Method for estimating locking pressure in a braking system of a multi-axle vehicle

Abstract

The invention relates to a method for estimating the locking pressure in a brake system of a multi-axle vehicle. In the method for estimating the locking pressure in a brake system of a multi-axle vehicle in the case of a dynamic axle load transfer, the locking pressure during the axle load transfer and the wheel normal force at two points in time during the axle load transfer are determined, and the locking pressure at the later of the points in time is determined therefrom.

Description

Technical Field

[0002] The invention relates to a method for estimating the locking pressure in a brake system of a multi-axle vehicle in the event of a dynamic axle load transfer. Technical Field

[0003] Hydraulic brake systems in vehicles are known which are equipped with an anti-lock braking system (ABS), by means of which the locking of the braked wheels is to be prevented. In the event of locking, the braked wheels are so far in the range of sliding friction that the controllability of the vehicle is not guaranteed as long as the locked wheels are used for steering the vehicle, since in this case no lateral guiding forces can be transmitted from the wheels to the ground. In order to prevent this state, the anti-lock braking system is designed in such a way that the brake pressure may only be increased until the wheels have a tendency to lock, in which case the wheel slip reaches a given limit value. The wheel slip is determined based on a comparison of the wheel speed, which is determined by means of wheel speed sensors, with a reference speed of the vehicle. After the locking tendency has been detected, the brake pressure in the brake system is reduced so that the controllability of the vehicle remains guaranteed in the case of steerable wheels.

[0004] The magnitude of the transferable braking force depends on the current friction coefficient between the wheel and the ground, which is defined as the quotient of the braking force and the normal force. In the case of dynamic axle load transfers, which occur with high braking forces, for example during braking, the normal force at the vehicle wheel changes, and thus also the friction coefficient. Since the longitudinal slip also changes with respect to the µ-slip curve, the brake pressure level must also be adapted in order to reliably prevent wheel locking. Summary of the invention

[0005] The method according to the invention makes it possible to estimate the locking pressure in the wheel brakes of a hydraulic brake system of a multi-axle vehicle in the event of a dynamic axle load transfer. This makes it possible to adapt the brake pressure levels in the wheel brakes of the vehicle wheels in such a way that locking of the vehicle wheels is prevented in the event of an axle load transfer. In the case of steerable wheels, the steerability remains.

[0006] In order to reliably prevent locking, it is necessary to know the current locking pressure in the wheel brakes of the relevant vehicle wheels. The locking pressure depends on the current normal force at the vehicle wheel, which changes in the case of a dynamic axle load transfer. In order to estimate the locking pressure during the axle load transfer, the locking pressure is first determined at a first point in time, preferably when a maximum friction coefficient is reached, in particular in a brake pressure control cycle, at the beginning of an ABS (anti-lock braking system) pressure reduction in the case of an impending locking tendency. If the longitudinal slip at the vehicle wheel exceeds a limiting value, the impending locking tendency can be identified according to the µ-slip curve. The longitudinal slip can be determined based on the wheel speed that can be detected via the wheel speed sensor and a reference speed of the vehicle, the longitudinal slip being defined as the quotient of the wheel speed and the reference speed.

[0007] Knowing the locking pressure at a first time for a given axle load distribution, the locking pressure at a subsequent second time can be estimated within a continuous brake pressure control, in particular an ABS control, but during a continuous dynamic axle load transfer during a subsequent brake pressure control cycle.

[0008] An ABS control cycle can be understood as a pressure modulation between two pressure reduction phases, which begin when the adhesion limit (maximum friction coefficient) is reached.

[0009] In the method, a locking pressure determined at a first point in time (during a first brake pressure control cycle) and a ratio of a wheel normal force at a second point in time to the wheel normal force at the first point in time during a subsequent brake pressure control cycle are determined. Based on these variables, the locking pressure can be estimated at a second point in time during an axle load transfer. This estimation can be performed without a brake pressure control process being performed at the second point in time during an axle load transfer at a brake pressure in the range of wheel instability.

[0010] The method is carried out during a continuous braking process, in particular a brake pressure regulation, preferably an ABS regulation. The first time point is in a first brake pressure regulation cycle within the brake pressure regulation, and the second time point is in a subsequent second brake pressure regulation cycle within the same brake pressure regulation. During the brake pressure regulation, advantageously a plurality of successive brake pressure regulation cycles are carried out. If necessary, further intermediate brake pressure regulation cycles may be between the first and second brake pressure regulation cycles. In the first brake pressure regulation cycle, the locking pressure is preferably determined when the maximum value of the friction coefficient is reached. The brake pressure regulation is a brake pressure regulation process.

[0011] This procedure has the advantage that the locking pressure at a second point in time can be estimated during a dynamic axle load transfer and the pressure level in the vehicle brake system can be adapted accordingly. In particular, it is possible to optimally utilize the current friction coefficient so that a high braking effect is achieved without the risk of the vehicle wheels locking. The brake pressure in the wheel brake system can be increased quickly with knowledge of the locking pressure without the risk of wheel instability.

[0012] This procedure is possible both when the wheel normal force is falling and when the wheel normal force is rising. When the wheel normal force is falling, the brake force build-up can be slowed down because wheel instability can be anticipated earlier, thereby avoiding or at least reducing pressure jumps and high pressure gradients. Conversely, when the wheel normal force is rising, the brake pressure build-up can be accelerated because vehicle instability can be anticipated later, wherein a correspondingly higher pressure build-up gradient also does not lead to wheel instability due to the higher normal force.

[0013] The variables required for determining the locking pressure during an axle load transfer can be detected by means of sensors. For example, it is possible to determine the wheel normal force at the vehicle wheel by means of a height sensor, which measures the vehicle body movement between the vehicle axle on which the vehicle wheel is arranged and the roadway. The change in the normal force can be inferred from the height change measured in the height sensor. The determination of the absolute normal force level is also possible. Axle load transfers occurring during braking or during steering movements can also be detected by means of a height sensor.

[0014] It may be expedient to assume an axle load transfer only if a change in the wheel normal force or a vehicle body movement exceeds a threshold value.

[0015] According to a preferred embodiment, during the axle load transfer, the locking pressure is calculated at the desired second time point from the product of the ratio of the locking pressure of the previous ABS control cycle (first time point) to the wheel normal force (the current normal force at the second time point to the normal force at the first time point during the last ABS reduction in the previous ABS control cycle). Accordingly, the locking pressure during the axle load transfer is determined by extrapolating the locking pressure of the last ABS control cycle with the help of the wheel normal force ratio. Since the wheel normal force at any time point can be determined, for example, via a height sensor from the vehicle body movement, it is possible to determine the locking pressure at any time point. The only prerequisite is that the locking pressure at a specific first time point is determined and the wheel normal force is also present at this time point. If these variables are given, the current locking pressure can also be determined at any later time point via the current wheel normal force. The ratio of the current wheel normal force to the wheel normal force at the first time point at which the locking pressure was determined describes the axle load transfer.

[0016] According to another advantageous embodiment, the locking pressure gradient is also taken into account when setting the current brake pressure. The locking pressure gradient can be calculated based on the gradient of the wheel normal force, given knowledge of the locking pressure at an earlier first time and the wheel normal force at this earlier time. Given knowledge of the locking pressure gradient, the dynamics of the pressure increase can be set in an optimal manner.

[0017] The method is advantageously carried out during a continuous brake pressure control process of an anti-lock braking system or ESP system (Electronic Stability Program), which is particularly involved in a control process. The determination of the locking pressure during the axle load transfer is carried out during the continuous brake pressure control process. This assumes that the road conditions and therefore also the friction coefficient do not change or at least do not change significantly between the first time and the second time within the time period of the continuous brake pressure control process.

[0018] Another aspect of the invention relates to a method for improving a braking process, wherein firstly the locking pressure is estimated during a dynamic axle load transfer as described above, wherein the estimated locking pressure is provided as an input variable to a brake pressure control system, in particular an anti-lock braking system or an ESP system. This allows a maximum deceleration of the vehicle without the risk of vehicle instability.

[0019] Another aspect relates to a control device for actuating adjustable components of a hydraulic brake system, wherein the above-described method steps are carried out in the control device.

[0020] Furthermore, the invention relates to a hydraulic brake system in a vehicle which is equipped with a corresponding control device for carrying out the method.

[0021] The invention further relates to a vehicle having a hydraulic brake system as described above. The vehicle has at least two vehicle axles between which a dynamic axle load distribution can be performed. The vehicle is preferably designed as a multi-track vehicle, in particular a motor vehicle or a commercial vehicle. However, it is also conceivable to use it in single-track vehicles such as motorcycles or scooters.

[0022] The invention further relates to a computer program product having a program code which is designed to carry out the method steps described above. The computer program product runs in a control device described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further advantages and advantageous embodiments can be gathered from the further claims, the description of the figures and the drawings.

[0024] Figure 1 A schematic diagram showing a hydraulic brake system in a vehicle,

[0025] Figure 2 The µ-slip curve with the variation of the friction coefficient according to the longitudinal slip is shown,

[0026] Figure 3 A diagram is shown with the time profile of the wheel normal force at the vehicle wheel as a function of the dynamic axle load transfer and with the profile of the brake pressure. DETAILED DESCRIPTION

[0027] exist Figure 1 The hydraulic brake system 1 for a vehicle shown in FIG. 1 comprises two brake circuits 2 and 3, for example, a front axle brake circuit 2 and a rear axle brake circuit 3, for supplying brake fluid under hydraulic pressure to a wheel brake device 9 at each wheel of the vehicle and for actuating the wheel brake device. The two brake circuits 2 and 3 are connected to a common master brake cylinder 4, which is supplied with brake fluid via a brake fluid reservoir 5. The master brake cylinder 4 is actuated by the driver via a brake pedal 6, and the pedal travel applied by the driver is measured via a pedal travel sensor 7. A brake booster 10 is located between the brake pedal 6 and the master brake cylinder 4, and the brake booster 10 comprises, for example, an electric motor, which actuates the master brake cylinder 4 via a transmission (iBooster). The actuating movement of the brake pedal 6 measured by the pedal travel sensor 7 is transmitted as a sensor signal to a control device 11, in which a control signal for actuating the power booster 10 is generated. The supply of brake fluid to the wheel brake device 9 is carried out in each brake circuit 2 and 3 via different switching valves, which belong to the brake hydraulic system 8, including other assemblies. Furthermore, brake hydraulic system 8 includes a hydraulic pump which is a component of an anti-lock braking system (ABS) or an electronic stability program (ESP).

[0028] A method for estimating the locking pressure in the brake system of a multi-axle vehicle during a dynamic axle load transfer is described below. The locking pressure is the brake pressure in a hydraulic brake system that locks the vehicle wheels. In particular, in the case of steerable vehicle wheels, the vehicle wheels become unstable when they lock and are no longer steerable.

[0029] In accordance with Figure 2 In the µ-slip diagram, the maximum of the curve is at a critical point at a longitudinal slip s of about 0.2, from which the vehicle wheels face locking and become unstable with increasing slip s. In the case of dynamic axle load transfer, the normal force F acting on the wheel N When the friction coefficient µ remains constant, the braking force transmittable via the vehicle wheel in the longitudinal direction of the wheel also changes.

[0030] The maximum value of the µ-slip curve is assigned a locking pressure pbl , which must not be exceeded in the wheel brakes of the vehicle wheels in order to avoid wheel instabilities. bl During a braking process with active brake pressure control, in particular in an active anti-lock braking system ABS, it can be determined that the hydraulic brake pressure moves up to a maximum value of the µ-slip curve, but does not exceed this maximum value. If the anti-lock braking system ABS is involved in a controlled manner in the braking system of the vehicle, the locking pressure p can therefore be determined. bl .

[0031] Locking pressure p bl Depends on the wheel normal force F N The wheel normal force can also be detected at the time of determining the locking pressure, for example by means of a height sensor which measures the movement of the vehicle body between the vehicle axle on which the vehicle wheels are suspended and the roadway. Using this information, it is possible to infer the changed locking pressure which results during continuous brake pressure control after a dynamic axle load transfer. During strong braking or when cornering, the axle load distribution between the front and rear axles of the vehicle and therefore the wheel normal forces at the vehicle wheels of the front or rear axle change. This causes a change in the locking pressure at the vehicle wheels.

[0032] The changed locking pressure p at time T2 b1,T2 The locking pressure p at time point T1 can be understood b1,T1 In the case of , it is determined according to the following relationship:

[0033] .

[0034] Wheel normal force F at time T1 N,T1 It is known that the locking pressure P is also determined at the time T1. b1,T1 At time T2, the wheel normal force F can also be determined based on sensor information, in particular from a height sensor. N,T2 Therefore, in order to calculate the locking pressure p at time T2 b1,T2 , all necessary information is available. Locking pressure p b1,T2 The locking pressure at time T2 during the axle load transfer and can be used to further control the brake pressure control system. The knowledge of the locking pressure at time T2 improves the friction coefficient utilization and enables a rapid increase in braking force.

[0035] In principle, the time point T2 can be at any desired time distance from the time point T1 at which the locking pressure p b1,T1 and wheel normal force F N,T1, which assumes that the roadway friction coefficient does not change significantly during this time. This allows any axle load transfer to be taken into account and an adapted locking pressure to be determined. It is only necessary to determine the wheel normal force F at time T2 N , which can however be carried out without problems using a height sensor. However, it is expedient to determine a new locking pressure at the start of the pressure increase for each new ABS control cycle (time T2).

[0036] In addition, the locking pressure gradient can be calculated based on

[0037]

[0038] At time t=T2, the locking pressure gradient can be determined from the locking pressure and the wheel normal force at time T1 and from the gradient of the wheel normal force at time T2.

[0039] In accordance with Figure 3 In the diagram, the upper curve shows the change process of normal force F when the axle load changes. N At any assumed time T1, the normal force course is located shortly before the maximum value. If the anti-lock braking system in the braking system is activated at time T1, the associated locking pressure p can be determined. b1,T1 The associated normal force F N,T1 The brake pressure p includes the locking pressure p b1,T1 The relevant pressure change process is located according to Figure 3 The lower curve in the graph.

[0040] From the locking pressure p at the first time point T1 b1,T1 and the normal force F N,T1 Starting from the above, the locking pressure p at time T2 or at any arbitrary time, for example at time T2+Δt, can be determined. b1,T2 It is only necessary to determine the wheel normal force at time T2 or T2+Δt, and then the current locking pressure p can be determined based on the relationship described above. b1,T2 .

Claims

1. A method for estimating the locking pressure in a braking system (1) of a multi-axle vehicle in the event of a dynamic axle load transfer, wherein during a braking process, at a first point in time (T1) during the dynamic axle load transfer, the locking pressure (p bl,T1 ) and determining the wheel normal force (F) at the vehicle wheel at the same time point (T1) N,T1 ) and during the continuous braking process at the next second time point (T2), the locking pressure (p bl,T2 ) according to the locking pressure (p bl,T1 ) and according to the wheel normal force (F N,T2 ) and the wheel normal force (F N,T1 ) ratio to determine.

2. The method according to claim 1, characterized in that The locking pressure (p bl,T1 ) is determined during a brake pressure regulation process (ABS) in the brake system (1).

3. The method according to claim 1 or 2, characterized in that: The locking pressure (p bl,T2 ) According to the following relationship To determine, T1 represents the first time point during the dynamic axle load transfer, T2 represents the second time point during the dynamic axle load transfer, p bl,T1 represents the locking pressure at time point T1, p bl,T2 represents the locking pressure at time point T2, F N,T1 represents the normal force of the wheel at time point T1, F N,T2 represents the wheel normal force at time point T2.

4. The method according to claim 3, characterized in that When setting the brake pressure, Consider locking pressure gradients.

5. The method according to claim 1 or 2, characterized in that: The wheel normal force (F N,T1 , F N,T2 ) is determined with the aid of a height sensor which measures the movement of the vehicle body between a vehicle axle and the roadway.

6. The method according to claim 1 or 2, characterized in that: The dynamic axle load transfer is determined by means of a height sensor as a function of the vehicle body movement or as a function of the wheel normal forces.

7. The method according to claim 1 or 2, characterized in that Performed during a continuous brake pressure regulation process.

8. A method for improving a braking process, wherein the locking pressure (p bl,T2 ) is performed according to the method according to any one of claims 1 to 7, wherein the locking pressure (p bl,T2 ) is provided to the brake pressure control system as an input variable as a locking pressure prediction.

9. A control device for actuating adjustable components of a hydraulic brake system (1) in a vehicle in order to carry out the method according to claim 1.

10. A hydraulic brake system in a vehicle, comprising a control device (11) according to claim 9.

11. A vehicle having the hydraulic brake system according to claim 10. 12 . A computer program product having a program code which is designed to carry out the steps of the method according to claim 1 when the computer program product is run in a control device ( 11 ) according to claim 9 .

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