Hall effect sensor

A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications ^[1].

In its simplest form, the sensor operates as an analog transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.

Electricity carried through a conductor will produce a magnetic field that varies with current, and a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be measured.

Frequently, a Hall sensor is combined with circuitry that allows the device to act in a digital (on/off) mode, and may be called a switch in this configuration. Commonly seen in industrial applications such as the pictured pneumatic cylinder, they are also used in consumer equipment; for example some computer printers use them to detect missing paper and open covers. When high reliability is required, they are used in keyboards.

Hall sensors are commonly used to time the speed of wheels and shafts, such as for internal combustion engine ignition timing, tachometers and anti-lock braking systems. They are used in brushless DC electric motors to detect the position of the permanent magnet. In the pictured wheel with two equally spaced magnets, the voltage from the sensor will peak twice for each revolution. This arrangement is commonly used to regulate the speed of disk drives.

Hall probe

A Hall probe contains an indium compound semiconductor crystal such as indium antimonide, mounted on an aluminum backing plate, and encapsulated in the probe head. The plane of the crystal is perpendicular to the probe handle. Connecting leads from the crystal are brought down through the handle to the circuit box.

When the Hall probe is held so that the magnetic field lines are passing at right angles through the sensor of the probe, the meter gives a reading of the value of magnetic flux density (B). A current is passed through the crystal which, when placed in a magnetic field has a "Hall effect" voltage developed across it. The Hall effect is seen when a conductor is passed through a uniform magnetic field. The natural electron drift of the charge carriers causes the magnetic field to apply a Lorentz force (the force exerted on a charged particle in an electromagnetic field) to these charge carriers. The result is what is seen as a charge separation, with a buildup of either positive or negative charges on the bottom or on the top of the plate. The crystal measures 5 mm square. The probe handle, being made of a non-ferrous material, has no disturbing effect on the field.

A Hall probe can be used to measure the Earth's magnetic field. It must be held so that the Earth's field lines are passing directly through it. It is then rotated quickly so the field lines pass through the sensor in the opposite direction. The change in the flux density reading is double the Earth's magnetic flux density. A Hall probe must first be calibrated against a known value of magnetic field strength. For a solenoid the Hall probe is placed in the center.

Working principle

When a beam of charged particles passes through a magnetic field, forces act on the particles and the beam is deflected from its straight line path. The beam of charged particles refers to the electrons flowing through a conductor. When a current carrying conductor is placed in a magnetic field perpendicular to the path of the electrons, the electrons are deflected from its straight line path. Therefore, one side of the conductor becomes negative portion and the other side becomes positive one. The transverse voltage is measured and is known as Hall Voltage ^[2].

The charge separation continues until the force on the charged particles from the electric field balances the force produced by magnetic field. If the current is constant, then the Hall voltage is a measure of the magnetic flux density. There are two forms of Hall Effect Sensors. One is linear where the output voltage linearly varies with the magnetic flux density. The other is known as threshold where there is a sharp drop of output voltage at a particular magnetic flux density.

Materials for Hall effect sensors

The key factor deciding on sensitivity of Hall effect sensors is high electron mobility. As a result following materials are especially suitable for Hall effect sensors:

Signal processing and interface

Hall effect sensors are linear transducers. As a result such sensors require linear circuit for processing of sensor's output signal. Such linear circuit:

provide constant driving current to the sensors
amplify output signal

In some cases linear circuit may cancel the offset voltage of Hall effect sensors. Moreover, AC modulation of driving current may reduce the influence of offset voltage on characteristics of Hall effect sensor.
Recently, Hall effect sensors with linear transducers are commonly integrated with digital electronic. This enables advanced corrections of sensor's characteristics (e.g. temperature coefficient corrections) and digital interfacing to the microprocessor systems. In some solutions of IC Hall effect sensors DSP is implemented. The simplest digital processing circuit is electronic comparator enabling switch on/off operation useful in industrial automation applications.
The Hall effect sensors interfaces may include input diagnostics, fault protection for transient conditions, and short/open circuit detection. It may also provide and monitor the current to the hall effect sensor itself. There are precision IC products available to handle these features.

Advantages

Hall effect sensors may operate as an electronics switch.

Such switch cost less than mechanical switches and is much more reliable
It can be operated up to 100 kHz.
It does not suffer from contact bounce because a sequence of contacts are used rather than a single contact.
It will not be affected by environmental contaminants. Therefore it can be used under severe conditions.

In the case of linear sensor (for the magnetic field strength maesurements) Hall effect sensor:

can measure wide range of magnetic fields
can measure both AC and DC magnetic field
sensor can be flat

Disadvantages

Hall effect sensors provide much lower measuring accuracy than fluxgate magnetometers or magnetoresistance based sensors. Moreover, significant drift of Hall effect sensors have to be compensated.

Applications

Position sensing

Sensing the presence of magnetic object (connected with the position sensing) is the most common industrial application of Hall effect sensors, especially these operating in the switch mode (on/off mode). The Hall effect sensors are also used in the brushless DC motor to sense the position of the rotor and to switch the transistor in the right sequence.

DC current transformers

Hall effect sensors may be utilized for contactless measurements of DC current in current transformers. In such a case Hall effect sensor is mounted in the gap in magntic core arround the current conductor ^[4]. As a result, the DC magnetic flux can be measured, what enables calculation of DC current in the conductor.

Automotive fuel level indicator

The Hall sensor is used in some automotive fuel level Indicators. The main principle of operation of such indicator is position sensing of floating element ^[5].
This can either be done by using a vertical float magnet or a rotating lever sensor.

In a vertical float system a permanent magnet is mounted on the surface of a floating object. The current carrying conductor is fixed on the top of the tank lining up with the magnet. When the level of fuel rises, an increasing magnetic field is applied on the current resulting in higher Hall voltage. As the fuel level decreases, the Hall voltage will also decrease. The fuel level is indicated and displayed by proper signal condition of Hall voltage.
In a rotating lever sensor a diametrically magnetized ring magnet rotates about a linear hall sensor. The sensor only measures the perpendicular (vertical) component of the field. The strength of the field measured correlates directly to the angle of the lever and thus the level of the fuel tank.

References

^ Ed Ramsden (2006). Hall-effect sensors: theory and applications (2, illustrated ed.). Elsevier. ISBN 0-7506-7934-4.
^ R. S. Popović (2004). Hall effect devices (2, illustrated ed.). CRC Press. ISBN 0-7503-0855-9.
^ Petruk, O.; Szewczyk, R.; Ciuk, T.; et al. (2014). "Sensitivity and Offset Voltage Testing in the Hall-Effect Sensors Made of Graphene". Advances in Intelligent Systems and Computing. 267. Springer: 631. doi:10.1007/978-3-319-05353-0_60. {{cite journal}}: Explicit use of et al. in: |first3= (help)
^ Petruk, O.; Szewczyk, R.; Salach, J.; Nowicki, M. (2014). "Digitally Controlled Current Transformer with Hall Sensor". Advances in Intelligent Systems and Computing. 267. Springer: 641. doi:10.1007/978-3-319-05353-0_61.
^ https://www.infineon.com/dgdl/AppNote_Liquid_Level_Sensing_Rev.1.0.pdf?folderId=db3a30431ce5fb52011d4cae1f582dad&fileId=db3a30432313ff5e0123a385f3b2262d