Abstract:
Disclosed herein is a method of determining the brake pedal actuation force exerted by a vehicle driver on an actuating pedal in an electrohydraulic brake system which includes a brake master cylinder and an electromechanically driven booster stage connected upstream of the brake master cylinder. The brake pedal actuation force sought is determined according to the formula F Ped,Bet =F Bet,Hz −F Verst  by evaluating output signals of a pressure sensor which detects the pressure induced in the brake master cylinder, and of an auxiliary force determining device which detects the boost force made available by the booster stage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/064662, filed Oct. 29, 2008, which claims priority to German Patent Application No. 10 2007 055 509.3, filed Nov. 21, 2007, and German Patent Application No. 10 2008 039 306.1, filed Aug. 22, 2008, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a brake actuating unit consisting of a brake master cylinder which is actuatable by means of a brake pedal and to which wheel brakes of a motor vehicle are connected, of a booster stage which is connected upstream of the brake master cylinder and which additively superposes an auxiliary force on a brake pedal actuation force and actuates a brake master cylinder piston with the sum of both forces, of an electromechanical drive unit for generating the auxiliary force and of a device for determining a brake pedal actuation force. 
       BACKGROUND OF THE INVENTION 
       [0003]    A brake actuating unit of this type is known, for example, from EP 1 638 830 B1, which is incorporated by reference. The brake master cylinder and the booster stage with its electromechanical drive form a pressure generator which generates a braking pressure corresponding to a sum of an actuation force generated by the vehicle driver on a brake pedal and a boost force generated by the booster. The particular features of the previously known brake actuating unit are, inter alia, an input force sensor for detecting a signal representing the actuation force exerted by the driver, and an electronic control unit which activates the electromechanical drive of the booster stage as a function of signals supplied by the input force sensor. In the known actuating unit, the input force sensor is integrated in a piston rod which is coupled to the brake pedal and which, in the event of, inter alia, failure of the electronic control unit, enables an actuation of the brake master cylinder by the brake pedal. The input force sensor therefore moves with the piston rod upon each actuation of the brake pedal. The high manufacturing cost associated with the use of a force sensor is to be regarded as a particular disadvantage of the known brake actuating unit. The electrical connection of the force sensor to the electronic control unit is also very complex and costly, because, as already mentioned, the force sensor is integrated in the piston rod and therefore is arranged movably in relation to the electronic control unit. 
       SUMMARY OF THE INVENTION 
       [0004]    It is therefore an object of the present invention to propose a brake actuating unit of the type mentioned in the introduction in which precise determination of the pedal actuation force exerted by the vehicle driver is possible, which determination can be implemented at low cost and, in particular, requires no electrical connection of moving components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings. Included in the drawings is the following figures: 
           [0006]      FIG. 1  is a schematic representation of a first embodiment of the subject matter of the invention; 
           [0007]      FIG. 2  is a schematic representation of a second embodiment of the subject matter of the invention; 
           [0008]      FIG. 3  is a schematic representation of a second embodiment of the subject matter of the invention, and 
           [0009]      FIG. 4  is a schematic representation of a fourth embodiment of the subject matter of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    The brake actuating unit shown in a schematic representation in  FIG. 1  consists essentially of a brake master cylinder  1 , preferably a tandem master cylinder, a booster stage  2  operatively connected upstream of the brake master cylinder  1 , and a drive unit  3  which serves to drive the booster stage  2  and is formed by an electric motor. For activation of the brake actuating unit according to aspects of the invention there is provided a brake pedal  4 , to which is coupled a piston rod  23  which is in force-transmitting communication, via an interposed booster piston  13 , with a first piston or primary piston  22  of the brake master cylinder  1 . Together with a second or secondary piston (not shown) of the brake master cylinder, the primary piston  22  delimits pressure chambers (not shown) to which wheel brakes  7 ,  8 ,  9 ,  10  of a motor vehicle are connected via an interposed wheel pressure modulating device  24 . The booster piston  13  is guided in an axially displaceable manner in a booster housing  17  in which it delimits a hydraulic booster chamber denoted by reference numeral  18 . Force is transmitted from the drive stage  3  to the booster stage  2  by means of an electrohydraulic cylinder/piston arrangement  5 , the piston  6  of which is driven by an electromechanical actuator or electric motor. The brake actuating unit is supplied with electrical energy from an onboard electrical network (not shown) of the vehicle. In order to make the operation of the brake actuating unit independent of the availability of the vehicle onboard network, which according to breakdown statistics has a high failure rate in comparison to brake systems, there is provided an electrical energy accumulator  16  from which the electronic control unit  15  and the electric motor  3  are supplied with buffered electrical energy. A pressure chamber  37  of the cylinder/piston arrangement  5 , delimited by the piston  6 , is connected to the aforementioned booster chamber  18  by means of a connecting line  19  which enables the booster piston  13  to be subjected to a pressure induced in the pressure chamber  37 , from which results a boost force F Verst  acting on the booster piston  13 . A pressure sensor  12  connected to the connecting line  19  is used to detect the pressure induced in the pressure chamber  37 . In a rest state of the brake actuating unit, a line section  38  establishes a hydraulic connection of the hydraulic chambers  37  and  18 , connected by the line  19 , to an unpressurized pressure medium reservoir  20 , The line section  38  can be blocked by means of a preferably electromagnetically activatable 2/2-way valve  21 . A displacement sensor  11  is used to detect the distance s 1  traveled by the piston  6  of the cylinder/piston arrangement  5 , while the distance s 2  traveled by the booster piston  13  is detected by a second displacement sensor  14 . The integrity of the line connection  19  and the blocking function of the valve  21  can be determined in operation of the brake actuating unit by means of correlation of the two displacement signals. The pressure induced upon actuation of the brake master cylinder  1  is determined with a pressure sensor  30 , which in the example illustrated is integrated in the wheel pressure modulating device  24 . As is known to a person skilled in the art, the booster piston  13  performs the function of an addition element which transmits the total force 
         [0000]    
       
      
       F 
       Bet,Hz 
       =F 
       Ped,Bet 
       +F 
       Verst  
      
     
         [0000]    to the primary piston  22  of the brake master cylinder. The two aforementioned pressure sensors  12 ,  30  form a brake pedal actuation force detecting device, the function of which is explained in the following text. The above-described first variant of the invention has the advantage that both the actuation force F Bet,Hz  of the brake master cylinder  1  and the boost force F Verst  can be detected using pressure sensors well tried in motor vehicle brake technology. It is also advantageous that the electrohydraulic cylinder/piston arrangement  5  can be arranged in a separate module. For example, it can be allocated, together with the electronic control unit  15 , to the wheel pressure modulating device  24  connected downstream, which may be in the form of an ABS or ESP unit. It is therefore also possible to integrate the electronic control unit  15  in an electronic ABS/ESP control unit (not shown). 
         [0011]    The principle for determining the pedal actuation force F Ped,Bet  acting on the brake pedal  4  is the previously described equation according to which the pedal actuation force sought is 
         [0000]        F   Ped,Bet   =F   Bet,Hz   −F   Verst . 
         [0000]    The subtraction indicated is carried out mathematically in the aforementioned electronic control unit  15 . In this calculation, use is made of the fact that the total force F Bet,Hz  is proportional to the hydraulic pressure p Hz , measured by the second pressure sensor  39 , and the boost force F Verst  is proportional to the hydraulic pressure in the booster chamber  18  measured by the first hydraulic sensor  12 . The parameters required for the calculation, in particular proportionality factors, are stored in the electronic control unit  15 . Otherwise, the person skilled in the art is familiar with the operation of the brake system described, so that a detailed description is unnecessary. 
         [0012]    In the second embodiment of the invention, represented in  FIG. 2 , the booster stage  2  is formed by a reduction gear  25  which is in the form a ball screw drive  27 ,  28 . In this case the threaded nut  27  is driven by an electric motor  32  serving as a drive, while the threaded spindle  28  is connected in a force-transmitting manner to the brake pedal  4  on one side and to the primary piston  22  of the brake master cylinder  1  on the other. 
         [0013]    Upon activation of the brake actuating unit shown in  FIG. 2 , a force sensor  26  fixed to the housing determines the force F Abstütz  with which the axially immovable, driven threaded nut  27  is supported against a booster housing  29  accommodating the ball screw drive  25  and  27 ,  28  and the drive motor  32 . The support force mentioned is equal to the boost force generated by the ball screw drive  27 ,  28 . In order to determine the pedal actuation force F Bet,Ped  sought, this force, together with the output value of the pressure sensor  30  mentioned in connection with  FIG. 1 , is supplied to an electronic control unit  31 , which calculates the brake master cylinder actuation force F Bet,Hz  acting on the primary piston  22  from the pressure value P Hz  supplied by the pressure sensor  30 . 
         [0014]    In a concluding step for determining the brake pedal actuation force F Bet,Ped  sought, the boost force is subtracted from the brake master cylinder actuation force F Bet,Hz , in accordance with the equation established in the context of the above-described first embodiment of the invention. 
         [0015]    The structure of the third embodiment of the invention, shown in  FIG. 3 , corresponds extensively to the structure of the second embodiment described in the previous paragraph. The only difference is that, instead of the force sensor for determining the aforementioned support force, a device  33  for estimating said force is provided, the output value of the device  33  being used instead of the signal of the force sensor  26  in calculating the pedal actuation force F Bet,Ped . The output value of the electronic control unit  31  used for activating the drive motor  32  is supplied as an input value to the force-estimating device  33 . 
         [0016]    The fourth embodiment of the invention shown in  FIG. 4  has substantially all the technical features which have been described in connection with the first embodiment of the subject matter of the invention shown in  FIG. 1 , and the reference numerals of  FIG. 4  have been increased by  100  in relation to those in  FIG. 1 . In this illustration the proposed processing of the signals representing the brake master cylinder actuation force F Bet,Hz  and the boost force F Verst  and the subtraction thereof in order to form a signal for the pedal actuation force F Bet,Ped  are elaborated in particular. The components energy accumulator ( 16  in  FIG. 1 ) and the displacement sensor for detecting the travel of the piston  106  ( 11  in  FIG. 1 ), which are less important in this context, have been omitted.