Abstract:
A method and a device for compensating for an accumulator pressure in an electrohydraulic braking system of a motor vehicle, in which a valve arrangement controls an input of a hydraulic fluid from a pressure accumulator into wheel brake cylinders, the hydraulic fluid being deliverable by a pump into the pressure accumulator, in which a variable is defined that corresponds to a driving situation and/or a driver&#39;s command and that represents a setpoint wheel pressure with which at least one wheel brake cylinder is chargeable to achieve an optimal braking action, the variable is compared to another variable representing an accumulator pressure in the pressure accumulator to provide a comparison result, and the accumulator pressure is increased in response to the comparison result falling below a predefinable minimal difference between the variable representing the setpoint wheel pressure and the another variable representing the accumulator pressure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for compensating for the accumulator pressure in an electrohydraulic braking system. 
     BACKGROUND INFORMATION 
     An electrohydraulic braking system is described, for example, in German Patent Application No. 195 48 248, which describes controlling the input of hydraulic fluid from a hydraulic pressure accumulator, via valves, into the particular wheel brake cylinders, the accumulator being charged by a pump. 
     German Patent No. 196 44 880 describes a method for operating a vehicle&#39;s braking system, which includes a power brake unit, a pump for building up pressure in the wheel brakes, as well as a brake pedal. An operating force being applied to a brake pedal is recorded in a first step; the variable representing the braking action is recorded in a second step; it is determined in a third step, on the basis of the operating force and the variable representing the braking action, whether the braking action corresponds to the action expected by the brake pedal actuation, the braking action not corresponding to the expected action when the two variables deviate from one another; and a pressure buildup is triggered in a fourth step by actuation of the pump when the braking action does not correspond to the expected action. 
     In electrohydraulic braking systems (EHB), wheel pressures from a hydraulic high-pressure accumulator are applied via valves. Excessive strain on the brakes can lead to degradation of the braking performance (so-called brake fading). Given the same wheel circuit pressure, diminished braking action results in lower (or less effective) vehicle deceleration. When working with conventional braking systems, it is possible to compensate for this effect by applying an increased braking pedal force. In EHB systems, however, it can happen that the system&#39;s hydraulic accumulator pressure does not suffice for applying this necessary wheel pressure. 
     Electrohydraulic braking systems are also discussed in the article entitled “SAE Technical Paper Series 960991”, International Congress &amp; Exposition, Detroit, Mich., Feb. 26-29, 1996. This article discusses, inter alia, ascertaining a brake fading by observing a deceleration control. Provision can be made in such a case to raise the pressure level in the electrohydraulic braking system. It is believed that the method proposed in this article may be relatively expensive, since it may be necessary to compensate brake fading via special deceleration-control devices. 
     SUMMARY OF THE INVENTION 
     An object of an exemplary embodiment of the present invention is, therefore, to provide an electrohydraulic braking system, which will enable the required hydraulic accumulator pressure to be adjusted in a simple manner. 
     This object may be achieved by a method for compensating for the hydraulic accumulator pressure in an electrohydraulic braking system. 
     In accordance with an exemplary embodiment of the present invention, a method is provided for compensating for the accumulator pressure in an electrohydraulic braking system, in which a wheel setpoint pressure is compared in a simple manner to an active accumulator pressure, and in response to falling below (or failing to meet) a predefinable (or preset) minimum difference, the accumulator pressure is raised accordingly. 
     The variable representing the wheel setpoint pressure is expediently determined from a brake pedal travel, and/or a master brake cylinder pressure, and/or from the vehicle speed. These parameters are able to be determined relatively simply and make it possible for one to define an optimal wheel setpoint pressure with adequate precision. 
     It is also beneficial for a motor vehicle driver to be prompted when a compensation of accumulator pressure has taken place. It is believed that this measure raises the safety standard of the motor vehicle, since an occurring brake fading is able to be indicated, allowing the driver, if needed, to correct his driving technique or seek out a service station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an exemplary embodiment of an electrohydraulic braking system, where the hydraulic accumulator pressure is able to be compensated in accordance with an exemplary embodiment of the present invention. 
     FIG. 2 shows a flow chart illustrating an exemplary embodiment of the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The method according to an exemplary embodiment of the present invention will now be explained on the basis of the electrohydraulic braking system depicted in FIG.  1 . Again, it is emphasized that the electrohydraulic braking system depicted in FIG. 1 is merely an example of an electrohydraulic braking system, for which the method according to an exemplary embodiment of the present invention can be used. 
     FIG. 1 shows the important elements of an electrohydraulic braking system. A brake pedal is denoted by  100 . By way of the brake pedal, pressure can be built up in a master brake cylinder  110 . A pedal travel sensor  118  is able to record the movement of the brake pedal. Master brake cylinder  110  is in contact with a pressure fluid supply reservoir  115 . Master brake cylinder  110  is connected to a safety valve  120 , which, when not conducting current, is situated in the position shown. A pedal travel simulator  125  is connected in parallel to the safety valve. Arranged in the connecting line between master brake cylinder  10  and safety valve  120 , i.e., pedal travel simulator  125 , is a pressure sensor  130 , which makes available a signal indicating the pressure PHZ in master brake cylinder  110 . 
     In the non-current-conducting state, safety valve  120  releases the connection between the master brake cylinder and outlet valves  141  and  142 . In their non-current-conducting states, the outlet valves are likewise switched into their flow-through position and release the connection to the wheel brake cylinders. 
     Outlet valve  141  is assigned to wheel brake cylinder VR of the right front wheel, and outlet valve  142  is assigned to wheel brake cylinder VL of the left front wheel. The pressure prevailing in the wheel brake cylinders can be detected by sensors  151 ,  152 . 
     In addition, the wheel brake cylinders are in contact via inlet valves  161  and  162 , and a check valve  170 , with an accumulator  185 . The pressure in the accumulator can be detected by a pressure sensor  180 . Inlet valve  161  is assigned to the right front wheel, and inlet valve  162  to the left front wheel. 
     Furthermore, accumulator  185  is in contact via inlet valves  163  and  164  with wheel brake cylinder HL of the left rear wheel, i.e., with wheel brake cylinder HR of the right rear wheel. The wheel brake cylinders of the left rear wheel, i.e., of the right rear wheel, are in contact, in turn, via outlet valves  143 , i.e., outlet valves  144 , with supply reservoir  115 . 
     Outlet valves  141  and  142  can likewise be brought into contact via safety valve  120 , with supply reservoir  115 . 
     A pump  190 , which is operated by a pump motor  195 , delivers hydraulic fluid from supply reservoir  115  into accumulator  185 . In accordance with the delivery by pump  190 , the pressure prevailing in accumulator  185  is able to be raised or lowered. 
     A control device for controlling the depicted electrohydraulic braking system is schematically denoted by  300 . Through (likewise schematically depicted) lines  310 , control device  300  receives signals, which describe the active state of the motor vehicle, i.e., of the braking system, for example pressure signals from pressure sensors  118 ,  130 ,  151 ,  152 ,  153  and  154 . In addition, control unit  300  can receive other signals, for example pedal travel or the motor vehicle&#39;s speed, which are determined by appropriate sensors. Through additional lines  320 , likewise schematically shown, control unit  300  drives the valves in question, i.e., pump motor  195 , to operate the electrohydraulic braking system. For the sake of clarity, the signal lines going out from each of the components (pressure sensors, valves, motor) are not shown individually. 
     The depicted system operates in the following manner: during normal operation, safety valve  120  conducts current. Safety valve  120  releases the connection between supply reservoir  115  and the outlet valves, and interrupts the connection between master brake cylinder  110  and the outlet valves. If the driver actuates brake pedal  100 , then sensor  118  detects a signal corresponding to the pedal travel of brake pedal  100 , and/or sensor  130  supplies a pressure signal with respect to the pressure prevailing in the master brake cylinder. 
     On the basis of at least one of these signals, which correspond to the driver&#39;s command, and possibly to other operating parameters (such as the vehicle speed), control unit  300  defines a setpoint braking pressure, which is to be, i.e., should be applied to the wheel brake cylinders, and produces corresponding driving signals to be received by inlet valves  161 ,  162 ,  163 , and  164 , as well as by outlet valves  141 ,  142 ,  143 , and  144 . 
     By opening inlet valves  161  through  164 , and closing outlet valves  141  through  144 , the pressure prevailing in the wheel brake cylinders is increased as a function of the driver&#39;s command. The pressure required for this is made available by accumulator  185 , in which a preset normal system pressure prevails. The normal system pressure prevailing in accumulator  185  is reduced in response to the inlet valves opening to produce the desired wheel setpoint pressure. To re-establish or maintain the normal system pressure, current is applied to pump motor  195  to drive pump  190 , and hydraulic fluid is delivered by supply reservoir  115  into accumulator  185 . 
     By opening the outlet valves and closing the inlet valves, the pressure prevailing in the wheel brake cylinders can be reduced again in conformance with the pedal actuation. 
     In the case of brake fading at the wheel brake cylinders, it can happen that the determined “optimal” wheel setpoint pressure is very close to the normal system pressure, since it is necessary for a higher pressure to be applied to the wheel brake cylinders to achieve the same braking action. The situation can even arise that the setpoint wheel pressure reaches or exceeds the normal system pressure. To recognize this potentially dangerous situation, the setpoint wheel pressure determined by control unit  300  is compared to the accumulator pressure prevailing in accumulator  185 . If the difference between these two pressure values falls below a predefined minimum value, pump  190  is driven by motor  195  to increase the normal system pressure prevailing in accumulator  185 . A comparison of this kind can be carried out, for example, in regular intervals in response to an established braking command, but also without the existence of a specific braking command. 
     This method is explained once more on the basis of the flowchart of FIG.  2 . In a step  101 , the variable describing the setpoint wheel pressure is defined. Subsequently, in a step  102 , the defined variable is compared to the variable describing the accumulator pressure prevailing in accumulator  185 . When the difference between these two variables falls below the minimum predefinable difference, i.e., a predefinable minimum value, the accumulator pressure prevailing in the accumulator is increased, as described, in a step  103 . If, in carrying out step  102 , the difference between the two variables does not fall below the predefinable minimum difference, then the system branches back to step  101 . 
     Once step  103  is carried out to increase the accumulator pressure prevailing in accumulator  185 , the system branches back to step  102 , where the increased accumulator pressure is compared once again to the variable describing the setpoint wheel pressure. As indicated by the dotted line, it is also possible for this system to branch back directly from step  103  to step  101 . 
     In the case of an increase in the accumulator pressure, it is preferred that this be indicated to the driver of the motor vehicle in a step  104 . 
     This measure of increasing the accumulator pressure, i.e., the normal system pressure, will be further explained on the basis of two typical situations: if, for example, during vehicle travel (vehicle speed greater than a parameterizable minimum speed), in response to a pedal actuation by the driver, a specific setpoint wheel pressure is established, which is close to the active accumulator pressure, i.e., normal system pressure (for example, setpoint wheel pressure  130  bar, accumulator pressure  150  bar), the accumulator pressure prevailing in accumulator  185  is increased by driving pump  190  in such a way that ensures that the desired setpoint wheel pressure is adjustable on the basis of the accumulator pressure. It is conceivable, for example, to adjust the minimal difference between the accumulator pressure and the wheel setpoint pressure in such a way that, to ensure a reliable braking operation, even in the case of an additional, sudden increase in the setpoint wheel pressure, a sufficient “pressure buffer” remains between the accumulator pressure and the setpoint wheel pressure. 
     If, in addition, it is recognized, for example, in the case of low vehicle speeds, that, on the basis of an elevated setpoint wheel pressure at the driven axle and on the basis of the information from a traction control system (ASR), that an ASR control is active, in which the setpoint wheel pressure is approximately equal to the accumulator pressure (for example when starting from rest on a slope having different coefficients of friction on each side of the road), then the accumulator pressure in accumulator  185  is likewise increased (for example from a normal pressure of about  160  bar to about  180  through  200  bar, depending on the braking system). 
     Therefore, using the method according to an exemplary embodiment of the present invention, the operational state of a brake fading, for example, can be ascertained in a simple and reliable manner, since, under normal operational conditions, i.e., fully operational brakes, the setpoint wheel pressure always observes a sufficient difference from the normal accumulator pressure in accumulator  185 . The operational state “brake fading” can be indicated to the driver, so that he can correct his driving technique accordingly, for example by driving more slowly, by applying the engine brake, or, if necessary, by seeking out a service station. 
     Therefore, the method according to an exemplary embodiment of the present invention makes it possible in a simple and reliable manner to compensate for a fading braking action in accordance with a conventional braking system, by increasing the accumulator pressure. In this manner, the safety and availability of an electrohydraulic braking system is increased; in addition, the driver receives additional information on the status of the braking system of his vehicle.