Abstract:
The present invention relates to a method for regulating a predetermined modifiable brake pressure in the wheel brakes of a brake system, wherein input quantities determining the brake pressure in the individual wheel brakes are evaluated and correcting variables of hydraulic valves are defined in a control and/or data processing system. To obviate the need for additional pressure sensors which register the pressure in the wheel brakes, it is arranged for by the invention that a characteristic curve is stored in the control or data processing system, said curve correlating the valve characteristics of the hydraulic valve with a pressure difference of the hydraulic valve, and the correcting variable of the hydraulic valve is defined according to the characteristic curve.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for regulating a predetermined modifiable brake pressure in the wheel brakes of a brake system, wherein input quantities determining the brake pressure in the individual wheel brakes are evaluated and correcting variables of hydraulic valves are defined in a control and/or data processing system. 
   BACKGROUND OF THE INVENTION 
   Vacuum brake boosters require a vacuum supplied by the engine for boosting the pedal force to be generated by the driver. Depending on the engine, even relatively low pedal forces allow reaching a condition where further increase of the force applied to the actuating unit is possible only by an increase in the pedal force because the vacuum brake booster has reached the maximum possible boosting force. This condition is referred to as the point of maximum boosting of the booster. Braking operations that take place in excess of the point of maximum boosting of the vacuum brake booster place high demands on the driver in terms of the pedal force to be generated. For this reason, brake systems (OHB-V) are employed at an increasing rate supporting the driver by means of an active hydraulic brake pressure build-up. Brake systems of this type generally include a motor-and-pump assembly and a hydraulic unit being controlled by an electronic control such as ESP, ABS, TCS, and similar systems, to comply with the desired brake force boosting. 
   When pressure is built up in the wheel brakes by means of hydraulic boosting beyond the point of maximum boosting of the vacuum brake booster, the (analogized) separating valves are closed, and the hydraulic pump delivers brake fluid from the tandem master cylinder (TMC) into the wheel brakes. In the pressure reduction phase, it is required to discharge the additionally generated pressure into the tandem master cylinder by way of the separating valves in a manner comfortable with respect to pedal feeling and braking effect. This action usually requires two additional pressure sensors at the wheel brakes, one per brake circuit, in order to determine the wheel pressure. The result is a higher requirement of components causing an increase in the system&#39;s costs. 
   SUMMARY OF THE INVENTION 
   In view of the above, an object of the invention is to provide a method for regulating a predetermined variable brake pressure in the wheel brakes of a brake system, wherein brake pressures can be adjusted according to input quantities without any knowledge about a measured wheel pressure. 
   According to the invention, this object is achieved in that a characteristic curve is stored in the control or data processing system, said curve correlating the valve characteristics or the valve current of the hydraulic valve with the pressure difference of the hydraulic valve, and the correcting variable of the hydraulic valve is defined according to the characteristic curve. Advantageously, hydraulic boosting of the brake pressure introduced into the brake system is brought about by means of the predetermined variable brake pressure. 
   It is favorable that the correcting variable is an electric valve current by means of which the hydraulic valve is controlled in analog manner. 
   With the knowledge of the valve characteristic curve of a separating valve, the tandem maser cylinder pressure and the nominal pressure for the wheel brakes, it is possible to define and adjust a valve current in such a fashion that a defined wheel pressure is achieved, and falling short of said pressure will not occur without the need for additional pressure sensors measuring the wheel pressures. 
   Advantageously, the correcting variable is determined from the pressure difference between the nominal pressure for the wheel brakes and the pressure of an actuating device (pressure difference of the hydraulic valve). The pressure of the actuating device (TMC=tandem master cylinder) is determined by way of a pressure sensor provided in a brake circuit in an ESP control, and the nominal pressure is determined in a known model. 
   To reduce leakages, on the one hand, and allow limiting the maximum pressure in response to the situation, on the other hand, at least two strategies are provided for regulating the brake pressure. While the valve current follows the course of the characteristic curve according to the first strategy, the second strategy arranges for jump functions at the maximum actuating current or the actuating current represented by the characteristic curve. The maximum actuating current is limited corresponding to a function according to I max =f (P max , P TMC , P PLV )=min(P max , P TMC , P PLV ) (PLV=pressure limiting valve). 
   A device of the invention for implementing the method is designed so that the brake pressure generated by actuation of a pump is controlled in such a way that a desired hydraulic boosting of the introduced brake pressure is achieved in each brake circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of the invention is illustrated in the accompanying drawings and will be described in detail in the following. 
     In the drawings, 
       FIG. 1  is a view showing a brake system with two brake circuits. 
       FIG. 2  is a schematic view of the signal variation of the actuating current of a hydraulic valve and the pressure according to the method of the invention. 
       FIG. 3  is a characteristic curve of the invention. 
       FIG. 4  is a schematic view of the signal variation of the actuating current with an additional energization. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The one brake circuit of a brake system for motor vehicles with two brake circuits as shown in  FIG. 1  consists of an actuating unit  1 , e.g. a brake cylinder, and a brake force booster  2  actuated by a brake pedal  3 . Arranged at the actuating unit  1  is a supply tank  4  that contains pressure fluid volume and is connected to the working chamber of the actuating unit in the brake&#39;s release position. The one brake circuit shown includes a brake line  5  connected to a working chamber of the actuating unit  1  and containing a separating valve  6  that provides in its inactive position an open passage for the brake line  5 . The separating valve  6  is typically actuated electromagnetically. 
   The brake line  5  branches into two brake lines  8 ,  9  respectively leading to a wheel brake  10 ,  11 . Brake lines  8 ,  9  respectively contain an electromagnetically operable inlet valve  12 ,  19 . Said valves are open in their inactive position and can be operated to adopt their closed position by energizing of the actuating magnet. Connected in parallel to each inlet valve  12 ,  19  is a non-return valve  13  that opens in the direction of the brake cylinder  1 . A so-called return circuit that comprises return lines  15 ,  32 ,  33  with a pump  16  is connected in parallel to wheel brake circuits  26 ,  27 . By way of each one outlet valve  14 ,  17  and through return lines  32 ,  33 , the wheel brakes  10 ,  11  are connected to the return line  15  and, hence, to the suction side of the pump  16  whose pressure side is connected to the brake pressure line  8  in a mouth E between the separating valve  6  and the inlet valves  12 ,  19 . 
   Pump  16  is designed as a reciprocating piston pump with a pressure valve (not shown) and a suction valve. A low-pressure accumulator  20 , composed of a housing  21  with a spring  22  and a piston  23 , is arranged at the suction side of the pump  16 . 
   A biased non-return valve  34  opening towards the pump is interposed into the connection between the low-pressure accumulator  20  and the pump  16 . 
   The suction side of the pump  16  is further connected to a low-pressure damper  18  by way of a suction line  30  and to the brake cylinder  1  by way of a change-over valve  31 . In addition to the hydraulic unit  43 , the brake force transmission circuit also includes a device  28  for controlling the brake system. Said device basically is an ESP control unit  45 , associated with which is a model  41  for determining the nominal brake pressure and an accumulator  42  for storing the valve characteristic curve that describes the valve current and the corresponding difference in pressure at which the separating valve  6  opens. The pressure sensor  40  detecting the pressure of the actuating unit  1  is arranged in the brake line  5  between the brake cylinder  1  and the change-over valve  31  or the separating valve  6 , respectively. Wheel speed sensors associated with the wheels are designated by reference numerals  50 ,  51 . Input quantities that are sent to the ESP control unit  45 , such as the signals of the rotational speed sensors, at least one yaw sensor, of one acceleration sensor or pressure sensor  40  are exemplarily designated by reference numerals  55  to  57 . 
   The brake system operates as follows: 
   When braking, for example, the driver increases the brake pressure in the hydraulic unit  43  by way of the pedal  3  and the actuating unit  1  with the vacuum brake booster  2 , without the vehicle decelerating corresponding to the pedal force. When braking by pedal depression, the device  28  evaluates the pressure of the actuating unit  1  determined by the pressure sensor  40  or the brake pressure introduced into the brake line  5 . When the pressure reaches a limit value which describes the maximum boosting pressure of the actuating unit or the point of maximum boosting of the vacuum brake booster  2 , the transition from the pneumatic brake force boosting by way of the vacuum brake booster  2  to the active brake force boosting by means of the pump  16 , is carried out in particular according to the relation P nominal =P point of max. boosting +factor K×(P TMC −P point of max. boosting ), with P nominal =nominal pressure, P point of max. boosting =pressure at the point of maximum boosting of the brake booster, P TMC =pressure of the actuating unit. To this end, the (analogized) separating valves  6  are closed in the pressure build-up with the change-over valve  31  open, and the hydraulic pump  16  delivers brake fluid out of the brake cylinder  1 , e.g. a tandem master cylinder (TMC) into the wheel brakes  10 ,  11 . The inlet valves  12  or  19 , respectively, are open, while the outlet valves  14  or  17 , respectively, are closed. In the pressure reduction phase, the additionally produced pressure is discharged through the analogized separating valves  6  into the brake cylinder  1  in a manner that is comfortable with respect to pedal feeling and braking effect. 
     FIG. 2  shows a schematic view of the signal variation of the actuating current I 0 , I of the separating valve  6  and the differential pressure P 0 , P TMC , P nominal . 
   This actuating current is determined from the difference between the nominal pressure P nominal  for the wheel brakes  10 ,  11  and the measured pressure of the brake cylinder (TMC)  1 . If a higher pressure prevails in the brake cylinders of the wheel brakes  10 ,  11 , the separating valve  6  will open, and fluid propagates back into the brake cylinder  1  until the desired pressure has been adjusted in the hydraulic unit  43 . When pressure balance prevails between the lines  5  and  8 ,  9  or between the inlet and outlet of the separating valve  6 , the separating valve  6  will close and the pressure is maintained. To speed up the adjustment operation, the nominal pressure gradient of the brake pressure P nominal  can be evaluated and, hence, the valve current I modified accordingly. This achieves a greater volume flow through the separating valve  6 . This means, the reaction to quick pressure increase requirements is that the separating valve  6  is opened to a larger extent. 
   To actuate the separating valve  6 , the valve characteristic curve  70  ( FIG. 3 ) is stored in non-volatile memory  42  and describes the valve current I 0 , I, I max  and the corresponding pressure difference P 0 , P nominal , P TMC , at which the separating valve  6  opens. Said characteristic curve  70  can be determined by a measurement of the valves or by a calibration at the end of the assembly line. The calibration at the end of the assembly line represents a favorable variant because the complete chain of influencing factors (drivers, coils, valve, etc.) is included in the stored characteristic curve at this point. Said characteristic curve permits controlling the pressure produced by actuation of the pump  16  in such a fashion that the desired hydraulic boosting can be adjusted to an appropriately greater accuracy without the driver being able to notice the difference compared to the controlled system (with wheel pressure sensors). 
   Principally, braking operations can be subdivided into the sections pressure increase, maintaining the pressure constant, and pressure reduction. In each of the braking cycles, two strategies are generally provided to regulate the brake pressure. According to the first strategy, the brake pressure follows the course of the characteristic curve  70 , while the second strategy arranges for jump functions at the maximum actuating current or the actuating current represented by the characteristic curve. 
   I. Pressure Increase 
   Strategy A: The valve current I used to adjust the separating valve  6  is modified corresponding to the characteristic curve I=I (P nominal ) shown in  FIG. 3 . 
   Advantage: The pressure adjusted in wheel brakes  10 ,  11  is limited in its magnitude. When the pump  16  builds up too much pressure in at least one brake circuit, the portion in excess of the nominal pressure P nominal  can flow off through the separating valve  6 . This case can occur especially at the rear axle in brake systems with black-and-white circuit allotment. 
   Possibility B: Jump to the making current for differential pressure zero (I 0 ) and, subsequently, a quick ramp  1  to the maximum current I max , according to the relation I max =f(P max , P TMC , P PLV )=min (P max −P TMC , P PLV ) with P PLV =pressure limiting valve pressure shut off, P max =maximum pressure in the brakes, P TMC =pressure of the actuating unit. A zero point adjustment is required because the correlation between the pressure difference at the separating valve  6  and the current at which the separating valve  6  starts to open at this pressure difference is stored in the controller. Once open, the separating valve  6  will close due to hysteresis effects, such as friction, only when a lower pressure difference prevails again. To safely close an open separating valve  6  again, it must be energized corresponding to a pressure difference that is higher by a pressure value, e.g. 30 bar approximately. 
   Advantage: The separating valve  6  exhibits only low leakages. There are no closing noises. 
   Depending on the situation, the optimally suitable of the two strategies is chosen per brake circuit. Strategy A is used with greatly different volume characteristic curves, for example
         Systems with black-and-white circuit allotment   With ESP—OHB (optimized hydraulic brake) superposition   With ABS—OHB superposition       

   In this arrangement, the maximum wheel pressure can be predefined, with the pump running. 
   Otherwise, strategy B is used to keep the separating valve  6  closed in the best possible manner. 
   II. Maintaining the Pressure Constant 
   Possibility A: 
   The valve current I used to adjust the separating valve  6  is modified corresponding to the characteristic curve I=I (P nominal ) illustrated in  FIG. 3 . 
   Advantage: The real pressure is limited in an upward direction. Excess pressure can be reduced. This may be necessary in the event of superposition with other control functions of the device  28  such as ESP, ABS, etc. 
   Possibility B: 
   The valve current remains at I max . 
   Advantage: The valve is maximally seal-tight. 
   A pressure limitation of the hydraulic unit and the wheel brakes takes place at a predefined pressure of the pressure limiting valve. PLV pressure P PLV  implies a pressure limitation that prevents a malfunction of the brake system or parts of the brake system. As this occurs, the separating valve  6  assumes a pressure limiting function according to the relation I=I[min(P max −P TMC , P PLV )], and the separating valve  6  will open when the maximum pressure is exceeded, and pressure will discharge into the actuating unit  1 . Below the maximum pressure, the separating valve  6  is being closed, or remains closed. The control unit  45  controls the separating valve  6  according to the differential pressures resulting in front of and behind the separating valve  6  or the pressure (behind the separating valve  6 ) in the hydraulic unit  43  and the wheel brakes  10 ,  11 . 
   Possibility C: 
   When equal or rising differences in nominal pressure are detected for a certain time or number of loops in strategy A, the separating valve  6  is energized corresponding to an additional pressure difference in order to close it. With decreasing nominal pressure differences, there is a switch-back into the status according to strategy A: energization according to the nominal pressure.  FIG. 4  depicts the variation of the nominal pressure P nominal  and the variation of the actual pressure P actual . The separating valve  6  is energized at time t 1  according to I=I (P nominal +ΔP) in order to close it. The variation of P actual  without this current increase, which would result due to leakages, is referred to by reference numeral  80 . 
   III. Commencement of Pressure Reduction: 
   Possibility A: 
   Jump to the valve characteristic curve 
   Advantage: quick reaction 
   Possibility B: 
   Jump close to I=I (P nominal +ΔP) of the valve characteristic curve. Thereafter follows a smooth, adapted transition by means of ramp  2  into or onto the valve characteristic curve, i.e. I=I (P nominal ) 
   Advantage: errors of the characteristic curve will no longer be noticeable. 
   Termination of boosting, the pressure of the actuating unit  1  is the wheel pressure again. 
   Current ramp according to I=0 (ramp  3 ). 
   Ramps: They are advantageous because errors in the characteristic curves are not noticeable as an impact at the pedal but rather are removed by slow transitions that remain unnoticed. Further, noise development is greatly reduced.