Method for controlling a braking system

Abstract

The invention relates to a method for controlling a brake system comprising a hydraulic pump and a normally open isolating valve between the pressure side and the suction side of the pump. To regulate a hydraulic pressure to a setpoint, the isolating valve is supplied with a valve flow based on the magnitude of the setpoint, while the hydraulic pump delivers brake fluid to at least one wheel brake. To lock the hydraulic pressure in the at least one wheel brake with the lowest possible pressure loss, the pump is switched off and the isolating valve is closed. To close the isolating valve, the valve flow is reduced by a value ΔI in a first step, and a closing pulse with a higher current is applied in a second step. The value ΔI is selected based on the setpoint of the hydraulic pressure.

Description

[0001] The invention relates to a method for controlling a brake system comprising a hydraulic pump and a normally open isolating valve between the pressure side and the suction side of the pump, wherein, for the control of a hydraulic pressure, the isolating valve is supplied with a current based on the magnitude of the hydraulic pressure, while the hydraulic pump delivers brake fluid to the wheel brake.

[0002] Conventional braking systems feature a master brake cylinder operated by the brake pedal, often designed as a tandem master cylinder to supply two separate brake circuits. During normal braking, this cylinder directly affects the wheel brakes, meaning that when the brake pedal is pressed, brake fluid is forced into the wheel brakes, building up brake pressure.

[0003] To perform driver-independent braking, as is the case with adaptive cruise control, emergency braking systems, stability control, hill hold functions, and many other assistance systems, an electrically operated hydraulic pump is also provided. To precisely regulate the desired target pressure and pressure profile in the wheel brakes with such a pump, a separating valve is also provided, connecting the pressure side of the pump to its suction side. With such a valve, a differential pressure across the valve can be set very precisely by supplying it with a coil current that still allows a slight flow rate, thus establishing a predefined differential pressure.

[0004] A method for calibrating an inlet valve is known from DE 10 2005 014 097 A1. For this purpose, the valve is briefly opened while a hydraulic pump builds up a pressure differential. Subsequently, the volume that flowed through the inlet valve into the wheel brake is pumped back and its volume is determined. The flow opening can then be determined from this volume.

[0005] DE 10 2008 013 043 A1 further discloses a method for controlling analog isolation valves of a brake system, wherein an upper and a lower limit value are assigned to take into account tolerances of a characteristic curve, wherein in a special mode, when a valve request is zero, the current is reduced to the lower limit value.

[0006] Furthermore, WO 02090159A1 discloses a method for holding a vehicle on an incline by applying brake pressure to the wheel brakes. A pressure drop due to leaks in the brake system is detected, and the pressure is increased to such an extent that holding the vehicle is ensured.

[0007] If, after setting the pressure in the wheel brakes to a target value, the pressure needs to be kept constant for an extended period, the shut-off valve could continue to regulate to the corresponding target value, allowing the pump to deliver brake fluid, which would then flow completely back through the shut-off valve. However, this should be avoided due to the noise of the pump and the thermal stress. Instead, the pump is deactivated, and usually, taking into account the pump's overrun time, the shut-off valve is completely closed, thus trapping the brake fluid in the wheel brake and maintaining a constant pressure. If the valve does not close completely, however, leakage will occur, which will nevertheless reduce the pressure in the wheel brakes.

[0008] It is therefore an object of the present invention to minimize leakage flows and thus to keep the pressure as constant as possible.

[0009] The problem is solved by a method according to claim 1 for controlling a brake system comprising a hydraulic pump and a normally open isolating valve between the pressure side and the suction side of the pump, wherein, to regulate a hydraulic pressure to a setpoint, the isolating valve is supplied with a valve flow based on the magnitude of the setpoint, while the hydraulic pump delivers brake fluid to at least one wheel brake, and to lock the hydraulic pressure in the at least one wheel brake, the pump is switched off and the isolating valve is closed, wherein, to close the isolating valve, the valve flow is reduced by a value Delta I in a first step, and in a second step a closing pulse with a higher current is applied, wherein the value Delta I is selected based on the hydraulic pressure.During regulation to a setpoint, the valve is in an intermediate state, almost completely closed, due to the valve flow being appropriate for the pressure. The valve stem is therefore only a very small distance from the valve seat. Reducing the valve flow causes the stem to move further away from the valve seat. When the closing pulse is subsequently applied, the valve stem must travel a greater distance and is thus accelerated for a longer period by the magnetic field generated by the valve coil. This results in the valve stem striking the valve seat with higher kinetic energy, leading to a more effective valve closure and minimizing leakage. Since the valve stem assumes different positions depending on the valve flow and thus the set hydraulic pressure, the reduction in flow is selected according to the invention based on the set pressure.This ensures that, especially at higher pressures, the return movement of the valve stem is sufficient to guarantee reliable valve closure. At the same time, it ensures, particularly at lower pressures, that the descent and thus the return movement of the valve stem does not become excessive, as this would lead to loud valve switching noises that are uncomfortable for the driver. The closing current can be, for example, the valve's highest holding current value, such as the current value for 180 bar. The duration of the closing pulse is preferably between 10 and 30 ms.

[0010] In a preferred embodiment of the invention, the value ΔI, i.e., the reduction of the valve flow in a first range, with a hydraulic pressure lower than a pressure limit, is chosen to be smaller than in a second range, with a pressure higher than the pressure limit. Since the valve stem already has a greater distance from the valve seat in this first range, i.e., at lower pressures, a smaller reduction is sufficient.

[0011] In a preferred embodiment of the invention, the pressure limit is between 10 and 50 bar. Preferably, the pressure limit is between 15 and 30 bar, particularly at 20 bar. It has been found that this provides a particularly advantageous division into the two ranges for most valves.

[0012] In a preferred embodiment of the invention, the value of Delta I is larger and thus the reduction of the valve flow is deeper the higher the set pressure.

[0013] In a further preferred embodiment of the invention, the reduction of the valve flow occurs for a period of less than 100 ms. There is therefore only a short reduction pulse. Preferably, this pulse has a duration of 5 to 30 ms, particularly preferably 10 to 20 ms. This limits the additional volume of brake fluid flowing out due to the valve movement, and it provides just enough time for the valve stem to move into the position that enables a secure and tight closure.

[0014] In a further preferred embodiment of the invention, the current of the isolation valve for regulating the hydraulic pressure to the setpoint is determined from a differential pressure-current characteristic curve of the isolation valve.

[0015] In a particularly preferred embodiment of the invention, the value Delta I in the first region is determined from the differential pressure-flow characteristic of the isolation valve, wherein the flow reduction corresponds to a predetermined pressure reduction. For example, the flow value can be selected for a pressure 1 bar lower than the regulated setpoint. This can be implemented in a particularly cost-effective and resource-efficient manner.

[0016] In a particularly preferred embodiment of the invention, the value Delta I is determined in the second section from a pressure-volume curve of the supplied wheel brakes and the differential pressure-current characteristic of the isolating valve, wherein the current reduction corresponds to a predetermined volume flow rate. While the differential pressure-current characteristic indicates the differential pressure across the isolating valve when it is supplied with a specific electrical current, the pressure-volume curve indicates the volume of brake fluid that must be supplied to the system with the wheel brakes to achieve a specific pressure. In this way, a volume can be calculated, in particular, from the predetermined volume flow rate, which is stored in a storage device of the brake system, and the pulse duration of the current reduction.If one reads the resulting pressure from the pressure-volume curve, which results from the outflow of this volume, one can read a flow rate from the differential pressure-flow characteristic curve of the separating valve to which the valve flow is reduced.

[0017] In a particularly preferred embodiment of the invention, a holding current of the isolating valve is set after the closing pulse. This keeps the closed valve in this state.

[0018] The problem is also solved by a hydraulic motor vehicle brake system according to claim 10 comprising a hydraulic pump and a separating valve, wherein a control unit is provided which is configured to carry out one of the methods described above.

[0019] Further features, advantages, and applications of the invention will also become apparent from the following description of exemplary embodiments and the drawings. All features described and / or illustrated, both individually and in any combination, are part of the subject matter of the invention, even independently of their compilation in the claims or their cross-references. Fig. Figure 1 schematically shows a brake system according to the invention, Fig. Figure 2 shows a diagram with a typical differential pressure-flow characteristic of a separating valve, Fig. Figure 3 shows a diagram with a typical pressure-volume characteristic of a braking system and the derivation of the current characteristic from Fig. 2, Fig. Figure 4 shows the course of the valve flow during the method according to the invention;

[0020] The in Fig. The brake system shown in Figure 1 comprises a master brake cylinder 8, which is designed as a tandem master brake cylinder. This is operated by a driver by means of a brake pedal. Fig. Figure 1 shows one of two chambers of the tandem master brake cylinder 8, which is connected to the upper brake circuit, which operates a diagonal of wheel brakes 10a, 10b.

[0021] During normal braking, the hydraulic valve 15 is closed and the isolating valve 11 is open. Similarly, the inlet valves 12a and 12 are open and the outlet valves 13a and 13b are closed. The master brake cylinder 8 is thus directly connected to the wheel brakes 10.

[0022] When ABS is active, the inlet valve 12a, 12b of a locked wheel is closed and the outlet valve 13a, 13b is opened to reduce wheel pressure. This allows brake fluid to flow into the low-pressure reservoir 14. Subsequently, the piston pump 9 is activated to pump the brake fluid from the low-pressure reservoir 14 back into the brake circuit.

[0023] To perform driver-independent braking, the piston pump 9 is also activated to draw brake fluid from the master cylinder 8 when the hydraulic valve 15 is open. For this purpose, the isolating valve 11 is operated in overcurrent mode by being controlled based on a characteristic curve 20 to maintain a predetermined differential pressure. This is now described below using the example of an automatic holding function. When the driver brakes the vehicle to a standstill, they build up pressure in the wheel brakes 10 via the master cylinder 8 as described above. This can be, for example, a pressure of 50 bar. When the vehicle comes to a standstill, the automatic holding function is activated. For this, the hydraulic valve 15 is opened and the piston pump 9 is activated. At the same time, the isolating valve is supplied with an electrical current, which is controlled according to the pressure differential-current characteristic curve 20 (see Fig. 2) The differential pressure of the separating valve 11 corresponds to 60 bar. The pump 9 thus delivers brake fluid to the wheel brakes 10a, 10b, thereby increasing their pressure. At a pressure of 60 bar, the entire delivered volume flows back to the suction side of the piston pump 9 via the separating valve 11.

[0024] Fig. Figure 4 shows the electrical currents flowing through the isolation valve 11. This is the supply of the isolation valve with the holding current, which is in Fig. 4 is labelled with Px.

[0025] Now the piston pump is deactivated and the isolation valve 11 is completely closed. To do this, the valve flow of the isolation valve 11 is reduced in a first step, as shown in Fig. 4 is shown. Since, as in Fig. As shown in section 3, where the set pressure of 60 bar is located in the second area, a fixed volume flow of 10 cm³ is used to determine the flow rate reduction. 3 / s is used. This is combined with the pulse duration of the current reduction of 10ms (see Fig. 4) multiplied by a volume of 0.1 cm³ 3 to obtain. Now, let's consider the pressure-volume characteristic curve 21 of the brake system, starting from the current pressure of 60 bar and thus a volume of 2.4 cm³. 3 A downward displacement of 0.1 cm³ corresponds to a pressure of 50 bar. The pressure is therefore reduced according to the differential pressure-current characteristic curve 20 to a current value for 50 bar. After 10 ms, the isolation valve is subjected to a closing pulse, also lasting 10 ms and with a current value corresponding to 180 bar. To keep the valve closed, a holding current value is set that is 30 bar higher than the applied differential pressure.

[0026] By reducing the flow before applying the closing flow, the valve stem is moved slightly away from the valve seat. By selecting the reduction according to the invention based on the setpoint of the hydraulic pressure, this movement is precisely adjusted so that the valve closes reliably without unnecessary noise.

Claims

[1] Method for controlling a brake system comprising a hydraulic pump and a normally open isolating valve between the pressure side and the suction side of the pump, wherein, to control a hydraulic pressure to a setpoint, the isolating valve is supplied with a valve flow based on the magnitude of the setpoint, while the hydraulic pump delivers brake fluid to at least one wheel brake, characterized by , that to lock in the hydraulic pressure in at least one wheel brake the pump is switched off and the isolation valve is closed, wherein to close the isolation valve the valve flow is reduced by a value Delta I in a first step, and in a second step a closing pulse with a higher current is applied, wherein the value Delta I is chosen based on the setpoint of the hydraulic pressure. [2] Method according to claim 1, characterized by, that the value Delta I in a first area, with a hydraulic pressure less than a pressure limit, is smaller than in a second area, greater than the pressure limit. [3] Method according to claim 2, characterized by that the pressure limit is between 10 and 50 bar. [4] Method according to any one of the preceding claims, characterized by , that the value Delta I is greater the higher the set pressure. [5] Method according to any one of the preceding claims, characterized by that the reduction of the valve flow occurs for a period of time shorter than 100 ms. Preferably 5 to 30 ms, particularly preferably 10 to 20 ms. [6] Method according to any one of the preceding claims, characterized by , that the flow rate of the isolation valve for regulating the hydraulic pressure to the setpoint is determined from a differential pressure-flow characteristic curve of the isolation valve. [7] Method according to any one of the preceding claims, characterized by, that in the first area the value Delta I is determined from the differential pressure-flow characteristic of the separating valve, whereby the flow reduction corresponds to a predetermined constant pressure reduction. [8] Method according to any one of the preceding claims, characterized by , that in the second area the value Delta I is determined from a pressure-volume curve of the supplied wheel brakes and the differential pressure-flow characteristic of the separating valve, whereby the flow reduction corresponds to a predetermined constant volume flow. [9] Method according to any one of the preceding claims, characterized by , that a holding current of the isolation valve is set after the closing pulse. [10] Hydraulic motor vehicle brake system comprising a hydraulic pump and a separating valve, characterized by a control unit configured to carry out a method according to any one of claims 1 to 9.

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