Tilt-Sensing with Kionix MEMS Accelerometers

Transcription

1 Tilt-Sensing with Kionix MEMS Accelerometers Introduction Tilt/Inclintion sensing is common ppliction for low-g ccelerometers. This ppliction note describes how to use Kionix MEMS low-g ccelerometers to enble tilt sensing. Applicble theory, plots nd equtions re provided with this note s guidelines. Tilt Sensing Applictions Accelerometers hve countless potentil tilt-sense pplictions in tody s motion-enbled world. Tilt-sensing opportunities exist in vriety of industries, such s utomotive, consumer electronics nd militry/erospce, nd include: Vehicle stbility systems Inclinometers Cell phone/pda screen nvigtion Motion-enbled gme ply Tilt-enbled computer mouse/pointer Tilt-compensted electronic compss Some of these pplictions currently utilize dul-xis ccelerometers tht, t times, re dequte for the job. A tri-xis ccelerometer, however, cn enble dditionl functionlity, ccurcy nd precision. Dul-Axis Tilt Sensing Presently, low-g dul-xis (, ) ccelerometers re often used in tilt-sensing pplictions where the force of grvity is used s n input to determine the orienttion of n object. The sensor is most responsive to chnges in tilt when the sensitive xis is perpendiculr to the force of grvity, yielding sensitivity of pproximtely 17.45mg/ of tilt. The sensor is lest responsive to chnges in tilt when the sensitive xis is oriented in its +1g or 1g position, yielding n pproximte sensitivity of only 0.15mg/ of tilt. When horizontlly mounted, s shown in Figure 1, dul-xis ccelerometer s diminishing sensitivity through incresed tilt prevents ccurte tilt sensing when the ccelerometer is tilted beyond 45 long either xis. Also, the bsence of -xis sensor mens the ccelerometer cnnot detect n inversion, cusing improper functioning. 1g Erth s Surfce Figure 1: Horizontlly Mounted Dul-Axis Accelerometer. The g vector indicted is the pprent ccelertion mesured by -xis sensor due to the sttic grvittionl force. 36 Thornwood Dr. - Ithc, N tel: fx: info@kionix.com Pge 1 of 6

2 These problems cn be ddressed by mounting the ccelerometer verticlly, s shown in Verticlly Mounted Dul-Axis Accelerometer, creting n, sensor. Unfortuntely, n dditionl ccelerometer will be required to sense both -tilt nd -tilt, incresing the bill of mterils nd potentilly doubling cost. 1g Erth s Surfce Figure : Verticlly Mounted Dul-Axis Accelerometer The integrtion of Kionix tri-xis (,, ) ccelerometer into this ppliction provides the llimportnt -xis tht functions with the tilted xes to mintin constnt sensitivity nd sense the inversion of the ccelerometer, thus permitting full, unrestricted rnge of tilt. Tri-Axis Tilt Sensing Kionix technology provides for, nd -xis sensing on single, silicon chip. When using Kionix tri-xis ccelerometer, the -xis cn be combined with both of the tilting xes to improve tilt sense precision nd ccurcy. For this note, we will follow the tilt ssignments described below in Figure 3. 1g No -tilt or -tilt; level with the ground. -tilt () is the ngle of the x xis reltive to the ground. is the ngle of the z xis reltive to grvity. -tilt () is the ngle of the xis reltive to the ground. is the ngle of the z xis reltive to grvity. Erth s Surfce Figure 3: -tilt nd -tilt Assignments Reltive to Ground Bsic tilt ngles cn be generted from the ccelerometer outputs using Eqution 1 since the - nd -xes follow the sine function nd the -xis follows the cosine function. It should be noted tht these ngles re not the pitch nd roll ngles typiclly used in eronutics, nor re they the direction cosine ngles for describing which direction vector is pointing. The ngles nd re the ngles tht the nd ccelerometer xes mke with the fixed reference plne. Pge of 6

3 rcsin rcsin rccos Eqution 1. Clculting ngle from ccelertion (g) where x, y, nd z re the ccelertions in the respective xis. x y z For exmple, s Kionix tri-xis ccelerometer is rotted 360 round the -xis, s shown in Figure 4, ccelertion due to tilt on the - nd -xes will chnge ccording to the tble in Figure 5. ou cn see tht the -xis output is t its minimum sensitivity when the -xis output is t its mximum sensitivity. Therefore, mximum sensitivity cn be mintined if both the nd outputs re combined. Note tht this method is vlid when rotting 360 round ny xis. 90 Accelerometer End View 360 of Tilt 0, 360 1g 70 Figure 4: Kionix Tri-Axis Accelerometer, 360 of Tilt Pge 3 of 6

4 Tilt Sensitivity (mg/deg) AN 005 Tri-Axis Tilt Clcultions - xis Figure 5: Tilt Tble The -xis of Kionix tri-xis ccelerometer cn be combined with the - nd/or -xes to mintin constnt sensitivity through ll 360 of tilt. Figure 6 shows the expected tilt sensitivity through the first 90 of tilt when using just the tilt xis, the -xis nd the combintion of the two. Tilt Sensitivity vs. Tilt Angle - xis -tilt () Accelertion Chnge per Accelertion Chnge per in ( ) (g) of Tilt (mg) (g) of Tilt (mg) : ArcSin() : ArcCos() 3: ArcTn(/) Tilt Angle (deg) Figure 6: Tilt Sensitivity vs. Tilt Angle Pge 4 of 6

5 Accelertion (g) AN 005 As you cn see, pproximtely mg/ of sensitivity cn be mintined t ny tilt orienttion when combining n or tilt xis with the -xis. This method llows tilt ngles greter thn 45 to be sensed ccurtely nd precisely. Both -tilt () nd -tilt () cn be sensed simultneously using the outputs of ll three xes, s shown in Eqution. rctn x y z rctn x y z Eqution : Combined -tilt () nd -tilt () Clcultions Sign Recognition There is still n dditionl wy to improve tilt sense precision. As shown in Figure 7, when the - xis tilt is pproximtely 90, the sme -xis sensor output voltge cn represent two different ngles. To better illustrte this sitution, Figure 7 presents 360 of ccelerometer tilt divided into four qudrnts, Q1, Q, Q3, nd Q4. Accelertion vs. Tilt Angle Q1 Q Q3 Q Axis -Axis Tilt Angle (degrees) Figure 7: Tilt Qudrnts Through 360 of Pitch Sign recognition is impertive becuse, for exmple, tilts of 45 nd 135 re represented by the sme sensor output,.707g. The methods presented up to now do not explin how to tell the difference between two tilt ngles tht result in the sme sensor output. This mbiguity cn be resolved by monitoring the sign of the -xis output. In this cse, when the ccelerometer is tilted t 45, the -xis output is positive vlue. When the ccelerometer is tilted to 135, the -xis output is negtive vlue. The sign of the -xis nd the -xis will indicte in which qudrnt the ccelerometer is being tilted, consequently indicting inversion. The tble in Figure 8 shows the expected signs for ech output in ech qudrnt. Pge 5 of 6

6 Figure 8: Accelertion Signs Through Ech Qudrnt Accelerometer errors One detil so fr unmentioned is the ccelerometer s bility to crete ccurte tilt ngles. Clibrting ccelerometers is mostly fctory test problem. End-users cn re-zero (level) -tilt nd -tilt in their production line. The individul tilt offsets cn be found by plcing the phone or circuit bord ssembly on known flt surfce nd mesuring the vlues with ner zero expecttion for the nd vlues. The offset error cn be zeroed out fter instlltion nd will include ny pltform leveling error. Sensitivity or scle fctors could be done for ccelerometer nd tilt outputs by roboticlly tipping the ssembly nd mtching the resulting output with the desired output. Obviously test costs must be ssessed with the desired compss ccurcy nd the qulity of ccelerometers. Additionl informtion on the sources of error nd how to minimize them cn be found in Appliction Note AN01 Accelerometer Errors. The Kionix Advntge The Kionix tri-xis ccelerometers cn mesure 360 of tilt round ny xis with gret precision. Kionix technology provides for, nd -xis sensing on single, silicon chip. One ccelerometer cn be used to enble vriety of simultneous fetures including, but not limited to: Tilt-screen nvigtion Gme plying Imge stbility, screen orienttion Drop force modeling for wrrnty protection HDD shock protection Theft, mn-down, ccident lrm Computer pointer Nvigtion, mpping Automtic sleep mode Theory of Opertion Kionix MEMS liner tri-xis ccelerometers function on the principle of differentil cpcitnce. Accelertion cuses displcement of silicon structure resulting in chnge in cpcitnce. A signl-conditioning CMOS technology ASIC detects nd trnsforms chnges in cpcitnce into n nlog output voltge, which is proportionl to ccelertion. These outputs cn then be sent to micro-controller for integrtion into vrious pplictions. For product summries, specifictions, nd schemtics, plese refer to the Kionix MEMS ccelerometer product sheets t Pge 6 of 6