Gary Pepka Abstract Hall-effect (magnetic field) sensing app

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Gary Pepka Abstract Hall-effect (magnetic field) sensing app

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Position Level Sensing Using Hall Effect Sensing Technology
Gary Pepka
Abstract
Hall-effect (magnetic field) sensing applications have become practical recently through advancements supporting technnologies. This paper introduces Hall-effect technology, then explores been applied, particular, differentiating between primary types Hall sensors, highly differentiated range sensing behaviors they provide. addition, explores some enabling technologies, such advances signal processing, that have made this technology much more robust than earliest days. This allows application extreme high-reliability benefit contactless Hall sensing broader range applications than ever before. addition improvements supporting technologies, Hall-effect devices themselves have advanced, contributing designs complete solutions. These advances include power space reduction, well integration diagnostic protection functions that allow Hall sensor provide advanced data-driven features that becoming more demand miniaturized portable consumer electronics, automobiles, other growing industries.
Figure Hall effect, magnetic flux perpendicular flow electrical current results measurable voltage.
Hall Effect Benefits
Introduction
With extensive variety solutions available position sensing level sensing, designers select optimum technologies packages meet their commercial engineering goals. these solutions, Hall-effect technology, with application contactless magnetic sensing, provides exceptional value reliability. This application note examines benefits Hall-effect technology latest developments these devices enhance position level sensing results.
There almost many means sensing position level there applications requiring these functions. Inductive, capacitive, mechanical, magneto- resistive, Hall effect, optical, name just few, viable sensing options list continues expand. designer, there always remains same critical elements that need addressed that inevitably mate requirements application appropriate sensing technology. Critical requirements, such cost, distance travel (effective operating gap), resolution, accuracy, often times cost again, need determined effectively efficiently select proper sensing technology. course, constructing answers each these elements always straightforward task. Here, though, flexibility Hall-effect sensing technology most advanta-
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geous. High reliability, small size, production-viable cost, wide operating voltage ranges, variety output options, ease implementation allow Hall-effect sensing technology service applications most every market.
Overview Hall Technology
First, brief tutorial Hall-effect technology works. Simply stated, Hall effect, named after Edwin Hall discovered 1879, refers measurable voltage across conductive material, example silicon (Si) gallium arsenide (GaAs), that occurs when electric current flowing through conductor influenced magnetic field (see figure This transverse force created magnetic field known Lorentz force. Therefore, Hall-effect sensor requires magnetic field order actuate device. Although quite common today, Hall-effect technology truly begin gain mass acceptance until 1980s. This because voltage potential across Hall element minuscule, easily influenced outside forces, such temperature package stresses. shown figure more recent devices incorporate advances ability amplify signal, addition utilization on-chip, offset cancellation techniques, which have allowed Hall-effect sensing technology employed even under extreme environmental conditions, such under-hood applications automobiles. Furthermore, "non-contacting" operation Hall-effect sensors affords user nearly infinite life with regard actuation switching.
either digital analog output. former option optimal sensing discrete positions, while latter affords user relatively infinite number positions greater resolution. Some examples applications requiring discrete position level sensing are: automotive shift selectors, seat belt buckle switches, seat position sensors, cellular flip phones, brushless motor commutation, windshield wiper fluid reservoirs, tanks, name just few. high reliability, Hall-effect technology used replace reed switches mechanical switches these applications. Most Hall-effect switches have output structures that open drain provide resistance, thus simplifying interface most microprocessors other digital electronics (threshold comparators, multiplexers, basic gates, forth). Typical open-drain outputs, once switched "on," output voltage Hall-effect device transitions from high low. This being said, there abundance variations Hall-effect sensors order service plethora position level sensing applications, each with nuances. These variations include features such micro-power consumption, magnetic pole-independent sensing, user-programmable options, two-wire current sourced output devices, magnetic bias sensing ferrous targets, inverted outputs. These cannot adequately discussed sitting, purpose this article, focus will standard devices: their operation application uses.
Standard Hall Device Characteristics
Hall Device Options
Further investigating elements that require consideration position level sensing application, Hall-effect sensors provide designer with multitude features variations, including
There three common variations standard digital position level sensors: unipolar, latching, bipolar. With unipolar switches actuation caused magnetic field sufficient strength turn device "on." Typically Bsouth indicates magnetic flux density) must greater than magnetic operate
Regulator subcircuits Sample Hold Dynamic Offset Cancellation VOUT Control Signal Amplifier Low-Pass Filter
Chopper Stabilization Offset Cancellation
Currrent Limit
Figure Modern Hall-effect sensors integrate signal conditioning amplification techniques make practical devices.
Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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point, BOP, device order turn these devices Once magnetic field reduced below magnetic release point, BRP, device, these devices return "off" state. Latching sensors turn manner similar unipolar switches. However, latching sensors only turned (unlatched) when device sees sufficient magnetic field strength opposite polarity, Bnorth. Bipolar switches akin latching devices that they opposing magnetic polarities turn off. high sensitivity these devices, they cannot guaranteed operate latch. some cases, bipolar switches have switchpoints (BOP BRP) that cause them function standard unipolar switch even negative switch (switching only presence sufficient north magnetic polarity).
example, five nine. Simple Binary Coded Decimal (BCD) systems, more advanced systems such Gray code Densely Packed Decimal (DPD), used decode logic acquire positional information. Similarly this tactic could used sense fluid levels tank means flotation device with magnet inside, illustrated figure magnet floats down with changes level fluid, discrete levels determined which sensor state.
High Resolution Applications
Resolution Applications
seen very quickly from shift selector example that discrete position level sensing ideal when only positions required. This method adding sensor each position very quickly becomes cost prohibitive spatially challenging when application requires finer resolution. Enter linear Hall-effect sensor with analog output. Similar digital switches, there abundance features available linears; example, ratiometric outputs, user programmability, digital outputs (such PWM), unidirectional bidirectional sensing. Like preceding description sensors discrete positions levels, this discussion will concentrate only standard linear Hall-effect sensors: their means operation application uses. Most standard linear Hall-effect sensors have ratiometric outputs (0.5 VDD) that respond proportionately magnetic field strength. These devices generally require regulated supply (quiescent voltage output, VOUT(Q))
excellent example application that uses discrete position sensing automobile shift selector. shift selectors there commonly five discrete positions (Park, Reverse, Neutral, Drive Low). With unipolar switch placed each individual position each switch only turns when magnet shifter moved directly adjacent switch, shown figure Should designer require additional positions, spacing between sensors reduced create "cross talk" between sensors. this manner additional positions obtained when magnet sufficiently close devices such that they both turned thereby increasing number positions from,
Figure Hall sensors used proximity switches, matched 1-to-1 with sensed positions, arrayed provide additional sensing positions through analysis magnetic cross-talk using multiple sensors.
Figure Level sensing application fluid tank; spherical float with button magnet inside rides fluid surface, while Hall sensors wiring fully isolated separate chamber.
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Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
when there significant magnetic field present (see figure output voltage increases when sensing increasing magnetic field from south pole magnet, approaching Conversely, output voltage will decrease when sensing increasing magnetic field from north pole magnet, approaching There common configurations applications linear devices, which form foundation most designs. These techniques termed slide-by head-on.
A-to-D converter standard microprocessor simple lookup table then employed convey actual position. this situation, resolution (the number positions that detected) predicated resolving capabilities A-to-D converter, analog signal provides relatively infinite number positions. example application that slide-by sensing valve position, diagrammed figure this application often magnet two-pole ring magnet that rotates front (slides face) Hall-effect sensor. opposing magnetic fields pass front sensor, voltage output changes proportionately change field strength. means precise sensing, position valve controlled dictate more accurately flow substance through carrier.
Slide-By Configurations
Flux Density,
standard slide-by application, magnet moves across face sensor, such that Hall element senses both magnetic poles, shown figure There effectively three positions which voltage output zero: before magnet close enough field sensed device, once zero crossing between poles directly adjacent Hall element, once magnet moved past device enough that there longer sufficient field detectable element. Effectively, change output voltage from (assuming that north pole magnetic field passes face sensor, from south pole passes face sensor. This typically labeled bidirectional sensing. certainly also possible sense change only pole across device, although this could limit available range. Known unidirectional sensing, change output then limited only standard linears. obtain full range operation, would have employ user-programmable linear with this feature. change voltage output from Hall-effect sensor field changes across face then used determine relative position moving magnet.
(mm)
Figure Slide-by application configuration response curve, showing separate nodes peaks north pole south pole.
Saturation Output Voltage, VOUT
Quiescent Output Voltage, VOUT(Q)
Saturation Flux Density,
Figure Linear Hall-effect devices respond throughout range sensed magnet flux, outputting ratiometric analog signal.
Figure Valve position sensing proven application slide-by Hall sensor configurations.
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Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
Head-On Configurations
Head-on position sensing very similar unidirectional sensing slide-by configuration. essence linear sensor only differentiates change magnetic field strength magnetic pole, which either north south polarity. detection pattern straightforward. magnet approaches device, field detected sensor increases, field strength decreases magnet removed, shown figure Detecting height deck treadmill illustrates well uses head-on sensing technique. When height deck altered change gradient runner, linear Hall sensor used detect displacement deck. Typically magnet attached deck itself while sensor remains stationary. runner increases decreases gradient deck, sensor provides feedback control module relative displacement, means change field strength witnessed sensor.
Therefore discrete position sensing, always good practice determine effective gap, from face sensor magnet, required switching position, then determine maximum minimum field strengths, over rated temperature range, that distance. This value should then compared maximum rated operating switchpoint each alternative device. chart formula estimating field degradation effective provided figure This change calculated using formula below: Fieldx= where: Residual Magnetic Inductance material, Length magnet, Distance between surface magnet sensor, Radius magnet, chart reflects typical results button magnet, similar that used figure composed NdFe, rated (oersted; microtesla, µT), with radius, thickness.
Determining Field Specifications
with technology, there some specific considerations when designing application using Hall-effect sensor. Careful selection magnet utmost importance, including shape placement, shown figure Magnetic field strength decreases exponentially over distance. Furthermore, magnets have temperature coefficients that need considered.
Flux Density,
Effective (mm)
10.0
Figure Head-on application configuration response curve, showing monotonic characteristic regardless pole orientation.
Figure This model depicts change field strength button magnet (similar that used figure 10). arrows represent magnetic flux lines. closer lines magnet, stronger field strength.
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Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
good rule thumb designer make certain that, required position device switch, there least more field strength than required maximum rated switchpoint. example, required unipolar switch with BOP(max) turn certain distance, then field strength that distance should less than under conditions.
Then: VOUT VOUT2 VOUT1 4000 1000 3000 (full linear range), Bapplied(G) Bmax Bmin (-100 Note: algebraic convention applied positive values denote south polarity, negative values denote north polarity. Entering these into general equation: Gain (mV/G) 3000 mV/G course, real world applications transfer functions perfectly linear there inherent offset system. this reason, further consideration must given accuracy required application, well resolution capabilities A-to-D converter similar device that must read output, temperature coefficient magnet. helpful these situations consider: change quiescent output voltage function temperature, VOUT(Q)(TA), change sensitivity (gain) function temperature, Vsens(Q)(TA), linearity device over given range magnetic field strength.
Designing Linear Applications
Unlike digital Hall-effect switches, which require only certain strength polarity field order actuate, linear devices require little more application specification order achieve satisfactory results. gain sensor device determines resolution given distance. Therefore regardless whether application slide-by head-on, must select appropriate gain. order this, known points required resolution (number data points) must established. following brief example determining appropriate gain. Assuming that requirements application depicted figure useable linear range would full range magnet travels across device would (gauss; millitesla, mT). Dividing change output voltage, VOUT, change applied field, Bapplied, provides appropriate gain linear Hall-effect device this application. greater clarity, here equations results this example. general equation Gain (mV/G) VOUT (mV) Bapplied example data, first convert VOUT from
2500 2000 1500 1000
Output Voltage, VOUT Quiescent Output Voltage, VOUT(Q)
VOUT2
Flux Density,
Full Travel VOUT1
-100
Effective (mm)
Figure This graph depicts exponential decrease field strength (BZ, gauss) versus distance NdFe magnet.
Figure Example dynamic range linear Hall-effect sensor.
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Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
Linear Hall-effect sensors back-biased with magnetic field order sense ferrous targets. example, Hall sensors widely accepted automotive industry accurately sense position lobes speed crankshafts engines, order improve timing thereby grant more efficient consumption fuel. high bandwidth capability many Hall-effect linears allows them used sense changes current dc-to-dc converters battery management systems hybrid vehicles.
Summary
Obviously these simplified examples applications that employ Hall-effect sensors, very compressed descriptions capabilities features offered this technology. Other interesting examples important Hall sensor options include: current source outputs two-wire devices ideal safety-critical applications, such seat position seat belt buckle sensors. This because these devices output distinct current levels indicate states.
output that deviates from these levels fault condition, affording user with inherent diagnostics. Extremely current draw permits Hall-effect sensors open/closed circuit sensors. This particularly valuable battery-operated applications that sensitive power loss, example: cellular flip phones, laptop computers, pagers. flexibility these sensors further enhanced assortment package options. Some micro-leaded packages (MLP, also known leadless packages) small while others large enough include samarium cobalt magnet back-bias sensor. myriad applications that served Hall-effect technology that drives ever-increasing diversity these devices. result, technology continues evolve. ongoing reductions size continual increase capabilities lend Hall technology viable solution almost position level sensing application.
Copyright ©2006, 2007, Allegro MicroSystems, Inc. products described herein manufactured under more following U.S. patents: 5,045,920; 5,264,783; 5,442,283; 5,389,889; 5,581,179; 5,517,112; 5,619,137; 5,621,319; 5,650,719; 5,686,894; 5,694,038; 5,729,130; 5,917,320; other patents pending. Allegro MicroSystems, Inc. reserves right make, from time time, such departures from detail specifications required permit improvements performance, reliability, manufacturability products. Before placing order, user cautioned verify that information being relied upon current. Allegro's products used life support devices systems, failure Allegro product reasonably expected cause failure that life support device system, affect safety effectiveness that device system. information included herein believed accurate reliable. However, Allegro MicroSystems, Inc. assumes responsibility use; infringement patents other rights third parties which result from use. latest version this document, visit website: www.allegromicro.com
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Allegro MicroSystems, Inc. Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

Gary Pepka Abstract Hall-effect (magnetic field) sensing app

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