A Hall-effect sensor (also known as a Hall sensor or Hall probe) is any sensor based on the Hall effect (named after physicist Edwin Hall), in which a voltage is produced proportional to one axial component of the magnetic field vector .
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 (ICs) 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 (voltage) 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 (the Hall voltage) 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. (Inductive sensors, in contrast, only respond to changes in fields.)
The Hall voltage is also temperature-dependent and can have an offset. Due to the proportionality of the Hall voltage to the charge carrier mobility and the concentration of the majority charge carriers, the Hall effect is an established method for determining these parameters in semiconductor technology.)
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 (IC) 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 (PWM) 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 (on and off) that output a binary digital signal.
Their outputs may be open collector NPN transistors (or open drain n-type MOSFETs) 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 Schmitt trigger filtering may be applied (or integrated into the IC) to provide a clean digital output that is robust against sensor noise. The hysteresis thresholds for switching (specified as B and B) categorize digital Hall ICs as either unipolar switches, omnipolar switches, or bipolar switches, which may sometimes be called latches. Unipolar (e.g., A3144) 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 (and thus require both positive and negative magnetic fields to operate). The difference between B and B tends to be greater for bipolar switches described as latches, which remain in one state much longer (i.e. they latch onto their last value) 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 (e.g. in ) 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 (unlike potentiometers, electromechanical reed switches, relays, or other mechanical switches and sensors). 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 (when appropriately packaged) 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 (such as other wires) 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 (e.g., absolute and incremental 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: Gallium arsenide (GaAs) Indium arsenide (InAs) Indium phosphide (InP) Indium antimonide (InSb) Graphene
Applications
Hall-effect sensors may be used in various sensors such as rotating speed sensors (bicycle wheels, gear-teeth, automotive speedometers, electronic ignition systems), 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 (that includes a small magnet) 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.Tank Sensors & Probes , Electronic Sensors, Inc., retrieved August 8, 2018.
Magnetometers Hall-effect magnetometers (also called tesla meters or gauss meters) use a "Hall probe", with a Hall element, to measure magnetic fields or inspect materials (such as tubing or pipelines) 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 (a shunt, required for the most common current sensing method) 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 ratio Integrating a Hall sensor into a ferrite ring (as shown) concentrates the flux density of the current's magnetic field along the ferrite ring and through the sensor (because flux flows through ferrite much better than through air), 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.