Method and device for operating a brake system, computer program and computer program product, brake system

Abstract

A method for operating a brake system of a motor vehicle. The motor vehicle has a vehicle body and multiple wheels mounted relative to the vehicle body by a wheel suspension on the vehicle body. The vehicle body is capable of executing a pitching movement by the wheel suspension. The brake system has a wheel-individual wheel brake for at least some of the wheels. A pitch angle of the vehicle body is monitored, and the wheel brakes are actuated as a function of the acquired pitch angle. The pitch angle is calculated as a function of normal forces acting on the wheels.

Claims

1. A method for operating a brake system of a motor vehicle, the motor vehicle having a vehicle body and multiple wheels mounted relative to the vehicle body by a wheel suspension on the vehicle body, the vehicle body being capable of executing a pitching movement by the wheel suspension, and the brake system having, for each of at least two of the wheels, a respective wheel brake, the method comprising the following steps: monitoring a pitch angle of the vehicle body; and for each respective one of the at least two of the wheels: based on the pitch angle of the vehicle body determined from the monitoring, determining a respective normal force acting on the respective wheel; setting a respective brake force for the respective wheel based on an association of the respective wheel with the respective normal force determined to be acting on the respective wheel; and actuating the respective wheel brake of the respective wheel as a function of the respective brake force set for the respective wheel.

2. The method as recited in claim 1, wherein the pitch angle is ascertained using a programmed model, the programmed model being a one-track model.

3. The method as recited in claim 2, wherein the programmed model takes vertical translation movements and rotary movements of the vehicle body into account.

4. The method as recited in claim 3, wherein, for the each respective one of the at least two wheels, the determination of the respective normal force acting on the respective wheel is made additionally based on a braking torque compensation factor, which is a function of the wheel suspension.

5. The method as recited in claim 3, wherein vertical dynamics in a center of gravity of the vehicle body are ascertained to determine the vertical translation movements.

6. The method as recited in claim 5, wherein, as a function of the ascertained vertical dynamics and using a calculation of a principle of angular acceleration, pitching dynamics in the center of gravity of the vehicle body is ascertained as a function of braking forces acting at the wheels in a driving direction.

7. A method for operating a brake system of a motor vehicle, the motor vehicle having a vehicle body and multiple wheels mounted relative to the vehicle body by a wheel suspension on the vehicle body, the vehicle body being capable of executing a pitching movement by the wheel suspension, and the brake system having, for each of at least two of the wheels, a respective wheel brake, the method comprising the following steps: ascertaining pitching dynamics in a center of gravity of the vehicle body as a function of ascertained vertical dynamics, a calculation of a principle of angular acceleration, and braking forces acting at the wheels in a driving direction; transforming the ascertained vertical dynamics to wheel-individual compression travels of the wheel suspension as a function of the ascertained pitching dynamics; and actuating the wheel brakes as a function of the wheel-individual compression travels of the wheel suspension.

8. The method as recited in claim 7, wherein: respective wheel-individual vertical forces between each respective wheel and the vehicle body are calculated as a function of the compression travels, a wheel-individual spring force, and a damper force; and the actuating is performed as a function of the wheel-individual vertical forces.

9. The method as recited in claim 8, wherein wheel-individual normal forces are calculated as a function of the respective vertical forces and respective wheel masses of the wheels, and the actuating is performed as a function of the wheel-individual normal forces.

10. A non-transitory machine-readable memory medium on which is stored a computer program for operating a brake system of a motor vehicle, the motor vehicle having a vehicle body and multiple wheels mounted relative to the vehicle body by a wheel suspension on the vehicle body, the vehicle body being capable of executing a pitching movement by the wheel suspension, and the brake system having, for each of at least two of the wheels, a respective wheel brake, the computer program, when executed by a computer, causing the computer to perform the following steps: monitoring a pitch angle of the vehicle body; and for each respective one of the at least two of the wheels: based on the pitch angle of the vehicle body determined from the monitoring, determining a respective normal force acting on the respective wheel; setting a respective brake force for the respective wheel based on an association of the respective wheel with the respective normal force determined to be acting on the respective wheel; and actuating the respective wheel brake of the respective wheel as a function of the respective brake force set for the respective wheel.

11. A device for operating a motor vehicle, the motor vehicle having a vehicle body and multiple wheels mounted relative to the vehicle body via a wheel suspension on the vehicle body, and a brake system, the vehicle body being capable of performing a pitching movement by the wheel suspension, and the brake system having, for each of at least two of the wheels, a respective wheel brake, the device comprising: a control unit configured to operate the brake system, the control unit configured to: monitor a pitch angle of the vehicle body; and for each respective one of the at least two of the wheels: based on the pitch angle of the vehicle body determined from the monitoring, determine a respective normal force acting on the respective wheel; set a respective brake force for the respective wheel based on an association of the respective wheel with the respective normal force determined to be acting on the respective wheel; and actuate the respective wheel brake of the respective wheel as a function of the respective brake force set for the respective wheel.

12. A brake system for a motor vehicle, the motor vehicle having a vehicle body and multiple wheels mounted relative to the vehicle body by a wheel suspension on the vehicle body, the vehicle body being capable of executing a pitching movement by the wheel suspension, wherein the brake system comprises: a wheel-individual wheel brake for each of at least two of the wheels; and a control unit configured to operate the brake system, the control unit configured to: monitor a pitch angle of the vehicle body; and for each respective one of the at least two of the wheels: based on the pitch angle of the vehicle body determined from the monitoring, determine a respective normal force acting on the respective wheel; set a respective brake force for the respective wheel based on an association of the respective wheel with the respective normal force determined to be acting on the respective wheel; and actuate the respective wheel brake of the respective wheel as a function of the respective brake force set for the respective wheel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A to 1C show a simplified representation of a vehicle dynamics of a motor vehicle.

(2) FIG. 2 shows a simplified physical model of the motor vehicle.

(3) FIG. 3 shows a flow diagram in order to describe an advantageous method for operating a brake system of the motor vehicle, in accordance with an example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(4) In a simplified representation, FIGS. 1A to 1C show a motor vehicle 1 having a brake system 2, which has an individually actuable wheel brake 3 for each wheel of the motor vehicle. The wheels are individually mounted on a vehicle body 5 of the motor vehicle by a wheel suspension 4 so that the wheels are able to move relative to the vehicle body independently of one another. A booster spring 6 and a damper 7, which jointly form a spring damper system of wheel suspension 4 for the respective wheel, are assigned to each wheel through wheel suspension 4. The center of gravity of the motor vehicle is denoted by S, and the mass of motor vehicle 1 by m.sub.Fzg. In the idle state of motor vehicle 1 as shown in FIG. 1A, the center of gravity S lies at a height h.sub.S above the axes of rotation of wheels 3 and at a vertical distance l.sub.h to a wheel positioned in the rear in the driving direction and at a distance l.sub.v from a wheel positioned in the front in the driving direction.

(5) FIG. 1B shows motor vehicle 1 in a second state in which vehicle body 5 is compressed in the direction of the road surface or the wheels, which means that distance h.sub.s is reduced. Because this involves a purely vertical movement, wheel suspension 4 is compressed to the same extent at both wheels 3. Wheel springs 6 and dampers 7 are loaded to equal degrees at the front and back or at all wheels.

(6) FIG. 1C shows motor vehicle 1 in a state in which vehicle body 5 executes a pitching movement, as indicated by arrow 9. During the pitching movement, vehicle body 5 pivots about a horizontal axis that extends through center of gravity S and is aligned transversely to the driving direction. In the exemplary embodiment illustrated in FIG. 1C, the pitching movement occurs because of a braking intervention that decelerates motor vehicle 1. Due to the deceleration, vehicle body 5 pivots in the downward direction at its front end and in an upward direction at its rear end.

(7) FIG. 2 shows a simplified physical model of motor vehicle 1, which is embodied as a one-track model. In the following text, an advantageous method that ensures that wheel brakes 3 supply the best possible braking power at all times will be described based on the model and the flow diagram shown in FIG. 3. The method is particularly executed by a control unit of the motor vehicle, which includes a non-volatile memory in which the present method is stored in the form of a computer program. The method starts upon the initial operation of the motor vehicle in step S1.

(8) To begin with, in a step S2, braking force Fx in the longitudinal direction is calculated for each wheel with the aid of the moment balance at the respective wheel, which results from the principle of angular acceleration, so that a longitudinal force F.sub.xh for the rear wheel according to the single-track model and a longitudinal force F.sub.xv for the front wheel are calculated. From the principle of angular acceleration for this particular example, the following results for the wheel on the front left (index: FL=front left, FR=front right, RL=rear left, RR=rear right), for example:

(9) $\begin{array}{cc}{\stackrel{.}{}}^{\mathrm{FL}}=\frac{-F_x^{\mathrm{FL}}.Math.r-M_{\mathrm{br}}^{\mathrm{FL}}+M_{\mathrm{motor}}}{J_{\mathrm{wheel}}}& \left(1\right)\end{array}$

(10) With the aid of the known variables such as drive torque M.sub.motor, the wheel brake pressure, the wheel moment of inertia J.sub.wheel, the wheel acceleration and drive torque, the brake force in the longitudinal direction is calculated following a rearrangement of the equation (1). Brake torque M.sub.br required for the calculation is especially ascertained from the wheel brake pressure and a linear conversion factor cp.

(11) In the next step S3, a differential equation is set up for the vertical dynamics of vehicle body 5 in the vehicle center of gravity S:

(12) $\begin{array}{cc}\stackrel{.Math.}{z}=\frac{d\left(z\right)}{\mathrm{dt}}=\frac{F_z^{FL}+F_z^{FR}+F_z^{RL}+F_z^{RR}}{m_{Fzg}}-g.Math.\cos \left(\right)& \left(2\right)\end{array}$

(13) As may be gathered from the equation (2), the acceleration of vehicle body 5 in center of gravity S is calculated with the aid of the vertical forces between the vehicle body and wheel. In the process, vertical forces F.sub.zh, F.sub.zv that are acting on the respective wheel are initially assumed as known variables, in particular as the variables calculated in the preceding calculation cycle. Through an integration in step S4, velocity z of vehicle body 5 in center of gravity S in the vertical direction is calculated:
={umlaut over (z)}(t).Math.dt(3)

(14) A still further integration of the velocity in step S5 yields the compression travel of vehicle body 5 at the center of gravity in comparison with the initial position:
z=(t).Math.dt(4)

(15) In order to calculate the pitch dynamics of vehicle body 5 in center of gravity S, the torque balance is preferably set up in step S6 via the rotation behavior of vehicle body 5 using a calculation of the principle of the angular momentum:

(16) $\begin{array}{cc}\stackrel{.Math.}{}=\frac{d\left(\stackrel{.}{}\right)}{\mathrm{dt}}=\frac{\begin{array}{c}\left(F_z^{\mathrm{RL}}+F_z^{\mathrm{RR}}\right).Math.l_{\mathrm{rear}}^{\mathrm{CG}}-\left(F_z^{\mathrm{FL}}+F_z^{\mathrm{FR}}\right).Math.l_{\mathrm{front}}^{\mathrm{CG}}-\\ \left(F_x^{\mathrm{FL}}+F_x^{\mathrm{FR}}+F_x^{\mathrm{RL}}+F_x^{\mathrm{RR}}\right).Math.h^{\mathrm{CG}}\end{array}}{\mathrm{J\_Fzg}}& \left(5\right)\end{array}$

(17) The differential equation provides angular acceleration {umlaut over ()} of vehicle body 5. The variables required for this purpose such as braking forces in longitudinal direction F.sub.xh, F.sub.xv already result from the equation (1). The likewise required vertical forces F.sub.zh, F.sub.zv between vehicle body 5 and the wheel are initially assumed as known variables, as in the equation (2), in particular from the preceding cycle.

(18) Analogous to the vertical velocity, an integration of the angular acceleration in step S7 results in angular velocity {dot over ()}:
{dot over ()}={umlaut over ()}(t).Math.dt(6)

(19) A further integration of angular velocity {dot over ()} in step S8 results in the pitch angle :
={dot over ()}(t).Math.dt(7)

(20) With the aid of pitch angle , in step S9, the transformation of the compression travel in center of gravity S of vehicle body 5 is now transformed to the wheel-individual compression travels of wheel suspension 4 at the axles of motor vehicle 1. The same applies to the compression rate:
z.sup.FL=z.sup.FR=z.sup.CGl.sub.front.sup.CG.Math.sin(.sup.CG)
z.sup.RL=z.sup.RR=z.sup.CGl.sub.rear.sup.CG.Math.sin(.sup.CG)(8)
.sup.FL=.sup.FR=.sup.CGl.sub.front.sup.CG.Math.sin(.sup.CG)
.sup.RL=.sup.RR=.sup.CGl.sub.rear.sup.CG.Math.sin(.sup.CG)(9)

(21) Spring force F.sub.c between vehicle body 5 and the wheel, in particular wheel-individual spring force F.sub.ch, F.sub.cv, is then calculated in step S10 with the aid of compression travel z and spring stiffness c, in the following manner:
F.sub.c.sup.FL=c.sup.FA.Math.z.sup.FL(8)

(22) The calculation for all wheels of the motor vehicle is carried out as shown in equation (8).

(23) In step S11, damper force F.sub.d between the vehicle body and the wheel is calculated, especially individually for each wheel, as damper force F.sub.dv and F.sub.dh based on spring compression rate and damper constant d:
F.sub.d.sup.FL=d.sup.FA.Math..sup.FL(9)

(24) Here, too, the calculation for all wheels of motor vehicle 1 is carried out in the same way as in the equation (9).

(25) For damper constant d, a distinction can be made between a traction and pressure stage. In addition, it is possible that the damper constant has a non-linear characteristic which is a function of the compression rate. However, this depends on the respective vehicle chassis of motor vehicle 1 and/or respective wheel suspension 4.

(26) In step S12, the vertical forces between vehicle body 5 and respective wheel F.sub.zh, F.sub.zv are subsequently calculated with the aid of the spring and damper forces and the influence of a braking torque compensation that results from the wheel suspension (in the example, from the wheel at the front left (FL)):
F.sub.z.sup.FL=F.sub.c.sup.FL+F.sub.d.sup.FL+k.sub.Mbr.sup.FA.Math.F.sub.x.sup.FL(10)

(27) This calculation, too, is performed for each wheel of the motor vehicle as shown in equation (10).

(28) The factor of braking torque compensation k.sub.Mbr depends on the design of wheel suspension 4 and is preferably calculated as described before.

(29) In step S13, wheel-individual normal forces F.sub.N are subsequently calculated using vertical forces F.sub.z and wheel mass m.sub.R:
F.sub.N=F.sub.zv+m.sub.vR.Math.g(11)

(30) Equation (11) is used in a similar manner for each wheel of the motor vehicle.

(31) In this way, the normal forces and thus the tire contact forces of the wheel on the road are calculated for each wheel of motor vehicle 1 and supplied to the brake system so that the brake system optimally sets the particular brake forces that achieve the best possible braking power, individually for each wheel.