Method for detecting a deflated tire on a vehicle

• US 6,222,444 B1
• Filed: 04/03/2000
• Issued: 04/24/2001
• Est. Priority Date: 04/03/2000
• Status: Active Grant

• Current Assignee: Robert Bosch Corporation (Robert Bosch GmbH)
• Original Assignee: Robert Bosch Corporation (Robert Bosch GmbH)
• Inventors: Wang, Orson Szu-Han
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): TANG, SON M
• Application Number:
  US09/542,488
• Time in Patent Office: 386 Days
• Field of Search: 340/442, 340/443, 340/444, 340/441, 340/466
• US Class Current: 340/442
• CPC Class Codes: B60C 23/0408 transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
• CPC Class Codes: B60C 23/061 by monitoring wheel speed measuring distance traversed on the ground by vehicles G01C22/00

Abstract

Wheel speed values for each of four wheels are collected and statistically analyzed axle by axle for a difference which could indicate low tire pressure. Prior to analysis, and following reset of the system, calibration factors are determined for each axle to compensate for rolling radius variation, and are subsequently used to correct the percentage difference values for the two wheels on any one axle. When a sufficient number of values have been collected, a t₀ value is calculated for each axle according the paired t-test statistical method or a slight variation thereof The t₀ value for each axle is then compared to a respective empirical value based on a predetermined pressure loss. This comparison can provide the basis for a driver warning. Various types of filters can be used prior to calculating the t₀ values to eliminate data that may lead to improper deflation detection.

Claims

1. A method for detecting loss of pressure in a tire of a motor vehicle having four wheels and a tire on each wheel, the method comprising:

measuring the wheel speed of a first wheel on an axle a plurality of times n;

measuring the wheel speed of a second wheel on the axle a plurality of times n; and

using the measured wheel speeds in a paired t-test statistical analysis to determine whether either wheel has a deflated tire.

2. The method of claim 1, wherein the paired t-test statistical analysis utilizes the equation $t_0=\frac{\stackrel{\_}{D}}{S_D/\sqrt{n}}$

 

,

 

wherein {overscore (D)} is the sample mean difference of the wheel speeds for the first and second wheels, S_D is the standard deviation of the wheel speed differences for the first and second wheels, and t₀ is the t₀ value of the paired t-test.

3. The method of claim 2, wherein {overscore (D)} is calculated by calculating a percentage difference between the first and second wheel speeds for each time set n, adding the percentage difference values to form a tire inflation monitoring sum TIM SUM, and calculating {overscore (D)} according to $\stackrel{\_}{D}=\frac{\mathrm{TIM}}{}$

 



SUMn.

4. The method of claim 2, wherein S_D is calculated according to $S_D=\sqrt{\frac{\sum _{}^{}}{}}$

j=1n

Dj2-[(∑

j=1n

Dj)2/n]n-1,

wherein D_j is the difference between the wheel speeds for the first and second wheels.

5. The method of claim 4, wherein S_D is calculated using a floating point microprocessor.

6. The method of claim 2, wherein S_D is calculated according to $S_D=\frac{\mathrm{RANGE}}{}$

 



SUMd2

k,

wherein $\text{\hspace{1em}<br><br>}<br><br>k=\frac{n}{10},$

d₂ is the mean of the relative range distribution and is a constant supplied from a statistics table, and RANGE SUM is the range of normalized percentage difference values between wheel speeds for the first and second wheels.

7. The method of claim 6, wherein the microprocessor is an integer microprocessor.

8. The method of claim 2, wherein {square root over (n)} is supplied from a table stored in a microprocessor.

9. The method of claim 8, wherein the microprocessor is an integer microprocessor.

10. The method of claim 2, wherein the t₀ value is compared to a predetermined upper limit to determine whether either tire is deflated.

11. The method of claim 10, further including actuating a driver warning device when t₀ exceeds the upper limit.

12. The method of claim 2, wherein the t₀ value is compared to a predetermined lower limit to determine whether either tire is properly inflated.

13. The method of claim 1, wherein the axle is a first axle and the method further comprises:

measuring the wheel speed of a first wheel on a second axle a plurality of times n;

measuring the wheel speed of a second wheel on the second axle a plurality of times n; and

using the measured wheel speeds from the second axle in the paired t-test statistical analysis to determine whether either wheel on the second axle has a deflated tire.

14. The method of claim 13, wherein the paired t-test statistical analysis isolates the axles by using the measured wheel speeds from the first axle independently of the measured wheel speeds from the second axle.

15. A method for detecting loss of pressure in a tire of a motor vehicle having four wheels and a tire on each wheel, the method comprising:

measuring the wheel speeds V_i1, V_i2, Y_i3 and V_i4 a plurality of times n;

calculating the normalized percentage difference values NORM_i1 between the wheel speeds V_i1 and V_i2 on a first axle, where ${\mathrm{NORM}}_{\mathrm{i1}}=\frac{\left(V_{\mathrm{i1}}-V_{\mathrm{i2}}\right)}{V_{i<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{ref}}}$

calculating the normalized percentage difference values NORM_i2 between the wheel speeds V_i3 and V_i4 on a second axle, where ${\mathrm{NORM}}_{\mathrm{i2}}=\frac{\left(V_{\mathrm{i3}}-V_{\mathrm{i4}}\right)}{V_{i<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{ref}}}$

adding the NORM_i1 values to form a tire inflation monitoring sum TIM SUM₁;

adding the NORM_i2 values to form a tire inflation monitoring sum TIM SUM₂;

calculating a first sample mean difference value {overscore (D)}₁ where ${\stackrel{\_}{D}}_1=\frac{\mathrm{TIM}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{SUM}}_1}{n}$

calculating a second sample mean difference value {overscore (D)}₂ where ${\stackrel{\_}{D}}_2=\frac{\mathrm{TIM}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{SUM}}_2}{n}$

calculating a first standard deviation value S_D1 where $S_{\mathrm{D1}}=\frac{\mathrm{RANGE}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{SUM}}_1}{d_2<br><br>k}$

wherein $k=\frac{n}{10},$

d₂ is the mean of the relative range distribution and is a constant supplied from a statistics table, and RANGE SUM₁ is the range of NORM_i1 values;

calculating a second standard deviation value S_D2 where $S_{\mathrm{D2}}=\frac{\mathrm{RANGE}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{SUM}}_2}{d_2<br><br>k}$

wherein $k=\frac{n}{10},$

d₂ is the mean of the relative range distribution and is a constant supplied from a statistics table, and RANGE SUM₂ is the range of NORM_i2 values;

calculating a first t₀ value t₀₁ for the first axle where $t_{01}=\frac{{\stackrel{\_}{D}}_1}{S_{\mathrm{D1}}/\sqrt{n}}$

wherein {square root over (n)} is supplied from a second table in the microprocessor;

calculating a second t₀ value t₀₂ for the second axle where $t_{02}=\frac{{\stackrel{\_}{D}}_2}{S_{\mathrm{D2}}/\sqrt{n}}$

wherein {square root over (n)} is supplied from the second table in the microprocessor;

comparing t₀₁ to a predetermined upper limit to determine whether the first axle has a deflated tire thereon; and

comparing t₀₂ to a predetermined upper limit to determine whether the second axle has a deflated tire thereon.

16. The method of claim 15, further comprising actuating a driver warning device when t₀₁ or t₀₂ exceeds the respective predetermined upper limits.

17. The method of claim 15, further comprising

comparing t₀₁ to a predetermined lower limit to determine whether the tires on the first axle are properly inflated; and

comparing t₀₂ to a predetermined lower limit to determine whether the tires on the second axle are properly inflated.

18. The method of claim 15 further comprising

measuring the wheel speeds V_i1, V_i2, V_i3 and V_i4 a plurality of times m in a calibration time interval prior to the plurality of times n;

determining the first axle calibration factor MOD₁, the axle calibration factor MOD, discounting variations in rolling radii between the two wheels on the first axle;

modifying the normalized percentage difference NORM_i1 by MOD₁ during the plurality of times n;

determining the second axle calibration factor MOD₂, the axle calibration factor MOD₂ discounting variations in rolling radii between the two wheels on the second axle; and

modifying the normalized percentage difference NORM_i2 by MOD₂ during the plurality of times n.

19. The method of claim 15 further comprising determining when the vehicle is cornering and excluding wheel speeds measured during cornering from the steps used to calculate the respective t₀₁ and t₀₂ values.

20. The method of claim 15 further comprising determining when the vehicle is accelerating or decelerating within predetermined limits and excluding wheel speeds measured during acceleration or deceleration outside the predetermined limits from the steps used to calculate the respective t₀₁ and t₀₂ values.