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 B. 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 BOP and BRP) 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 BOP and a negative BRP (and thus require both positive and negative magnetic fields to operate). The difference between BOP and BRP 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 § Dual Hall sensor ICs) 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.
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