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

Abstract

Method for estimating the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up of a motor vehicle, comprising the steps: a) Adjusting a braking torque on a wheel of a first axle or on a first axle of the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; b) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; c) Determination of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle; d) Applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system; e) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; f) Estimating the actual braking torque via the inverse of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle determined in step c); (g) Calculation of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the first axle of the motor vehicle, wherein the method also includes the determination of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for at least one further axle of the motor vehicle.

Description

[0001] The invention relates to a method for estimating the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up in a motor vehicle. Furthermore, the invention relates to a method for adjusting the target braking torque of a hydraulic brake system with axle-specific pressure build-up in a motor vehicle in order to obtain a desired actual braking torque.

[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 2011 085 984 A1 discloses a method for determining the coefficient of friction for each wheel in vehicles, in which the frictional torque is specified according to a defined torque profile at a wheel of the vehicle and the wheel speed is determined in response to the wheel torque. The current coefficient of friction can be determined from the phase shift between wheel torque and wheel speed.

[0006] DE 10 2014 226 290 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 chassis of the vehicle 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 set.

[0007] 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 based on the deformation of the brake holder caused by the frictional force in a direction tangential to the brake disc surface.

[0008] German patent application DE 10 2016 220 415 presents a method for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle. In this method, a braking torque is set via a precise actuator, the vehicle or wheel deceleration is measured, and the braking torque-deceleration transmission behavior is determined. Subsequently, a braking torque is set via the hydraulic braking system, and the deceleration is measured. The actual braking torque can then be determined and the coefficient of friction of the brake estimated using the inverse of the determined braking torque-deceleration transmission behavior.

[0009] This method applies only to braking systems with pressure build-up at the master cylinder level. For braking systems with axle-specific pressure build-up, the friction coefficients of the individual axles cannot be determined using this method. This is because this approach assumes an overall vehicle friction coefficient (based on a fixed ratio of axle usage, usually the installed distribution). However, with axle-specific pressure build-up, the distribution of braking torque across the axles can vary. This can lead to different deviations from the standard friction coefficient at the brakes of the two axles. Therefore, the vehicle's reaction does not allow conclusions to be drawn about the friction coefficients of the individual axles. In other words, if a specific distribution is used as a basis and an overall vehicle friction coefficient is estimated, it is not known which axle is causing a deviation from the standard friction coefficient, nor how large that deviation is.

[0010] DE 10 2016 220 415 A1 discloses a system for estimating the coefficient of friction of a hydraulic braking system of a motor vehicle, comprising: at least one hydraulic braking system acting on a wheel or axle of the motor vehicle; at least one further actuator system capable of applying a braking torque to the wheel or axle with a higher positioning accuracy than the hydraulic braking system; at least one sensor; and a unit for calculating an estimated coefficient of friction from the sensor data.

[0011] Against this background, the task was to provide a method for estimating the coefficient of friction of a hydraulic braking system with axle-specific pressure build-up in a motor vehicle, without requiring the use of vehicle parameters such as vehicle mass. A braking system with axle-specific pressure build-up acts separately on each axle of the vehicle.

[0012] Within the scope of the present invention, a method for estimating the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up of a motor vehicle, comprising the features of claim 1, is presented. Embodiments are described in the dependent claims and the description.

[0013] Furthermore, a method with the features of claim 6 for adjusting a target braking torque of a hydraulic braking system with axle-specific pressure build-up of a motor vehicle in order to obtain a desired actual braking torque is presented. Embodiments are described below.

[0014] The invention provides a solution for estimating the coefficient of friction of a hydraulic braking system with axle-specific pressure build-up in a motor vehicle, which does not require the use of vehicle parameters, such as the vehicle mass. A braking system with axle-specific pressure build-up acts separately on each axle of the vehicle. The method according to the invention allows the determination of the coefficients of friction for the individual axles of the motor vehicle. Actuating and measuring systems, which 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.

[0015] The system used in the process comprises at least one hydraulic braking system with axle-specific pressure build-up, acting on a wheel or axle of the motor vehicle; and at least one further actuator system, which can apply a braking torque to the wheel or axle with a higher positioning accuracy than the hydraulic braking system. Furthermore, the system comprises at least one sensor and a unit for calculating an estimated coefficient of friction from the sensor data.

[0016] In one embodiment, the system has at least one actuator system on each axle, which can apply a braking torque to the axle or a wheel connected to the axle with a higher positioning accuracy than the hydraulic brake system with axle-specific pressure build-up.

[0017] In one embodiment, the additional actuator system comprises at least one axle motor. In another embodiment, the additional actuator system comprises at least one EM single-wheel motor.

[0018] The system includes at least one suitable sensor, e.g., for measuring vehicle deceleration. In one embodiment, the system includes at least one wheel speed sensor. In another embodiment, the system includes at least one wheel velocity sensor. In yet another embodiment, the system includes at least one brake pressure sensor.

[0019] The inventive method for estimating the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up of a motor vehicle comprises the following steps: A braking torque (target = actual) is applied to a first wheel or axle via an actuator system that has a higher positioning accuracy than the hydraulic brake system (hereinafter also referred to as the "precise actuator"). The deceleration of the wheel or axle or the vehicle caused by the braking torque is measured, and the transmission behavior between the braking torque and the deceleration of the wheel or axle or the vehicle is determined. A target braking torque is applied to the first wheel or axle via the hydraulic brake system, and the deceleration of the wheel or axle or the vehicle caused by the braking torque is measured. Then, the inverse transmission behavior between the braking torque and the deceleration of the wheel or axle or the vehicle can be used to determine the coefficient of friction.The actual braking torque and thus the actual coefficient of friction of the hydraulic braking system with axle-specific pressure build-up for the first axle of the motor vehicle are calculated.

[0020] If the wheel brakes on an axle are coupled, e.g., hydraulically, then the braking torque at the wheel is half the braking torque at the axle. If the axles are coupled via the braking system, e.g., via the hydraulically installed distribution, then the braking torque at the respective axle can be calculated using this distribution and the total braking torque.

[0021] In one embodiment, the method according to the invention also includes determining the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for at least one further axle of the motor vehicle.

[0022] One embodiment of the method is based on the idea that the axles must be considered separately. The axles must therefore be stimulated independently. A braking torque (target = actual) is set via a precise actuator, the vehicle or wheel deceleration is measured, and the torque-deceleration transmission behavior is determined. Subsequently, a target braking torque is applied separately via the hydraulic braking system, first to the first axle and then to at least one other axle, e.g., the second axle, and the deceleration is measured in each case. Using the inverse torque-deceleration transmission behavior, the actual braking torque, and thus the coefficient of friction of the brake, can be estimated for each axle. When the hydraulic braking torque is applied, a deceleration torque can be applied to the other axle or the first axle by the axle motor, for example, to maintain driving stability.Since this deceleration torque is precisely known, it can be subtracted from the calculated actual braking torque.

[0023] Due to the unknown coefficient of friction of the braking system, the target braking torque usually deviates from the actual braking torque. Other actuator systems (e.g., individual wheel motors, axle motors) are known that can apply a braking torque with higher positioning accuracy than the braking system. The target braking torque thus corresponds closely to the actual braking torque. A braking torque is set at a first wheel or axle via an actuator system with high positioning accuracy (target braking torque = actual braking torque), and the wheel or vehicle deceleration is measured. The transmission behavior between braking torque and wheel or vehicle deceleration is then determined. A target braking torque is then applied to the first axle via the hydraulic braking system, and the vehicle or wheel deceleration is measured. Using the inverse transmission behavior of torque and deceleration, the actual torque of the friction brake on this axle is then calculated.Knowing the target torque and the nominal coefficient of friction, the actual coefficient of friction can be deduced. Actual coefficient of friction = Actual braking torque / Target braking torque * Nominal coefficient of friction.

[0024] Alternatively, the actual coefficient of friction can be calculated by measuring the brake pressure: Actual coefficient of friction = Actual braking torque / brake pressure.

[0025] The same procedure is then carried out for a braking torque on another axle of the motor vehicle, e.g. the second axle, and the coefficient of friction for this axle is determined.

[0026] Determining the coefficient of friction for at least one additional axle of the motor vehicle comprises the following steps: Setting a braking torque at a wheel of an additional axle or at an additional axle of the motor vehicle via an actuator system with higher positioning accuracy than the hydraulic braking system; measuring the deceleration of the wheel or axle or the motor vehicle caused by the braking torque; and determining the transmission behavior between the braking torque and the deceleration of the wheel or axle or the motor vehicle. Subsequently, applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system; measuring the deceleration of the wheel or axle or the motor vehicle caused by the braking torque; and estimating the actual braking torque using the inverse of the previously determined transmission behavior between the braking torque and the deceleration of the wheel or axle or the motor vehicle.of the motor vehicle, and finally calculation of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the other axle of the motor vehicle.

[0027] For a precise determination of the coefficient of friction, it is advantageous to have an actuator system on each axle that has a higher positioning accuracy than the hydraulic braking system. However, it is also possible to determine the coefficient of friction of both axles of a two-axle vehicle with a single system in which only one axle has an additional actuator system that has a higher positioning accuracy than the hydraulic braking system. In this embodiment, measurements are taken on the first axle, and the result is then calculated for the second axle, for example, by analyzing slippage.

[0028] In another embodiment, the fact that the two axles of a motor vehicle possess different driving dynamic potential is utilized. In the event of deceleration, the front axle, in particular, is more stable and can be subjected to a higher braking torque. Since the coefficients of friction on both axles cannot be estimated together, the coefficient of friction of the more stable axle is estimated first (i.e., a braking torque is applied to this axle alone). Knowing this coefficient of friction, a braking torque can be applied to both axles under a safe brake force distribution, and the second coefficient of friction can be estimated. With this approach, it is not necessary to apply a braking torque solely to the (unstable) rear axle; instead, a stable brake force distribution can be assumed for estimating the coefficient of friction on the rear axle.When the hydraulic braking torque is applied, a deceleration torque can be applied to the other axle or the first axle by the axle motor, for example, to maintain driving stability. Since this deceleration torque is precisely known, it can be subtracted from the calculated actual braking torque. A further advantage of this embodiment is that only an additional high-precision actuator system is required on the front axle.

[0029] In this embodiment, a braking torque (target = actual) is set via a precise actuator, the vehicle or wheel deceleration is measured, and the braking torque-deceleration relationship is determined. Subsequently, a target braking torque is applied via the hydraulic braking system, first to the first axle, preferably the one with the higher driving dynamics potential, and the deceleration is measured. Using the inverse braking torque-deceleration relationship, the actual braking torque, and thus the coefficient of friction of the brake for the first axle, can then be estimated. Next, a target braking torque is applied to both axles at a specific ratio, and the deceleration is measured. Using the inverse torque-deceleration relationship and the previously estimated coefficient of friction for the first axle, the actual braking torque, and thus the coefficient of friction of the brake for the second axle, can then be estimated.

[0030] Due to the unknown coefficient of friction of the braking system, the target braking torque usually deviates from the actual braking torque. Other actuator systems (e.g., electronic actuators) are known that can apply a braking torque with higher positioning accuracy than the braking system. The target braking torque thus corresponds closely to the actual braking torque. A braking torque is set via the precise actuator (target braking torque = actual braking torque), and the vehicle or wheel deceleration is measured, and the torque-deceleration transmission behavior is determined.

[0031] A target braking torque is then applied to the first axle via the hydraulic braking system, and the vehicle or wheel deceleration is measured. Using the inverse relationship between braking torque and deceleration, the actual braking torque of the friction brake on that axle is then calculated. Knowing the target braking torque and the nominal coefficient of friction, the actual coefficient of friction can be determined. Actual coefficient of friction = Actual torque / Target torque * Nominal coefficient of friction.

[0032] Alternatively, the actual coefficient of friction can be calculated by measuring the brake pressure: Actual coefficient of friction = Actual torque / brake pressure.

[0033] Now, a target braking torque is applied to both axles with a specific ratio (e.g., stable brake force distribution), and the deceleration is measured. Then, using the inverse transmission behavior of brake torque and deceleration, as well as the previously estimated coefficient of friction for the first axle and the set distribution, the actual braking torque, and thus the coefficient of friction of the brake for the second axle, can be estimated.

[0034] Determining the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up for the second axle of a two-axle motor vehicle therefore comprises the following steps: Applying a target braking torque in a predetermined distribution ratio to both axles of the motor vehicle via the hydraulic brake system, measuring the deceleration of the second axle or the motor vehicle caused by the braking torque, estimating the actual braking torque of the second axle via the inverse of the transmission behavior between braking torque and deceleration of a wheel of the first axle or the first axle or the motor vehicle, as well as the calculated coefficient of friction of the first axle and the predetermined distribution ratio of the applied target braking torque, and finally calculating the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the second axle of the motor vehicle.

[0035] In many cases, the coefficient of friction is not of interest; rather, the focus is on the ratio between the target and actual braking torque of the braking system per axle. The target braking torque for each axle can then be multiplied by this factor to obtain the desired torque per axle. To do this, the actual braking torque of the braking system for each axle is determined as described above and related to the target braking torque.

[0036] Another problem solved within the scope of the present invention is to set a desired actual braking torque using a hydraulic braking system at a wheel or axle. In this case, the coefficient of friction is not of interest, but rather the ratio between the target and actual braking torque of the hydraulic braking system per axle. As the quotient of the target and actual braking torque, this ratio corresponds to a factor by which the target braking torque of the braking system per axle can then be multiplied to obtain the desired braking torque per axle. To do this, the actual braking torque of the braking system per axle is determined as described above and related to the target braking torque of the braking system.

[0037] The invention therefore also relates to a method for adjusting the target braking torque of a hydraulic braking system with axle-specific pressure build-up in a motor vehicle to achieve a desired actual braking torque on one axle of the motor vehicle. The desired actual braking torque corresponds to the original target braking torque.

[0038] The method comprises setting a braking torque (target = actual) at a wheel or axle of the motor vehicle via an actuator system that has a higher positioning accuracy than the hydraulic braking system, and measuring the deceleration of the wheel or axle or the motor vehicle caused by the braking torque, followed by determining the transmission behavior between the braking torque and the deceleration of the wheel or axle or the motor vehicle. Furthermore, the method comprises applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system, measuring the deceleration of the wheel or axle or the motor vehicle caused by the braking torque, and estimating the actual braking torque via the inverse of the transmission behavior between the braking torque and the deceleration of the wheel or axle.The ratio between the target and actual braking torque of the hydraulic brake system for the axle is then calculated, and the target braking torque of the hydraulic brake system for the axle is multiplied by the obtained factor to obtain a target braking torque that generates an actual braking torque which corresponds to the original target braking torque.

[0039] The method utilizes a system for adjusting the target braking torque of a hydraulic braking system with axle-specific pressure build-up in a motor vehicle to achieve a desired actual braking torque at a given axle. The desired actual braking torque corresponds to the original target braking torque. The system comprises at least one hydraulic braking system with axle-specific pressure build-up acting on a wheel or axle of the motor vehicle, and at least one additional actuator system capable of applying a braking torque to the wheel or axle with higher positioning accuracy than the hydraulic braking system. The system also includes at least one sensor, a unit for calculating an actual braking torque from the sensor data, and a unit for modifying the target braking torque of the hydraulic braking system.

[0040] Examples of suitable additional actuator systems include axle motors and EM single-wheel motors. Suitable sensors include, for example, wheel speed sensors, wheel velocity sensors, sensors for measuring vehicle deceleration, or brake pressure sensors.

[0041] 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] Method for estimating the coefficient of friction of a hydraulic brake system with axle-specific pressure build-up of a motor vehicle, comprising the steps: a) Adjusting a braking torque on a wheel of a first axle or on a first axle of the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; b) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; c) Determination of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle; d) Applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system; e) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; f) Estimating the actual braking torque via the inverse of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle determined in step c); (g) Calculation of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the first axle of the motor vehicle, wherein the method also includes the determination of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for at least one further axle of the motor vehicle. [2] Method according to claim 1, wherein the determination of the coefficient of friction for at least one further axle of the motor vehicle comprises the following steps: h1) Adjusting a braking torque on a wheel of another axle or on another axle of the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; i1) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; j1) Determination of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle; k1) Applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system; l1) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; m1) Estimating the actual braking torque via the inverse of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle determined in step j1); n1) Calculation of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the other axle of the motor vehicle. [3] The method of claim 1, wherein the motor vehicle has two axles and the further axle is the second axle of the motor vehicle and the determination of the coefficient of friction comprises the following steps: h2) Applying a target braking torque in a predetermined distribution ratio to both axles of the motor vehicle via the hydraulic braking system; i2) Measurement of the deceleration of the second axle or the motor vehicle caused by the braking torque; j2) Estimating the actual braking torque via the inverse of the transmission behavior determined in step c) between braking torque and deceleration of the wheel of the first axle or the first axle or the motor vehicle, as well as with the coefficient of friction of the first axle calculated in step g) and the specified distribution ratio of the applied target braking torque; k2) Calculation of the coefficient of friction of the hydraulic brake system with axle-specific pressure build-up for the second axle of the motor vehicle. [4] Method according to one of claims 1 to 3, wherein the coefficient of friction is calculated from the target braking torque and the actual braking torque and the nominal coefficient of friction of the hydraulic braking system according to Coefficient of friction = Actual braking torque / Target braking torque * Nominal coefficient of friction. [5] Method according to one of claims 1 to 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 braking torque of a hydraulic brake system with axle-specific pressure build-up of a motor vehicle in order to achieve a desired actual braking torque on an axle of the motor vehicle, comprising the steps: a) Adjusting a braking torque on a wheel of the axle or the axle of the motor vehicle via an actuator system which has a higher positioning accuracy than the hydraulic braking system; b) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; c) Determination of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle; d) Applying a target braking torque to the wheel or axle of the motor vehicle via the hydraulic braking system; e) Measurement of the deceleration of the wheel or axle or motor vehicle caused by the braking torque; f) Estimating the actual braking torque via the inverse of the transmission behavior between braking torque and deceleration of the wheel or axle or motor vehicle determined in step c); g) Calculation of the ratio between the target and actual braking torque of the hydraulic braking system for the axle; h) Multiply the target braking torque of the hydraulic brake system for the axle by the obtained factor to obtain a target braking torque corresponding to the desired actual braking torque.