WO2009080409A1

WO2009080409A1 - A method and a device for checking a vehicle efficiency

Info

Publication number: WO2009080409A1
Application number: PCT/EP2008/065495
Authority: WO
Inventors: Remo Corghi
Original assignee: Corghi SpA
Current assignee: Corghi SpA
Priority date: 2007-12-19
Filing date: 2008-11-13
Publication date: 2009-07-02
Anticipated expiration: 2010-06-19
Also published as: ITRE20070128A1

Abstract

A method and a device for checking a vehicle, the method comprising a stage of measuring a wear parameter of at least one tyre of the vehicle, and a stage of using the measurement to estimate a limit to future use of the vehicle, up to which limit the vehicle can be predicted to remain efficient.

Description

A Method and a Device for Checking a Vehicle Efficiency

Technical Field

This invention concerns a method and a device for checking the efficiency of vehicles. Background Art

As is known, a vehicle's efficiency is strongly influenced by tyre wear, upon which many important factors affecting road-holding and driving comfort depend, such as for example the vehicle's grip on the road, and wheel drift. Wheel drift is the phenomenon whereby some vehicles tend to move spontaneously to the left or to the right even when they are proceeding with wheels oriented straight ahead on a perfectly level surface. Wheel drift can be due to uneven wear on the tyres, the rolling of which on the road surface generates a system of uncompensated forces and moments, which can make the vehicle deviate from the desired direction. Wheel drift can also have constructional causes, such as for example the conicity of the wheels and/or the conformation of the layers of rubber in the tyres: such causes are however not constant but vary as tyres wear over time.

That is why it is very important to check tyre wear periodically, for example in order to replace excessively worn tyres or to modify the relative positioning of the wheels on a vehicle, so as to reduce the negative effects worn tyres have on a vehicle's dynamic behaviour and thus on its efficiency. A correct, more even tyre wear also enables a vehicle's efficiency to be improved by reducing fuel consumption and the emission of pollutants caused by rubber dust, which as a result of wear is released into the atmosphere. Tyre wear can be checked by measuring one of the effects it causes, with the variation of the effect being used over time to evaluate the progress of wear and future wear tendencies.

The effects of wear can either be dimensional, for example the average tread depth (which is dependent on wear), or they can be measurable physical effects which occur while the tyre is in use, for example the effect of wheel drift.

Thus wear is expressed numerically by means of parameters which are characteristic of these effects, for example direct measurements (e.g. average tread depth) or alternatively, by indices which are calculated on the basis of one or more measurements using appropriate mathematical formulae (e.g. drift indices).

Merely as an example, wheel drift can be measured using special balancing machines which, in addition to being provided with a rotating shaft onto which the wheel to be measured is keyed, and with means for detecting the transverse vibrations of the shaft while the wheel is rotating, are also provided with a rotating roller which is in contact with the tyre tread, thus simulating the rolling of the wheel on the ground. As the wheel rotates, appropriate sensors detect the axial forces which are unloaded onto the rotating roller and/or the rotating shaft which supports the wheel, and send the detected measurements to a processor which calculates an index of the drift effect relative to the wheel being measured, on the basis of known relations. In particular, the drift index of each wheel can be the axial force value which is measured for that wheel. Alternatively, the drift index could be the inclination of the wheel footprint on the road relative to the wheel's axis of rotation, which inclination can be measured using apparatus which are known in the sector. Average tread depth can be measured manually by an operator, with the aid of a calliper or other suitable measuring instruments. A limitation which is exhibited by presently-used tyre wear checking systems is that they provide no information relative to future tyre wear; and thus to the impact that such wear will have on the vehicle's efficiency, since the information they provide is valid only at the time of checking. The aim of the invention is to provide a tyre wear checking system which provides information about future wear, for example in order to allow the strategy for positioning the wheels on a vehicle to be more effective, thus at least partially obviating the limitations of known systems. A further aim of the invention is to achieve the above-mentioned aim within the ambit of a simple, rational and economical solution. The aims are reached thanks to the characteristics of the invention which are disclosed in the independent claims. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention. Disclosure of Invention

In particular, the invention provides a method for checking the efficiency of vehicles, which comprises the stage of acquiring at least one value of a characteristic parameter of the wear of at least one tyre of a vehicle, and the stage of using this value to estimate a limit to future use of the vehicle, prior to which limit the vehicle can be predicted to remain efficient. The efficiency limit can be expressed as a time interval, but is more preferably expressed as a number of kilometres to be travelled. Thanks to this solution, the estimated efficiency limit can for example be the number of kilometres which can be travelled before checking the tyre anew, or before carrying out procedures to re-position the wheels on the vehicle in order to improve its efficiency, or again it can be the residual lifespan of the tyres prior to their replacement. According to the invention, the wear parameter can be any parameter which describes an effect of tyre wear, for example tread depth, preferably average depth, or wheel drift.

In a preferred embodiment of the method of the invention, at least two wear parameter values relating to different states of wear of the tyre are considered, an interpolating function is defined for these measurements relative to the times in use of the tyre, and the interpolating function is calculated for a predetermined limit value of the considered parameter. Note that in this description, the terms "state of use" and "time in use" of a tyre are generally intended as meaning the time, or more preferably the number of kilometres, travelled by the tyre since it was new. Once the interpolating function has been defined, it is for example possible to set a limit value for average tread depth which must be reached before performing a new check, or before the tyre is completely worn out, and to use the interpolating function to calculate the number of kilometres which it is estimated can be travelled before this tread depth limit is reached. Since the interpolating function is dependent on the wear parameter values, which directly or indirectly derive from measurements performed on the tyre, it is not arbitrary, given that it takes into account the real wear of the tyre to which it refers, which wear also depends on the type of use to which the tyre is subjected (fast or slow driving style, use on motorway as opposed to in an urban environment) and/or any constructional defects. According to the invention, the interpolating function can be any mathematical function which comprises the values of the wear parameter or which approximates the progress thereof as nearly as possible: for example the interpolating function can be a linear function or a polynomial function. In general, the efficiency of a vehicle is not only influenced by the wear of each individual tyre, but also by whether or not the wear to which the tyres are subjected is uniform, which often depends on the relative positioning of the wheels of the vehicle.

For this reason, in the method of the invention the value of the wear parameter is preferably acquired for all the vehicle's wheels, and the future efficiency limit is calculated on the basis of these values and, possibly also on the basis of the positioning of the wheels of the vehicle.

In this way the method can also estimate the efficiency limit for each possible positioning of the wheels of the vehicle, and provide operators with the wheel positioning which maximises the efficiency limit. Further, the invention provides a device for checking vehicles, which comprises a computer configured in such a way as to implement the above- outlined method, to which device the means for measuring the magnitudes necessary for acquiring the wear parameter values are also preferably associated. The vehicle checking device can be effectively integrated into a balancing machine which is provided with means for detecting wheel imbalances, and into a tyre-removing machine provided with means for dismounting tyres from rims; or else into a wheel alignment device provided with means for measuring the characteristic angles of the wheels of the vehicle. Further, a computer program is provided, which when run on a computer, makes the computer implement the phases of the method of the invention. Brief description of the Drawings

Further characteristics and advantages of the invention will emerge from the following description, with the aid of the appended figures of the drawings, provided by way of a non-limiting example. Figure 1 schematically shows a checking device of the invention. Figures 2 to 5 show four diagrams relative to the estimation methods which are used in the device of figure 1.

Figure 6 shows schematically a measuring device according to an alternative embodiment of the invention. Figures 7 and 8 show two diagrams relating to an estimation method used by the device of figure 6.

Best Mode for Carrying Out the Invention

The method for checking the vehicle of the invention is based on the consideration that a vehicle's progressive loss of efficiency, not only in road- holding and driving comfort terms, but also in terms of wear and polluting emissions, depends to a fundamental, albeit not exclusive, extent on the progressive wear of tyres.

In completely general terms, the method of the invention evaluates the progressive wear of tyres in use, and based on this evaluation, estimates a time limit to future use of the vehicle, prior to which limit the vehicle will remain efficient.

This limit to future use, which can expressed either in terms of time, or preferably in terms of number of kilometres travelled, can advantageously be used as the limit beyond which the user, typically the vehicle's owner, is advised have the vehicle checked anew (for example after having covered between 5000 and 6000 kilometres. Naturally the method also includes calculation of two limits of future use of the vehicle, such as to be able to advise the user to request a fresh check-up of the vehicle withing a certain future use limit (for example after having covered between 5000 and 6000 kilometres).

The method of the invention principally consists of two phases. In the first phase, at least one value of a characteristic parameter of the wear of at least one tyre is acquired, while in the second phase the at least one value is processed in such a way as to obtain a future use limit for the vehicle, prior to which limit it can be forecast that the vehicle will remain efficient. The value can be acquired by direct measurement if the wear parameter is a directly measurable magnitude (for example average tread depth), or it can be calculated, if the parameter is an index obtained on the basis of one or more measurements and/or of one or more magnitudes (for example the index of wheel drift effect).

In a preferred aspect of the method of the invention, use is made during the estimation phase not only of the value(s) of the wear parameter, but in various ways also of data relative to one or more wear-influencing factors. It is in fact known that tyre wear, commonly caused by the interaction between the tyre and the road, is influenced by numerous other factors including for example: the tyre type, the attitude parameters of the vehicle, tyre pressure, the type of road and the driving style, for example whether the vehicle is used prevalently for urban as opposed to motorway journeys, and the environmental conditions under which the vehicle is used. These factors can be used in the estimation phase, for example by first of all mathematically processing the wear parameter value(s), and subsequently multiplying the results of this first processing operation by one or more corrective coefficients, each of which is dependent on a respective value of these factors.

More in particular, in the case of attitude parameters it is known that the damping inefficiency of shock absorbers, mechanical play and wear and tear in other components of the suspensions (swinging arms, steering linkage, steering organs, elastic couplings), together with incorrect values of the characteristic angles of the wheels (camber and convergence), are causes which lead to irregular tyre rotation (with vibration and shaking of the steering wheel) and irregular or anomalous tyre wear.

Correctly aligned tyres are therefore essential for improving the overall performance of the vehicle, whatever the driving conditions and speed. Alignment is incorrect when the suspension and steering systems are not functioning correctly; this can lead to excessive or irregular wear and reduce a tyre's lifespan by up to 70%. Further, excessive positive or negative camber increases tread wear on the road- or kerb-side of the tyre.

The method of the invention therefore calculates coefficients which take into account the values of the characteristic attitude parameters (preferably in relation with the set nominal values for a particular vehicle), and uses the coefficients during the estimation phase.

The characteristic attitude parameters can be measured by means of alignment devices which are well known in the sector. As regards tyre type, it is known that not all tyres wear in the same way. In the USA, tyres are graded according to the UTQG (Uniform Tyre Quality

Grading) system which, among the various parameters, specifies a tread wear index. UTQG indexes can be found on the side of a tyre.

The index is obtained by comparing tyre wear under particular test conditions (journey of 9600 km on a circuit with controlled environmental conditions) with those of a reference tyre which by convention has an index of 100. The index ranges between 60 and 620.

For example, a grade 50 tyre will wear twice as fast as a reference tyre subjected to the same conditions. Therefore in general, the higher the tread wear index, the longer the lifespan of the tyre will be.

The method of the invention calculates coefficients which take into account the tread wear index, which can be read off the tyre, and is used during the estimation phase. In particular the tread wear index can be used to estimate an initial interval in kilometres, before subjecting the tyre to the first check. For example, the first check can be linearly dependent on the tread wear index, as shown as an example in the following table:

Tyre inflation pressure (the nominal value of which is usually set by the vehicle maker in agreement with the tyre maker) is a highly important technical parameter for obtaining good kilomethc performance and for correct tyre use, but above all for driver and passenger safety.

Prolonged use of an incorrect inflation pressure causes rapid and irregular tyre wear patterns. For example, if inflation pressure is excessive, irregular wear will be accentuated along the central zone of the tread, thus reducing tyre life. On the contrary, when pressure is low, wear will be accentuated near the tyre shoulder.

In particular, curves and tables (for example provided by the makers) are known which indicate the percentages of reduction of tyre lifespan in function of differences between actual and ideal inflation pressure.

The method of the invention therefore calculates coefficients which take into account tyre inflation pressure, and they are used during the estimation phase.

For vehicles which are fitted with run-flat tyres, which usually mount tyre pressure sensors, these values can be acquired directly from the vehicle's electronic control unit, using known diagnostic devices.

It is also possible to calculate these coefficients on the basis of the inflation pressure of all the wheels on a vehicle, should it not be the same for all the wheels, in this way highlighting any significant differences between one wheel and another which might accelerate vehicle efficiency loss.

As regards the type of roads used and the style of driving, it is known for example that a set of tyres which has always travelled on smooth asphalted routes (for example motorways) lasts 100% more than tyres which have covered a mix of roads with rough uneven surfaces, in country areas, or which in any case are badly maintained. It is also known that high vehicle speeds give rise to significantly higher temperatures within a tyre and consequently reduce its performance in terms of kilometres. In fact a tyre wears twice as fast at 120 km/h as at 70 km/h. Sudden acceleration and frequent sharp braking (with locked wheels) also greatly influence early and sometimes irregular wear.

The method of the invention therefore calculates coefficients which take into account (historical and projected) information relative to the type of journey covered and the driving style, which information can for example be provided by the driver when the car is being checked, and the coefficients are used during the estimation stage.

Examples of possible corrective coefficients for tyre lifespan which are based on the type of road and driving style are shown in the following table:

As regards ambient conditions of use, it is known for example that ambient temperature has a significant influence on tread wear. In summer, when road surfaces are hot, tyre compound degrades rapidly due to the heat produced, which breaks down the molecular bonds within the rubber, giving rise to a significant decline in its physical, mechanical and elastic properties (elasticity, flexibility). In summer tyres wear up to three times more quickly than in winter. The method of the invention therefore calculates coefficients which take into account the future ambient conditions in which the vehicle will be used, which can be obtained for example from meteorological offices, and used during the estimation phase. More simply, coefficients can be set for the different periods of the year. Other factors can influence tyre wear, among which the distribution of heat within the tyre, and load distribution on a tyre.

For example, the correct distribution of heat within, and loads on, a tyre will be responsible for slower, more uniform wear, while non-uniform distribution will lead to a vehicle losing efficiency more rapidly. Temperature distribution can be measured by performing an analysis after the vehicle has been in use for a certain period of time. Appropriate measuring mats are known, upon which the vehicle is positioned to detect load distribution. The method of the invention therefore calculates coefficients which take into account information relative to the distribution of temperatures in, and/or loads on, tyres and are used during the estimation phase. A more detailed description is now provided of a first embodiment of the method for checking a vehicle according to the invention. As shown in figure 1 , the method is preferably implemented using a device 1 , substantially in the form of an appropriately programmed computer schematically comprising an electronic processor 2, a memory unit 3 for storing data, and operator interface means, which comprise a keyboard 4, via which the operator can enter data into the processor, and a monitor 5, via which the processor 2 makes the results of its calculations available to the operator.

The device 1 is associated to measuring means 6, which directly or indirectly measure a wear parameter of the vehicle's tyres, in this particular case a manual caibrator for measuring the average depth of the tread. Alternatively, the measuring means 6 could comprise electromechanical or opto-electronic instruments connected to the processor 2, so as to transmit effected measurements directly to the processor 2. The device 1 can further be associated to other measuring means (not shown) to measure one or more factors which influence tyre wear, among which in particular those mentioned previously (inflation pressure, attitude, etc.). In the method, the device 1 initially identifies the vehicle to be checked. Identification can be performed by the operator, who recognises the vehicle and enters the information into the processor 2 using the keyboard 4. Alternatively, the vehicle can be identified automatically, for example by means of a video camera which views the vehicle's registration plate and transmits the image thereof to the processor 2, which recognises the registration number and associates it to the relative vehicle; or else by connecting up with the vehicle's electronic control unit, thanks to which the processor 2 can read a vehicle identification serial number in the control unit, for example the VIN. When the identification phase is concluded, the processor 2 recovers from the memory unit 3 the number of kilometres travelled and the average tyre tread depth of at least one tyre, which were measured and recorded during a previous vehicle check, typically the most recent one. If no data relative to a previous check are available, the processor 2 could use an a priori, known measurement of the average tread depth, for example the tread depth when the tyre was new, which can be provided directly by the tyre maker and stored in the memory unit 3 during the programming stage. In this way, the processor 2 knows a unique first real point P1 in the two- dimensional space of a Cartesian coordinate system shown in figure 2, where the axes represent the number of kilometres travelled by the tyre and the average tread depth in millimetres.

At this point, the processor 2 acquires the present values of the number of kilometres travelled and the average tread depth relative to the vehicle to be checked, so as to have a second real point P2 in the two-dimensional system of figure 2.

The number of kilometres travelled can be read off the speedometer by the operator, and entered into the processor via the keyboard 4; alternatively, the number of kilometres travelled can be detected directly by the processor 2, for example via a link with the electronic control unit of the vehicle. As mentioned previously, measurement of the average tread depth is performed by the operator using the calliper 6, and transmitted to the processor 2 via the keyboard 4. Obviously, if the measuring means 6 are electromechanical or opto-electronic, they could perform the measurement automatically and transmit it to the processor 2 using appropriate communication systems.

The processor 2 then calculates an interpolating function which passes through both real points P1 and P2, over the number of kilometres travelled and the average tread depth.

In the example shown, the interpolating function is a simple linear function; however it could be replaced by any other function which defines a curve in the bi-dimensional system which effectively approximates tyre wear during use of the vehicle. After having obtained the interpolating function, the processor 2 can then calculate the value of the function for a limit value L of the average tread depth, within which limit value L the vehicle can be presumed to remain efficient, thus deriving the number of kilometres Q which it is estimated can be travelled in future before the average tread depth limit L is reached. The processor 2 then provides the operator with the result, displaying the number of kilometres Q on the monitor 5 and/or printing a paper copy. The limit value L of average tread depth can be the value at which the tyre is completely worn out and needs replacing, but could also be an intermediate value at which point it is suggested that the tyre check should be repeated, in order for example to modify the relative positioning of the wheels on the vehicle. The limit value L of average tread depth can be chosen arbitrarily by the operator and be entered into the electronic processor 2 via the keyboard 4; otherwise it can be a characteristic value of the tyre, and be stored in the memory unit 3. Note that if more than two real measurements are available, a more accurate estimate of the future wear of the tyre can be obtained, and further, it is possible to evaluate any anomalies and deviations that are departures from a constant pattern of wear. In the example shown in figure 3, it is supposed that the new tyre has an average tread depth of 7.6 mm, and that the aim is to estimate the duration of the tyre before reaching an average tread depth of 1.6 mm, which is the minimum limit allowed in many European countries.

Of course, any other limit depth could be set, for example many European tyre makers advise that their tyres should be replaced when tread depth is 3 mm.

Performing an initial measurement at 12,000 km, the average tread depth measured is around 5.5 mm, so that linear interpolation between points P1 and P2 provides an estimate of the overall lifespan of the tyre of approximately 34,000 km. Performing a second measurement at 22,000 km, the average measured tread depth is approximately 3.9 mm and linear interpolation between points P3 and P2 provides an estimate of the overall lifespan of the tyre as being approximately 37,000 km. Finally, performing a third measurement after 30,000 km, the average tread depth measured is 2.5 mm and linear interpolation between P4 and P3 provides a final estimate of the overall duration of the tyre of approximately 40,000 km.

In practice, the trend of the average tread depth in function of the number of kilometres travelled is represented by a spline with linear segments, the nodes of which P1 -P4 correspond to the actually measured values of average tread depth. According to the invention, when N real measurements of the average tread depth are available relative to different numbers of kilometres travelled, the interpolating function can also be a polynomial function of degree N — 1. It has in fact been demonstrated that given N known points in a two- dimensional space, a polynomial of degree N — 1 always exists which defines a curve passing through all these points. So for example, with three real measurements of the average tread depth, an interpolating parabola which passes through these three points (see figure 4) can be determined.

According to the invention, the interpolating function can also be a polynomial of a lower degree than N — 1. In general, the curve defined by this polynomial will pass through all the known points N of the two-dimensional space, but the processor 2 can use a method of numerical approximation to extract the interpolating function which most closely approximates the trend of the real measured values. So for example if four or more measurements of the average tread depth are known, it is equally possible to determine an interpolating parabola which, although not passing exactly through these points, approximates the trend in a sufficiently accurate way (see figure 5). Note that at present numerous numerical approximation methods exist which are suitable for this task, all of which are widely known and extremely reliable.

As already mentioned, tyre wear is influenced by numerous factors, among which those mentioned at the beginning of this description, for example inflation pressure and the normal conditions of use, for example whether tyres are used prevalently for urban or motorway journeys. One or more of these factors can be considered by the processor 2 when estimating tyre wear. In particular, in the method, the device 1 initially acquires data relative to one of these factors, which can be entered by the user via the keyboard 4 or directly by appropriate electronic measuring means if present; from this information the processor 2 then calculates an appropriate corrective coefficient, using curves or tables of the described previously type, and multiplies the corrective coefficient to the estimated value Q provided by the interpolating curve. More simply, the corrective coefficients can be stored in the memory unit 3 during the programming phase, so that the processor has only to associate the corresponding coefficient to the acquired data.

Note that it is preferable to implement the above-described estimation method, which has been described in relation to only one tyre of the vehicle, on all the tyres of the vehicle, in order for example to establish appropriate strategies for wheel positioning, so that tyre wear is more uniform and overall tyre lifespan is increased. A second embodiment of the method for checking according to the invention is described below.

The second example can be performed by the device 1 shown in figure 6, which differs from the previous device in that it comprises measuring means 6' which measure the drift effect of a wheel 100. The measuring means 6' generally comprise a motorised rotating shaft 60 to which the wheel to be measured is keyed, and a rotating roller 61 which is in contact with the tread of the tyre as the shaft 60 rotates, thus simulating the rolling of the wheel 100 on the ground. The measuring means 6' comprise further appropriate sensors 62, which are fitted on the rotating roller 61 and connected directly to the electronic processor 2, which sensors 62 detect the axial forces exerted on the rotating rollers 61 as the wheel 100 rotates.

Based on the forces which the sensors 62 detect, the processor 2 uses known modalities to calculate an index of the drift effect relative to the wheel 100 being measured. In particular, the index of the drift effect of each wheel can be the axial force value for the wheel in question. According to the invention, the processor 2 can estimate the vehicle's future efficiency limit by using the wheel drift index as a wear parameter, adopting the same modalities which were described above with reference to average tread depth. In general however, the efficiency of a vehicle, and thus the number of kilometres that it can travel before it is subjected to a check or any other maintenance intervention depend not only on the wear of each individual tyre, but also on whether the tyres do or do not wear uniformly, which in turn often depends on the relative positioning of the wheels on the vehicle. For this reason, in a preferred embodiment of the invention, the processor 2 calculates the value of the drift index for all the wheels on the vehicle, and estimates an efficiency limit for the vehicle which is based not only on these values, but also on the relative positioning of the wheels on the vehicle. For example, the efficiency limit of the vehicle can be estimated by the processor 2 using the formula:

K = K₀ + K' * F

where K₀ is a fixed quantity representing the minimum limit of kilometres which can be travelled, K' is a maximum increase beyond which it can be hypothesised that a new check on the tyres is necessary, while F is a corrective function which is variable between the values 0 and 1 , and is dependent on the values of the drift index of all the wheels on the vehicle and the camber/toe-in characteristics of the wheels.

If αi and ci2 are the values of the drift effect index of the front wheels, and α₃ and α₄ are the values of the drift effect indices of the rear wheels, the corrective function can for example be expressed by the formula:

F= 1-[min(αi2, d34ymax(αi₂, Ci₃₄)]

where 012 = (αi + ci2)/2 is the average value of the drift index of the front wheels, while O₃₄ = (α₃ + α₄)/2 is the average value of the drift index of the rear wheels.

This corrective function is justified by the observation that if the average values of the drift indices are sufficiently close to each other, sufficiently similar tyre wear can be predicted and a more remote efficiency limit can therefore be estimated. If on the other hand the values of the indices differ greatly from each other, non-uniform wear of tyres can be forecast and consequently a closer efficiency limit. The minimum limit K₀ and the maximum increase K' can be arbitrarily set by the operator and entered via the keyboard 4 into the processor 2, or more preferably they can be characteristic values of the tyre, or values which were appropriately chosen during the programming phase, and therefore stored in the memory unit 3.

The values K₀ and K' can further differ in accordance with the other factors influencing tyre wear, such as those mentioned at the beginning of this description, for example the prevalent type of use to which the vehicle is subjected (motorway/urban cycle). In this context, the invention further provides the possibility for the processor 2 to calculate the efficiency limit of the vehicle for all possible positionings of the wheels, and then selects, and suggests to the operator, the positioning which maximises the efficiency limit, for example by displaying the information on the monitor 5 or by printing a paper copy. Obviously when using the estimation procedure delineated above, which takes into account the positioning of the wheels on the vehicle, wheel drift indices can be replaced by other magnitudes which are indicative of tyre wear, for example the measurements of average tread depth. Note however that the estimation procedure outlined above does not take into account values of the drift index acquired during previous vehicle checks, which was instead the case for the average depth of the tread in the first- described example: it takes into account only the values which are present at the time of estimating. In other words, when a user returns to have the vehicle checked, the estimation of the efficiency limit is performed from scratch, using new values as the initial conditions for the tyre wear parameters.

A different more accurate mathematical model, using tyre drift to estimate the vehicle efficiency time limit, is described below with the aid of figures 7 and 8. In the first place, this model uses a predetermined criterion to establish the optimal positioning of the tyres on the vehicle, based on the drift index values of all the wheels. If for example the drift index represents the magnitude of the drift force which is applied to the tyre, the criterion can be that of establishing, as the optimal positioning of the tyres, the positioning for which the overall resultant of the drift forces applied on the vehicle is reduced to a minimum.

The efficiency estimation thus model acquires the values A1 , A2, A3 and A4 relative to the drift index of each wheel, in similar ways to those already described.

In this way, in the Cartesian coordinate system shown in the figures, in which the drift index is on the Y-axis and the time of the relative tyre has been in use (preferably expressed in number of kilometres) is on the X-axis, four initial points are known at x-coordinate to at which the drift values are acquired.

At this point, the above-mentioned evaluation criterion is used to establish the optimal positioning of the wheels on the vehicle, based on the acquired drift index values A1...A4. Subsequently, for each wheel the function of future variation of the drift index is estimated, represented on the graph by a curve issuing from the relative initial point AL..A4.

In general, the trend for each individual index can be estimated on the basis of the acquired initial value and the initial values of the parameters relative to the other wheels of the vehicle, for example from the difference between the individual value and the average of the values of all the wheels on the vehicle.

More in particular, considering the average value of the drift indices as being:

n ~f

(with n = 4 for a four-wheel vehicle), the speed of variation of the drift index of each wheel can be estimated, that is to say, the inclination of its representative curve in the Cartesian coordinate system, making this speed directly proportional to the difference between the initial value and the average value of the indices of all the wheels of the vehicle. In mathematical terms:

V₁ = ^ = K - [A₁ - A₁) with

A₁\_t0=A₁misurati

where ki is a corrective coefficient which can take into account one or more of the factors influencing tyre wear, illustrated at the outset of the description (attitude, suspensions, tyre pressure, etc.).

In this way, the estimated trend of the drift index for each wheel of the vehicle in the Cartesian coordinate system can be traced. After having estimated the speed of initial variation based on the values measured at t₀, a subsequent time ti is set and for each wheel and the value which the relative drift index would assume at time ti is calculated, based on the previously estimated speed of variation.

In this way, four new points are obtained in the Cartesian system in figure 7, which represent the estimated values AL..A4 _ti of the drift indices of the wheels at X coordinate ti. Based on these estimated A1...A4 _ti values, the expected optimal positioning of the wheels of the vehicle at time ti is calculated, using the previously-used evaluation criterion.

If the optimal positioning still coincides with the initial positioning, then a time t₂ which is subsequent to ti is set, and the previous phases are repeated. Preferably, based on the estimated AL..A4 _ti values of the drift index at ti, the previously estimated variation function for the drift index of each wheel is corrected according to a relation which is similar to the previous relation:

V₁ = ^ = K - [A₁ - A₁) with A₁ ^A^stimati

In this way, the curves in the Cartesian system which represent the speed of variation of the drift index and which emerge from the estimation of each point at time t1 will in general exhibit a different slope compared with the previous ones (see figure 8).

On the basis of the variation functions of the drift index, for each wheel the value which the relative drift index would have at time t₂ is calculated, and thus four new points are obtained in the Cartesian system of figure 7, which represent estimated values A1...A4 _t2 for the drift indices of the wheels at X- coordinate t₂.

On the basis of these estimated AL..A4 _t2 values and using the same previously-used criterion of evaluation, the optimal arrangement of the wheels which is expected on the vehicle at time t₂ is established. If the optimal positioning differs from the initial positioning, t₂ will be considered as being the limit of future use for the vehicle, prior to which limit it is presumed that the vehicle will remain efficient, and the user will be informed of this limit. If on the contrary the optimal positioning of the wheels still coincides with the initial positioning thereof, then the procedure will be repeated iteratively, progressively setting successive times t₃...t_n, and for each successive time, the above-described phases will be repeated, until a time t_n is obtained at which the optimal arrangement of the wheels estimated with the selected criterion differs from the initial one. The time t_n thus obtained will represent the limit to future use of the vehicle, prior to which it is presumed that the vehicle will remain efficient. This time will therefore be communicated to the user by the processor 2 via the monitor 5, as being the advised number of kilometres which can be travelled before proceeding to check the tyres again. Naturally the times ti...t_n are set a priori and, compatibly with the processing capacity of the processor 2, the difference between each of these moments in time and the previous moment can be chosen to be as small as is desired, so that the polyline representing the variation of the drift index of each wheel approximates increasingly to a continuous curve, thus gradually making the estimation more accurate and reliable.

Note that the values of the drift index of the wheels at times ti...t_n are estimated values. When the user returns for a vehicle check, the procedure starts from scratch using the newly acquired values as new initial conditions. At this point, the corrective values ki can be modified, by means of a more accurate estimation of the factors which determine them (pressure, etc.), based on a historical analysis of these factors, for example using similar modalities to those previously described for estimating average tread depth. In conclusion, in the method of the invention, the processor 2 can perform an estimation with several variables, that is, taking into account in the calculation two or more characteristic wear parameters, for example both the average tread depth and the drift index. Finally, note that the measurements performed by each of the measuring instruments mentioned in the above description, including the measuring instruments 6 and 6' and any measuring instruments which measure wear- influencing factors (such as attitude angles, inflation pressure, state of suspensions, etc,) can be entered manually by the operator or, more efficiently, by an appropriate direct communication system between the measuring instruments and the processor can be provided. The following are some examples: serial connection or more in general a connection via cable, wireless connection, use of an internet connection (useful for example if the measurements are performed in physically separate locations).

The information can also be stored on a device which accompanies the vehicle, the wheel or even the user. For example RFID tags can be mounted on the wheels or else all the measurements can be stored on an appropriate pen-drive which is then given to the user. According to the invention, the above-described devices 1 can preferably be integrated into more complex wheel-servicing machines, such as balancing machines, tyre removing machines, or aligner machines for measuring the characteristic angles of the wheels.

The above-described methods for checking the efficiency of vehicles can be implemented by the processor 2 by means of a computer program which, when run on a computer, makes the computer perform the phases of the methods.

Obviously a person skilled in the art could introduce numerous modifications of a technical and applicational character to the checking device 1 and the estimation method it implements, without forsaking the ambit of the invention as claimed below.

Claims

1 ). A method for checking a vehicle efficency, characterised in that it comprises a stage of acquiring at least one value of a parameter which is characteristic of a state of wear of at least one tyre of the vehicle, and a stage of using the at least one value to estimate a limit to future use of the vehicle, up to which limit the vehicle can be predicted to remain efficient. 2). The method of claim 1 , characterised in that the tyre wear parameter is a tread depth.

3). The method of claim 1 , characterised in that the tyre wear parameter is a wheel drift index. 4). The method of claim 1 , characterised in that at least two wear parameter values relatiing to different aspects of evidence of wear at two stages of use of the at least one tyre are acquired, and in that in a wear estimation stage an interpolating function is defined in order to calculate values for expressing a state of wear of the tyre, and in that the interpolating function is calculated for a predetermined limit value of the wear parameter value.

5). The method of claim 4, characterised in that the interpolating function is a linear function. 6). The method of claim 4, characterised in that the interpolating function is a polynomial function.

7). The method of claim 1 , characterised in that at least a wear parameter value is acquired for all the tyres of the vehicle, and in that the limit to future use is estimated on a basis of the acquired at least a value. 8). The method of claim 7, characterised in that the limit to future use is estimated in respect of the positioning of the wheels on the vehicle.

9). The method of claim 8, characterised in that the limit to future use is estimated for each possible positioning of the wheels on the vehicle, and in that a positioning of the wheels which maximises the limit to future use is selected. 10). The method of claim 8, characterised in that in the estimation stage a minimum limit to future use is imposed, and a variable increase is added to the minimum limit, which variable increase is dependent on the values of the wear parameter and on the positioning of the wheels on the vehicle. 11 ). The method of claim 7, characterised in that in the estimation stage comprises:

- establishing, using a predetermined criterion, an optimal positioning of the tyres on the vehicle, on a basis of acquired wear parameter values of all the tyres of the vehicle; - setting a wear parameter variation function for each tyre of the vehicle;

- setting a series of successive time deadlines during future use of the tyres;

- calculating, for each successive time deadline, the wear parameter variation function of each tyre of the vehicle, such as to obtain, for each successive time deadline, an expected wear parameter value for each tyre of the vehicle; - setting, using the same predetermined criterion, an optimal positioning of the tyres on the vehicle for each successive time deadline, based on the expected values of the wear parameter of all the tyres of the vehicle at the successive deadline time;

- providing, as a limit for future use of the tyre, an immediately successive first deadline at which optimal positioning of the tyres on the vehicle differs from an original optimal positioning thereof.

12). The method of claim 11 , characterised in that the wear parameter variation function of each tyre of the vehicle is dependent on values of the wear parameter of all the tyres of the vehicle, and in that the estimation stage includes correcting the variation function at each successive time deadline, on a basis of expected values of the wear parameter of all the tyres of the vehicle at the successive time deadline.

13). The method of claim 1 , characterised in that it comprises a stage of acquiring at least a datum relating to at least a factor which influences tyre wear, and that it uses the at least a datum in estimating a limit to future use. 14). A computer program, which when run on a computer (2) causes the computer (2) to implement a method for checking the efficiency of a vehicle of any of the previous claims.

15). A diagnostic device for checking a vehicle efficiency, comprising a computer (2) and user interface means (5), characterised in that the computer (2) is configured in such a way as to acquire at least a value of a characteristic wear parameter of at least a tyre and to use the at least a value to estimate a future use limit of the vehicle, prior to which limit the vehicle can be predicted to remain efficient, and to communicate the future use limit to the user via the interface means (5).

16). The device of claim 15, characterised in that the tyre wear parameter is a tread depth.

17). The device of claim 15, characterised in that the tyre wear parameter is wheel drift. 18). The device of claim 15, characterised in that the computer (2) is configured in such a way as to use at least two values of the wear parameter relating to different states of use of the at least one tyre; to define an interpolating function for the at least two values during the times of use of the tyre; and to calculate the interpolating function for a predetermined limit value of the parameter.

19). The device of claim 15, characterised in that the computer (2) is configured in such a way that it acquires at least a value of the tyre wear parameter values for all the tyres of the vehicle, and estimates the limit to future use of the vehicle on the basis of the values. 20). The device of claim 19, characterised in that the computer (2) is configured in such a way that it uses the positioning of the wheels on the vehicle to estimate, and provide the user with, the limit to future use of the vehicle. 21 ). The device of claim 20, characterised in that the computer (2) is configured in such a way as to estimate the limit to future use for each possible positioning of the wheels on the vehicle, providing the operator, via the interface means, with an optimum positioning of the wheels that maximises the limit to future use.

22). The device of claim 19, characterised in that the computer (2) is configured in such a way as to: - set a predetermined criterion with which to establish the optimal positioning of the tyres on the vehicle, based on the wear parameter values acquired for all the tyres of the vehicle;

- set a wear parameter variation function for each tyre of the vehicle;

- select a series of successive time deadlines in the use of the tyres; - calculate the wear parameter variation function of each tyre of the vehicle for each successive time deadline, thus obtaining a value for the expected wear parameter of each tyre of the vehicle, for each successive time deadline;

- use the previous predetermined criterion to set the optimal position of the tyres on the vehicle at each successive time deadline, based on expected values of the wear parameter of all the tyres of the vehicle at that deadline;

- provide the first successive time deadline at which the optimal positioning of the tyres on the vehicle differs from the initial positioning as the limit to future use of the vehicle. 23). The device of claim 22, characterised in that the wear parameter variation function of each tyre of the vehicle is dependent on the values of the wear parameter of all the tyres of the vehicle, and in that the computer (2) is configured in such a way as to correct the variation function at every successive time deadline, based on expected values of the wear parameter of all the tyres of the vehicle at that deadline.

24). The device of claim 15, characterised in that the computer (2) is configured in such a way as to acquire at least one datum relating to at least one factor which influences the wear of a tyre, and use the at least one datum to estimate the limit to future use. 25). The device of claim 15, characterised in that it comprises a memory unit (3) which stores the at least one wear parameter. 26). The device of claim 15, characterised in that it comprises means (6, 6') for measuring the wear parameter.

27). A machine for balancing the wheels of vehicles comprising the device (1 ) of claim 15. 28). An aligner for checking the attitude of vehicles comprising the device (1 ) of claim 15. 29). A tyre removing machine comprising the device (1 ) of claim 15.