Download pdf - CAUSE & EFFECT: TROUBLESHOOTING HALL EFFECT SENSORS · PDF file52 September 2006 CAUSE & EFFECT: TROUBLESHOOTING HALL EFFECT SENSORS CAUSE & EFFECT: BY BERNIE THOMPSON The predictable

52 September 2006

CAUSE & EFFECT:

TROUBLESHOOTING HALL EFFECT SENSORS

CAUSE & EFFECT:

TROUBLESHOOTING HALL EFFECT SENSORSBY BERNIE THOMPSON

The predictable effect caused by theintroduction of a magnetic field to acurrent flowing through a conductor has

been harnessed to create a range of Hall Effect sensors. We’ll explain how these

sensors work, to help you identify possible causes when they don’t.

BY BERNIE THOMPSON

The predictable effect caused by theintroduction of a magnetic field to acurrent flowing through a conductor has

been harnessed to create a range of Hall Effect sensors. We’ll explain how these

sensors work, to help you identify possible causes when they don’t.

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53September 2006

The whale oil lamp illumi-nated the area over thekitchen table where Ed-win was working on athin rectangular strip ofgold foil. He could see

his reflection in the strip and his minddrifted for a moment as he thoughtabout how tired he looked. It was verylate, but Edwin was onto somethingnew, something very new. Edwin Hallhad been working on Kelvin’s theory ofelectron flow, which had been present-ed some 30 years earlier in 1849. As heworked, he happened to notice that ifcurrent was flowing through the goldstrip and a magnetic field was placedperpendicular to one side of the strip, adifference in electrical potential was de-tected at the strip’s edges. This discov-ery was credited to Dr. Edwin Hall andit is now referred to as the Hall Effect.

As with many discoveries, Dr. Hall’sbrilliant observation came not by look-ing for it, but by noticing somethingunusual and then acting upon it. TheHall Effect has been known for over100 years, but applications for its useweren’t developed until the last fewdecades. The automotive industry hasapplied this technology to many sys-

tems used in modern vehicles, includ-ing powertrain, body control, tractioncontrol and antilock braking systems.To cover these various systems, theHall Effect sensor is configured in sev-eral different ways—switching, analogand digital. They are proximity sensors;they make no direct contact, but use amagnetic field to activate an electroniccircuit.

The Hall Effect can be produced byusing conductors such as metals andsemiconductors, and the quality of theeffect changes depending on the mater-ial in the conductor. The material willdirectly affect the electrons, or positiveions, flowing through it. The automotiveindustry commonly uses three types ofsemiconductors to make the Hall ele-ment—gallium arsenide (GaAs), indi-um antimonide (InSb) and indium ar-senide (InAs). The most common ofthese semiconductors is indium ar-senide. Just as in Dr. Hall’s experiment,it’s important for the conductor to berectangular and very thin. This allowsroom for the carriers flowing through itto separate and pool at the edges.

Now let’s look at the Hall Effectprinciple (Figs. 1 and 2 above). If cur-rent is flowing through a conductorand a magnetic field (magnetic flux) isallowed to move through the conduc-

tor perpendicular to the current flow,the charged particles drift to the edgesof the rectangular strip. These chargedparticles pool at the surface edges. Themagnetic flux imparts a force on theconductor, which causes the voltage(positive force) to drift to one edgewhile the electrons (negative force)drift to the opposite edge. The forceexerted on the current flow is calledthe Lorentz Force.

While the magnetic force is appliedto the conductor, the carriers stay atopposite sides, setting up a voltagedrop across the conductor. This voltagedifferential is the Hall voltage. It’s pro-portional to the current flowingthrough it, the intensity of the magnet-ic field and the type of conductor ma-terial. If any of these three variableschanges, the voltage differential acrossthe conductor also will change. This iswhy the Hall element must have a reg-ulated voltage applied to the currentpath. If the current is regulated andthe material of the conductor is given,the only thing left to change is themagnetic intensity. As the magnetic in-tensity changes to a 90° angle to thecurrent path, the voltage drop acrossthe conductor also changes. The moreintense the magnetic flux, the largerthe voltage drop across the conductor.

The Hall voltage that’s generated isan analog signal. This Hall signal is verysmall—usually about 30 microvoltswith a 1 gauss magnetic field. Due tothe small voltage generated, the Hallsignal must be amplified if the device isto be used for practical applications.

The type of amplifier that is best suit-ed for use with the Hall element is thedifferential amplifier (Fig. 3 on page56), which amplifies only the potentialdifference between the positive andnegative inputs. If there is no voltagedifference between the positive andnegative inputs to the amplifier, therewill be no output voltage from the am-plifier. However, if there is a voltage dif-ferential, this difference will have a lin-ear amplification. The amount of ampli-fication is given by the differential am-plifier used in the circuit.

The Hall element is connected di-rectly to the differential amplifier, so ac-tivity in the Hall element is mirrored bythe amplifier. When the magnetic fieldis absent in the Hall element, there isno Hall voltage created and no outputvoltage from the amplifier. As a magnet-ic field is applied to the Hall element, aHall voltage is created across the ele-ment. The differential amplifier detectsthis voltage differential and amplifies it.

The way in which a Hall sensor is

54 September 2006


No Magnetic Flux Hall Effect

Magnetic Flux Hall Effect

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Fig. 1 Fig. 2

used determines the circuitry changesneeded to allow the proper output tothe control device. This output may beanalog, such as an acceleration positionsensor or throttle position sensor, ordigital, such as a crankshaft orcamshaft position sensor.

Let’s examine these different Hallsensor configurations. When the Hallelement is to be used for an analogsensor, which could be used for a tem-perature dial on a climate control sys-tem or a throttle position sensor on apowertrain control system, the circuitmust first be modified. The Hall ele-ment is connected to the differentialamplifier and the amplifier is connect-ed to an NPN transistor (Fig. 4). Themagnet is attached to the rotationalshaft. As the shaft turns, the magneticfield intensifies on the Hall element.The Hall voltage created is proportion-al to the intensity of the magnetic field.

If a throttle shaft were to be moni-tored by the PCM, the magnet wouldbe rotated with the throttle shaft. At

idle, the throttle plate would be closed.In this case, the magnetic field intensi-ty would be low and the Hall voltagecreated would be low. The differentialamplifier would have a small potentialdifference and the amplifier outputwould be low. The NPN transistor basewould receive the amplifier’s output.

Because the voltage at the base islow, the NPN transistor amplificationalso is low. In this condition, the TPSoutput voltage would be on the orderof 1 volt. As the engine is put underload, the throttle shaft rotates to openthe throttle plate. As the throttle shaftrotates, the magnetic field intensifieson the Hall element. The Hall voltagecreated increases in proportion to themagnetic field intensity. As the Hallvoltage increases, the differential am-plifier receives its potential difference.The amplifier then amplifies the differ-ence between the negative and posi-tive inputs. This increasing output issent to the base of the NPN transistor,which then amplifies the signal, creat-

ing the throttle position sensor’s out-put. This linear output is proportionalto the rotation of the throttle shaft.

The output from the TPS is sent tothe PCM, where it reports the throttleshaft angle. The PCM’s microprocessorcannot directly read the analog voltagesent from the TPS. This signal must bemodified to a binary format—1s and0s. To accomplish this, a device calledan analog-to-digital converter is used.In most cases an 8-bit A/D converter isused. This device converts a voltagelevel to a series of 1s and 0s that themicroprocessor can decode and use forthe actual throttle shaft angle.

When the Hall element is to beused for a digital signal, such as in acrankshaft or camshaft position sensoror a vehicle speed sensor, the circuitmust first be modified. The Hall ele-ment is connected to the differentialamplifier, which is connected to aSchmitt trigger. In this configuration,the sensor outputs a digital on/off sig-nal. In most automotive circuits, the

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Hall Effect With Amplifier

Fig. 3

Hall sensor is a current sink or appliesa ground to the signal circuit. To ac-complish this, an NPN transistor isconnected to the Schmitt trigger’s out-put (Fig. 5). The magnet is placedacross from the Hall element. A triggerwheel, or target, is positioned so the

shutter can come between the magnet-ic field and the Hall element.

When the shutter is not between themagnet and the Hall element, themagnetic field penetrates the Hall ele-ment, creating a Hall voltage. Thisvoltage is sent to the positive and nega-

tive inputs of the differential amplifier.The amplifier boosts this differentialvoltage and sends it out to the input ofthe Schmitt trigger (a digital triggeringdevice). As the voltage from the differ-ential amplifier increases, it reaches aturn-on threshold, or operate point. Atthis operate point, the Schmitt triggerchanges its state, allowing a voltage sig-nal to be sent out.

The release (turn-off) point is set ata lower voltage than the turn-on point.The purpose of this differential be-tween the turn-on and turn-off points(hysteresis) is to eliminate false trigger-ing that can be caused by minor varia-tions from the differential amplifier.The Schmitt trigger is turned on andthe output voltage is sent to the base ofan NPN transistor. When voltage ispresent at the base of the transistor,the transistor is turned on.

The control unit’s voltage regulatorfeeds voltage to a resistor or load. Theresistor circuit is connected to the col-lector of the NPN transistor, and whenthe NPN is turned on, the currentflows into the collector and out theemitter to ground. In this condition,the signal is pulled to ground. Sincethe resistor is inside the control unit,the voltage is on the ground leg andwill drop very close to ground voltage.

As the trigger wheel rotates, theshutter moves between the magnetand the Hall element. Since the triggerwheel is made of a ferrous material, itpulls the magnetic field to the shutter.At this point the Hall element nolonger has a magnetic field penetratingit and no Hall voltage is created. With-out Hall voltage, the differential ampli-fier has no output to the Schmitt trig-ger. In turn, the Schmitt trigger has novoltage output to the base of the NPNtransistor and the transistor changesstate and shuts off. The ground is thenremoved from the load. This creates anopen circuit. In an open circuit, sourcevoltage is present. If the voltage regu-lator were a 5-volt source, then thevoltage in the open circuit would be 5volts. As the shutter rotates, it movesout from between the magnet and theHall element. The circuit is turned on,which completes the ground leg fromthe load. Thus, the signal voltage drops

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Hall Effect Throttle Position Sensor

Shielded-Field Hall Effect Sensor

Fig. 4

Fig. 5

very close to ground. This cycle is re-peated to create the digital signal fromthe shielded-field Hall Effect sensor.

The gear-tooth Hall Effect sensor(Fig. 6) is another type of digital on/offsensing device. A bias magnet is placedover the Hall element. In this sensor,the magnetic field always penetratesthe Hall element and Hall voltage is al-ways present. As a gear tooth, or tar-get, passes under the Hall element, themagnetic field intensifies in the ele-

ment. As the magnetic field intensifies,the Hall voltage increases. This voltageis sent to a circuit that compares theHall non-tooth voltage output to theHall tooth voltage output.

For this sensor to operate, the targetmust move past the Hall element. Inthe non-tooth position, a capacitor ischarged to store the non-tooth Hallvoltage so it can compare it to thetoothed Hall voltage. As the leadingedge of the tooth approaches the sen-

sor, the Hall voltage increases to a pre-determined operate point. At thispoint, the comparator sends a signal tothe trigger circuit. The trigger applies avoltage signal to the NPN transistorand turns it on. The NPN transistor isconnected to the resistor circuit in thecontrol unit.

One side of the resistor is connectedto the voltage regulator, the other sideto the collector of the NPN transistor.When the transistor changes state andturns on, the signal voltage is pulled toground. As the target rotates and thetrailing edge of the tooth passes theHall sensor, the voltage drops below apredetermined release point and thecomparator releases the voltage to thetrigger circuit and turns off the NPNtransistor. The transistor then changesstate and opens the circuit. Sourcevoltage is now present in the signal cir-cuit. If the regulator is a 5-volt source,the signal voltage is now 5 volts. As thetooth passes beneath the Hall sensor,the circuit becomes activated and pullsthis 5-volt signal to ground. This cyclerepeats itself to create the digital out-put from the gear-tooth Hall Effectsensing device.

To troubleshoot these circuits (seeFigs. 7 and 8), a voltage drop measure-ment should be taken at the power,ground and signal. If the signal is cor-rect at the low and high outputs, thepower and ground will be good as well.If the power source is battery voltage,the voltage regulator is located insidethe Hall sensor. If power is suppliedfrom an electronic module, the voltageregulator is located in that module. Ifthe power source drops from a voltagedrop (resistance) or from a regulatorproblem, the output signal also willdrop. If the power supply increases, theoutput signal also will increase. If theground voltage increases due to a volt-age drop (resistance), the output signalalso will increase.

With an analog Hall sensing device,if there’s a voltage drop or open circuitbetween the Hall sensor device andthe control module, the signal voltagewill be correct at the sensor, but incor-rect at the module. If the voltage iscorrect at the module and the scan toolvoltage is incorrect, then the A/D con-

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Gear-Tooth Hall Effect Sensor

Fig. 6

verter inside the control unit couldhave a problem. Always check thepowers, grounds and signals at the con-trol module before replacing the unit.

An oscilloscope is needed to trou-bleshoot a digital sensor. The followingguidelines will aid your diagnosis:

•With a digital Hall Effect sensingdevice, if the signal at the sensor ishigh, failing intermittently or has failedcompletely, the circuit from the controlmodule is good.

•Different control units use differ-ent signal voltage levels; 5, 8, 9 and 12volts are all common. This signal volt-age level must be within 10% of thetarget voltage or the control unit willnot detect the voltage change of state.

•If the signal is low, failing intermit-tently or completely inoperative, thevoltage regulator or circuit in the con-trol unit could be bad, the signal wirecould be open or grounded or the HallEffect sensor could be bad and pullingthe signal to ground.

•If the sensor’s ground voltage levelis not within 10% of the vehicle’sground voltage, the control unit willnot detect the signal’s change of state.

•If the voltage is stuck high or low,make sure the target is moving.

•When multiple Hall Effect sensorsfail, make sure a target is not hittingone of them.

•When the Hall signal wire is shortedor is intermittently or permanentlyshorted to a power source, it will burnup the electronic circuits inside the Hallsensor and usually pull the signal toground. The Hall sensor is designed toflow 20 milliamps or less. The resistor islocated in the signal circuit so it can lim-it the current flowing through that cir-cuit. If this resistor drops its resistance,the current flow would increase, creat-ing multiple Hall sensor failures.

There are many Hall Effect sensingdevice configurations. All of these de-vices work on the same basic principlescovered here. When you’re working inyour service bay, let your brilliance shinelike Dr. Edwin Hall’s. Pay attention towhat’s unusual and act upon it.

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Fig. 7: The yellow trace in the top graph is the Hall Effect signal at the sensor.This is an indication that the circuit from the PCM is good. The problem is in theHall Effect sensor or its circuits—power or ground. The red trace in the bot-tom graph is another Hall Effect signal, also at the sensor. The signal is failinglow (0 volts). The problem could be located in the PCM, signal wire or sensor.

Fig. 8: The yellow trace in the top graph is the Hall Effect signal at the sensor.The signal is breaking down and failing high (5 volts). This indicates that thecircuit from the PCM is good. The problem is in the Hall Effect sensor or itspower circuit. The red trace in the bottom graph is another Hall Effect signal,also at the sensor. The signal is riding off ground at 3 volts. This problemcould be caused by the Hall Effect sensor or the sensor’s ground.

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