Method Of Monitoring Tire Pressure In A Motor Vehicle

Abstract

A method of monitoring tire pressure in a motor vehicle employs an indirectly measuring tire pressure monitoring system (DDS) and a directly measuring tire pressure monitoring system (TPMS) with two pressure sensors for determining tire pressure and/or tire pressure loss. Depending on the arrangement (axlewise, sidewise or diagonal) of the pressure sensors at the vehicle wheels, a reference value (DIAG, SIDE, AXLE) of the indirectly measuring tire pressure monitoring system (DDS) is used as the main reference value for detecting tire inflation pressure loss.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The indirectly measuring tire pressure monitoring system (DDS) detects tire pressure loss by way of a variation of the rolling circumferences or the wheel speeds of the vehicle wheels, respectively, the wheel speeds being measured by means of wheel rotational speed sensors. This indirectly measuring tire pressure monitoring system (DDS) is combined with a directly measuring tire pressure monitoring system (TPMS), which includes only two pressure sensors. This combination renders it possible to reliably monitor all vehicle tires with respect to tire pressure loss, without using a complete directly measuring tire-pressure monitoring system, i.e. equipped with four pressure sensors.

The indirectly measuring tire pressure monitoring system (DDS) normally employs for a tire pressure check non-dimensional reference values, which are determined from the wheel rotational speeds. The wheel rotational speeds n depend on the tire rolling circumferences A_j (j=1, 2, 3, 4) and on the vehicle speed V:

$\begin{array}{cc}n=\frac{V}{A_j}& \left(1\right)\end{array}$

Any reference value D_i desired can therefore be expressed as a function F of the tire rolling circumferences A_j (j=1, 2, 3, 4).

D _i =F(A₁, A₂, A₃, A₄)  (2)

i=1, 2, 3

As the rolling circumferences A_j (j=1, 2, 3, 4) depend on the tire pressures P, and hence also on the variations of the tire pressures ΔP_j (j=1, 2, 3, 4), the optional reference value D_i can also be written as function Φ of the variations of the tire pressures ΔP_j (j=1, 2, 3, 4).

D _i=Φ(ΔP₁, ΔP₂, ΔP₃, ΔP₄)  (3)

i=1, 2, 3

Because it is not possible to obtain more than three independent non-dimensional values from four wheel rotational speeds, only three equations are available for the four unknown tire rolling circumferences or pressure variations, respectively, out of the indirectly measuring tire pressure monitoring system (DDS). Therefore, two pressure sensors of a directly measuring tire pressure monitoring system (TPMS) are evaluated in addition in order to reliably detect inflation pressure loss. These two pressure sensors may be arranged at any location in or at the vehicle tires. Thus, the pressure sensors can be disposed in each case at the two wheels of one axle (axlewise arrangement) or at respectively one wheel of the front axle and one wheel of the rear axle. In this respect, the pressure sensors can be arranged at the left or the right vehicle side (sidewise arrangement) or diagonally (diagonal arrangement) at the vehicle, e.g. one pressure sensor at the left front wheel and one pressure at the right rear wheel.

The rolling circumference A of the tire in a first approximation depends on the sum of the basic rolling circumference

A₀ of the tire and the product of a proportionality coefficient k and the tire pressure P. The basic rolling circumference A₀ describes the rolling circumference at a tire pressure of P=0.

A≈A ₀ +k·P  (4)

The relative variation of the rolling circumference ΔA/A thus depends linearly on the relative variation of the pressure ΔP/P:

ΔA/A≈k·ΔP/P  (5)

The variations of the three reference values ΔD_i (i=1, 2, 3) depend on the pressures of all four wheels δP_j=ΔP_j/P_j(j=1, 2, 3, 4):

ΔD_i=f(δP₁, δP₂, δP₃, δP₄)≈f₁(δP₁)+f₂(δP₂)+f₃(δP₃)+f₄(δP₄)  (6)

The functions f₁ to f₄ are also linear in a first approximation, therefore, it is possible to use three linear equations for the calculation of the pressure variations:

$\begin{array}{cc}\Delta D_i\approx k_{i1}·\delta P_1+k_{i2}·\delta P_2+k_{i3}·\delta P_3+k_{i4}·\delta P_4=\sum _{j=1}^4k_{\mathrm{ij}}·\delta P_ji=1,2,3& \left(7\right)\end{array}$

The coefficients k_i1 to k_i4 depend on the properties of the tires and must be determined empirically. When the tire pressure in a wheel has been measured directly, this tire pressure can be considered a known quantity in the three equations 7. In this case, the three linear equations 7 form a defined system, what means that the pressure variations on all wheels can be determined (checked).

When a tire pressure check system M (M≧1) includes directly measuring pressure sensors, the system can be outlined according to the equations 7 as follows:

$\begin{array}{cc}\Delta D_{\mathrm{icomp}}=\Delta D_i-\sum _{j=1}^Mk_{\mathrm{ij}}·\delta P_j=\sum _{j=1}^{4-M}k_{\mathrm{ij}}·\delta P_ji=1,2,3& \left(8\right)\end{array}$

Each compensated reference value ΔD_icomp according to equation 8 depends on (4−M)-tire pressures in the tires without pressure sensors. In order to calculate the tire pressures, only (4−M)-equations can be used by the system according to equation 8. These equations must be linearly independent.

When the directly measuring tire pressure monitoring system (TPMS) includes two pressure sensors, the following arrangements of the two pressure sensors at the vehicle or at the vehicle wheels are possible:

pressure sensors in a diagonal arrangement

pressure sensors in an axlewise arrangement

pressure sensors in a sidewise arrangement

For each of the above-mentioned positions of the pressure sensors, one of the three reference values (DIAG, SIDE, AXLE) is chosen as a main reference value for the monitoring operation. In this case, the reference values (DIAG, SIDE, AXLE) are basically composed of the wheel revolution times T of the individual wheels. The wheel revolution times T are determined from the wheel rotational speeds n of the wheels.

In case that the pressure sensors are arranged on the vehicle diagonal, the reference value DIAG is used as the main reference value:

$\begin{array}{cc}\mathrm{DIAG}=\frac{T_{\mathrm{FL}}+T_{\mathrm{RR}}}{T_{\mathrm{FR}}+T_{\mathrm{RL}}}-1T=\frac{1}{n}& \left(9\right)\end{array}$

The indices FL, FR, RL, RR refer to the front left (FL), front right (FR), rear left (RL) and rear right (RR) wheels.

In case that the pressure sensors are arranged on one vehicle side, the reference value SIDE is used as the main reference value:

$\begin{array}{cc}\mathrm{SIDE}=\frac{T_{\mathrm{FL}}+T_{\mathrm{RL}}}{T_{\mathrm{FR}}+T_{\mathrm{RR}}}-1& \left(10\right)\end{array}$

In case that the pressure sensors are arranged on one vehicle axle, the reference value AXLE is used as the main reference value:

$\begin{array}{cc}\mathrm{AXLE}=\frac{T_{\mathrm{FL}}+T_{\mathrm{FR}}}{T_{\mathrm{RL}}+T_{\mathrm{RR}}}-1& \left(11\right)\end{array}$

To detect tire pressure loss, three compensated reference values ΔD_DIAGcomp, ΔD_SIDEcomp and ΔD_AXLEcomp with ΔD_DIAG=DIAG, ΔD_SIDE=SIDE AND ΔD_AXLE=AXLE are produced from the equations 9 to 11 by using equation 8. It further applies in the directly measuring tire pressure monitoring system (TPMS) with two pressure sensors that M=2.

A warning with regard to tire pressure loss must be given, for example, when one or several pressure reductions δP_j exceed a defined threshold (e.g. 25%).

For the reference value (DIAG, SIDE, AXLE) used as the main reference value, two different threshold values (low threshold value, high threshold value) are defined for the detection of tire pressure loss. These threshold values are determined empirically. In this arrangement, the high threshold value is e.g. twice as high as the low threshold value. When the result of the analysis of all three compensated reference values (ΔD_DIAGcomp, ΔD_SIDEcomp and ΔD_AXLEcomp) is that pressure loss occurs in one vehicle tire, a warning with regard to tire pressure loss is given when the main reference value (DIAG, SIDE or AXLE) exceeds the low threshold value for this main reference value. If the analysis shows that pressure loss occurs in two vehicle tires without pressure sensors, then a warning with regard to tire pressure loss is given when the main reference value exceeds the high threshold value. When pressure loss is detected by a pressure sensor at a wheel, the warning with regard to tire pressure loss is given when a defined threshold (e.g. pressure loss is higher than 25%) is exceeded. This pressure loss can also be tested by the above method in order to preclude a defect of a pressure sensor, for example.

A simplified linear model is used to detect the tire pressure variations of the system according to equation 8.

Claims

1.-9. (canceled)

10. A method of monitoring tire pressure in a motor vehicle employing an indirectly measuring tire pressure monitoring system (DDS) and a directly measuring tire pressure monitoring system (TPMS) with two pressure sensors for determining tire pressure and tire pressure loss, comprising the steps of generating a reference value (DIAG, SIDE, AXLE) of the indirectly measuring tire pressure monitoring system (DDS) depending on the arrangement (axlewise, sidewise or diagonal) of the pressure sensors at the vehicle wheels,detecting tire inflation pressure loss using the reference value as a main reference value, andgenerating warning in case of a tire inflation loss.

11. The method as claimed in claim 10, wherein the reference value DIAG is used as the main reference value when each one pressure sensor is arranged at the left front wheel (FL) and the right rear wheel (RR), or at the right front wheel (RF) and the left rear wheel (LR), wherein

12. The method as claimed in claim 10, wherein the reference value SIDE is used as the main reference value when each one pressure sensor is arranged at the left front wheel (FL) and the left rear wheel (RL), or at the right front wheel (FR) and the right rear wheel (RR), wherein

13. The method as claimed in claim 10, wherein the reference value AXLE is used as the main reference value when each one pressure sensor is arranged at the left front wheel (FL) and the right front wheel (FR), or at the right rear wheel (RR) and the left rear wheel (RL), wherein

14. The method as claimed in claim 10, wherein two differently high, empirically determined threshold values (low threshold value, high threshold value) are used for the reference value in order to detect tire pressure loss.

15. The method as claimed in claim 14 for a vehicle with four tire pressure sensors, wherein three reference values (DIAG, SIDE, AXLE) are calculated with different pairs of tire pressure sensors, comprising the step ofgenerating a compensated reference value (ΔDDIAGcomp, ΔDSIDEcomp and ΔDAXLEcomp) from each reference value (DIAG, SIDE, AXLE) to detect a tire pressure loss, the compensated value being obtained in each case by means of a linear function.

16. The method as claimed in claim 15, wherein a tire inflation pressure loss is detected when all three compensated reference values (ΔDDIAGcomp, ΔDSIDEcomp and ΔDAXLEcomp) identify a vehicle tire as the tire with pressure loss, and when the main reference value exceeds the high threshold value.

17. The method as claimed in claim 14, wherein tire pressure loss at two vehicle tires without pressure sensors is detected when the main reference value exceeds the high threshold value.