applying linear output hall effect transducers -...

Solid State SensorsApplying Linear Output Hall Effect Transducers

84 Honeywell 1 Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 Canada

APPLICATION DATA

INTRODUCTIONThe SS9 Series Linear Output Hall EffectTransducer (LOHETTM) provides mechan-ical and electrical designers with signif-icant position and current sensing capa-bilities. Sensor characteristics and appli-cations are discussed in this section.

SENSOR DESCRIPTIONPhysical dimensions, magnetic charac-teristics and electrical parameters arecovered on page 19.

Figure 2 shows the block diagram of theSS9. The elements which make up thesetransducers are: a Hall effect element,temperature compensating amplifier andoutput transistor. Three thick film resis-tors are incorporated in the design. Sensi-tivity adjustment and temperature com-pensation is provided, and one resistor istrimmed for the offset voltage.

Figure 1Linear Output Hall Effect Transducer (LOHETTM)

Figure 2Block Diagram

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Honeywell 1 Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 Canada 85

APPLICATION DATA

MAGNETICSThe SS9 is magnetically actuated. Figure3 through Figure 6 represent a few of theways a magnetic system can be present-ed to the LOHETTM for position measure-ment. The method of actuation will bedetermined based upon cost, perform-ance, accuracy and other requirementsfor a given application.

Head-on sensingA simple method of position sensing isshown in Figure 3. One pole of a magnetis moved directly to or away from thesensor. This is a unipolar head-on posi-tion sensor. When the magnet is farthestaway from the sensor, the magnetic fieldat the sensing face is near zero gauss. Inthis condition, the sensor’s nominal out-put voltage will be six volts with a 12 voltsupply. As the south pole of the magnet

approaches the sensor, the magneticfield at the sensing surface becomesmore and more positive. The output volt-age will increase linearly with the magnet-ic field until a +400 gauss level or nominaloutput of 9 volts is reached. The output asa function of distance is nonlinear, butover a small range may be consideredlinear.

Figure 3Unipolar Head-On Position Sensor

Bipolar head-on sensingBipolar head-on sensing is shown in Fig-ure 4. When the magnets are moved tothe extreme left, the SS9 is subjected to astrong negative magnetic field by magnet#2, forcing the output of the sensor to anominal 3.0 volts. As magnet #1 movestoward the sensor, the magnetic fieldbecomes less negative, until the fieldsof magnet #1 and magnet #2 canceleach other, at the midpoint between

the two magnets. The sensor output willbe a nominal 6.0 volts. As magnet #1continues toward the sensor, the field willbecome more and more positive until thesensor output reaches 9.0 volts. This ap-proach offers high accuracy and goodresolution as the full span of the sensor isutilized. The output from this sensor islinear over a range centered around thenull point.

Figure 4Bipolar Head-On Position Sensor

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APPLICATION DATA

Biased head-on sensingBiased head-on sensing, a modified formof bipolar sensing, is shown in Figure 5.When the moveable magnet is fully re-tracted, the SS9 is subjected to a negativemagnetic field by the fixed bias magnet.As the moveable magnet approaches the

sensor, the fields of the two magnetscombine. When the moveable magnet isclose enough to SS9, the sensor will‘‘see’’ a strong positive field. This ap-proach features mechanical simplicity,and utilizes the full span of the SS9.

Figure 5Biased Head-On Position Sensor

Slide-by sensingSlide-by actuation is shown in Figure 6. Atightly controlled gap is maintained be-tween the magnet and the SS9. As themagnet moves back and forth at that fixedgap, the field seen by the sensor be-comes negative as it approaches thenorth pole, and positive as it approachesthe south pole. This type of position sen-sor features mechanical simplicity andwhen used with a long enough magnet,

can detect position over a long magnettravel. The output characteristic of a bipo-lar slide-by configuration is the most line-ar of all systems illustrated, especiallywhen used with a pole piece at each poleface. However, tight control must bemaintained over both vertical positionand gap to take advantage of this sys-tem’s characteristics.

Figure 6Slide-By Position Sensor

LINEARIZING OUTPUTThe output of the sensor as a function ofmagnetic field is linear, while the outputas a function of distance may be quitenonlinear as shown in Figure 3. Severalmethods of converting sensor output toone which compensates for the non-lin-earities of magnetics as a function of dis-tance are possible. One involves convert-ing the analog output of the SS9 to digitalform. The digital data is fed to a micro-processor which linearizes the outputthrough a ROM look-up table, or transferfunction computation techniques. A sec-ond method involves implementing ananalog circuit which has the necessarytransfer function to linearize the sensor’soutput. Figure 7A diagrams the micro-processor approach, and Figure 7B dia-grams the analog circuit approach.

Figure 7AMicroprocessor Linearization

Figure 7BAnalog Linearization

A third method for linearizing the SS9output can be realized through magneticdesign by altering the geometry and posi-tion of the magnets used. These types ofmagnetic assemblies are not normallydesigned using theoretical approaches.In most instances, it is easier to designmagnetics empirically by measuring themagnetic curve of the particular assem-bly. By substituting a calibrated Hall ele-ment for the variety of magnetic systemsavailable, the designer can develop sys-tems which perform a wide variety ofsensing functions.PDFINFO p a g e - 0 8 6



APPLICATION DATA

SENSOR APPLICATIONSLiquid level measurementDetermining the height of a float is onemethod of measuring the level of liquid ina tank. Figure 8 illustrates an arrange-ment of a LOHET and a float in a tankmade of non-ferrous material (alumi-num). As the liquid level goes down, themagnet moves closer to the sensor, caus-ing an increase in output voltage. Thissystem allows liquid level measurementwithout any electrical connections insidethe tank.

Flow meterFigure 9 shows how LOHET could beused to make a flow meter. As the flowrate through the chamber increases, aspring loaded paddle turns a threadedshaft. As the threaded shaft turns, it raisesa magnetic assembly that actuates thesensor. When flow rate decreases, thecoil spring causes the assembly to lower,reducing the output. The magnetic andscrew assemblies of the flow meter aredesigned to provide a linear relationshipbetween the measured quantity, flowrate, and the output voltage of the sensor.

Current sensingLOHET sensors need not be used exclu-sively with permanent magnets. Since themagnetic field in an unsaturated electro-magnet varies linearly with current, aLOHET may be used to sense current.Figure10 illustrates a simple current sen-sor. The coil around the torroid is placedin series with the line and the sensor isplaced in the gap. The magnetic field inthis gap varies linearly with current, thusproducing a voltage ouput proportionalto the current. This type of sensor couldbe used in applications such as a motorcontrol with current feedback.

The magnetic field in an electromagnet isnot only a function of current, but also ofthe number of turns on the core. If thecurrent to be measured is greater than 30amperes, a single turn design can beused, such as shown in Figure 11. Thistype of sensor is particularly useful in highcurrent systems where broad dynamicrange, low series resistance, and a linearcurrent measurement are required.

Figure 8LOHETTM Float Height Detector

Figure 9LOHETTM Float Meter

Figure 10LOHETTM Current Sensor

Figure 11LOHETTM High Current Sensor

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APPLICATION DATA

MagneticsFigure 12 is a semi-logarithmic graph ofgauss versus distance for various barmagnets. Each curve is from a singlemagnet in the head-on mode of oper-ation. The most stable operation at anygiven distance is obtained by using themagnet that provides the greatest rate ofchange in gauss at that distance. Thebest accuracy for any give magnet in thehead-on mode of operation is at 400gauss (40.0 mT).

Although the output is linear as a functionof magnetic field, it is not linear as a func-tion of distance. Therefore, the head-onmode of operation does not provide alinear output voltage versus distance. Inan application requiring use of the head-on mode of operation, a microcomputerwith a look-up table can be used to con-vert the LOHETTM output to a linear volt-age.

Gauss patterns for typical ring magnetsare shown in Figures 13A and 13B. Thereis an angular distance around zero gausslevel where the gauss versus degrees ofrotation approaches linearity. The num-ber of poles on the magnet determinesthe number of degrees of rotation wherethis relationship holds true. The spacingbetween the magnet and the sensor de-termines the gauss level at which the rela-tionship between gauss and degrees ismost linear.

Figure 12

Figure 13A

Figure 13B




APPLICATION DATA

Figure 14 illustrates the use of two mag-nets to obtain a linear relationship be-tween distance and gauss. The distanceover which the relationship is most nearlylinear depends on the magnets used, andthe gap length between the magnets. Theassembly in Figure 14 moves perpendic-ularly to the LOHETTM. If travel is limited toprevent the magnets from touching theLOHETTM, the assembly can be used inangular measurements. Non-magneticmaterial such as aluminum or brassshould be used for the magnet mountingbracket.

Two-magnet arrangements are alsoshown in Figure 15A and 15B. The spac-ing between the magnets and theLOHETTM must be held constant for re-peatable operation. Curves are shown forseveral gap spacings between the mag-nets and the LOHETTM. These assembliesare most useful when a high rate ofchange in gauss over a short travel isrequired.

Figure 14

Figure 15A

Figure 15B

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APPLICATION DATA

Relatively long distances with a linear re-lationship can be realized with the ar-rangement shown in Figures 16A and16B. The pole piece (flux concentrator)mounted behind the LOHETTM should beequal to or greater than twice the length ofthe magnet. The pole pieces at each endof the magnet extend above the magnet.The area of extension is approximately35% of the cross sectional area of themagnet. The magnet is usually 50% long-er than the distance over which the linearrelationship is desired. The relative sizesof the parts are shown in Figure 16A and16B.

By using precisely placed magnets, thearrangements shown in Figures 15 and16 allow accurate measurement over ashort distance when total travel is large,as shown in Figure 17.

Figure 16A

Figure 16B

Figure 17




APPLICATION DATA

APPLICATIONAn arm is rigidly attached to a shaft thatrotates 90° (Figure18). The movement ofthe arm is rapid until it approaches thefinal position. Then it is to move slowly tothe exact position required. A microcom-puter based control system is used.

SolutionAt zero gauss (center point of the mag-net), the variations of Ig due to setup willnot change the gauss level. When the armrotates the full 90°, the gauss level at theLOHETTM will be zero.1. At some time during the machine cy-

cle, when the magnet is away from theLOHETTM, read the voltage through anA/D converter (either on board thecomputer, or a separate device). Thisreading serves as the reference for thiscycle.

2. Monitor the output voltage during thecycle or generate an interrupt as thezero degree point is approached.

3. When the linear region is reached, theoutput voltage can be converted bythe microcomputer to degrees rota-tion or distance, as desired.

4. When the LOHETTM output voltagematches the reference from step 1, thearm is at the desired point.

This method provides continuous calibra-tion so that any changes due to temper-ature variations of the A/D conversion orof the LOHETTM, do not influence the mea-surement. An electromagnet driven bythe microcomputer can be used in placeof the permanent magnet.

Figure 18

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