Method and system for ascertaining a pressure ratio between a setpoint tire pressure and an actual tire pressure for tire of a vehicle

Abstract

A pressure ratio between a setpoint tire pressure and an actual tire pressure for a tire of a vehicle is ascertained by the following steps: Ascertaining a wheel load which is acting on the tire. Ascertaining a dynamic tire radius of the tire. Ascertaining the pressure ratio as a function of the wheel load and the dynamic tire radius.

Claims

1. A method for indicating a vehicle tire pressure, the vehicle including a control device, a force sensor, a wheel sensor, a velocity sensor, and a display mounted in the vehicle, the method comprising: measuring, by the force sensor, a wheel load acting on the tire based on a deflection of a suspension element; measuring, by the wheel sensor, a number of rotations of the tire, and ascertaining, by the control device, a dynamic tire radius of the tire based on the number of rotations and a distance traveled by the vehicle; measuring, by the velocity sensor, a velocity of the vehicle; ascertaining by the control device, a tire rigidity based on a predefined tire rigidity function using the velocity of the vehicle and the wheel load; ascertaining by the control device, a pressure ratio between a setpoint tire pressure and an actual tire pressure for the vehicle, as a function of the tire rigidity, the wheel load and the dynamic tire radius, and; displaying, on the display, the pressure ratio.

2. The method according to claim 1, wherein the pressure ratio is ascertained, by the control device, as a function of a constant of the tire, which indicates a setpoint ratio between the dynamic tire radius and an unloaded tire radius.

3. The method according to claim 2, wherein: the pressure ratio is calculated, by the control device, in accordance with the following equation:
p.sub.Rel=(1c.sub.Actual)(1c.sub.Ideal), p.sub.Rel being the pressure ratio, c.sub.Actual being a ratio between the dynamic tire radius and the unloaded tire radius, and c.sub.Ideal being the constant of the tire.

4. The method according to claim 1, further comprising: ascertaining, by a pressure sensor, an actual tire pressure of the tire; and ascertaining, by the control device, a setpoint tire pressure of the tire, by multiplying the pressure ratio by the actual tire pressure.

5. The method according to claim 1, further comprising: ascertaining a tire rigidity; ascertaining a difference between an unloaded tire radius of the tire in an unloaded state and the dynamic tire radius as a function of the wheel load and the tire rigidity; and ascertaining the unloaded tire radius from a sum of the dynamic tire radius and the difference.

6. The method according to claim 5, further comprising: predefining a tire rigidity function; detecting, by a pressure sensor, a tire pressure of the tire; ascertaining, by a velocity sensor, a velocity of the vehicle; and ascertaining, by the control device, the tire rigidity based on the tire rigidity function as a function of the tire pressure, the velocity, and the wheel load.

7. The method according to claim 5, further comprising: acquisition, by the control device, of measured values of an actual wheel load, by the force sensor, and an actual velocity of the vehicle, by a velocity sensor, at different points in time, ascertaining a tire rigidity function, by the control device, which indicates the tire rigidity as a function of speed, the wheel load, and the tire pressure, based on the measured values recorded at different instants; and ascertaining the tire rigidity, by the control device, as a function of the tire rigidity function.

8. A system for indicating a vehicle tire pressure, comprising: a force sensor adapted to measure a wheel load acting on the tire based on a deflection of a suspension element; a wheel sensor adapted to measure a number of rotations of the tire; a velocity sensor adapted to measure a velocity of the vehicle; and a control device adapted to: ascertain a dynamic tire radius of the tire based on the number of rotations of the tire the distance traveled by the vehicle; ascertain a tire rigidity based on a predefined tire rigidity function using the velocity of the vehicle and the wheel load; and ascertain a pressure ratio between a setpoint tire pressure and an actual tire pressure for the vehicle, as a function of the tire rigidity, the wheel load and the dynamic tire radius; and a display, adapted to display the pressure ratio.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Using FIG. 1, the terms static, dynamic and unloaded tire radius are explained.

(2) FIG. 2 illustrates examples of the tire rigidity (ratio between the difference between dynamic and unloaded tire radius and the wheel load).

(3) FIG. 3 illustrates the flow chart of a method according to an example embodiment of the present invention for ascertaining the pressure ratio between setpoint tire pressure and an actual tire pressure.

(4) FIG. 4 schematically shows a vehicle having a system according to an example embodiment of the present invention.

DETAILED DESCRIPTION

(5) FIG. 1 shows a tire 5, which has a static tire radius r.sub.stat, a dynamic tire radius r.sub.dyn and an unloaded tire radius r.sub.0. Static tire pressure r.sub.stat defines the shortest distance between center axis 6 of tire 5 and the road on which the vehicle is standing. Dynamic tire radius r.sub.dyn is able to be calculated on the basis of the rolling circumference of the tire. Unloaded tire radius r.sub.0 is the largest radius measurable on tire 5 and corresponds to the particular tire radius that tire 5 assumes in the unloaded state (i.e., no wheel load is acting on the tire). In summary, the following in equation (5) applies:
r.sub.stat<r.sub.dyn<r.sub.0(5)

(6) Reference symbol d denotes the difference or the distance between unloaded tire radius r.sub.0 and dynamic tire radius r.sub.dyn.

(7) In FIG. 2, the approximately linear relation or ratio m between difference d and wheel load f acting on individual wheel 5 is shown for a particular tire or tire type. Ratio m or the relation between d and f depends on the actual tire pressure and on the speed at which the vehicle is driving. FIG. 2 shows ratio m for pressures 1.7 bar, 2.2 bar and 2.7 bar and for a speed of 0 km/h (i.e., for standstill), and for a speed of 200 km/h.

(8) FIG. 3 shows a flow chart of a method according to an example embodiment of the present invention for ascertaining pressure ratio p.sub.Rel and for ascertaining ideal tire pressure p.sub.Ideal.

(9) In first step S1, dynamic tire radius r.sub.dyn is ascertained. In general, it holds true that the lower the tire pressure and the higher the wheel load, the more the individual tire will be deformed in the region in which the tire contacts the road (i.e., the greater the difference between dynamic tire radius r.sub.dyn and unloaded tire radius r.sub.0, and thus difference d).

(10) In second step S2, wheel load f acting on the individual tire is ascertained. Wheel load f may be recorded by corresponding sensors, which in case of an air suspension, for instance, measure the air pressure within the corresponding suspension element. In a conventional suspension system of the vehicle, wheel load f can be detected based on the deflection of the corresponding suspension element.

(11) If no corresponding sensors are available, wheel load f acting on the individual tire is able to be input manually by the driver of the vehicle, for instance. Simplifications are possible, for example in that only one load per axle of the vehicle is predefined and a symmetrical load distribution is assumed, so that the predefined axle load is acting on the individual tire by half a share.

(12) In third step S3, the actual tire pressure of the individual tire is detected by pressure sensors installed inside the tire, for example. This step S3 is optional and required in particular when an absolute value is to be calculated for the setpoint tire pressure. Even without this step S3, the method still allows a relative tire pressure p.sub.rel to be determined. Relative tire pressure p.sub.rel supplies information, for instance that the tire pressure of the affected tire is too high or too low by 10%.

(13) In fourth step S4, a tire rigidity function is ascertained, by which tire rigidity m (ratio of the difference between dynamic tire radius r.sub.dyn and unloaded tire ratio r.sub.0 and wheel load f acting on the particular tire) is able to be ascertained as a function of the wheel load, the tire pressure and the speed of the vehicle.

(14) To do so, measured values are recorded or ascertained for the wheel load and the speed for each tire of the vehicle across a certain period of time. If the actual tire pressure is available as well, corresponding measured values are recorded in addition.

(15) For example, using a Kalman filter (or a similar approach), the constants or parameters of a function m=func (f, p, v) are able to be ascertained in advance using the previously recorded measured values. The parameters estimated with the aid of the Kalman filter, for instance, describe certain characteristics of the individual tire or tire type and define the tire rigidity function, so that ratio m is able to be ascertained for the variables of wheel load f, tire pressure p, and velocity v.

(16) As an alternative, the function may also be provided in the form of a database, in which the previously described function with the aid of which the tire rigidity is able to be calculated as a function of the wheel load, the tire pressure and the vehicle velocity, is stored for the various types of tires. Furthermore, there is the option of storing the data that describes the tire rigidity function on an RFID tag directly in or on the tire, for example, and of reading out these data in a contactless manner.

(17) As soon as this function is known or has been determined, tire rigidity m is able to be ascertained in following step S5 as a function of wheel load f and tire pressure p.sub.Actual with the aid of the tire rigidity function. In following step S6, difference d results from the product of tire rigidity function m and wheel load f. Difference d may be used in following step S7 for calculating unloaded tire radius r.sub.0 by adding dynamic tire radius r.sub.dyn and difference d.

(18) Since dynamic tire radius r.sub.dyn and unloaded tire radius r.sub.0 are now known, ratio c.sub.Actual can then be ascertained in following step S8. Given knowledge of setpoint ratio c.sub.Ideal this ratio may be used in step S9 for ascertaining relative pressure ratio p.sub.Rel, so that ideal tire pressure p.sub.Ideal is able to be calculated in step S10 with the aid of the product of relative pressure ratio p.sub.Rel and actual tire pressure p.sub.Actual.

(19) FIG. 4 schematically shows a vehicle having a system 20 according to an example embodiment of the present invention. The system 20 includes a control 1, a tire pressure sensor 2, a force sensor 3, and a velocity sensor 4.

LIST OF REFERENCE CHARACTERS

(20) 1 control 2 tire pressure sensor 3 force sensor 4 velocity sensor 5 tire 6 tire center axis 10 vehicle 20 system c.sub.Actual ratio between dynamic tire radius and unloaded tire radius c.sub.Ideal optimal ratio between dynamic tire radius and unloaded tire radius d difference (r.sub.0r.sub.dyn) f wheel load m tire rigidity p.sub.Actual actual tire pressure p.sub.Ideal setpoint tire pressure p.sub.Rel pressure ratio r.sub.0 tire radius in the unloaded state r.sub.dyn dynamic tire radius r.sub.stat static tire radius S.sub.1-S.sub.10 method step