Hall effect sensor

A Hall effect sensor is any sensor incorporating one or more Hall elements, each of which produces a voltage proportional to one axial component of the magnetic field vector using the Hall effect.
Hall sensors are used for proximity sensing, positioning, speed detection, and current sensing applications and are common in industrial and consumer applications. Hundreds of millions of Hall sensor integrated circuits are sold each year by about 50 manufacturers, with the global market being valued at around a billion dollars.

Principles

In a Hall sensor, a fixed DC bias current is applied along one axis across a thin strip of metal called the Hall element transducer. Sensing electrodes on opposite sides of the Hall element along another axis measure the difference in electric potential across the axis of the electrodes. The current's charge carriers are deflected by the Lorentz force in the presence of a magnetic field perpendicular to their flow. The sensing electrodes measure the potential difference proportional to the axial component of the magnetic field that is perpendicular to both the current's axis and the sensing electrodes' axis.
Hall effect sensors respond both to static magnetic fields and to changing ones.

Amplification

Hall effect devices produce a very low signal level and thus require amplification. The vacuum tube amplifier technology available in the first half of the 20th century was too large, expensive, and power-consuming for everyday Hall effect sensor applications, which were limited to laboratory instruments. Even early generation transistor technology was unsuited; it was only with the development of the low-cost silicon chip-based integrated circuit micro-technology that the Hall effect sensor became suitable for mass application. Devices sold as Hall sensors nowadays contain both the sensor as described above and a high gain IC amplifier in a single package. These Hall sensor ICs may add a stable voltage regulator in addition to the amplifier to allow operation over a wide range of supply voltage and boost the Hall voltage for a convenient analog signal output proportional to the magnetic field component. In some cases, the linear circuit may cancel the offset voltage of Hall sensors. Moreover, AC modulation of the driving current may also reduce the influence of this offset voltage.
Hall sensors are called linear if their output is proportional to the incident magnetic field strength. This output signal can be an analog voltage, a pulse-width modulation signal, or be communicated digitally over a modern bus protocol. Hall sensors may also be ratiometric if their sensitivity is also proportional to their supply voltage. With no magnetic field applied, their quiescent output voltage is typically half of the supply voltage. They may have rail-to-rail output.

Hall switch

While the Hall element is an analog device, Hall switch ICs often additionally incorporate threshold detection circuitry to form an electronic switch which has two states that output a binary digital signal.
Their outputs may be open collector NPN transistors for compatibility with ICs that use different supply voltages. Rather than a voltage being produced at the Hall sensor signal output wire, an output transistor is turned on, providing a circuit to ground through the signal output wire.

Hysteresis

filtering may be applied to provide a clean digital output that is robust against sensor noise. The hysteresis thresholds for switching categorize digital Hall ICs as either unipolar switches, omnipolar switches, or bipolar switches, which may sometimes be called latches. Unipolar refers to having switching thresholds in only one polarity of the magnetic field. Omnipolar switches have two sets of switching thresholds, for both positive and negative polarities, and so operate alternatively with a strong positive or a strong negative magnetic field.
Bipolar switches have a positive B and a negative B. The difference between B and B tends to be greater for bipolar switches described as latches, which remain in one state much longer and require a greater field strength to change states than bipolar switches require. The naming distinction between "bipolar" and "latch" may be a little arbitrary, for instance, the datasheet for the Honeywell SS41F describes it as "bipolar", while another manufacturer describes their SS41F with comparable specifications as a "latch".

Characteristics

Directionality

Hall elements measure only the sensing axis component of the magnetic field vector. Because that axial component may be positive or negative, some Hall sensors can sense the binary direction of the axial component in addition to its magnitude. An additional perpendicularly-oriented Hall element must be incorporated to determine a 2-D direction, and another perpendicularly-oriented Hall element must be added to detect the full 3-D components of the magnetic field vector.

Solid state

Because Hall sensor ICs are solid-state devices, they are not prone to mechanical wear. Thus, they can operate at much higher speeds than mechanical sensors, and their lifespan is not limited by mechanical failure. However, Hall sensors can be prone to thermal drift due to changes in environmental conditions and to time drift over the lifetime of the sensor.
Hall effect devices are immune to dust, dirt, mud, and water. These characteristics make Hall effect devices better for position sensing than alternative means such as optical and electromechanical sensing.

Bandwidth

The bandwidth of practical Hall sensors is limited to the hundreds of kilohertz, with commercial silicon ones commonly limited to 10–100 kHz., the fastest Hall sensor available in the market has a bandwidth of 1 MHz but uses non-standard semiconductors.

Susceptibility to external fields

Magnetic flux from the surroundings may diminish or enhance the field the Hall probe intends to detect, rendering the results inaccurate. Hall sensors can detect stray magnetic fields easily, including that of Earth, so they work well as electronic compasses: but this also means that such stray fields can hinder accurate measurements of small magnetic fields. To solve this problem, Hall sensors are often integrated with magnetic shielding of some kind.
Mechanical positions within an electromagnetic system can instead be measured without the Hall effect using optical position encoders and induced voltage by moving the amount of metalcore inserted into a transformer. When Hall is compared to photo-sensitive methods, it is harder to get an absolute position with Hall.

Differential Hall sensors

While a single Hall element is susceptible to external magnetic fields, a differential configuration of two Hall elements can cancel stray fields out from measurements, analogous to how common mode voltage signals are canceled using differential signaling.

Materials

The following materials are especially suitable for Hall effect sensors:

Hall effect sensors may be used in various sensors such as rotating speed sensors, fluid flow sensors, current sensors, and pressure 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.
Common applications are often found where a robust and contactless alternative to a mechanical switch or potentiometer is required. These include: electric airsoft guns, triggers of electropneumatic paintball guns, go-kart speed controls, smartphones, and some global positioning systems.

Position sensing

One of the most common industrial applications of Hall sensors used as binary switches is in position sensing.
Hall effect sensors are used to detect whether a smartphone's cover is closed.
Some computer printers use Hall sensors to detect missing paper and open covers and some 3D printers use them to measure filament thickness.
Hall sensors are used in some automotive fuel-level indicators by detecting the position of a floating element in the fuel tank.
Hall sensors affixed to mechanical gauges that have magnetized indicator needles can translate the physical position or orientation of the mechanical indicator needle into an electrical signal that can be used by electronic indicators, controls, or communications devices.

Magnetometers

use a "Hall probe", with a Hall element, to measure magnetic fields or inspect materials using the principles of magnetic flux leakage. A Hall probe is a device that uses a calibrated Hall-effect sensor to directly measure the strength of a magnetic field. Since magnetic fields have a direction as well as a magnitude, the results from a Hall probe are dependent on the orientation, as well as the position, of the probe.

Ammeters

Hall sensors may be utilized for contactless measurements of direct current in current transformers. In such a case the Hall sensor is mounted in a gap in the magnetic core around the current conductor. As a result, the DC magnetic flux can be measured, and the DC in the conductor can be calculated.
When electrons flow through a conductor, a magnetic field is produced. Thus, it is possible to create a non-contacting current sensor or ammeters. The device has three terminals. A sensor voltage is applied across two terminals and the third provides a voltage proportional to the current being sensed. This has several advantages; no additional resistance needs to be inserted in the primary circuit. Also, the voltage present on the line to be sensed is not transmitted to the sensor, which enhances the safety of measuring equipment.

Improving signal-to-noise

Integrating a Hall sensor into a ferrite ring concentrates the flux density of the current's magnetic field along the ferrite ring and through the sensor, which greatly reduces the relative influence of stray fields by a factor of 100 or better. This configuration also provides an improvement in signal-to-noise ratio and drift effects of over 20 times that of a bare Hall device.
The range of a given feedthrough sensor may also be extended upward and downward by appropriate wiring. To extend the range to lower currents, multiple turns of the current-carrying wire may be made through the opening, each turn adding to the sensor output the same quantity; when the sensor is installed onto a printed circuit board, the turns can be carried out by a staple on the board. To extend the range to higher currents, a current divider may be used. The divider splits the current across two wires of differing widths and the thinner wire, carrying a smaller proportion of the total current, passes through the sensor.