September 2006 Articles & Features
By Bernie Thompson | September 2006
The effect caused by the introduction of a magnetic field to a 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.
The whale oil lamp illuminated the area over the kitchen table where Edwin was working on a thin rectangular strip of gold foil. He could see his reflection in the strip and his mind drifted for a moment as he thought about how tired he looked. It was very late, but Edwin was onto something new, something very new. Edwin Hall had been working on Kelvin's theory of electron flow, which had been presented some 30 years earlier in 1849. As he worked, he happened to notice that if current was flowing through the gold strip and a magnetic field was placed perpendicular to one side of the strip, a difference in electrical potential was detected at the strip's edges. This discovery was credited to Dr. Edwin Hall and it is now referred to as the Hall Effect.
As with many discoveries, Dr. Hall's brilliant observation came not by looking for it, but by noticing something unusual and then acting upon it. The Hall Effect has been known for over 100 years, but applications for its use weren't developed until the last few decades. The automotive industry has applied this technology to many systems used in modern vehicles, including powertrain, body control, traction control and antilock braking systems. To cover these various systems, the Hall Effect sensor is configured in several different ways� switching, analog and digital. They are proximity sensors; they make no direct contact, but use a magnetic field to activate an electronic circuit.
The Hall Effect can be produced by using conductors such as metals and semiconductors, and the quality of the effect changes depending on the material in the conductor. The material will directly affect the electrons, or positive ions, flowing through it. The automotive industry commonly uses three types of semiconductors to make the Hall element�gallium arsenide (GaAs), indium antimonide (InSb) and indium arsenide (InAs). The most common of these semiconductors is indium arsenide. Just as in Dr. Hall's experiment, it's important for the conductor to be rectangular and very thin. This allows room for the carriers flowing through it to separate and pool at the edges.
Now let's look at the Hall Effect principle (Figs. 1 and 2 above). If current is flowing through a conductor and a magnetic field (magnetic flux) is allowed to move through the conductor perpendicular to the current flow, the charged particles drift to the edges of the rectangular strip. These charged particles pool at the surface edges. The magnetic flux imparts a force on the conductor, which causes the voltage (positive force) to drift to one edge while the electrons (negative force) drift to the opposite edge. The force exerted on the current flow is called the Lorentz Force.
While the magnetic force is applied to the conductor, the carriers stay at opposite sides, setting up a voltage drop across the conductor. This voltage differential is the Hall voltage. It's proportional to the current flowing through it, the intensity of the magnetic field and the type of conductor material. If any of these three variables changes, the voltage differential across the conductor also will change. This is why the Hall element must have a regulated voltage applied to the current path. If the current is regulated and the material of the conductor is given, the only thing left to change is the magnetic intensity. As the magnetic intensity changes to a 90° angle to the current path, the voltage drop across the conductor also changes. The more intense the magnetic flux, the larger the voltage drop across the conductor.
The Hall voltage that's generated is an analog signal . This Hall signal is very small�usually about 30 microvolts with a 1 gauss magnetic field. Due to the small voltage generated, the Hall signal must be amplified if the device is to be used for practical applications.
The type of amplifier that is best suited for use with the Hall element is the differential amplifier (Fig. 3 on page 56), which amplifies only the potential difference between the positive and negative inputs. If there is no voltage difference between the positive and negative inputs to the amplifier, there will be no output voltage from the amplifier. However, if there is a voltage differential, this difference will have a linear amplification. The amount of amplification is given by the differential amplifier used in the circuit.
The Hall element is connected directly to the differential amplifier, so activity in the Hall element is mirrored by the amplifier. When the magnetic field is absent in the Hall element, there is no Hall voltage created and no output voltage from the amplifier. As a magnetic field is applied to the Hall element, a Hall voltage is created across the element. The differential amplifier detects this voltage differential and amplifies it.
The way in which a Hall sensor is used determines the circuitry changes needed to allow the proper output to the control device. This output may be analog, such as an acceleration position sensor or throttle position sensor, or digital, such as a crankshaft or camshaft position sensor.
Let's examine these different Hall sensor configurations. When the Hall element is to be used for an analog sensor, which could be used for a temperature dial on a climate control system or a throttle position sensor on a powertrain control system, the circuit must first be modified. The Hall element is connected to the differential amplifier and the amplifier is connected to an NPN transistor (Fig. 4). The magnet is attached to the rotational shaft. As the shaft turns, the magnetic field intensifies on the Hall element. The Hall voltage created is proportional to the intensity of the magnetic field.
If a throttle shaft were to be monitored by the PCM, the magnet would be rotated with the throttle shaft. At idle, the throttle plate would be closed. In this case, the magnetic field intensity would be low and the Hall voltage created would be low. The differential amplifier would have a small potential difference and the amplifier output would be low. The NPN transistor base would receive the amplifier's output.
Because the voltage at the base is low, the NPN transistor amplification also is low. In this condition, the TPS output voltage would be on the order of 1 volt. As the engine is put under load, the throttle shaft rotates to open the throttle plate. As the throttle shaft rotates, the magnetic field intensifies on the Hall element. The Hall voltage created increases in proportion to the magnetic field intensity. As the Hall voltage increases, the differential amplifier receives its potential difference. The amplifier then amplifies the difference between the negative and positive inputs. This increasing output is sent to the base of the NPN transistor, which then amplifies the signal, creating the throttle position sensor's output. This linear output is proportional to the rotation of the throttle shaft.
The output from the TPS is sent to the PCM, where it reports the throttle shaft angle. The PCM's microprocessor cannot directly read the analog voltage sent from the TPS. This signal must be modified to a binary format-1s and 0s. To accomplish this, a device called an analog-to-digital converter is used. In most cases an 8-bit A/D converter is used. This device converts a voltage level to a series of 1s and 0s that the microprocessor can decode and use for the actual throttle shaft angle.
When the Hall element is to be used for a digital signal, such as in a crankshaft or camshaft position sensor or a vehicle speed sensor, the circuit must first be modified. The Hall element is connected to the differential amplifier, which is connected to a Schmitt trigger. In this configuration, the sensor outputs a digital on/off signal. In most automotive circuits, the Hall sensor is a current sink or applies a ground to the signal circuit. To accomplish this, an NPN transistor is connected to the Schmitt trigger's output (Fig. 5). The magnet is placed across from the Hall element. A trigger wheel, or target, is positioned so the shutter can come between the magnetic field and the Hall element.
When the shutter is not between the magnet and the Hall element, the magnetic field penetrates the Hall element, creating a Hall voltage. This voltage is sent to the positive and negative inputs of the differential amplifier. The amplifier boosts this differential voltage and sends it out to the input of the Schmitt trigger (a digital triggering device). As the voltage from the differential amplifier increases, it reaches a turn-on threshold, or operate point. At this operate point, the Schmitt trigger changes its state, allowing a voltage signal 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 variations 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, the voltage is on the ground leg and will drop very close to ground voltage.
As the trigger wheel rotates, the 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. Without 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 the load. Thus, the signal voltage drops very close to ground. This cycle is repeated to create the digital signal from the shielded-field Hall Effect sensor.
The gear-tooth Hall Effect sensor (Fig. 6) is another type of digital on/off sensing device. A bias magnet is placed over the Hall element. In this sensor, the magnetic field always penetrates the Hall element and Hall voltage is always present. As a gear tooth, or target, passes under the Hall element, the magnetic field intensifies in the element. As the magnetic field intensifies, the Hall voltage increases. This voltage is sent to a circuit that compares the Hall non-tooth voltage output to the Hall tooth voltage output.
For this sensor to operate, the target must move past the Hall element. In the non-tooth position, a capacitor is charged to store the non-tooth Hall voltage so it can compare it to the toothed Hall voltage. As the leading edge of the tooth approaches the sensor, the Hall voltage increases to a predetermined operate point. At this point, the comparator sends a signal to the trigger circuit. The trigger applies a voltage signal to the NPN transistor and turns it on. The NPN transistor is connected to the resistor circuit in the control unit.
One side of the resistor is connected to the voltage regulator, the other side to the collector of the NPN transistor. When the transistor changes state and turns on, the signal voltage is pulled to ground. As the target rotates and the 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 measurement 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 voltage is incorrect, then the A/D converter 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 Effect sensor could be bad and pulling the signal to ground.
• If the sensor's ground voltage level is not within 10% of the vehicle's 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 limit 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 devices work on the same basic principles covered here. When you're working in your service bay, let your brilliance shine like Dr. Edwin Hall's. Pay attention to what's unusual and act upon it.