Estimation method for the coefficient of friction of a hydraulic braking system

Abstract

System for estimating a coefficient of friction (Cp value) between brake pad and brake disc of a hydraulic braking system of a motor vehicle, comprising: at least one hydraulic braking system that acts on at least one wheel of the motor vehicle; at least one further actuator system comprising an EM single-wheel motor, which can apply a torque to at least one wheel with a higher positioning accuracy than the hydraulic braking system, causing a yaw moment on the vehicle; at least one sensor; a unit for calculating an estimated coefficient of friction (Cp value) from the sensor data.

Description

[0001] The invention relates to a system and a method for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle. Furthermore, the invention relates to a system and a method for adjusting the target yaw moment generated by a hydraulic braking system of a motor vehicle in order to obtain a desired actual yaw moment.

[0002] The coefficient of friction (also known as Cp value) of a hydraulic braking system between the brake pad and brake disc determines the generation of braking torque and is proportional to the brake pressure. Since only the brake pressure can be measured, it is important to estimate the coefficient of friction as accurately as possible for a precise understanding of the braking torque at the wheels. The coefficient of friction can be calculated by measuring the vehicle deceleration and the brake pressure, and by knowing vehicle parameters such as the vehicle mass. There is also a method for estimating the coefficient of friction that does not require knowledge of the vehicle mass. This method compares the longitudinal deceleration produced by the friction brake with that produced by an electric motor whose torque is precisely known.

[0003] DE 35 02 050 A1 proposes to measure the temperature generated in the braking device that produces the braking torque as a characteristic parameter for the braking torque and to evaluate it in an evaluation device.

[0004] DE 100 11 270 A1 discloses a method and a device for determining a characteristic value of a wheel brake, which represents the relationship between braking torque and braking pressure. The determination of the brake characteristic value is carried out individually for each wheel by calculation based on wheel-specific parameters such as wheel slip, wheel contact force, wheel brake pressure, and wheel radius.

[0005] DE 10 2014 226 290 A1 teaches a method for adjusting or controlling a brake actuation variable BR(FZ), which is either the clamping force F, the brake pressure PR, or the clamping travel X (xs) of a motor vehicle brake with at least one braked wheel R, within an electro-hydraulic or electromechanical vehicle brake that is attached to a vehicle chassis with a brake caliper and brakes the wheel via friction linings according to the brake actuation variable BR(FZ), wherein the wheel circumferential force F (FU) resulting from contact of the braked wheel with a road surface is measured, and the braking force F (FU) is influenced as a function of the slope of the wheel circumferential force F (ΔFU / Δs), where the actual value of the control is the slope of the wheel circumferential force F (ΔFU / Δs), and this is adjusted by setting the brake actuation variable BR(FZ) to a predetermined setpoint. TVR(OP(ΔFU / Δs)) or a setpoint range TUR(OP(ΔFU / Δs) + / -R(ΔFU / Δs)) is regulated.

[0006] From DE 10 2010 043 320 A1, a device and a method for determining a measure of the frictional force acting on a disc brake of a motor vehicle are known. A sensor device arranged on the disc brake measures a displacement of the brake holder caused by the braking force in a direction perpendicular to the axial direction of the brake disc. A measure of the acting braking torque is determined from the deformation of the brake holder caused by the frictional force in a direction tangential to the brake disc surface.

[0007] Against this background, the task was to provide a new system and method for estimating the coefficient of friction of a hydraulic brake system of a motor vehicle, which do not require the use of vehicle parameters, such as vehicle mass.

[0008] Within the scope of the present invention, a new system for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle, comprising the features of claim 1, is presented. Embodiments are described in the dependent claims and the description. Furthermore, a method for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle, comprising the features of claim 3, is presented. Embodiments are described in the dependent claims and the description.

[0009] In addition, a method with the features of claim 6 for adjusting a target yaw moment of a motor vehicle generated by a hydraulic braking system in order to obtain a desired actual yaw moment, and a suitable system for this purpose with the features of claim 7, are presented. Embodiments are described in the dependent claims, the description, and the drawings.

[0010] The invention is schematically illustrated with reference to embodiments in the drawings and is described schematically and in detail with reference to the drawings. It shows: Fig. 1 a flowchart of the inventive method for estimating the coefficient of friction of a hydraulic brake system of a motor vehicle, in which the method steps and data flows are illustrated; Fig. 2 a flowchart of the inventive method for adjusting the target yaw moment of a motor vehicle generated by a hydraulic braking system in order to obtain a desired actual yaw moment, in which the method steps and data flows are illustrated.

[0011] The invention provides a solution for estimating the coefficient of friction of a hydraulic braking system in a motor vehicle that does not require the use of vehicle parameters, such as the vehicle mass. Actuating and measuring systems that are typically present in the vehicle can be used within the scope of the invention. Further advantages and embodiments of the invention will become apparent from the following description.

[0012] The system according to the invention comprises at least one hydraulic braking system with the capability of individual wheel pressure build-up, which acts on at least one wheel of the motor vehicle; and at least one further actuator system, which can apply a torque to at least one wheel with higher positioning accuracy than the hydraulic braking system, thereby generating a yaw moment on the motor vehicle. Furthermore, the system according to the invention comprises at least one sensor and a unit for calculating an estimated coefficient of friction from the sensor data.

[0013] Hydraulic braking systems typically offer the capability of individual wheel pressure build-up, for example via an ESP pump and appropriate valve switching. By building up pressure at individual wheels, a yaw moment can be applied to the vehicle, which improves driving dynamics or stability. This is known as brake torque vectoring (BTV).

[0014] In one embodiment, the further actuator system comprises at least one EM single-wheel motor. The EM single-wheel motor can also be operated as a generator. It can therefore apply both a positive and a negative torque to the wheel, thus generating a yaw moment. In another embodiment, the further actuator system comprises at least one sport differential, i.e., a differential that can apply a differential torque to the wheels on one axle.

[0015] The system according to the invention includes at least one suitable sensor. In one embodiment, the system comprises at least one yaw rate sensor. The inventive method for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle comprises the following steps: A braking torque (target braking torque = actual braking torque) is set at at least one wheel of the motor vehicle via an actuator system with high positioning accuracy, thereby generating a yaw moment on the vehicle. The yaw reaction caused by the yaw moment is measured, and the transmission behavior between the yaw moment and the yaw reaction is determined. A target yaw torque is applied to the vehicle via the hydraulic braking system, and the yaw reaction is measured. The actual yaw torque can then be calculated using the inverse transmission behavior between the yaw moment and the yaw reaction.From the yaw moments on the vehicle, the braking torques at the wheels and thus the actual coefficient of friction of the hydraulic braking system can be calculated using the vehicle dimensions.

[0016] To illustrate, the process according to the invention is shown in Fig. 1 also represented as a flowchart, in which the individual process steps and data flows are shown.

[0017] Due to the unknown coefficient of friction of the braking system, the target yaw moment usually deviates from the actual yaw moment. Other actuator systems (e.g., electric motors for individual wheels, sport differentials) are known that can apply a yaw moment to the vehicle with higher positioning accuracy than the braking system. The target yaw moment thus corresponds well to the actual yaw moment. A braking torque is applied to the wheel via an actuator system with high positioning accuracy, generating a yaw moment on the vehicle (target yaw moment = actual yaw moment). The yaw response is measured, and the transmission behavior between yaw moment and wheel deceleration is determined. Alternatively, a target yaw moment is applied to the vehicle via the hydraulic braking system, and the yaw response is measured. Using the inverse transmission behavior between yaw moment and yaw response, the actual torque of the friction brake at the wheel is deduced. Knowing the target torque and the nominal coefficient of friction, the actual coefficient of friction can be calculated. Actual coefficient of friction = Actual yaw moment / Target yaw moment * Nominal coefficient of friction.

[0018] Alternatively, the actual coefficient of friction can be calculated by measuring the brake pressure: Actual coefficient of friction = Actual yaw moment / brake pressure.

[0019] Another problem solved within the scope of the present invention is to adjust a desired actual yaw moment on the vehicle via at least one wheel with a hydraulic braking system. In this case, the coefficient of friction is not of interest, but rather the ratio between the target and actual yaw moment generated by the hydraulic braking system. This factor can then be used to multiply the target yaw moment generated by the hydraulic braking system to obtain the desired yaw moment on the vehicle. To do this, as described above, the actual yaw moment generated by the braking system is determined and related to the target yaw moment generated by the braking system on the vehicle.

[0020] The invention therefore also relates to a system and a method for adjusting the target yaw moment generated by a motor vehicle's hydraulic braking system to achieve a desired actual yaw moment on the motor vehicle. The desired actual yaw moment corresponds to the original target yaw moment. The method comprises setting a yaw moment on the motor vehicle via an actuator system that has a higher positioning accuracy than the hydraulic braking system and measuring the yaw reaction of the motor vehicle caused by the yaw moment, followed by determining the transfer behavior between yaw moment and yaw reaction. Furthermore, the method comprises applying a target yaw moment to the motor vehicle via the hydraulic braking system, measuring the yaw reaction caused by the yaw moment, and estimating the actual yaw moment using the inverse of the transfer behavior between yaw moment and yaw reaction.The ratio between the target and actual yaw moment generated by the hydraulic braking system is then calculated, and the target yaw moment generated by the hydraulic braking system is multiplied by the obtained factor to obtain a target yaw moment that generates an actual yaw moment which corresponds to the original target yaw moment.

[0021] To illustrate, the process according to the invention is shown in Fig. 2 is also represented as a flowchart, in which the individual process steps and data flows are shown.

[0022] Furthermore, the invention provides a system for adjusting the target yaw moment generated by a motor vehicle's hydraulic braking system to obtain a desired actual yaw moment. The desired actual yaw moment corresponds to the original target yaw moment. The system comprises at least one hydraulic braking system acting on at least one wheel of the motor vehicle and at least one further actuator system capable of applying a yaw moment to the vehicle with higher positioning accuracy than the hydraulic braking system. The system also includes at least one sensor, a unit for calculating an actual yaw moment from the sensor data, and a unit for modifying the target yaw moment generated by the hydraulic braking system.

[0023] Examples of suitable additional actuator systems include sport differentials and EM individual wheel motors. Suitable sensors include, for example, yaw rate sensors.

[0024] It is understood that the aforementioned features can be used not only in the combinations specified above, but also in other combinations or individually, without departing from the scope of the present invention.

Claims

[1] System for estimating a coefficient of friction (Cp value) between brake pad and brake disc of a hydraulic braking system of a motor vehicle, comprising: at least one hydraulic braking system that acts on at least one wheel of the motor vehicle; at least one further actuator system comprising an EM single-wheel motor, which can apply a torque to at least one wheel with a higher positioning accuracy than the hydraulic braking system, causing a yaw moment on the vehicle; at least one sensor; a unit for calculating an estimated coefficient of friction (Cp value) from the sensor data. [2] System according to claim 1, comprising at least one yaw rate sensor. [3] Method for estimating a coefficient of friction (Cp value) between brake pad and brake disc of a hydraulic braking system of a motor vehicle, comprising the steps: a) Adjusting a yaw moment on the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; b) Measurement of the yaw response of the vehicle caused by the yaw moment; c) Determination of the transfer behavior between yaw moment and yaw response of the vehicle; d) Applying a target yaw moment to the motor vehicle via the hydraulic braking system; e) Measurement of the yaw response of the vehicle caused by the yaw moment; f) Estimating the actual yaw moment via the inverse of the transfer behavior between yaw moment and yaw response of the vehicle determined in step c); g) Calculation of the actual coefficient of friction (Cp value) between brake pad and brake disc of the hydraulic brake system. [4] Method according to claim 3, wherein the coefficient of friction is calculated from the target yaw moment and the actual yaw moment and the nominal coefficient of friction of the hydraulic brake system according to Actual coefficient of friction = Actual yaw moment / Target yaw moment * Nominal coefficient of friction. [5] Method according to claim 3, wherein the coefficient of friction is calculated from the actual braking torque and the braking pressure of the hydraulic braking system according to Coefficient of friction = Actual braking torque / Brake pressure. [6] Method for adjusting a target yaw moment of a motor vehicle generated by a hydraulic braking system in order to achieve a desired actual yaw moment, comprising the steps: a) Adjusting a yaw moment on the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; b) Measurement of the yaw response of the vehicle caused by the yaw moment; c) Determination of the transfer behavior between yaw moment and yaw response of the vehicle; d) Applying a target yaw moment to the motor vehicle via the hydraulic braking system; e) Measurement of the yaw response of the vehicle caused by the yaw moment; f) Estimating the actual yaw moment via the inverse of the transfer behavior between yaw moment and yaw response of the vehicle determined in step c); g) Calculation of the ratio between the target and actual yaw moment generated by the hydraulic braking system; h) Multiplying the target yaw moment generated by the hydraulic braking system by the obtained factor to obtain a target yaw moment corresponding to the desired actual yaw moment. [7] System for adapting a target yaw moment of a motor vehicle generated by a hydraulic braking system to a desired actual yaw moment, comprising: at least one hydraulic braking system that acts on at least one wheel of the motor vehicle; at least one further actuator system comprising an EM single-wheel motor, which can apply a torque to at least one wheel with a higher positioning accuracy than the hydraulic braking system, causing a yaw moment on the vehicle; at least one sensor; a unit for calculating an actual yaw moment from the sensor data; and a unit for changing the target yaw moment generated by the hydraulic braking system. [8] System according to claim 7, comprising at least one yaw rate sensor.

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