Method, control device, and system for determining a profile depth of a profile of a tire

Abstract

A method for determining a tread depth of a tread of a tire during operation of a vehicle having the tire, a control device for a vehicle for determining a tread depth of a tread of a tire of the vehicle, and a system for a vehicle having such a control device and at least one electronic wheel unit, are provided. Provision is made to determine the tread depth based on a determined instantaneous dynamic wheel radius of a wheel, having the tire, of the vehicle and a determined instantaneous dynamic inside radius of the tire. In addition, at least one further first operating parameter of the tire, selected from the group including an instantaneous roadway gradient, an instantaneous vehicle drive mode and an instantaneous tire material expansion, is determined and taken into consideration.

Claims

1. A method for determining a tread depth of a tread of a tire during operation of a vehicle having the tire, the method comprising the following steps: determining an instantaneous rotational speed of a wheel having the tire on the vehicle based on data determined by at least one first sensor; determining an instantaneous speed of the vehicle based on data determined by at least one second sensor being different than the at least one first sensor; determining an instantaneous dynamic wheel radius of the wheel having the tire based on the determined instantaneous rotational speed and the determined instantaneous speed of the vehicle; determining at least one first operating parameter of the tire selected from the group consisting of an instantaneous tire temperature, an instantaneous tire pressure and an instantaneous tire load; determining an instantaneous dynamic inside radius of the wheel based on the at least one determined first operating parameter, and determining the inside radius of the wheel as a distance between a wheel center and a tire-side start of the tread; determining a tread depth of the tread of the tire based on the determined instantaneous dynamic wheel radius and the determined instantaneous dynamic inside radius; determining at least one further first operating parameter of the tire selected from the group consisting of an instantaneous roadway gradient, an instantaneous vehicle drive mode and an instantaneous tire material expansion; and further determining the instantaneous dynamic inside radius based on the at least one determined further first operating parameter.

2. The method according to claim 1, which further comprises: determining the instantaneous roadway gradient as the at least one further first operating parameter; and rejecting results of at least one of the instantaneously carried out determination or steps of the method based on the instantaneously carried out determination as invalid, upon the determined roadway gradient exceeding a predetermined roadway gradient threshold value.

3. The method according to claim 1, which further comprises determining the instantaneous roadway gradient as the at least one further first operating parameter, and carrying out a correction being linearly dependent on the determined instantaneous roadway gradient when determining the instantaneous dynamic inside radius.

4. The method according to claim 1, which further comprises determining the instantaneous roadway gradient based on a vehicle acceleration measured by using an acceleration sensor of the vehicle and on a vehicle longitudinal acceleration determined by evaluating at least one rotational speed of a wheel of the vehicle.

5. The method according to claim 1, which further comprises determining the instantaneous vehicle drive mode as the at least one further first operating parameter, and carrying out a correction being linearly dependent on a drive torque determined in dependence on the determined instantaneous vehicle drive mode when determining the instantaneous dynamic inside radius.

6. The method according to claim 1, which further comprises determining the instantaneous vehicle drive mode as the at least one further first operating parameter, and carrying out a correction implemented by using a lookup table depending on the determined instantaneous vehicle drive mode and the determined instantaneous speed of the vehicle when determining the instantaneous dynamic inside radius.

7. The method according to claim 1, which further comprises determining the instantaneous tire material expansion based on a tire age of the tire as the at least one further first operating parameter, and carrying out a correction depending on the determined instantaneous tire material expansion when determining the instantaneous dynamic inside radius.

8. The method according to claim 7, which further comprises determining the instantaneous tire material expansion based on a predetermined model function for mapping the instantaneous tire age of the tire onto the instantaneous tire material expansion, and providing the model function as a concave function.

9. The method according to claim 8, which further comprises predetermining the model function depending on tire information data about the tire.

10. The method according to claim 7, which further comprises: determining the tread depth based on the instantaneous tire age of the tire in a first phase immediately after installation of a new tire; learning an influence of the speed of the vehicle on the instantaneous dynamic wheel radius in a second phase; and determining the tread depth based on the determined instantaneous dynamic wheel radius and the determined instantaneous dynamic inside radius in a third phase following the second phase.

11. The method according to claim 10, which further comprises further determining the model function in the second phase by evaluating variables determined in the first phase and the second phase.

12. The method according to claim 1, which further comprises: determining the tread depth by taking into consideration a correction variable determined in advance for the type of tire; and using the correction variable to characterize a relationship between a change in the tread depth and a change resulting from the change in the tread depth in a difference between the dynamic wheel radius and the dynamic inside radius.

13. The method according to claim 12, which further comprises using a correction constant characteristic of the type of tire as the correction variable, and using the correction constant to represent a relationship between the change in the tread depth and the change resulting from the change in the tread depth in the difference between the dynamic wheel radius and the dynamic inside radius.

14. The method according to claim 12, which further comprises using a correction function being characteristic of the type of tire for mapping a difference between the dynamic wheel radius and the dynamic inside radius onto a change in the tread depth as the correction variable.

15. The method according to claim 1, which further comprises: additionally determining at least one second operating parameter selected from the group consisting of an instantaneous acceleration of the vehicle, an instantaneous yaw rate of the vehicle, an instantaneous steering angle, an instantaneous torque of a drive motor of the vehicle and an operating state of a braking device of the vehicle; and additionally determining the tread depth in dependence on the at least one determined second operating parameter.

16. A control device for a vehicle for determining a tread depth of a tread of a tire of the vehicle, the control device comprising: a reception device configured to receive an instantaneous rotational speed of a wheel of the vehicle having the tire, an instantaneous speed of the vehicle and at least one first operating parameter of the tire selected from the group consisting of an instantaneous tire temperature, an instantaneous tire pressure and an instantaneous tire load; a first determination device configured to determine an instantaneous dynamic wheel radius of the wheel having the tire based on the received instantaneous rotational speed and the received instantaneous speed of the vehicle; a second determination device configured to determine an instantaneous dynamic inside radius of the wheel based on the at least one received first operating parameter, the inside radius of the wheel being a distance between a wheel center and a tire-side start of the tread; a third determination device configured to determine a tread depth of the tread of the tire based on the determined instantaneous dynamic wheel radius and the determined instantaneous dynamic inside radius; said reception device being further configured to receive at least one further first operating parameter of the tire selected from the group consisting of an instantaneous roadway gradient, an instantaneous vehicle drive mode and an instantaneous tire material expansion; and said second determination device being further configured to determine the instantaneous dynamic inside radius additionally based on the at least one determined further first operating parameter.

17. A system for a vehicle for determining a tread depth of a tread of a tire of the vehicle, the system comprising: a control device according to claim 16; and at least one electronic wheel unit configured to be disposed in the tire, said at least one electronic wheel unit having at least one sensor selected from the group consisting of a temperature sensor, a pressure sensor, an acceleration sensor and a deformation sensor.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 shows a flowchart for a method for determining a tread depth of a tread of a tire,

(2) FIG. 2 shows a schematic plan view of a vehicle having a system by means of which the method of FIG. 1 can be carried out,

(3) FIG. 3 shows a schematic side view of a wheel of the vehicle of FIG. 2,

(4) FIG. 4 shows a schematic side view of a tire of the wheel of FIG. 3 for explaining geometrical relationships according to a tire model in a first limit case,

(5) FIG. 5 shows a schematic side view of the tire of the wheel of FIG. 3 for explaining geometrical relationships according to the tire model in a second limit case,

(6) FIG. 6 shows a block diagram of an electronic wheel unit used in the system of the vehicle of FIG. 2,

(7) FIG. 7 shows an illustration showing tire operation on a vehicle on a roadway with a roadway gradient which is different from zero,

(8) FIG. 8 shows an illustration showing various vehicle drive modes in a vehicle, and

(9) FIG. 9 shows an exemplary graph of an additional contribution, caused by tire material expansion, to the dynamic inside radius of a tire depending on a mileage of the tire.

DETAILED DESCRIPTION OF THE INVENTION

(10) FIG. 1 shows a flowchart of a method for determining a tread depth of a tread of a tire during operation of a vehicle having the tire according to one exemplary embodiment.

(11) In a step S1, an instantaneous rotational speed of a wheel, having the tire, of the vehicle is determined based on data determined by at least one first sensor. By way of example, an instantaneous angular speed ω of the wheel is determined. To this end, the at least one first sensor is preferably designed as a rotational speed sensor.

(12) In a step S2, an instantaneous speed v of the vehicle (vehicle longitudinal speed) is determined based on data determined by at least one second sensor which is different from the at least one first sensor. Determining the instantaneous speed v typically involves determining a value of a distance covered by the vehicle in a specific time interval based on the data determined by the at least one second sensor. To this end, the at least one second sensor is designed, for example, as a satellite-assisted position determination sensor (for example as a GPS device). Furthermore, the at least one second sensor can be designed, for example, as a radar sensor, lidar sensor, ultrasound sensor or optical camera in order to thereby determine, for example, a distance of the vehicle from objects identified as being stationary at different times and to determine the distance covered by the vehicle therefrom.

(13) In a step S3, an instantaneous dynamic wheel radius r1.sub.dyn of the wheel having the tire is determined based on the determined instantaneous rotational speed, that is to say the angular speed ω in the exemplary embodiment shown, and the determined instantaneous vehicle speed v. In the exemplary embodiment shown, this is performed by means of the relationship v=r1.sub.dyn.Math.ω, where, as already mentioned, v denotes the instantaneous speed of the vehicle, r1.sub.dyn denotes the instantaneous dynamic wheel radius of the wheel and ω denotes the instantaneous angular speed of the wheel.

(14) In a step S4, at least one first operating parameter of the tire selected from the group consisting of an instantaneous tire temperature T, an instantaneous tire pressure p and an instantaneous tire load RL is determined. At least two, in particular all three of said parameters, are preferably determined in this case. In particular, said parameters can be determined, for example, by means of an electronic wheel unit arranged in the tire in question, as is explained in yet more detail in connection with the further figures.

(15) In addition, three further first operating parameters of the tire (which can also be called “further operating parameters” for short), specifically an instantaneous roadway gradient SL, an instantaneous vehicle drive mode DT and an instantaneous tire material expansion BE, are further determined in step S4 in the exemplary embodiment shown.

(16) In the illustrated example, the instantaneous roadway gradient SL is defined as the instantaneous angle of inclination as viewed in the direction of travel, that is to say the angle of the roadway with respect to the horizontal direction.

(17) In the illustrated example, the instantaneous vehicle drive mode DT is an item of operating state information which indicates how a drive torque generated by a drive device of the vehicle is instantaneously distributed between the wheels of the vehicle.

(18) In the illustrated example, the instantaneous tire material expansion BE is defined as the instantaneous extent of an additional contribution r2.sub.dyn,BE to the dynamic inside radius r2.sub.dyn of the tire that results overall.

(19) In a step S5, the instantaneous dynamic inside radius r2.sub.dyn of the wheel is determined based on the at least one determined first operating parameter (T and/or p and/or RL) and the determined further (first) operating parameters (SL and DT and BE), wherein the inside radius of the wheel is the distance between the wheel center and the tire-side start of the tread.

(20) Here, the instantaneous dynamic inside radius r2.sub.dyn can additionally be determined based on the determined instantaneous speed v of the vehicle and/or additionally based on the type of tire since these parameters also have an influence on the inside radius of the tire that results in an operating situation. The type of tire can be stored, for example, in a memory device of an electronic wheel unit in question, for example together with an initial tread depth.

(21) The type of tire or data stored in respect of this can specify, in particular, for example whether said tire is a summer tire, a winter tire or an all-year tire and/or specify whether the tire material is relatively hard or relatively soft (in particular also in quantitative terms) and/or specify a type of tread.

(22) The type of the tread or data stored in respect of this can specify, in particular, for example the tread pattern defined by elevations and depressions and/or physical properties of the tread that result therefrom.

(23) The instantaneous dynamic inside radius r2.sub.dyn is preferably determined by means of at least one characteristic curve stored in a memory device.

(24) The dynamic inside radius r2.sub.dyn of the wheel typically increases as the tire temperature T increases and as the tire pressure p increases, whereas an increasing tire load RL typically leads to a reduction in the dynamic inside radius r2.sub.dyn. As the speed v of the vehicle increases, the dynamic inside radius r2.sub.dyn typically increases, wherein the increase in the inside radius r2.sub.dyn typically transitions into a saturation when a specific speed range is reached.

(25) Said dependencies can advantageously be determined beforehand, for example, by a corresponding measurement of a specimen of a tire of the type of tire in question on a test bench in order to determine one or preferably a plurality of characteristic curves or a characteristic curve diagram for the type of tire in question. When carrying out the method according to the invention during operation of the vehicle, it is possible to then use such characteristic curves stored in a memory device when determining the dynamic inside radius r2.sub.dyn.

(26) The determination of the instantaneous dynamic inside radius carried out in the step S4 is additionally based on the further (first) operating parameters SL, DT and BE. In the illustrated example, the instantaneous roadway gradient SL is firstly taken into consideration in such a way that, when an absolute value of SL exceeds a predetermined roadway gradient threshold value, the recording situation is judged to be highly unfavorable and all of the instantaneously determined variables for determining the tread depth are rejected as invalid. On the other hand, if the absolute value of SL does not exceed the roadway gradient threshold value, an additional contribution r2.sub.dyn,SL is calculated taking into consideration SL and at least the speed v of the vehicle and is added to a value of r2.sub.dyn calculated beforehand without taking into consideration SL (correction of r2.sub.dyn based on the roadway gradient SL).

(27) In the illustrated example, the instantaneous vehicle drive mode DT can be determined from so-called “torque vectoring” data of a vehicle electronics system and therefore a total drive torque (positive or negative) of the vehicle and also the distribution of said total drive torque between the individual wheels of the vehicle can be known. In the illustrated example, DT is then taken into consideration in a similar way to that for SL, specifically by calculating an additional contribution r2.sub.dyn,DT for the further correction of r2.sub.dyn.

(28) The additional contribution based on DT is generally different from wheel to wheel, depending on DT, and can be calculated in good approximation (just like the additional contribution based on SL) as a linear correction term, that is to say proportionally to a drive torque or an additional drive torque of the wheel in question.

(29) In the illustrated example, the instantaneous tire material expansion BE is determined based on a predetermined model function which maps a mileage, in the illustrated example an accumulated previous distance traveled, of the tire in question onto the instantaneous tire material expansion BE, and in which said model function is a concave function, similarly to a function which maps a value onto the square root of said value.

(30) This specifically selected model function or functional parameters which determine it can be stored, for example, in a memory device of the electronic wheel unit in question and therefore can be used after being called up by a control device.

(31) In a departure from this exemplary embodiment, provision can also be made, for example, to determine and/or to update (to improve) the model function by way of an influence of the speed v of the vehicle on the instantaneous dynamic wheel radius r1.sub.dyn being learnt at least periodically during operation of the vehicle in order to draw a conclusion about which point on the graph the model function is instantaneously located at on the basis of a quantitative change in said influence considered over relatively large tire age intervals (for example a mileage of at least 1000 km), and in order to define or to update the model function to be used in future on the basis of the data collected in such a learning phase. After the learning phase is complete, the tread depth can then be determined in a following further phase (in the last step of the method according to the invention) based on r1d.sub.yn and r2.sub.dyn, wherein the model function is accordingly taken into consideration when determining r2.sub.dyn. If no other model function which can be used for this is defined, the tread depth can be determined in another way, for example calculated as decreasing proportionally to the mileage.

(32) In a step S6, the tread depth td of the tread of the tire is determined based on the determined instantaneous dynamic wheel radius r1.sub.dyn and the determined instantaneous dynamic inside radius r2.sub.dyn.

(33) According to the teaching of DE 10 2012 217 901 B3 already mentioned at the outset, this tread depth determination (step S6) would take place based on both of said variables on the basis of the following relationship:
td=r1.sub.dyn−r2.sub.dyn

(34) Even though this relationship appears to yield good results for many types of tire and many operating situations with regard to tire and/or vehicle, it has been found that the tread depth determination based at least on dynamic wheel radius r1.sub.dyn and dynamic inside radius r2.sub.dyn can be further improved by a measure according to which the tread depth td is determined taking into consideration a correction variable determined beforehand for the type of tire, wherein the correction variable characterizes the relationship between a change in the tread depth td, on the one hand, and a change resulting therefrom in the difference between the dynamic wheel radius r1.sub.dyn and the dynamic inside radius r2.sub.dyn, on the other hand.

(35) Therefore, while it is a prerequisite according to said document for the tread depth determination for all types of tire that a change in the difference between r1.sub.dyn and r2.sub.dyn is always equivalent to a change of exactly the same magnitude in td (Δtd=Δ(r1.sub.dyn−r2.sub.dyn)), use is made in the tread depth determination according to this exemplary embodiment of the finding that this relationship does constitute a good approximation, but this does not always yield satisfactory results in particular for specific types of tire and/or in specific operating situations of the tire and/or of the vehicle.

(36) In the exemplary embodiment, a more or less large systematic error in the tread depth determination is advantageously partially compensated for by way of taking into consideration, as mentioned, the correction variable in order to accordingly improve the determination result.

(37) Before discussing further possible refinements of the method according to the invention in detail, a vehicle 1, which is equipped with a system for carrying out the method according to the invention, is first described with reference to FIGS. 2 and 3.

(38) FIG. 2 shows a vehicle 1 which has four vehicle wheels W1-W4 in the illustrated example. The vehicle 1 is, for example, a passenger car.

(39) The reference numerals of components that are present multiple times in one embodiment but have the same effect (for example the wheels W1-W4) are numbered consecutively and each have a hyphen and a consecutive number added. Reference is also made below to individual such components or to all such components by the reference numeral which does not have a hyphen and a consecutive number added.

(40) FIG. 3 shows, by way of example, a wheel W of the vehicle 1 (FIG. 2), which wheel may be any of the wheels W1-W4 shown in FIG. 2.

(41) The wheel W is formed by an air-filled tire 2 and a rim 3, wherein the tire 2 has a tread 4, schematically illustrated in FIG. 3, having a tread depth td.

(42) During operation of the vehicle 1 having the tire 2, the wheel W rotates about its wheel center 5. The arrow 7 symbolizes this wheel rotation, in which the radially outer end (facing away from the tire) of the tread 4 rolls on the driving surface. The radially inner end of the tread 4, that is to say the tire-side start of the tread 4, is denoted by the reference numeral 6.

(43) A tire load RL exerted by the vehicle 1 on the wheel W and therefore the tire 2 causes a certain deformation of the tire 2 in which, as illustrated in FIG. 3, a more or less large tire footprint is formed between the tire 2 and the driving surface, the length of which footprint is denoted L in FIG. 3.

(44) The wheel W is furthermore equipped with an electronic wheel unit 12 arranged thereon (cf. electronic wheel units 12-1 to 12-4 in FIG. 2), by means of which wheel operating parameters of the wheel W can be recorded and corresponding wheel operating data can be transmitted to a control device 20 of the vehicle 1 (FIG. 2) in the form of a periodically transmitted radio signal R (cf. radio signals R1-R4 in FIG. 2).

(45) In the illustrated example according to FIG. 2, a tire pressure monitoring system (TPMS) of the vehicle 1 is formed with the aid of the electronic wheel units 12-1 to 12-4. Accordingly, the electronic wheel units 12-1 to 12-4 each record at least the tire pressure p of the vehicle wheel in question from amongst the vehicle wheels W1-W4, and the electronic wheel units 12-1 to 12-4 transmit wheel operating data containing at least one item of information about the tire pressure p to the control device 20 of the vehicle 1.

(46) The electronic wheel units 12-1 to 12-4 are of identical design and are arranged on an inner side of a tire tread surface of the respective tire 2 in the illustrated example.

(47) With reference to FIG. 6, which shows the design of such an electronic wheel unit 12 (cf. electronic wheel units 12-1 to 12-4 in FIG. 2) by way of example, each of the wheel units 12 comprises a pressure sensor 14 for measuring the tire pressure p and for providing a sensor signal representing the tire pressure p, an acceleration sensor 15 for measuring a radial acceleration a at the mounting site of the electronic wheel unit 12 and for providing a sensor signal representing this radial acceleration a, and a temperature sensor 16 for measuring a tire temperature T in the interior of the tire 2 and for providing a sensor signal representing this tire temperature T.

(48) Said sensor signals are fed for processing to a program-controlled computer unit 18, the operation of which is controlled by a program code stored in an associated digital memory unit 17.

(49) The computer unit 18 generates the wheel operating data to be transmitted, which data is sent to the vehicle-side control device 20 in the form of the radio signals R.

(50) The vehicle-side control device 20 is connected for this purpose to a radio receiver 30 for receiving the radio signals R from all of the electronic wheel units 12.

(51) In the illustrated example, the measurement of the radial acceleration a and provision of the corresponding sensor signal is used to determine, by way of evaluating the sensor signal characteristic, values of an instantaneous rotational speed and/or rotational position of the wheel W in question and to incorporate corresponding information about this into the wheel operating data (radio signals R).

(52) The radio receiver 30 decodes the received data telegrams and forwards the information contained therein to a reception device 21 of the control device 20.

(53) In the exemplary embodiment shown, the vehicle 1 is further equipped with a satellite-assisted position determination unit in the form of a GPS device 8 which is designed to determine position data relating to the current position of the vehicle 1 by way of receiving and evaluating radio signals from a plurality of satellites S. The GPS device 8 can constitute a component of a navigation system that is often provided in modern vehicles in any case and transmits the determined position data and/or a speed v of the vehicle 1 determined therefrom to the reception device 21 of the control device 20.

(54) The vehicle 1 is further equipped with a vehicle acceleration sensor 9 which is arranged fixed to the vehicle and is designed to determine an instantaneous acceleration A.sub.veh of the vehicle 1 and to output corresponding data to the reception device 21 of the control device 20.

(55) The vehicle 1 is further equipped with rotational speed sensors 10-1 to 10-4 which are each arranged fixed to the vehicle and designed to determine values of the instantaneous rotational speed and/or rotational position of the vehicle wheel in question from amongst the vehicle wheels W1-W4 and to output said values to the reception device 21 of the control device 20 in the form of rotational speed sensor data D1, D2, D3 or D4.

(56) The control device 20 further has a program-controlled computer unit 22 and an associated memory unit 28, wherein, in particular, a program code used to operate the computer unit 22 can be stored in the memory unit 28.

(57) The computer unit 22 of the control device 20 in connection with said program code in the memory unit 28 at the same time implements the determination devices 23, 24 and 25 symbolically indicated in FIG. 2 and described below with regard to their function.

(58) The manner of operation of the control device 20 for the vehicle 1 in order to determine the tread depth td of the tread 4 of a tire 2 can be described as follows:

(59) The reception device 21 receives the instantaneous rotational speed, for example the angular speed co, of the wheel W having the tire 2 in question, the instantaneous speed v of the vehicle 1 and at least one operating parameter of the tire, selected from the group consisting of the instantaneous tire temperature T, the instantaneous tire pressure p and the instantaneous tire load RL.

(60) In the illustrated example, the rotational speed is determined based on the rotational speed sensor data D of the rotational speed sensor in question from amongst the rotational speed sensors 10-1 to 10-4. As an alternative or in addition, the radio signal R in question could also be used for this purpose.

(61) The instantaneous speed v of the vehicle 1 is determined based on that data which is determined by the GPS device 8.

(62) In the illustrated example, the electronic wheel unit 12 in question, which here delivers data relating to the instantaneous tire temperature T and relating to the instantaneous tire pressure p in particular, is used to determine the at least one first operating parameter of the tire 2. This data can also contain for example, information about the instantaneous tire load RL, which may be achieved in the illustrated example, for example, by way of the instantaneous length L of the tire footprint first being determined, for example, by suitable evaluation of the sensor signal provided by the acceleration sensor 15 in order to determine the tire load RL therefrom taking into consideration further parameters, for example the tire temperature T and/or the tire pressure p in particular.

(63) In a departure from this exemplary embodiment, instead of the acceleration sensor 15, a deformation sensor could also be used, for example, in order to accordingly determine the described parameters L and RL by evaluating the sensor signal profile of the deformation sensor.

(64) The reception device 21 further receives all three of the further first operating parameters of the tire 2 used in this example, that is to say the instantaneous roadway gradient SL, the instantaneous vehicle drive mode DT and the instantaneous tire material expansion BE.

(65) In the illustrated example, the instantaneous roadway gradient SL is calculated based on the vehicle acceleration A.sub.veh measured by means of the vehicle acceleration sensor 9 of the vehicle 1 and a vehicle longitudinal acceleration determined by evaluating the rotational speeds of the wheels W1-W4, wherein the rotational speeds from the rotational speed sensor data D1-D4 are used (here the rotational angular speeds ω).

(66) FIG. 7 illustrates, by way of example, a situation in which the vehicle 1 is traveling uphill, wherein SL represents the angle of inclination as viewed in the direction of travel which is to be determined by said vehicle from the abovementioned vehicle accelerations. The acceleration sensor 9 measures the effective vehicle acceleration A.sub.veh which is given as the vectorial sum of the vehicle longitudinal acceleration A.sub.lon and acceleration due to gravity A.sub.g. In contrast, the vehicle acceleration determined using the rotational speed sensor data D1-D4 is equal to the vehicle longitudinal acceleration A.sub.lon, and therefore the angle of inclination SL can be calculated on the basis of a vectorial comparison of A.sub.veh and A.sub.lon.

(67) The vehicle drive mode DT is transmitted to the reception device 21 from another part (not illustrated) of the vehicle electronics system of the vehicle 1.

(68) FIG. 8 shows, by way of example, two different vehicle drive modes on the basis of the drive forces F1-F4 exerted on the roadway by the wheels W1-W4 in each mode. In FIG. 8, the forces F1-F4 are illustrated using solid lines for an all-wheel drive mode, whereas drive forces F3 and F4 for a rear-wheel drive mode are indicated using dashed lines.

(69) If a changeover is made, for example, from the rear-wheel drive mode to the all-wheel drive mode during driving, this leads to the slip increasing, for example, at the wheel W1. However, the influence on the result of the tread depth determination actually caused in this way is corrected or at least partially compensated for by taking into consideration DT when determining r2.sub.dyn.

(70) The tire material expansion BE is determined in the control device 20 using the computer unit 22 on the basis of a model function, wherein the model function or functional parameters determining this model function can be stored, for example, in the memory unit 28 and optionally periodically updated.

(71) FIG. 9 shows, by way of example, a typical profile of the instantaneous tire material expansion BE depending on the mileage dist of the tire in question. During normal operation, there is typically initially a linear increase in BE as the mileage dist increases, wherein the increase in BE generally decreases and is very small toward the end of the service life of the tire. With such a convex model function as shown in FIG. 9 by way of example, the instantaneous tire material expansion BE can be determined from a tire age in question which is defined as the mileage dist here. This tire material expansion BE is then used as an additional contribution r2.sub.dyn,BE due to the tire material expansion when determining the dynamic inside radius r2.sub.dyn of the tire in question.

(72) The first determination device 23, which is formed by a first partial functionality of the control program running in the computer unit 22 in the exemplary embodiment shown, determines an instantaneous dynamic wheel radius r1.sub.dyn of the wheel W having the tire 2 in question based on the received instantaneous rotational speed (here: angular speed ω) and the received instantaneous speed v of the vehicle 1. This is performed in accordance with the relationship r1.sub.dyn=v/ω.

(73) The second determination device 24, which is formed by a second partial functionality of the control program in the exemplary embodiment shown, determines the instantaneous dynamic inside radius r2.sub.dyn of the wheel W based on the at least one received first operating parameter (here, in particular, for example the tire temperature T, the tire pressure p and the tire load RL) and further based on the further (first) operating parameters roadway gradient SL, vehicle drive mode DT and tire material expansion BE.

(74) The dynamic inside radius r2.sub.dyn can be determined, for example, by way of determining the dynamic inside radius r2.sub.dyn beforehand as the difference between the dynamic wheel radius r1.sub.dyn and the defined tread depth td.sub.0 (for example td.sub.new) in the case of a tire having a defined tread depth td.sub.0, for example a new tire with a new tread depth td.sub.new (r2.sub.dyn=r1.sub.dyn−td.sub.0), and then storing this inside radius r2.sub.dyn determined thereby, for example, at the beginning of the tire life, for example, for the entire remainder of the tire life as a base value in the control device 20 or its memory unit 28 and, when the method is carried out, further correcting said inside radius by way of the second determination device 24 based on the at least one first operating parameter and the further (first) operating parameters in order to determine the instantaneous dynamic inside radius r2.sub.dyn.

(75) The third determination device 25, which is formed by a third partial functionality of the control program in the exemplary embodiment shown, determines the tread depth td of the tread 4 of the tire 2 based at least on the determined values r1.sub.dyn and r2.sub.dyn.

(76) One particular feature of the determination device 25 or of the tread depth determination is that in this case a correction variable F determined beforehand for the type of tire 2 in question is taken into consideration, which correction variable characterizes the relationship between a change Δtd in the tread depth td, on the one hand, and a change Δ (r1.sub.dyn−r2.sub.dyn) resulting therefrom in the difference between the dynamic wheel radius r1.sub.dyn and the dynamic inside radius r2.sub.dyn, on the other hand.

(77) In the illustrated example, this correction variable F, which is described in even more detail below, is stored in the memory unit 28 of the control device 20 and was determined beforehand by measuring the type of tire in question on a tire test bench.

(78) With regard to the correction variable F, two limit cases of a mathematical-physical model of a tire 2 installed, for example, on the vehicle 1 of FIG. 2 are explained below with reference to FIG. 4 and FIG. 5.

(79) Although this mathematical-physical model may explain the significance and the advantage of the process according to the invention of taking into consideration the correction variable F, the model should in no way be understood as restricting the present invention, but rather merely to be an illustrative explanatory approach for the advantage achieved by way of the invention.

(80) FIG. 4 and FIG. 5, in a schematic side view corresponding to FIG. 3, once again each show, in isolation, the tire 2 in an operating situation, that is to say rolling on a driving surface and in this case loaded by a wheel load RL acting from above. The tread 4 or its tread depth td is illustrated with an exaggerated size in FIGS. 4 and 5 for the sake of clarity.

(81) In addition to a geometric wheel radius r1 and a geometric inside radius r2, which can be measured on the unloaded tire 2 for example, an outer circumference U1 at the outer tread end and an inner circumference U2 at the tread start 6 of the tire 2 are also indicated.

(82) FIG. 4 shows a case in which the tire 2 has only slight deformation in the region of its contact with the driving surface and therefore a small tire footprint. In this case, the cross section of the tire 2 can accordingly be considered to be approximately a circular object, in which circumferential sections 11 on the outer circumference U1 and 12 on the inner circumference U1 situated in the contact region between the tire 2 and the driving surface, as illustrated, have different lengths (11>12).

(83) In contrast, FIG. 5 shows a case in which, depending on the type of tire, the tire 2 is deformed on the bottom side in the contact region between the tire 2 and the driving surface to such an extent that the rolling of the tread 4 exhibits a similarity to the rolling of the track of a tracked chassis or of a tracked vehicle (for example a tank, a bulldozer, a snowmobile etc.).

(84) Using this tank track analogy, FIG. 5 indicates two conceptual tracked chassis rollers over which the tread 4 of the tire 2 runs like a track of a tracked chassis provided with these rollers.

(85) In this case, the circumferential sections 11 on the outer circumference U1 and 12 on the inner circumference U2 situated in the contact region between the tire 2 and the driving surface are of substantially the same length. This description or this limit case is also referred to below as tank track limit case.

(86) It is pointed out at this juncture that, in particular depending on the type of tire 2 and on specific operating parameters of the operation of the tire 2 and of the vehicle 1, cases that lie as it were between these two limit cases according to FIG. 4 and FIG. 5 can be imagined and are even particularly relevant in practice.

(87) With regard to the dynamic radii, a difference Δr.sub.dyn with respect to measurements on the outer circumference U1 and inner circumference U2 may be defined as Δr.sub.dyn=11−12.

(88) However, to determine the tread depth td or a change Δtd in the tread depth td from which the tread depth td can be easily determined, the geometric radii of the tire 2 or a corresponding difference Δr.sub.geom is critical: Δtd=Δr.sub.geom, where Δr.sub.geom=r1−r2, and where r1 denotes the geometric wheel radius and r2 denotes the geometric inside radius of the wheel 2.

(89) However, in the tread depth determination according to the invention (cf. step S6 in FIG. 1), the dynamic difference Δr.sub.dyn which can be obtained from the determined values of r1.sub.dyn and r2.sub.dyn can advantageously be mapped onto the ultimately critical difference Δr.sub.geom (=Δtd).

(90) In the limit case of FIG. 4, a change in a geometric radius is linked directly to a change of identical magnitude in the corresponding dynamic radius, giving: Δr.sub.dyn=Δr.sub.geom.

(91) However, in the limit case of FIG. 5 (tank track limit case), a change Δr.sub.geom or equivalently a tread depth change Δtd does not result in any corresponding change Δr.sub.dyn in the dynamic radius. It holds true that Δr.sub.dyn=0.

(92) This can be clearly seen in FIG. 5: In the tank track limit case, a change Δtd in the tread depth td does not bring about a change in the relationship between the rotational speed (for example angular speed co) of the tire 2 and the speed v of the vehicle 1. Rather, the value of the tread depth td in this case influences only the vertical distance between the wheel center 5 and the driving surface, but not said relationship.

(93) Based on the above observations, for example according to one particularly simple embodiment, a linear relationship or linear mapping between a dynamic radius or a change in a dynamic radius and the associated geometric radius or the change in the associated geometric radius can be assumed, which linear relationship can be expressed, with reference to corresponding differences, by the following relationship:
Δr.sub.geom=F.Math.Δr.sub.dyn
where F represents a correction constant characteristic of at least the type of tire 2 and which represents the relationship between the difference or change Δtd in the tread depth td, on the one hand, and the corresponding difference or change Δ(r1.sub.dyn−r2.sub.dyn) resulting therefrom, on the other hand.

(94) In this embodiment, for the case according to FIG. 4, it accordingly holds true that F=1, whereas, for the case according to FIG. 5 (tank track limit case), the correction constant F diverges (F=∞).

(95) In this limit case (FIG. 5), a determination (calculation) of the geometric tread depth change Δtd fails in theory due to a determined change Δ(r1.sub.dyn−r2.sub.dyn) in the dynamic radius difference r1.sub.dyn−r2.sub.dyn alone.

(96) In practice, however, it is advantageous, for common tires 2, for the behavior to lie rather between the described limit cases according to FIG. 4 and FIG. 5, such that the limit case F=∞ does not occur in the mentioned linear mapping. In this respect, it is feasible, in the linear mapping, to provide a correction constant F which is finite but which may be considerably greater than 1. In one embodiment, the correction constant is selected so as to lie in the range of from 2 to 4.

(97) Independently of this, in practice, in the tread depth determination according to the invention based on the determined instantaneous dynamic wheel radius r1.sub.dyn and the determined instantaneous dynamic inside radius r2.sub.dyn, provision can be made for the tread depth td to be determined, in the case of which td is not determined exclusively depending on the difference r1.sub.dyn−r2.sub.dyn, but rather one or more further variables of the type already described, in particular operating parameters of the tire 2 in question and/or of the vehicle 1 in question, are taken into consideration or jointly taken into consideration.

(98) Therefore, provision can be made, for example, for the tread depth td to be determined, as an alternative or in addition to said dependency on the difference r1.sub.dyn−r2.sub.dyn, depending on the dynamic wheel radius r1.sub.dyn and/or the dynamic inside radius r2.sub.dyn.

(99) As an alternative or in addition, further variables such as, in particular, for example operating parameters of the vehicle operation, for example the speed v of the vehicle 1, and/or operating parameters of the tire operation, for example tire temperature T, tire pressure p and/or tire load RL, may be provided as variables, on which the result of the tread depth determination (step S6 in FIG. 1) depends.

(100) As an alternative to using a correction constant, a correction function F1, characteristic at least of the type of tire 2, for mapping the difference r1.sub.dyn−r2.sub.dyn onto a tread depth change Δtd can also be used as the correction variable F.

(101) In this embodiment, for example a change Δr.sub.geom in the geometric radius and accordingly a geometric change Δtd in the tread depth td can be calculated as a value of the function F1 which depends at least on the change Δr.sub.dyn in the dynamic radius: Δr.sub.geom=F1(Δr.sub.dyn).

(102) Such a mapping function F1 can be stored, for example, by means of one or more characteristic curves in a memory device of the system used for carrying out the method according to the invention, for example in the form of a lookup table.

LIST OF REFERENCE SIGNS

(103) 1 Vehicle v Speed of the vehicle W1 to W4 Wheels of the vehicle ω Angular speed of the wheel 2 Tire RL Tire load 3 Rim L Length of the tire footprint 4 Tread of the tire td Tread depth 5 Wheel center 6 Start of the tread 7 Wheel rotation S Satellites 8 GPS device 9 Vehicle acceleration sensor A.sub.veh Vehicle acceleration A.sub.g Acceleration due to gravity A.sub.lon Vehicle longitudinal acceleration 10-1 to 10-4 Rotational speed sensors D1 to D4 Rotational speed sensor data 12-1 to 12-4 Electronic wheel units R1 to R4 Radio signals 14 Pressure sensor p Pressure 15 Acceleration sensor a Radial acceleration 16 Temperature sensor T Tire temperature 17 Memory unit 18 Computer unit 19 Radio transmitter 20 Control device 21 Reception device 22 Computer unit 23 First determination device 24 Second determination device 25 Third determination device 28 Memory unit 30 Radio receiver U1 Outer circumference 11 Circumferential section (on outer circumference) r1 Geometric wheel radius r1.sub.dyn Dynamic wheel radius U2 Inner circumference 12 Circumferential section (on inner circumference) r2 Geometric inside radius r2.sub.dyn Dynamic inside radius SL Instantaneous roadway gradient C.sub.SL Coefficient r2.sub.dyn,SL Additional contribution based on roadway gradient DT Instantaneous vehicle drive mode Tq (Additional) drive torque of the wheel F1 to F4 Drive forces C.sub.Tq Coefficient r2.sub.dyn,DT Additional contribution based on vehicle drive mode BE Instantaneous tire material expansion dist Mileage of the tire (in kilometers) r2.sub.dyn,BE Additional contribution based on tire material expansion