DE102012207553A1 - Method for operating brake system used in motor vehicle e.g. car, involves determining defect of vacuum sensor based on comparison of measured negative pressure with ambient pressure measured by ambient pressure sensor - Google Patents

Abstract

A brake booster (1) is divided by a movable partition (4) into a vacuum chamber (3) and a working chamber (2). The vacuum chamber is connected with a vacuum source to establish a negative pressure. A vacuum sensor (8) is connected to vacuum chamber. The defect of the vacuum sensor is determined based on comparison of the measured negative pressure with ambient pressure measured by an ambient pressure sensor. An independent claim is included for brake system.

Description

The invention relates to a method according to the preamble of claim 1 and a brake system according to the preamble of claim 13 and its use.

Modern motor vehicles must meet high demands in terms of comfort and safety. In order to achieve even greater vehicle deceleration with a reasonable pedal effort, the force applied by the driver operating force on the brake pedal is amplified by the auxiliary power of a brake booster. Vacuum and vacuum brake booster are particularly popular, which use a negative pressure (or the pressure difference between a vacuum chamber and a ventilated in accordance with the brake pedal operation working chamber) as an energy source. This can be generated or maintained via a suction pipe of an internal combustion engine or a motor-driven vacuum pump. Without the continuous evacuation of the vacuum chamber (s), a vacuum brake booster would not be able to fulfill its function after a few braking operations because air flows in during each braking process.

From the DE 10 2007 027 768 A1 a method for providing negative pressure of a brake actuation device of a motor vehicle brake system is known which comprises a pneumatic brake booster whose interior is divided into at least one vacuum chamber and a working chamber. A vacuum sensor detects a pressure level in the vacuum chamber and / or the pressure difference between the vacuum chamber and the working chamber. As soon as a first vacuum level in the vacuum chamber is undershot (or the pressure in the vacuum chamber is too high), a pneumatic motor-pump unit is activated; upon reaching a second vacuum level (or below an absolute pressure threshold) in the vacuum chamber, the motor-pump unit is turned off. Such a method for controlling a motor-pump unit or an electric vacuum pump can be used, for example in hybrid vehicles, to ensure a sufficient brake boost even during a period in which only the electric drive drives the vehicle (so-called "sailing operation").

For safety-relevant components or systems in motor vehicles, concrete requirements criteria are defined for fault scenarios and fallback levels, such as in the European standard ECE R13H for car brake systems. In order to be able to ensure a required minimum delay even in the event of a fault, it may therefore be necessary to detect a defect of the vacuum sensor and / or to at least approximately determine the negative pressure in the vacuum or vacuum brake booster even in the case of a defective vacuum sensor. Thus, measures to ensure the required braking delay can be initiated. When using a single vacuum sensor without intrinsic safety or redundancy, the given reliability requirements can not be met without further measures. For this reason, two vacuum sensors, double sensors or intrinsically safe vacuum sensors for measuring the negative pressure in the vacuum brake booster are used in the prior art, for example.

The DE 10 2007 003 741 A1 discloses a method of operating a vacuum brake booster of a vehicle brake system, comprising a housing partitioned by a movable partition (or membrane) into at least one vacuum chamber and at least one working chamber. A sensor unit senses the pressure in the vacuum chamber and supplies it to an electronic control unit, which calculates the control point of the vacuum brake booster solely on the basis of the pressure prevailing in the vacuum chamber. The Aussteuerpunkt denotes a state in which a further increase in the brake pressure only by increasing the pedal force is possible because the vacuum brake booster has reached the maximum possible support force. In order to determine possible defects of the sensor unit or of the vacuum brake booster (or of the vacuum pump), a plausibility check of the pressure value measured by the sensor unit is carried out by forming a model which, on the basis of empirically determined data in conjunction with fluidic and thermodynamic processes, obtains the state variables in the vacuum chamber and the working chamber estimates.

The object of the present invention is to provide an alternative method for checking the plausibility of the measured pressure in a vacuum chamber of a brake booster.

This object is achieved by a method according to claim 1.

It is thus a method for operating a brake system with a brake booster, which is divided by at least one movable partition in at least one vacuum chamber and at least one working chamber, wherein at least one vacuum chamber with a vacuum source to build A negative pressure is connected or can be connected, in which a defect of the vacuum sensor and / or the vacuum system is detected on the basis of a comparison of the measured vacuum or a determined from the measured negative pressure achievable negative pressure with a measured ambient pressure.

• • Eine bisher eingesetzte redundante Messwerterfassung des Vakuums bzw. Drucks in der Unterdruckkammer kann vereinfacht werden, indem bereits für andere Einsatzzwecke vorhandenen Sensoren mitbenutzt werden. Dies verringert die Kosten des Bremssystems, ohne dass die Zuverlässigkeit abnimmt.
• • Durch den Wegfall eines redundanten Sensors verringert sich sowohl das Systemgesamtgewicht als auch der benötigte Bauraum, so dass Fahrzeuge mit gleicher Funktion kleiner und leichter werden, was Agilität und Umweltverträglichkeit erhöht.
• • Auch gegenüber einem System mit einer aufwendigen Modellberechnung verringert sich der benötigte Entwicklungs- und Produktionsaufwand.

The process according to the invention has a number of advantages:

• • A previously used redundant data acquisition of the vacuum or pressure in the vacuum chamber can be simplified by sharing already existing for other applications sensors. This reduces the cost of the brake system without decreasing the reliability.
• • The elimination of a redundant sensor reduces both the overall system weight and space requirements, making vehicles with the same functionality smaller and lighter, increasing agility and environmental sustainability.
• • Compared to a system with a complex model calculation, the required development and production costs are reduced.

Preferably, a defect of the vacuum sensor is recognized by the fact that the amount of the measured negative pressure or the achievable negative pressure by at least one predetermined threshold value is greater than the measured ambient pressure. Thus, an obviously too large value of the measured negative pressure can be detected.

Expediently, an efficiency or a suction capacity of the vacuum source and / or a tolerance of a check valve connected to the at least one vacuum chamber and / or a tolerance of the ambient pressure sensor and / or a tolerance of the vacuum sensor are taken into account. By assuming respectively minimum and maximum values of the deviations, a tolerance band with an upper and a lower limit for the negative pressure can be determined and a defect of the vacuum sensor or the vacuum system can be recognized if the measured negative pressure or the achievable negative pressure lies outside the tolerance band in particular after the vacuum source has been connected to the vacuum chamber for at least a predetermined duration and has been active. Thus, it can also be checked whether the vacuum sensor erroneously displays too low values for the negative pressure.

It is advantageous if at least one vacuum chamber is connected to the vacuum sensor and to a motor-pump unit, which builds a vacuum in the vacuum chamber as the sole or additional negative pressure source when activated, and that the motor-pump unit is activated when the amount of the measured negative pressure is lower than a first negative pressure threshold value or the measured absolute pressure in the negative pressure chamber is greater than a first absolute pressure threshold value. Thus, e.g. an electrical vacuum pump can be controlled according to demand.

It is particularly advantageous if the motor-pump unit is operated until the amount of the measured negative pressure exceeds a second negative pressure threshold or the amount of measured negative pressure corresponds to the measured ambient pressure to a tolerance threshold or exceeds or until the measured absolute pressure in the vacuum chamber is greater than a second absolute pressure threshold, which preferably corresponds to the measured ambient pressure less a tolerance threshold. This makes it possible to avoid a problem of vacuum supply occurring in higher regions with low ambient pressure (for example in high mountains). Otherwise, the vacuum pump may operate for too long, without it being possible to achieve the desired vacuum.

Preferably, the achievable negative pressure is determined from the measured negative pressure and the time since activation of a motor-pump unit. By evaluating several successive measurements of the vacuum before or during the activation of an electric vacuum pump, the achievable negative pressure can be extrapolated, even if the electric vacuum pump is switched off before this pressure is reached.

If the behavior of the electrical vacuum pump or of the overall system deviates from the ideal behavior, a suitable characteristic curve and / or a characteristic diagram can be determined in test measurements and the influence thus compensated or modeled.

Conveniently, when considering several consecutive measurements during an activity of the motor-pump unit, a quantity representing the efficiency of the motor-pump unit is monitored, in particular the speed of the motor-pump unit or the motor supplied power. Then, on the basis of fluctuations of this magnitude, which exceed a predetermined fluctuation threshold, a disturbance of the measurements or changing conditions can be detected.

The invention further relates to a brake system for a motor vehicle, which is a brake booster, which is divided by at least one movable partition in at least one vacuum chamber and at least one working chamber, wherein at least one vacuum chamber is connected to a vacuum source for establishing a negative pressure and a vacuum sensor or connected can, a motor-pump unit as a sole or additional negative pressure source, at least one connected to the brake booster master cylinder, is constructed in accordance with a brake pedal brake pressure, and at least one associated with the master cylinder wheel brake and an electronic control unit which receives information about the ambient pressure and a method according to any one of the preceding claims. Such a brake system realizes the advantages of the method according to the invention.

According to a preferred embodiment of the invention, both a sensor that detects the pedal angle or pedal travel of a brake pedal operation and a sensor for detecting the built-up brake pressure are present, and the electronic control unit is connected to both. Thus, it can be recognized when the gain of the brake booster decreases, whereby appropriate measures can be taken.

It is advantageous if a hydraulic pump which can be connected to at least one wheel brake is present. Thus, a driver-independent build-up of brake pressure to compensate for a low gain.

Furthermore, the invention relates to the use of a brake system according to the invention in a motor vehicle, which is driven by an internal combustion engine and / or at least one electric machine.

Further preferred embodiments will become apparent from the subclaims and the following description of an embodiment with reference to figures.

Show it

1 a master cylinder with upstream vacuum brake booster,

2 a scheme for checking the plausibility of a vacuum sensor,

3 a time course of the vacuum during the evacuation of a vacuum chamber, wherein different conditions are described over a time constant,

4 a first diagram for estimating the achievable negative pressure,

5 a second diagram for estimating the achievable negative pressure, and

6 a diagram for determining a time constant from the ratio of two successive pressures.

1 shows a master cylinder 8th with upstream vacuum or vacuum brake booster 1 which is also called a "booster". The vacuum brake booster 1 has a housing 5 on that in a working chamber 2 and a vacuum chamber 3 is divided. This is done by a movable partition 4 , which is provided with an axially movable rubber diaphragm. Center in the vacuum brake booster 1 is a hub of control 9 arranged, whose function will be explained in more detail below. The power is delivered via a power output member 14 that is above a reaction disk 15 at a stage 16 supported. On the other side, the control hub penetrates 9 the housing 5 and is to the atmosphere via a filter 17 axially open. By means of a form-fitting inserted seal 18 becomes the working chamber 2 sealed to the environment. The power transmission to the reaction disc 15 via a valve piston 19 which is on a ball head of a piston rod 7 is clamped.

The piston rod 7 penetrates an airspace 21 and communicates with an unillustrated operation pedal. In the airspace 21 is one of the piston rod 7 penetrated poppet valve 22 used. The poppet valve 22 is arranged so that it is the airspace 21 separates to the inside of the amplifier, as in the rest position of the vacuum brake booster shown here 1 the case is. In this rest position, the air supply to the working chamber 2 shut off. Thus prevails in the working chamber 2 a negative pressure, since these via openings with the vacuum chamber 3 is connected and there the vacuum chamber 3 via a vacuum connection 10 is connected to a vacuum source, not shown, preferably an electric vacuum pump.

With the help of a vacuum sensor 6 becomes the pressure in the vacuum chamber 3 measured.

Becomes one with the piston rod 7 connected brake pedal actuated and thus the valve piston 19 moved, so will the poppet valve 22 operated and the vacuum chamber 3 and the working chamber 2 are no longer in contact with each other. In the further course of the movement, a connection between the working chamber 2 and the outside air through the poppet valve 22 open. By the at the movable partition 4 applied pressure difference between working chamber 2 and vacuum camera 3 the input force is assisted by the brake pedal and the vacuum brake booster 1 downstream master cylinder 8th by means of the power output element 14 actuated. In this standby position, any slight change in the pedal force will increase or decrease the difference between the two sides of the bulkhead 4 applied pressures and effected via the master cylinder 8th an increase or reduction of the hydraulic pressure in the brake system and thus a controlled deceleration of the motor vehicle.

The maximum possible support force of the vacuum brake booster 1 is given when the working chamber 2 is fully ventilated and atmospheric pressure prevails. This condition is called a trigger point. In the control point is thus the maximum pressure difference between the working chamber 2 and the vacuum chamber 3 reached. Another increase in the force on the master cylinder piston 8th who joined the power delivery member 14 connects, is only possible by an even greater pedal force of the driver, with the hydraulic pressure in the brake system only increases unreinforced. This leads to the fact that after exceeding the Aussteuerpunktes a further increase in the braking force requires a significantly increased force on the brake pedal.

To remedy this problem, it is known to switch to a hydraulic gain when reaching the Aussteuerpunktes and to control a hydraulic pump, which builds up an additional brake pressure. However, for this additional brake power assistance, it is necessary to accurately recognize the Aussteuerpunkt to turn on the additional hydraulic gain as needed. Since the vacuum brake booster 1 only a vacuum sensor 6 to determine the pressure in the vacuum chamber 3 has the trigger point estimated or calculated.

Defects of the vacuum sensor 6 must not lead to a faulty detection of the control point, and also a high pressure in the vacuum chamber (or lack of negative pressure) must be reliably detected. Therefore, preferably, a plausibility check of the vacuum sensor 6 measured pressure value to reliably detect possible defects, ie sensor error or failure of the vacuum brake booster and taken appropriate countermeasures and / or a warning of the driver can be made.

In order to achieve a high gain in a small space, the brake system may also include a tandem brake booster, which corresponds to two vacuum brake boosters connected in series and thus has two vacuum chambers and two working chambers. Also in these tandem brake boosters, the inventive method is applicable accordingly.

With each brake application, the vacuum brake booster "consumes" a certain amount of its vacuum supply, ie the pressure in the vacuum chamber 3 is increasing. In conventional gasoline engines, the intake manifold of the internal combustion engine serves as a vacuum source, while diesel engines often use a mechanically driven vacuum pump. These work many times continuously, whereby the vacuum level is constantly maintained at a nearly constant value and thus always the greatest possible auxiliary power is available. In some brake system configurations, however, the vacuum is generated alone or in addition via an electric vacuum pump (EVP). This allows, for example in hybrid vehicles with electric auxiliary drive, the temporary shutdown of the engine to drive to reduce or avoid emissions alone by means of electric motor can (one speaks of "sailing operation"). So that in the sailing operation even after several braking maneuvers nor the full brake boosting is ensured, the electric vacuum pump is activated and evacuated the vacuum chamber. In order to avoid a continuous operation of the electric vacuum pump, their control is preferably as hysteresis with executed an upper and a lower switching point, wherein the signal of a vacuum sensor is evaluated. The vacuum pump control ensures that there is always a certain level of vacuum available as an energy source for the brake booster, even when the internal combustion engine is not in operation.

In a hysteresis control, the vacuum sensor measures 8th So the pressure in at least one vacuum chamber 3 of the brake booster 1 , If the pressure exceeds a first sub-atmospheric threshold p _min , the remaining negative pressure is expected to cause a decrease in the amplification effect, then the electric vacuum pump (not shown) is activated. Based on the atmosphere, the first vacuum threshold p _min can be between -600 mbar and -750 mbar, for example. The electric vacuum pump reduces the pressure in the brake booster, which approaches a saturation pressure which is influenced by the pumping speed of the vacuum pump and the leakage rate of the vacuum chamber, that is to say a maximum attainable negative pressure. As soon as the pressure has fallen below a second vacuum threshold p _max , the electric vacuum pump is deactivated. Based on the atmosphere, the second vacuum threshold p _{max can be,} for example, between -750 mbar and -850 mbar. By operating with a hysteresis, the control reduces the frequency of activation of the electric vacuum pump, each providing sufficient vacuum in the vacuum chamber for multiple braking operations.

According to a particularly preferred embodiment of the invention, both a sensor for detecting the brake pedal angle and a sensor for detecting the hydraulic pressure in the master cylinder are present. From a comparison of the master cylinder pressure with the brake pedal angle, a reaching of the output point of the brake booster can then be detected, so that e.g. a hydraulic pump for brake assistance can be activated.

An inventive idea is to plausibilize the information of the vacuum sensor in a suitable manner with an absolute ambient pressure, so that a reliable and reliable statement about the available vacuum can be given even without expensive additional components. Thus, safety-critical driving conditions such as overbraking (at incorrectly too low measured pressure) and under braking (by lacking negative pressure) are avoided. A key point of the invention is that the relative to the ambient pressure relative vacuum or the negative pressure in terms of amount can never be greater than the absolute pressure of the environment: | P _{vacuum_rel} | ≤ | P _{ambient_abs} |

With this idealized plausibility relationship, the influences of the atmospheric pressure on the relative vacuum signal can be used to check the plausibility of the vacuum sensor.

2 shows a scheme for plausibility of a vacuum sensor, wherein various factors are shown as boxes. As already explained, the ambient pressure P _{ambient_abs limits} the maximum achievable negative pressure and thus represents a significant quantity. It is therefore measured by an absolute pressure sensor, which is also used, for example, to control an internal combustion engine and is therefore frequently already present. Since the pressure P _{ambient_sensor_abs} measured by the sensor never exactly corresponds to the actual ambient pressure, the tolerance P _{ambient_sensor_tolerance of} the absolute pressure sensor must also be taken into account. The efficiency η _{vacuum_source of} the vacuum source, or the ratio of the suction power of the vacuum source and the leakage rate of the vacuum chamber or the housing, limits the achievable negative pressure. In order to prevent the air from flowing back, a check valve is arranged between the vacuum source and the vacuum chamber. The tolerance of the check valve P _{check_valve_tolerance} or a pressure difference caused _{thereby also has} an effect on the pressure P _vacuum in the vacuum chamber or in the brake booster. The vacuum _sensor measures the pressure P _{vac_sensor_rel} , which is still corrupted by a tolerance P _{vac_sensor_tolerance of} the vacuum _sensor .

In the overall view of the vacuum control and signal chain so all components and sensors must be considered, such as the efficiency of the vacuum pump and the tolerances of sensors and check valves. Furthermore, when the final value or saturation value of the vacuum source, that is to say after a prolonged activity, e.g. the vacuum pump, can never be greater than the surrounding air pressure or ambient pressure. After reaching the saturation value, which occurs after a corresponding time, thus the quality of the information of the vacuum sensor or the established vacuum can be checked.

Taking into account the efficiency of the vacuum source and the tolerance of the check valve, the plausibility relationship (after saturation of the vacuum source) is: | P _{vac_rel} | = | (η _{vacuum_source} · P _{ambient_abs} ) - P _{check_valve_tolerance} |

Taking into account the tolerance of the pressure sensors, the plausibility relationship with the influence of vacuum source, check valve, vacuum sensor and absolute pressure sensor (after saturation of the vacuum source) is: | _{Vac_sensor_rel} | = | (η _{vacuum_source} · (P _{ambient_sensor_abs} ± P _{ambient_sensor_tolerance} )) ± P _{check_valve_tolerance} ± P _{vac_sensor_tolerance} |

By assuming the most favorable possible value for all tolerances, a minimum pressure or a best possible vacuum can be calculated (after saturation of the vacuum source): | _{Vac_sensor_rel_max} | = | (η _{vacuum_source_max} · (P _{ambient_sensor_abs} + P _{ambient_sensor_tolerance_max} )) + (P _{check_valve_tolerance_max} = 0) + P _{vac_sensor_tolerance_max} |

Correspondingly, the worst possible vacuum is obtained if, for each of the tolerances, the worst-case possible value is assumed (after saturation of the vacuum source): | _{Vac_sensor_rel_min} |. _Vac = | (η _{vacuum_source_min} · (P _{ambient_sensor_abs} + P _{ambient_sensor_tolerance_min} )) + P _{check_valve_tolerance_min} + P _{vac_sensor_tolerance_min} |

If the pressure in the vacuum chamber is measured after saturation of the vacuum source, the following plausibility check of the vacuum pressure against the ambient pressure can be made, whereby a reliable and reliable statement about the available vacuum can be made: | _{Vac_sensor_rel_min} |. _Vac <P _{vac_sensor_rel} (measured value) <| P _{vac_sensor_rel_max} |

When considering the implementation of the method on a particular vehicle model, the tolerances are appropriate to select in order for the o.g. Tolerance bands of the vacuum signal to ensure a reliable detection of vacuum errors and vacuum signal errors while maximum robustness.

Without this error detection, the control unit of the brake system would expect a high vacuum brake assist in case of a wrongly measured vacuum signal and shift the thresholds for the additional brake assist by the hydraulic pump in the direction of a higher pedal force. At the same time, the electric vacuum pump would erroneously be turned on too late, thus just creating the situation in which a hydraulic assistance is needed. In the worst case, therefore, the legally prescribed minimum delay could not be met with the defined reference force.

If the vacuum signal without o.g. Fault detection would be measured too small, the brake system would move the additional hydraulic brake assist in the direction of a lower pedal force despite high vacuum brake assist. This can lead to overbraking of the vehicle and in the worst case to the loss of vehicle control.

The mechanically and electrically driven vacuum pumps or motor pump units used in today's motor vehicles have charging curves with an approximately exponential course.

3 shows a family of vacuum charging curves with different time constants τ, normalized to the difference between the vacuum starting value and the vacuum saturation; ie 100% corresponds to reaching the final value, the minimum possible pressure or the maximum possible negative pressure. The final value is for the most part temperature-dependent, while the characteristic time constant (τ) of the exponential charge curve is determined by the rotational speed of the pump.

As already explained, electric motor-pump units for vacuum generation are often operated with a hysteresis control. Thus, a saturation of the vacuum source or the maximum negative pressure is not achieved. In the following embodiments of the invention are described which allow an estimate of the achievable negative pressure despite aborted charging of the vacuum source. With the described method for vacuum final value / target value determination, a plausibility of a pressure sensor with the absolute ambient pressure can be made without the actual achievement of saturation. Thus, the vacuum charging phase of a motor-pump unit can be monitored, which opens up further applications for the use and protection of vacuum sensors or vacuum signals.

4 shows a first diagram for estimating the achievable negative pressure, wherein the time course of the achieved pressure difference between the environment and the vacuum chamber or the amount of the achieved negative pressure is shown. If the operation of a motor-driven pump unit or an electric vacuum pump is aborted before reaching the final value, the saturation value must be extrapolated from the previously observed course in order to be able to carry out the already explained plausibility check. Due to the known exponential curve of the charging curve, this extrapolation can take place on the basis of a few measuring points, as explained below:

wird die Ableitungsfunktion gebildet:
Im Zeitpunkt t₁ entspricht die Steigung der im Ursprung einer Sprungfunktion, die auf dem Startlevel P(t₁) beginnt, aber den gleichen Zielwert P_sat anstrebt:Based on the representation of the exponential function

the derivative function is formed:
At time t ₁ , the slope corresponds to that at the origin of a jump function which starts at the start level P (t ₁ ) but aims at the same target value P _sat :

womit nun alle Werte aus dem Signalverlauf ermittelbar sind. Im Übrigen ist das Verhältnis der Steigungen gleich der Exponentialfunktion:

If the slopes are set in proportion, results

with which now all values from the signal course can be determined. Incidentally, the ratio of the slopes is equal to the exponential function:

was unabhängig vom Wert von σ identisch dem Verhältnis der exakten Ableitungen ist. The slope at a point on the curve is usually approximated from the measured time course by the observation of a small time interval (ΔP / Δt) , Since a measurement of noise, measurement and quantization error is superimposed and therefore inaccurate, in principle a longer observation interval to reduce these influences is advantageous. Here the duration σ of the observation interval is insignificant, because the following applies:

which, whatever the value of σ, is identical to the ratio of exact derivatives.

5 shows a second diagram for estimating the achievable negative pressure, in which the time course of the amount of negative pressure is shown upon activation of the motor-pump unit. According to a preferred embodiment of the method according to the invention, the observed time interval T, which is limited for example by the times at which the electric vacuum pump or the motor-pump unit has been activated or deactivated, halved and the values at the points t = 0, t = t / 2 and t = T used.

oder alternativ

The estimated saturation value for extrapolation of the function then becomes too

or alternatively

Mathematically, one obtains the second solution from the choice of a negative σ, practically any of the two formulas can be used to calculate the achievable negative pressure (or a calculation using both formulas with subsequent averaging).

6 shows a diagram for determining a time constant from the ratio of two successive values of the pressure reached after a predetermined pumping time, on the one hand a set of vacuum charging curves with different time constants τ, which are normalized to the difference between the vacuum start value and vacuum saturation, and on the other hand, the ratio of two successive values of the currently achieved negative pressure are shown. This is a set of exponentially falling curves showing the ratio of the vacuum increment continuously since the beginning of the charge or the activity of the electric vacuum pump in the particularly preferred time interval t to t / 2 as a function of different time constants τ.

According to a particularly preferred embodiment of the method according to the invention, the characteristic time constant τ of the vacuum charging curve is determined and from this the achievable negative pressure is calculated. Similar to an exponential curve, charging curves with a fixed time constant τ always have a defined relative vacuum increase, which depends on the initial vacuum value and the attainable final vacuum value. Since the initial value of the vacuum is known (by a measurement) and the relative vacuum increase is determined / measured, the achievable negative pressure or final vacuum value can be calculated.

Specifically, in a first step after measuring the vacuum, the ratio of vacuum gain since the start of charging is calculated from time P1 (p _{vac @ t} , t) to time P1 (p _{vac @ t} , t / 2). Appropriately, a previously stored (eg in a non-volatile memory of the electronic control unit of the brake system) map based on the time t and the ratio p _vac (t) / p _vac (t / 2) allows the determination of the characteristic charging time constant τ.

It is advantageous to make the determined charging time constant plausible with minimum and maximum limit values for possible charging time constants and / or the rotational speed of the vacuum pump.

After the charging time constant has been determined, the achievable negative pressure or saturation value of the charging curve can be calculated in a second step.

Assuming an ideal exponential charge curve, the maximum possible vacuum gain can be calculated by the following formula:

To calculate the absolute absolute value of the vacuum, the initial value of the vacuum must be taken into account:

Assuming a non-ideal exponential characteristic charge curve of the vacuum source or the motor-pump unit, a second map for the maximum percentage vacuum increase based on the measured at time t vacuum value and the charging time constant must be known, eg when programming the electronic Control unit of the brake system have been stored in a flash memory.

By means of the calculated charging time constant and the current time value t, the maximum percentage increase in vacuum can be determined by means of the second characteristic map.

Then can be calculated from the negative pressure upon activation of the vacuum source or the vacuum initial value and the vacuum increase an achievable vacuum or saturation value.

With the determined final vacuum value or saturation value, the functioning of the vacuum sensor system, vacuum source and vacuum system can then be checked and ensured on the basis of the plausibility formulas described above, also in the vacuum loading phase or during evacuation.

In order to be able to suitably process noisy or disturbed vacuum input signals, preferably input filters are used, in particular a low-pass filter. For a further increase in robustness, it is expedient to use an output filter for filtering the determined charging time constant or the determined final vacuum value.

The described methods are not limited to a one-time calculation, rather they can be carried out repeatedly and, in particular, averaging can take place. If the charging time constant changes during a continuous evaluation, one possible cause is e.g. a speed change of the vacuum pump. In this case, the vacuum initial value is particularly preferably set again and the charge time constant is redetermined. As long as the determined charging time constant retains an approximately constant value, the value pairs may be used to calculate the final vacuum value. As a result, a high accuracy of the estimate of the achievable negative pressure can be achieved.

Thus, the operation of the vacuum sensor, vacuum source, and vacuum system can also be checked if the charge phase is aborted due to reaching a level expected by the pump controller and before reaching a saturation value, i. there is a hedge of the vacuum signal during the vacuum charging phase. An advantage of the invention is the increase in the signal reliability, which results essentially from plausibility of the vacuum sensor with the absolute ambient pressure.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• DE 102007027768 A1 [0003]
• DE 102007003741 A1 [0005]

Cited non-patent literature

• European Standard ECE R13H [0004]

Claims (16)

Method for operating a brake system with a brake booster, which is divided by at least one movable partition in at least one vacuum chamber and at least one working chamber, wherein at least one vacuum chamber with a vacuum source for establishing a negative pressure and with a vacuum sensor is connected or can be connected, characterized in that a defect of the vacuum sensor and / or of the vacuum system is detected on the basis of a comparison of the measured negative pressure or an achievable negative pressure determined from the measured negative pressure with a measured ambient pressure. A method according to claim 1, characterized in that a defect of the vacuum sensor is detected when the amount of the measured negative pressure or the achievable negative pressure by at least a predetermined threshold value is greater than the measured ambient pressure. A method according to claim 1 or 2, characterized in that an efficiency or a suction capacity of the vacuum source and / or tolerance of a connected to the at least one vacuum chamber check valve and / or a tolerance of the ambient pressure sensor and / or a tolerance of the vacuum sensor is taken into account Tolerance band is determined with an upper and a lower limit for the negative pressure and a defect of the vacuum sensor and / or the vacuum system is detected when the measured negative pressure or the achievable negative pressure is outside the tolerance band, in particular after the vacuum source for at least a predetermined period with the Low pressure chamber connected and active. A method according to claim 3, characterized in that the negative pressure is measured after the vacuum source for at least a predetermined duration connected to the vacuum chamber and was active, and that a tolerance band for the amount of the measured negative pressure or the achievable negative pressure according to | _{Vac_sensor_rel} | = | (η _{vacuum_source} · (P _{ambient_sensor_abs} ± P _{ambient_sensor_tolerance} )) ± P _{check_valve_tolerance} ± P _{vac_sensor_tolerance} | calculated, with the amount of the measured negative pressure or the achievable negative pressure | P _{vac_sensor_rel} |, the efficiency of the vacuum _source η _{vacuum_source} , the measured ambient pressure P _{ambient_sensor_abs} , the tolerance of the ambient pressure _sensor P _{ambient_sensor_tolerance} , the tolerance of the check valve P _{check_valve_tolerance} and the tolerance of the vacuum _sensor P _{vac_sensor_tolerance} . Method according to at least one of claims 1 to 4, characterized in that at least one vacuum chamber with the vacuum sensor and a motor-pump unit is connected, which builds as a sole or additional negative pressure source when activated a negative pressure in the vacuum chamber, and that the motor-pump unit is activated when the amount of the measured negative pressure falls below a first negative pressure threshold value or the measured absolute pressure in the negative pressure chamber is greater than a first absolute pressure threshold value. A method according to claim 5, characterized in that the motor-pump unit is operated until the amount of the measured negative pressure exceeds a second negative pressure threshold or the amount of measured negative pressure corresponds to the measured ambient pressure to a tolerance threshold or exceeds or until the measured absolute pressure in the negative pressure chamber is greater than a second absolute pressure threshold, which preferably corresponds to the measured ambient pressure minus a tolerance threshold value. A method according to claim 5 or 6, characterized in that both a sensor for detecting the brake pedal angle or brake pedal travel and a sensor for detecting the braking force in a brake booster downstream brake pressure constructed are provided, and that a comparison of the sensor data is made, wherein by means of a With at least one wheel brake connectable additional pressure source, a build-up of braking torque in at least one wheel brake of the vehicle is effected when the determined from a comparison of the sensor data gain of the brake booster falls below a predetermined gain threshold. Method according to at least one of claims 5 to 7, characterized in that the achievable negative pressure from the measured negative pressure and the time since the activation of a motor-pump unit is determined. A method according to claim 8, characterized in that the achievable negative pressure P _sat according to

and or

is determined, measured at P (0) negative pressure at the beginning of the activation of the motor-pump unit, P (T) measured negative pressure after a time period T has elapsed since activation of the motor-pump unit, P (T / 2) measured negative pressure after half period of time. A method according to claim 8, characterized in that the achievable negative pressure according to

is determined, measured at P (0) negative pressure at the beginning of the activation of the motor-pump unit, P (T) measured vacuum after a period T has elapsed since activation of the motor-pump unit, and wherein the time constant τ from the ratio of two consecutive measured negative pressure values is determined. A method according to claim 10, characterized in that the time constant τ is determined from the ratio of two successive measured negative pressure values, in particular P (T) / P (T / 2), and the time period T since activation of the motor-pump unit by a first Characteristic map is evaluated with values for the time constant τ as a function of the time duration T and the ratio of successive measured negative pressure values, and preferably from a further ratio of successively measured negative pressure values or the pressure P (T) and the time constant τ, a correction value from a second characteristic field is determined. A method according to claim 10 or 11, characterized in that a value representing the efficiency of the motor-pump unit is monitored, in particular the speed of the motor-pump unit or the current supplied to the motor, the time constant being determined again if two consecutive values for the size representing the efficiency of the motor-pump unit deviate from one another by more than a predetermined fluctuation value. A braking system for a motor vehicle, comprising a brake booster, which is divided by at least one movable partition into at least one vacuum chamber and at least one working chamber, wherein at least one vacuum chamber is connected to a vacuum source for establishing a negative pressure and a vacuum sensor or can be connected, an engine Pump unit as a sole or additional vacuum source, at least one connected to the brake booster master cylinder in which brake pressure is built up in accordance with a brake pedal actuation, at least one connected to the master cylinder wheel brakes, characterized by an electronic control unit which receives information about the ambient pressure and a method according to one of the preceding claims. Brake system according to claim 13, characterized in that both a sensor which detects the pedal angle or pedal travel of a brake pedal operation, as well as a sensor for detecting the built-up brake pressure are present and the electronic control unit is connected to both. Brake system according to claim 13 or 14, characterized in that a connectable with at least one wheel brake hydraulic pump is present. Use of a brake system according to at least one of claims 13 to 15 in a motor vehicle which is driven by an internal combustion engine and / or at least one electric machine.

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