Troubleshooting Hall Effect Sensors

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CAUSECAUSE && EFFECT:EFFECT: TROUBLESHOOTINGTROUBLESHOOTING HALLHALL EFFECTEFFECT SENSORSSENSORS BY BERNIE THOMPSON The predictable effect caused by the introductionintroduction ofof aa magneticmagnetic fieldfield toto aa current 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.

52 September 2006

Illustration: Harold A. Perry; concept Camaro: Weick Media Services T applied thistechnologytomany sys- decades. Theautomotiveindustry has weren’t developed untilthelastfew 100 years,butapplicationsfor itsuse Hall Effecthasbeenknown forover unusual andthenactingupon it.The ing forit,butbynoticingsomething brilliant observationcamenotbylook- it isnowreferredtoastheHallEffect. ery wascreditedtoDr. EdwinHalland tected atthestrip’s edges.Thisdiscov- difference inelectricalpotentialwasde- perpendicular toonesideofthestrip,a strip andamagneticfieldwasplaced current wasflowingthroughthegold worked, hehappenedtonoticethatif ed some30yearsearlierin1849.Ashe electron flow, which hadbeenpresent- had beenworkingonKelvin’s theoryof new, something very new. EdwinHall late, butEdwinwasontosomething about howtiredhelooked.Itwasvery drifted foramomentashethought his reflectioninthestripandmind As withmanydiscoveries,Dr. Hall’s September 2006 gold foil.Hecouldsee thin rectangularstripof win wasworkingona kitchen tablewhereEd- nated theareaover he whaleoillampillumi- 53 TROUBLESHOOTING HALL EFFECT SENSORS

No Magnetic Flux Magnetic Flux Hall Effect Hall Effect Illustrations: Bernie Thompson and Harold A. Perry Bernie Thompson Illustrations: Fig. 1 Fig. 2 tems used in modern vehicles, includ- tor perpendicular to the current flow, The Hall voltage that’s generated is ing powertrain, body control, traction the charged particles drift to the edges an analog signal. This Hall signal is very control and antilock braking systems. of the rectangular strip. These charged small—usually about 30 microvolts To cover these various systems, the particles pool at the surface edges. The with a 1 gauss magnetic field. Due to Hall Effect sensor is configured in sev- magnetic flux imparts a force on the the small voltage generated, the Hall eral different ways—switching, analog conductor, which causes the voltage signal must be amplified if the device is and digital. They are proximity sensors; (positive force) to drift to one edge to be used for practical applications. they make no direct contact, but use a while the electrons (negative force) The type of amplifier that is best suit- magnetic field to activate an electronic drift to the opposite edge. The force ed for use with the Hall element is the circuit. exerted on the current flow is called differential amplifier (Fig. 3 on page The Hall Effect can be produced by the Lorentz Force. 56), which amplifies only the potential using conductors such as metals and While the magnetic force is applied difference between the positive and semiconductors, and the quality of the to the conductor, the carriers stay at negative inputs. If there is no voltage effect changes depending on the mater- opposite sides, setting up a voltage difference between the positive and ial in the conductor. The material will drop across the conductor. This voltage negative inputs to the amplifier, there directly affect the electrons, or positive differential is the Hall voltage. It’s pro- will be no output voltage from the am- ions, flowing through it. The automotive portional to the current flowing plifier. However, if there is a voltage dif- industry commonly uses three types of through it, the intensity of the magnet- ferential, this difference will have a lin- semiconductors to make the Hall ele- ic field and the type of conductor ma- ear amplification. The amount of ampli- ment—gallium arsenide (GaAs), indi- terial. If any of these three variables fication is given by the differential am- um antimonide (InSb) and indium ar- changes, the voltage differential across plifier used in the circuit. senide (InAs). The most common of the conductor also will change. This is The Hall element is connected di- these semiconductors is indium ar- why the Hall element must have a reg- rectly to the differential amplifier, so ac- senide. Just as in Dr. Hall’s experiment, ulated voltage applied to the current tivity in the Hall element is mirrored by it’s important for the conductor to be path. If the current is regulated and the amplifier. When the magnetic field rectangular and very thin. This allows the material of the conductor is given, is absent in the Hall element, there is room for the carriers flowing through it the only thing left to change is the no Hall voltage created and no output to separate and pool at the edges. magnetic intensity. As the magnetic in- voltage from the amplifier. As a magnet- Now let’s look at the Hall Effect tensity changes to a 90° angle to the ic field is applied to the Hall element, a principle (Figs. 1 and 2 above). If cur- current path, the voltage drop across Hall voltage is created across the ele- rent is flowing through a conductor the conductor also changes. The more ment. The differential amplifier detects and a magnetic field (magnetic flux) is intense the magnetic flux, the larger this voltage differential and amplifies it. allowed to move through the conduc- the voltage drop across the conductor. The way in which a Hall sensor is

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Hall Effect With Amplifier

Fig. 3 used determines the circuitry changes idle, the throttle plate would be closed. ing the throttle position sensor’s out- needed to allow the proper output to In this case, the magnetic field intensi- put. This linear output is proportional the control device. This output may be ty would be low and the Hall voltage to the rotation of the throttle shaft. analog, such as an acceleration position created would be low. The differential The output from the TPS is sent to sensor or throttle position sensor, or amplifier would have a small potential the PCM, where it reports the throttle digital, such as a crankshaft or difference and the amplifier output shaft angle. The PCM’s microprocessor camshaft position sensor. would be low. The NPN transistor base cannot directly read the analog voltage Let’s examine these different Hall would receive the amplifier’s output. sent from the TPS. This signal must be sensor configurations. When the Hall Because the voltage at the base is modified to a binary format—1s and element is to be used for an analog low, the NPN transistor amplification 0s. To accomplish this, a device called sensor, which could be used for a tem- also is low. In this condition, the TPS an analog-to-digital converter is used. perature dial on a climate control sys- output voltage would be on the order In most cases an 8-bit A/D converter is tem or a throttle position sensor on a of 1 volt. As the engine is put under used. This device converts a voltage powertrain control system, the circuit load, the throttle shaft rotates to open level to a series of 1s and 0s that the must first be modified. The Hall ele- the throttle plate. As the throttle shaft microprocessor can decode and use for ment is connected to the differential rotates, the magnetic field intensifies the actual throttle shaft angle. amplifier and the amplifier is connect- on the Hall element. The Hall voltage When the Hall element is to be ed to an NPN transistor (Fig. 4). The created increases in proportion to the used for a digital signal, such as in a magnet is attached to the rotational magnetic field intensity. As the Hall crankshaft or camshaft position sensor shaft. As the shaft turns, the magnetic voltage increases, the differential am- or a vehicle speed sensor, the circuit field intensifies on the Hall element. plifier receives its potential difference. must first be modified. The Hall ele- The Hall voltage created is proportion- The amplifier then amplifies the differ- ment is connected to the differential al to the intensity of the magnetic field. ence between the negative and posi- amplifier, which is connected to a If a throttle shaft were to be moni- tive inputs. This increasing output is Schmitt trigger. In this configuration, tored by the PCM, the magnet would sent to the base of the NPN transistor, the sensor outputs a digital on/off sig- be rotated with the throttle shaft. At which then amplifies the signal, creat- nal. In most automotive circuits, the

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Hall sensor is a current sink or applies shutter can come between the magnet- tive inputs of the differential amplifier. a ground to the signal circuit. To ac- ic field and the Hall element. The amplifier boosts this differential complish this, an NPN transistor is When the shutter is not between the voltage and sends it out to the input of connected to the Schmitt trigger’s out- magnet and the Hall element, the the Schmitt trigger (a digital triggering put (Fig. 5). The magnet is placed magnetic field penetrates the Hall ele- device). As the voltage from the differ- across from the Hall element. A trigger ment, creating a Hall voltage. This ential amplifier increases, it reaches a wheel, or target, is positioned so the voltage is sent to the positive and nega- turn-on threshold, or operate point. At this operate point, the Schmitt trigger changes its state, allowing a voltage sig- Hall Effect Throttle Position Sensor nal to be sent out. The release (turn-off) point is set at a lower voltage than the turn-on point. The purpose of this differential between the turn-on and turn-off points (hysteresis) is to eliminate false triggering that can be caused by minor varia- tions from the differential amplifier. The Schmitt trigger is turned on and the output voltage is sent to the base of an NPN transistor. When voltage is present at the base of the transistor, the transistor is turned on. The control unit’s voltage regulator feeds voltage to a resistor or load. The resistor circuit is connected to the collector of the NPN transistor, and when the NPN is turned on, the current flows into the collector and out the emitter to ground. In this condition, the signal is pulled to ground. Since the resistor is inside the control unit, Fig. 4 the voltage is on the ground leg and will drop very close to ground voltage. As the trigger wheel rotates, the Shielded-Field Hall Effect Sensor shutter moves between the magnet and the Hall element. Since the trigger wheel is made of a ferrous material, it pulls the magnetic field to the shutter. At this point the Hall element no longer has a magnetic field penetrating it and no Hall voltage is created. With- out Hall voltage, the differential amplifier has no output to the Schmitt trigger. In turn, the Schmitt trigger has no voltage output to the base of the NPN transistor and the transistor changes state and shuts off. The ground is then removed from the load. This creates an open circuit. In an open circuit, source voltage is present. If the voltage regulator were a 5-volt source, then the voltage in the open circuit would be 5 volts. As the shutter rotates, it moves out from between the magnet and the Hall element. The circuit is turned on, which completes the ground leg from Fig. 5 the load. Thus, the signal voltage drops

58 September 2006 TROUBLESHOOTING HALL EFFECT SENSORS very close to ground. This cycle is re- ment. As the magnetic field intensifies, sor, the Hall voltage increases to a pre- peated to create the digital signal from the Hall voltage increases. This voltage determined operate point. At this the shielded-field Hall Effect sensor. is sent to a circuit that compares the point, the comparator sends a signal to The gear-tooth Hall Effect sensor Hall non-tooth voltage output to the the trigger circuit. The trigger applies a (Fig. 6) is another type of digital on/off Hall tooth voltage output. voltage signal to the NPN transistor sensing device. A bias magnet is placed For this sensor to operate, the target and turns it on. The NPN transistor is over the Hall element. In this sensor, must move past the Hall element. In connected to the resistor circuit in the the magnetic field always penetrates the non-tooth position, a capacitor is control unit. the Hall element and Hall voltage is al- charged to store the non-tooth Hall One side of the resistor is connected ways present. As a gear tooth, or tar- voltage so it can compare it to the to the voltage regulator, the other side get, passes under the Hall element, the toothed Hall voltage. As the leading to the collector of the NPN transistor. magnetic field intensifies in the ele- edge of the tooth approaches the sen- When the transistor changes state and turns on, the signal voltage is pulled to ground. As the target rotates and the Gear-Tooth Hall Effect Sensor trailing edge of the tooth passes the Hall sensor, the voltage drops below a predetermined release point and the comparator releases the voltage to the trigger circuit and turns off the NPN transistor. The transistor then changes state and opens the circuit. Source voltage is now present in the signal circuit. If the regulator is a 5-volt source, the signal voltage is now 5 volts. As the tooth passes beneath the Hall sensor, the circuit becomes activated and pulls this 5-volt signal to ground. This cycle repeats itself to create the digital output from the gear-tooth Hall Effect sensing device. To troubleshoot these circuits (see Figs. 7 and 8), a voltage drop measure- ment should be taken at the power, ground and signal. If the signal is correct at the low and high outputs, the power and ground will be good as well. If the power source is battery voltage, the voltage regulator is located inside the Hall sensor. If power is supplied from an electronic module, the voltage regulator is located in that module. If the power source drops from a voltage drop (resistance) or from a regulator problem, the output signal also will drop. If the power supply increases, the output signal also will increase. If the ground voltage increases due to a voltage drop (resistance), the output signal also will increase. With an analog Hall sensing device, if there’s a voltage drop or open circuit between the Hall sensor device and the control module, the signal voltage will be correct at the sensor, but incorrect at the module. If the voltage is correct at the module and the scan tool Fig. 6 voltage is incorrect, then the A/D con-

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verter inside the control unit could have a problem. Always check the powers, grounds and signals at the control module before replacing the unit. An oscilloscope is needed to troubleshoot a digital sensor. The following guidelines will aid your diagnosis: •With a digital Hall Effect sensing device, if the signal at the sensor is high, failing intermittently or has failed completely, the circuit from the control module is good. •Different control units use different signal voltage levels; 5, 8, 9 and 12 volts are all common. This signal voltage level must be within 10% of the target voltage or the control unit will not detect the voltage change of state. •If the signal is low, failing intermittently or completely inoperative, the voltage regulator or circuit in the control unit could be bad, the signal wire could be open or grounded or the Hall Screen captures: Bernie Thompson captures: Screen Fig. 7: The yellow trace in the top graph is the Hall Effect signal at the sensor. Effect sensor could be bad and pulling This is an indication that the circuit from the PCM is good. The problem is in the the signal to ground. Hall Effect sensor or its circuits—power or ground. The red trace in the bot- •If the sensor’s ground voltage level tom graph is another Hall Effect signal, also at the sensor. The signal is failing is not within 10% of the vehicle’s low (0 volts). The problem could be located in the PCM, signal wire or sensor. ground voltage, the control unit will not 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 sensors fail, make sure a target is not hitting one of them. •When the Hall signal wire is shorted or is intermittently or permanently shorted to a power source, it will burn up the electronic circuits inside the Hall sensor and usually pull the signal to ground. The Hall sensor is designed to flow 20 milliamps or less. The resistor is located in the signal circuit so it can lim- it the current flowing through that circuit. If this resistor drops its resistance, the current flow would increase, creating multiple Hall sensor failures. There are many Hall Effect sensing device configurations. All of these de- vices work on the same basic principles covered here. When you’re working in your service bay, let your brilliance shine Fig. 8: The yellow trace in the top graph is the Hall Effect signal at the sensor. like Dr. Edwin Hall’s. Pay attention to The signal is breaking down and failing high (5 volts). This indicates that the what’s unusual and act upon it. circuit from the PCM is good. The problem is in the Hall Effect sensor or its power circuit. The red trace in the bottom graph is another Hall Effect signal, Visit www.motor.com to download also at the sensor. The signal is riding off ground at 3 volts. This problem a free copy of this article. could be caused by the Hall Effect sensor or the sensor’s ground.

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