GB2403520A

GB2403520A - Determination of brake fluid consumption in electrohydraulic braking systems for automobiles

Info

Publication number: GB2403520A
Application number: GB0315347A
Authority: GB
Inventors: Iryna Deineko; Peter Martin; Alan Leslie Harris
Original assignee: TRW Ltd
Current assignee: TRW Ltd
Priority date: 2003-07-01
Filing date: 2003-07-01
Publication date: 2005-01-05
Anticipated expiration: 2023-07-01
Also published as: GB2403520B; GB0315347D0

Abstract

An electrohydraulic braking (EHB) system comprising a brake fluid reservoir, a pump connected to the reservoir for providing a supply of pressurized brake fluid, an accumulator connected to the pump for storing pressurized brake fluid, control valves coupled to the accumulator and reservoir for selectively supplying pressurized brake fluid from the accumulator to a brake actuator for application of a vehicles brake and discharging brake fluid from the brake actuator back to the reservoir for release of the brake, and an electronic control unit (ECU) which evaluates the driver's braking demands and controls the valves and pump accordingly, and wherein, for providing an indication of brake fluid consumption, a comparison is made during a braking operation of the volume of brake fluid which is taken from the accumulator with the volume of brake fluid which is supplied to the brake actuator.

Description

DESCRIPTION

DETERMINATION OF BRAKE FLUID CONSUMPTION IN

ELECTROHYDRAULIC BRAKING SYSTEMS FOR AUTOMOBILES

The present invention relates to electrohydraulic braking (EHB) systems and is concerned in particular with a method and apparatus for determining brake fluid consumption in such systems.

F,lectrohydraulic braking (EHB) systems generally comprise a reservoir which stores brake fluid, a pump connected to the reservoir for providing a supply of pressurized brake fluid, a gas-charged accumulator connected to the pump for storing pressurized brake fluid, control valves coupled to the accumulator and reservoir for selectively supplying pressurized brake fluid from the accumulator to brake actuators for application of the vehicle brakes and discharging brake fluid from the brake actuators back to the reservoir for release of the brakes, and an electronic control unit (ECU) which evaluates the driver's braking demands and the individual brake pressures and controls the valves and pump accordingly.

Faults associated with the brake fluid itself, such as leakage of brake fluid from the system or air within the brake fluid system can give rise to dangerous conditions, not only for those inside the car but also for pedestrians and other road users. Therefore, in order to maximise system safety, it would be advantageous to be able to have a method for detecting such faults. [ In accordance with the present invention, for providing an indication of brake fluid consumption, a comparison is made during a braking operation of the volume of brake fluid which is taken from the accumulator with the volume of brake fluid which is supplied to the brake actuator. I Advantageously, the EHB system has a first pressure sensing means which provides a measure of the accumulator pressure and the volume of brake fluid which is taken from the accumulator is established using the expressions: VACCU(t) VACCU MAX ( I (PPRECHARGE /P ACCU(t)) ) where: VAccu(t) = volume of brake fluid taken from the accumulator at a point in time t VACCU MAX = the nominal volume of the accumulator, PPRECHARGE = the precharge pressure of the gas in the accumulator, Y = the ratio for specific heats of the gas in the accumulator, and PACCI.J() = the accumulator pressure at the corresponding point in time t as measured by said pressure sensing means.

The system can include means for compensating for the effect of temperature on

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the precharge pressure PPRECI-IARCE The temperature compensation can be based on a measurement of the temperature using a temperature sensor associated with said accumulator pressure sensing means.

Advantageously, the system includes means for correcting for the degradation with time of the accumulator gas pressure.

The correction for degradation of the accumulator gas pressure can be based on a 1 mathematical model and/or a sensor means for determining the value of the precharge pressure with time.

Preferably, the system includes a second pressure sensing means which provides a measure of the pressure at said brake actuator and wherein the volume of brake fluid which is supplied to the brake actuator is established from a knowledge that the latter volume is proportional to the pressure at the brake actuator, as measured by said second pressure sensing means. I In some embodiments, said comparison comprises subtracting from the calculated volume of brake fluid which is taken form the accumulator, the calculated volume of brake fluid which is supplied to the brake actuator. t

In other embodiments, the comparison comprises establishing the ratio of the calculated volume of brake fluid which is taken from the accumulator with the calculated volume of brake fluid which is supplied to the brake actuator.

Advantageously, if the result of the comparison exceeds a predetermined threshold, then a warning is given automatically to the driver and/or a failsafe operating mode of the braking system is actuated.

In the case of the comparison being made when it is detected that the vehicle is cornering or running over rough road, then the threshold level is preferably increased.

In the condition that the pump motor is running, it is preferred that the determination is made of whether: AVACCTJ + AVPUMP AVBRAKE > LIMIT where t2 AVPIJMP = Q (I) dt tl and Q is the pump flow rate.

The invention is described further hereinafter, by way of example only, with r reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic illustration of a typical conventional electrohydraulic braking system (EHB) to which the present application is applicable; I Fig. 2 is a simplified schematic representation of a section of an electrohydraulic braking system for use in explaining the present invention; and Fig. 3 is a graphical representation of the characteristics of the system shown in Fig. 2 under a variety of operational circumstances.

Referring first to Fig. I of the drawings, there is shown a typical EHB system wherein a driver's braking demand is inputted via a foot pedal 10 and detected by a pedal displacement transducer 12 whose output is transmitted to an ECU 13.

The ECU provides control signals for proportional solenoid valves 14a, 14b, 14c, 1 4d which are supplied with hydraulic fluid by means of a pump 16 driven by an electric motor 17, the wheel brakes 1 8a, 1 8b of one axle being supplied with hydraulic fluid by electrically actuated brake channels 20a, 20b and the wheel brakes 1 8c, 1 ad of the other axle being supplied by electrically actuated channels 20c, 20d. Hydraulic fluid for the system is stored in a reservoir 21.

Under normal breaking conditions, brake pressure modulation in the electrically t actuated brake channels 20a, 20b, 20c, 20d is effected in a known ("brake-by wire") manner by means of the proportional solenoid control valves 14a, 14b, 14c and 14d, the brake pressure being provided by a pressure accumulator 22 whose pressure is maintained by the pump 16 operated by the electric motor 17. The accumulator 22 can be selectively isolated by a solenoid operated valve 40 and a manually operable valve 42.

Pressure sensors (not shown) monitor the hydraulic pressure (P2, P3) at the wheel brakes 18a, 18b of the front axle and pressure sensors (not shown) monitor the hydraulic pressure (P4, P5) at the wheel brakes 18c, 18d of rear axle. Further pressure sensors monitor the pressure within a push-through circuit 27 for the right and left front wheel brakes and the supply pressure in the circuit of pump 16.

Respective solenoid valves 29 and 31 enable the brake channels 20a, 20b and 20c, 20d to be coupled together.

The push-through circuit 27 includes solenoid controlled valves 32a, 32b to enable these circuits to be closed (open-circuited) during normal brake by wire operation.

The push-through arrangement includes a master-cylinder 34 coupled to the brake pedal 10 and to the circuit 27, the master cylinder enabling the front brakes to be actuated manually in the event of failure of the brakeby-wire system. Coupled to r the circuit 27 is a travel simulator 38 which is activated hydraulically by master- cylinder pressure to give "feel" to the driver during operation of the brakes in brake-by-wire mode.

In operation, the system is controlled by means of the ECU 13 whereby to control the brake actuators 18a-18d in accordance with the driver's braking demand established by monitoring the actuation of the foot pedal 10 using the travel sensor 12. Fluid pressure for operating the brake actuators is supplied by the pressurised accumulator 12.

The typical EHB system of Fig. 1 is shown in simplified form in Fig. 2. A brake actuator 40 is supplied with brake fluid from an accumulator 42 which is charged by means of a pump 44. The inlet of the pump 44 is connected to a brake fluid reservoir 46. The pump 44 is driven by an electric motor 54 under the control of an ECU 56 which also controls the opening and closing of solenoid valves 48, 50 which selectively connect the brake actuator 40 to the accumulator 42 and reservoir 46, respectively.

A pressure sensor means 52 senses the accumulator pressure PACCIJ' the pressure sensor 52 being electrically connected to the ECU 56. The pump 44 is driven by the electric motor 54 which is electronically controlled by the ECU 56 such that the electric motor 54 is energised for charging the accumulator 42 whenever the

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accumulator pressure PACCU falls below a minimum pressure PACCU MrN (for example 140 bar). The electric motor can be stopped as soon as the accumulator 42 has been charged to its maximum pressure PACCU MAX for example 1 80bar) .

The brake pressure PBRAKE at the brake actuator 40 is sensed by a pressure sensor means 58 which is also electrically connected to the ECU 56. The ECU controls the brake pressure PBRAKE via the electronically controlled valve arrangement 48, comprising a normally-closed 2/2-way inlet valve 50 located between the accumulator 42 and the brake actuator 40 and a normally-open 2/2-way outlet valve 48 located between the brake actuator 40 and the reservoir 46.

The control of the brake pressure PBRAKE is executed as follows: - To increase the brake pressure PBRAICE the normally-closed 2/2-way inlet valve 50 is switched to its open position and the normally open 2/2- way outlet valve 48 is switched to its closed position.

- To hold the brake pressure PBRAKE the normally-closed 2/2-way inlet valve SO is switched back to its closed position and the normally-open 2/2-way outlet valve 48 is left in its closed position.

- To decrease the brake pressure PBRAKE the normally-closed 2/2-way inlet valve 50 is left in its closed position and the normally-open 2/2-way outlet valve * 48 is switched back to its open position.

Fig. 3 shows a schematic timing diagram of an EHB system according to Fig. 2 for explaining the function of such a system by way of example: - At to the system is in a standby mode, i.e. the pump motor 54 is switched off, the inlet valve 50 is closed, the outlet valve 48 is opened, the accumulator pressure PACCU is between the minimum pressure PACCU M1N and the maximum pressure PAcctJ MAX and the brake pressure PBRAKE is zero - At to a braking operation begins by opening the inlet valve 50 and closing the outlet valve 48. As a consequence the accumulator pressure PACCU begins to fall off, while the brake pressure PBRAKE begins to increase.

- At t2, when the brake pressure PBRAICE has reached the value of PBRAICE2, the inlet valve 50 is closed, so that this value is held. Also the falling off of the accumulator pressure PACCU stops.

At t3 the inlet valve 50 is opened again, whereby the accumulator pressure PACCTJ begins to fall off further and the brake pressure PBRAKE further increases.

At t4 the accumulator pressure PACC[] reaches the minimum pressure PACCU MIN The pump motor 54 is therefore turned on to drive the pump 44 for recharging the accumulator 42 by increasing the accumulator pressure PACCTJ.

- At t5 the inlet valve 50 is closed again, so that the value PBRAKE 5 of the brake pressure PBRAICE is held.

At t6, when the accumulator 42 has been recharged to the maximum pressure PACCU MAX the pump motor 54 is switched off.

At t7, when the outlet valve 48 is opened, the braking operation ends, so that the brake pressure PBRAICE steeply decreases. The system is now back in the standby mode as it was at to.

For the purpose of performing the present method, the volume of brake fluid which is taken from the accumulator 42 during a braking operation is compared with the volume of brake fluid supplied to the brake actuator 40. If there is a difference between the two, then this can give an indication of the level of leakage.

The volume of brake fluid which is taken from the accumulator 42, AVACCU, during a time interval from t' to t2 can be calculated as the difference, preferably the absolute difference, between the volume at to (VAcct,(t)) and the volume at t2 (VAccu(t2)) i.e.: t AVACCU = I VACCU(tl) VACCU (t2) 1 (1) Generally, the volume at a point in time VAccu(t) can be calculated by the formula: VACCU(t) VACCU MAX ( I (Ppc /PAccu(t)) ) (2) wherein PrC = the precharge pressure of the accumulator In this formula, VACCU MAx is the nominal volume of the accumulator 42, e.g. 200 cc, which is a constant system parameter (and equal to the precharge volume of the gas in the accumulator).

PA is also a constant system parameter, representing the precharge pressure of the gas in the condition that no brake fluid is stored in the accumulator 42, e.g. 85 bar.

The value is provided by the manufacturer.

Furthermore Y is a constant system parameter determined by the type of gas in the accumulator and corresponds to its ratio of specific heats, e. g. Y = l represents isothermal (very low) changes, while Y = 1.4 represents adiabatic (very fast) changes. In practice Y = 1.2 has turned out to be a good compromise, and is used preferentially in the present model.

Thus VAccu(t) can be directly calculated in dependence from the accumulator pressure, PACCU(t), at time t. Since the accumulator pressure PACCU is directly measured by pressure sensor means 52, a precise calculation of VAccu(t) and thus of AVAccJ can be obtained using formula (2).

To improve the calculation of VAccJ(t) and thus of AVACCIJ the influence of temperature on the precharge pressure PA can be compensated. Since the pressure sensor 52 normally includes a temperature sensor means, this is possible without l additional expenditure.

Calculation of VAccu(t) can be further improved by considering the degradation of the gas. Degradation particularly occurs in accumulators using a diaphragm as a partition between the gas and brake fluid chamber. (It should be noted that accumulators using a metal bellow as the partition are not affected by this problem). Due to the degradation, the precharge pressure Pp' decreases over the lifetime of the accumulator 42. Therefore, it is advantageous if a mathematical model and/or a sensor means for determining the precharge pressure PpC is incorporated into the calculations of the present method.

The volume of brake fluid which is supplied to the brake actuator(s) 40, AVBRAKE, during a time interval t'-t2 can be calculated from the known caliper displacement characteristic of the brake(s) 1. The caliper displacement characteristic is determined by the volume of brake fluid which is supplied to the brake actuator 40, which depends on the brake pressure PBRAKE' i e f VBRAKE(t) = function ( PHRAKE (t) ) ( ) An example of a typical caliper displacement characteristic is shown in Fig. 3.

From this it follows that AVBRAKE can be calculated in the time interval from t' to t2 as the difference, preferably the absolute difference, between the volume at t,, VBRAKE (t,) and the volume at t2, VBRAKE(t2).

VBRAKE = I VsRAKE(tl) VsRAKE(t2) 1 ( ) The caliper displacement characteristic can be implemented in the ECU as a mathematical formula or a look-up table. VBRAKE(t) can be directly calculated depending on the brake pressure pBRAKE(t) at time t. Since the brake pressure PBRAKE is directly measured by pressure sensor means 58, a precise calculation of VBRAKE (t) and thus of AVBRAKE can be obtained.

The comparison of the volume of brake fluid which is taken from the accumulator 42, AVACCU, with the volume of brake fluid supplied to the brake actuator(s) 40, AVRAKF, can be done either by checking, if the difference of VACCU and AVBRAKE exceeds a predefined threshold value V' ANT, i.e. AVACCU AVBRAKL > AVLIMIT (5) t or by checking, if the ratio of AVACCU and AVBRAICE exceeds a predefined percentage threshold value AVI IMlT%,i.e.

AVACCU /AVBRAICE > AVI.IMIT% (6) If the predefined threshold value AVLI5, lIT, AVLIMIT8/O respectively, is exceeded, then more brake fluid as usual has been required for the brake actuator(s) 40. This indicates that the system has a leakage or that air which must be compressed by the additional brake fluid is in the system.

The levels of the thresholds determining AVLIMIT and VLIMIT % can preferably be set individually for a given system.

In the event that the predetermined threshold is exceeded, indicative of an undesired level of leakage and/or air in the system, a warning can be given to the driver by some suitable means, ea. visual and/or audible, and/or the system can be switched automatically into a failsafe mode.

Advantageously, the described fault detection system is applied to the front brakes only so as to perform a check on the push-through circuit.

When it is detected that the vehicle is cornering or running over a rough road surface, for example using information from the vehicle ABS, the thresholds for AVLPV[IT AVLDA,IT6/6 can be automatically increased of the fault detection algorithm disabled altogether during these operational circumstances.

The test is normally made during the period t' - t2 when the pump motor is not running. If the pump motor is running then an additional factor must be taken into account since in this case the determination being made is whether: AVACCU + AVPUh4P AVBRAICE: AVLIMIT where t2 AVPUMP = J Q (t) dt tl in this case Q. the pump flow rate, can be determined from the knowledge that Q is a function of motor speed and the viscosity of the brake fluid. Motor speed can be evaluated by using, for example at least one of the motor current, the motor voltage and the time behaviour of the motor's P,,,,, control signal. The fluid viscosity can be obtained from a mathematical model. In some cases it can be preferable for the aforegoing check to be evaluated over a time period.

Claims

f4Oq / CLAIMS

1. An electrohydraulic braking (EHB) system comprising a brake fluid reservoir, a pump connected to the reservoir for providing a supply of pressurized brake fluid, an accumulator connected to the pump for storing pressurized brake fluid, control valves coupled to the accumulator and reservoir for selectively supplying pressurized brake fluid from the accumulator to a brake actuator for application of a vehicle brake and discharging brake fluid from the brake actuator back to the reservoir for release of the brake, and an electronic control unit (ECU) which evaluates the driver's braking demands and controls the valves and pump accordingly, and wherein, for providing an indication of brake fluid consumption, a comparison is made during a braking operation of the volume of brake fluid which is taken from the accumulator with the volume of brake fluid which is supplied to the brake actuator.

2. An EHB system as claimed in claim 1, having a first pressure sensing means which provides a measure of the accumulator pressure and the wherein volume of brake fluid which is taken from the accumulator is established using the expressions: VACCU(t) VACCU MAX ( I (PPReCuARGE /P ACCU(t)) ) where: VAccu(t) = volume of brake fluid taken from the accumulator at a point in time t VACCU MAX = the nominal volume of the accumulator, PPRECHARGI = the precharge pressure of the gas in the accumulator, = the specific heat ratio of the gas in the accumulator, and PACK = the accumulator pressure at the corresponding point in time t as measured by said pressure sensing means.

3. An EHB system as claimed in claim 2, including means for compensating for the effect of temperature on the precharge pressure PPRECHARGE.

4. An EHB system as claimed in claim 3, wherein said temperature compensation is based on a measurement of the temperature using a temperature sensor associated with said accumulator pressure sensing means.

5. An EHB system as claimed in claim 2, 3 or 4, including means for correcting for the degradation with time of the accumulator gas pressure.

6. An EHB system as claimed in claim 5, wherein said correction for degradation of the accumulator gas pressure is based on a mathematical model and/or a sensor means for determining the value of the precharge pressure with time.

7. An EHB system as claimed in any of claims 1 to 6, having a second pressure 2 sensing means which provides a measure of the pressure at said brake actuator and wherein the volume of brake fluid which is supplied to the brake actuator is established from a knowledge that the latter volume is proportional to the pressure at the brake actuator, as measured by said second pressure sensing means.

8. An EHB system as claimed in any of claims I to 7, wherein said comparison comprises subtracting from the calculated volume of brake fluid which is taken from the accumulator, the calculated volume of brake fluid which is supplied to the brake actuator.

9. An EHB system as claimed in any of claims I to 7, wherein said comparison comprises establishing the ratio of the calculated volume of brake fluid which is taken from the accumulator with the calculated volume of brake fluid which is supplied to the brake actuator.

10. An EHB system as claimed in any of claims 1 to 9, wherein if the result of the comparison exceeds a predetermined threshold, then a warning is given automatically to the driver and/or a failsafe operating mode of the braking system is actuated.

I 1. An EHB system as claimed in claim 10, wherein in the case of the comparison being made when it is detected that the vehicle is cornering or running over a rough road, then said threshold level is increased.

12. An EHB system as claimed in any of claims I to 11, wherein in the condition that the pump motor is running, it is preferred that the determination is made of whether: AVACCU + AVPUMP AVBRAKE > AVLIMIT where t2 VI,u,p = J Q (t) dt t, and Q is the pump flow rate.

13. An EHB system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.