SE2050171A1

SE2050171A1 - Method and control arrangement for determining momentary tire wear rate of a wheel of a vehicle

Info

Publication number: SE2050171A1
Application number: SE2050171A
Authority: SE
Inventors: Linus Flodin
Original assignee: Scania Cv Ab
Priority date: 2020-02-17
Filing date: 2020-02-17
Publication date: 2021-08-18
Also published as: DE102021101595A1; SE544696C2

Abstract

A method and control arrangement (4) for determining momentary tire wear rate of a wheel (2) of a vehicle (1). The method comprises calculating (S5), for a plurality of points in time, longitudinal wheel slip values (Kx(tx)) for a wheel slip between a surface of the wheel (2) and a corresponding road surface (10); and determining (S6) a momentary tire wear rate indicator (TW) based on a set of the calculated longitudinal wheel slip values (Kx (tx)) corresponding to different points in time (tx) during a time period (Δt), wherein the momentary tire wear rate indicator (TW) is correlated with momentary tire wear rate established during the time period (Δt).

Description

Method and control arrangement for determining momentary tire wear rate of a wheel of a vehicle Technical Field The present disclosure relates to vehicles, and in particular to methods for determining momentary wear rate of a wheel of a vehicle.

BackgroundHeavy vehicles such as trucks may comprise a tractor, and also one or more trailers. One or two driven axles are used for propelling the vehicle. The drivenaxle(s) may carry a relatively small portion of the weight of the vehicle.Consequently, the normal force on the driven axle(s) available for generatingforce for propelling and braking the vehicle is relatively low compared to the inertiaof the vehicle which may result in high wheel slip. Trucks are also often equippedwith auxiliary brakes in addition to the service brakes. Auxiliary brakes are oftenpart of the drivetrain and thus only act on the driven wheels on the vehicle. lt isdesired to use the auxiliary brakes as much as possible to spare the servicebrakes. lt is also desired to utilize the auxiliary brake(s) of vehicles with brakeenergy regeneration as much as possible, in order to recover energy. However,auxiliary brakes will only act on the driven wheels which may cause a high wheel slip and thereby wear to the tire.

Longitudinal wheel slip is a relative movement between the wheel surface and theroad surface the wheel is rolling on. Longitudinal wheel slip is often denoted as apercentage difference between a longitudinal peripheral speed of the tire and alongitudinal speed of the vehicle itself. When exerting the wheel for a highlongitudinal force relative to the normal force from the ground, there is a risk ofcausing high longitudinal wheel slip, which over time may lead to high tire weardue to increased tire temperature (caused by internal friction in the rubber) andabrasion between tire and road surface. This is especially a problem when drivingwith an unfavorable ratio between drive axle load and train inertia (meaning thatthe drive axle load is low in relation to the train inertia) and/or when driving withtires with relatively low slip stiffness (“soft tires”).

SummaryThus, trucks are often heavy and may carry heavy weight. However, the driven wheels often only carry a small part of this weight. When the trucks are operatedwith high longitudinal forces exerted on the driven wheels for propelling or brakingthe vehicle, the driven wheels may exhibit high wheel slip levels. High longitudinalwheel slip over time may lead to high tire wear. However, the longitudinal slip mayhave different wear effects on the tire depending on e.g. for how long time thewheel has been exposed to the slip, and tire characteristics. lt is an object of the disclosure to determine how sensitive the tire is to wear at acertain point in time when exposed to longitudinal wheel slip. lt is a further objectof the disclosure to detect when the wear becomes high, based on how sensitive the tire is to wear.

These objects and others are at least partly achieved by the arrangement and themethod according to the independent claims, and by the embodiments accordingto the dependent claims.

According to a first aspect, the disclosure relates to method for determiningmomentary tire wear rate of a wheel of a vehicle. The method comprisescalculating, for a plurality of points in time, longitudinal wheel slip values for awheel slip between a surface of the wheel and a corresponding road surface. Themethod further comprises determining a momentary tire wear rate indicator basedon a set of the calculated longitudinal wheel slip values corresponding to differentpoints in time during a time period, wherein the momentary tire wear rate indicator is correlated with momentary tire wear rate established during the time period.

With the method, an indicator of momentary tire wear rate of a tire can bedetermined and analyzed. The momentary tire wear rate describes how sensitivethe tire is to wear at a certain moment in time. Circumstances causing undesirable momentary high tire wear rates may then be mitigated or adapted to avoid excessive tire wear. Thus, the knowledge of an undesirable momentary high tirewear rate can be used to operate the vehicle in such a way that the driven wheelsare not subjected to momentary high wear for long time periods. This can beachieved by indications to the driver, or automatically by limiting an admissiblebraking force or torque on the driven wheels, depending on the determinedmomentary tire wear rate. Momentary tire wear rate could alternatively be referred to as momentaneous or instantaneous tire wear rate.

According to some embodiments, the method comprises performing an actionupon that the momentary tire wear rate indicator meets one or morepredetermined criteria. Thus, an action might be taken in response to thedevelopment of the momentary tire wear rate indicator to avoid high levels.

According to some embodiments, the one or more predetermined criteriacomprises a tire wear rate threshold that the momentary tire wear rate indicatorshould exceed. Thus, if the momentary tire wear rate indicator becomes too high,an action to mitigate the wear may be taken. The action is taken e.g. to generallyreduce the use of auxiliary brakes, or to adapt the speed and/or distance to apreceding vehicle in case using look-ahead functionality (brake-related function),to reduce the need of using auxiliary brakes.

According to some embodiments, the action comprises indicating to the driver thatthe wheel has a high momentary tire wear rate. Thereby, the driver may take anaction to mitigate high wear.

According to some embodiments, the action comprises controlling a brake-relatedvehicle function. According to some embodiments, the controlling comprisesreducing the use of the brake-related vehicle function. According to someembodiments, the brake-related vehicle function comprises one or more auxiliarywheel braking functions. As auxiliary brakes are acting on driven wheels, andservice brakes are acting on all wheels, the braking force is distributed on more wheels when using service brakes which leads to a reduced slip. Thus, by reducing the use of auxiliary brakes when the momentary tire wear rate is high, and instead use service brakes, the tire wear may be reduced.

According to some embodiments, the determining comprises time filtering the setof calculated longitudinal wheel slip values using a sliding window algorithm.Thus, the momentary tire wear rate indicator can then be determined in a timewindow that, for example, has a predetermined length but is continuously updatedin time. The momentary tire wear rate indicator will then be continuously or repeatedly updated.

According to some embodiments, the determining comprises determining themomentary tire wear rate indicator using a tire wear rate model comprising afunction of previous longitudinal wheel slip values over time, wherein the tire wearrate model takes the set of calculated longitudinal wheel slip values as input, andoutputs the momentary tire wear rate indicator. Thus, the behavior of the tire whenit is exposed to wheel slip can be determined in terms of momentary tire wearrate. Thereby, the method takes into account previous or historical exertion tohigh slip levels.

According to some embodiments, the function is a function of previouslongitudinal wheel slip values and slip stiffness over time. Thus, characteristics fordifferent types oftires are taken into account when determining the momentary tire wear rate indicator.

According to some embodiments, the tire wear rate model outputs the momentarytire wear rate indicator as one or more momentary tire wear rate values, whereeach momentary tire wear rate value represents accumulated longitudinal wheelslip values up to and including a longitudinal wheel slip value at a selected point intime. Hence, the model captures the effect of the longitudinal slip over time.

According to some embodiments, the function comprises a weight for eachlongitudinal wheel slip value. Hence, the model is adapted based on tirecharacteristics, and thus more precise.

According to some embodiments, the tire wear rate model is modellingtemperature of the tire over time, and outputs the tire rate wear indicator as one ormore momentary tire wear rate values where each momentary tire wear rate valuerepresents a temperature of the tire at a selected point in time. Hence, the modeldescribes how the temperature of the tire develops for a certain slip level.

According to some embodiments, the tire wear rate model is also based on one ormore forces acting on the tire. The method comprising obtaining one or more tireforces acting on the wheel during the time period, and using the obtained one ormore tire forces as input to the tire wear rate model. Hence, the model takes tireforces into account that may affect the determination of the momentary tire wearrate indicator.

According to some embodiments, the tire wear rate model is also based onambient temperature. The method comprises obtaining ambient temperateindicating the ambient temperature during the time period, and using the obtainedambient temperature as input to the tire wear rate model. Hence, the model takesambient temperature that may affect the determination of the momentary tire wear rate indicator into account.

According to some embodiments, the method comprises obtaining wheel speedproperties indicative of wheel speed of a wheel, obtaining vehicle speedproperties indicative of corresponding speed of the vehicle, and wherein thecalculating comprises calculating the Iongitudinal wheel slip values based on thewheel speed properties and the corresponding vehicle speed properties. Thus, values for calculating the Iongitudinal wheel slip values are obtained.

According to some embodiments, the obtaining of wheel speed propertiescomprises obtaining wheel speed properties indicative of a predicted wheel speedof the wheel along a route on a road ahead; the obtaining of vehicle speedproperties comprises obtaining vehicle speed properties indicative ofcorresponding vehicle speed on a route on a road ahead; and wherein thecalculating comprises calculating future longitudinal wheel slip values based onthe wheel speed properties and the corresponding vehicle speed properties.Thus, values for calculating the future longitudinal wheel slip values are obtained.

According to a second aspect, the disclosure relates to a control arrangement fordetermining momentary tire wear rate of a wheel of a vehicle, wherein the controlarrangement is configured to execute the method according to any one of thepreceding claims.

According to a third aspect, the disclosure relates to a vehicle comprises a controlarrangement according to the second aspect, and a wheel speed sensorconfigured to sense a speed indicative of the speed of a drive wheel.

According to a fourth aspect, the disclosure relates to a computer programcomprising instructions, which, when the program is executed by a controlarrangement, cause the control arrangement to carry out the method according tothe first aspect.

According to a fifth aspect, the disclosure relates to a computer-readable mediumhaving stored thereon the computer program according to the fourth aspect.

Brief description of the drawinqs Fig. 1 illustrates a vehicle according to an example embodiment.

Fig. 2 illustrates forces acting on a driven wheel and resulting speeds duringoperation of the vehicle.

Fig. 3 illustrates a flow chart of a method for determining momentary tire wear rateindicator of a wheel of a vehicle according to some embodiments.

Fig. 4 is a graph of slip for different types of tires.

Fig. 5 is a graph of slip stiffness for different types of tires.

Fig. 6 is a graph of tire temperature for different types of tires.

Fig. 7 illustrates a vehicle approaching a downhill.

Fig. 8 illustrates a control arrangement for determining momentary tire wear rate indicator of a wheel of a vehicle according to some embodiments.

Detailed descriptionln the following disclosure a method and control arrangement for determining a momentary tire wear rate indicator for a wheel of a vehicle are described. Themethod is especially relevant for heavy vehicles. Thus, the vehicle is for examplea heavy vehicle, such as a truck, that typically comprises a tractor and optionallyone or more trailers. The driven wheels of the vehicle may be exposed to highwear, and the present method aims to detect such situations by estimating themomentaneous rate of the tire wear. As previously described, the momentary tirewear rate describes how sensitive the tire is to wear at a certain moment in time.Thus, different tires may be differently sensitive to wear, and thus have differentmomentary tire wear rates during similar operating conditions. A high momentarytire wear rate should be avoided, but may be acceptable for a short while, until theaccumulated wear of the tire is considered too large. On the other hand, a lowermomentary tire wear rate may be acceptable for a longer time, as it takes longer time for the accumulated wear to reach a high value. ln the following a vehicle, where the herein disclosed method and controlarrangement may be implemented, will be described. The vehicle will beexplained with reference to the illustrated vehicle in Fig. 1, and thereafter forcesacting on a driven wheel and resulting speeds during operation of the vehicle will be explained with reference to Fig. 2.

The vehicle 1 in Fig. 1 is a heavy vehicle, also referred to as a heavy commercialvehicle, being operated along a road surface 10. The vehicle 1 has one drivenaxle (not shown) arranged for transferring power from an engine (not shown) to two driven wheels; the foremost located wheels of the vehicle. Thus, the drivenaxle is connected to and driven by the drivetrain of the vehicle. Only one of thetwo driven wheels 2 is illustrated in the figure. The vehicle 1 also has one rear tagaxle that is used to support the weight of the vehicle 1. This rear axle is connectedto two wheels 3. The rear axle is not connected to the drivetrain. The wheels 2, 3have tire surfaces 11 that contact the road surface 10. lt should be understoodthat this vehicle 1 is only one example, and the proposed solution may of coursealso be used in a vehicle 1 comprising more than two axles, more driven axlesand wheels, and/or which comprises axles with differentials etc. A wheel is hereindefined to comprise a rim and a tire. A tire is the rubber part of the wheel thatgrips the road.

The vehicle 1 comprises a control arrangement 4 configured to implement theproposed technique. The control arrangement 4 will be more explained in thefollowing, but may be implemented as an Electronic Control Unit, ECU. Thevehicle 1 also comprises sensors 5 such as a vehicle speed sensor configured tosense the speed of the vehicle, a weight sensor configured to sense the weight ofthe vehicle 1 or at least the load of the vehicle 1. The weight of the vehicle 1without load is normally a parameter that is known and saved in a memory in thevehicle 1. Thus, as the total weight of the vehicle 1 is known, the weight on eachwheel 2, 3 of the vehicle 1 may be determined and thus also the resulting normal force acting on each wheel 2, 3.

An ECU is basically a digital computer that controls one or more electricalsystems (or electrical sub systems) of the vehicle 1 based on e.g. informationread from sensors and meters placed at various parts and in different componentsof the vehicle 1. ECU is a generic term that is used in automotive electronics forany embedded system that controls one or more functions of the electrical systemor sub systems in a transport vehicle. A vehicle typically comprises a plurality ofECUs that communicate over a Controller Area Network, CAN. The CAN is a network that is used to handle communication between the various control arrangements in the vehicle 1. Alternatively, the vehicle 1 may use an ethernet based protocol for communication. ln Fig. 2, depicting one of the drive wheels 2 of the vehicle 1 in Fig. 1, a load forcem and a corresponding normal force FN acts on the wheel 2. The driven axlemakes the wheel rotate with an angular velocity w. The effective radius of thewheel 2 is denoted r. The wheel center speed and its direction are illustrated as avector u. Thus, u represents the velocity of the vehicle 1 in relation to the roadsurface 10. The free-rolling speed of the wheel 2 is the speed at which the wheel(and tire) would spin if no brake or drive force are applied to it. The wheel 2 isexerted to a longitudinal force f; and thereby spins at a speed different from its“free-roiiing speed”. The iongitudinai force F; is acting on the tire surface ”itreiative to the road surface 10. The iongitudinai force f; creates friction anddeformation of the rubber of the fire. The longitudinal force Fx may be known inbeforehand or estimated from forces exerted by drivetrain and service brakes(measured with one or more force sensors 5c, Fig. 8). The difference in speeds isdescribed as longitudinal wheel slip, orjust “slip ratio”. Hence, a iongitudinaiwheei siip K' of the tire can be defined as the difference between the tire tangentiaispeed (w - r) and the speed of the axle (u) relative to the road surface 10. The longitudinal wheel slip K, or siip ratio, may be determined as: which gives a value of the wheei K slip in a range between fi. A siip ratio inpercent is obtained by muitipiying the vaiue with 1GB. When the iongitudinai siip iszero there is no siip. When the slip is non-zero there is thus a siip. Theiongitudinai siip may be considered high when it is ciose to +1 or -1. A wheei siipmeans iost traction on the road. A positive iongitudinal wheei siip means that thewheei 2 is spinning, typicaiiy caused by appiying too much throttle. A negativeiongitudinal wheel siip means that wheei 2 is skidding, typicaily caused byappiying too much braking force.

En the following, a method for determining momentary tire wear rate indicators of awheel of a vehicle, typically a drive wheel of a vehicle, using longitudinal wheelslip, will be explained. The vehicle is for example the vehicle illustrated in Fig. 1.The method will be explained with reference to the flow chart in Fig. 3, thediagrams in Figs. 4-6 and the illustration in Fig. 7. The method may beimplemented as a computer program comprising instructions, which, when theprogram is executed by a control arrangement, cause the control arrangement tocarry out the method. The control arrangement may be a control arrangement 4 ina vehicle 1 as illustrated in Fig. 1. Alternatively, the control arrangement 4 may bea control arrangement that is remote from the vehicle 1, typically an off-boardcomputer. The method may be executed using real-time sensed values from thevehicle 1, or using values that are obtained from a vehicle model that models thebehavior of the vehicle when driving along a road. lt is then possible to predict themomentary tire wear rate along an upcoming or simulated route. The method isexplained in relation to one wheel, but it should be understood that the methodmay be applied to all driven wheels of a vehicle in parallel.

The method makes use of longitudinal wheel slip values K(tx). These values maybe known in beforehand in the vehicle 1, or calculated from measured orestimated values of the wheel speed and vehicle speed. ln order to determine momentary tire wear rate indicators in advance, the futureroute needs to be known. The method may then include obtaining S0 informationof the road ahead, where the information includes route information. The routeinformation may include geographical data and altitude data of the route ahead.Upcoming braking forces may be predicted using the weight of the vehicle and thealtitude of the route, and a vehicle model. The vehicle model models how muchforce the wheel is exposed to, to keep a certain speed given a certain roadinclination, etc., as known in the art. With the assumption that the vehicle shallhave a certain speed along the route, or not go over a certain speed, or otherassumption, it is possible to calculate that the wheel will be exposed to a certain sequence of longitudinal forces and thereby a sequence of slip levels during a certain time. The vehicle speed may thus be set in advance, and the predictedwheel speed determined from the calculated longitudinal forces etc. Thus, insome embodiments, the method comprises obtaining S1 wheel speed propertiesindicative of a predicted wheel speed of the wheel 2 along the route on a roadahead. Further, the method comprises obtaining S2 vehicle speed properties indicative of corresponding vehicle speed on the route on a road ahead. ln case the method is performed while driving, the wheel speed and the vehiclespeed may already be available in the vehicle on a communication network in thevehicle 1 such as CAN. Thus, the method may include obtaining S1 wheel speedproperties indicative of wheel speed of the wheel 2, and obtaining S2 vehiclespeed properties indicative of corresponding speed of the vehicle 1. Alternatively,they may be determined. The wheel center speed u of the vehicle 1 correspondsto the speed of the vehicle 1, and may be measured using another speed sensor,estimated using the position of the vehicle using the Global Positioning System(GPS), or estimated using radar measurements etc. The wheel center speed urepresents the tangential free rolling speed of the wheel 2. Hence, the sensedwheel speed together with knowledge or estimations of the longitudinal forcesexerted on the driven wheel (from drivetrain and service brakes), the free rollingwheel speed can be estimated, and thus the wheel center speed. The methodmay then comprise obtaining S3 one or more tire forces F(t) acting on the wheel2. ln some embodiments, the method may make use of the temperature of theambient, i.e., the environment. The method may then comprise obtaining S4ambient temperate indicating the ambient temperature during a time period. Theambient temperature is for example measured using a temperature sensor in the vehicle 1.

The longitudinal wheel slip values K(1:x) are for example calculated using theequation (1), using the obtained properties. The effective radius r of the vehicle 1 is a known parameter in the vehicle 1 and the angular velocity w may be measured using a wheel speed sensor. ln other words, the method comprisescalculating S5 for a plurality of points in time, longitudinal wheel slip values K(tx)for a wheel slip between a surface of the wheel 2 and a corresponding roadsurface 10. The wheel 2 is here a driven wheel of the vehicle.

From the step S5, a plurality oftime dependent longitudinal slip values K(tx) areobtained. The step S5 may be continuously performed during operation of thevehicle 1, or simulated or predicted for a future road ahead and then calculatingfuture longitudinal wheel slip values. The method further comprises determiningS6 a momentary tire wear rate indicator TW based on a set of the calculatedlongitudinal wheel slip values K(tx) corresponding to different points in time txduring a time period At. The determining is typically based on a model thatdescribes how wear properties of a tire is affected by longitudinal wheel slip. Theset may be a part of, or all of, the calculated longitudinal wheel slip values K(tx)calculated during the time period At. lt may for example be every second value,every third value, etc., and may be adapted based on the computational load.However, the set should represent the time period At. Every calculatedlongitudinal wheel slip value K(tx) corresponds to a unique point in time duringthe time period At. The momentary tire wear rate indicator TW is correlated withmomentary tire wear rate established during the time period A. The momentarytire wear rate indicator indicates how fast the tire is worn out during the timeperiod. The momentary tire wear rate indicator TW may include one or moremomentary tire wear rate values established during the time period At, or be afunction of the momentary tire wear rate during the time period At. Themomentary tire wear rate indicator TW may be a maximum or average value ofthe momentary tire wear rate values determined during the time period At. Duringthe time period At, a plurality of momentary tire wear rate values may beestablished. Data may be collected from a plurality of such time periods At. Thetime period At may have the same length as the time period for driving a route of the vehicle 1, thus, from a start of operation until the vehicle 1 stops operating, or a predicted time for driving a predicted route ahead. Alternatively, the time period A1: may have predetermined length and be repeated continuously along the route.

Momentary (or instantaneous) tire wear rate is an estimate of how fast the tire isbeing worn out at a given time point, a particular instance, or for a short timeperiod. A high rate means that the tire is worn out fast, a low rate means that thetire is worn out at a slower pace. The momentary or instantaneous tire wear rateis thus an estimate of the rate of the tire wear at a given instance. lt describeshow sensitive the tire is to wear at a certain point in time. The momentary tirewear rate indicates the resistance or sensibility of the tire to wear at a certain timepoint. lt also describes how durable the tire is to wear at a certain time point. Fig.5 illustrates slip stiffness for a plurality ofdifferent tires A, B, C: a soft tire A, amiddle-soft tire B and a hard tire C. The soft tire A has a flatter inclination than thetires B and C, and thus a higher slip for the same longitudinal force. This means ahigher heat development because of inner friction of the tire A, which means ahigher wear for the same longitudinal force than tire B and C. At lower normalforce FN, the inclination becomes flatter. At high normal force (heavy vehicle), it isneeded to stay higher up on the curve to generate the same braking power FZ,which gives a higher heat development and thus higher wear. Generally, heavyvehicles have a relatively low normal force FN on driven wheels, and thus often problem with high heat development in their driven wheels.

As explained, the longitudinal slip values may be continuously, continually orregularly calculated while driving based on measured values or based on avehicle model. Fig. 4 is a graph of longitudinal wheel slip K(tx) for different typesoftires A, B, C over time tx. For a soft tire as illustrated by A, the slip is typicallyhigher than for a hard tire C or a semi-hard tire B. The determining S6 is typicallyalso repeatedly performed, for example at each time instant tx. ln someembodiments, the determining S6 comprises time filtering the set of calculatedlongitudinal wheel slip values K(tx) using a sliding window algorithm. Then, longitudinal wheel slip values within the sliding window are used for determining the momentary tire wear rate indicator. The time period At will then have the samelength as the length of the sliding window. The sliding window may have a lengthof for example 1-10 minutes, more specified between for example 2-5 minutes. ltwill then capture the calculated longitudinal wheel slip values K(tx) from the last1-10 (or 2-5) minutes of travelling, or alternatively for the upcoming 1-10 (or 2-5)minutes of travelling if predicting using e.g. look-ahead functionality. The timefiltering is then continuously performed during actual or predicted travelling alongthe road. Thus, the longitudinal wheel slip values are continuously calculated, andthe sliding window algorithm selects which values that should be used for determining the momentary tire wear rate indicator.

The momentary tire wear rate indicator TW may be determined using a tire wearrate model. The tire wear rate model models how sensitive the tire is to wear at acertain point in time when exposed to longitudinal wheel slip. The tire wear ratemodels how internal friction develops in the tire when exerted to longitudinal slip.ln some embodiments, the determining S6 comprises determining the momentarytire wear rate indicator using a tire wear rate model comprising a function TW(tx) = f(Kx(t1: tx)) of previous longitudinal wheel slip values K(tx) over timetx. The tire wear rate model takes the set of calculated longitudinal wheel slipvalues K(tx) as input, and outputs the momentary tire wear rate indicator TW. lfthe time period for example comprises ten time points, then x will be incrementedfrom zero to 10 by one. ln some embodiments, the momentary tire wear rateindicator TW is calculated as the accumulated sum of previous longitudinal wheelslip values K(tx) during the time period. So, if the time period comprises threetime points, the function TW(tx) = f(Kx(t1: tx)) will give TW(1:1) = Kx(t1),TW(1:2) = TW(1:1) + Kx(t2) and TW(1:3) = TW(t2) + Kx(t3). The momentary tirewear rate indicator TW may then include all these values. When using a slidingwindow algorithm, the time point 1:1 is typically the first time point in the window. lfthe time period is the whole length of the route, then 1:1 may be the starting pointof the route. tx will go from “1” to the end time point of the time period. Turning to Fig. 4 and using the tire wear rate model, the momentary tire wear rate indicator TW is calculated as the accumulated sum of previous longitudinal wheel slipvalues K(tx) during the time period. Thus, if the time period At comprises the timepoints 1:1, 1:2 and 1:3, the momentary tire wear rate indicator TW here in thisexample comprises three values: for 1:1 the value is K(t1), for 1:2 it is K(1:1) +K(t2), and for 1:3 it is K(t1) + K(t2) + K(t3). However, the indicator may be onlya subset of the values, for example the maximum value of the values. Thus, insome embodiments, the tire wear rate model outputs the momentary tire wearrate indicator TW as one or more momentary tire wear rate values TW(tx) whereeach momentary tire wear rate value TW(tx) represents accumulated longitudinalwheel slip values up to and including a longitudinal wheel slip value TW(tx) at aselected point in time tx. The selected point in time is in the illustrated example then any of the time points 1:1, 1:2 and 1:3.

As explained, there are different kinds of tires characterized as soft or hard orsomething in-between. lt is possible to take such characteristics into account bymeans of slip stiffness, S, in determining the momentary tire wear rate indicator.Slip stiffness is defined as the ratio between the normalized longitudinal force Fx(thus divided with the normal force FN) and wheel slip K(tx). As previouslydescribed, Fig. 5 is a graph of slip stiffness Sfor different types of tires A, B, C.Thus, in some embodiments, the function is a function TW(1;x) = f(Kx(1:1: tx), S)of previous longitudinal wheel slip values K(tx) and slip stiffness S over time tx.ln more detail, based on the slip stiffness S, a weight W(Kx(tx),S) for eachlongitudinal slip value may be determined. Thus, in some embodiments, thefunction comprises a weight W(Kx(tx),S) for each longitudinal wheel slip valueK(tx). The weight may be positive or negative and may increase for an increasedvalue of K(tx). The arrows in the figure illustrate how the weight may increase foran increased longitudinal slip. Alternatively, the weight may be independent fromthe longitudinal wheel slip and/or the slip stiffness, and for example be constant.Generally, the weight is intended to mirror when it is desired to increase ordecrease the tire wear indicator. The weight should be positive when the wear ofthe tire increases, and negative when the wear decreases. The weight should be greater the faster the wear increases (meaning that the momentary tire wear rateindicator becomes greater, typically more than a predetermined threshold). Thismeans that the weight typically is negative when the wheel is free rolling, butpositive when the momentary tire wear rate indicator is greater than thepredetermined threshold (then, there is heat development in the tire). When thetire rate indicator increases from a previous tire rate indicator value, the When thevehicle is driven in steady-state (typically constant speed driving, e.g. high-waydriving), the weight should be close to zero, or zero. For example, if the timeperiod At comprises the time points 1:1, 1:2 and 1:3, the momentary tire wear ratevalue for 1:1 is TW(1:1) = TW(1:0) + K(t1) - W(Kx(t1), S), for 1:2 the momentarytire wear rate value is TW(1:2) = TW(1:1) + K(t2) - W(Kx(t2), S) and for 1:3 themomentary tire wear rate value is TW(1:3) = TW(1:2) + K(t3) - W(Kx(t3), S). Themomentary tire wear rate indicator TW may be determined for example as all thevalues, the maximum value or an average value of the values, etc. As a summary,for each time point tx there is a certain longitudinal slip value Kxütx). The slipstiffness S, has previously been estimated and may be considered constant duringthe time period At in question for the accumulation. The weight W is a function ofslip stiffness S and longitudinal slip Kxüïx). For each time point tx, a momentarytire wear rate TW(tx) is accumulated, during the time period, that correlates orgives an indication of the rate of tire wear. Thus, for each time point tx, a newmomentary tire wear rate TW(1:x) is accumulated and thus determined. lt hasbeen established through experiments and simulations that this accumulatedvalue has a strong correlation to how fast the tire is worn out at the present timepoint. Thus, using the present slip, high tire wear can be recognized using a tire wear model by accumulating a weighted longitudinal slip level over time.

The above-mentioned model describes a relation between longitudinal slip andmomentary tire wear rate, that is correlated with how internal friction develops inthe tire when exerted to longitudinal slip. ln the following another model will bedescribed, where the actual temperature of the tire is modelled. ln theseembodiments, the tire wear rate model is a temperature model. This tire wear rate model is modelling temperature of the tire over time, and outputs the tire rate wearindicator TW as one or more momentary tire wear rate values TW(tx) where eachmomentary tire wear rate value TW(tx) represents a temperature of the tire at aselected point in time tx. Fig. 6 is a graph of tire temperature for different types oftires A, B, C. ln this embodiment, the tire wear rate model outputs an estimatedtemperature of the tire for each time point. Also in this embodiment, the function isa function TW(tx) = f(Kx(t1: tx), S) of previous longitudinal wheel slip valuesK(tx) and slip stiffness S over time tx. For each time point tx, the modelestimates a tire temperature T. The temperature is correlated with the momentarytire wear rate. Typically, the momentary tire wear rate indicator TW comprises thelatest estimated temperature during the time period At. ln more detail, thelongitudinal wheel slip Kx(tx) and the slip stiffness S are used to model how thetire is heated up and cooled (typically the tire is heated when exposed to high sliplevels and is cooled if exposed to low or no force or if the vehicle is standing still).The tire wear rate model may also include parameters with coefficientsdetermining how fast the tire is heated up and how fast it is cooled down. Thesecoefficients may be used to determine weights W(Kx(tx)) to be used in thecalculations. For example, if the weight is positive, the momentary tire wear ratevalue TW(tx) will increase (the tire is heated up), and if the weight is negative, themomentary tire wear rate value TW(1;x) will decrease (the tire is cooled). ln someembodiments, the tire wear rate model is also based on one or more forces actingon the tire. The method then comprises obtaining S3 one or more tire forces F(t)acting on the wheel 2 during the time period and using the obtained one or moretire forces F(t) as input to the tire wear rate model. The tire forces are forexample the longitudinal force Fx and the normal force FN (see Fig. 2). The modelmay also use ambient temperature and/or speed of the vehicle. Thus, in someembodiments, the tire wear rate model is also based on ambient temperature, andwherein the method comprising obtaining S3 ambient temperate indicating theambient temperature during the time period, and using the obtained ambienttemperature as input to the tire wear rate model. The ambient temperature is for example measured using a temperature sensor in the vehicle. Hence, in summary, the tire wear rate model may estimate the tire temperature as a functionof Iongitudinal wheel slip, slip stiffness, one or more forces acing on the driven wheel 2 and ambient temperature, over time.

The momentary tire wear rate indicator may be used for a plurality of purposes.For example, the method may comprise determining a trend of the tire wear ratefrom the momentary tire wear rate indicator or indicators. Thereby, is can berecognized e.g. when high momentary tire wear rate occurs. ln someembodiment, the method comprises performing S7 an action upon that themomentary tire wear rate indicator TW meets one or more predetermined criteria.For example, the one or more predetermined criteria may comprise a tire wearrate threshold that the momentary tire wear rate indicator TW should exceed.When the threshold is exceeded, one or more actions are performed. The tirewear rate threshold is typically predetermined. The tire wear rate threshold mayfor example be related to a maximum allowed accumulated tire wear rate thatshould not be exceed. When the threshold is exceeded, one or more actions areperformed. The aim of the action is typically to make the momentary tire wear rateindicator TW to go below the threshold. The value or values of the momentary tirewear rate indicator may thus be compared to the threshold as soon as they areestablished, and as soon as a value goes above the threshold, one or moreactions may be performed. ln one embodiment, the method comprisesdetermining the maximum of the momentary tire wear rate values determinedduring the time window and determining if this value exceeds the tire wear rate threshold. lf exceeding the threshold, the method includes performing an action.

The action may comprise indicating S7a to the driver that the wheel 2 has a highmomentary tire wear rate. The indication is for example a voice indication, agraphical indication, or a tactile indication (via a suitable interface such as aloudspeaker, dashboard, mobile device or similar). ln response, the driver mayreduce use of auxiliary wheel braking functions, depending on the drivingsituation. By the actions performed by the driver, the momentary tire wear rateshall be reduced (as the Iongitudinal force on driven wheels will decrease and thus also the Iongitudinal wheel slip). This may also be indicated to the driver ifsuccessful, that is, that the wear is reduced. Alternatively, or in combination withthe indicating, the action comprises controlling S7b a brake-related vehiclefunction. For example, the controlling S7b comprises reducing the use of thebrake-related vehicle function. The brake related functions may for exampleinclude adaptive speed control and constant speed control. The brake-relatedvehicle function typically comprises one or more auxiliary wheel braking functions.The auxiliary wheel braking functions comprises for example retarder and exhaustbrake. Thus, when using adaptive speed control and constant speed control, theretarder or exhaust brake is typically used, or a brake blending of the retarder andthe exhaust brake (and the conventional braking system including the servicebrakes). The controlling S7b may be automatically performed by the vehicle 1.Thus, when controlling S7b the vehicle 1 to use the auxiliary brakes less, the conventional braking system is used more.

Fig. 7 is illustrating a vehicle 1 at a time point t0 just before a downhill start. lnorder to avoid excessive wear, the method may be executed before the downhillstarts, and a control strategy for the vehicle 1 may be determined in advance,based on the result of the method, that mitigates or prevents momentary tire wearrate indicators over the threshold. The control strategy may include speed,braking force, throttle commands etc. along the upcoming route. The time periodAt should preferably capture events such as a downhill or an uphill, e.g. thedownhill 13 in Fig. 7. The time period is here from t1 to t4, and x = 1: 4. Betweeneach time point there is one minute, and the time period At is thus three minutes.lf using a sliding window, the length of the sliding window should be at least threeminutes, to capture the downhill, such that it is possible to adapt the control of thevehicle 1 before the downhill. However, this is merely an example to illustrate theprinciple, and typically there are many more time points than the four time pointsillustrated, for example one every second. ln some embodiments, the time periodAt (and sliding window) has such length such that it captures the vehicle 1 beforethe downhill 13 (or uphill), in order to adapt the behavior of the vehicle 1 beforethe downhill (or uphill) starts.

Upcoming slip levels may be predicted from estimated slip stiffness of the wheeland upcoming braking forces the wheel will be exerted to along the route. Apredicted momentary tire wear rate indicator may be determined in beforehandand compared with different alternative scenarios, for example such that thevehicle speed is decreased before the downhill. The vehicle may then increase itsspeed in the downhill without braking it, or with a reduced need to brake it. Thedetermining of predicted momentary tire wear rate may be incorporated with otherdriver assistance functionality where the vehicle behavior depends on predictionsof the road ahead, such as cruise control with active prediction. Avoiding high tirewear could be a factor that is taken into account when planning braking orpropelling the vehicle 1 ahead. Alternatively, the longitudinal wheel slip values arecalculated using real sensed values, and the control of the vehicle 1 is configured “on the fly", to avoid more tire wear.

The disclosure also relates to a control arrangement for determining momentarytire wear rate indicators of a wheel 2 of a vehicle 1. Fig. 8 illustrates such a controlarrangement 4. The control arrangement 4 may be arranged in a vehicle 1, or inan off-board remote computer. The control arrangement 4 is configured toexecute the method according to any one of the embodiments as explainedherein. The control arrangement 4, or more specifically a processor 6 of thecontrol arrangement 4, is configured to cause the control arrangement 4 toperform all aspects of the method described herein. This is typically done byrunning computer program code stored in a memory 6 in the processor 5 of thecontrol arrangement 4. The computer program may also be stored on a computer-readable medium. ln some embodiments, the control arrangement 4 is a “unit” ina functional sense. Hence, in some embodiments the control arrangement 4 is acontrol arrangement comprising several physical control arrangements thatoperate in corporation. The control arrangement 4 comprises hardware andsoftware. The hardware comprises various electronic components on PrintedCircuit Board, PCB. The most important of those components is typically one ormore processors e.g. a microprocessor, along with memory e.g. EPROM or a Flash memory chip. ln this example, the control arrangement 4 comprising theprocessor 6, the memory 7 and a communication interface 8. The processor 6may include one or processing units. The memory 7 may include one or morememory units. The communication interface 8 is for example configured tocommunicate with other devices e.g. with other subsystems of the vehicle 1 orwith devices outside the vehicle 1. lf the control arrangement 4 is arranged remotefrom the vehicle 1, the communication interface 8 may be configured tocommunicate with devices in the vehicle 1. The software (also called firmware) istypically lower-level software code that runs in the microprocessor. The controlarrangement 4 is for example configured to receive and/or collect data from oneor more sensors 5 in the vehicle 1, for example one or more wheel speed sensors5a, a vehicle speed sensor 5b and/or one or more force sensor 5c. The wheelspeed sensor 5a is configured to sense a speed indicative of the speed of a drivewheel 2. The vehicle 1 may comprise one such sensor for each drive wheel of thevehicle 1. The control arrangement 4 may also be configured to communicatedata e.g. control data to a vehicle function 12 of the vehicle 1. The controlarrangement 4 may also be configured to communicate an indication to the drivervia an interface such as an indication device 9. The indication device 9 is forexample a loudspeaker, a dashboard, a mobile device or similar.

The present disclosure is not limited to the above-described preferredembodiments. Various alternatives, modifications and equivalents may be used.Therefore, the above embodiments should not be taken as limiting the scope ofthe disclosure, which is defined by the appending claims.

Claims

claims

1. A method for determining momentary tire wear rate of a wheel (2) of a vehicle (1), the method comprising: - calculating (S5), for a plurality of points in time, Iongitudinal wheel slip values(Kx(1:x))) for a wheel slip between a surface of the wheel (2) and acorresponding road surface (10); and - determining (S6) a momentary tire wear rate indicator (TW) based on a set ofthe calculated Iongitudinal wheel slip values (Kx(tx))) corresponding todifferent points in time (tx) during a time period (At), wherein the momentarytire wear rate indicator (TW) is correlated with momentary tire wear rate established during the time period (At).

2. The method according to claim 1, comprising- performing (S7) an action upon that the momentary tire wear rate indicator (TW) meets one or more predetermined criteria.

3. The method according to claim 2, wherein the one or more predeterminedcriteria comprises a tire wear rate threshold that the momentary tire wear rate indicator (TW) should exceed.

4. The method according to claim 2 or 3, wherein the action comprises indicating(S7a) to the driver that the wheel (2) has a high momentary tire wear rate.

5. The method according to any one of the claims 2 to 4, wherein the actioncomprises controlling (S7b) a brake-related vehicle function.

6. The method according to claim 5, wherein the controlling (S7b) comprises reducing the use of the brake-related vehicle function.

7. The method according to claim 5 or 6, wherein the brake-related vehicle function comprises one or more auxiliary wheel braking functions.

8. The method according to any one of the preceding claims, wherein thedetermining (S6) comprises time filtering the set of calculated longitudinal wheel slip values (Kx(tx)) using a sliding window algorithm.

9. The method according to any one of the preceding claims, wherein thedetermining (S6) comprises: - determining the momentary tire wear rate indicator using a tire wear ratemodel comprising a function (TW(1:x) = f(Kx(t1: tx)) of previous longitudinalwheel slip values (Kx(tx))) over time (t(x)), wherein the tire wear rate modeltakes the set of calculated longitudinal wheel slip values (Kx(1;x)) as input, and outputs the momentary tire wear rate indicator.

10. The method according to claim 9, wherein the function is a function ((TW(tx) = f(Kx(t1: tx),S)) of previous longitudinal wheel slip values (Kx(tx)) and slip stiffness (S) over time (tx).

11. The method according to claim 9 or 10, wherein the tire wear rate modeloutputs the momentary tire wear rate indicator (TW) as one or moremomentary tire wear rate values (TW(tx)) where each momentary tire wearrate value (TW(1:x)) represents accumulated longitudinal wheel slip values upto and including a longitudinal wheel slip value (Kx(tx)) at a selected point in time (tx).

12.The method according to claim 10 and 11, wherein the function comprises a weight (W(Kx(tx))) for each longitudinal wheel slip value (Kx(tx)).

13.The method according to claim 10, wherein the tire wear rate model is modelling temperature of the tire over time, and outputs the momentary tirerate wear indicator (TW) as one or more momentary tire wear rate values(TW(1:x)) where each momentary tire wear rate value (TW(1:x)) represents a temperature of the tire at a selected point in time (t(x)).

14.The method according to claim 13, wherein the tire wear rate model is also based on one or more forces acting on the tire, and wherein the methodcomprising: - obtaining (S4) one or more tire forces (F(tx)) acting on the wheel (2) duringthe time period, and using the obtained one or more tire forces (F(tx)) as input to the tire wear rate model.

15.The method according to claim 12 or 13, wherein the tire wear rate model is also based on ambient temperature, and wherein the method comprising:- obtaining (S3) ambient temperate indicating the ambient temperature duringthe time period, and using the obtained ambient temperature as input to the tire wear rate model.

16.The method according to any one of the preceding claims, comprising - obtaining (S1) wheel speed properties indicative of wheel speed of the wheel(2): - obtaining (S2) vehicle speed properties indicative of corresponding speed ofthe vehicle (1 ); and wherein the calculating (S5) comprises calculating thelongitudinal wheel slip values based on the wheel speed properties and the corresponding vehicle speed properties.

17.The method according to claim 16, wherein the obtaining (S1) of wheel speed properties comprises obtaining wheel speed properties indicative of apredicted wheel speed of the wheel (2) along a route on a road ahead; andwherein the obtaining (S2) of vehicle speed properties comprises obtainingvehicle speed properties indicative of corresponding vehicle speed on a routeon a road ahead; and wherein the calculating (S5) comprises calculating futurelongitudinal wheel slip values based on the wheel speed properties and the corresponding vehicle speed properties.

18. A control arrangement (4) for determining momentary tire wear rate of a wheel(2) of a vehicle (1), wherein the control arrangement (4) is configured toexecute the method according to any one of the preceding claims.

19. A vehicle (1) comprising- a control arrangement (4) according to claim 18, and- a wheel speed sensor (5a) configured to sense a speed indicative of thespeed of a drive wheel (2).

20.A computer program comprising instructions, which, when the program isexecuted by a control arrangement (4), cause the control arrangement (4) tocarry out the method according to any one of the claims 1 to 17.

21. .A computer-readable medium having stored thereon the computer program of claim 20.