DE19600603B4 - Hydraulic brake system for a motor vehicle - Google Patents

Abstract

Hydraulic brake system for a motor vehicle
with a master brake cylinder, to which the wheel brakes for the wheels of the front axle and rear axle are connected via brake lines, the wheel brakes of the wheels being connected to low-pressure accumulators via return lines,
with inlet valves arranged in the brake lines and outlet valves arranged in the return lines, check valves being connected in parallel with the inlet valves and opening towards the master cylinder,
with a control unit (60) for actuating the valves in order to be able to set a pressure in the wheel brakes which is independent of the master cylinder pressure,
characterized in that for emptying that low pressure accumulator (37) associated with the wheel brakes of the rear axle, the associated exhaust valve (34) is opened after braking and the associated intake valve (20) is kept closed so that pressure fluid from the low pressure accumulator (37) via the parallel to the inlet valve (20) connected check valve (23) flows to the master cylinder (1).

Description

The The invention relates to a hydraulic brake system for a motor vehicle, according to the generic term of claim 1.

With such a brake system, inter alia, an electronic brake force distribution can be realized. From the DE 40 34 112 A1 For example, a generic anti-lock hydraulic multi-circuit brake system is known.

A brake force distribution is understood to mean the following:
In order to brake a vehicle stable track, it is necessary to design the hydraulic actuation of the wheel brakes so that higher braking forces are transmitted to the brakes of the front axle, as to the brakes of the rear axle. This avoids that the wheels of the rear axle temporally blocked before the wheels of the front axle. In addition, the hydraulic brake system should be designed so that at the wheels of both axes at each deceleration of the vehicle as equal adhesion utilization is achieved.

Under these premises receives a so-called ideal brake force distribution. The actual Brake force distribution is generally adjusted so that the braking force at the rear axle is smaller than the ideal brake force distribution would allow.

In This is achieved by a so-called brake force control valve in conventional brake systems realized, which is installed in the brake line to the rear axle.

It has also been suggested in brake systems with valves and pumps are provided for regulating the pressure in the wheel brakes, the electronic brake force distribution by controlling the valves to realize and to a separate brake force control valve without.

The mentioned Pressure control systems are also referred to as anti-lock braking systems, since they are used to block the wheels during a braking operation to prevent.

she generally consist of an inlet valve in the brake line and a return pump, the pressure fluid from the wheel brakes in the master cylinder promotes. Between the wheel brake and the suction side of the pump is a so-called. Auslaßventil and a low pressure accumulator provided, the amount of pressure medium picks up over the exhaust valve is released from the wheel brake and not immediately by the pump be promoted back can.

If with such a system an electronic brake force distribution is realized is generally waived, the pressure medium zurückzufördern by means of the pump in the master cylinder, as this with a considerable noise connected is. This can be for the rarely occurring decelerations of the vehicle with high delay, which would lead to a locking of the wheels without brake pressure control, accepted become; not for the common occurring braking in the partial braking area in which the electronic Brake force distribution begins.

There the regulation of the brake pressure during a electronic brake force distribution can be quite rough needs the wheel brake only little pressure fluid are removed during braking can be accumulated in the low-pressure accumulator.

To a brake must the Low-pressure accumulator be emptied, however, so he at a following braking again can absorb pressure medium.

So far became the exhaust valve open, so that Pressure medium from the low-pressure accumulator via the outlet valve the inlet valve and the central valves in the master cylinder to the reservoir of Brake system can flow back. This however, has the following disadvantage. If during such an emptying process, which can take up to 2 seconds, a braking takes place, that would Pressure medium, by an actuation of the Master cylinder are displaced from the master cylinder into the wheel brakes should, led into the low-pressure accumulator, which has the consequence that the operating pedal falls through and no more brake pressure can be built. You could Now imagine that with Help of a sensor or switch, e.g. the brake light switch, it is determined whether a braking is initiated, and the exhaust valve to close again, as soon as a corresponding signal is present. But that depends the safety of the brake system from the functioning of the brake light switch from.

The The invention is therefore based on the task of a safe brake system to create, in which the low-pressure accumulator safely emptied can.

To It is proposed to drain the low-pressure accumulator, the exhaust valve to open and the inlet valve close, the pressure medium over a parallel to the inlet switched check valve drain to the master cylinder can.

He follows in this case during the emptying of the low-pressure accumulator braking, so can too here due to the signal of the brake light switch the original State, i. closed exhaust valve and open intake valve be restored. But should not the brake light switch work, so would have This merely means that the Brake line to the rear axle for the short emptying time is locked. After this time namely switch the valves in their original one Standing back, So that the Wheel brakes of the rear axle can be pressurized. The delay in pressure build-up, since they only affect the brakes of the rear axle, be accepted.

advantageously, the emptying of the low pressure accumulator is caused by a signal, that triggered will, as soon as the vehicle over is accelerated beyond a certain threshold.

This Acceleration signal is from a sensor with a downstream Evaluation unit generated. The sensor can be a Act acceleration sensor. Preferably, the acceleration signal becomes but from wheel sensors that sense the angular velocity of the wheels, derived. These wheel sensors are also used for anti-skid control needed.

The Of course, emptying is needed only to be initiated if previously an electronic brake force distribution took place. For this purpose, in the control unit a corresponding Condition signal set as soon as during an EBV control outlet valve open becomes.

at a brake force distribution, in which the wheel brakes of the rear axle are connected together to a brake circuit is generally the Brake pressure in the wheel brakes regulated together. It is a single one Valve pair consisting of an inlet and an outlet valve intended. For emptying these two valves are switched. In a so-called. Diagonalaufteilung, are the wheel brakes the rear axle in different brake circuits and both have each a valve pair. The case may occur that the electronic brake force distribution only the valves of a wheel brake of the rear axle are controlled so that too only the corresponding one Low pressure accumulator filled becomes. Nevertheless, in the emptying phase both valve pairs controlled, so that in case that one Braking during the emptying phase is done and not by a response of the brake switch is interrupted, both wheel brakes of the rear axle are separated from the master cylinder and only after the expiration of the emptying time be connected to the brake circuits. This avoids that the vehicle when braking "wrong pulls ".

The Invention will be described below with reference to a hydraulic circuit diagram explained in more detail.

The figure shows in the upper part of a tandem master cylinder 1 , which has two working chambers 2 . 3 features. These working chambers are via central valves 4 . 5 with a storage container 6 connected. The representation of the central valves 4 . 5 takes place here purely schematically. In fact, these valves are generally located in the pistons of the master cylinder, which are the chambers 2 . 3 limit.

To the chambers 2 . 3 close brake lines 10 . 11 on, with the brake line 10 to the wheel brakes 12 . 13 the rear axle leads while the other brake line 11 branches, with each branch to each wheel brake 14 . 15 the front axle leads.

In the brake line to the rear brakes and in the branch lines to the front brakes is ever an inlet valve 20 . 21 . 22 provided, to each of which a check valve 23 . 24 . 25 is connected in parallel.

The Wheel brakes are still over a return line with a return pump connected.

The rear wheel brakes 12 . 13 have a common return line 30 on that to a first pump 31 leads. The wheel brakes of the rear axle 14 . 15 are over a branching return line 32 with a second pump 33 connected. In the return line 30 and the branches of the return line 32 is ever an exhaust valve 34 . 35 . 36 arranged. At every return line 30 . 32 closes a low-pressure accumulator 37 . 38 at.

The pumps 31 . 33 are over pressure lines 40 . 41 with the brake lines 10 respectively. 11 connected between the master cylinder and the respective intake valve.

The pumps 31 . 33 are powered by a common motor.

The brake system is equipped with additional valves that allow the wheel brakes 14 . 15 to supply the front axle independent of an actuation of the master cylinder with pressure medium. This makes it possible to perform a traction control. A line connects to it 50 the brake line 11 to the front brakes with the suction side belonging to this brake circuit pump 33 , being in this line 50 a changeover valve 51 is provided. This is normally open and closes due to pressure in the brake pipe 11 , It is also located between the junction of the pressure line 40 in the brake line 11 and in the master cylinder 1 a separating valve 52 , which blocks in the case of traction control, so that the pumped by the pump pressure fluid is not in the master cylinder 1 flowing back. With this isolation valve 52 is a pressure relief valve 53 connected in parallel, which determines the pump pressure in the ASR case.

The system is controlled by means of an electronic evaluation unit 60 , including the signals of a brake light switch 61 contains. This switch registers the movement of the brake pedal 62 for actuating the master cylinder 1 , In addition, the evaluation unit, which is not shown here, receives the signals of the wheel sensors. It generates signals for the intake and exhaust valves, which are actuated electromagnetically. The intake valves are normally open, the exhaust valves closed normally.

An electronic brake force distribution is now carried out in such a way that when braking (brake light switch 61 closed) to prevent further pressure increase in the wheel brakes of the rear axle, the inlet valve 20 is closed. If it becomes necessary to lower the pressure, the exhaust valve becomes 34 open, with pressure medium from the connected wheel brakes 12 . 13 in the low-pressure accumulator 37 flows.

After completion of braking the low-pressure accumulator must be emptied, this is done according to the invention in that the control unit 60 both a signal for the exhaust valve 34 as well as for the inlet valve 20 generated. As a result, the exhaust valve opens and the intake valve closes. The spring in the low-pressure accumulator now presses this in the low-pressure accumulator 37 contained pressure medium via the open outlet valve, the check valve 23 , which is parallel to the inlet valve 20 is switched to the corresponding chamber 2 of the master cylinder 1 and over the open central valve 4 in the reservoir 6 , The inlet valve 20 is therefore closed, because it is conceivable that during this discharge time, the brake is actuated.

Is the brake light switch 61 okay, the electronic evaluation unit generates a reset signal during brake actuation, that the inlet valve and exhaust valve switches back, so that the brake line is released. The pressure medium that in the low pressure accumulator 37 is left, is taken after completion of this braking by re-initiating a discharge process the low-pressure accumulator.

Should while such a braking has not been regulated, it takes subsequent Emptying process only the remaining emptying time. Otherwise is for the emptying process again set the full scheduled time.

Should the brake light switch 61 be defective, can the electronic evaluation unit 60 do not output a reset signal. This would mean that the inlet valve remains closed, so that at least for the remainder of the ongoing emptying time, the wheel brakes of the rear axle can not be supplied with pressure medium. However, this can be tolerated because the wheel brakes of the rear axle usually do not contribute much to the overall deceleration of the vehicle.

At the Expiration of a specified emptying time ends the electronic Control unit the emptying process by the intake and exhaust valve switches back again, so now for the Rest of the braking applied to the rear brake normally with pressure can be.

Should while such braking, whose introduction is not registered by means of the brake light switch can be initiated, anti-skid control is so generated with the initiation of the control a reset signal. The Emptying of the low-pressure accumulator is then mittes a corresponding Program that is part of the anti-skid control.

The special feature of the system is also that the discharge phase is not activated due to the brake light switch, but due to an acceleration signal. This acceleration signal is in the electronic evaluation unit 60 generated and output based on the wheel sensor signals when the calculated vehicle acceleration is above a threshold. Although there is a risk here that the wheel sensors fail. However, this would mean that the electronic braking force distribution as a whole would fail. In this circumstance, the driver is warned by appropriate warning devices.

Claims (7)

Hydraulic brake system for a motor vehicle with a master cylinder to which the wheel brakes for the wheels of the front and rear axles are connected via brake lines, the wheel brakes of the wheels being connected via return lines to low pressure accumulators, with intake valves arranged in the brake lines Tilen and arranged in the return lines exhaust valves, wherein parallel to the intake valves check valves are connected, which open to the master cylinder, with a control unit ( 60 ) for actuating the valves in order to be able to set a pressure independent of the master cylinder pressure in the wheel brakes, characterized in that for emptying that low-pressure accumulator ( 37 ), which is assigned to the wheel brakes of the rear axle, after braking the associated exhaust valve ( 34 ) and the associated inlet valve ( 20 ) is kept closed, so that pressure medium from the low pressure accumulator ( 37 ) via the parallel to the inlet valve ( 20 ) switched check valve ( 23 ) to the master cylinder ( 1 ) drains off. Hydraulic brake system according to claim 1, characterized in that the switching state of the valves ( 34 . 20 ) is maintained for emptying the low-pressure accumulator over a predetermined period of time (emptying time). Hydraulic brake system according to claim 1 or 2, characterized in that a Sensor is provided for detecting the vehicle acceleration, the controller for draining the low-pressure accumulator only then is initiated when the vehicle acceleration over a certain threshold. Hydraulic brake system according to claim 3, characterized in that the circuit of the valves for emptying the low-pressure accumulator is only carried out when the exhaust valve ( 34 ) has been switched to control the braking force distribution. Brake system according to one of the preceding claims, characterized in that the brake system has a brake light switch ( 61 ), which registers the master cylinder actuation, wherein the emptying of the low-pressure accumulator is canceled as soon as the brake light switch ( 61 ) appeals. Brake system according to one of the preceding claims, characterized characterized in that Wheel brakes of the rear axle assigned to different brake circuits are, and that everyone Wheel brake of the rear axle, an intake and an exhaust valve and a low-pressure accumulator is associated with, for emptying of at least one low-pressure accumulator both valve pairs are controlled. Brake system according to Claim 5, characterized in that, after an emptying of the low-pressure accumulator (interrupted by braking), 37 ) is emptying time limited to the remainder of the remainder of the emptying time after this braking.