DE102010031673B4 - Method for automatically stopping a vehicle at a standstill - Google Patents

Abstract

Method for automatically holding a vehicle at a standstill by means of a brake system (17) which comprises a master brake cylinder (4) and a plurality of wheel brakes (11), a setpoint pressure (psoll) being determined and by corresponding energizing of a pressure control valve (8a, 8b) on at least one of the wheel brakes (11) is set, with the valve current (iusv) to be set at the pressure control valve (8a, 8b) being determined as a function of the setpoint pressure (psoll), a brake pressure (pMC) prevailing at the master brake cylinder (4) and an additional correction term, with a pre-pressure gradient is determined and the correction term is calculated as a function of the pre-pressure gradient according to the following formula: dpMCdtΔtDELAYwhere pMC is the pre-pressure and Δtdelay is a time delay.

Description

The invention relates to a method for automatically stopping a vehicle at a standstill.

State of the art

Modern vehicles with a driving dynamics control system, such as ESP (electronic stability program), include a specially adapted braking system for carrying out automatic, wheel-specific braking interventions on the wheels. Such braking systems usually have several valves to switch between normal braking (foot braking) and automatic braking.

From the WO 02/090159 A1 a method for holding a vehicle on a slope by controlling a brake pressure in the wheel brakes and a traction aid is known. The method comprises the steps a) determining the brake pressure required for stopping, b1) determining a period of time based on a reduction in brake pressure, in particular due to leakage in the brake system, from the release of the brake pedal or b2) comparing the brake pressure determined with the actual brake pressure, from the release of the brake pedal and c) increasing the brake pressure in the wheel brakes, by c1) switching on a pressure generator, c2) opening the switching valve, c3) controlling a valve in such a way that at least the brake pressure required for holding is set in the wheel brakes.

From the DE 103 43 985 A1 discloses a method for preventing a vehicle from rolling away on an incline, using an analog or analogized valve, which is closed when stationary to maintain a brake actuation pressure. In order to avoid leaks in the valve or to reduce the brake pressure losses occurring as a result of the leaks, it is provided that the closing force of the valve is increased when the valve is closed when the vehicle is stationary to maintain a brake actuation pressure or when the valve is closed when the vehicle is stationary to maintain it a brake actuation pressure is to be closed.

From the DE 10 2006 020 520 A1 discloses a device for controlling the brake pressure in a hydraulic brake system by means of a pressure-limiting valve, which limits the brake pressure to a predetermined threshold value and is controlled for this purpose by an electronic device according to a valve characteristic. The positioning accuracy can be significantly improved if an estimation unit for estimating the brake pressure, a sensor system for measuring the brake pressure, a unit for determining a differential pressure from the measured and the estimated brake pressure, and a controller unit are provided, which controls the pressure relief valve as a function of the differential pressure.

From the DE 197 07 960 A1 a method for controlling the pressure in at least one wheel brake is known. A setpoint brake pressure is specified and an actual brake pressure is determined. A valve arrangement is controlled for pressure build-up and pressure reduction as part of a pressure regulation. The at least one control signal for the valve arrangement is formed on the basis of the deviation of the actual pressure from the setpoint pressure and on the basis of a stored relationship between the control signal and the pressure conditions in the area of the valve arrangement. It is provided here that the connection between the control signal and the pressure conditions is stored in characteristic curves for the pressure build-up and/or the pressure reduction, which describe the at least one control signal as a function of the pressure difference across the valve arrangement. The control signals derived from the characteristic are corrected as a function of a correction signal.

From the DE 10 2006 022 806 A1 a device for adjusting a pressure relief valve of a hydraulic brake system is known, which limits the brake pressure to a predetermined threshold value. The positioning accuracy can be significantly improved if an estimation unit for estimating the brake pressure, a sensor system for measuring the brake pressure, a unit for determining a differential pressure from the measured and the estimated brake pressure, and a controller unit are provided, which controls the pressure relief valve as a function of the differential pressure.

1 1 shows a hydraulic brake system 17 known from the prior art, which is set up to carry out vehicle dynamics control. The brake system 17 includes two brake circuits 19a, 19b in X or || distribution, which are symmetrically formed. In the following, therefore, only the part 19a shown on the left in the figure is referred to.

The brake system comprises a foot brake pedal 1, a brake booster 2 with a master brake cylinder 4 connected to it, on which a brake fluid container 3 is arranged. When the foot brake pedal 1 is actuated, a corresponding pressure is generated in the main brake lines 5a, 5b, which pressure acts on the brakes 11 of the wheels 12 via a switching valve 8a and the two inlet valves 10a, 10b. The path in which pressure builds up when the foot brake pedal 1 is actuated is indicated by arrows b. A high-pressure switching valve 7a is closed in this state.

When the driving dynamics control intervenes, the brake pressure is automatically built up with the aid of a hydraulic pump 9a, which is controlled by a control unit (not shown). During the regulation, the switching valve 8a is closed and the high-pressure switching valve 7a is mostly open. The hydraulic pump 9a then delivers the hydraulic fluid along the paths a to the brakes 11. The hydraulic fluid thus flows from the brake fluid reservoir 3 through the main brake line 5a, the high-pressure switching valve 7a, an intake line 6a, through the hydraulic pump 9a and further through the inlet valves 10a, 10b the brakes 11. The braking pressure is modulated by means of the inlet valves 10a, 10b and the outlet valves 13a, 13b, brief pressure peaks being buffered in an equalizing tank 14a.

An additional function of the braking system 17, which is also referred to as a hold or hill hold function, makes it possible to automatically hold a vehicle at a standstill without the driver having to press the foot brake for this purpose. Various holding functions, such as HHC (Hill Hold Control) or AVH (Automatic Vehicle Hold), are known from the prior art, which basically work as follows: During a braking process in which the driver brakes the vehicle to a standstill, the brake pressure is locked in the wheel brakes with the help of switching valves. For this purpose, the valves are actuated accordingly by a control unit in which the HHC function is stored. As a result, the brake pressure is automatically maintained in the wheel brakes, even if the driver takes his foot off the brake pedal.

$$\Delta {\text{p}}_{\text{USV}}={\text{p}}_{\text{soll}}-{\text{p}}_{\text{MC}}.$$

In order to hold the vehicle automatically, the holding function specifies a setpoint pressure, which is usually dependent on the slope or other influencing factors. The target pressure is then adjusted with the help of the switching valves 8a, 8b. For this purpose, the valves 8a, 8b are supplied with a corresponding current. Basically, a higher current closes the valve further and thus a higher brake pressure is maintained in the wheel brake 11, while a lower current causes the valve 8a, 8b to open further. The valve current I required to set the setpoint pressure depends on the pressure difference at the switching valve 8a, 8b, where: $$\text{I}=\text{f}\left(\Delta {\text{p}}_{\text{UPS}}\right)\text{with}$$

$$\Delta {\text{p}}_{\text{UPS}}={\text{p}}_{\text{should}}-{\text{p}}_{\text{MC}}.$$

In this case, Δpusv is the pressure difference at the switching valve 8a, 8b, psoll _is the desired pressure and _pMC is the brake pressure prevailing at the master brake cylinder 4. As can be seen from equations (1) and (2), the setting accuracy of the target pressure p set _is decisively influenced by the accuracy of the form p _MC .

The master brake cylinder pressure p _MC , also called admission pressure below, is measured by means of an admission pressure sensor 18 . Due to the signal processing of the admission pressure signal and the associated time delays, the measured value does not correspond to the actually prevailing admission pressure. The measured value p _MC lags behind the actual admission pressure. In a phase of a braking maneuver in which the driver releases the brake pedal again, the measured admission pressure p _MC will therefore always be greater than the actually prevailing admission pressure. Thus, the valve current determined at the switching valve 8a, 8b according to equation (1) does not correspond to the actually desired pressure difference Δpusv, and the wheel pressure does not _correspond to the desired setpoint pressure p setpoint . In the worst case, it can happen that the wheel pressure is too low and the vehicle unintentionally rolls back.

Disclosure of Invention

It is therefore the object of the present invention to improve the setting accuracy of the setpoint pressure to be set on a wheel brake.

This object is achieved according to the invention by the features specified in claim 1. Further configurations of the invention are the subject matter of dependent claims.

According to the invention, it is proposed to correct the error in the determination of the valve current, which occurs as a result of the deviation between the measured admission pressure value and the admission pressure actually prevailing. As a result, the setpoint pressure required by the hold function of the vehicle dynamics control system can be set much more precisely. The correction is preferably implemented by introducing an additional correction term to calculate the pressure difference Δpusv. The correction term corrects the deviation between the measured admission pressure and the actually prevailing admission pressure at the master brake cylinder.

The correction term is a function of the master cylinder pressure gradient. According to the invention, the correction term is calculated according to the following formula: $$\frac{\mathrm{i.e}{p}_{MC}}{\mathrm{i.e}t}\Delta t_{DELAY}$$

The pressure control valve according to the invention is preferably a switching valve of a hydraulic brake system.

wobei p_soll ein vorgegebener Solldruck an einer Radbremse, p_MC der Vordruck und Δt_delay eine konstante Verzögerungszeit ist. Der Ventilstrom wird dann vorzugsweise aus einer Tabelle, abhängig von Δpusv, ausgelesen oder analytisch berechnet.According to a preferred embodiment of the invention, the pressure difference across the valve is calculated from the following equation: $$\Delta p_{uSV}=p_{sOll}-p_{MC}+\left|\frac{\mathrm{i.e}{p}_{MC}}{\mathrm{i.e}t}\right|\Delta t_{\mathrm{i.e}elay}$$

where p _is a predetermined target pressure at a wheel brake, p _MC is the admission pressure and Δt _delay is a constant delay time. The valve current is then preferably read out from a table, depending on Δpusv, or calculated analytically.

• 1 ein aus dem Stand der Technik bekanntes hydraulisches Bremssystem, das für eine Fahrdynamikregelung ausgelegt ist;
• 2a den Verlauf des Ventilstroms iusv an einem Umschaltventil gemäß dem Stand der Technik und gemäß der Erfindung;
• 2b den Verlauf des gemessenen und des tatsächlichen Vordrucks p_MC und des Solldrucks p_soll; und
• 2c den zeitlichen Verlauf des Solldrucks p_soll und eines Rad-Bremsdrucks p_Rad.

The invention is explained in more detail below by way of example with reference to the attached drawings. Show it:

• 1 a hydraulic brake system known from the prior art, which is designed for vehicle dynamics control;
• 2a the course of the valve current iusv on a switching valve according to the prior art and according to the invention;
• 2 B the progression of the measured and the actual admission pressure p _MC and the target pressure p _desired ; and
• 2c the time profile of the setpoint pressure p _setpoint and a wheel brake pressure p _wheel .

Regarding the explanation of 1 reference is made to the introduction to the description.

2a shows the time curve 20 of the valve current iusv at the switching valve 8a, 8b in a known brake system and the corresponding curve 21 in a brake system according to the invention. As can be seen, the valve current 21 is significantly higher than the valve current 20. The changeover valve 8a, 8b is closed further as a result, so that a higher braking pressure is maintained at the wheel.

wobei p_soll ein vorgegebener Solldruck an einer Radbremse, p_MC der Vordruck und Δt_delay eine konstante Verzögerungszeit ist. Der letzte Term der Gleichung ist ein Korrekturterm, der vom Gradienten des Vordrucks p_MC abhängt. Der Korrekturterm dient dazu, eine Abweichung zwischen dem gemessenen Vordruck p_MC und dem tatsächlich herrschenden Vordruck ${\text{p}}_{\text{MC}}^{\text{real}}$

zu korrigieren.The characteristic curve 21 can be determined, for example, according to the following relationship: $$\Delta p_{uSV}=p_{sOll}-p_{MC}+\left|\frac{\mathrm{i.e}{p}_{MC}}{\mathrm{i.e}t}\right|\Delta t_{\mathrm{i.e}elay}$$

where p _is a predetermined target pressure at a wheel brake, p _MC is the admission pressure and Δt _delay is a constant delay time. The last term of the equation is a correction term that depends on the gradient of the admission pressure p _MC . The correction term serves to calculate a deviation between the measured admission pressure p _MC and the actually prevailing admission pressure ${\text{p}}_{\text{MC}}^{\text{real}}$

to correct.

hinterher. In einer Phase in der der Fahrer das Fuß-Bremspedal wieder löst und Bremsdruck abbaut, ist der gemessene Vordruck p_MC größer als der tatsächlich herrschende Vordruck ${\text{p}}_{\text{MC}}^{\text{real}}.$

2 B shows the progression of the admission pressure p _MC measured by the sensor 18 and the actually prevailing admission pressure at the master brake cylinder 4. As can be seen, the measured value p _MC corresponds to the actually present value ${\text{p}}_{\text{MC}}^{\text{real}}$

after. In a phase in which the driver releases the foot brake pedal again and brake pressure is reduced, the measured admission pressure p _MC is greater than the actually prevailing admission pressure ${\text{p}}_{\text{MC}}^{\text{real}}.$

As in 2c As can be seen, the setpoint value p setpoint specified by the system _can be set very precisely at a wheel brake 11 . On the other hand, without correction of the admission pressure p _MC measured by sensors, the pressure p _{wheel set on the wheel} is smaller than the target value p _setpoint , as can be seen from the dashed line 22 .

Claims (3)

Method for automatically holding a vehicle at a standstill by means of a brake system (17) which comprises a master brake cylinder (4) and a plurality of wheel brakes (11), with a setpoint pressure (p _setpoint ) being determined and, by correspondingly energizing a pressure control valve (8a, 8b) at least one of the wheel brakes (11) is set, with the valve current (i _usv ) to be set at the pressure control valve (8a, 8b) depending on the setpoint pressure (p _set ), a brake pressure (p _MC ) prevailing at the master brake cylinder (4) and an additional correction term is determined, whereby an admission pressure gradient is determined and the correction term is calculated as a function of the admission pressure gradient according to the following formula: $$\frac{\mathrm{i.e}{p}_{MC}}{\mathrm{i.e}t}\Delta t_{DELAY}$$

where p _MC is the admission pressure and Δt _delay is a time delay. procedure after claim 1 , characterized in that the pressure control valve (8a, 8b) is a changeover valve. procedure after claim 1 or 2 , characterized in that the valve current (iusv) is calculated using the following equation: $$\Delta p_{uSV}=p_{sOll}-p_{MC}+\left|\frac{\mathrm{i.e}{p}_{MC}}{\mathrm{i.e}t}\right|\Delta t_{\mathrm{i.e}elay}$$

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