Indirect Tire Pressure Monitoring Using Sensor Fusion

Transcription

1 --5 Indiec Tie Pessue Monioing Using Senso Fusion Copyigh Sociey of Auomoie Enginees, Inc. Niclas Pesson, Fedik Gusafsson Depamen of Elecical Engineeing, inköping Uniesiy Makus Deö Nia Dynamics AB ABSTRACT Vehicle handling depends ciically on he ie-oad conac pach. When he ie inflaion pessue changes he conac pach is no longe opimal and he handling popeies deeioae. Fuhemoe fuel consumpion inceases and he lifeime of he ies deceases. Theefoe i is ey impoan ha he ies ae coecly inflaed. We hee focus on an indiec ie pessue monioing sysem, whee no pessue sensos ae needed. The sysem is based on ibaion and wheel adius analysis. These wo appoaches ae combined fo opimal pefomance concening sensiiiy o deec pessue losses and obusness o diffeen diing condiions. When hese wo appoaches ae combined, i is possible o deec pessue losses lage han 5% in one, wo, hee, o fou (diffusion) ies wihin minue. I is also possible o deec which of he ies ha ae undeinflaed. INTRODUCTION The impoance of coec ie inflaion pessue is well known. Vehicle handling is decided upon he ie-oad conac pach. Fuhemoe fuel consumpion inceases and ie lifeime deceases wih % fo eey. ba unde-inflaion. I is also known [] ha ie poblems ae he hid mos common beakdown fo passenge ehicles. 75% of all ie flas ae peceded by undeinflaion o slow leakage. Wih a ie pessue monioing sysem seee accidens can be aoided and boh economical and enionmenal benefis can be made. Thee ae wo ways of monioing he ie pessue. One way is o moun a pessue senso a he im on each ie and ia a communicaion-link (usually a adio-link) ansfe he senso alue o a cenal uni. This is called diec ie pessue monioing. Due o he ea hadwae needed his mehod is ey epensie. The ohe alenaie, called indiec ie pessue monioing uses eising sensos and sofwae algoihms. The indiec sysems ae ey cos-effecie (no ea hadwae). Thee ae pleny of on-going R&D pojecs (see e.g. []) on indiec Tie Pessue Monioing Sysems (TPMS), efleced in oe paens. Thee ae wo classes of indiec TPMS.. Vibaion analysis using he fac ha he ubbe in he ie eacs like a sping when ecied by oad oughness. The ibaion analysis can be pefomed by FFT-based mehods o by paameic mehods (using an auo-egessie model). The idea is o monio he esonance fequency, which is coelaed wih he ie pessue (see e.g. [5] and [6]).. Wheel adius analysis using he fac ha he ie pessue affecs he effecie olling adius of he ie. The mos common suggesion is o monio a esidual, based on a saic non-linea ansfomaion of wheel speeds, which should be close o zeo when he ies ae equally lage, i.e. hae equally ie pessue (see e.g. [7]). Mos of he on-going R&D pojecs ae in he lae class. These wo classes hae some diffeen popeies egading obusness and sensiiiy, fo insance o aying elociy, and abiliy o deec pessue changes. Typically wheel adius analysis is ey sensiie o diffeen elociies, bu on he ohe hand i esponds ey quickly o pessue changes. The ibaion analysis is sensiie o diffeen oad condiions, bu is insensiie o aious elociies. The idea hee is o combine hese wo classes in a way such ha boh obusness and sensiiiy popeies ae enhanced. The mehod poposed fo he wheel adius analysis uses boh longiudinal and laeal ehicle dynamic models. The longiudinal model compaes he dien and non-dien wheels, using a ficion model, while he laeal model compaes he lef and igh wheels, using a yaw ae model. The analysis is based on [], [] and []. The ibaion analysis includes a pepocessing uni, see [], o impoe he signal qualiy fom he ABS-sensos, which makes he analysis

2 feasible. This mehod uses ABS-senso signals only o monio he ie pessue. TIRE MODE The infomaion souces fo indiec TPMS ae wheel speed sensos and pehaps ineial sensos like aegyos and acceleomees. The main souces ae he wheel speed sensos; in moden ehicles hese sensos ae inegaed in he ABS-sysem. The ie can be modeled as a sping-dampe sysem boh in eical and osional diecion, see Figue. wheel speeds. Again, he ibaion fequency is deceased when he ie inflaion pessue is deceased. Fo his ibaion he mos significan ibaion mode is abou -5 Hz. TPMS USING VIBRATION ANAYSIS The idea hee is o monio he ibaion fequency fom he eical and osional sping-dampe sysems and o deec abnomal alues of he fequency. Hee we hae only used he osional ibaion. The fequency conen (FFT) of he wheel speed signal fo hee diffeen es uns (wih diffeen pessue) is shown in Figue. As is eiden fom he figue he coelaion beween esonance peak fequency and inflaion pessue can be used fo ie pessue monioing. In he figue, baches of seconds es uns ae used. k Figue : Tie modeled as sping-dampe sysem in boh eical and osional diecion. The eical sping-dampe sysem is he souce of wo phenomena. Fis, he effecie olling adius is dependen on he inflaion pessue and if he pessue is deceased he adius is deceased. This causes he ie o oae fase. In he equaion denoes he change of adius affeced by deflaion. Secondly, he ie ibaes in he eical diecion ecied by oad oughness and causes he effecie olling adius o flucuae wih a specific esonance fequency. Indiecly his esuls in a flucuaion of he wheel speed a he same esonance fequency. When he ie inflaion pessue deceases he sping-consan is deceased,, and his yields a lowe esonance k fequency. The mos significan mode fo his ibaion is abou - Hz. esonance k In he equaion N denoes he nomal foce applied o he wheel. The osional sping-dampe sysem is also ecied by oad oughness. The ie ibaes in he osional diecion and he ibaion diecly affecs he N k Figue : Smoohed FFT of wheel speed signal fo hee diffeen es uns wih %, 85% and 7% of he nominal inflaion pessue, especiely. The FFT is a bach-wise daa pocessing and consumes a lage amoun of memoy and causes a ime delay. An alenaie is o esimae he esonance fequency ecusiely using a second ode dampe-sping model of he fom y + a q + a q whee y is he wheel speed and q is he shif opeao q y y. Model-based paamee esimaion can easily be implemened wih sandad ecusie mehods, such as ecusie leas squaes (RS) o Kalman file. The paamees a and a ae ansfomed by means of a non-linea funcion o he fequency alue of he esonance peak. Using a ecusie file, he esonance peak fequency becomes a funcion of ime and is shown in Figue.

3 ie loses pessue. This is pehaps he mos inuiie way of deecing unde-inflaion by monioing wheel speeds, bu i has boh adanages and disadanages. Mos of he eising appoaches o TPMS use saic nonlinea elaions o deec unde-inflaion. One eample is. Figue : Recusie esimaes of osional ibaion fequency fo hee diffeen es uns wih %, 85% and 7% of he nominal inflaion pessue, especiely. To ealuae he pefomance of he file, es uns wee pefomed. The fis hae nominal pessue, he en in he middle ae 5 % unde-inflaed and he las ae % unde-inflaed. The es uns include diffeen diing syles, sufaces and elociies. An aeage of he ecusie esimae fo each es un is shown in Figue. A pessue decease of 5 % is deecable wihin seconds, bu a moe ealisic heshold is 5 % wihin one minue. The wheel speeds i ae enumeaed as fon lef (), fon igh (), ea lef (), and ea igh (). This enumeaion is used in he es of he pape. A pessue loss is deeced when he es saisic,, is non-zeo. As an inhei disadanage, his saic consisency es canno deec equal pessue losses on he same ale o side. Assuming ha all ies ae aeling wih he same elociy his is seen fom. An equal pessue loss in fo eample boh ea ies does no affec he es saisic,. Insead of using a saic non-lineaiy, we use boh longiudinal and laeal ehicle dynamics. The longiudinal dynamics compaes he dien and non-dien wheel speeds, using a ficion model, while he laeal dynamics compaes lef and igh wheel speeds, using a yaw ae model. ONGITUDINA DYNAMICS As suggesed in [8] a ficion model can be used o deec unde-inflaion. The ficion model is based on he linea pa of he classic longiudinal slip model, Figue 5, descibed in fo insance [9] and []. The wheel speeds of fon (dien) and ea (non-dien) wheels ae compaed o compue he slip. Figue : Aeage of pessue indicaion fo es uns wih %, 85% and 7% of he nominal inflaion pessue, especiely. The pessue indicaion is he nomalized ibaion fequency. Vibaion analysis has he adanage o independenly ealuae eey ie and heefoe i is also possible o deec pessue losses in all fou ies (diffusion). TPMS USING WHEE RADIUS ANAYSIS TPMS based on wheel adius analysis uses he fac ha he effecie olling adius of he wheel deceases if he Figue 5: Slip cues fo asphal, gael and snow. The slope of he linea pa (s small) is used in he TPMS.

4 The wheel speed and engine oque ae compued fom sandad sensos. The wheel slip is defined as he diffeence beween he cicumfeenial and longiudinal elociy. s Fo small alues of he nomalized acion foce, µ F F z, he wheel slip is a linea funcion of he nomalized acion foce and he elaie diffeence in ie adius beween he fon and ea wheel s µ k + whee k is he inese of he longiudinal siffness and, fo he lef side, is defined as T Using he sae eco ( / k l ) he discee ime sae space model fo he lef side of he ehicle is s + + ( µ l ) + e, An analogous and independen model is used fo he igh side of he ehicle. The offse,, is caused by unequal ie adius and can be used in he TPMS as an indicao of he ie inflaion pessue. The sae space model can be diecly applied o a Kalman file and wih pope uning a decen ade-off beween noise aenuaion and acking speed can be achieed. Epeimenal esuls In ode o ealuae he pefomance of esimaing (lef side) and (igh side), a seies of es uns wee conduced. The es uns included 5 diffeen scenaios.. Nominal pessue in all ies.. ow pessue in fon lef fom he beginning.. ow pessue in fon lef afe seconds.. ow pessue in ea lef fom he beginning. 5. ow pessue in ea lef afe seconds. The esul is shown in Figue 6. The esponse o he pessue change is fas and accuae and i akes only abou 6 seconds o ack he igh leel. Figue 6: Recusie esimaes of longiudinal dynamic elaie adius paamees. The solid line epesens nominal pessue; he dash doed low pessue in fon lef and he doed low pessue in ea lef. ATERA DYNAMICS Insead of compaing fon and ea ies, lef and igh ies can be compaed. Addiional o wheel speed sensos a ae gyo is needed. Rae gyos hae become moe and moe common in moden ehicles due o he fac ha he numbe of sabiliy assisan sysems inceases and hey demand ae gyos. Using a laeal dynamic model i is possible o compue he yaw ae fom wheel angula speeds. Dynamic model The well-known elaion beween yaw ae, longiudinal elociy and cue adius R (see e.g. [9] and []) is R The elociy a he cene of he ea ale is + The cue adius is defined as he disance beween he cene of he ea ale and he Insananeous Cene of Moion, ICM. R ( ) ea ( + )

5 Wih he definiion and wih ea denoing he disance beween he ea wheels, yaw ae is compued as + ) ( ) ( ea ea The ie adius is unknown and a good woking appoimaion is o use he nominal ie adius n. ψ & + + & n n n ψ m, ea ea ea ea whee m indicaes a compuable alue. Again, an analogous model is used fo he fon ale. ea I is hee assumed ha he unknown inpu only affecs he yaw acceleaion, which is common fo moion models. The discee ime model is applied by a Kalman file and he ie inflaion pessue coelaed paamees and can be esimaed. Epeimenal esuls Figue 7 illusaes he esimaed -paamees fo he laeal dynamics model. The same es uns as in he longiudinal dynamics ae used. Again, he acking is boh fas and eliable. The same pefomance as fo he longiudinal dynamics is achieed. As can be seen he esimaed paamees ae non-zeo een wih nominal pessue in all ies. The eason fo his is diffeen wea and ea fo all ies. A calibaion ouine is needed o compue he nominal leel of he esimaed paamees. The gyo signal is subjec o an offse eo and needs o be modeled. A good woking appoimaion of gyo impefecions is &. ψ + gyo o Kalman file A sae space model fo he laeal dynamic models (fon and ea) and gyo model uses he sae eco ( ψ & & ψ ) T o and he measuemens ae aanged as y ( ψ & gyo m, ea m, fon ) Wih hese wo ecos a discee ime sae space model can be deied as y + Ts n ea Ts Ts + n fon + e Figue 7: Recusie esimaion of laeal dynamics elaie adius paamees fo he 5 es uns defined aboe. The solid line epesens nominal pessue; he dash doed low pessue in fon lef and he doed low pessue in ea lef. FUSION OF VIBRATION AND WHEE RADIUS ANAYSIS Boh ibaion and wheel adius analysis cay infomaion abou he cuen ie inflaion pessue. They can be used independenly, bu fo opimal pefomance hey should be combined. Wheel adius analysis esimaes he elaie adii beween fon, ea, lef and igh wheel pais, especiely. Using simple logisic i is possible o deec whehe one, wo o hee ies ae unde-inflaed. Boh ibaion and wheel adius analysis need calibaion afe he pessue is changed in one o moe ies. The calibaion ouine compues he nominal alue of he esonance fequency and elaie ie adius. Neihe of he poposed mehods includes a sae fo he absolue ie pessue and he sandad fusion fomula canno be applied. Insead a oing scheme has o be applied.

6 Signal sensos Vibaion Analysis Wheel Radius Analysis Voing Scheme Audio/Visual Waning Figue 8: A oing scheme is used fo he fusion of ibaion and wheel adius analysis. Boh ibaion and wheel adius analysis delie wo signals.. A flag i (i,) indicaing which ie is/ae unde-inflaed.. A confidence c i (i,) leel of he fis signal. The oing scheme woks as follows: If boh flags indicae he same unde-inflaion,, he mean of he confidence leels is esed agains a pedeemined heshold, (c +c )/ > h. In case he es is posiie a waning is applied. If he flags indicae diffeen unde-inflaions hey ae eaed sepaaely. Each confidence leel is esed agains a pedeemined heshold c >h and c >h. If any of he ess is posiie a waning is applied. If none of he flags indicaes undeinflaion no waning is applied. An impoan noe hee is ha h, h > h, i.e. if boh ibaion and wheel adius analysis indicae ha he same ie (o ies) is undeinflaed he heshold leel is lowe. This means ha he confidence is highe and smalle pessue changes can be deeced fase and moe accuae. I also impoes he false deecion ae. In Figue 9 waning signals fo he hee diffeen appoaches ae illusaed fo a es un wih % unde-inflaion. The deecion ime fo he fusion algoihm is wihin minue. CONCUSIONS A noel appoach fo deecing low/changed pessue wihou pessue sensos has been oulined. The appoach uses boh ibaion and wheel adius analysis fo opimal pefomance and only eising sensos in moden ehicles ae used. Independenly he appoaches can deec a pessue loss of 5% wihin one minue, bu if hey ae combined a pessue loss of 5% in one, wo, hee o fou ies (diffusion) can be deeced wihin one minue. To combine he wo appoaches a oing scheme was poposed. REFERENCES. F. Gusafsson, M. Deo, U. Fossell, M. ofgen, N. Pesson, and H. Quicklund, Viual senso of ie pessue and oad ficion, SAE pape ,.. F. Gusafsson, S. Ahlqis, U. Fossell, and N. Pesson, Senso fusion fo accuae compuaion of yaw ae and absolue elociy, SAE pape - -6,.. N. Pesson e al., Tie Pessue Esimaion, Inenaional paen applicaion WO8767, N. Pesson and F. Gusafsson, Een Based Sampling wih Applicaion o Vibaion Analysis in Pneumaic Ties, in ICASSP. 5. T. Takeyasu e al., Tie Pneumaic Pessue Deeco, EP 7798, T. Naio e al., Tye ai pessue esimaing appaaus, EP 9596, F. Baun e al., Mehod and appaaus fo monioing he ye pessue of moo ehicle wheels, EP 98987, F. Gusafsson, Slip-based esimaion of ie-oad ficion, Auomaica, (6), 87-99, J.Y. Wong, Theoy of gound ehicles, nd ed. John Wiley and Sons Inc., 99.. T.D. Gillespie, Fundamenals of Vehicle Dynamics, SAE Inenaional, 99.. SchadeBidgepo Sandad Thomson. hp:// Docke Managemen Sysem, Docke numbe NHTSA hp://dms.do.go/, --6. CONTACT Figue 9: Waning signals fo wheel adius analysis, ibaion analysis and fusion of boh mehods. Zeo and one means no waning and waning, especiely. As can be seen in he figue he deecion ime is shoe when boh wheel adius and ibaion analysis is used. Conac auho: Niclas Pesson, M.Sc., inköping Uniesiy, Dep. of E.E., SE-588 inköping, Sweden. Phone: +6 () Mobile: +6 ()7 58. Fa. +6 () pesson@isy.liu.se. UR:

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