Hall effect sensor

A Hall effect sensor is a device that is used to measure the magnitude of a magnetic field. Its output voltage is directly proportional to the magnetic field strength through it.

A wheel containing two magnets passing by a Hall effect sensor
The magnetic piston (1) in this pneumatic cylinder will cause the Hall effect sensors (2 and 3) mounted on its outer wall to activate when it is fully retracted or extended.
Engine fan with Hall effect sensor
Commonly used circuit symbol

Hall effect sensors are used for proximity sensing, positioning, speed detection, and current sensing applications.[1]

Frequently, a Hall sensor is combined with threshold detection so that it acts as and is called a switch. 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. They can also be used in computer keyboards, an application that requires ultra-high reliability. Another use of a Hall Sensor is in the creation of MIDI organ pedal-boards, where the movement of a 'key' on the pedal-board is translated as an on/off switch via Hall Sensors.

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 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 should be calibrated against a known value of magnetic field strength. For a solenoid the Hall probe is placed in the centre.

Working principle

In a Hall effect sensor, a thin strip of metal has a current applied along it. In the presence of a magnetic field, the electrons in the metal strip are deflected toward one edge, producing a voltage gradient across the short side of the strip (perpendicular to the feed current). Hall effect sensors have an advantage over inductive sensors in that, while inductive sensors respond to a changing magnetic field which induces current in a coil of wire and produces voltage at its output, Hall effect sensors can detect static (non-changing) magnetic fields.

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.

When a beam of charged particles passes through a magnetic field, forces act on the particles and the beam is deflected from a straight path. The flow of electrons through a conductor form a beam of charged carriers. When an conductor is placed in a magnetic field perpendicular to the direction of the electrons, they will be deflected from a straight path. As a consequence, one plane of the conductor will become negatively charged and the opposite side will become positively charged. The voltage between these planes is called the Hall voltage.[2]

When the force on the charged particles from the electric field balances the force produced by the magnetic field, the separation of them will stop. If the current is not changing, then the Hall voltage is a measure of the magnetic flux density. Basically, there are two kinds of Hall effect sensors: linear, which means the output of voltage linearly depends on magnetic flux density; and threshold, which means there will be a sharp decrease of output voltage at each magnetic flux density. This experiment was the one to demonstrate that there are only negative charges free to move in a conductor. Before this, it was believed that positive charges move in a current carrying conductor. This experiment is known as the Hall Experiment.

Materials

The key factor determining sensitivity of Hall effect sensors is high electron mobility. As a result, the 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 a linear circuit for processing of the sensor's output signal. Such a linear circuit:

  • provides a constant driving current to the sensors
  • amplifies the output signal

In some cases, the linear circuit may cancel the offset voltage of Hall effect sensors. Moreover, AC modulation of the driving current may also reduce the influence of this offset voltage.

Hall effect sensors with linear transducers are commonly integrated with digital electronics.[4] This enables advanced corrections to the sensor's characteristics (e.g. temperature coefficient corrections) and digital interfacing to microprocessor systems. In some solutions of IC Hall effect sensors a DSP is used, which provides for more choices among processing techniques.[1]:167

The Hall effect sensor 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

A Hall effect sensor may operate as an electronic switch.

  • Such a switch costs less than a mechanical switch and is much more reliable.
  • It can be operated at higher frequencies than a mechanical switch.
  • It does not suffer from contact bounce because a solid state switch with hysteresis is used rather than a mechanical contact.
  • It will not be affected by environmental contaminants since the sensor is in a sealed package. Therefore, it can be used under severe conditions.

In the case of linear sensor (for the magnetic field strength measurements), a Hall effect sensor:

  • can measure a wide range of magnetic fields
  • is available that can measure either North or South pole magnetic fields
  • can be flat

Disadvantages

Hall effect sensors provide much lower measuring accuracy than fluxgate magnetometers or magnetoresistance-based sensors. Moreover, Hall effect sensors drift significantly, requiring compensation.

Applications

Position sensing

Sensing the presence of magnetic objects (connected with the position sensing) is the most common industrial application of Hall effect sensors, especially those 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 transistors in the right sequence.

Smartphones use hall sensors to determine if the Flip Cover accessory is closed.[5] See Galaxy S4 Accessories.

Direct current (DC) transformers

Hall effect sensors may be utilized for contactless measurements of DC current in current transformers. In such a case the Hall effect sensor is mounted in the gap in magnetic core around the current conductor.[6] As a result, the DC magnetic flux can be measured, and the DC current in the conductor can be calculated.

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 a floating element.[7] 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.

Keyboard switch

Hall Effect switches for computer keyboards were developed in the late 1960s by Everett A. Vorthmann and Joseph T. Maupin at Honeywell.[8] Due to high manufacturing costs these keyboards were often reserved for high-reliability applications such as aerospace and military. As mass-production costs have reduced increasing number of consumer models have become available.[9] Various mechanical keyboards are noted to use Hall Effect switches.

Electric Treadmill

In electric treadmills, hall sensors may be used as speed sensors and in an emergency stop pull cord. The pull cord is attached to a magnet on one side and to the waistband of the user on the other side. The circuit remains closed through this magnet. If the user falls, the magnet is pulled off the contact and an interruption of power supply occurs which initiates an emergency stop.

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References
  1. Ramsden, Edward (2006). Hall-effect sensors: theory and applications (2, illustrated ed.). Elsevier. ISBN 978-0-7506-7934-3.
  2. Popović, R. S. (2004). Hall effect devices (2, illustrated ed.). CRC Press. ISBN 978-0-7503-0855-7.
  3. Petruk, Oleg; Szewczyk, Roman; Ciuk, Tymoteusz; et al. (2014). Sensitivity and Offset Voltage Testing in the Hall-Effect Sensors Made of Graphene. Advances in Intelligent Systems and Computing. 267. Springer. p. 631. doi:10.1007/978-3-319-05353-0_60. ISBN 978-3-319-05352-3.
  4. "Hall Effect Sensor Voltage Regulation and Power Management". phareselectronics.com. Retrieved 26 May 2015.
  5. "ZenFone 5 (A500CG)". asus.com. Retrieved 2 September 2017.
  6. Petruk, O.; Szewczyk, R.; Salach, J.; Nowicki, M. (2014). Digitally Controlled Current Transformer with Hall Sensor. Advances in Intelligent Systems and Computing. 267. Springer. p. 641. doi:10.1007/978-3-319-05353-0_61. ISBN 978-3-319-05352-3.
  7. "Liquid Level Sensing: Measuring Liquid Levels Using Hall Effect Sensors" (PDF). infineon.com. 12 February 2009. Retrieved 2 September 2017.
  8. Vorthmann, Everett A.; Maupin, Joseph T. (May 1969). "Solid state keyboard". AFIPS Spring Joint Computing Conference 1969: 149–159. doi:10.1145/1476793.1476823.
  9. Limmen, Calder (May 10, 2019). "What are Hall Effect keyboard Switches". Wooting Developer Blog. Retrieved July 11, 2019.

Further reading
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