Abstract:
A method is described for determining a vehicle tire tread depth using at least one sensor situated in or on the vehicle tire, the tire tread depth being determined as a function of a tire contact area detected by the sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for determining a vehicle tire tread depth. 
     BACKGROUND INFORMATION 
     German printed patent publication DE 103 04 126 A1, for example, discusses an electronic device for measuring a tire tread depth and a pressure loss of a vehicle tire, a reference value of a distance traveled being compared to the wheel revolutions of an ABS wheel speedometer sensor system and the actual tire diameter and the tread depth being inferred on the basis of correlation algorithms. A disadvantage is that this method and respectively this device is too inaccurate to allow for a comparatively precise and reliable determination of the vehicle tire tread depth. 
     SUMMARY OF THE INVENTION 
     The method of the present invention for determining a vehicle tire tread depth in accordance with the independent claims has the advantage over the related art that it allows for the tire circumference to be determined directly in a particularly simple manner using only one single sensor. This is achieved by the fact that the sensor measures both the tire revolution time of the vehicle tire as well as the tire contact time. The tire revolution time is proportional to the tire circumference, while the tire contact time is essentially constant even when the tire circumference changes and/or the change of the tire contact time is less than the change of the tire revolution time since the tire contact time in the first approximation depends primarily on the weight of the vehicle. The change in the proportion of the tire contact time to the tire revolution time is thus a measure for the changing tire circumference. The sensor may be situated in a tire section that has a radius unequal to zero with respect to the tire axle such that a sensor tire section denotes a section of a tire tread that has an overlap with the sensor in the radial direction of the vehicle tire. In the sense of the present invention, the tire contact area comprises an area of a contact region between the vehicle tire and a roadway. The tire contact time and the tire revolution time are measured for example by measuring an action of force on the sensor, the sensor tire section in a first time interval being in the tire contact area such that essentially the force of gravitation acts on the sensor and the sensor tire section in a second time interval being outside of the tire contact area such that essentially a centrifugal force caused by the rotation of the vehicle tire acts on the sensor. The first time interval thus comprises the tire contact time, while a sum of the first and the second time interval comprises the tire revolution time. The first, the second and/or the sum of the first and the second time interval are determined by measuring time values at points in time at which the action of force on the sensor exhibits a comparatively great change, i.e. in a transitional range from the gravitational force to the centrifugal force or vice versa. In a particularly advantageous manner, the measurement of the tire contact area additionally makes it possible to determine the loading state of the vehicle. 
     According to another exemplary embodiment, for determining the average vehicle tire tread depth, an average is formed over many individual determinations of the vehicle tire tread depths such that the accuracy of the tire tread depth determination is increased in a particularly advantageous manner. In particular, an averaging over many individual determinations is provided, which are ascertained by different measuring methods. 
     According to another exemplary embodiment, the vehicle tire tread depth is determined as a function of first correction data of a memory, the memory having characteristic curves for the vehicle tire and the characteristic curves may include information regarding the dimensions, rolling properties, rolling radius changes and factory tire tread depths of various tire types. Particularly advantageously, this information about the vehicle tire allows for a correction of the tire tread depth so as to increase its accuracy. In particular, the dimension information of a new tire makes it possible to determine or detect an absolute tire tread depth, for example of a minimum tread depth of 1.6 mm. Furthermore, a changing rolling radius of the vehicle tire is taken into account. 
     According to another exemplary embodiment, the vehicle tire tread depth is determined as a function of second correction data of a tire pressure sensor. In addition to depending on the vehicle&#39;s weight, the tire contact area or tire contact time has a comparatively low dependence on the tire pressure such that taking into account the tire pressure with the aid of a tire pressure sensor advantageously increases the accuracy of the vehicle tire tread depth determination. 
     According to another exemplary embodiment, the vehicle tire tread depth is determined as a function of third correction data, the third correction data comprising differences between the tire revolution times of a plurality of tires of the vehicle, which occur in particular in a steering operation. Advantageously, another correction of the tire tread depth is thus effected since in particular when cornering the tire revolution time of the vehicle tires may vary independently of a tire circumference change. 
     According to another exemplary embodiment, the vehicle tire tread depth is determined as a function of fourth correction data of a traction control system, of inclination sensors and/or of displacement sensors such that advantageously an additional correction of the tire tread depth may be achieved to increase its accuracy. In particular, the correction data of the inclination sensors allow for a detection of the loading state of the vehicle, which has a comparatively great influence on the size of the tire contact area and thus on determining the tread depth. 
     According to another exemplary embodiment, the vehicle tire tread depth is communicated to a driver, a warning signal is produced when the vehicle tire tread depth falls below a limit value and/or a maximum range of the vehicle is determined and/or communicated to the driver before the vehicle tire tread depth falls below the limit value. In a particularly advantageous manner, the driver is thus constantly informed about the current tire tread depth of his vehicle, in particular visually and/or acoustically, such that the accident and breakdown risk as a result of tire damage and/or lack of tire tread depth is significantly reduced. Furthermore, on the basis of the calculation of the remaining maximum range, the driver is able to plan an impending tire change early, in particular prior to longer trips. 
     According to another exemplary embodiment, the vehicle tire tread depth is determined in a central control unit, which may receive the tire revolution time, the speed value, the tire contact area, the average value, the vehicle tire tread depth, the range, the limit value, the warning signal, the first correction data, the second correction data and/or the third correction data inductively and/or electromagnetically. In a particularly advantageous manner, the central control unit may thus be situated in a location in the motor vehicle that is comparatively easy to reach and/or inexpensive to contact, in particular by supply lines. Such a wireless transmission of the data also allows for the sensors to be situated at a distance from the control unit such that in particular an acceleration sensor may be situated in the tire of the vehicle. 
     Another subject matter of the exemplary embodiments and/or exemplary methods of the present invention is a sensor system for determining a vehicle tire tread depth, in particular for implementing a method as recited in one of the preceding claims, the sensor being situated in or on a vehicle tire and detecting the tire contact time of the vehicle tire and the sensor may include an acceleration sensor. As described above, an acceleration sensor fastened in the sensor tire section of a vehicle tire allows for the tire contact time and/or the tire revolution time to be determined by time measurement when comparatively great changes of actions of force on the sensor occur. Advantageously, a vehicle tire tread depth determination using only one single sensor is thus realized that has a markedly increased precision compared to the related art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a schematic graphical representation of an action of force on an acceleration sensor in a method according to an exemplary specific embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic graphical representation of an action of force on an acceleration sensor in a method according to an exemplary specific embodiment of the present invention, an acceleration sensor  1  being situated in a region of a vehicle tire  2 , which has a radius  12  greater than zero with respect to a tire axle  11  such that sensor  1  is located in particular in the region of a tire tread  17 . A tire contact area  40  is formed by a contact region between tire tread  17  and a road surface  18 . A sensor tire section includes a section of tire tread  17 , which overlaps sensor  1  essentially in a radial direction, parallel to radius  12 . If the sensor tire section is situated in tire contact area  40  as a result of a tire rotation  41  about tire axle  11 , then essentially the gravitational force (1 g) acts on sensor  1 , while essentially a centrifugal force  37  caused by tire rotation  41  acts on sensor  1  as soon as the sensor tire section is situated outside of tire contact area  40 . The graphical representation shows the curve of action of force  8  on sensor  1  over a unit of time, an action of force scale being plotted on an ordinate  8 ′ and a time scale being plotted on an abscissa  20 . In a transition of the sensor tire section into tire contact area  40  and in a transition of the sensor tire section out of tire contact area  40 , curve of action of force  8  respectively undergoes a comparatively great change  9 , a difference of the points in time between an entry  20 ′ and an exit  20 ″ of the sensor tire section into or out of tire contact area  40  representing the first time interval or the tire contact time  4  and a difference of the points in time between the entry  20 ′ and another subsequent entry  20 ′″ of the sensor tire section into tire contact area  40  representing the sum of the first and a second time interval or tire revolution time  3 . The great action of force changes  9  become detectable in a comparatively simple manner in particular by a mathematical derivation of action of force curve  8 . Tire revolution time  3  is proportional to the tire circumference, while tire contact time  4  is essentially constant even when the tire circumference changes. The change in the proportion of tire contact time  4  to tire revolution time  3  is thus a measure for the changing tire circumference and hence for the tire tread depth. With the aid of dimension information about the brand-new vehicle tire, i.e. about the tire circumference having the maximum tire tread depth, it is possible to convert the tire tread depth into an absolute tire tread depth value. Using correction data, which take into account for example the loading of the vehicle and/or the tire pressure, a correction of the determined tire tread depth may be provided for increasing the accuracy.