Method for ascertaining the coefficient of friction between a vehicle wheel and the roadway

Abstract

In a method for ascertaining the coefficient of friction between a vehicle wheel and the roadway, at least one active vehicle unit influencing the longitudinal dynamics of the vehicle is controlled during the trip in such a manner, that the longitudinal wheel force acting upon at least one vehicle wheel is increased, and/or the contact patch force of the wheel is decreased, while the characteristic of the sum of all of the longitudinal forces acting upon the vehicle remains unchanged.

Claims

1. A method for ascertaining a coefficient of friction between a vehicle wheel and a roadway, the method comprising: controlling, during a trip, at least one active vehicle unit influencing longitudinal dynamics of the vehicle so that a longitudinal wheel force acting upon at least one vehicle wheel is increased, and/or a contact patch force acting upon the at least one vehicle wheel is decreased, while a characteristic of a sum of all longitudinal forces acting upon the vehicle remains unchanged, and determining an actual coefficient of friction currently used from the longitudinal wheel force and the contact patch force of the wheel; wherein the coefficient of friction at a wheel is calculated from ${\mu }_{\mathrm{act}}=\frac{F_x}{F_N},$ where μ.sub.act is the coefficient of friction currently used, F.sub.x is the longitudinal wheel force, and F.sub.N is the contact patch force of the wheel, so that the coefficient of friction currently used μ.sub.act increases when the longitudinal wheel force F.sub.x is increased and/or the contact patch force F.sub.N of the wheel is decreased, so that during a stable trip, the coefficient of friction currently used μ.sub.act is below a maximum possible coefficient of friction, wherein at an automatic activation of drive and braking systems at a first time and at an automatic deactivation of the drive and braking systems at a second time, the forces acting upon the vehicle have the expected characteristic of the sum of all the longitudinal forces acting upon the vehicle, so that the activation and deactivation are not perceived by the driver, wherein between times t.sub.1 and t.sub.2, the longitudinal wheel force F.sub.x is increased and the contact patch force F.sub.N of the wheel is reduced at least slightly, so that the currently used coefficient of friction μ.sub.act at the vehicle wheels of the rear axle is increased, and consequently, a higher percentage of the maximum coefficient of friction is used, so as to improve a prediction of the maximum coefficient of friction, and wherein a time between t.sub.1 and t.sub.2, a torque characteristic, which represents a totality of running resistances, is obtained automatically from a sum of a drive torque automatically applied to a front axle and a braking torque automatically applied to a rear axle, so that the driver does not perceive any of the automatic intervention in the drive and braking systems of the vehicle.

2. The method as recited in claim 1, wherein the coefficient of friction is ascertained in a time interval of increasing the longitudinal wheel force acting upon the at least one vehicle wheel and/or decreasing the contact patch force acting upon the at least one vehicle wheel.

3. The method as recited in claim 1, wherein at least one vehicle wheel is decelerated for ascertaining the coefficient of friction.

4. The method as recited in claim 1, wherein two vehicle wheels on a common vehicle axle are decelerated or accelerated.

5. The method as recited in claim 1, wherein a subset of wheels of the vehicle is decelerated, and a drive torque is applied to another subset of the wheel of the vehicle.

6. The method as recited in claim 1, wherein the sum of all of the longitudinal forces acting upon the vehicle assumes a constant characteristic during the ascertainment of the coefficient of friction.

7. The method as recited in claim 1, wherein the sum of all of the longitudinal forces acting upon the vehicle remains constant during the ascertainment of the coefficient of friction.

8. The method as recited in claim 1, wherein the longitudinal wheel force and/or the contact patch force at at least one vehicle wheel is changed during a trip at a constant or approximately constant speed.

9. The method as recited in claim 1, wherein the longitudinal wheel force and/or the contact patch force at at least one vehicle wheel is changed during an acceleration or braking phase.

10. The method as recited in claim 1, wherein the longitudinal wheel force and/or the contact patch force at at least one vehicle wheel is changed while traveling straight ahead.

11. The method as recited in claim 1, wherein the longitudinal wheel force and/or the contact patch force at at least one vehicle wheel is changed during cornering.

12. An apparatus for controlling a vehicle unit influencing longitudinal dynamics of a vehicle, comprising: a control unit configured to ascertain a coefficient of friction between a vehicle wheel and a roadway, by performing the following: controlling, during a trip, at least one active vehicle unit influencing longitudinal dynamics of the vehicle so that a longitudinal wheel force acting upon at least one vehicle wheel is increased, and/or a contact patch force acting upon the at least one vehicle wheel is decreased, while an expected characteristic of a sum of all longitudinal forces acting upon the vehicle remains unchanged, and determining an actual coefficient of friction currently used from the longitudinal wheel force and the contact patch force of the wheel; wherein the coefficient of friction at a wheel is calculated from ${\mu }_{\mathrm{act}}=\frac{F_x}{F_N},$ where μ.sub.act is the coefficient of friction currently used, F.sub.x is the longitudinal wheel force, and F.sub.N is the contact patch force of the wheel, so that the coefficient of friction currently used μ.sub.act increases when the longitudinal wheel force F.sub.x is increased and/or the contact patch force F.sub.N of the wheel is decreased, so that during a stable trip, the coefficient of friction currently used μ.sub.act is below a maximum possible coefficient of friction, and wherein at an automatic activation of drive and braking systems at a first time and at an automatic deactivation of the drive and braking systems at a second time, the forces acting upon the vehicle have the expected characteristic of the sum of all the longitudinal forces acting upon the vehicle, so that the activation and deactivation are not perceived by the driver, wherein between times t.sub.1 and t.sub.2, the longitudinal wheel force F.sub.x is increased and the contact patch force F.sub.N of the wheel is reduced at least slightly, so that the currently used coefficient of friction μ.sub.act at the vehicle wheels of the rear axle is increased, and consequently, a higher percentage of the maximum coefficient of friction is used, so as to improve a prediction of the maximum coefficient of friction, and wherein a time between t.sub.1 and t.sub.2, a torque characteristic, which represents a totality of running resistances, is obtained automatically from a sum of a drive torque automatically applied to a front axle and a braking torque automatically applied to a rear axle, so that the driver does not perceive any of the automatic intervention in the drive and braking systems of the vehicle.

13. A vehicle, having comprising: a control unit; and at least one vehicle unit, which is controllable by the control unit and which is configured for influencing longitudinal dynamics of the vehicle; wherein the control unit is configured to ascertain a coefficient of friction between a vehicle wheel and a roadway, by performing the following: controlling, during a trip, the at least one active vehicle unit for influencing the longitudinal dynamics of the vehicle so that a longitudinal wheel force acting upon at least one vehicle wheel is increased, and/or a contact patch force acting upon the at least one vehicle wheel is decreased, while a characteristic of a sum of all longitudinal forces acting upon the vehicle remains unchanged, and determining an actual coefficient of friction currently used from the longitudinal wheel force and the contact patch force of the wheel; wherein the coefficient of friction at a wheel is calculated from ${\mu }_{\mathrm{act}}=\frac{F_x}{F_N},$ where μ.sub.act is the coefficient of friction currently used, F.sub.x is the longitudinal wheel force, and F.sub.N is the contact patch force of the wheel, so that the coefficient of friction currently used μ.sub.act increases when the longitudinal wheel force F.sub.x is increased and/or the contact patch force F.sub.N of the wheel is decreased, so that during a stable trip, the coefficient of friction currently used μ.sub.act is below a maximum possible coefficient of friction, and wherein at an automatic activation of drive and braking systems at a first time and at an automatic deactivation of the drive and braking systems at a second time, the forces acting upon the vehicle have the expected characteristic of the sum of all the longitudinal forces acting upon the vehicle, so that the activation and deactivation are not perceived by the driver, wherein between times t.sub.1 and t.sub.2, the longitudinal wheel force F.sub.x is increased and the contact patch force F.sub.N of the wheel is reduced at least slightly, so that the currently used coefficient of friction μ.sub.act at the vehicle wheels of the rear axle is increased, and consequently, a higher percentage of the maximum coefficient of friction is used, so as to improve a prediction of the maximum coefficient of friction, and wherein a time between t.sub.1 and t.sub.2, a torque characteristic, which represents a totality of running resistances, is obtained automatically from a sum of a drive torque automatically applied to a front axle and a braking torque automatically applied to a rear axle, so that the driver does not perceive any of the automatic intervention in the drive and braking systems of the vehicle.

14. A non-transitory computer readable storage medium on which is stored a computer program, which is executable by a control unit having a processor, comprising: a program code arrangement having program code for ascertaining a coefficient of friction between a vehicle wheel and a roadway, the program code, by performing the following: controlling, during a trip, at least one active vehicle unit influencing longitudinal dynamics of the vehicle so that a longitudinal wheel force acting upon at least one vehicle wheel is increased, and/or a contact patch force acting upon the at least one vehicle wheel is decreased, while a characteristic of a sum of all longitudinal forces acting upon the vehicle remains unchanged, and determining an actual coefficient of friction currently used from the longitudinal wheel force and the contact patch force of the wheel; wherein the coefficient of friction at a wheel is calculated from ${\mu }_{\mathrm{act}}=\frac{F_x}{F_N},$ where μ.sub.act is the coefficient of friction currently used, F.sub.x is the longitudinal wheel force, and F.sub.N is the contact patch force of the wheel, so that the coefficient of friction currently used μ.sub.act increases when the longitudinal wheel force F.sub.x is increased and/or the contact patch force F.sub.N of the wheel is decreased, so that during a stable trip, the coefficient of friction currently used μ.sub.act is below a maximum possible coefficient of friction, and wherein at an automatic activation of drive and braking systems at a first time and at an automatic deactivation of the drive and braking systems at a second time, the forces acting upon the vehicle have the expected characteristic of the sum of all the longitudinal forces acting upon the vehicle, so that the activation and deactivation are not perceived by the driver, wherein between times t.sub.1 and t.sub.2, the longitudinal wheel force F.sub.x is increased and the contact patch force F.sub.N of the wheel is reduced at least slightly, so that the currently used coefficient of friction μ.sub.act at the vehicle wheels of the rear axle is increased, and consequently, a higher percentage of the maximum coefficient of friction is used, so as to improve a prediction of the maximum coefficient of friction, and wherein a time between t.sub.1 and t.sub.2, a torque characteristic, which represents a totality of running resistances, is obtained automatically from a sum of a drive torque automatically applied to a front axle and a braking torque automatically applied to a rear axle, so that the driver does not perceive any of the automatic intervention in the drive and braking systems of the vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows two graphs representing a trip at initially constant speed in a vehicle, with manipulation of the accelerator pedal for generating a drive torque during a first phase, and a subsequent phase without a drive torque, to allow the vehicle to roll without propulsion.

DETAILED DESCRIPTION

(2) The two graphs stand for a trip at initially constant speed in a vehicle, with manipulation of the accelerator pedal for generating a drive torque during a first phase, and a subsequent phase without a drive torque, in order to allow the vehicle to roll without propulsion. In the period of time between 0 and t.sub.1, the vehicle initially still travels, using a constant drive torque, which the driver selects by manipulating the accelerator pedal. At time t.sub.1, the driver goes from the accelerator pedal and ends the manipulation of the accelerator pedal, with the intention of allowing the vehicle to roll without applying a drive torque. If no active vehicle unit influencing the longitudinal dynamics of the vehicle is automatically controlled, then running resistance M.sub.W, which includes the air resistance, the rolling friction, as well as the engine drag torque, acts upon the vehicle. Based on his/her experience, the driver expects a torque characteristic in accordance with running resistance M.sub.W.

(3) The characteristic of running resistance M.sub.W may be simulated by simultaneously applying a drive torque M.sub.A to the vehicle wheels of the front axle and a braking torque M.sub.B to the vehicle wheels of the rear axle. This takes place for the purpose of generating an increased longitudinal wheel force at the vehicle wheels of the rear axle, the increased longitudinal wheel force being advantageous for ascertaining the currently utilized coefficient of friction at the vehicle wheels of the rear axle. By reducing the speed of the vehicle, the load on the rear axle is additionally removed, which means that the contact patch force at the vehicle wheels of the rear axle is reduced, which is also advantageous in view of the determination of the currently utilized coefficient of friction at the wheels of the rear axle.

(4) The coefficient of friction at a wheel is generally calculated from

(5) ${\mu }_{\mathrm{act}}=\frac{F_x}{F_N}.$
In this, ρ.sub.act denotes the coefficient of friction currently used, F.sub.x denotes the longitudinal wheel force, and F.sub.N denotes the contact patch force of the wheel. Accordingly, the coefficient of friction currently used μ.sub.act increases when longitudinal wheel force F.sub.x is increased and/or contact patch force F.sub.N of the wheel is decreased. During a stable trip, the coefficient of friction currently used μ.sub.act is below the maximum possible coefficient of friction.

(6) Since, between times t.sub.1 and t.sub.2, longitudinal wheel force F.sub.x is increased and contact patch force F.sub.N of the wheel is reduced at least slightly, then on the whole, the currently utilized coefficient of friction μ.sub.act at the vehicle wheels of the rear axle is increased, and consequently, a higher percentage of the maximum coefficient of friction is already utilized. This facilitates and improves the determination of the maximum coefficient of friction; prediction of the maximum coefficient of friction is made easier. Using this information, for example, driver assistance systems, such as an electronic stability program or the like, may be parameterized.

(7) In the time between t.sub.1 and t.sub.2, torque characteristic M.sub.W, which represents the totality of the running resistances, is yielded from the sum of drive torque M.sub.A automatically applied to the front axle and braking torque M.sub.B automatically applied to the rear axle. This means that the driver does not perceive any of the automatic intervention in the brake and drive systems of the vehicle.

(8) During the increase of engine drive torque M.sub.A at the wheels of the front axle, in the interval between times t.sub.1 and t.sub.2, longitudinal slip λ.sub.FA at the front axle wheels correspondingly increases, as well. Due to the braking torque M.sub.B applied to the wheels of the rear axle, a negative longitudinal slip λ.sub.RA is generated there.

(9) At time t.sub.2, the automatic intervention in the drive system and the brake system is ended, so that the vehicle continues to roll without a drive torque and without a braking torque. The forces and torques acting upon the vehicle are yielded from the entirety of the running resistances M.sub.W for the air resistance, the rolling friction and the engine drag torque and also correspond to the characteristic subjectively expected by the driver, just as in the preceding time interval between times t.sub.1 and t.sub.2; the transition in the phase between t.sub.1 and t.sub.2, which includes the automatic intervention in the drive and braking systems, and also the transition into the following phase subsequent to time t.sub.2, proceeding continuously. Through this, it is ensured that at both the automatic activation of the drive and braking systems at time t.sub.1 and the automatic deactivation of the drive and braking systems at time t.sub.2, the forces acting upon the vehicle have the expected characteristic, so that the activation and deactivation are not perceived by the driver.