Pump monitoring with pressure fluctuations

Abstract

The invention relates to a method for controlling a hydraulic motor vehicle braking system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake, wherein a fault situation of the hydraulic piston pump is detected by measuring a pressure built-up by the hydraulic piston pump using a pressure sensor and analyzing it for pressure oscillations that have a frequency which corresponds to the rotation period of the hydraulic piston pump and / or a characteristic multiple of the rotation period of the hydraulic piston pump.

Description

[0001] The invention relates to a method for controlling a hydraulic motor vehicle braking system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake and such a motor vehicle braking system.

[0002] Older vehicle braking systems, such as those described in DE 10 2007 050 662, often feature vacuum brake boosters. A brake pedal equipped with such a booster is directly mechanically connected to the wheel brakes via a master cylinder. A piston pump, often integrated with the system, serves solely as a return pump for ABS control. Its suction side is located on the pressure side of the master cylinder. If air accumulates there, it is compressed by the master cylinder.

[0003] In modern braking systems, an increasing number of functions, especially safety-critical ones, are performed by the piston pump. It is therefore essential to monitor and ensure its proper functioning. Air ingress can dangerously reduce the efficiency of the piston pump.

[0004] From DE 10 2004 062 029 A1 it is known to determine the frequency of pressure oscillations of a multi-piston pump in order to detect the failure of one of the several pistons in case of deviations from an expected frequency.

[0005] From DE 10 334 817 A1 it is known, in particular, to detect instabilities of a pump drive by detecting vibrations at the natural frequency of the pump drive.

[0006] It is therefore an object of the present invention to provide a method for a motor vehicle braking system which can detect faults.

[0007] The problem is solved by a method for controlling a hydraulic motor vehicle braking system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake. A fault condition of the hydraulic piston pump is detected by measuring the pressure built up by the hydraulic piston pump using a pressure sensor and analyzing it for pressure oscillations that have a frequency corresponding to the rotation period of the hydraulic piston pump and / or a characteristic multiple thereof. The occurrence of pressure oscillations at the output side of a piston pump is due to its physical operating principle. It has been found that these artifacts, also known as pressure ripples, are an excellent indicator of the operating condition of the piston pump.

[0008] According to the invention, this detects a fault situation in which air is present in the intake path of the hydraulic piston pump.

[0009] In a preferred embodiment of the invention, the magnitude of the pressure oscillations is determined and compared with a threshold value, whereby a fault situation is detected if the threshold value is exceeded. The pressure profile can, for example, be evaluated as the magnitude, and the so-called peak-to-peak value determined. It has been shown that, particularly with air in the intake path, the compression of which leads to an amplification of the pressure oscillations.

[0010] In a particularly preferred embodiment of the invention, the threshold value is between 5 and 20 bar, preferably at 10 bar.

[0011] In a further preferred embodiment of the invention, the threshold value is selected depending on the pump speed and / or the temperature of the brake fluid. In this way, by adjusting the threshold value, it can be taken into account that the magnitude of the pressure fluctuations is greater at lower temperatures and higher speeds.

[0012] In a further preferred embodiment of the invention, the analysis of pressure fluctuations is performed during braking. The piston pump is activated to implement a deceleration request. Thus, the piston pump can be monitored for malfunctions during normal operation. Alternatively or additionally, a hydraulic self-test can be performed; that is, the pump is activated without a deceleration request, solely for monitoring purposes. In this case, very specific control signals can be implemented, allowing for particularly precise monitoring. Furthermore, the availability of the piston pump can be ensured during periods without a deceleration request.

[0013] In a further preferred embodiment of the invention, the analysis of pressure oscillations includes determining the rotational speed from the period of the pressure oscillations. Depending on the precise mechanical principle of the piston pump, a specific number of pressure oscillations occur per revolution. In the case of a two-piston pump, this is two pressure oscillations per revolution. The rotational speed can therefore be easily determined by halving the pressure oscillation repetition rate. Furthermore, the delivered volume of the hydraulic piston pump is determined, and the rotational speed and volume are compared with a stored characteristic value to detect a fault. For example, a characteristic curve can be stored that specifies a target delivery volume for a particular rotational speed. A deviation from this value is considered a fault.

[0014] In a particularly preferred embodiment of the invention, the fault situation is detected when the conveyed volume per revolution is less than a second threshold value.

[0015] In a particularly preferred embodiment of the invention, the pumped volume is directed into a linear actuator, the piston of which is pushed back so that it can receive the volume. The volume received, and thus the volume pumped, is determined by the piston position or its change and the cross-sectional area of ​​the piston.

[0016] In a particularly preferred embodiment of the invention, the pumped volume is determined from the resulting pressure and a pressure-volume characteristic curve. Specifically, if at least one wheel brake is connected to the hydraulic piston pump, the pumped volume is directed into the wheel brake. This brake exhibits a specific relationship between the applied volume and the resulting pressure, which is defined in the pressure-volume characteristic curve. Therefore, the pumped volume can be determined by measuring the pressure.

[0017] In a further preferred embodiment of the invention, when a fault is detected, an additional pressure supply device is activated to purge the suction path of the hydraulic piston pump, in particular by pumping brake fluid backwards towards a container. This allows air in the suction path not only to be detected but also to resolve the problem, so that the brake system is fully functional again.

[0018] In a further preferred embodiment of the invention, the pump speed is increased when a fault is detected. In particular, the relationship between the pump speed to be set based on the delay or pressure requirement is modified, so that a higher pump speed is set than in the case where no fault has been detected. For example, the pump speed can be multiplied by a factor.

[0019] In a further preferred embodiment of the invention, when a fault is detected, braking functions that require the pump are deactivated and / or a warning lamp is activated. This improves the availability of the actual core functions that can bring the vehicle to a standstill.

[0020] The problem is also solved by a hydraulic motor vehicle braking system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake and a control device for controlling the hydraulic piston pump, wherein the control device is configured to carry out the above method.

[0021] 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.

[0022] Fig. Figure 1 schematically shows a brake system according to the invention,

[0023] In Fig. Figure 1 shows a redundant hydraulic braking system for motor vehicles. By way of example, the braking system is designed to actuate four hydraulically actuated wheel brakes 8; expansion to more wheel brakes is easily possible. By way of example, the two upper wheel brakes (HL, HR) in the figure, referred to here as the first wheel brakes, are assigned to the rear axle, and the wheel brakes (VL, VR), referred to here as the second wheel brakes, are assigned to the front axle of the vehicle.

[0024] The brake system comprises a first component, which is exemplified as a first electro-hydraulic brake control unit with a valve block and a first electronic control unit ECU1, and a second component, which is exemplified as a second electro-hydraulic brake control unit with a valve block and a second electronic control unit ECU2. Each valve block forms its own housing.

[0025] The first assembly unit has a pressure medium reservoir 4 with two chambers, the first chamber being assigned a first reservoir connection and the second chamber a second reservoir connection. Reservoirs with three or more chambers are also possible. The second assembly unit does not have its own pressure medium reservoir.

[0026] In the first assembly unit, a first electrically actuated pressure source 5 is arranged.

[0027] In the second assembly unit, a second electrically actuated pressure source 2 and wheel-specific brake pressure modulation valves are arranged, which are designed as an electrically actuated inlet valve 6 and an electrically actuated outlet valve 7 for each wheel brake 8.

[0028] The first pressure source 5 and the second pressure source 2 are connected on the pressure side to a brake supply line, to which the four inlet valves 6 are connected. This allows all four wheel brakes 8 to be actuated by means of the first pressure source 5 and / or by means of the second pressure source 2.

[0029] An electrically actuated circuit isolator valve 40 is arranged in the brake supply line, and thus in the second component unit. When the circuit isolator valve 40 is closed, the brake supply line is divided into a first line section, to which the inlet valves 6 and the first wheel brakes 8 of the rear axle are connected, and a second line section, to which the inlet valves 6 and the second wheel brakes 8 of the front axle are connected. The second pressure source 2 is hydraulically connected to the first line section, and the first pressure source 5 is hydraulically connected to the second line section. With the circuit isolator valve 40 closed, the brake system is thus divided into two hydraulic brake circuits, I and II, or a first sub-circuit and a second sub-circuit.In the first brake circuit I, pressure source 2 (via the first line section) is connected only to the rear axle wheel brakes 8, and in the second brake circuit II, pressure source 5 (via the second line section) is connected only to the front axle wheel brakes 8. The circuit isolating valve 40 is advantageously designed to be normally open (de-energized). Such an operating mode can be called circuit isolating mode or ACS for "Active Circuit Separation".

[0030] As already mentioned, the brake system comprises, for each hydraulically actuated wheel brake 8, an inlet valve 6 and an outlet valve 7, which are hydraulically connected in pairs via center connections and each pair is connected to a hydraulic wheel connection of the second assembly, to which the corresponding wheel brake 8 is connected. A check valve opening towards the brake supply line is connected in parallel to each inlet valve 6. The outlet connections of the outlet valves 7 are connected to the pressure medium reservoir 4 or its second chamber via a common return line. The inlet connections of all inlet valves 6 can be supplied with pressure via the brake supply line (i.e., with the circuit separator valve 40 open), which is provided by the first pressure source 5 or, for example, in the event of a failure of the first pressure source 5, by the second pressure source 2.

[0031] The first electrically controlled pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electrohydraulic actuator (linear actuator)), whose piston can be actuated by a schematically indicated electric motor via a similarly schematically depicted rotary-translational transmission, in particular by moving it back and forth to build up and release pressure in a pressure chamber. The piston defines the pressure chamber of pressure source 5. A rotor position sensor, also only schematically indicated, is provided to control the electric motor. This sensor allows the piston position and speed, and thus the volume of brake fluid delivered or received, to be determined.

[0032] A section of system pressure line is connected to the pressure chamber of the first electrically controlled pressure source 5. This line section connects the pressure source 5, or rather its pressure chamber, to a hydraulic connection of the first component, which in turn is connected via a hydraulic connecting element to a hydraulic connection of the second component. This connection constitutes the only hydraulic pressure connection between the first and second components. It is a hydraulic connection for transmitting brake pressure to actuate the wheel brakes 8.

[0033] The pressure chamber is connected to the pressure medium reservoir 4 via a (suction) line, regardless of the piston's actuation state. A check valve 53, closing towards the pressure medium reservoir 4, is arranged in the line and connected to the second chamber. An electrically switchable valve 23 forms a further connection to this line, which is connected to the output port of the linear actuator 5. This isolating valve 23 is normally open, so that in the de-energized state, the wheel brakes 8 are connected to the brake fluid reservoir 4. The cylinder-piston assembly 5, for example, has no vent holes.

[0034] The second electrically controlled pressure source 2 of the second assembly is, for example, designed as a two-piston pump whose two pressure sides are connected together. The suction sides are connected to the return line and, via a so-called suction line or feedline, to the pressure medium reservoir 4. This feedline (FL) is routed externally, is flexible, and is usually made of plastic for cost reasons. The pressure sides are connected to the first section of the brake supply line.

[0035] In addition to the pressure source 2 and the brake pressure modulation valves 6, 7, the second assembly includes, for example, an electrically actuated isolation valve 26, which is advantageously normally open. The isolation valve 26 is hydraulically positioned between the connection and the second section of the brake supply line. Thus, the first pressure source 5, and therefore the entire first assembly, is disconnected from the second section of the line, i.e., the brake supply line, via the isolation valve 26.

[0036] The brake system includes, for example, a pressure sensor 19 in brake circuit I, which is thus assigned to the second pressure source 2. This is advantageous for burst protection when the circuit isolating valve 40 is closed. However, the pressure sensor can also be located in brake circuit II. In this case, pressure sensor 19 directly measures the pressure supplied by the second pressure source.

[0037] For example, the brake system for leakage monitoring includes a level measuring device for determining a pressure medium level in the pressure medium reservoir 4.

[0038] Each valve block is assigned an electronic control unit (ECU1) and an electronic control unit (ECU2). Each electronic control unit comprises electrical and / or electronic elements (e.g., microcontrollers, power components, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors—that is, the entire respective assembly. Advantageously, the valve block and the electronic control unit are designed as a single electrohydraulic unit, as is known.

[0039] The first electronic control device controls the first pressure source 5. For example, the first pressure source 5 is supplied with energy (from a first electrical energy source) via the first electronic control device.

[0040] The second electronic control device controls the second pressure source 2. For example, the second pressure source 2 is supplied with energy (from a second electrical energy source) via the second electronic control device.

[0041] For example, the first pressure source or primary pressure source 5 can be controlled exclusively by the first electronic control device, and the second pressure source or secondary pressure source 2 can be controlled exclusively by the second electronic control device.

[0042] Under normal operating conditions, the pressure in the wheel brakes is built up by the primary pressure source 5. The pressure in the primary pressure source 5 is reduced by retracting the piston. The pressure is modulated individually for each wheel as needed by the inlet and outlet valves. If necessary, the isolation valve 26 is closed to allow the primary pressure source 5 to draw in additional volume.

[0043] When a particularly high flow rate is required, both pressure sources 5 and 2 operate simultaneously in parallel. When a particularly high pressure is required, the isolation valve 26 is closed, and the secondary pressure source 2 increases the pressure above the pressure of the primary pressure source 5. Outside of braking situations, atmospheric pressure equalization can be permanently ensured via the isolation valve 23 and the isolation valve 26.

[0044] The control units ECU1 and ECU2 are configured to implement pressure build-up using the linear actuator 5 and / or the piston pump 2 based on pressure requirements. These requirements can originate from automatic functions, assistance programs, and, in particular, from the driver, who communicates their wishes via the brake pedal. This information is transmitted to the control units as a brake pedal actuation signal. The brake pedal actuation signal can be brake pedal travel, brake pedal force, and / or similar parameters.

[0045] In the event of a leak in the brake system, the circuit isolating valve 40 closes, thereby dividing the system into two independent brake circuits, I and II. The leak can be monitored via various monitors. A Volume Deviation Monitor (VDM) is a leak detection device that can detect hydraulic leaks based on the pressure-volume ratio (PV ratio). The volume is determined by sensing the linear actuator or pedal position. The expected PV behavior can be stored using functions or predefined characteristic curves. A low brake fluid level (BFLS) in the reservoir or excessively low pressure also triggers leak detection.

[0046] Preferably, the isolation valve 26 is controlled by the secondary ECU. The following description of operation in the event of a fault refers to this valve assignment.

[0047] If the secondary system fails electrically, particularly the secondary ECU or its power supply, the pressure is built up and released via the primary pressure source 5 as in normal operation. Individual wheel pressure control is not possible, but collective modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by wheel lock-up.

[0048] If the primary system fails electrically, particularly the primary ECU or its power supply, the secondary ECU switches to a fallback mode in which the second unit becomes the sole pressure source for the vehicle's wheel brakes 8. To achieve this, it closes the isolation valve 26 to allow pressure to build up via the secondary pressure source 2. Without the isolation valve 26 closed, the brake fluid would continue to flow into the reservoir via the normally open shut-off valve 23, thus preventing pressure build-up. Pressure is released via the outlet valves 7. Preferably, the inlet and outlet valves are controlled by the secondary ECU so that the pressure can be modulated individually for each wheel.

[0049] To maintain this fallback mode, the hydraulic piston pump 2 must be sufficiently functional. For this purpose, it is monitored for possible malfunctions.

[0050] In Fig. Figure 2 illustrates the basic operating principle of a two-piston pump. There are two piston emptying cycles per motor rotation. Due to the geometry, the emptying of the pistons results in pulsed volume flow, as shown in the upper diagram. Fig. Figure 2 illustrates this. This results in pressure fluctuations or pump pressure ripples at the outlet side of the piston pump 2. Due to its two-part design, the suction line SL1 of the hydraulic piston pump 2 is relatively long and therefore must also have a large diameter to avoid unnecessary suction resistance. Since this line may also contain bends, it is susceptible to air accumulation. Such air accumulation in the suction line SL1 can, however, have a very negative impact on the delivery rate of the hydraulic piston pump.

[0051] This is in Fig. 3 shown. While the upper part of the Fig. Figure 3 again illustrates the error-free flow pattern, while the lower part schematically shows the flow rate with air in the intake path. An air-drawn pump fills the pistons with air and brake fluid. During rotation, the air is initially compressed, and the subsequent brake fluid flow rate has a higher gradient because the piston is already moving at a higher speed. The resulting brake fluid flow rate is therefore subject to longer pauses, followed by a steeper flow gradient. This resulting flow rate from the air-drawn pump leads to greater pressure ripples compared to a pump that does not draw air.

[0052] The pressure profile during a pressure build-up is crucial for a flawless scenario in Fig. Figure 4 shows that the pressure rises rapidly and exhibits small pressure oscillations or pressure ripples, which have twice the frequency of the piston pump's rotational speed.

[0053] The pressure profile with air in the intake path is in Fig. Figure 5 shows that the pressure increases significantly more slowly, which is reflected in the shallower slope. However, the pressure fluctuations or pressure ripples are much more pronounced.

[0054] This behavior can now be used according to the invention to detect a possible fault condition of the hydraulic piston pump.

[0055] Firstly, the exact frequency of pressure oscillations during braking can be determined, thus providing a very precise reading of the hydraulic piston pump's rotational speed. Simultaneously, the pump's flow rate is determined from the pressure profile using a pressure-volume characteristic curve. The relationship between flow rate or pressure and the pump's rotational speed can be compared to a target value, and any deviation indicates a fault.

[0056] This can also be performed as part of a hydraulic self-test outside of braking operations. For this purpose, the hydraulic piston pump 2 can, for example, with the inlet valves 6 closed, pump fluid through the open circuit separator valve 40 and isolation valve 26 into the linear actuator 5. This pushes the actuator's piston back, and the pumped volume can be determined very precisely via the piston position and geometry, such as the piston's cross-sectional area. The rotational speed of the piston pump 2, determined as above, can then be used to verify normal operating behavior.

[0057] The piston pump can be designed, for example, to displace 100 mm³ per piston stroke. With 10 revolutions, it should therefore deliver 2000 mm³ (2 cm³) due to the two pistons. If less than 80% of the expected volume is delivered during pressure build-up, a fault is considered detected.

[0058] Before a hydraulic self-test, pressure can be built up using the linear actuator 5 with the inlet valves closed to check for air pockets on the pressure side. If air pockets are present, the compression of this air during the advance of the linear actuator would result in a shallower pressure rise.

[0059] Such a hydraulic self-test can be performed at regular intervals, after a detected leak or after filling with brake fluid.

[0060] Furthermore, instead of the frequency of the pressure ripples, their amplitude can be evaluated. If the amplitude of the pressure ripples, for example the peak-to-peak value, exceeds a threshold, preferably 10 bar, a fault situation is detected.

[0061] In the event of a detected fault, the linear actuator is activated to pump fluid backwards through the suction line SL1 into the pressure medium reservoir 4 via at least one open inlet valve 6 and an open outlet valve 7 on the same wheel brake 8. This removes any air pockets. The pumping speed of the linear actuator 5 is selected to be low enough that the pressure build-up in the wheel brake due to back pressure does not result in any safety-relevant braking. This speed can be set to be speed-dependent.

Claims

[1] Method for controlling a hydraulic motor vehicle brake system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake, characterized by , that a fault situation of the hydraulic piston pump, in which air is present in the intake path of the hydraulic piston pump, is detected by measuring a pressure built up by the hydraulic piston pump using a pressure sensor and analyzing it for pressure oscillations that have a frequency which corresponds to the rotation period of the hydraulic piston pump and / or a characteristic multiple of the rotation period of the hydraulic piston pump. [2] Method according to any of the preceding claims characterized by , that the magnitude of the pressure oscillations is determined and compared with a threshold value, whereby the fault situation is detected if the threshold value is exceeded. [3] Method according to claim 2 characterized bythat the threshold value is between 5 and 20 bar, preferably at 10 bar. [4] Method according to claim 2 or 3 characterized by , that the threshold value is chosen depending on the pump speed and / or the temperature of the brake fluid. [5] Method according to any of the preceding claims characterized by that the analysis of pressure oscillations is carried out during braking and / or during pump operation independently of braking as a hydraulic self-test. [6] Method according to any of the preceding claims characterized by , that the analysis of the pressure oscillations includes determining the rotational speed from the period of the pressure oscillations, as well as determining the delivered volume of the hydraulic piston pump, whereby the rotational speed and the volume are compared with a stored parameter in order to detect a fault situation. [7] Method according to claim 6 characterized by, that the error situation is detected when the conveyed volume per revolution is less than a second threshold. [8] Method according to claim 6 or 7 characterized by , that the conveyed volume is directed into a linear actuator, whereby a piston position is used to determine the conveyed volume. [9] Method according to claim 6 or 7 characterized by that the pumped volume is determined from the resulting pressure and a pressure-volume characteristic curve. [10] Method according to any of the preceding claims characterized by , that in the event of a detected fault situation, a further pressure supply device is activated to flush the suction path of the hydraulic piston pump, in particular in which brake fluid is conveyed backwards towards a container. [11] Method according to any of the preceding claims characterized by , that the pump speed is increased when an error situation is detected. [12] Method according to any of the preceding claims characterized by , that if a fault situation is detected, brake functions which require the pump are deactivated and / or a warning light is activated. [13] Hydraulic motor vehicle braking system comprising a hydraulic piston pump for pressure build-up in at least one wheel brake and a control device for controlling the hydraulic piston pump characterized by that the control device is configured to carry out a method according to one of claims 1 to 12.

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