DE102021214028A1 - Method for determining a loss of pressure medium in a hydraulic brake system of a motor vehicle and a brake system - Google Patents

Abstract

Method for determining a loss of pressure medium in a hydraulic brake system (1) of a motor vehicle, the brake system (1) having a master brake cylinder (3) which can be actuated by a brake pedal (2), and a simulation device (4) which is controlled hydraulically by a simulator release valve ( 15) can be connected to the master brake cylinder (3), a first volume of pressure medium being determined, which represents a first quantity of pressure medium displaced from the master brake cylinder, and a second volume of pressure medium being determined, which via the simulator release valve (15) into the simulation device (4) shifted second quantity of pressure medium, the first volume of pressure medium being compared with the second volume of pressure medium, and if a difference was determined, then the loss of pressure medium is inferred, and a brake system of a motor vehicle, comprising a master brake cylinder (3) which is activated by a brake pedal (2) can be actuated, a simulation device (4) which can be connected hydraulically to the master brake cylinder (3) by a simulator release valve (15), a pressure medium reservoir (9) at atmospheric pressure for storing a hydraulic pressure medium, and an electronic control and regulating unit (13), wherein the electronic control and regulation unit (13) is set up to carry out the method according to one of the preceding claims.

Description

The invention relates to a method for determining a loss of pressure medium in a hydraulic brake system of a motor vehicle according to the preamble of claim 1 and a brake system for carrying out the method according to the preamble of claim 10.

In so-called "brake-by-wire" braking systems, a hydraulic pedal unit with a simulator is used. In normal operation, the pedal unit is separated from the wheel brakes by means of an isolating valve and connected to the simulator via a simulator valve. This is used to display a pedal force-displacement characteristic. If the brake system fails, the isolating valve is opened in the hydraulic fallback level and the simulator valve is closed, so that the driver now moves the brake fluid from the master brake cylinder directly into the wheel brakes. The minimum legal requirements for the braking effect must be complied with. In order for these to be met, the master brake cylinder must be sufficiently functional. This must not have a leak to the reservoir via the seal of the primary sleeve, nor must it have a gas pocket. Therefore, special methods are required that detect a loss of pressure medium in the brake system.

From the US 2021/0179041 A1 a brake system for actuating four hydraulically actuated wheel brakes is known, comprising an electrically controllable pressure modulation device with individual wheel inlet and outlet valves, a master brake cylinder that can be actuated by a brake pedal and is hydraulically connected to the pressure modulation device, a simulation device that is hydraulically connected to the master brake cylinder, an electrically controllable pressure supply device, which is also hydraulically connected to the pressure modulation device, and a pressure medium reservoir under atmospheric pressure.

This document describes a method for detecting a leak in the brake system, in which a pressure sensor is used to measure the brake pressure at the pressure supply device during a braking process. A pressure medium volume that has been shifted into the two brake circuits can be derived from this brake pressure. With the help of an electronic control and regulation unit, a model value for the pressure medium volume is determined as a function of the brake pressure, which has been shifted to the wheel brakes. This model value represents a pressure medium volume that was required to build up the measured brake pressure. Furthermore, a further pressure medium volume is determined with a rotor position sensor of the electrically controllable pressure supply device, which volume was shifted by the pressure supply device to the wheel brakes during the braking process. The model value is then compared with the value of the pressure medium volume, which was determined by the rotor position sensor. If the discrepancy or the difference exceeds a specific limit value, it is assumed that there is a leak in the brake system. Otherwise there is no leakage. If a leak is detected, for example during an ABS control intervention, the wheel brake at which the leak is occurring can be determined with the aid of the electronic control and regulation unit. To do this, the wheel that is slipping is identified using the wheel speed sensors. If, for example, a leak is determined on just one wheel, the associated wheel brake is excluded from the braking process by closing the associated inlet valve.

Another method for determining a leak in a vehicle brake system is from DE 10 2020 214 119 A1 known. A diagnostic method for identifying a leak in a simulator valve is described therein, for example. To perform a diagnostic test for a leak in the simulator valve, a simulator test valve and a pump valve are opened (energized) while other valves remain closed to retain brake fluid from the booster assembly in a booster circuit. The booster is then actuated to compress the brake fluid to a predetermined pressure level. The intensifier plunger is then held in position and if the pressure in the intensifier circuit (as measured by a pressure sensor on the intensifier) decreases at or above a predetermined rate, then a leak in the simulator valve is identified. Alternatively, if the intensifier's plunger must travel more than a predetermined distance to achieve a particular pressure, a leak in the simulator valve is also inferred.

It is the object of the invention to provide a method for determining a pressure medium loss in a hydraulic brake system of a motor vehicle, which reliably and safely detects a pressure medium loss and to provide a brake system with which a pressure medium loss can be reliably and safely detected.

According to the invention, a method for determining a loss of pressure medium in a hydraulic brake system of a motor vehicle is provided, the brake system having a master brake cylinder which can be actuated by a brake pedal, and a simulation device which hydraulically connectable to the master brake cylinder by a simulator release valve, wherein a first pressure medium volume is determined, which represents a first pressure medium quantity displaced from the master brake cylinder, and a second pressure medium volume is determined, which represents a second pressure medium quantity displaced via the simulator release valve into the simulation device, wherein the first volume of pressure medium is compared to the second volume of pressure medium, and if a difference was determined, then the loss of pressure medium is inferred.

The method according to the invention has the advantage that, during normal operation of the brake system, a plausibility check is carried out as to whether there is sufficient pressure medium in the master brake cylinder for a braking process in the fallback level. Due to leaks in the master brake cylinder, it can happen that the pressure medium partially escapes from the pressure chamber of the master brake cylinder during a brake application. Initially, this does not have a negative effect in normal operation because the braking process is carried out by the pressure supply device. However, so that there is sufficient pressure medium in the master brake cylinder for braking by the driver in the fallback level, a leak in the brake system must be ruled out. A leak in the brake system can be inferred from the fact that during normal operation the first volume of pressure medium, which is displaced from the master brake cylinder in the direction of the simulation device, is compared with the second volume of pressure medium, which reaches the simulation device via the simulator release valve. Both pressure medium volumes are determined in different ways and compared with one another. If there is a deviation, then the loss of pressure medium is inferred. The loss of pressure medium is preferably only inferred when the determined difference exceeds a predetermined limit value. The deviation can be minimal or large depending on the pressure built up during normal operation. To ensure that not every deviation leads to a trigger, it is advisable to specify a limit value. This limit value can also be adaptively adapted to the different pressures.

Accordingly, according to a preferred embodiment, a warning signal is emitted when the loss of pressure medium has been concluded. The warning signal can be used to alert the vehicle operator to a fault in the brake system, for example, which requires the brake system to be checked by a specialist. Countermeasures can also be taken internally in the system, such as, for example, the targeted refilling of the master brake cylinder with pressure medium from a pressure medium reservoir.

According to a further preferred embodiment, the first pressure medium volume is determined in that the displacement path of a master brake cylinder piston of the master brake cylinder is multiplied by a cross-sectional area of the master brake cylinder. The first pressure medium volume is determined from the volume which is displaced from the master brake cylinder by actuation of the master brake cylinder piston. With the known cross-sectional area of the master brake cylinder and the travel path of the master brake cylinder piston, the theoretically shifted pressure medium volume is known. If there is a pressure medium loss or a gas lock in the master brake cylinder, then in reality less pressure medium is displaced than was determined with this calculation. For this purpose, the travel path is preferably calculated from a first path point and a second path point of the master brake cylinder piston, the first path point being defined by a first actuation of the master brake cylinder piston and the second path point being defined when the pressure applied to the master brake cylinder is zero.

According to a further preferred embodiment, the second pressure medium volume is determined in that a volume flow flowing through the simulator release valve is integrated over time. The second pressure medium volume is determined from the volume flow which flows through the simulator release valve into the simulation device. The actual amount of pressure medium entering the simulation device is determined via the volume flow according to the so-called orifice equation. The second pressure medium volume is preferably determined finally when the pressure present at the simulator release valve is zero.

In a further preferred embodiment, a gas volume enclosed in the brake master cylinder is determined by subtracting a maximum simulation device volume from a maximum shifted brake master cylinder volume and subtracting the difference between the first pressure medium volume and the second pressure medium volume.

The maximum displaced master cylinder volume is preferably determined by multiplying a travel path of a master brake cylinder piston of the master brake cylinder by a cross-sectional area of the master brake cylinder, the travel path being calculated from a first path point and a second path point of the master brake cylinder piston, the first path point being determined by a first actuation of the master brake cylinder piston is and the second waypoint is set when a on pressure applied to the master brake cylinder has reached its maximum, and that the maximum simulator volume is determined by integrating a volume flow through the simulator release valve over time when the pressure applied to the simulator release valve has reached its maximum.

Furthermore, it is possible not only to determine an escape of pressure medium during the actuation process, but also to identify a gas pocket in the master brake cylinder. To do this, a maximum pressure medium volume displaced by the master brake cylinder must first be determined, which is calculated from the cross-sectional area of the master brake cylinder and the travel of the master brake cylinder piston, with the travel being determined by when the measured pressure has reached its maximum. If there is a maximum pressure, the travel has also reached its maximum. A simulation device volume is subtracted from this maximum pressure fluid volume displaced by the master brake cylinder, which is also calculated by the volume flow using the orifice equation, with the maximum pressure also determining the measurement window here. The previously determined leakage volume, which resulted from the difference between the first and the second pressure medium volume, is also subtracted from this.

The method is used in a brake system, comprising a master brake cylinder, which can be actuated by a brake pedal, a simulation device, which can be connected hydraulically to the master brake cylinder by a simulator release valve, a pressure medium reservoir under atmospheric pressure for storing a hydraulic pressure medium, and an electronic control and regulating unit , which is set up to carry out the method according to the invention.

Further preferred embodiments of the invention result from the following description of an exemplary embodiment with reference to the figures.

• 1 eine Kraftfahrzeugbremsanlage,
• 2 einen Ausschnitt der Kraftfahrzeugbremsanlage, und
• 3 einen beispielsgemäßem Signalverlauf eines Drucksensors und eines Wegsensors im Falle eines Druckmittelverlustes.

Show it:

• 1 a motor vehicle brake system,
• 2 a section of the motor vehicle brake system, and
• 3 an exemplary signal curve of a pressure sensor and a displacement sensor in the event of a loss of pressure medium.

In the 1 shows the circuit diagram of a brake system 1 for a motor vehicle, which is operated with a hydraulic pressure medium. Brake system 1 comprises a brake master cylinder 3, which can be actuated by means of a brake pedal 2, a simulation device 4 that is hydraulically connected and interacts with the brake master cylinder 3, an electrically controllable pressure supply device 6 for generating a hydraulic pressure, and a pressure modulation device 7 for setting wheel-specific brake pressures on four wheel brakes 8a-8d , and a pressure medium reservoir 9.

Master brake cylinder 3, simulation device 4, pressure supply device 6 and pressure modulation device 7 are combined to form a structural electrohydraulic control and regulation unit 10, for example. This structural unit 10 is characterized in that it contains the aforementioned components of the brake system as a metal mounting block. I.e. master brake cylinder 3, simulation device 4, pressure supply device 6 are integrated in this receiving block. In addition, a wide variety of valves and sensors are integrated into the mounting block. The hydraulic connections of the individual components via the valves are realized by channels or bores in the receiving block. Pressure medium reservoir 9 is also placed on top of electrohydraulic control and regulation unit 10 .

In the present case, wheel brake 8a is assigned to the left front wheel VL, wheel brake 8b to the right front wheel VR, wheel brake 8c to the left rear wheel HL and wheel brake 8d to the right rear wheel HR, with left and right front wheels VL and VR being assigned to the front axle VA and left and right rear wheels HL and HR are assigned to the rear axle HA.

Master brake cylinder 3 includes a master brake cylinder piston 32 which is coupled to brake pedal 2 via a piston rod 31 . When the vehicle operator actuates the brake pedal 2 , the master cylinder piston 32 is axially displaced in a housing 35 . Housing 35 delimits a pressure chamber 34 with master brake cylinder piston 32, in which hydraulic pressure medium is located, whereby a single-circuit master brake cylinder 3 is represented. In addition, a restoring spring 33 is accommodated in the housing 35, which moves the unactuated master brake cylinder piston 32 into its initial position. Housing 35 is represented by the structural unit of electrohydraulic control and regulation unit 10 .

A travel sensor 51 is provided on master brake cylinder 3 , which detects the actuation path or travel path of master brake cylinder piston 32 . Displacement sensor 51 is arranged in electrohydraulic control and regulation unit 10 . displacement sensor 51 is preferably a magnetic field sensor and detects a change in the magnetic field emanating from the master brake cylinder piston 32 in order to determine the travel. The measured distance represents the driver's braking request. In addition, a first pressure sensor 53 is provided, which detects the pressure built up in the pressure chamber 34 of the master brake cylinder 3 as a result of a displacement of the master brake cylinder piston 32 . First pressure sensor 53 is arranged in electrohydraulic control and regulation unit 10 .

Simulation device 4 includes a simulator piston 42 located in a simulator housing 41, which separates a simulator pressure chamber 45 and a simulator rear chamber 44 from one another. Simulator pressure chamber 45 is set up to hold hydraulic pressure medium and simulator rear chamber 44 is dry. Simulator pressure chamber 45 can be connected hydraulically to master brake cylinder 3 via a simulator release valve 15 . The hydraulic pressure medium presses on the simulator piston 42, which is supported on a simulator spring 43 arranged in the rear chamber 44 of the simulator.

The pressure modulation device 7 arranged in the electrohydraulic control and regulating unit 10 comprises an inlet valve 18a-18d and an outlet valve 19a-19d for each hydraulically actuated wheel brake 8a-8d, which are hydraulically interconnected in pairs via central connections and connected to wheel brakes 8a-8d. A check valve opening toward the brake supply line is connected in parallel to each of the inlet valves 18a-18d. Inlet valves 18a-18d of pressure modulator device 7 can be switched on or off from master brake cylinder 3 by a separating valve 14. Furthermore, a circuit separating valve 16 is provided, which can separate the inlet valves 18c and 18d of the rear axle HA from the inlet valves 18a and 18b of the front axle VA. Thus, when the circuit separating valve 16 is closed, no hydraulic connection is possible from the master brake cylinder 3 to the wheel brakes 8c and 8d of the rear axle HA. If, on the other hand, the circuit separating valve 16 is open, hydraulic pressure from the master brake cylinder 3 is also applied to these wheel brakes.

Electrically controllable pressure supply device 6 is designed as a linear actuator. An electric motor 61 drives a piston 63 with the interposition of a rotation-translation gear 62, which pressurizes the hydraulic pressure medium in a pressure chamber 64. Pressure supply device 6 can be switched on via a hydraulic connection to the inlet valves 18a-d of pressure modulator device 7 in order to build up braking pressure at wheel brakes 8a-8d. For the precise control of piston 63, the rotor position of electric motor 61 is detected with a rotor position sensor 52. A sequence valve 17 is provided downstream of the pressure supply device 6 . When the sequence valve 17 is open, the pressure modulation device 7 is supplied with hydraulic pressure medium from the pressure supply device 6 . A second pressure sensor 54 is arranged downstream of the switching valve 17 and upstream of the pressure modulation device 7 to measure the pressure provided by the pressure supply device 6 . Second pressure sensor 54 is arranged in electrohydraulic control and regulation unit 10 .

Pressure medium reservoir 9 is under atmospheric pressure and serves as a store and as an expansion tank for the hydraulic pressure medium. For this purpose, the pressure medium reservoir 9 is placed on the electrohydraulic control and regulation unit 10 so that the hydraulic pressure medium can flow downwards. Pressure medium reservoir 9 comprises a first connection 56 and a second connection 57, to each of which a pressure medium line is connected in order to supply the components of brake system 1 with the pressure medium and thus also pressure medium from the components can flow back into pressure medium reservoir 9. Pressure medium reservoir 9 includes a level sensor 55. Level sensor 55 is used to detect the level or also functions as a level warning device. If necessary, this sensor can also be equipped with one, two or more switching states.

A first electronic control and regulation unit 12 and a second electronic control and regulation unit 13 are also assigned to the electrohydraulic control and regulation unit 10 . First electronic control and regulation unit 12 is used to control the pressure modulator device 7, i.e. the electrically operable inlet valves 18a-18b and the electrically operable outlet valves 19a-19d. The measurement signals from the first pressure sensor 53 and from the second pressure sensor 54 and the level sensor 55 are preferably processed in the first electronic control and regulation unit 12 . Second electronic control and regulation unit 13 is used to control pressure supply device 6 and switching valves 14, 15, 16 and 17. The measurement signals from displacement sensor 51 and rotational position sensor 52 are preferably processed in second electronic control and regulation unit 13.

A first pressure medium line 21 leads from master brake cylinder 3 to simulator release valve 15. In detail, first pressure medium line 21 is connected to an outlet of pressure chamber 34 of master brake cylinder 3 and branches off with a first line section 21a to an input side of simulator release valve 15. From Simulator release valve 15 leads a second pressure medium line 22 to simulation device 4. In detail, second pressure medium line 22 is connected to an output side of simulator release valve 15 and to simulator pressure chamber 45 of simulation device 4.

In addition, first pressure medium line 21 branches off with a second line section 21b to an input side of isolating valve 14 . A first pressure sensor 53 is arranged at the junction of the first and second line sections 21a and 21b.

Furthermore, a third pressure medium line 23 connects pressure medium reservoir 9 to master brake cylinder 3 . This is used to refill the pressure chamber 34 of the master brake cylinder 3 with hydraulic pressure medium. In addition, a throttle check valve device 28 is provided in the third pressure medium line 23 , the check valve of which opens in the direction of flow from the pressure medium reservoir 9 to the master brake cylinder 3 . Furthermore, the outlet valves 19a and 19b are connected to the third pressure medium line 23 via a line section in order to return pressure medium from the wheel brakes 8a and 8b to the pressure medium reservoir 9 .

Pressure modulator device 7 is connected to an output side of isolating valve 14 via a fourth pressure medium line 24 (brake supply line). Fourth pressure medium line 24 is divided by circuit valve 16 into a first line section 24a and a second line section 24b. Inlet valves 18a and 18b are connected to the first line section 24a. Inlet valves 18c and 18d are connected to the second line section 24b. In addition, the fourth pressure medium line 24 is connected to the sequence valve 17 via the second line section 24b. Second pressure sensor 54 is located on second line section 24b.

Pressure supply device 6 is connected to the sequence valve 17 via a fifth pressure medium line 25 . In detail, the fifth pressure medium line 25 is connected to an outlet of the pressure chamber 64 of the pressure supply device 6 . Inlet valves 18a-d of pressure modulator device 7 can be switched on by open switching valve 17 of pressure supply device 6, so that hydraulic pressure medium can be displaced in wheel brakes 8a-8d to build up brake pressure by actuating piston 64. When the circuit separating valve 16 is open, all four inlet valves 18a-18d of the wheel brakes 8a-8d are hydraulically connected to the pressure supply device 6, as a result of which a brake pressure is built up at the wheel brakes 8a-8d by means of the pressure supply device 6. When the circuit separating valve 16 is closed, only the wheel brakes 8c and 8d of the rear axle HA are hydraulically connected to the pressure supply device 6 to build up brake pressure.

A sixth pressure medium line 26 (suction line) is provided for refilling the pressure chamber 64 of the pressure supply device 6 and leads from the second connection 57 of the pressure medium reservoir 9 to the pressure supply device 6 . This line is connected to a return connection from the pressure chamber 64 of the pressure supply device 6 . At this point there is also a spring-loaded check valve 27 which opens in the direction of the pressure supply device 6 . Hydraulic pressure medium can be sucked back in by moving the piston 63 back when the sequence valve 17 is closed. As a result, the pressure medium can flow from the pressure medium reservoir 9 into the pressure chamber 64 .

If an actuation of brake pedal 2 is detected by travel sensor 51, separating valve 14 is closed (energized) and simulator release valve (15) is opened (energized). This creates a pressure medium path from master brake cylinder 3 to simulation device 4 . The actuation of the master brake cylinder piston 32 shifts hydraulic pressure medium into the simulator pressure chamber 45 and acts there on the simulator piston 42 , which is supported on the simulator spring 43 . As a result, a brake actuation on brake pedal 2 is simulated. I.e. a brake pressure build-up is communicated to the vehicle operator when brake pedal 2 is actuated. However, the actual brake pressure build-up is effected by pressure supply device 6. With the sequence valve 17 open, pressure supply device 6 transfers pressure medium via inlet valves 18a-18 from pressure modulator device 7 to wheel brakes 8a-8d.

Based on 2 the problem of pressure medium loss in a hydraulic brake system of a motor vehicle is to be explained in more detail.

Shown is a section of motor vehicle brake system 1 with a focus on master brake cylinder 3 and simulation device 4. In order to ensure the functioning of the legally prescribed hydraulic fallback level, it must be ensured that there is sufficient pressure medium in master brake cylinder 3. Ie there must not have been a loss of pressure medium in master brake cylinder 3. Such a loss of pressure medium can occur, for example, via the sealing collar on the return connection 29 . If this is damaged or there is a leak, pressure medium can flow from pressure chamber 34 of master brake cylinder 3 in Overflow pressure medium reservoir 9. Not only is this undesirable, but it can also go undetected. Consequently, there is too little pressure medium in pressure chamber 34. However, it can also happen that air gets into pressure chamber 34 of master brake cylinder piston 32 via the sealing collar. Such a gas pocket 5 also reduces the effective amount of pressure medium in pressure chamber 34. If there is less hydraulic medium in pressure chamber 34 than it should actually be, this can lead to a loss of braking power at the wheel brakes in the event of a braking process in the fallback level. Less pressure medium is then transferred from pressure chamber 34 via separating valve 14 and fourth pressure medium line 24 to the downstream wheel brakes, as a result of which only a small braking pressure can be built up by the vehicle operator. For this reason, a loss of pressure medium must already be detected during normal operation. This is carried out, for example, by means of a path-printing plausibility check.

According to the example, first a first pressure medium volume is determined, which is shifted from pressure chamber 34 of master brake cylinder 3 via first pressure medium line 21 into simulator pressure chamber 45 of simulation device 4 when the brake pedal is actuated. When the brake pedal is actuated, a first path point x ₁ is detected or stored with the aid of path sensor 51 as soon as separating valve 14 is closed and master brake cylinder piston 32 has passed return connection 29 . The actuation of master brake cylinder piston 32 builds up a hydraulic pressure which is detected by pressure sensor 53 . As the actuation continues, the hydraulic pressure increases to a maximum and the master cylinder piston 32 no longer moves. If, for example, there is now a leak in the sealing sleeve at the return connection 29 , hydraulic medium flows into the pressure medium reservoir 9 or emerges completely from the master brake cylinder 3 . As a result, the hydraulic pressure drops during the actuation process. Master brake cylinder piston 32 is partially pushed back by return spring 33 . As soon as the hydraulic pressure is at zero (measured with pressure sensor 53), a second path point x ₂ of master brake cylinder piston 32 is detected or stored with travel sensor 51. With the known diameter of master brake cylinder 3, its cross-sectional area A _HZ can be multiplied by the difference between the second waypoint _x2 and the first waypoint _x1 . Ie the first pressure medium volume DV ₁ is calculated according to the following formula: $$\mathrm{<figure-callout\; id="1"\; label="D\; V"\; filenames="DE102021214028A1\_0007.png"\; state="\{\{state\}\}">D</figure-callout>}{V}_{}{}_1=A_{HZ}\ast \left(x_2-x_1\right)$$

D.h. mit bekannter Querschnittsfläche A_SFV von Simulatorfreigabeventil 15 und den Messwerten von Drucksensor 53 während dem Messfenster, wird das zweite Druckmittelvolumen DV₂ fortlaufend berechnet. Die Messung hierfür erfolgt ebenfalls ab der Betätigung vom Bremspedal und endet, wenn ein an dem Simulatorfreigabeventil 15 anliegender Druck von null mit Drucksensor 53 gemessen wird. Ab dem ersten Wegpunkt x₁ wird eine Druckmittelmenge unter ansteigendem Druck durch Simulatorfreigabeventil 15 in Simulationseinrichtung 4 vorschoben. Mit den Messwerten von Drucksensor 53 und der Querschnittsfläche von Simulatorfreigabeventil 15 wird während dessen die in Simulationseinrichtung 4 verschobene Druckmittelmenge berechnet. Sobald der Hydraulikdruck auf 0 abgefallen ist, wird die bis dahin verschobene Druckmittelmenge als das zweite Druckmittelvolumen abgespeichert.A second pressure medium volume, which was shifted into the simulation device 4 via the simulator release valve 15, is determined for the purpose of adjustment. The second pressure medium volume results from the volume flow of the pressure medium, which flows through the simulator release valve 15 within a period of time in the simulation device 4 . The second pressure medium volume is the integrated volume flow Q over time. The volume flow Q indicates how much volume of a medium was transported through a specified cross-section per period of time and is calculated using the changed pressure. The following equation is used for the second pressure medium volume DV ₂ : $$\mathrm{<figure-callout\; id="2"\; label="D\; V"\; filenames="DE102021214028A1\_0005.png"\; state="\{\{state\}\}">D</figure-callout>}{V}_{}{}_2={\displaystyle {\int }_0^T{A}_{SfV}\ast \sqrt{\left|\mathrm{\Delta }p\right|}}\mathrm{i.e}t$$

That is, with a known cross-sectional area A _SFV of simulator release valve 15 and the measured values of pressure sensor 53 during the measuring window, the second pressure medium volume DV ₂ is continuously calculated. The measurement for this also takes place from the actuation of the brake pedal and ends when a pressure of zero present at the simulator release valve 15 is measured with the pressure sensor 53 . From the first waypoint x _{1 ,} a quantity of pressure medium is pushed forward through the simulator release valve 15 into the simulation device 4 with increasing pressure. The measured values from pressure sensor 53 and the cross-sectional area of simulator release valve 15 are meanwhile calculated for the quantity of pressure medium displaced in simulation device 4 . As soon as the hydraulic pressure has dropped to 0, the amount of pressure medium that has been shifted up to that point is stored as the second volume of pressure medium.

A check then takes place as to whether the expected amount of pressure medium was actually pushed forward in the simulation device 4 by the measured displacement path of the master brake cylinder piston 32 and the calculated volume flow during the brake actuation. If, for example, a quantity of pressure medium escapes via the sealing sleeve at return connection 29 while the pedal is being actuated, the theoretically shifted pressure medium volume, which was determined via the travel of master brake cylinder piston 32, does not match the actually shifted pressure medium quantity. Because less pressure medium is shifted in the direction of the simulation device 4 than was determined with the calculation.

The pressure medium quantity actually displaced by the master brake cylinder piston 32 is determined as the second pressure medium volume with the aid of the pressure sensor 53 and the volume flow through the simulator release valve 15 . So if the first volume of pressure medium is compared to the second volume of pressure medium and there is a discrepancy, this plausibility check concludes that there is a loss of pressure medium. So the leakage volume is calculated as follows: $$V_{Leak}=\mathrm{<figure-callout\; id="1"\; label="D\; V"\; filenames="DE102021214028A1\_0007.png"\; state="\{\{state\}\}">D</figure-callout>}{V}_{}{}_1-\mathrm{<figure-callout\; id="2"\; label="D\; V"\; filenames="DE102021214028A1\_0005.png"\; state="\{\{state\}\}">D</figure-callout>}{V}_{}{}_2$$

The 3 shows in a diagram the signal curve over time of the displacement sensor and the pressure sensor when the master brake cylinder is actuated when there is or is a loss of pressure medium. The path s and the pressure p are plotted on the ordinate and the time t is plotted on the abscissa. The solid line represents the displacement signal from the displacement sensor and the dashed line is the pressure signal from the pressure sensor.

The exemplary determination of a loss of pressure medium begins at reference number 71. When the brake pedal is actuated, as soon as the brake master cylinder piston has passed the snifter hole and the isolating valve is closed at the same time, the displacement path of the brake master cylinder piston is recorded with the travel sensor. That is, at 71 the position of the master brake cylinder piston within the master brake cylinder is recorded as waypoint x ₁ . When the brake pedal is actuated, the master brake cylinder piston moves within the pressure chamber of the master brake cylinder and compresses the pressure medium therein. Accordingly, the displacement path of master brake cylinder pistons increases to s _max , as shown at reference number 72 , and the hydraulic pressure measured with the pressure sensor also increases, as indicated by reference number 73 , to maximum pressure p _max . The signal values for travel and pressure are at their respective maxima at reference number 74 .

In the further course, the master brake cylinder piston remains in its end position at reference number 75 and the pressure decreases due to a leak. This is illustrated by the pressure drop at reference number 76 . If the pressure drops, the master brake cylinder piston is at least partially pushed back by the return spring. This is represented by the reference number 77, after which the travel path of the master brake cylinder piston decreases. As soon as the measured pressure is at zero (p ₀ ), the path point x ₂ of the master brake cylinder piston is recorded and the measurements are complete. This is represented by reference number 78 . This diagram illustrates how the travel path s of the master brake cylinder piston is determined in order to determine the first pressure medium volume.

The second pressure medium volume is determined from the changed pressure p over time t during the actuation process. This means that the second pressure medium volume is continuously calculated from the initial actuation (x ₁ ) of the master brake cylinder piston until the pressure drops to p ₀ . These two volumes are then compared.

The method according to the example is also supplemented in that a check for gas inclusions in the master brake cylinder is carried out. A volume of gas trapped in the master cylinder is calculated using the following formula: $$V_{L\mathrm{and}ft}=\left(V_{HZ\left(Pmax\right)}-V_{SIM\left(Pmax\right)}-V_{Leak}\right)\ast \frac{Pmax+1}{Pmax}$$

The maximum pressure medium volume displaced by the master brake cylinder piston is used as the V _HZ . That is, the travel of the master cylinder piston is measured from the first waypoint x ₁ to the waypoint when the maximum pressure p _max is reached. The same also applies to the simulator volume V _SIM , which is then also determined from the volume flow through the simulator release valve while the pressure change has reached its maximum p _max within a time window after the initial actuation. The previously calculated leakage volume, which resulted from the first and the second pressure medium volume, is used as the V _Leak .

Due to the different zero points of hydraulic pressure and atmospheric pressure, the gas volume to be determined is multiplied by the factor P _max +1/P _max . A measured hydraulic pressure of 0 bar corresponds to approximately 1 bar of atmospheric pressure. As a result, a gas volume possibly enclosed in the master brake cylinder is obtained from the hydraulic pressure measurement.

Reference List

1

braking system

2

brake pedal

3

master cylinder

4

simulation facility

5

gas lock

6

print staging facility

7

pressure modulation device

8a-d

wheel brakes

9

pressure fluid reservoir

10

electrohydraulic control and regulation unit

12

first electronic control and regulation unit

13

second electronic control and regulation unit

14

isolation valve

15

simulator release valve

16

circuit isolation valve

17

sequence valve

18a-d

intake valves

19a-d

exhaust valves

21

first pressure line

21a

first line section

21b

second line section

22

second pressure line

23

third pressure line

24

fourth pressure medium line

24a

first line section

24b

second line section

25

fifth pressure line

26

sixth pressure medium line

27

check valve

28

Throttle check valve device

29

feedback port

31

piston rod

32

master cylinder piston

33

return spring

34

pressure chamber

35

Housing

41

simulator housing

42

simulator piston

43

simulator spring

44

simulator rear chamber

45

simulator pressure chamber

51

displacement sensor

52

rotor position sensor

53

first pressure sensor

54

second pressure sensor

55

level sensor

56

first connection

57

second connection

61

electric motor

62

rotary translation gear

63

Pistons

64

pressure room

71

begin

72

rise way

73

increase pressure

74

travel and pressure maximum

75

End position master brake cylinder piston

76

pressure drop

77

Return master cylinder piston

78

End

HA

rear axle

HL

back left

MR

back right

v.a

front axle

VL

front left

VR

front right

x1

first waypoint

x2

second waypoint

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 2021/0179041 A1 [0003]
• DE 102020214119 A1 [0005]

Claims (10)

Method for determining a loss of pressure medium in a hydraulic brake system (1) of a motor vehicle, the brake system (1) having a master brake cylinder (3) which can be actuated by a brake pedal (2), and a simulation device (4) which is controlled hydraulically by a simulator release valve ( 15) can be connected to the master brake cylinder (3), characterized in that a first volume of pressure medium is determined, which represents a first quantity of pressure medium displaced from the master brake cylinder, and a second volume of pressure medium is determined, which enters the simulation device via the simulator release valve (15). (4) represents shifted second quantity of pressure medium, wherein the first volume of pressure medium is compared with the second volume of pressure medium, and if a difference was determined, then the loss of pressure medium is inferred. procedure after claim 1 , characterized in that the loss of pressure medium is only concluded when the determined difference exceeds a predetermined limit value. procedure after claim 1 or 2 , characterized in that when the loss of pressure medium has been concluded, a warning signal is emitted. Procedure according to one of Claims 1 until 3 , characterized in that the first pressure medium volume is determined in that the travel of a master brake cylinder piston (32) of the master brake cylinder (3) is multiplied by a cross-sectional area of the master brake cylinder (3). procedure after claim 4 , characterized in that the travel path is calculated from a first path point (x ₁ ) and a second path point (x ₂ ) of the master brake cylinder piston (32), the first path point (x ₁ ) being defined by a first actuation of the master brake cylinder piston (32) and the second waypoint (x ₂ ) is defined when there is zero pressure present at the master brake cylinder (3). Procedure according to one of Claims 1 until 5 , characterized in that the second pressure medium volume is determined in that a volume flow flowing through the simulator release valve (15) is integrated over time. procedure after claim 6 , characterized in that the determination of the second pressure medium volume takes place conclusively when a pressure of zero applied to the simulator release valve (15) is present. Procedure according to one of Claims 1 until 7 , characterized in that a gas volume (5) enclosed in the master brake cylinder (3) is determined in that a maximum simulation device volume is subtracted from a maximum shifted master brake cylinder volume, and the difference between the first pressure medium volume and the second pressure medium volume is subtracted from this. procedure after claim 8 , characterized in that the maximum displaced master brake cylinder volume is determined by multiplying a travel path of a master brake cylinder piston (32) of the master brake cylinder (3) by a cross-sectional area of the master brake cylinder (3), the travel path consisting of a first way point (x ₁ ) and a second waypoint (x ₂ ) of the master brake cylinder piston (32) is calculated, the first way point (x ₁ ) being determined by a first actuation of the master brake cylinder piston and the second way point (x ₂ ) being determined when a brake master cylinder (3) is present pressure has reached its maximum (p _max ), and that the maximum simulation device volume is determined by integrating a volume flow flowing through the simulator release valve (15) over time when the pressure (p _max ) applied to the simulator release valve (15) is maximum (p _max ) has been reached. Brake system (1) of a motor vehicle, comprising a master brake cylinder (3) which can be actuated by a brake pedal (2), a simulation device (4) which can be hydraulically connected to the master brake cylinder (3) by a simulator release valve (15), an atmospheric pressure Pressure medium reservoir (9) for storing a hydraulic pressure medium, and an electronic control and regulation unit (13), characterized in that the electronic control and regulation unit (13) is set up to carry out the method according to one of the preceding claims.

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