Method for determining the location of a vehicle's center of gravity

Abstract

Method for determining the position of a center of gravity (S) of a vehicle (1) having a chassis (3) with several wheels (4), wherein the chassis (3) enables a pitching motion of the vehicle due to acceleration in the longitudinal direction and / or a rolling motion of the vehicle due to acceleration in the lateral direction, comprising the following steps: a) Monitoring the vehicle for acceleration in the longitudinal and / or lateral direction, b) Detection of wheel contact forces (F FL ,F FR ,F RL ,F RR ) the wheels (4) of the chassis (3), c) Determining the position of the center of gravity (S) as a function of the recorded accelerations and the resulting wheel contact forces (F) FL , F FR , F RL , F RR), whereby it is first checked whether the vehicle is in a quasi-stationary state, whereby the vehicle must be in a steady state with respect to the pitch and roll dynamics, whereby the magnitude of the respective detected acceleration is checked, whereby the signs of the detected accelerations are checked and care is taken to ensure that accelerations with both signs are selected equally.

Description

[0001] The invention relates to a method for determining the position of a center of gravity of a vehicle having a chassis with several wheels, wherein the chassis enables a pitching motion of the vehicle due to acceleration in the longitudinal direction of the vehicle and / or a rolling motion of the vehicle due to acceleration in the lateral direction of the vehicle. State of the art

[0002] Methods of the type mentioned above are known from the prior art. The location of a motor vehicle's center of gravity has a significant influence on its driving dynamics. Therefore, knowledge of the center of gravity is highly advantageous for the design and operation of vehicle safety systems, particularly those that influence the vehicle through control interventions, such as ESP (Electronic Stability Program) or rollover prevention systems. However, the center of gravity of a vehicle is not constant or fixed, but depends primarily on the number and arrangement of the occupants and the vehicle's load. Direct determination of the center of gravity using a sensor is not possible. Therefore, estimation methods are typically used that attempt to determine the vehicle's load.

[0003] For example, a method for determining the center of gravity is known from the patent application DE 10 2004 056 108 A1, in which the position of the center of gravity in the longitudinal direction is determined depending on the longitudinal tire forces, the longitudinal acceleration, the road inclination and the vehicle parameters wheelbase and center of gravity height, which are assumed to be known.

[0004] The publication DE 10 2005 062 285 A1 discloses a method for determining the position of the center of gravity of a vehicle having a chassis with multiple wheels, wherein the chassis allows a pitching motion of the vehicle due to acceleration in the longitudinal direction and / or a rolling motion of the vehicle due to acceleration in the lateral direction, wherein the vehicle is monitored for acceleration in the longitudinal and / or lateral direction, wheel contact forces of the wheels of the chassis are recorded, and the position of the center of gravity is determined as a function of the recorded accelerations and the resulting wheel contact forces.

[0005] The known methods all share the common feature of inferring the position of the center of gravity from a calculated wheel slip. However, in practical applications, inaccuracies in these estimates arise due to model and measurement inaccuracies, particularly when determining the height of the center of gravity. Because of the insufficiently precise knowledge of the center of gravity's position, safety precautions must generally be taken in the design and operation of existing vehicle safety systems, thus limiting the theoretically achievable performance of these systems. Disclosure of the invention

[0006] The method according to the invention, with the features of claim 1, has the advantage that, in particular, the height of the center of gravity is determined with sufficiently high accuracy, so that vehicle safety systems can be optimally utilized. The method according to the invention makes it possible to determine the position of the center of gravity in all three spatial axes with high accuracy in a simple manner. According to the invention, the vehicle is monitored for acceleration in the longitudinal and / or lateral direction, while the wheel contact forces of the chassis are recorded. The position of the center of gravity is then determined as a function of a recorded acceleration (in the longitudinal and / or lateral direction) and the resulting wheel contact forces. The position of the center of gravity is thus determined by the relationship between the vehicle accelerations and the resulting (re)distribution of the wheel contact forces.A quasi-stationary mathematical model is particularly preferred for this purpose, which describes the relationships depending on the position of the center of gravity. The model is based on the consideration of the moment equilibrium with respect to the pitch and roll axes of the vehicle, whereby the only unknown values ​​in the model are the positional coordinates of the center of gravity, while all other quantities can be determined as constant (vehicle parameters) or time-varying (accelerations and wheel contact forces). The wheel contact forces are preferably measured directly at the respective wheel using one or more suitable sensors. In particular, it is provided that the current wheel contact force is determined as a function of the deflection of a wheel relative to the chassis or vehicle and as a function of a spring force acting on that wheel.

[0007] According to an advantageous embodiment of the invention, the spring stiffnesses of the chassis are taken into account when determining the location of the center of gravity. In the simplest case, the model is based on the assumption that the chassis suspension is infinitely stiff. Preferably, however, spring stiffnesses of the chassis are considered, which are present in every vehicle and influence the distribution or redistribution of the wheel contact forces depending on longitudinal or lateral acceleration. The model particularly considers the steady-state suspension behavior at each wheel during pitching and rolling of the vehicle and thus the associated kinematic changes. By taking the spring stiffnesses into account, the accuracy in determining the center of gravity is further increased.

[0008] According to a preferred embodiment of the invention, at least one acceleration profile is compared with the wheel contact forces, in particular with the wheel contact force profiles, when determining the position of the center of gravity. By considering the acceleration profile and the wheel contact force profiles, it is possible, particularly with the aid of an identification method, to determine the value for the position of the center of gravity that leads to optimal agreement between the model and the measurement. Furthermore, considering the profiles also allows for an assessment of the quality of the method itself. Preferably, the method is only carried out if the profiles indicate that the vehicle is in a steady state.As long as the center of gravity cannot be determined with sufficient accuracy, the vehicle safety systems will continue to operate preferably with a so-called "worst-case" parameterization. Only when the method according to the invention has been carried out and the position of the center of gravity has been determined with sufficient accuracy will the vehicle safety systems use this determined position and adjust their control interventions accordingly, or only intervene when actually needed. Taking the curves into account requires a quasi-stationary state of the vehicle in order to carry out the method according to the invention. The vehicle must be in a steady state with respect to its pitch and roll dynamics to ensure correct evaluation of the model.At the same time, high accelerations and the resulting wheel contact forces are given preferential consideration for the model, since the following applies: The greater the magnitude of the respective acceleration, the more accurately the position, in particular the height of the center of gravity, can be determined.

[0009] Preferably, the comparison of the curves takes into account the wheelbase, track width, and / or the lever arm of a stabilizer bar of the vehicle. These vehicle parameters are assumed to be constant and can be easily incorporated into the model equation. These parameters can be determined, in particular, from the vehicle's design data. The vehicle mass required for carrying out the method is preferably calculated directly from the wheel contact forces and therefore does not need to be applied or estimated separately during operation. The method thus has the advantage that it does not require parameters that are difficult to determine or that vary during operation, such as tire stiffness, wheel moment of inertia, or dynamic wheel radius.

[0010] According to a preferred embodiment of the invention, the comparison and / or execution of the method take into account the road gradient, in particular a longitudinal gradient and / or a transverse gradient. This allows the influence of the road gradient on the suspension behavior to be considered, thereby enabling the position of the vehicle's center of gravity to be reliably determined at any time.

[0011] Furthermore, it is preferably intended that the procedure is carried out regularly, and in particular continuously, during vehicle operation. Specifically, the wheel contact forces and accelerations are continuously recorded during driving and, if necessary, low-pass filtered, particularly with a slight low-pass filter. Preferably, a decision is made at each point in time, depending on the driving situation, as to whether the current measurement data are helpful or can be used to determine the position of the center of gravity.For this purpose, the following criteria, some of which have already been indicated above, are evaluated separately for longitudinal and lateral directions: If the vehicle is in a quasi-stationary, i.e., steady-state condition, the recorded accelerations are sufficiently large in magnitude to be able to determine the position of the center of gravity with sufficient accuracy, and / or accelerations of both signs are recorded equally to facilitate an accurate determination of the position, in particular the height, of the center of gravity.

[0012] Furthermore, the invention includes a computer program characterized by the fact that it performs all steps of the inventive method when the program is running on a computer.

[0013] Furthermore, the invention relates to a computer program product characterized by a program code stored on a machine-readable medium for carrying out the method according to the invention when the program is executed on a computer.

[0014] Furthermore, the invention relates to a device for operating a vehicle, in particular a vehicle safety system, which is characterized by a specially designed control unit that executes the method according to the invention.

[0015] The invention will now be explained in more detail using an exemplary embodiment. For this purpose, we will show... Fig. 1. A schematic diagram of the moment equilibrium of a motor vehicle, Fig. 2 the result of a determination of the location of the vehicle's center of gravity.

[0016] Fig. Figure 1 shows a simplified representation of a motor vehicle 1, which has a body 2 supported by a chassis 3. The chassis 3 has several wheels 4 in contact with a road surface 5. Each wheel 4 is held to the vehicle 1 by the chassis 3 via a spring-damper system, allowing the vehicle 1 to compress relative to the wheels 4 and the road surface 5, resulting in roll and pitch. The chassis 3 has a sensor assigned to each wheel 4, which detects the contact force of the wheels 4 on the road surface 5. Specifically, the sensors detect the displacement of each wheel 4 from a rest position, which is determined by the weight of the vehicle body 2, the inclination of the vehicle body 2, and the spring stiffness of the chassis.

[0017] To operate vehicle safety systems, such as an ESP system (ESP = Electronic Stability Program), it is important to know the position of the vehicle's center of gravity S. However, the center of gravity S is not solely determined by the vehicle parameters, but also by the number and arrangement of the occupants and the vehicle's load. Therefore, the center of gravity S is not in a constant position, but rather a variable one. For optimal operation of the vehicle safety systems, it is advantageous to determine the current position of the center of gravity S quickly and with high accuracy. A corresponding method for this purpose will be described below.

[0018] The method is based on the idea of ​​creating a quasi-stationary mathematical model that describes the relationships between the vehicle accelerations occurring in the longitudinal and / or lateral direction and the resulting (re)distribution of the wheel contact forces as a function of the position or coordinates of the center of gravity S. The model specifically considers the steady-state suspension behavior of the chassis 3 during pitching or rolling of the vehicle and thus the associated kinematic changes.

[0019] The model is based on the in Fig. Figure 1 shows the moment equilibrium with respect to the pitch and roll axes. The moment-generating forces and the acceleration a are depicted. x for the pitch axis and the terminal acceleration g assuming infinitely stiff suspension of the chassis 3, as well as the resulting front wheel impact forces F z,f and behind F z,rThe following formula applies here. [mg−max][xz]=L2(Fz,f−Fz,r).

[0020] To eliminate the errors of this highly simplified consideration, the influence of the suspension or spring stiffness of the chassis 3 and any existing stabilizers on the kinematics is additionally considered, resulting in the following extensive equations: (2cFcRlmg(−m2g2+2cRlmax)cF+(FFL+FFR)mg(cF+cR))T⋅(xz)=−cFcRl2(mg−2(FFL+FFR)) (2(cF+2csF)mgw2(FFL−FFR)mg+2(cF+2csF)mayw)T⋅(yz) =(FFL−FFR)w(w(cF+cR)+2wsFcsF+2wsRcsR)

[0021] The following values ​​or quantities are used: The spring stiffness at the front c F and at the back c R , the wheelbase L of the vehicle, the track width w of the vehicle, the vehicle mass m, the acceleration due to gravity g, the stabilizer stiffnesses at the front C sF and at the back C sR, the associated front stabilizer lever arm w sF and behind w sR , the longitudinal acceleration a x , the lateral acceleration a y , the front left wheel contact forces F FL , front right F FR , rear left F RL and rear right F RR .

[0022] It is also possible to consider the influence of road gradient (longitudinal and / or lateral slope) in the model and / or equations. The equations derived from the moment equilibrium analysis also contain the unknown coordinates (x, y, z) of the center of gravity or center of mass of vehicle 1. The model parameters to be applied are therefore limited to the stiffnesses of the vehicle suspension and stabilizers, as well as the kinematic quantities wheelbase L, track width w, and effective stabilizer lever arm w. sThese parameters can all be determined from the design data of vehicle 1. The required actual vehicle mass m can be calculated directly from the wheel contact forces and therefore does not need to be applied or estimated separately during operation.

[0023] At the start of the journey, the position of the center of gravity S must be assumed to be unknown, as it may have changed due to loading / unloading. As long as the position of the center of gravity S cannot be determined, vehicle safety systems must continue to operate with a worst-case safety parameter setting. Once the position of the center of gravity S has been determined, the vehicle safety systems can use this information and adjust their control interventions accordingly, or even optimally, and only when actually needed.

[0024] During the journey, the wheel contact forces and the vehicle's longitudinal and lateral accelerations are continuously recorded and, if necessary, slightly low-pass filtered. At any given time, depending on the driving situation, a decision is made as to whether the current measurement data is helpful for determining the position of the center of gravity S. The following criteria are applied, each considered separately for the longitudinal and lateral directions: First, it is checked whether the vehicle 1 is in a quasi-stationary state. For this to be the case, the vehicle 1 must be in a steady state with respect to its pitch and roll dynamics. Next, the magnitude of each recorded acceleration is checked. The greater the magnitude of the acceleration, the more accurately the position of the center of gravity S, and in particular its height, can be determined. Finally, the signs of the recorded accelerations are checked.Care should be taken to select accelerations of both signs equally, which facilitates a more accurate determination of the position of the center of gravity S, in particular the height of the center of gravity.

[0025] The set of measurement data selected according to these criteria is used to determine the location of the center of gravity. Each measurement data set (consisting of the wheel contact forces and the accelerations) is inserted into the model equations, resulting in an overdetermined system of equations. Using known identification methods (such as recursive or non-recursive least-squares methods), the unknown coordinates of the center of gravity S can then be calculated.

[0026] When using a recursive method, a currently selected measurement data set is processed immediately and does not need to be stored. After a sufficient number of measurement data points have been processed, the output of the identification converges to the coordinates of the center of mass S.

[0027] When using a non-recursive method, selected measurement data sets are stored. As soon as a sufficient number are available, the coordinates or the position of the center of gravity S can be determined in a single calculation step.

[0028] The accuracy of the identification is improved by normalizing the model equations, as all measurement data sets are then weighted equally in the identification process. In particular, normalization to the Hessian normal form is performed as follows: (n1n2)T⋅(x1x2)=b⇒(n1n2)Tn12+n22⋅(x1x2)=bn12+n22

[0029] Fig.Figure 2 shows the results of determining the position of the center of gravity S for the x and z coordinates, i.e., the position of the center of gravity in the longitudinal direction (x) of the vehicle and in height (z), for numerous driving situations with three different load states B1, B2, and B3, and thus different actual positions of the center of gravity S, using a non-recursive least-squares method. It is evident that the x-coordinate, i.e., the position of the center of gravity S in the longitudinal direction of the vehicle 1, yields fairly consistent results, whereas the significantly more difficult-to-determine z-coordinate, i.e., the height of the center of gravity S, shows greater variation. Therefore, it is preferable to average the results of several driving situations so that the height of the center of gravity S is also determined with sufficient accuracy.

[0030] Overall, the advantageous method results in a particularly accurate determination of the position of the center of gravity S in all three spatial directions x, y, z, which enables optimal operation of vehicle safety systems.

Claims

[1] Method for determining the location of a center of gravity (S) of a vehicle (1) having a chassis (3) with several wheels (4), wherein the chassis (3) enables a pitching motion of the vehicle due to acceleration in the longitudinal direction and / or a rolling motion of the vehicle due to acceleration in the lateral direction, comprising the following steps: a) Monitoring the vehicle for acceleration in the longitudinal and / or lateral direction, b) Detection of wheel contact forces (F FL ,F FR ,F RL ,F RR ) the wheels (4) of the chassis (3), c) Determining the position of the center of gravity (S) as a function of the recorded accelerations and the resulting wheel contact forces (F) FL , F FR , F RL , F RR), whereby it is first checked whether the vehicle is in a quasi-stationary state, whereby the vehicle must be in a steady state with respect to the pitch and roll dynamics, whereby the magnitude of the respective detected acceleration is checked, whereby the signs of the detected accelerations are checked and care is taken to ensure that accelerations with both signs are selected equally. [2] Method according to claim 1, characterized by , that when determining the location of the center of gravity (S) spring stiffnesses of the chassis (3) are taken into account. [3] Method according to any one of the preceding claims, characterized by , that when determining the position of the center of gravity (S) at least one acceleration profile is compared with the wheel contact forces, in particular with the wheel contact force profiles of the wheels (4). [4] Method according to any one of the preceding claims, characterized bythat the procedure is only carried out if it is detected that the vehicle is in a steady state. [5] Method according to any one of the preceding claims, characterized by that in the comparison a wheelbase (L), a track width (w) and / or a lever arm (w) SF ,w SR ) of a stabilizer of the chassis (3) shall be taken into account. [6] Method according to any one of the preceding claims, characterized by that the road gradient is taken into account in the comparison. [7] Method according to any one of the preceding claims, characterized by , that the procedure is carried out regularly during the operation of the vehicle (1). [8] Computer program that performs all steps of a method according to any one of claims 1 to 7 when the program is running on a computer. [9] Computer program product comprising program code stored on a machine-readable medium for carrying out the method according to any one of claims 1 to 7 when the program is executed on a computer. [10] Device for operating a vehicle (1), characterized by , that a specially designed control unit is provided which performs the method according to one of claims 1 to 7.

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