Method for controlling a hydraulic vehicle braking system

Abstract

Method for adjusting the air clearance and / or the brake pedal travel of a hydraulic vehicle brake system (1) having a controllable brake booster (3), wherein the air clearance of wheel brakes (5) is reduced by adjusting an initial position of the brake booster (3), characterized in that a movement of a brake pedal (15) is measured until a measurable pressure increase of a hydraulic pressure in the vehicle brake system (1) and the initial position of the brake booster (13) is adjusted.

Description

Prior ArtThe invention relates to a method for controlling a hydraulic vehicle brake system having a brake booster, having the features of the preamble of claim 1. "control" within the meaning of the invention is also to be understood as a regulation.Hydraulic vehicle brake systems are known per se. They have a master brake cylinder to which one or more wheel brake cylinders of wheel brakes are connected, wherein here, for brevity, only wheel brakes will be mentioned. The actuation is carried out by foot by means of a brake pedal or manually with a brake lever. The master brake cylinder usually has a vacuum brake booster.Hydraulic vehicle brake systems with wheel slip control, for example a (brake) anti-lock control, a traction slip control and / or a driving dynamics control, are also known. The abbreviations ABS, ASR, FDR and ESP are customary for these regulations. For regulation, the vehicle brake systems have wheel brake pressure modulation valve arrangements and one hydraulic pump per brake circuit, with which the wheel brake pressures can be / modulated individually for the wheel, and in simpler systems can also be regulated / modulated partially on an axle-by-axle basis. The hydraulic pump of a wheel slip control is often referred to as a return pump. In such vehicle brake systems, it is possible and known to feed brake fluid into the wheel brakes with the hydraulic pump before a brake actuation in order to reduce a clearance in the wheel brakes, preferably to apply friction brake linings of the wheel brakes to a brake disc, a brake drum or another brake body to be braked. The clearance is a gap between the friction brake linings and the brake body when the brake is not actuated. The friction brake linings can be applied to the brake body without force or pressed against it with a low clamping force. During a subsequent brake actuation, the air clearance does not have to be overcome, a clearance during the brake actuation shortens, the brake actuation takes place more quickly. A further advantage is that a greater actuation path is available for the brake actuation.The reduction of the clearance takes place in response to a possible braking request of a vehicle driver. Such a braking request is typically considered to be a rapid release of an accelerator pedal or accelerator grip of a motor vehicle. "Rapid" in this case means a more rapid, in particular a considerably more rapid, release of the accelerator pedal or accelerator grip than usual, wherein the usual release speed of the accelerator pedal or accelerator grip can be individual to the driver, namely can be measured during the driving operation. In the event of a rapid release of the accelerator pedal or of the throttle grip, a subsequent brake actuation, often also a strong brake actuation, is to be expected. The reduction of the clearance can also take place at each release of the accelerator pedal or accelerator grip independently of the speed of the release. A possible braking request can also be determined by a distance radar which is known for maintaining a speed-dependent minimum distance from a vehicle driving ahead. If the vehicle approaches a vehicle traveling ahead at a considerably higher speed or approaches an obstacle, the clearance in the wheel brakes can be reduced by delivering brake fluid using the hydraulic pump in order to shorten the time of the brake operation.International patent application WO 98 / 43 857 A1 discloses a hydraulic vehicle brake system having a wheel slip control device and having an auxiliary-force-actuatable master brake cylinder which has a controllable vacuum brake booster. "Controllable" means that a pressure difference between a working chamber and a vacuum chamber of the brake booster is controllable by means of a solenoid valve, so that an actuating force can be exerted on the master brake cylinder independently of a muscle-force actuation. This enables a pre-filling of wheel brakes of the vehicle brake system.In order to shorten an actuation path of a master brake cylinder, German Patent Application DE 198 13 031 A1 provides solenoid valves in each brake circuit between a brake fluid reservoir and the master brake cylinder, which solenoid valves are closed in the event of an imminent brake actuation, so that master brake cylinder pistons do not first have to pass sniffer bores of the master brake cylinder in order to hydraulically separate the master brake cylinder from the brake fluid reservoir, but instead displace brake fluid into the vehicle brake system from the beginning of a piston stroke.Disclosure of the InventionThe invention provides a controllable brake booster, wherein controllably means controllability independent of muscle force actuation in order to actuate the brake booster. In particular, the brake booster is electrically or electronically controllable.For adjusting the air clearance and / or adjusting the brake pedal travel, the method according to the invention provides for reducing the air clearance of the wheel brakes by actuating the brake booster independently of a possible braking request. The air clearance adjustment can be carried out individually for each wheel if wheel slip control is present. A constructional clearance of the wheel brakes can be reduced, even individually for each wheel.In response to a possible braking request, the brake booster can be actuated, so that the air clearance in the wheel brakes of the vehicle brake system is reduced, in particular reduced to "zero". A certain prestress of the vehicle brake system, i.e. a pressing of the friction brake linings of the wheel brakes against their brake disks, brake drums or other brake bodies with a low clamping force, i.e. a slight brake actuation, is also possible. The invention shortens the actuation travel during a brake actuation, in particular a free travel is shortened until the vehicle brake system "responds", i.e. brakes. The free travel can be shortened to "zero"; as already stated, even the vehicle brake system can be "prestressed", i.e. a low braking force can be built up.A considerable advantage of the invention is that the return pump, i.e. the hydraulic pump of a wheel slip control, is not used in order to reduce the air clearance of the wheel brakes in preparation for a brake actuation. The service life of hydraulic pumps of wheel slip controls is not designed for use with each brake actuation, but only for rare brake actuations, because wheel slip control occurs rarely in all-day traffic. The reduction of the air clearance with the hydraulic pump of a wheel slip control therefore requires the vehicle brake system to be equipped with hydraulic pumps which have a service life which is several times longer. Brake boosters, on the other hand, are used for each brake actuation and are therefore designed from the outset for a corresponding service life or number of actuations.The dependent claims are subject matter of advantageous embodiments and developments of the invention.As already explained, a release or a rapid release of an accelerator pedal or of an accelerator grip of a motor vehicle equipped with the vehicle brake system can be considered as a possible braking request. The distance to a vehicle driving ahead or to an obstacle, in particular as a function of the driving speed, is another criterion of a possible braking request, wherein the change in the distance, i.e. the speed of approach to the vehicle driving ahead or to the obstacle, can also form the criterion for the possible braking request. Claim 3 provides a sensor system which detects an approach of a driver's foot to a brake pedal. The approach of the driver's foot to the brake pedal is considered a possible braking request, which triggers the actuation of the brake booster in order to reduce the air clearance of the wheel brakes. An optical sensor, for example with a light barrier, an ultrasonic sensor, a capacitive sensor, is possible. The enumeration is not exhaustive; any sensor system with which the approach of the driver's foot to the brake pedal can be recognized is considered for use in the invention.The vehicle brake system may have a valve, by means of which at least one wheel brake is hydraulically separable from a master brake cylinder of the vehicle brake system. Vehicle brake systems with wheel slip control have such valves, namely usually a isolating valve for each brake circuit, with which the brake circuit is connected to the master brake cylinder, and a brake pressure build-up valve for each wheel brake, and in the case of simple wheel slip control systems also a brake pressure build-up valve for the wheel brakes of a vehicle axle. In response to the possible braking request, the valve can be closed and the brake booster can be actuated as described, so that the air clearance of the wheel brakes is reduced. If the valve has a bypass, it can be closed before, during or after the actuation of the brake booster. Such a bypass, namely a nonreturn valve through which flow can take place from the direction of the master brake cylinder in the direction of the wheel brakes, usually has the isolating valve. Without bypass, the valve can be closed only after the brake booster has been actuated and the air clearance of the wheel brakes has been reduced. The brake booster is then reset back into its starting position. An advantage of this embodiment of the method according to the invention is that the brake booster has its initial position at the beginning of a brake actuation, so that the entire actuation path of the brake booster is available.If the valve has a bypass, it does not have to be opened during a brake actuation. However, the valve can be opened upon a brake actuation, wherein the valve is preferably only opened when the pressure in the master brake cylinder is approximately as high or even greater than in the wheel brakes.The method according to the invention can be used in principle with any controllable brake booster. For this purpose, a vacuum brake booster has, for example, a controllable valve for ventilating a working chamber, in addition to a servo valve which is present and which ventilates the working chamber as a function of the path of a piston rod and / or a muscle force exerted on it. Claim 5 provides an electromechanical brake booster which is electrically or electronically controllable on the basis of its construction. Electromechanical brake force boosters are known; reference is made, for example, to the laid-open specification DE 100 57 557 A1.BRIEF DESCRIPTION OF THE DRAWINGThe invention is explained in more detail below with reference to the drawings. The single FIGURE shows a circuit diagram of a hydraulic vehicle brake system for explaining the method according to the invention.Embodiment of the InventionThe hydraulic vehicle brake system 1 according to the invention shown in the drawing has a wheel slip control 2 (anti-lock control ABS; traction slip control ASR; driving dynamics control FDR, ESP). It is designed as a dual-circuit brake system having two brake circuits I, II which are connected to a master brake cylinder 3. Each brake circuit I, II is connected to master brake cylinder 3 via a isolating valve 4. The separating valves 4 are open 2 / 2-way solenoid valves in their normally open basic position. A check valve 6 through which the master brake cylinder 3 can flow to wheel brakes 5 is hydraulically connected in parallel with the separating valves 4. The wheel brakes 5 are connected to the isolation valve 4 of each brake circuit I, II via brake pressure build-up valves 7. The brake pressure build-up valves 7 are open 2 / 2-way solenoid valves in their normally open basic position. Connected in parallel with them are check valves 8, through which the wheel brakes 5 can flow in the direction of the master brake cylinder 3.A brake pressure reducing valve 9 is connected to each wheel brake 5, which are connected together to a suction side of a hydraulic pump 10, which is also referred to as a return pump. The brake pressure reducing valves 9 are designed as 2 / 2-way solenoid valves which are closed in their normally open basic position. A pressure side of the hydraulic pump 10 is connected between the brake pressure build-up valves 7 and the isolation valves 4, i.e. the pressure side of the hydraulic pump 10 is connected via the brake pressure build-up valves 7 to the wheel brakes 5 and via the isolation valve 4 to the master brake cylinder 3. The brake pressure build-up valves 7 and the brake pressure decrease valves 9 are proportional valves because of the better controllability and controllability.Each of the two brake circuits I, II has a hydraulic pump 10, which can be driven together with an electric motor 11. The suction sides of the hydraulic pumps 10 are connected to the brake pressure reducing valves 9. On the suction side of the hydraulic pumps 10, hydraulic accumulators 12 are provided for receiving and temporarily storing brake fluid, which flows out of the wheel brakes 5 by opening the brake pressure reducing valves 9 during a slip control.The brake pressure build-up valves 7 and the brake pressure decrease valves 9 form wheel brake pressure modulation valve arrangements with which, when the hydraulic pump 10 is driven, wheel-specific brake pressure control for slip control is possible in a manner known per se and not to be explained here. The isolation valves 4 are closed during a slip control, i.e. the vehicle brake system 1 is hydraulically isolated from the master brake cylinder 3.The master brake cylinder 3 has a brake booster 13, in the exemplary embodiment an electromechanical brake booster 13, which generates an auxiliary force with the aid of an electric motor 14, which auxiliary force actuates the master brake cylinder 3 together with a muscle force which is applied via a brake pedal 15. The electric motor 14, shown symbolically, is integrated into the brake booster 13. The electric motor 14 can be a rotary motor, the rotational movement of which is reduced by a transmission and converted into a translatory movement for actuating the master brake cylinder 3. An embodiment of the brake booster 13 with an electric linear motor or an electromagnet is also possible. The enumeration is not exhaustive. An electromechanical brake booster 13 is also not absolutely necessary for the invention, and other brake boosters controllable with an electronic control unit 16 are also possible.For controlling or regulating the vehicle brake system 1 including the brake booster 13, the aforementioned electronic control unit 16 is present. A muscle force or pedal force exerted on the brake pedal 15 can be measured with a force sensor 17 and a position and also a speed or acceleration of the brake pedal 15 can be measured with a travel sensor 18. The position of an accelerator pedal or its actuation and, in particular, release speed can be measured with an accelerator pedal sensor 19. Furthermore, a foot sensor 20 is provided, with which a foot 21 of a vehicle driver or the movement thereof in the direction of the brake pedal 15 can be measured. The foot 21 is symbolically represented by a shoe. For the foot sensor 20, for example, an optical sensor or a light barrier, an ultrasonic sensor or a capacitive sensor can be considered. The enumeration is not exhaustive; any sensor or sensor system is possible with which the approach of an object, in the present case the foot 21 of a vehicle driver, to the brake pedal 15 can be recognized. Furthermore, a distance sensor 22 is present, with which the distance of a vehicle equipped with the vehicle brake system 1 from a vehicle driving ahead or from an obstacle can be measured. Such distance sensors 22 are used for speed-dependent distance control. The foot sensor 20 and the distance sensor 22 are provided for specific embodiments or refinements of the method according to the invention.When the vehicle brake system 1 is (yet) not actuated, the brake booster 13 is activated in response to a possible braking request of a vehicle driver. That is, brake booster 13 is actuated without a brake actuation, but rather in anticipation of an imminent brake actuation, and in turn actuates master brake cylinder 3; master brake cylinder 3 is actuated in such a way that a clearance in wheel brakes 5 is reduced; the clearance is preferably eliminated, i.e., friction brake linings of wheel brakes 5 contact brake disks, brake drums or other brake bodies of wheel brakes 5. The friction brake linings can be pressed against the brake bodies almost without force or even with a low clamping force, a low clamping force meaning a slight brake actuation. A possible braking request is considered to be, for example, a removal of an accelerator pedal into an idle position, which is measured by the accelerator pedal sensor 19. The criterion of a possible braking request can also be the speed of the removal of the accelerator pedal, in particular an unusually rapid removal of the accelerator pedal. Likewise, the approach of the foot 21 of the vehicle driver to the brake pedal 15, which can be detected by the foot sensor 20, or the approach of the vehicle to a vehicle driving ahead or to an obstacle, which is measured by the distance sensor 22, can be considered as a possible braking request. The enumeration of circumstances which are considered to be a possible braking request is not exhaustive. By reducing or preferably eliminating the clearance of the wheel brakes 5 already in response to the possible braking request, a subsequent, actual brake actuation takes place more quickly, because no idle travel of the brake pedal 15 is necessary to overcome the clearance of the wheel brakes 5.In response to the possible braking request, not only the brake booster 13 can be actuated, but also either the isolating valves 4 or the brake pressure build-up valves 7 can be closed. Because the isolation valves 4 with the check valves 6 connected in parallel with them have a bypass through which flow can take place in the direction of the wheel brakes 5, the isolation valves 4 can be closed before, simultaneously or even after actuation of the brake booster 13. The brake pressure build-up valves 7 are closed only after actuation of the brake booster 13, that is to say after reduction or elimination of the clearance of the wheel brakes 5, because the check valves 8 connected in parallel with them cannot be flowed through in the direction of the wheel brakes 5. By closing the isolation valves 4 or the brake pressure build-up valves 7, the volume of brake fluid displaced by the master brake cylinder 3 into the wheel brakes 5 by actuation of the brake booster 13 in response to the possible braking request is enclosed in the wheel brakes 5 or the vehicle brake system 1. The brake booster 13 is reset to its initial position. The volume of brake fluid enclosed in the wheel brakes 5 or the vehicle brake system 1 keeps the air clearance of the wheel brakes 5 reduced or eliminated. The isolation valves 4 or the brake pressure build-up valves 7 are opened again if, after actuation of the brake booster 13, a braking operation actually takes place by actuation of the brake pedal 15 in response to the possible braking request. The isolation valves 4 or the brake pressure build-up valves 7 can be opened immediately upon actuation of the brake pedal 15 or even only when the pressure in the master brake cylinder 3 has approximately reached the pressure in the vehicle brake system 1 or in the wheel brakes 5. The isolation valves 4 or the brake pressure build-up valves 7 are opened again even when the possible braking request no longer exists, i.e. for example when a certain period of time has elapsed after releasing the accelerator pedal or the accelerator pedal is actuated again or when the foot 21 of the vehicle driver is removed from the brake pedal 15.According to the invention, an air clearance setting or a brake pedal travel setting is also possible independently of a possible braking request. A clearance of the wheel brakes 5, which clearance is caused by design, for example, is reduced by actuating the brake booster 13. For this purpose, for example, the movement of the brake pedal 15 can be measured with the travel sensor 18 during a brake actuation from the starting position of the brake pedal 15 up to a measurable increase in the pressure of the hydraulic pressure in the vehicle brake system 1. The pressure increase indicates the contact of the friction brake linings of the wheel brakes 5 with their brake bodies. By actuating the brake booster 13, i.e. adjusting its starting position, this path can be shortened and thus the air clearance of the wheel brakes 5 can be reduced. Preferably, when the vehicle is stationary, wheel-specific measurement of the clearance until the friction brake linings of the wheel brakes 5 are applied to their brake bodies is also possible. In this case, a wheel-specific air clearance adjustment is possible individually for the wheel by actuating the brake booster 13, in that all brake pressure build-up valves 7 except for one are closed and the brake pressure build-up valve 7 which remains open is likewise closed after the air clearance adjustment. The method is carried out successively for each wheel brake 5.

Claims

Method for adjusting the air clearance and / or adjusting the brake pedal travel of a hydraulic vehicle brake system (1) having a controllable brake booster (3), wherein the air clearance of wheel brakes (5) is reduced by adjusting an initial position of the brake booster (3), characterized in that a movement of a brake pedal (15) up to a measurable increase in pressure of a hydraulic pressure in the vehicle brake system (1) is measured and the initial position of the brake booster (13) is adjusted.Method according to Claim 1, characterized in that the air clearance is set individually for each wheel by closing all brake pressure build-up valves (7) except for one which is connected to the vehicle brake system (1) by the wheel brakes (5), actuating the brake booster (13) and likewise closing the brake pressure build-up valve (7) which has remained open after the air clearance setting.Method according to claim 2, characterised in that the method is carried out successively for each wheel brake (2).Method according to Claim 1, characterized in that the vehicle brake system (1) has an electromechanical brake booster (13).

Images