Abstract:
A method and a system for ascertaining a mass value representing the vehicle mass of a motor vehicle, particularly of a commercial vehicle, having a drive unit. An acquisition at least of a first and a second acceleration value is provided for this determination. These acceleration values represent the vehicle acceleration at a first and a second point of time. Further, at least one first and one second drive value are acquired. These drive values represent the drive force or the drive torque of the drive unit at the first and the second point of time. At least as a function of the acquired acceleration values and the acquired drive values, at least a first and a second driving-resistance value are then determined. The mass value is ascertained at least as a function of a comparison of the determined first driving-resistance or mass estimated value to the determined second driving-resistance or mass estimated value. A roadway slope is detected using the comparison, whereby an erroneous mass determination caused by the roadway slope is avoided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a system for determining a mass value representing a mass of a vehicle. 
     BACKGROUND INFORMATION 
     Systems for the open-loop and closed-loop control, respectively, of the driving dynamics of automobiles are known. The control of the braking system is the priority here. In such systems, the most precise information possible about the vehicle mass is of major significance. 
     If the motor vehicle is a commercial vehicle having a towing vehicle and a trailer/semi-trailer, an optimal coordination of the braking forces with regard to economic efficiency, safety and driving comfort can be attained if the masses of the towing vehicle and the trailer/semi-trailer are known as accurately as possible. If the mass of the entire truck with trailer is known, then, given the known mass of the towing vehicle, the mass of the trailer/semi-trailer can be determined. Since, however, according to its intended purpose, large differences can occur in the payload of commercial vehicles and thus in the total mass of the vehicle, the total mass and the distribution of the mass between the towing vehicle and the trailer/semi-trailer must continually be redetermined. Thus, the driving stability can be increased by suitably distributing the braking torque on the individual wheel brakes. 
     German Patent Application No. 42 28 413 describes a determination of the total mass of a vehicle, in which the longitudinal acceleration of the vehicle and the appertaining driving and propulsive forces are measured at two different points of time in brief succession during an acceleration process of the vehicle. The vehicle mass can then be determined as a function of these measured variables. It is assumed here that the driving resistance while determining the mass does not change significantly, for example due to a change in the roadway slope. 
     An object of the present invention is to determine mass as simply as possible and with greatest possible precision in view of a possibly sloped roadway. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and a system for determining a mass value of a motor vehicle representing the vehicle mass, particularly of a commercial vehicle, having a drive unit. For this, provision is made for acquisition of at least a first and a second acceleration value. These acceleration values represent the vehicle acceleration at a first and a second point of time. Furthermore, at least a first and a second drive value are detected. These drive values represent the drive force or the drive torque of the drive unit at a first and at a second point of time. At least as a function of the acquired acceleration values and the acquired drive values, at least a first and a second driving resistance or mass estimated value is then determined. According to the present invention, the determination of the mass value occurs at least as a function of a comparison of at least the determined first driving-resistance or mass estimated value with the determined second driving-resistance or mass estimated value. 
     A roadway slope is detected using the comparison in accordance with the present invention, whereby an erroneous determination of mass due to a roadway slope is avoided without necessitating an additional sensor for determining the mass and/or for determining the roadway slope. The mass can be determined during a single acceleration process, the method according to the present invention being simple to apply. In so doing, the result of the mass estimation achieved according to the present invention is sufficiently precise for a practical use. 
     In another embodiment of the present invention, the points of time are selected such that the acquired drive values vary from one another in a predeterminable manner. In particular, the difference or differences between the acquired drive values exceed(s) or fall(s) below a predeterminable first threshold value. The background for this embodiment is that a roadway descending gradient or roadway incline is having a particularly significant affect on the acceleration values of the vehicle if highly variable drive values exist. 
     In addition, or particularly as an alternative to the last cited variation, it can be provided that a signal representing the gear ratio of the vehicle transmission is generated. The points of time at which the acceleration and drive values are acquired can then be selected as a function of the generated signal. In so doing, different transmission ratios exist at these points of time. In this way, it can be ensured in a simple manner that the drive forces, which are drawn upon to determine the driving resistance or mass estimated values, are sufficiently different. The differing drive forces, which are necessary to recognize the slope, are realized in this embodiment by using “gear-speed tracking”. This has a basis in that, during the start-up (e.g., drive off), the transmission ratio becomes smaller, and thus smaller forces act on the drive axle as the vehicle speed increases. 
     For the comparison according to the present invention, it is ascertained whether the determined driving resistance or mass estimated values are within a predeterminable range. In particular, it can be ascertained whether the difference or differences between the determined driving resistance or mass estimated values exceeds or falls below or exceed or fall below a predeterminable second threshold value. In this manner, it can be determined whether the vehicle is currently located on a sloped roadway, because in this case, the driving-resistance or mass estimated values, which were ascertained for different drive values, vary significantly. 
     To compensate for short-term fluctuations, the determined driving-resistance or mass estimated values are low-pass filtered. 
     In particular, the mass value is ascertained only if the determined driving-resistance or mass estimated values are within a predeterminable range, thus if there is no or only a slight roadway slope. This ensures the reliable determination of a mass value. 
     In a further embodiment of the present invention, to ascertain the mass value, at least one of the determined driving-resistance or mass estimated values is also drawn upon, provision being made in particular that the average value from at least two of the determined driving-resistance or mass estimated values is ascertained as the mass value. This increases the quality of the mass value determined according to the present invention. 
     If a value representing the vehicle speed and/or a value representing the rotational speed of the vehicle wheels is (are) also drawn upon to determine the driving-resistance or mass estimated values, the influences of the aerodynamic drag and/or moments of inertia of the vehicle wheels can be considered in the mass determination according to the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a general block diagram of the present invention. 
     FIG. 2 shows a detailed block diagram of an exemplary embodiment according to the present invention. 
     FIG. 3 shows a dependence of a mass determination on a drive force in a case of different roadway slopes. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates, with blocks  10   ij , wheel speed sensors which detect the rotational speeds of the vehicle wheels. Wheel speed signals Nij are supplied to block  101 , which determines a mass value M ges  representing the total mass of the vehicle, and this value is supplied to block  102 . In block  102 , braking system  11   ij , in particular the individual wheel braking systems, are driven by driving signals Aij as a function of total mass M ges , wheel speeds Nij, and possibly as a function of additional signals. 
     To determine the mass, drive force F antr  and the drive torque, respectively, determined in block  104 , are also supplied to block  101 . A signal I which represents the current gear ratio of the vehicle transmission can optionally be fed to block  101 . 
     Following, mass determination  101  of the vehicle or vehicle combination (towing vehicle plus trailer or semi-trailer) shall be described in greater detail with reference to FIG.  2 . 
     The starting point for determining mass M ges  of a vehicle is a force balance or an energy balance in the longitudinal direction of the vehicle movement. Used for that purpose are operational phases in which braking and drive torques acting on the wheels are known. 
     The determination of mass M ges  for a vehicle acceleration a Fhzg  is described below. For an acceleration process, the force balance is: 
     
       
           M   ges   *a   Fhzg   =F   antr   −F   Roll   −F   Luft   −F   Hang   −F   Rot   (1) 
       
     
     where: 
     a Fhzg  is the vehicle acceleration, 
     F antr  is the drive force, 
     F Roll  is the rolling resistance force, 
     F Luft  is the aerodynamic-drag force, 
     F Hang  is the downgrade force, and 
     F Rot  is the force to accelerate rotating masses (wheels, transmission, . . . ) 
     The current vehicle acceleration a Fhzg =a i  at point of time t i  is formed in block  21  from wheel speeds Nij in a conventional manner using differentiation. The formation of current drive force F antr =F antri  at point of time t i  is determined in block  104  (FIG. 1) in general as a function of the data present in the engine control device. This will be described later in this exemplary embodiment. 
     Aerodynamic-drag force F Luft  can be determined using the equation: 
     
       
           F   Luft =½ *C   W *ρ Luft   *A*V   Fnzg   2   (2) 
       
     
     plausible approximate values being used for C W  and ρ Luft . Vehicle longitudinal speed V Fhzg  is also formed from the wheel speeds in block  21  in the known manner. 
     Value F Rot  results from measured wheel speeds N Rad =Nij and the total moment of inertia of all wheels J Rad :                F   Rot     =              N   Rad            t       *     J   Rad     *     1     r   Rad                 (   3   )                                
     For vehicle groupings (towing vehicle with semi-trailer or trailer) having continually changing trailers or semi-trailers, a default value must be assumed for the moment of inertia of the trailer or semi-trailer wheels, respectively. 
     Rolling resistance F Roll  is not considered in this exemplary embodiment. 
     Given a level street, a value M i , representing the vehicle mass at a point of time t i  can be determined according to the equation:                M   i     =         F   antri     -     F   Lufti     -     F   Roti         a   i               (   4   )                                
     with values F antri , a i , F Roti ,F Lufti  current at point of time t i . This occurs in block  22 , initially independently of the roadway slope. In block  24 , mass value M i  attained in this way is low-pass filtered to become filtered mass value M f . 
     If the vehicle is traveling on a roadway sloped in the driving direction (incline or decline), then equation (4)—as well as any other physical balance equation—leads to a considerable estimation error because the driving-resistance is altered significantly by the slope. If a roadway is sloped, mass value M i  determined according to equation (4) includes a significant driving-resistance component. 
     Therefore, a method is necessary in which the driving-resistance change caused by too great a slope is taken into account, and the calculated mass estimated value is rejected or corrected. 
     FIG. 3 shows, in a simplified manner, which mass values M i  are determined in block  22  during a start-up process or during an acceleration process according to equation (4). Accordingly, on a level street (curve a), estimated mass M i  is independent thereof, at which drive force F antri  the measurement is taken. On a slope (curve b), however, given different drive forces F antri , different masses M i  are also estimated. This is used to determine the slope. 
     The slope is considered during an acceleration phase by determining the vehicle mass. It is assumed here that initially there is a large transmission ratio during the start-up. As the vehicle speed increases and the transmission ratios decrease, the drive force is thereby reduced on the driven axle. 
     If there is only little driving resistance during the start-up process (roadway is relatively level), then no or only a small estimated mass difference ΔM can be determined between phases, in which large and small drive forces F antri  act (see curve a in FIG.  3 ). 
     During start-up, if there is, however, a greater difference ΔM determined between the estimated masses in phases of high and lower drive forces F antri , then it is assumed that the driving resistances for this start-up process cannot be disregarded. The mass value thus determined must then be corrected or declared as invalid. 
     As illustrated in FIG. 2, the mass estimated and driving-resistance values M i  are determined in block  22  according to equation (4) only if sufficiently high vehicle acceleration a i  and sufficiently variable drive values F antri  during an acceleration action exist. To ensure this, drive values F antri  and/or acceleration values a i  are compared to predeterminable thresholds in block  23 . Signal S generated in block  23  controls the formation of values M i  in block  22  from these standpoints. 
     It is to be pointed out here that the function of block  22  is not limited to above equation (4); any other estimation method can be used in block  22 . 
     To determine whether there is a significant roadway slope, the differences ΔM of mass values M i  that were low-pass filtered in block  24  are formed in block  25 . At least one difference ΔM must be determined from two estimated values M f , and specifically, one estimated value each for large and small drive forces F antri , respectively. To ensure that difference formation  25  only forms differences from mass values with sufficiently varying drive values, difference formation  25  is controlled by signal R from block  23 . 
     In block  26 , it is determined whether difference value ΔM determined in this way is within a predeterminable range. This can be implemented such that difference ΔM is compared to a predeterminable threshold or, depending on the sign of ΔM, to predeterminable thresholds. 
     If it is determined in block  26  that difference ΔM is outside of the range (exceeded or fell below the corresponding thresholds), which represents travel in the plane, then block  27  is controlled such that no value M ges  is formed for the mass. 
     If it is determined in block  26  that difference ΔM is within the range (exceeded or fell below the corresponding thresholds), which represents travel in the plane, then block  27  is controlled such that filtered value M f  is drawn upon as value M ges  for the mass. It is also advantageous to derive the average value from a plurality of values M f  as mass value M ges . 
     The determination of drive force F antri  in block  104  is discussed below. Drive force F antr , which is necessary for estimation, can be calculated as below from the engine torque supplied by the engine control, taking into account the transmission ratio as well as the losses in the engine and transmission: 
     Engine torque M Mot     —     EDC  put out by engine control EDC is made up of drive torque M Mot     —     Antr , engine torque loss M Mot     —     Verl  and vehicle torque loss M Fhzg     —     Verl . 
     
       
           M   Mot     —     EDC   =M   Mot     —     Antr   +M   Mot     —     Verl   +M   Fhzg     —     Verl   (5) 
       
     
     In this context, M Mot     —     Antr  is the drive torque acting on the transmission input. M Mot     —     Verl  is the portion that constitutes engine friction losses M Mot     —     Reib  and engine acceleration losses M Mot     —     θ  (including clutch). 
     
       
           M   Mot     —     Verl   =M   Mot     —     Reib   +M   Mot     —     θ   (6) 
       
     
     Engine losses M Mot     —     Veri  can be described by friction losses M Mot     —     Reib  and losses due to engine acceleration M Mot     —     θ . The engine friction losses are a function of engine speed n Mot  and water temperature t Wasser . 
     
       
           M   Mot     —     Reib   =f ( n   Mot   ,t   Wasser )  (7) 
       
     
     The losses, which occur due to engine acceleration (M Mot     —     θ ), result from the engine speed acceleration and moment of inertia J Mot , which includes the engine as well as parts of the drive train.                M       Mot   –        θ       =       f        (            n   Mot            t       )       =              ω   Mot            t       *     J   Mot                 (   8   )                                
     In view of the above-specified losses, a moment which acts on the drive wheels can then be calculated from the engine drive torque using the gear ratio i ges  (transmission, differential . . . ).                M   Antr     =         M       Mot   –        Antr       *     η   Getr         i   ges               (   9   )                                
     In this context, η Getr  corresponds to the moment loss in the transmission and differential. 
     The total gear ratio is determined from the relationship of engine speed n Mot  to the wheel speed of driven wheels n Rad .                i   ges     =       n   Mot       n   Rad               (   10   )                                
     Drive force F antr  is determined from the moment that acts on the drive wheels, over wheel radius r Rad .                F   antr     =       M   Antr       r   Rad               (   11   )                                
     Possible variations of the method described thus far are described in the following embodiments: 
     Different drive forces F antri , which are needed to determine the roadway slope, are realized using a “gear-speed tracking.” For this, as illustrated in FIGS. 1 and 2, signal i, which represents the transmission ratio, is supplied to block  22 . At this point, driving-resistance or mass estimated values M i  are formed only if different transmission ratios are present. 
     Likewise, the difference formation can be controlled in block with signal i such that only differences ΔM are formed from values M i  that were determined for varying transmission ratios. 
     This has a basis in that during the start-up, the transmission ratio becomes smaller and thus, as the vehicle speed increases, lower forces F antri  act on the drive axle. Thus, the described control of blocks  22  and  25  by signal S and R, respectively, can be omitted, for example. 
     If it is determined during the accelerated phase that other variables are influencing the drive force, the estimation of the mass is interrupted or canceled. 
     Mass difference ΔM, which is needed to determine the driving-resistances, is yielded from the result of estimator  22 . Block  25  can be configured, for example, such that the calculated mass is saved in a sample-and-hold. In so doing, it is sufficient to use only one single estimator for the high and lower force ranges. 
     The estimation can be improved by considering the operating conditions to which the vehicle is subjected. If, e.g., there is excessive drive slip present during a start-up process, such that a drive slip control must be used, for example, then this start-up process should not be permitted for determining the mass. 
     Further, it is advantageous to consider an already existing value for the vehicle mass as a starting value. A measured value for the axle load (ALB value) can be drawn upon, for example, as such a starting value. The mass determination according to the invention can be optimized in this way. 
     In summary, it can be stated that with the determination of the vehicle mass according to the present invention, a parameter estimation method is used for only one parameter. By using only one parameter, this is simplified in contrast to the other known methods and comes the closest to the requirements of an implementation useful in practice. Estimator  22 , having one parameter, is regulated or controlled by trigger signals S and I, respectively. For additional requirements, it can be useful to conduct the mass determination using a plurality of estimators. 
     According to the present invention, vehicle total mass M ges  is estimated for high and low values of the drive force. For this, in accordance with the present invention, criteria for evaluating the estimated mass difference and thus a triggering of the estimation method are introduced. 
     Thus, the present invention has at least the following advantages: 
     No additional sensor is needed for determining the mass. 
     The vehicle mass is determined during a single acceleration process. 
     The algorithm according to the present invention is easy to apply. 
     The result of the mass estimation achieved according to the present invention is sufficiently precise for practice.