Method and system for estimating the severity of tire usage conditions

Abstract

A method for estimating the severity of conditions of use of a tire installed on a vehicle comprises the following steps: a step of measuring the speed of the vehicle and the load of the vehicle, a step of evaluating, as a function of the measurements performed, the power of the internal heat dissipations of the tyres, and a step of determining, as a function of this power, the internal temperature of the tyre, a step of recording the number of wheel revolutions performed and/or the time spent in conditions of use corresponding to a given temperature interval. There is also a system that makes it possible to implement said method.

Claims

1. A method for estimating the severity of conditions of use of a tire installed on a vehicle, the method comprising the following steps: measuring a speed of the vehicle and a load of the vehicle; evaluating, as a function of the measurements performed, a power of internal heat dissipations of the tire; determining, as a function of the power, an internal temperature of the tire; and recording at least one of a number of wheel revolutions performed and a time spent in conditions of use corresponding to a given temperature interval.

2. The method according to claim 1 further comprising a step of measuring ambient temperature, wherein ambient temperature is taken into account in the evaluation of thermal power.

3. The method according to claim 1 further comprising a step of measuring a temperature of a rim on which the tire is installed.

4. The method according to claim 1 further comprising a step of resetting records to zero performed when the installation of a new tire is detected.

5. The method according to claim 1 further comprising a step of determining, as a function of recordings made, a need for maintenance or inspection of the tire.

6. A system for estimating the severity of conditions of use of a tire installed on a road vehicle, the system comprising a braking computer comprising: means for measuring a speed of the vehicle and a load of the vehicle; means for evaluating, as a function of the measurements performed, a power of the internal heat dissipations of the tire; means for determining, as a function of the power, an internal temperature of the tire; and means for recording at least one of a number of wheel revolutions performed and a time spent in conditions of use corresponding to a given temperature interval.

7. The system according to claim 6 further comprising an ambient temperature sensor installed in the braking computer.

8. The system according to claim 6 further comprising a temperature sensor for sensing a temperature of a rim on which the tire is installed.

9. The system according to claim 6 further comprising means for alerting a driver or a user of the vehicle of a need for maintenance or inspection of the tire.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other objectives and advantages of the invention will emerge clearly from the following description of the preferred but nonlimiting embodiment, illustrated by the following figures in which:

(2) FIG. 1 shows a tyre section,

(3) FIG. 2 shows an example of isotherm curves in a tyre,

(4) FIG. 3 shows a diagram of the heat exchanges within a tyre,

(5) FIG. 4 shows the trend of the internal temperature of a tyre during a rolling sequence.

DESCRIPTION OF THE BEST EMBODIMENT OF THE INVENTION

(6) FIG. 1 shows a meridian section of a heavy truck tyre and the simplified architecture of its components. FIG. 2 shows the embedding of the isotherms around a temperature extremum situated at the shoulder. The cross section CC passes through the extremum zone. Along this line, the temperature gradients in the circumferential and meridian directions are very weak and the heat fluxes are substantially collinear to the line C-C. FIG. 3 schematically shows the heat exchanges within a tyre similar to that of FIG. 1. The references used in FIGS. 1 and 3 therefore represent the same elements.

(7) The simplified thermal operation of this zone is represented, reduced to one dimension such as a stacking of layers of materials each characterized (a) by their thermal properties (conductivity, volume heat capacity) and (b) by an estimator of the volume sources of dissipation as a function of the load, speed, local current temperature and the braking of the tyre. This function is established by real measurement or based on comprehensive digital simulations.

(8) Typically, the thermal conductivity of the rubber mixture has the value 0.25 to 0.30 W/(K.Math.m), its density is close to 1100 kg/m.sup.3, its mass heat capacity is of the order of 1470 J/Kg/K.

(9) It is possible to express this function as the product of a single function P(z,v) dependent on the load z of the tyre and its speed v, of a factor Q dependent on the local temperature and of a constant coefficient K.sub.i per material: P.sub.i(z,v,T)=K.sub.i.Math.P(z,v,).Math.Q(T)

(10) Typically, the source P for a tyre rolling at 10 m/s (36 km/h) at 60° C. has the value 2.5 E+4 W/m3 in the tread, i.e. a dissipation of 7500 J/m3 per cycle. This source decreases with temperature, and increases with load and speed.

(11) FIG. 1 represents an example of a tyre that is not limiting in that the invention can be applied to tyres having different architectures in terms of number and nature of the components.

(12) Thus, the outermost layer of the tyre is the tread BR, which is directly in contact with the ambient air A around the vehicle on which a system according to the invention is installed. It is specified here that, in the context of the invention, the temperature of this ambient air is for example measured by a sensor installed on the vehicle, and can be taken into account in the determination of the internal temperature of the tyre.

(13) The tread is generally attached to a sublayer SC, ensuring the bond with the crown plies. The sublayer generally has a different composition from the tread. This sublayer is installed on crown plies NS, which comprise, for example, fabric or metallic reinforcing elements. These crown plies NS are terminated by crown feet PS, positioned above the casing plies, which rest on the inner lining GI of the internal gas volume G, which are in direct contact with the internal gas G. Thus, a temperature gradient is created within the architecture of the tyre, from the block BR to the internal gas.

(14) The internal gas, also called inflation gas, is trapped between the tyre and the wheel. Heat exchanges therefore take place between this gas G and the wheel R, which is itself subjected to exchanges with the ambient air A.

(15) There now follows a description of the implementation of a method according to the invention by a braking computer, in a tyre having an architecture similar to that of FIG. 1.

(16) The resolving principle is as follows: On each finite time interval dt, for example 1 s, the braking computer updates the average load per tyre and the speed per tyre. It computes the heat sources per material zone, and the exchange coefficients at the walls. The ambient temperature is possibly available as previously indicated.

(17) The equation of discretised transient heat is integrated for the duration of dt, which provides an estimation of the temperature in the standard section:

(18) $\frac{\partial T}{\partial t}=\frac{\lambda }{\rho C}\frac{{\partial }^{2}T}{\partial z^2}+\frac{P}{\rho C}$
The unknowns are the temperatures T(z,t) at the time t and at the depth z, counted from the outer surface of the section (z=0) and the temperature of the inflation gas T.sub.G(t) at the time t.
The conditions at the flow limits on the ambient air side are Φa=H.sub.a(T(0, t)−T.sub.a). The exchange coefficient H.sub.a at the outer wall in line with the section CC depends on the speed v of the vehicle and on T.sub.a. It is typically 50 W/m.sup.2/K at 80 km/h.
And on the internal gas side Φ.sub.G=H.sub.G(T(z.sub.G, t)−T.sub.G).
The temperature T.sub.G of the internal gas originates from a TPMS sensor if available, otherwise it is estimated by the method described hereinbelow.

(19) The initial temperatures are established at ambient temperature or at a default value.

(20) The internal gas G of mass m.sub.G is considered at constant temperature T.sub.G. It exchanges heat with all of the walls delimiting the internal volume

(21) $m_G{C}_V\frac{\partial T_G}{\partial t}=S_G{H}_i\left(T^{*}-T_G\right)$
in which H.sub.i is the exchange coefficient between the internal wall and the internal gas, C.sub.v is the heat capacity of the volume, S.sub.G is the section, and T* is the effective temperature of the inner wall, defined as a weighted average of the ambient temperature T.sub.A and of the temperature T(z.sub.G) of the internal wall of the section CC.
T*=[βT(z.sub.G)+(1−β)T.sub.A]
Typically β=0.12.

(22) The section CC is discretised in slices delimited by nodes according to the conventional finite elements methods.

(23) The equation can then be resolved by using an explicit Euler resolution scheme as follows by retaining a fairly small dt for the scheme to be stable (Δt«(Δz/α).sup.0.5, in which Δz is the smallest interval and α is the corresponding diffusivity):

(24) $T_i^{k+1}=T_i^k+\frac{\Delta t}{\Delta x^2}\frac{{\lambda }_i^k}{\rho C_p}\left[T_{i+1}^k+T_{i-1}^k-2T_i^k\right]+\frac{\Delta {\mathrm{tP}}_i^k}{\rho C_p}$

(25) Advantageously, if the embedded computer has enough memory and power, other known schemes ensure an improved stability for great intervals dt (Crank-Nicolson, Adams-Moulton, Gear, Newmark).

(26) In the case where the thickness intervals are unequal, the laplacien ΔT is estimated according to the finite elements method with functions of appropriate form, for example “cap” functions.

(27) It is then possible to establish a temperature map over a rolling sequence, as shown in FIG. 4. The bottom curve shows the speed of the vehicle over a studied rolling sequence. The top map shows the internal temperature of the tyre over this same rolling sequence. The abscissa common to the two curves shows the time. The temperature of the tyre is expressed according to the depth, this depth being computed relative to the surface of the tread.

(28) In this example, it is observed on this curve that, after 1650 s, the speed of the vehicle is zero, and the tyre cools. The hot point of the tyre is situated approximately at 11 mm in the depth, which means that it is the material situated at this point which will be the most subject to ageing over this sequence.

(29) On each interval dt, the temperature profile found is used to update two histograms: (a) the time spent in a temperature class and (b) the number of wheel revolution cycles in a temperature class.

(30) The histogram (b) is used by performing a weighting:

(31) $Y={.Math.}_i\frac{n_i}{N_i}$
in which the n.sub.i are the sizes of the classes and N.sub.i are characteristic constants of the temperature class T.sub.i. A score

(32) $S_1=\frac{{.Math.}_i{n}_i}{Y}$
is determined.

(33) The histogram (a) is used according to an Arrhenius law. Conventionally, the rate of ageing

(34) $w=A\exp \left(\frac{-E}{\mathrm{RT}}\right)$
in which T is the temperature expressed in ° K, E is an activation energy typically between 40 and 80 kJ/mol, R is the constant of the ideal gases and A is a constant. The score

(35) $S_2=\frac{\int w.Math.\mathrm{dt}}{\int \mathrm{dt}}.$

(36) The system can also provide a score S.sub.3 linked to the energy efficiency of the tyre, because the rolling resistance of a tyre is lower when it is fairly hot. This score is constructed with the histogram (b) by weighting the dissipations d.sub.i by cycles that are a function of the temperature T.sub.i of the class i according to their frequency of occurrence: S.sub.3=Σ.sub.id.sub.in.sub.i/Σ.sub.in.sub.i. This score is particularly useful for characterizing a use in predominantly transient state.

(37) The embedded computer keeps the histograms up to date. The computation of the scores can take place remotely, in diagnostic software or a diagnostic server.

(38) Advantageously, the tyre model to the embedded or remote system to apply specific parameters instead of generic parameters.