Measuring and testing – Tire – tread or roadway – Tire inflation testing installation
Reexamination Certificate
2001-09-28
2003-12-30
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Tire, tread or roadway
Tire inflation testing installation
C073S146000, C073S146200, C073S146300, C073S078000, C152S415000
Reexamination Certificate
active
06668637
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Present Invention
The present invention relates to a tire air pressure estimating apparatus and, more particularly, to a tire air pressure estimating apparatus which estimates a friction state estimation value representing friction state between a tire and a road surface and which estimates a reduction of the tire air pressure based on the estimated friction state estimation value, without using a sensor for directly measuring the tire air pressure.
2. Description of the Related Art
For the purpose of detecting tire air pressure, it has been proposed to provide a pressure sensor in the tire to detect the tire air pressure directly. However, this method of directly detecting the tire air pressure has a problem in that a cost increase results from the need to provide a sensor in each tire. Further, when a vehicle is traveling at high speed, detection signals from the sensors can not be efficiently transmitted to the body of the vehicle, which makes it difficult to detect the tire air pressures.
Japanese Patent Application Laid-Open (JP-A) No. 8-164720 disclosed a technique of detecting an increase in angular velocity of rotation of a tire (wheel angular velocity), to thereby detect any reduction in the dynamic load radius of the tire, which is attributable to a reduction in the tire air pressure. This technique is now described in more detail. The following relationship exists between wheel angular velocity &ohgr;, vehicle speed v, slip rate s (−1<s<0 during driving), and dynamic load radius r during driving.
s
=(
v−r&ohgr;
)
r&ohgr;
(1)
Equation (1) can be re-arranged as follows with respect to the wheel angular velocity.
&ohgr;=
v
/{(1
+s
)
r}
(1′)
Therefore, in a low speed range at which great driving force is not required, the slip rate is small, so that a reduction of the dynamic load radius is detected as an increase in the wheel angular velocity. It is, for example, possible to detect a reduction in air pressure of one of left and right wheels caused by a puncture or the like, by detecting a difference between the angular velocities of the wheels.
However, as will be understood from the relationship shown in
FIG. 1
between tire air pressure, frictional force between the tire and a road surface (which is equivalent to a driving force), and slip rate, the slip rate is changed significantly by a reduction in tire air pressure during high speed travel at which a greater driving force is required to overcome air resistance, compared to change of the slip rate during low speed travel at which this great driving force is not required. As the slip rate s (−1<s<0) consequently approaches 0, the term (1+s) in Equation (1′) increases.
Accordingly, during high speed travel, the effect of a reduction in the dynamic load radius attributable to the reduction in tire air pressure and the effect of the increase of the term (1+s) are opposed by each other, and the reduction in the tire air pressure does not noticeably affect the wheel angular velocity of the driving wheel during high speed travel. This results in a problem that the accuracy of detection of the reduction in the tire air pressure of the driving wheel is reduced during high speed travel at which the greater driving force is required.
SUMMARY OF THE PRESENT INVENTION
The present invention has been conceived to solve the above problem, and it is an object of the present invention to provide a tire air pressure estimating apparatus capable of estimating tire air pressure of a driving wheel, even during high speed travel, without using a sensor that detects the tire air pressure directly.
The principle of the present invention will now be described. An increase in ground contact area of a tire as a result of a reduction in tire air pressure appears as a change in friction state between the tire and a road surface. As shown in
FIG. 2
, which is similar to
FIG. 1
, the gradient of a tangent of a curve representing a relationship between a frictional force between the tire and the road surface and a slip rate (or slip speed), which is a &mgr;-gradient of the road surface, increases as the tire air pressure decreases. That is, the &mgr;-gradient of the road surface increases as the tire air pressure decreases.
An increase in the &mgr;-gradient of the road surface can be estimated such that &mgr;-gradient of the road surface is estimated using a road surface &mgr;-gradient estimating technique, which is based on a wheel deceleration model, and an increase thereof is estimated. Alternatively, an increase in the &mgr;-gradient of the road can be estimated by estimating an increase in break point frequency that results from the increase in the &mgr;-gradient of the road surface, as will be described later, or by detecting a reduction in a vibration level at a special frequency. A gradient of braking force is represented by a gradient of a tangent of a curve that represents a relationship between slip speed (or, slip rate) and braking force. A gradient of driving force applied to the tire is represented by a gradient of a tangent of a curve that represents a relationship between slip speed (or, slip rate) and driving force. The gradient of braking force and the gradient of driving force are both physical quantities representing slipperiness between the tire and the road surface, or an estimation value of friction state representing friction state between the tire and the road surface. The gradient of braking force and the gradient of driving force are physical quantities equivalent to the &mgr;-gradient of the road surface, which represents grip state of the tire. Therefore, estimation of an increase in the &mgr;-gradient of the road surface will be described by describing estimation of an increase in the gradient of braking force.
As shown in
FIG. 3
, a dynamic model of a wheel resonance system can be represented by a model in which torsional spring elements
14
and
16
of a tire, having respective spring constants K
1
and K
2
, are interposed between a rim
10
and a belt
12
and in which a suspension element, provided by connecting a spring element
18
having a spring constant K
3
in parallel with a damper
20
, is interposed between the rim
10
and a vehicle body. In this model, a disturbance from the road surface (road surface disturbance) is transmitted from the belt
12
through the spring elements
14
and
16
to the rim
10
, to affect a wheel speed &ohgr;, and is transmitted to the vehicle body through the suspension element.
A description is now given of characteristics of transmission from road surface disturbance to the wheel speed for the braking force gradient, using a fifth order full wheel model, in which a first order wheel decelerating motion, second order longitudinal direction suspension resonance, and second order tire rotation resonance are integrated.
FIG. 4
is a gain diagram showing frequency responses from road surface disturbance to the wheel speed for ranges from a limit braking range to a low slip range where there is some margin for tire characteristics (for ranges from a range at which the braking force gradient is 300 Ns/m to a range at which the braking force gradient is 10000 Ns/m). That is, the diagram shows the relationship between frequency and gain of amplitude of the wheel speed with respect to amplitude of the road surface disturbance.
The wheel speed frequency characteristics in
FIG. 4
indicate that, when the braking force gradient is relatively small, such as near the limit of friction force between a tire and a road, the gain is great in a low frequency range and is small in a high frequency range. Namely, for the range where the braking force gradient is small, there is a big difference between the gain in the low frequency range and the gain in the high frequency range.
In contrast, the gain in the low frequency range for the range where the braking force gradient is relatively large, such as a stationary traveling region,
Asano Katsuhiro
Ono Eiichi
Umeno Takaji
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