, yaw rate sensors, and headtracking for virtual reality.

Vehicle Detection

The Earth’s field provides a uniform magnetic field overa wide area—say several kilometers2. Figure 28 showshow a ferrous object, a car, creates a local disturbancein this field whether it is moving or standing still. AMRmagnetic sensors can detect the change in the Earth’sfield due to the vehicle disturbance for many types ofapplications.

Figure 28. Vehicle Disturbance In Earth’s FieldApplications for vehicle detection can take severalforms. A single axis sensor can detect if a vehicle ispresent, or not. The sensing distance from the vehiclecan extend up to 15 meters away depending on its fer-rous content. This may be useful for parking garages togive drivers entering it a choice of where the mostavailable spaces to park. Another use is to detect ap-proaching trains to control the crossing gates. In thisapplication, two sensors could be used to detect pres-ence, direction of travel, and speed.

Magnetic disturbances can be used for vehicle classifi-cation for toll road application. A three axis AMR mag-netometer placed in the lane of traffic will provide a richsignal output for vehicle passing over it. Figure 29shows a magnetometer output for three vehicles drivingover it at roughly 1, 3, and 5 seconds on the time axis.The type of vehicle (car, truck, bus, etc) can be classi-fied through pattern recognition and matching algorithms.

Figure 29. Magnetic Variations For Vehicle Detection

17

Electronic Compass Using AMR Sensors

The Earth’s magnetic field intensity is about 0.5 to 0.6gauss and has a component parallel to the Earth’ssurface that always point toward magnetic north. Thisis the basis for all magnetic compasses. AMR sensorsare best suited for electronic compasses since theirrange of sensitivity is centered within the earth’s field[24].

The Earth’s magnetic field can be approximated withthe dipole model shown in Figure 30. This figure illus-trates that the Earth’s field points down toward north inthe northern hemisphere, is horizontal and pointingnorth at the equator, and point up toward north in thesouthern hemisphere. In all cases, the direction of theEarth’s field is always pointing to magnetic north. It isthe components of this field that are parallel to theEarth’s surface that are used to determine compassdirection. The vertical portion of the Earth’s magneticfield is ignored.

Figure 30. Earth’s Magnetic Field Dipole Model

To achieve a one degree accurate compass requires amagnetic sensor that can reliably resolve angularchanges to 0.1 degrees. The sensors must also exhibitlow hysteresis (<0.05%FS), a high degree of linearity(<0.5%FS error) and be repeatable. The magneticfields in the X and Y plane will typically be in the 200 to300 milligauss range—more at the equator, less at thepoles. Using the relationship:

Azimuth = arcTan (y/x) (4)

the required magnetometer resolution can be esti-mated. To resolve a 0.18 change in a 200milligaussfield would require a magnetic sensitivity of better than0.35 milligauss. Solid state MR sensors are availabletoday that reliably resolve 0.07 milligauss signals givinga five times margin of detection sensitivity.

Most often compasses are not confined to a flat andlevel plane. They are often hand held, attached to anaircraft, or on a vehicle in an uneven terrain. Thismakes it more difficult to determine the azimuth, orheading direction, since the compass is not alwayshorizontal to the Earth’s surface. Errors introduced bytilt angles can be quite large depending on the amountof the Dip angle. A typical method for correcting thecompass tilt is to use an inclinometer, or tilt sensor, todetermine the roll and pitch angles [25]. The terms rolland pitch are commonly used in aviation: roll refers tothe rotation around the X, or forward direction, andpitch refers to the rotation around the y, or left-right,direction as in Figure 31.

Liquid filled tilt sensors, resembling a glass “thimble”,use electrodes to monitor the fluid movement as thesensor changes angles. Newer solid state acceler-ometer tilt sensors are available that measure theEarth’s gravitational field by means of an electrome-chanical circuit [26]. The output of these devices are anelectrical signal equivalent to the angle of tilt.

φ

θ

ForwardLevel

roll

pitch

Com

pass

RightLevel

x

y

z

Figure 31. Compass Tilt Referenced To The Earth’sHorizontal Plane

To compensate a compass for tilt, knowing the roll andpitch is only half the battle. The magnetometer mustnow rely on all three magnetic axes (X, Y, Z) so that theEarth’s field can be fully rotated back to a horizontalorientation. In Figure 31, a compass is shown with roll(θ) and pitch (φ) tilt angles referenced to the right andforward level directions of the observer or vehicle. TheX, Y, and Z magnetic readings can be transformedback to the horizontal plane (XH, YH) by applying therotational equations shown below:

XH = X*cos(φ) + Y*sin(θ)*sin(φ) - Z*cos(θ)*sin(φ) YH = Y*cos(θ) + Z*sin(θ) (5)

Azimuth = arcTan (YH / XH)

18

Once the X and Y magnetic readings are in the hori-zontal plane, equations (4) can be used to determinethe azimuth. For speed in processing the rotational op-erations, a sine and cosine lookup table can be storedin program memory to minimized computation time.A block diagram for a tilt compensated compass isshown in Figure 32 with a serial bus interface. After theazimuth is determined, the declination correction canbe applied to find true north according to the geo-graphic region of operation.

X

Y3 - Axis

MagneticSensor

Analog to Digital

Converter

SerialBus

Tilt Sensor Pitch

Roll

Z

Figure 32. Tilt Compensated Compass System

SUMMARY

Magnetic field detection has vastly expanded as indus-try has utilized a variety of magnetic sensors to detectthe presence, strength, or direction of magnetic fieldsnot only from the Earth, but also from permanent mag-nets, from magnetized soft magnets, and from themagnetic fields associated with current. These sensorsare used as proximity sensors, speed and distancemeasuring devices, navigation compasses, and currentsensors. They can measure these properties withoutactual contact to the medium being measured and be-come the eyes of many control systems. This paperhas described the present state of several methods ofmagnetic sensing and how they are used in variousapplications.

Unit conversion from SI to Gaussian: 79.6 A/m = 1 oersted 1 gauss = 1 oersted (in free air) 1 gauss = 10-4 tesla = 105 gamma 1 nanotesla = 10 microgauss = 1 gamma

REFERENCES

[1] J.E. Lenz, “A Review of Magnetic Sensors”, Pro-ceedings of the IEEE, vol. 78, no.6, (June 1990) 973-989.

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