Patent Publication Number: US-2017355234-A1

Title: Method and system for determining a wheel load acting on a tire of a vehicle

Description:
The present invention relates to a method and a system for determining a wheel load acting on a tire of a vehicle. 
     Modern vehicles often contain a tire pressure monitoring system (TPMS) for measuring actual tire pressures of the tires mounted on the respective wheels of the vehicle. This enables e.g. to provide a warning about an incorrect tire pressure. 
     However, the wheel load acting on a tire of the vehicle significantly influences the required tire pressure. For example, the correct tire pressure of an empty vehicle will be incorrect when the vehicle is fully loaded e.g. by additional passengers and/or luggage in the vehicle, and vice versa. Insofar, there is also a need for determining the wheel load e.g. for evaluating an actual tire pressure in terms of correctness. 
     U.S. Pat. No. 8,255,114 B1 discloses a method and a system for determining a wheel load based on a measurement of the length of the footprint of the wheel&#39;s tire, i.e. the contact area between tire and ground. Namely, it is known that the wheel load affects the size and thus in particular the length of the footprint of the tire. In this prior art determination method, the wheel load is calculated using a set of equations and equation parameters, which have to be determined and stored in advance. This is accomplished by analyses of individual tires or tire types to determine a suitable set of equations or a suitable equation parameters set. Although, the known method and system takes into account that such suitable equations and parameters may change upon an exchange of the tires, the handling of this problem necessitates a laborious acquisition of the equations and equation parameters. 
     It is an object of the present invention, to provide an alternative method and a system for determining a wheel load acting on a tire of a vehicle, which can be employed more universally and which can also deal with the problem of varying tire characteristics, in the case of an exchange of tires. 
     According to an aspect of the invention, a method for determining a wheel load acting on a tire of a vehicle comprises:
     a) determining a footprint of the tire,   b) receiving a tire information that identifies a type of the tire and/or characterizes physical properties of the tire as such,   c) receiving tire operation conditions based on a measurement,   d) selecting one of a plurality of predetermined calculation models based on the received tire information, wherein each of the predetermined calculation models defines the wheel load as a function of the footprint and the tire operation conditions for a respective tire type and/or respective physical properties of the tire as such,   e) calculating the wheel load based on the determined footprint and the received tire operation conditions, using the selected calculation model.   

     As mentioned above, the method according to the invention calculates the wheel load based on a determined footprint by using a suitable calculation model. However, it is essential for the invention that the used calculation model is selected from a plurality of predetermined and e.g. stored calculation models, wherein this selection is based on a received tire information that identifies a type of the tire and/or characterizes physical properties of the tire as such. 
     As each of the predetermined calculation models individually defines the wheel load as a function of the footprint and the tire operation conditions, it can be safeguarded a suitable and universally employable determination of the wheel load taking into account all actual parameters that influence the relationship between wheel load and footprint. 
     Thus, the invention allows optimizing a load detection in a vehicle. 
     The invention relies on the fact that the wheel load can be calculated with high accuracy from the footprint, if the calculation takes into account the above mentioned “tire information” and “tire operation conditions” as the relevant impact factors in the relationship between wheel load and footprint. In the following, some of such impact factors are discussed in more detail. 
     Firstly, with respect to “vehicle velocity”, the tire will feel radial acceleration which is acting in outwards direction. This acceleration will increase with velocity and it is determined by the equation a r =v 2 /r, wherein a r  is the radial acceleration, v is the vehicle velocity, and r is the tire radius. This acceleration a r  will increase quadratic with velocity v, which means this acceleration will increase rapidly. As a result of very high acceleration, tire will experience high forces and it will expand. This phenomenon has an impact on footprint. The footprint will not be the same at low velocity and high velocity (when keeping e.g. same tire pressure and same wheel load). 
     The effect of “tire temperature” is related with the tire pressure and also with the properties of the tire material (e.g. a rubber compound). As tire temperature increases, tire pressure will increase which can be approximated by ideal gas law. Simultaneously rubber parts of tire will become softer. These effects of increasing of tire pressure and softening of tire material will affect the footprint. 
     Seasonal change in environment will affect the properties of the tire material straightaway. Therefore, tires are produced typically under summer, winter and all season tire labels. These labels typically indicate different rubber compounds which will be suitable in different temperature ranges. This means that the tire type in terms of the “season stipulated for the tire” is also affecting the footprint. 
     Another influencing factor is the tire wear. The “tread depth” is also affecting the footprint. Tire curvature which result in footprint, in case of low tread tire is changed. The footprint of a low tread tire will not be same as in case of the full tread tire. 
     The tire age is also one of the affecting factors. Tire curvature which result in footprint, in case of a few years old tire is changed. The footprint of a few years old tire will not be same as in case of the new tire. 
     Furthermore, the “tire dimension” such as in particular the “aspect ratio” has an influence on footprint. When a higher aspect ratio tire and a lower aspect ratio tire carry same load, the footprint will be different between these tires. 
     In an embodiment of the invention, in step a) the footprint is determined as a length of a footprint area of the tire. Such determination can be done e.g. by using a tire based sensor, i.e. a sensor arranged at the respective tire, as e.g. a deformation sensor (e.g. “bending element”), or an acceleration and/or shock sensor etc. 
     In an embodiment, step a) is conducted by means of a wheel unit disposed at the tire and comprising a sensor for measuring a deformation of the tire and a determining unit for determining the footprint based on the measured deformation of the tire. Such wheel units as such are known from the prior art and can also be used in the present invention. 
     In an embodiment, the tire information received in step b) comprises data related to at least one of: tire type, tire dimension, season stipulated for the tire, material and/or material properties, tread depth, age and/or manufacturing date. 
     In a preferred embodiment, the tire information received in step b) comprises at least data related to: tire type and/or tire dimension. In a development of this embodiment, all of the remaining tire information data mentioned above are included in the tire information received in step b). 
     The data related to the tire type may comprise e.g. tire code information. The data related to the tire dimension may include at least one of rim diameter, tire width, and tire aspect ratio. The data related to the season stipulated for the tire may define whether a tire is a summer tire, a winter tire or an all season tire. The data related to the material may include an indication about a chemical class to which the body material of the tire belongs. The data related to the material properties may include in particular an indication of the stiffness of the tire material. 
     The tire information may contain at least one indication based on a classification (e.g. summer tire, soft rubber, tire with run flat ability etc.) and/or at least one indication expressed by a corresponding value (tire diameter, tread depth, tire manufacturing date etc.). 
     In an embodiment, the tire information in step b) is received at least partially from a digital store disposed at the tire. 
     Preferably, such store is disposed in a wheel unit disposed at the tire and e.g. arranged in or on the tire in the region of the running surface of the tire. The store may be accessed via wireless communication preferably via LF and RF communications (e.g. as known from RFID systems). 
     Preferably, such wheel unit further comprises a sensor module having a sensor for providing a sensor signal dependent on a mechanical tire load (e.g. acceleration and/or deformation) at a predefined measurement location at the tire. Such sensor module can be advantageously used for determining the footprint of the tire and as the case may be, for further determining at least a part of the tire operation conditions needed in step c) of the inventive method. 
     In an embodiment, the tire information in step b) is received at least partially from a digital store disposed in a control unit of the vehicle. 
     Such digital store may be disposed in a central control unit (e.g. in-vehicle computer) of the vehicle, which also performs other control functions of vehicle as e.g. an electronic stability program (ESP). The central control unit may be equipped with a microcontroller for processing these control functions and for accessing the digital store containing (at least a part of) the tire information. This digital store may be a partition of a larger store that also contains software code for performing said control functions of the central control unit. 
     Preferably, the central control unit and thus the digital store storing the respective tire information can be accessed via a human machine interface (HMI) by the driver and/or service persons for setting or amending, respectively, the tire information stored therein. 
     In an embodiment, the tire operation conditions received in step c) comprise at least one of and preferably all of: tire pressure, tire temperature, vehicle velocity and/or tire rotational speed, vehicle acceleration, in particular vehicle lateral acceleration and vehicle longitudinal acceleration, and/or tire rotational acceleration. 
     At least the tire pressure and the tire temperature may be provided by corresponding sensors (pressure sensor and temperature sensor) disposed at the tire, wherein these sensors may be disposed at a sensor module disposed at the tire, e.g. the sensor module mentioned above, which also has the sensor for determining the footprint of the tire and as the case may be a digital store for storing at least a part of tire information. 
     If such sensor module comprises the above mentioned sensor for providing a sensor signal dependent on a mechanical tire load at a predefined measurement location at the tire, this sensor signal can be advantageously also used for determining tire rotational speed and/or tire rotational acceleration. This is because said sensor signal comprises more or less periodically appearing signal characteristics associated with the instances at which the measurement location of the rotating tire passes the footprint. Thus, an analysis of the sensor signal allows for a calculation of the rotational speed and/or acceleration of the tire. 
     In an embodiment, the measurement upon which the receiving of the tire operation conditions in step c) is based comprises a measurement conducted by a measurement unit (e.g. above mentioned sensor module) disposed at the tire. 
     Alternatively, or in addition, the tire operation conditions may also be provided by use of a central control unit (e.g. in-vehicle computer) of the vehicle, which processes results of measurements that are conducted also for the purpose of enabling the control functions of the vehicle provided by the central control unit. 
     In an embodiment, each of the predetermined calculation models used in step d) is represented by a respective set of digitally stored model parameters. 
     Each of the predetermined calculation models, e.g. digitally stored model parameters, may define one or more mathematical formulas enabling a calculation of the wheel load from the determined footprint, the received tire information and the received tire operation conditions, wherein the digitally stored model parameters may characterize the form of the mathematical formulas as such and/or parameters (function parameters) of these formulas. 
     In an embodiment, in step e) the wheel load is calculated based on the formula 
       WL= f 1(TP,FPA n ) 
     wherein WL is the wheel load, TP is the tire pressure, FPAn is a normalized footprint area of the tire, and f1 is a first function commonly used in the calculation models,
 
wherein the normalized footprint area is calculated based on the formula
 
       FPA n=f 2(FPA,ti,toc) 
     wherein FPA is a footprint area determined based on the determined footprint in step a) taking into account the tire information received in step b), ti is the tire information received in step b), and toc are the tire operation conditions received in step c). 
     In this embodiment, the (mathematical) functions f1 and f2 constitute the calculation model selected in step d) and used in step e). Accordingly, the selection performed in step d) may comprise a concrete definition of the functions f1 and/or f2 based on the received tire information ti. 
     Further, in this embodiment, the normalized footprint area FPAn can be regarded as an auxiliary quantity calculated in the inventive method, which represents a fictive footprint area of the tire when assuming predetermined values (as a norm) of parameters belonging to the tire information and/or tire operation conditions. 
     In a further development of this embodiment, the normalized footprint area FPAn used in step e) is calculated as a difference between the actual determined footprint area FPA and a third function g (normalization function) of the respective values. For example, the normalized footprint area can be calculated based on the formula 
       FPA n =FPA− g (ti1,ti2, . . . ;toc1,toc2, . . . )
 
     wherein ti1, ti2, . . . are one or more components of the tire information ti, and toc1, toc2, . . . are one or more components of the tire operation conditions toc. 
     The normalization function g defines the difference between the normalized footprint area FPAn and the actual footprint area FPA. 
     The normalization function g can be chosen, for example, dependent on tire season type and/or tire dimension. 
     In an embodiment, the component(s) ti1, ti2, . . . of the tire information ti used in above function g is at least one of the tread depth and the tire age. 
     In an embodiment, the component(s) toc1, toc2, . . . of the tire operation conditions toc used in above function g are at least: vehicle velocity (or tire rotational velocity) and tire temperature. 
     In the inventive method, the predetermined calculation models can contain a plurality of predetermined normalization functions g, which in this case are preferably “preclassified” based e.g. on different tire type classes and/or different tire dimensions classes. 
     With such preclassification method, different normalization functions g can be defined e.g. as illustrated by the following example: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 tire season  
                 tire dimension  
                   
               
               
                 type class:  
                 class:  
                 normalization function: 
               
               
                   
               
             
            
               
                 ″A″ 
                 ″A″  
                 gAA (vehicle velocity,  
               
               
                   
                   
                 tire temperature, tread  
               
               
                   
                   
                 depth)  
               
               
                 ″B″  
                 ″A″  
                 gBA (vehicle velocity,  
               
               
                   
                   
                 tire temperature, tread  
               
               
                   
                   
                 depth)  
               
               
                 ″A″  
                 ″B″  
                 gAB (vehicle velocity,  
               
               
                   
                   
                 tire temperature, tread  
               
               
                   
                   
                 depth)  
               
               
                 ″B″  
                 ″B″  
                 gBB (vehicle velocity,  
               
               
                   
                   
                 tire temperature, tread  
               
               
                   
                   
                 depth)  
               
               
                 . . .  
                 . . .  
                 . . . 
               
               
                   
               
            
           
         
       
     
     In the above example (table), each tire type class is defined by the season stipulated for the tire. Therefore, tire season class “A” may apply e.g. for summer tires and tire season class “B” may apply e.g. for winter tires. Each tire dimension class may be defined by corresponding ranges for at least one of: diameter, width, aspect ratio of the tire. Thus, tire dimension class “A” may apply for tires having a diameter, a width, and an aspect ratio each lying within a corresponding range for the respective parameter, wherein these ranges are defined by this tire dimension class “A”. 
     In an embodiment, the predetermined calculation models, and in particular e.g. above mentioned normalization functions (e.g. gAA, gBA, gAB, gBB in the above example) used in such calculation models, are initially determined in advance based on a fitting of previously recorded experimental data. 
     To this end, the mathematical form of equations used in the calculation models can be in particular designed based on physical model assumptions describing the influence of the relevant variables of the tire information ti and the relewant variables of the tire operation conditions toc on the relationship between wheel load WL and footprint (e.g. footprint area FPA, or footprint length L). 
     In this way, mathematical functions as e.g. the above mentioned functions f1, f2 and g can be designed in terms of their mathematical “form”, wherein respective function parameters (fixed or depending on tire information and/or tire operation conditions) can be determined by said fitting of recorded experimental data. 
     In the following, referring again to the above example of calculation models comprising normalization functions gAA, gBA, gAB, gBB etc., some aspects of said recording of experimental data and their use in predetermining calculation models are discussed. 
     In an experimental setup, keeping tire pressure, tire season type (e.g. winter, summer, all season tire) and tire dimension constant, the effects of individual variables such as vehicle velocity, tire temperature, tread depth and tire age can be studied and defined. In order to do this, one should consider the time frame on which these variables are varying in practice. 
     The “tread depth” and “tire age” will vary over longer time scale. Regular time interval monitoring on modified footprint over a longer time scale will show impact of tread depth and tire age on footprint. This effect can be eliminated by signal filtering such as high pass filtering with lower cut off frequency. 
     For remaining variables such as “vehicle velocity” and “tire temperature”, they will vary more quickly than tread depth. For this, in the calculation of a normalized footprint area FPAn, the following boundary condition can be taken: FPAn=constant, if vehicle velocity&gt;predetermined threshold velocity (e.g. 10 km/h). 
     The relation with different variables such as vehicle velocity and tire temperature can be learnt in advance under favourable condition such as varying predominantly one variable, e.g. either velocity or tire temperature, while rest are at minimum change. Thus, the velocity or tire temperature dependency can be learnt from experimentally recorded data. 
     As an example, learning can be done by fitting mechanism. For this, certain data have to be collected under favourable condition. By applying fitting mechanism to these data, behaviour of certain variables such as vehicle velocity and/or tire temperature can be defined. This behaviour can be e.g. linear, quadratic etc. When using a normalized footprint area FPAn as defined by above mentioned equation FPAn=FPA−g(ti1, ti2, . . . ; toc1, toc2, . . . ), the calculation of wheel load WL in step e) can be made free from all the relevant variables which have formerly affected the result of footprint acquisition. 
     The above learning process can be done in advance. In this case, the suitable designed calculation models can be preprogrammed in a control unit of the vehicle (e.g. “ECU”). But such learning process may also involve an “online learning”, which is done on the vehicle and is done automatically. 
     In an embodiment, step d) and/or step e) is conducted by a control unit (e.g. central control unit) of the vehicle using software running on that control unit. 
     In a respective further development of this embodiment, a wheel unit disposed at the tire is used for determining the footprint, and as the case may be provided at least a part of the tire information and/or the tire operation conditions, and communicates corresponding data (e.g. via wireless communication preferably via RF communication path) to the respective control unit of the vehicle. Based on such data, the control unit can perform the model selection of step d) and the wheel load calculation of step e). 
     In an embodiment, the conduction of step b) and step d) is initiated at the beginning of each driving period of the vehicle that follows a standstill period of at least a predetermined minimum standstill duration of the vehicle. 
     This embodiment takes into account the reasonable assumption that the tire information and thus the suitably selected calculation model (based on the tire information) can only change upon a longer lasting standstill period of the vehicle, namely enabling an exchange of one or more wheels or tires, respectively. 
     The minimum standstill duration may be chosen e.g. as being at least 1 min, or at least 2 min. On the other hand, a minimum standstill duration of e.g. up to 10 min, or up to 5 min can be chosen. In general, the minimum standstill duration may also be predefined dependent on the type of vehicle. 
     In another aspect of the present invention, a system for determining a wheel load acting on a tire of a vehicle is provided, which system comprises means for conducting the method according to the aspect of the invention as described above. 
     Accordingly, in an embodiment, a system for determining a wheel load acting on a tire of a vehicle comprises: 
     a) means for determining a footprint of the tire,
 
b) means for receiving a tire information that identifies a type of the tire and/or characterizes physical properties of the tire as such,
 
c) means for receiving tire operation conditions based on a measurement,
 
d) means for selecting one of a plurality of predetermined calculation models based on the received tire information, wherein each of the predetermined calculation models defines the wheel load as a function of the footprint and the tire operation conditions for a respective tire type and/or respective physical properties of the tire as such,
 
e) means for calculating the wheel load based on the determined footprint and the received tire operation conditions, using the selected calculation model.
 
     The embodiments of the method described above can be used accordingly as embodiments and developments of the system according to the invention. 
     The method and the system may be implemented using a computer program product or software code, respectively, adapted to perform the method. 
     In another aspect of the invention, a vehicle equipped with such system for determining a wheel load is provided. 
    
    
     
       The invention will now be described in more detail by way of example embodiments with reference to the accompanying drawings, in which 
         FIG. 1  illustrates a schematic block diagram of a system for determining a wheel load according an embodiment, 
         FIG. 2  illustrates a schematic block diagram of a wheel unit used in the system of  FIG. 1 , 
         FIG. 3  illustrates a schematic flow-chart of essential steps conducted in a method for determining a wheel load according to the invention, and 
         FIG. 4  illustrates a flow-chart of a method for determining a wheel load according to an embodiment. 
     
    
    
       FIG. 1  illustrates an embodiment of a system  10  for determining a wheel load acting on a tire  1  of a wheel  3  of a vehicle. 
     The system  10  comprises:
         a) means for determining a footprint of the tire  1 ,   b) means for receiving a tire information that identifies a type of the tire and/or characterizes physical properties of the tire as such,   c) means for receiving tire operation conditions based on a measurement,   d) means for selecting one of a plurality of predetermined calculation models based on the received tire information, wherein each of the predetermined calculation models defines the wheel load as a function of the footprint and the tire operation conditions for a respective tire type and/or respective physical properties of the tire as such,   e) means for calculating the wheel load based on the determined footprint and the received tire operation conditions, using the selected calculation model.       

     The above mentioned means of the system  10  can be described more detailed as follows: 
     a) Means for Determining a Footprint 
     The footprint is determined as a length L ( FIG. 1 ) of a footprint area of the tire  1  by means of a wheel unit  12  disposed at the tire  1 . The wheel unit  12  is illustrated in more detail in  FIG. 2  and comprises a sensor  14  for measuring a local deformation of the tire  1  at a predetermined location  5  at the tire (corresponding to the location of the wheel unit  12 ) and a determining unit  20 , implemented as a microcontroller, for determining the footprint length L (alternatively: e.g. footprint area) based on the measured deformation of the tire  1 . In  FIG. 2 , the sensor signal provided by the sensor  14  is designated as “def”. 
     The determining unit  20  generates data D, which are transferred to a transmitter  22  of the wheel unit  12 , which transmits the data D by means of a wireless signal preferably an RF signal  24  to a vehicle unit  30  configured to receive the RF signal  24  and to transfer the data D via a digital bus system  32  to a central control unit  34  (in-vehicle computer) of the vehicle. 
     The central control unit  34  comprises an electronic control unit (ECU)  36 , implemented as a microcontroller for conducting calculations, and a digital store  38 . 
     Thus, in the illustrated embodiment, the means for determining a footprint (footprint length L) are constituted by the deformation sensor  14  and the determining unit  20  in the wheel unit  12 . 
     b) Means for Receiving a Tire Information 
     The tire information is received e.g. at least partially from a digital store  26  arranged in the wheel unit  12 . The digital store  26  is connected to the determining unit  20 , so that tire information can be transmitted by transmitter  22  via wireless communication, preferably via RF communication. A receiver  28  has functionality for receiving new and/or modified tire information via wireless communication, preferably via LF communication by user inputs (e.g. by service personal) via a trigger tool (not shown). These received tire information are transferred via the determining unit  20  into the digital store  26 . The wheel unit  12  is configured to incorporate tire information from the digital store  26  into the data D, which are transmitted to the central control unit  34 . 
     A further possibility for storing tire information is provided by the digital store  38  of the central control unit  34 , which is also accessible by the ECU  36 . In this case, the tire information components stored in the digital store  38  can be defined by user inputs (e.g. by service personal) via a (not shown) human machine interface (HMI) of the central control unit  34 . 
     Thus, in the illustrated embodiment, the means for receiving a tire information are constituted by the digital store  26  of the wheel unit  12  and/or the digital store  38  of the vehicle&#39;s central control unit  34 . 
     The received tire information comprises data related to: tire type, tire dimension, season stipulated for the tire, material and/or material properties, tread depth, age and/or manufacturing date. 
     c) Means for Receiving Tire Operation Conditions 
     The measurement upon which the receiving of the tire operation conditions is based comprises a measurement conducted by means of two additional sensors integrated within the wheel unit  12 , namely a pressure sensor  16  for measuring a tire pressure TP in the tire  1  and a temperature sensor  18  for measuring a temperature T of the tire  1 . 
     The respective measurement results TP and T are also incorporated into the data D to be transmitted to the central control unit  34 . 
     Further, the wheel unit  12  can be configured to further incorporate a tire rotational speed (and/or based thereon a tire rotational acceleration) into these data D. This additional information can be determined by the determining unit  20  by way of an analysis of the sensor signal “def” provided by the deformation sensor  14 . Further, if the determining unit  20  knows the diameter of the tire  1 , it can e.g. also calculate vehicle velocity. 
     Such components of the tire operation conditions, here at least the tire pressure TP and the tire temperature T, which are provided by the wheel unit  12 , are incorporated into the data D to be transmitted to the central control unit  34 . Alternatively, or in addition, components of the tire operation conditions can be received from other systems (e.g. ESP) of the vehicle&#39;s control system. Such information component(s) may be available e.g. from the central control unit  34 . 
     Thus, in the illustrated embodiment, the means for receiving tire operation conditions are constituted by the wheel unit  12  and corresponding functionalities of the central control unit  34 . 
     The received tire operation conditions comprise: tire pressure, tire temperature, vehicle velocity and/or tire rotational speed, vehicle lateral and longitudinal accelerations and/or tire rotational acceleration. 
     d) Means for Selecting a Calculation Model 
     The store  38  of the central control unit  34  stores not only software code for operating the ECU  36 , but also a digital representation of the plurality of predetermined calculation models (e.g. mathematical modelling functions represented by digitally stored equation parameters). Based on the received tire information, the ECU  36  selects one of the stored calculation models by retrieving corresponding equation parameters etc. from the store  38 . 
     Thus, in the illustrated embodiment, the means for selecting a suitable calculation model are provided by the respective functionality of the ECU  36 . 
     e) Means for Calculating the Wheel Load 
     The ECU  36  using software running thereon conducts not only the above mentioned selection of the calculation model, but also the calculation of the wheel load based on the determined footprint length L (or e.g. subsequently based on the length L determined footprint area FPA) and the received tire operation conditions, using the selected calculation model. 
       FIG. 3  summarizes the essential steps a) to e), which are necessary for the determination of the wheel load “WL” according to the invention. 
     The steps a), b) and c), i.e. the determining of footprint (step a), the receiving of tire information “ti” (step b) and the receiving of tire operation conditions “toc” (step c) are substantially independent from one another, i.e. can be conducted in sequence or parallel. In the case, however, that the determination of the tire operation conditions (step c) uses any result of the steps a) or b), the step c) should be conducted after a previous conduction of step a) or step b), respectively. 
     The step d), i.e. the selection of the suitable calculation model “mod(ti)”, has to be conducted upon a previous conduction of step b). 
     The step e), i.e. the calculation of the wheel load WL, necessitates a previous conduction of steps a) to d). 
     The conduction of the steps a) to e) can be continuously repeated during driving periods of the vehicle. For saving calculation resources, however, steps b) and d) can often be omitted in such repeated processing according to  FIG. 3 . Namely, taking into account that a change in the tire information “ti” necessitates a preceding change of one or more tires or wheels, respectively, the steps b) and d) can be omitted by simply using the corresponding most recently selected calculation model in step e). 
       FIG. 4  is a more concrete flow-chart of a method for determining a wheel load WL, as can be performed e.g. by the system  10  of  FIG. 1 , in accordance to the more general flow-chat of  FIG. 3 . 
     In a step S 1  corresponding to the method step a), the footprint length L is determined. 
     In a step S 2 , it is determined whether a driving period of the vehicle that follows a standstill period of at least a predetermined minimum standstill duration of the vehicle has just begun. If so, a step S 3  corresponding to the method step b) is conducted (i.e. the tire information “ti” is received), followed by the conduction of a step S 4  corresponding to the method step c). Otherwise, step S 3  is omitted and the process jumps to step S 4 . 
     In step S 4  the tire operation conditions “toc” are received. 
     In a step S 5  it is determined whether a driving period of the vehicle that follows a standstill period of at least predetermined minimum standstill duration of the vehicle has just begun (To this end, the algorithm may simply resort to the result of step S 2 ). If so, the process continues with a step S 6  corresponding to the method step d), in which the selection of a calculation model “mod(ti)” in accordance with the received tire information “ti” is conducted, followed by a step S 7 . Otherwise, the step S 6  is omitted and the process jumps to step S 7 . 
     In step S 7 , which corresponds to the method step e) the wheel load WL is calculated based on the determined footprint length L and the received tire operation conditions, using the most recently selected calculation model “mod(ti)”. 
     In summary, the invention and the described embodiments provide a reliable method and system for determining a wheel load. The benefits of correct tire pressure at given tire load results in uniform tire wear, which means better usability of tire over its lifetime, reduction in fuel consumption, best safety, overload warning. In addition to that, the tire load information can be provided to different systems such as transmission control, engine control, power steering, vehicle dynamic control system for better driving experience and safety. 
     LIST OF REFERENCE SIGNS 
     
         
           1  tire 
           3  wheel 
           5  measurement location 
         L length of footprint 
           10  determining system 
           12  wheel unit 
           14  deformation sensor 
         def deformation sensor signal 
           16  pressure sensor 
         TP measured tire pressure 
           18  temperature sensor 
         T measured temperature of tire 
           20  determining unit 
         D data 
           22  transmitter 
           24  RF signal 
           26  digital store 
           28  receiver 
           30  vehicle unit 
           32  bus system 
           34  central control unit 
           36  ECU 
           38  digital store 
         ti tire information 
         toc tire operation conditions 
         mod(ti) selected calculation model 
         WL determined wheel load